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Is there a structural difference between Nerve cells in the Somatic and Autonomic Nervous Systems

Is there a structural difference between Nerve cells in the Somatic and Autonomic Nervous Systems


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Do the actual nerve cells in the Somatic and Autonomic Nervous Systems have a different structure or chemical make up or are they the same thing just separate systems?


The Structure of Reflexes

One difference between a somatic reflex, such as the withdrawal reflex, and a visceral reflex, which is an autonomic reflex, is in the efferent branch. The output of a somatic reflex is the lower motor neuron in the ventral horn of the spinal cord that projects directly to a skeletal muscle to cause its contraction. The output of a visceral reflex is a two-step pathway starting with the preganglionic fiber emerging from a lateral horn neuron in the spinal cord, or a cranial nucleus neuron in the brain stem, to a ganglion—followed by the postganglionic fiber projecting to a target effector. The other part of a reflex, the afferent branch, is often the same between the two systems. Sensory neurons receiving input from the periphery—with cell bodies in the sensory ganglia, either of a cranial nerve or a dorsal root ganglion adjacent to the spinal cord—project into the CNS to initiate the reflex (Figure 16.2.1). The Latin root “effere” means “to carry.” Adding the prefix “ef-” suggests the meaning “to carry away,” whereas adding the prefix “af-” suggests “to carry toward or inward.”

Figure 16.2.1 – Comparison of Somatic and Visceral Reflexes: The afferent inputs to somatic and visceral reflexes are essentially the same, whereas the efferent branches are different. Somatic reflexes, for instance, involve a direct connection from the ventral horn of the spinal cord to the skeletal muscle. Visceral reflexes involve a projection from the central neuron to a ganglion, followed by a second projection from the ganglion to the target effector.


BIO 140 - Human Biology I - Textbook

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Chapter 33

Basic Structure and Function of the Nervous System

  • Identify the anatomical and functional divisions of the nervous system
  • Relate the functional and structural differences between gray matter and white matter structures of the nervous system to the structure of neurons
  • List the basic functions of the nervous system

The picture you have in your mind of the nervous system probably includes the brain , the nervous tissue contained within the cranium, and the spinal cord , the extension of nervous tissue within the vertebral column. That suggests it is made of two organs&mdashand you may not even think of the spinal cord as an organ&mdashbut the nervous system is a very complex structure. Within the brain, many different and separate regions are responsible for many different and separate functions. It is as if the nervous system is composed of many organs that all look similar and can only be differentiated using tools such as the microscope or electrophysiology. In comparison, it is easy to see that the stomach is different than the esophagus or the liver, so you can imagine the digestive system as a collection of specific organs.

The Central and Peripheral Nervous Systems

The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is everything else (Figure 1). The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the CNS is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery&mdashmeaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal.

Figure 1: The structures of the PNS are referred to as ganglia and nerves, which can be seen as distinct structures. The equivalent structures in the CNS are not obvious from this overall perspective and are best examined in prepared tissue under the microscope.

Nervous tissue, present in both the CNS and PNS, contains two basic types of cells: neurons and glial cells. A glial cell is one of a variety of cells that provide a framework of tissue that supports the neurons and their activities. The neuron is the more functionally important of the two, in terms of the communicative function of the nervous system. To describe the functional divisions of the nervous system, it is important to understand the structure of a neuron. Neurons are cells and therefore have a soma , or cell body, but they also have extensions of the cell each extension is generally referred to as a process . There is one important process that every neuron has called an axon , which is the fiber that connects a neuron with its target. Another type of process that branches off from the soma is the dendrite . Dendrites are responsible for receiving most of the input from other neurons. Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons. These two regions within nervous system structures are often referred to as gray matter (the regions with many cell bodies and dendrites) or white matter (the regions with many axons). Figure 2 demonstrates the appearance of these regions in the brain and spinal cord. The colors ascribed to these regions are what would be seen in &ldquofresh,&rdquo or unstained, nervous tissue. Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But white matter is white because axons are insulated by a lipid-rich substance called myelin . Lipids can appear as white (&ldquofatty&rdquo) material, much like the fat on a raw piece of chicken or beef. Actually, gray matter may have that color ascribed to it because next to the white matter, it is just darker&mdashhence, gray.

The distinction between gray matter and white matter is most often applied to central nervous tissue, which has large regions that can be seen with the unaided eye. When looking at peripheral structures, often a microscope is used and the tissue is stained with artificial colors. That is not to say that central nervous tissue cannot be stained and viewed under a microscope, but unstained tissue is most likely from the CNS&mdashfor example, a frontal section of the brain or cross section of the spinal cord.

Figure 2: A brain removed during an autopsy, with a partial section removed, shows white matter surrounded by gray matter. Gray matter makes up the outer cortex of the brain. (credit: modification of work by &ldquoSuseno&rdquo/Wikimedia Commons)

Regardless of the appearance of stained or unstained tissue, the cell bodies of neurons or axons can be located in discrete anatomical structures that need to be named. Those names are specific to whether the structure is central or peripheral. A localized collection of neuron cell bodies in the CNS is referred to as a nucleus . In the PNS, a cluster of neuron cell bodies is referred to as a ganglion . Figure 3 indicates how the term nucleus has a few different meanings within anatomy and physiology. It is the center of an atom, where protons and neutrons are found it is the center of a cell, where the DNA is found and it is a center of some function in the CNS. There is also a potentially confusing use of the word ganglion (plural = ganglia) that has a historical explanation. In the central nervous system, there is a group of nuclei that are connected together and were once called the basal ganglia before &ldquoganglion&rdquo became accepted as a description for a peripheral structure. Some sources refer to this group of nuclei as the &ldquobasal nuclei&rdquo to avoid confusion.

Figure 3: (a) The nucleus of an atom contains its protons and neutrons. (b) The nucleus of a cell is the organelle that contains DNA. (c) A nucleus in the CNS is a localized center of function with the cell bodies of several neurons, shown here circled in red. (credit c: &ldquoWas a bee&rdquo/Wikimedia Commons)

Terminology applied to bundles of axons also differs depending on location. A bundle of axons, or fibers, found in the CNS is called a tract whereas the same thing in the PNS would be called a nerve . There is an important point to make about these terms, which is that they can both be used to refer to the same bundle of axons. When those axons are in the PNS, the term is nerve, but if they are CNS, the term is tract. The most obvious example of this is the axons that project from the retina into the brain. Those axons are called the optic nerve as they leave the eye, but when they are inside the cranium, they are referred to as the optic tract. There is a specific place where the name changes, which is the optic chiasm, but they are still the same axons (Figure 4). A similar situation outside of science can be described for some roads. Imagine a road called &ldquoBroad Street&rdquo in a town called &ldquoAnyville.&rdquo The road leaves Anyville and goes to the next town over, called &ldquoHometown.&rdquo When the road crosses the line between the two towns and is in Hometown, its name changes to &ldquoMain Street.&rdquo That is the idea behind the naming of the retinal axons. In the PNS, they are called the optic nerve, and in the CNS, they are the optic tract. Table 1 helps to clarify which of these terms apply to the central or peripheral nervous systems.

