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After completing this lesson, you should be able to:

  • Identify definitions of a neuron, nerve tissue, neuroglia, cell body, axon, dendrite, and synapse.
  • Identify the components of a neuron including the characteristics and functions of each.
  • Identify the types of neurons according to structure and function.
  • Identify the steps in the sequence of nerve impulse transmission.
  • Identify the locations of the major components of the central nervous system.
  • Identify the characteristics and functions of the major components of the central nervous system.
  • Identify the two subdivisions of the peripheral nervous system (PNS), including its characteristics, functions, and counter effects.
 

Contents

 

 

 

 

 

  1. Introduction
  2. Role of the Nervous System
  3. Organization of the Nervous System
  4. Cell Organization of the Nervous System
  5. Components of Neurons
  6. Types of Neurons
  1. Reflex Arc
  2. Nerve Impulse Transmission
  3. Major Components of the Central Nervous System
  4. Peripheral Nervous System
  5. Exercises

 

Human Anatomy & Physiology
The Nervous System

 

Introduction

The nervous system facilitates contact of the individual with his external and internal environments and aids in appropriate responses to these constantly changing environments. A general knowledge of the anatomy of the nervous system and an understanding of its physiology will help you to recognize and treat injuries and diseases of the nervous system.

 

Role of the Nervous System

The nervous system has three general functions that it performs in the role of the body's control center and communication network. The functions are:

  1. The nervous system is able to sense change both inside the body and change in the environment surrounding the body.
  2. The nervous system is able to interpret these changes.
  3. The nervous system causes the body to react to these changes by either muscular contraction or glandular secretion.

Homeostasis is a good example of the nervous system sensing change, interpreting change, and adjusting to change. (In homeostasis, the equilibrium of factors such as temperature, blood pressure, and chemicals are kept in relative balance.) In the case of homeostasis, the nervous system and the endocrine system operate together to maintain equilibrium in the body.

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Organization of the Nervous System

The nervous system has two main divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system is composed of the brain and the spinal cord. This system controls behavior. All body sensations are sent by receptors to the central nervous system to be interpreted and acted upon. All nerve impulses that stimulate muscles to contract and glands to secrete substances get the message from the central nervous system. The peripheral nervous system is composed of nerves. This system is a pathway to and from internal organs. PNS serves as a pathway to the brain for the five senses and helps humans adjust to the world around them. Further subdivision of the peripheral nervous system will be discussed later in this lesson.


Figure 1. Organization of the nervous system.

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Cell Organization of the Nervous System

Only two principal kinds of cells exist in the nervous system: neurons and neuroglia. Neuroglia cells (also called glial cells) act as connective tissue and function in the roles of support and protection. Some of these cells twine around nerve cells or line certain structures in the brain and spinal cord. Other neuroglia cells bind nervous tissue to supporting structures and attach neurons to their blood vessels. Other small neuroglia cells protect the central nervous system from disease by surrounding invading microbes and clearing away debris. Clinically, these cells are important because they are a common source of tumors of the nervous system. Neuron cells are nerve cells, the basic unit that carries out the work of the nervous system. Impulses from one body part to another body part are conducted by neurons.

Nervous System Cells

Neuron Cells

A nerve cell

Carries impulses from one body part to another

Neuoglia Cells

Acts as connective tissue

Supports and protects

Figure 2. Cells of the nervous system.

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Components of Neurons

The neuron, the basic unit that carries out the work of the nervous system, is a specialized conductor cell that receives and transmits electrochemical nerve impulses. In other words, neurons are nerve cells that conduct impulses from one body part to another body part. Each neuron is made up of three distinct parts: the cell body, dendrites, and an axon.

Cell Body, Dendrites, and Axon. The cell body contains a nucleus or control center. Also, a neuron usually has several highly branched, thick extensions of cytoplasm called dendrites. The exception is a sensory neuron that has a single, long dendrite instead of many dendrites. At the other extreme are motor neurons, each of which has many thick "tree-like" dendrites. The dendrite's function is to carry a nerve impulse toward the cell body. An axon is a