Figure 4: This drawing of the connections of the eye to the brain shows the optic nerve extending from the eye to the chiasm, where the structure continues as the optic tract. The same axons extend from the eye to the brain through these two bundles of fibers, but the chiasm represents the border between peripheral and central.

Table 1: Structures of the CNS and PNS

CNS PNS
Group of Neuron Cell Bodies (i.e., gray matter) Nucleus Ganglion
Bundle of Axons (i.e., white matter) Tract Nerve

Functional Divisions of the Nervous System

The nervous system can also be divided on the basis of its functions, but anatomical divisions and functional divisions are different. The CNS and the PNS both contribute to the same functions, but those functions can be attributed to different regions of the brain (such as the cerebral cortex or the hypothalamus) or to different ganglia in the periphery. The problem with trying to fit functional differences into anatomical divisions is that sometimes the same structure can be part of several functions. For example, the optic nerve carries signals from the retina that are either used for the conscious perception of visual stimuli, which takes place in the cerebral cortex, or for the reflexive responses of smooth muscle tissue that are processed through the hypothalamus.

There are two ways to consider how the nervous system is divided functionally. First, the basic functions of the nervous system are sensation, integration, and response. Secondly, control of the body can be somatic or autonomic&mdashdivisions that are largely defined by the structures that are involved in the response. There is also a region of the peripheral nervous system that is called the enteric nervous system that is responsible for a specific set of the functions within the realm of autonomic control related to gastrointestinal functions.

Basic Functions

The nervous system is involved in receiving information about the environment around us (sensation) and generating responses to that information (motor responses). The nervous system can be divided into regions that are responsible for sensation (sensory functions) and for the response (motor functions). But there is a third function that needs to be included. Sensory input needs to be integrated with other sensations, as well as with memories, emotional state, or learning (cognition). Some regions of the nervous system are termed integration or association areas. The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to produce a response.

Sensation. The first major function of the nervous system is sensation&mdashreceiving information about the environment to gain input about what is happening outside the body (or, sometimes, within the body). The sensory functions of the nervous system register the presence of a change from homeostasis or a particular event in the environment, known as a stimulus . The senses we think of most are the &ldquobig five&rdquo: taste, smell, touch, sight, and hearing. The stimuli for taste and smell are both chemical substances (molecules, compounds, ions, etc.), touch is physical or mechanical stimuli that interact with the skin, sight is light stimuli, and hearing is the perception of sound, which is a physical stimulus similar to some aspects of touch. There are actually more senses than just those, but that list represents the major senses. Those five are all senses that receive stimuli from the outside world, and of which there is conscious perception. Additional sensory stimuli might be from the internal environment (inside the body), such as the stretch of an organ wall or the concentration of certain ions in the blood.

Response. The nervous system produces a response on the basis of the stimuli perceived by sensory structures. An obvious response would be the movement of muscles, such as withdrawing a hand from a hot stove, but there are broader uses of the term. The nervous system can cause the contraction of all three types of muscle tissue. For example, skeletal muscle contracts to move the skeleton, cardiac muscle is influenced as heart rate increases during exercise, and smooth muscle contracts as the digestive system moves food along the digestive tract. Responses also include the neural control of glands in the body as well, such as the production and secretion of sweat by the eccrine and merocrine sweat glands found in the skin to lower body temperature.

Responses can be divided into those that are voluntary or conscious (contraction of skeletal muscle) and those that are involuntary (contraction of smooth muscles, regulation of cardiac muscle, activation of glands). Voluntary responses are governed by the somatic nervous system and involuntary responses are governed by the autonomic nervous system, which are discussed in the next section.

Integration. Stimuli that are received by sensory structures are communicated to the nervous system where that information is processed. This is called integration. Stimuli are compared with, or integrated with, other stimuli, memories of previous stimuli, or the state of a person at a particular time. This leads to the specific response that will be generated. Seeing a baseball pitched to a batter will not automatically cause the batter to swing. The trajectory of the ball and its speed will need to be considered. Maybe the count is three balls and one strike, and the batter wants to let this pitch go by in the hope of getting a walk to first base. Or maybe the batter&rsquos team is so far ahead, it would be fun to just swing away.

Controlling the Body

The nervous system can be divided into two parts mostly on the basis of a functional difference in responses. The somatic nervous system (SNS) is responsible for conscious perception and voluntary motor responses. Voluntary motor response means the contraction of skeletal muscle, but those contractions are not always voluntary in the sense that you have to want to perform them. Some somatic motor responses are reflexes, and often happen without a conscious decision to perform them. If your friend jumps out from behind a corner and yells &ldquoBoo!&rdquo you will be startled and you might scream or leap back. You didn&rsquot decide to do that, and you may not have wanted to give your friend a reason to laugh at your expense, but it is a reflex involving skeletal muscle contractions. Other motor responses become automatic (in other words, unconscious) as a person learns motor skills (referred to as &ldquohabit learning&rdquo or &ldquoprocedural memory&rdquo).

The autonomic nervous system (ANS) is responsible for involuntary control of the body, usually for the sake of homeostasis (regulation of the internal environment). Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue. The role of the autonomic system is to regulate the organ systems of the body, which usually means to control homeostasis. Sweat glands, for example, are controlled by the autonomic system. When you are hot, sweating helps cool your body down. That is a homeostatic mechanism. But when you are nervous, you might start sweating also. That is not homeostatic, it is the physiological response to an emotional state.

There is another division of the nervous system that describes functional responses. The enteric nervous system (ENS) is responsible for controlling the smooth muscle and glandular tissue in your digestive system. It is a large part of the PNS, and is not dependent on the CNS. It is sometimes valid, however, to consider the enteric system to be a part of the autonomic system because the neural structures that make up the enteric system are a component of the autonomic output that regulates digestion. There are some differences between the two, but for our purposes here there will be a good bit of overlap. See Figure 5 for examples of where these divisions of the nervous system can be found.

Figure 5: Somatic structures include the spinal nerves, both motor and sensory fibers, as well as the sensory ganglia (posterior root ganglia and cranial nerve ganglia). Autonomic structures are found in the nerves also, but include the sympathetic and parasympathetic ganglia. The enteric nervous system includes the nervous tissue within the organs of the digestive tract.

Visit this site linked to below to read about a woman that notices that her daughter is having trouble walking up the stairs. This leads to the discovery of a hereditary condition that affects the brain and spinal cord. The electromyography and MRI tests indicated deficiencies in the spinal cord and cerebellum, both of which are responsible for controlling coordinated movements. To what functional division of the nervous system would these structures belong?

Everyday Connection

How Much of Your Brain Do You Use?

Have you ever heard the claim that humans only use 10 percent of their brains? Maybe you have seen an advertisement on a website saying that there is a secret to unlocking the full potential of your mind&mdashas if there were 90 percent of your brain sitting idle, just waiting for you to use it. If you see an ad like that, don&rsquot click. It isn&rsquot true.

An easy way to see how much of the brain a person uses is to take measurements of brain activity while performing a task. An example of this kind of measurement is functional magnetic resonance imaging (fMRI), which generates a map of the most active areas and can be generated and presented in three dimensions (Figure 6). This procedure is different from the standard MRI technique because it is measuring changes in the tissue in time with an experimental condition or event.

Figure 6: This fMRI shows activation of the visual cortex in response to visual stimuli. (credit: &ldquoSuperborsuk&rdquo/Wikimedia Commons)

The underlying assumption is that active nervous tissue will have greater blood flow. By having the subject perform a visual task, activity all over the brain can be measured. Consider this possible experiment: the subject is told to look at a screen with a black dot in the middle (a fixation point). A photograph of a face is projected on the screen away from the center. The subject has to look at the photograph and decipher what it is. The subject has been instructed to push a button if the photograph is of someone they recognize. The photograph might be of a celebrity, so the subject would press the button, or it might be of a random person unknown to the subject, so the subject would not press the button.

In this task, visual sensory areas would be active, integrating areas would be active, motor areas responsible for moving the eyes would be active, and motor areas for pressing the button with a finger would be active. Those areas are distributed all around the brain and the fMRI images would show activity in more than just 10 percent of the brain (some evidence suggests that about 80 percent of the brain is using energy&mdashbased on blood flow to the tissue&mdashduring well-defined tasks similar to the one suggested above). This task does not even include all of the functions the brain performs. There is no language response, the body is mostly lying still in the MRI machine, and it does not consider the autonomic functions that would be ongoing in the background.

Chapter Review

The nervous system can be separated into divisions on the basis of anatomy and physiology. The anatomical divisions are the central and peripheral nervous systems. The CNS is the brain and spinal cord. The PNS is everything else. Functionally, the nervous system can be divided into those regions that are responsible for sensation, those that are responsible for integration, and those that are responsible for generating responses. All of these functional areas are found in both the central and peripheral anatomy.

Considering the anatomical regions of the nervous system, there are specific names for the structures within each division. A localized collection of neuron cell bodies is referred to as a nucleus in the CNS and as a ganglion in the PNS. A bundle of axons is referred to as a tract in the CNS and as a nerve in the PNS. Whereas nuclei and ganglia are specifically in the central or peripheral divisions, axons can cross the boundary between the two. A single axon can be part of a nerve and a tract. The name for that specific structure depends on its location.

Nervous tissue can also be described as gray matter and white matter on the basis of its appearance in unstained tissue. These descriptions are more often used in the CNS. Gray matter is where nuclei are found and white matter is where tracts are found. In the PNS, ganglia are basically gray matter and nerves are white matter.

The nervous system can also be divided on the basis of how it controls the body. The somatic nervous system (SNS) is responsible for functions that result in moving skeletal muscles. Any sensory or integrative functions that result in the movement of skeletal muscle would be considered somatic. The autonomic nervous system (ANS) is responsible for functions that affect cardiac or smooth muscle tissue, or that cause glands to produce their secretions. Autonomic functions are distributed between central and peripheral regions of the nervous system. The sensations that lead to autonomic functions can be the same sensations that are part of initiating somatic responses. Somatic and autonomic integrative functions may overlap as well.

A special division of the nervous system is the enteric nervous system, which is responsible for controlling the digestive organs. Parts of the autonomic nervous system overlap with the enteric nervous system. The enteric nervous system is exclusively found in the periphery because it is the nervous tissue in the organs of the digestive system.

Reference

Kramer, PD. Listening to prozac. 1st ed. New York (NY): Penguin Books 1993.


Functional Divisions of the Nervous System

In addition to the anatomical divisions listed above, the nervous system can also be divided on the basis of its functions. The nervous system is involved in receiving information about the environment around us (sensory functions, sensation) and generating responses to that information (motor functions, responses) and coordinating the two (integration).

Sensation. Sensation refers to receiving information about the environment, either what is happening outside (ie: heat from the sun) or inside the body (ie: heat from muscle activity). These sensations are known as stimuli (singular = stimulus) and different sensory receptors are responsible for detecting different stimuli. Sensory information travels towards the CNS through the PNS nerves in the specific division known as the afferent (sensory) branch of the PNS. When information arises from sensory receptors in the skin, skeletal muscles, or joints this is known as somatic sensory information when information arises from sensory receptors in the blood vessels or internal organs, this is known as visceral sensory information.

Response. The nervous system produces a response in effector organs (such as muscles or glands) due to the sensory stimuli. The motor (efferent) branch of the PNS carries signals away from the CNS to the effector organs. When the effector organ is a skeletal muscle, the information is called somatic motor when the effector organ is cardiac or smooth muscle or glandular tissue, the information is called visceral (autonomic) motor. Voluntary responses are governed by the somatic nervous system and involuntary responses are governed by the autonomic nervous system, which are discussed in the next section.

Integration. Stimuli that are received by sensory structures are communicated to the nervous system where that information is processed. This is called integration (see Figure 12.1.2 below). In the CNS, stimuli are compared with, or integrated with, other stimuli, memories of previous stimuli, or the state of a person at a particular time. This leads to the specific response that will be generated.

Figure 12.1.2 – Nervous System Function: Integration occurs in the CNS where sensory information from the periphery is processed and interpreted. The CNS then creates a motor plan that is executed by the efferent branch working with effector organs.

Chapter Review

The nervous system can be separated into divisions on the basis of anatomy and physiology. The anatomical divisions are the central and peripheral nervous systems. The CNS is the brain and spinal cord. The PNS is everything else and includes afferent and efferent branches with further subdivisions for somatic, visceral and autonomic function. Functionally, the nervous system can be divided into those regions that are responsible for sensation, those that are responsible for integration, and those that are responsible for generating responses.


Is pupillary reflex somatic or autonomic?

Pupillary Reflex Pathways. The pupil is under competing autonomic control in response to light levels hitting the retina. The sympathetic system will dilate the pupil when the retina is not receiving enough light, and the parasympathetic system will constrict the pupil when too much light hits the retina.

Also, what type of reflex is the pupillary light reflex? The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation of vision to various levels of lightness/

Accordingly, are reflexes autonomic or somatic?

There are two types: autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles). Autonomic reflexes sometimes involve the spinal cord and some somatic reflexes are mediated more by the brain than the spinal cord.

Is the pupillary reflex Monosynaptic?

The simplest reflexes are monosynaptic, such as the stretch or myotatic reflex. Ocular auto- nomic reflexes include the oculocardiac, pupillary, accommodative and lacrimatory reflexes.


Is there a structural difference between Nerve cells in the Somatic and Autonomic Nervous Systems - Psychology

The nervous system is a remarkable collection of cells that governs both involuntary and voluntary behavior, while also maintaining homeostasis. Functions of the nervous system include:

·&emspCognition (thinking) and problem-solving

·&emspExecutive function and planning

·&emspLanguage comprehension and creation

·&emspEmotion and emotional expression

·&emspRegulation of endocrine organs

·&emspRegulation of heart rate, breathing rate, vascular resistance, temperature, and exocrine glands

The human nervous system is a complex web of over 100 billion cells that communicate, coordinate, and regulate signals for the rest of the body. Mental and physical action occurs when the body can react to external stimuli using the nervous system. In this section, we will look at the nervous system and its basic organization.

Note: Much of the information contained in this section is also discussed in Chapter 1 of MCAT Behavioral Sciences Review.

CENTRAL AND PERIPHERAL NERVOUS SYSTEMS

Generally speaking, there are three kinds of nerve cells in the nervous system: sensory neurons, motor neurons, and interneurons. Sensory neurons (also known as afferent neurons) transmit sensory information from receptors to the spinal cord and brain. Motor neurons (also known asefferent neurons) transmit motor information from the brain and spinal cord to muscles and glands. Interneurons are found between other neurons and are the most numerous of the three types. Interneurons are located predominantly in the brain and spinal cord and are often linked to reflexive behavior.

Afferent neurons ascend in the spinal cord toward the brain efferent neurons exit the spinal cord on their way to the rest of the body.

Different types of information require different types of processing. Processing of stimuli and response generation may happen at the level of the spinal cord, or may require input from the brainstem or cerebral cortex. Reflexes, discussed later in this section, only require processing at the level of the spinal cord. For example, when a reflex hammer hits the patellar tendon, the sensory information goes to the spinal cord, where a motor signal is sent to the quadriceps muscle, causing the leg to jerk forward at the knee. No input from the brain is required. However, some scenarios require input from the brain or brainstem. When this happens, supraspinal circuits are used.

Let’s turn to the overall structure of the human nervous system, which is diagrammed in Figure 4.9.

Figure 4.9. Major Divisions of the Nervous System

The nervous system can be broadly divided into two primary components: the central and peripheral nervous systems. The central nervous system (CNS) is composed of the brain and spinal cord. The brain consists of white matter and grey matter. The white matter consists of axons encased in myelin sheaths. The grey matter consists of unmyelinated cell bodies and dendrites. In the brain, the white matter lies deeper than the grey matter. At the base of the brain is the brainstem, which is largely responsible for basic life functions such as breathing. Note that the lobes of the brain and major brain structures are discussed in Chapter 1 of MCAT Behavioral Sciences Review.

The spinal cord extends downward from the brainstem and can be divided into four divisions: cervical, thoracic, lumbar, and sacral. Almost all of the structures below the neck receive sensory and motor innervation from the spinal cord. The spinal cord is protected by the vertebral column, which transmits nerves at the space between adjacent vertebrae. Like the brain, the spinal cord also consists of white and grey matter. The white matter lies on the outside of the cord, and the grey matter is deep within it. The axons of motor and sensory neurons are in the spinal cord. The sensory neurons bring information in from the periphery and enter on the dorsal (back) side of the spinal cord. The cell bodies of these sensory neurons are found in the dorsal root ganglia. Motor neurons exit the spinal cord ventrally, or on the side closest to the front of the body. The structure of the spinal cord can be seen in Figure 4.10.

Figure 4.10. The Spinal Cord Sensory neurons transmit information about pain, temperature, and vibration up to the brain and have cell bodies in the dorsal root ganglia toward the back of the spinal cord the motor neurons run from the brain along the opposite side of the spinal cord in the ventral root and control movements of skeletal muscle and glandular secretions.

The peripheral nervous system (PNS), in contrast, is made up of nerve tissue and fibers outside the brain and spinal cord, such as the 12 pairs of cranial and 31 pairs of spinal nerves. The PNS thus connects the CNS to the rest of the body and can itself be subdivided into the somatic and autonomic nervous systems.

The somatic nervous system consists of sensory and motor neurons distributed throughout the skin, joints, and muscles. Sensory neurons transmit information through afferent fibers. Motor impulses, in contrast, travel along efferent fibers.

The autonomic nervous system (ANS) generally regulates heartbeat, respiration, digestion, and glandular secretions. In other words, the ANS manages the involuntary muscles associated with many internal organs and glands. The ANS also helps regulate body temperature by activating sweating or piloerection, depending on whether we are too hot or too cold. The main thing to understand about these functions is that they are automatic, or independent of conscious control. Note the similarity between the words autonomic and automatic. This association makes it easy to remember that the autonomic nervous system manages automatic functions such as heartbeat, respiration, digestion, and temperature control.

One primary difference between the somatic and autonomic nervous systems is that the peripheral component of the autonomic nervous system contains two neurons. A motor neuron in the somatic nervous system goes directly from the spinal cord to the muscle without synapsing. In the autonomic nervous system, two neurons work in series to transmit messages from the spinal cord. The first neuron is known as the preganglionic neuron, whereas the second is the postganglionic neuron. The soma of the preganglionic neuron is in the CNS, and its axon travels to a ganglion in the PNS. Here it synapses on the cell body of the postganglionic neuron, which then affects the target tissue.

KEY CONCEPT

The first neuron in the autonomic nervous system is called the preganglionic neuron. The second neuron is the postganglionic neuron.

THE AUTONOMIC NERVOUS SYSTEM

The ANS has two subdivisions: the sympathetic nervous system and the parasympathetic nervous system. These two branches often act in opposition to one another, meaning that they are antagonistic. For example, the sympathetic nervous system acts to accelerate heart rate and inhibit digestion, while the parasympathetic nervous system, in contrast, decelerates heart rate and increases digestion.

The main role of the parasympathetic nervous system is to conserve energy. It is associated with resting and sleeping states and acts to reduce heart rate and constrict the bronchi. The parasympathetic nervous system is also responsible for managing digestion by increasing peristalsis and exocrine secretions. Acetylcholine is the neurotransmitter responsible for parasympathetic responses in the body and is released by both preganglionic and postganglionic neurons. The vagus nerve (cranial nerve X), is responsible for much of the parasympathetic innervation of the thoracic and abdominal cavity. The functions of the parasympathetic nervous system are summarized in Figure 4.11.

Figure 4.11. Functions of the Parasympathetic Nervous System

In contrast, the sympathetic nervous system is activated by stress. This can include everything from a mild stressor, such as keeping up with schoolwork, to emergencies that mean the difference between life and death. The sympathetic nervous system is closely associated with rage and fear reactions, also known as “fight-or-flight” reactions. When activated, the sympathetic nervous system:

·&emspRedistributes blood to muscles of locomotion

·&emspIncreases blood glucose concentration

·&emspDecreases digestion and peristalsis

·&emspDilates the eyes to maximize light intake

·&emspReleases epinephrine into the bloodstream

Sympathetic and parasympathetic nervous systems:

·&emspSympathetic: “fight-or-flight

·&emspParasympathetic: “rest-and-digest

The functions of the sympathetic nervous system are summarized in Figure 4.12. In the sympathetic nervous system, preganglionic neurons release acetylcholine, while most postganglionic neurons release norepinephrine.

Figure 4.12. Functions of the Sympathetic Nervous System

Neural circuits called reflex arcs control reflexive behavior. For example, consider what occurs when someone steps on a nail. Receptors in the foot detect pain, and the pain signal is transmitted by sensory neurons up to the spinal cord. At that point, the sensory neurons connect with interneurons, which can then relay pain impulses up to the brain. Rather than wait for the brain to send out a signal, interneurons in the spinal cord can also send signals to the muscles of both legs directly, causing the individual to withdraw the foot with pain while supporting with the other foot. The original sensory information still makes its way up to the brain however, by the time it arrives there, the muscles have already responded to the pain, thanks to the reflex arc. There are two types of reflex arcs: monosynaptic and polysynaptic.

KEY CONCEPT

Consider the purpose of reflexes. Although it may be amusing to make your friends’ legs jump when you tap them, there is a more functional reason why this response occurs. The stretch on the patellar tendon makes the body think that the muscle may be getting overstretched. In response, the muscle contracts in order to prevent injury.

Monosynaptic

In a monosynaptic reflex arc, there is a single synapse between the sensory neuron that receives the stimulus and the motor neuron that responds to it. A classic example is the knee-jerk reflex, shown in Figure 4.13. When the patellar tendon is stretched, information travels up the sensory (afferent, presynaptic) neuron to the spinal cord, where it interfaces with the motor (efferent, postsynaptic) neuron that contracts the quadriceps muscle. The net result is extension of the leg, which lessens the tension on the patellar tendon. Note that the reflex is simply a feedback loop and a response to potential injury. If the patellar tendon or quadriceps muscles are stretched too far, they may tear, damaging the knee joint. Thus, the reflex serves to protect the muscle.

Figure 4.13. The Knee-Jerk Reflex The knee-jerk or knee extension reflex may be elicited by swiftly stretching the patellar tendon with a reflex hammer.

Polysynaptic

In a polysynaptic reflex arc, there is at least one interneuron between the sensory and motor neurons. A real-life example is the reaction to stepping on a nail described earlier, which involves the withdrawal reflex. The leg with which one steps on the nail will be stimulated to flex, using the hip muscles and hamstring muscles, pulling the foot away from the nail. This is a monosynaptic reflex, similar to the knee-jerk reflex described previously. However, if the person is to maintain balance, the other foot must be planted firmly on the ground. For this to occur, the motor neuron that controls the quadriceps muscles in the opposite leg must be stimulated, extending that leg. Interneurons in the spinal cord provide the connections from the incoming sensory information to the motor neurons in the supporting leg.

MCAT Concept Check 4.3:

Before you move on, assess your understanding of the material with these questions.

1. What parts of the nervous system are in the central nervous system (CNS)? Peripheral nervous system (PNS)?

2. What do afferent neurons do? Efferent neurons?

3. What functions are accomplished by the somatic nervous system? The autonomic nervous system?

4. What are the effects of the sympathetic nervous system? The parasympathetic nervous system?

5. What is the pathway of neural impulses in a monosynaptic reflex? In a polysynaptic reflex?


EXAMPLES

Examples of body processes controlled by the ANS include heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, urination, and sexual arousal.

The peripheral nervous system (PNS) is divided into the somatic nervous system and the autonomic nervous system. The somatic nervous system (SoNS) is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles.

The SoNS consists of efferent nerves responsible for stimulating muscle contraction, including all the non-sensory neurons connected with skeletal muscles and skin. The somatic nervous system controls all voluntary muscular systems within the body, and also mediates involuntary reflex arcs. The somatic nervous system consists of three parts:

The human nervous system: The major organs and nerves of the human nervous system.

  1. Spinal nerves are peripheral nerves that carry motor commands and sensory information into the spinal cord.
  2. Cranial nerves are the nerve fibers that carry information into and out of the brain stem. They include information related to smell, vision, eyes, eye muscles, the mouth, taste, ears, the neck, shoulders, and the tongue.
  3. Association nerves integrate sensory input and motor output these nerves number in the thousands.

The autonomic nervous system (ANS) is the part of the peripheral nervous system that acts as a control system, functioning largely below the level of consciousness and controlling visceral functions. The ANS affects heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), and sexual arousal.

Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind. The ANS is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS).

The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.


Neurotransmission: The Autonomic and Somatic Motor Nervous Systems

The autonomic nervous system (ANS a.k.a. the visceral, vegetative , or involuntary nervous system ) regulates autonomic functions that occur without conscious control. In the periphery, it consists of nerves, ganglia, and plexuses that innervate the heart, blood vessels, glands, other visceral organs, and smooth muscle in various tissues.

DIFFERENCES BETWEEN AUTONOMIC AND SOMATIC NERVES

• The efferent nerves of the ANS supply all innervated structures of the body except skeletal muscle, which is served by somatic nerves .

• The most distal synaptic junctions in the autonomic reflex arc occur in ganglia that are entirely outside the cerebrospinal axis . Somatic nerves contain no peripheral ganglia , and the synapses are located entirely within the cerebrospinal axis .

• Many autonomic nerves form extensive peripheral plexuses such networks are absent from the somatic system.

• Postganglionic autonomic nerves generally are nonmyelinated motor nerves to skeletal muscles are myelinated .

• When the spinal efferent nerves are interrupted, smooth muscles and glands generally retain some level of spontaneous activity, whereas the denervated skeletal muscles are paralyzed.

VISCERAL AFFERENT FIBERS. The afferent fibers from visceral structures are the first link in the reflex arcs of the autonomic system. With certain exceptions, such as local axon reflexes, most visceral reflexes are mediated through the central nervous system (CNS).

Information on the status of the visceral organs is transmitted to the CNS through 2 main sensory systems: the cranial nerve (parasympathetic) visceral sensory system and the spinal (sympathetic) visceral afferent system. The cranial visceral sensory system carries mainly mechanoreceptor and chemosensory information , whereas the afferents of the spinal visceral system principally convey sensations related to temperature and tissue injury of mechanical, chemical, or thermal origin.

Cranial visceral sensory information enters the CNS by 4 cranial nerves: the trigeminal (V), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. These 4 cranial nerves transmit visceral sensory information from the internal face and head (V) tongue (taste, VII) hard palate and upper part of the oropharynx (IX) and carotid body, lower part of the oropharynx, larynx, trachea, esophagus, and thoracic and abdominal organs (X), with the exception of the pelvic viscera. The pelvic viscera are innervated by nerves from the second through fourth sacral spinal segments. The visceral afferents from these 4 cranial nerves terminate topographically in the solitary tract nucleus .

Sensory afferents from visceral organs also enter the CNS from the spinal nerves and convey information concerned with temperature as well as nociceptive visceral inputs related to mechanical, chemical, and thermal stimulation. Those concerned with muscle chemosensation may arise at all spinal levels, whereas sympathetic visceral sensory afferents generally arise at the thoracic levels where sympathetic preganglionic neurons are found. The neurotransmitters that mediate transmission from sensory fibers have not been characterized unequivocally. Substance P and calcitonin gene-related peptide (CGRP), are leading candidates for neurotransmitters that communicate nociceptive stimuli from the periphery. Somatostatin (SST), vasoactive intestinal polypeptide (VIP), and cholecystokinin (CCK), also occur in sensory neurons. ATP appears to be a neurotransmitter in certain sensory neurons. Enkephalins, present in interneurons in the dorsal spinal cord, have antinociceptive effects both pre- and postsynaptically to inhibit the release of substance P. The excitatory amino acids glutamate and aspartate also play major roles in transmission of sensory responses to the spinal cord. These transmitters and their signaling pathways are reviewed in Chapter 14.

DIVISIONS OF THE PERIPHERAL AUTONOMIC SYSTEM. The ANS consists of 2 large divisions: the sympathetic and the parasympathetic (Figure 8–1).

Figure 8–1 The autonomic nervous system. Yellow , cholinergic red , adrenergic dotted blue , visceral afferent solid lines , preganglionic broken lines , postganglionic. The rectangle at right shows the finer details of the ramifications of adrenergic fibers at any 1 segment of the spinal cord, the path of the visceral afferent nerves, the cholinergic nature of somatic motor nerves to skeletal muscle, and the presumed cholinergic nature of the vasodilator fibers in the dorsal roots of the spinal nerves. The asterisk (*) indicates that it is not known whether these vasodilator fibers are motor or sensory or where their cell bodies are situated.

The neurotransmitter of all preganglionic autonomic fibers, most postganglionic parasympathetic fibers, and a few postganglionic sympathetic fibers is acetylcholine (ACh). Some postganglionic parasympathetic nerves use nitric oxide (NO) and are referred to as nitrergic . The majority of the postganglionic sympathetic fibers are adrenergic , in which the transmitter is norepinephrine (NE, noradrenaline). The terms cholinergic and adrenergic describe neurons that liberate ACh or NE, respectively. Substance P and glutamate may also mediate many afferent impulses.

SYMPATHETIC NERVOUS SYSTEM. The cells that give rise to the preganglionic fibers of this division lie mainly in the intermediolateral columns of the spinal cord and extend from the first thoracic to the second or third lumbar segment. The axons from these cells are carried in the anterior (ventral) nerve roots and synapse, with neurons lying in sympathetic ganglia outside the cerebrospinal axis. Sympathetic ganglia are found in 3 locations: paravertebral, prevertebral, and terminal.

The 22 pairs of paravertebral sympathetic ganglia form the lateral chains on either side of the vertebral column. The ganglia are connected to each other by nerve trunks and to the spinal nerves by rami communicantes . The white rami carry the preganglionic myelinated fibers that exit the spinal cord by the anterior spinal roots. The gray rami carry postganglionic fibers back to the spinal nerves for distribution to sweat glands and pilomotor muscles and to blood vessels of skeletal muscle and skin. The prevertebral ganglia lie in the abdomen and the pelvis near the ventral surface of the bony vertebral column and consist mainly of the celiac (solar), superior mesenteric, aorticorenal, and inferior mesenteric ganglia. The terminal ganglia are few in number, lie near the organs they innervate, and include ganglia connected with the urinary bladder and rectum and the cervical ganglia in the region of the neck. In addition, small intermediate ganglia lie outside the conventional vertebral chain, especially in the thoracolumbar region. They are variable in number and location but usually are in close proximity to the communicating rami and the anterior spinal nerve roots.

Preganglionic fibers from the spinal cord may synapse with the neurons of more than 1 sympathetic ganglion. Their principal ganglia of termination need not correspond to the original level from which the preganglionic fiber exits the spinal cord. Many of the preganglionic fibers from the fifth to the last thoracic segment pass through the paravertebral ganglia to form the splanchnic nerves. Most of the splanchnic nerve fibers do not synapse until they reach the celiac ganglion others directly innervate the adrenal medulla ( see below).

Postganglionic fibers arising from sympathetic ganglia innervate visceral structures of the thorax, abdomen, head, and neck. The trunk and the limbs are supplied by the sympathetic fibers in spinal nerves. The prevertebral ganglia contain cell bodies whose axons innervate the glands and smooth muscles of the abdominal and the pelvic viscera. Many of the upper thoracic sympathetic fibers from the vertebral ganglia form terminal plexuses, such as the cardiac, esophageal, and pulmonary plexuses. The sympathetic distribution to the head and the neck (vasomotor, pupillodilator, secretory, and pilomotor) is by means of the cervical sympathetic chain and its 3 ganglia. All postganglionic fibers in this chain arise from cell bodies located in these 3 ganglia all preganglionic fibers arise from the upper thoracic segments of the spinal cord, there being no sympathetic fibers that leave the CNS above the first thoracic level.

Pharmacologically, the chromaffin cells of the adrenal medulla resemble a collection of postganglionic sympathetic nerve cells. Typical preganglionic fibers that release ACh innervate these chromaffin cells, stimulating the release of epinephrine (EPI, adrenaline), in distinction to the NE released by postganglionic sympathetic fibers.

PARASYMPATHETIC NERVOUS SYSTEM. The parasympathetic nervous system consists of preganglionic fibers that originate in the CNS and their postganglionic connections. The regions of central origin are the midbrain, the medulla oblongata, and the sacral part of the spinal cord. The midbrain, or tectal, outflow consists of fibers arising from the Edinger-Westphal nucleus of the third cranial nerve and going to the ciliary ganglion in the orbit. The medullary outflow consists of the parasympathetic components of the VII, IX, and X cranial nerves.

The fibers in the VII (facial) cranial nerve form the chorda tympani, which innervates the ganglia lying on the submaxillary and sublingual glands. They also form the greater superficial petrosal nerve, which innervates the sphenopalatine ganglion. The autonomic components of the IX (glossopharyngeal) cranial nerve innervate the otic ganglia. Postganglionic parasympathetic fibers from these ganglia supply the sphincter of the iris (pupillary constrictor muscle), the ciliary muscle, the salivary and lacrimal glands, and the mucous glands of the nose, mouth, and pharynx. These fibers also include vasodilator nerves to these same organs. The X (vagus) cranial nerve arises in the medulla and contains preganglionic fibers, most of which do not synapse until they reach the many small ganglia lying directly on or in the viscera of the thorax and abdomen. In the intestinal wall, the vagal fibers terminate around ganglion cells in the myenteric and submucosal plexuses. Thus, in the parasympathetic branch of the autonomic nervous system, preganglionic fibers are very long, whereas postganglionic fibers are very short . The vagus nerve also carries a far greater number of afferent fibers (but apparently no pain fibers) from the viscera into the medulla. The parasympathetic sacral outflow consists of axons that arise from cells in the second, third, and fourth segments of the sacral cord and proceed as preganglionic fibers to form the pelvic nerves ( nervi erigentes ). They synapse in terminal ganglia lying near or within the bladder, rectum, and sexual organs. The vagal and sacral outflows provide motor and secretory fibers to thoracic, abdominal, and pelvic organs ( see Figure 8–1).

ENTERIC NERVOUS SYSTEM. The processes of mixing, propulsion, and absorption of nutrients in the GI tract are controlled through the enteric nervous system (ENS). The ENS consists of both afferent sensory neurons and a number of motor nerves and interneurons that are organized principally into 2 nerve plexuses: the myenteric (Auerbach) plexus and the submucosal (Meissner) plexus .

The myenteric plexus, located between the longitudinal and circular muscle layers, plays an important role in the contraction and relaxation of GI smooth muscle. The submucosal plexus is involved with secretory and absorptive functions of the GI epithelium, local blood flow, and neuroimmune activities. The ENS incorporates components of the sympathetic and parasympathetic nervous systems and has sensory nerve connections through the spinal and nodose ganglia ( see Figure 46–1). Parasympathetic preganglionic inputs are provided to the GI tract via the vagus and pelvic nerves. ACh released from preganglionic neurons activates nicotinic ACh receptors (nAChRs) on postganglionic neurons within the enteric ganglia. Excitatory preganglionic input activates both excitatory and inhibitory motor neurons that control processes such as muscle contraction and secretion/absorption. Postganglionic sympathetic nerves also synapse with intrinsic neurons and generally induce relaxation. Sympathetic input is excitatory (contractile) at some sphincters. Information from afferent and preganglionic neural inputs to the enteric ganglia is integrated and distributed by a network of interneurons. ACh is the primary neurotransmitter providing excitatory inputs between interneurons, but other substances such as ATP (via postjunctional P2X receptors), substance P (by NK 3 receptors), and serotonin (via 5HT 3 receptors) are also important in mediating integrative processing via interneurons.

The muscle layers of the GI tract are dually innervated by excitatory and inhibitory motor neurons with cell bodies primarily in the myenteric ganglia. ACh is a primary excitatory motor neurotransmitter released from postganglionic neurons. ACh activates M 2 and M 3 receptors in postjunctional cells to elicit motor responses. Pharmacological blockade of muscarinic cholinergic (mAChRs) receptors does not block all excitatory neurotransmission, however, because neurokinins (neurokinin A and Substance P) are also coreleased by excitatory motor neurons and contribute to postjunctional excitation. Inhibitory motor neurons in the GI tract regulate motility events such as accommodation, sphincter relaxation, and descending receptive relaxation. Inhibitory responses are elicited by a purine derivative (either ATP or β-nicotinamide adenine dinucleotide (β-NAD) acting at postjunctional P2Y 1 receptors) and NO. Inhibitory neuropeptides, such as VIP and pituitary adenylyl cyclase-activating peptide (PACAP), may also be released from inhibitory motor neurons under conditions of strong stimulation.

COMPARISON OF SYMPATHETIC, PARASYMPATHETIC, AND MOTOR NERVES (FIGURE 8–2)

• The sympathetic system is distributed to effectors throughout the body, whereas parasympathetic distribution is much more limited.

• A preganglionic sympathetic fiber may traverse a considerable distance of the sympathetic chain and pass through several ganglia before it finally synapses with a postganglionic neuron also, its terminals make contact with a large number of postganglionic neurons. The parasympathetic system has terminal ganglia very near or within the organs innervated and is generally more circumscribed in its influences.

• The cell bodies of somatic motor neurons reside in the ventral horn of the spinal cord the axon divides into many branches, each of which innervates a single muscle fiber, so more than 100 muscle fibers may be supplied by 1 motor neuron to form a motor unit. At each neuromuscular junction, the axonal terminal loses its myelin sheath and forms a terminal arborization that lies in apposition to a specialized surface of the muscle membrane, termed the motor end plate ( see Figure 11–3).

Figure 8–2 Wiring diagram for somatic motor nerves and the efferent nerves of the autonomic nervous system . The principal neurotransmitters, acetylcholine (ACh) and norepinephrine (NE), are shown in red . The receptors for these transmitters, nicotinic (N) and muscarinic (M) cholinergic receptors, t and adrenergic receptors, are shown in green .

• Somatic nerves innervate skeletal muscle directly at a specialized synaptic junction, the motor end plate, where ACh activates N m receptors.

• Autonomic nerves innervate smooth muscles, cardiac tissue and glands. Both parasympathetic and sympathetic systems have ganglia, where ACh is released by the preganglionic fibers ACh acts on N n receptors on the postganglionic nerves. ACh is also the neurotransmitter at cells of the adrenal medulla, where it acts on N n receptors to cause release of EPI and NE into the circulation.

• ACh is the dominant neurotransmitter released by postganglionic parasympathetic nerves and acts on muscarinic receptors. The ganglia in the parasympathetic system are near or within the organ being innervated with generally a one-to-one relationship between pre- and post-ganglionic fibers.

• NE is the principal neurotransmitter of postganglionic sympathetic nerves, acting on α- or β-adrenergic receptors. Autonomic nerves form a diffuse pattern with multiple synaptic sites. In the sympathetic system the ganglia are generally far from the effector cells (e.g., within the sympathetic chain ganglia). Preganglionic sympathetic fibers may make contact with a large number of postganglionic fibers.

RESPONSES OF EFFECTOR ORGANS TO AUTONOMIC NERVE IMPULSES. In most instances, the sympathetic and parasympathetic neurotransmitters can be viewed as physiological or functional antagonists (Table 8–1).

Responses of Effector Organs to Autonomic Nerve Impulses

Most viscera are innervated by both divisions of the autonomic nervous system, and their activities on specific structures may be either discrete and independent or integrated and interdependent. For example, the effects of sympathetic and parasympathetic stimulation of the heart and the iris show a pattern of functional antagonism in controlling heart rate and pupillary aperture, respectively, whereas their actions on male sexual organs are complementary and are integrated to promote sexual function.

GENERAL FUNCTIONS OF THE AUTONOMIC NERVOUS SYSTEM. The ANS is the primary regulator of the constancy of the internal environment of the organism.

The sympathetic system and its associated adrenal medulla are not essential to life in a controlled environment, but the lack of sympathoadrenal functions becomes evident under circumstances of stress. In the absence of the sympathetic system: body temperature cannot be regulated when environmental temperature varies the concentration of glucose in blood does not rise in response to urgent need compensatory vascular responses to hemorrhage, oxygen deprivation, excitement, and exercise are lacking and resistance to fatigue is lessened. Sympathetic components of instinctive reactions to the external environment are lost and other serious deficiencies in the protective forces of the body are discernible. The sympathetic system normally is continuously active, the degree of activity varying from moment to moment and from organ to organ, adjusting to a constantly changing environment. The sympathoadrenal system can discharge as a unit. Heart rate is accelerated blood pressure rises blood flow is shifted from the skin and splanchnic region to the skeletal muscles blood glucose rises the bronchioles and pupils dilate and the organism is better prepared for “fight or flight.” Many of these effects result primarily from or are reinforced by the actions of epinephrine secreted by the adrenal medulla.

The parasympathetic system is organized mainly for discrete and localized discharge. Although it is concerned primarily with conservation of energy and maintenance of organ function during periods of minimal activity, its elimination is not compatible with life. The parasympathetic system slows the heart rate, lowers the blood pressure, stimulates GI movements and secretions, aids absorption of nutrients, protects the retina from excessive light, and empties the urinary bladder and rectum.

NEUROTRANSMISSION

Nerve impulses elicit responses in smooth, cardiac, and skeletal muscles, exocrine glands, and postsynaptic neurons by liberating specific chemical neurotransmitters. Neurohumoral transmission relates to the transmission of impulses from postganglionic autonomic fibers to effector cells. Evidence supporting this concept includes:

• Demonstration of the presence of a physiologically active transmitter and its biosynthetic enzymes at appropriate sites

• Recovery of the transmitter from the perfusate of an innervated structure during periods of nerve stimulation but not (or in greatly reduced amounts) in the absence of stimulation

• Demonstration that the putative transmitter is capable of producing responses identical to responses to nerve stimulation

• Demonstration that the responses to nerve stimulation and to the administered transmitter candidate are modified in the same manner by various drugs, usually competitive antagonists

While these criteria are applicable for most neurotransmitters, including NE and ACh, there are now exceptions to these general rules. For example, NO has been found to be a neurotransmitter however, NO is not stored in neurons and released by exocytosis. Rather, it is synthesized when needed and readily diffuses across membranes. Synaptic transmission in many instances may be mediated by the release of more than 1 neurotransmitter.

STEPS INVOLVED IN NEUROTRANSMISSION

The sequence of events involved in neurotransmission is of particular importance because pharmacologically active agents modulate the individual steps.

AXONAL CONDUCTION. At rest, the interior of the typical mammalian axon is

70 mV negative to the exterior. In response to depolarization to a threshold level, an action potential is initiated at a local region of the membrane. The action potential consists of 2 phases. Following depolarization that induces an open conformation of the channel, the initial phase is caused by a rapid increase in the permeability and inward movement of Na + through voltage-sensitive Na + channels, and a rapid depolarization from the resting potential continues to a positive overshoot. The second phase results from the rapid inactivation of the Na + channel and the delayed opening of a K + channel, which permits outward movement of K + to terminate the depolarization.

The transmembrane ionic currents produce local circuit currents such that adjacent resting channels in the axon are activated, and excitation of an adjacent portion of the axonal membrane occurs, leading to propagation of the action potential without decrement along the axon. The region that has undergone depolarization remains momentarily in a refractory state.

The puffer fish poison, tetrodotoxin , and a close congener found in some shellfish, saxitoxin , selectively block axonal conduction by blocking the voltage-sensitive Na + channel and preventing the increase in Na + permeability associated with the rising phase of the action potential. In contrast, batrachotoxin , an extremely potent steroidal alkaloid secreted by a South American frog, produces paralysis through a selective increase in permeability of the Na + channel, which induces a persistent depolarization. Scorpion toxins are peptides that also cause persistent depolarization by inhibiting the inactivation process. Na + and Ca 2+ channels are discussed in more detail in Chapters 11, 14, and 20.


Chapter Review

The nervous system can be separated into divisions on the basis of anatomy and physiology. The anatomical divisions are the central and peripheral nervous systems. The CNS is the brain and spinal cord. The PNS is everything else. Functionally, the nervous system can be divided into those regions that are responsible for sensation, those that are responsible for integration, and those that are responsible for generating responses. All of these functional areas are found in both the central and peripheral anatomy.

Considering the anatomical regions of the nervous system, there are specific names for the structures within each division. A localized collection of neuron cell bodies is referred to as a nucleus in the CNS and as a ganglion in the PNS. A bundle of axons is referred to as a tract in the CNS and as a nerve in the PNS. Whereas nuclei and ganglia are specifically in the central or peripheral divisions, axons can cross the boundary between the two. A single axon can be part of a nerve and a tract. The name for that specific structure depends on its location.

Nervous tissue can also be described as gray matter and white matter on the basis of its appearance in unstained tissue. These descriptions are more often used in the CNS. Gray matter is where nuclei are found and white matter is where tracts are found. In the PNS, ganglia are basically gray matter and nerves are white matter.

The nervous system can also be divided on the basis of how it controls the body. The somatic nervous system (SNS) is responsible for functions that result in moving skeletal muscles. Any sensory or integrative functions that result in the movement of skeletal muscle would be considered somatic. The autonomic nervous system (ANS) is responsible for functions that affect cardiac or smooth muscle tissue, or that cause glands to produce their secretions. Autonomic functions are distributed between central and peripheral regions of the nervous system. The sensations that lead to autonomic functions can be the same sensations that are part of initiating somatic responses. Somatic and autonomic integrative functions may overlap as well.

A special division of the nervous system is the enteric nervous system, which is responsible for controlling the digestive organs. Parts of the autonomic nervous system overlap with the enteric nervous system. The enteric nervous system is exclusively found in the periphery because it is the nervous tissue in the organs of the digestive system.


The sympathetic nervous system, also part of the autonomic nervous system, originates in the spinal cord specifically in the thoracic and lumbar regions. It controls the body's "fight or flight" responses, or how the body reacts to perceived danger.

With sympathetic nervous responses, the body speeds up, tenses up and becomes more alert. Functions that are not essential for survival are shut down. Following are the specific reactions of sympathetic nervous system:

  • increase in the rate and constriction of the heart
  • dilation of bronchial tubes in the lungs and pupils in the eyes
  • contraction of muscles
  • release of adrenaline from the adrenal gland
  • conversion of glycogen to glucose to provide energy for the muscles.
  • shut down of processes not critical for survival
  • decrease in saliva production: the stomach does not move for digestion, nor does it release digestive secretions.
  • decrease in urinary output
  • sphincter contraction.

The parasympathetic nervous system counterbalances the sympathetic nervous system. It restores the body to a state of calm. The specific responses are:

  • decrease in heart rate
  • constriction of bronchial tubes in the lungs and pupils in the eyes
  • relaxation of muscles
  • saliva production: the stomach moves and increases secretions for digestion.
  • increase in urinary output
  • sphincter relaxation.


Structure

The peripheral nervous system is itself classified into two systems: the somatic nervous system and the autonomic nervous system. Each system contains afferent and efferent components.

The afferent arm consists of sensory (or afferent) neurons running from receptors for stimuli to the CNS. Afferent nerves detect the external environment via receptors for external stimuli such as sight, hearing, pressure, temperature etc. Afferent nerves exist in both the somatic and autonomic nervous systems as both can use sensory signals to alter their activity.

The efferent arm consists of motor (or efferent) neurons running from the CNS to the effector organ. Effector organs can either be muscles or glands.

The efferent nerves of the somatic nervous system of the PNS is responsible for voluntary, conscious control of skeletal muscles (effector organ) using motor (efferent) nerves.

The efferent nerves of the autonomic (visceral) nervous system control the visceral functions of the body. These visceral functions include the regulation of heart rate, digestion, salivation, urination, digestion and many more. The efferent arm of this system can be further subdivided into parasympathetic motor or sympathetic motor.

The enteric nervous system is sometimes classified as a separate component of the autonomic nervous system and is sometimes even considered a third independent branch of the PNS.

[caption align="aligncenter"] Fig 1 - Diagram showing the components that make up the somatic nervous system[/caption]



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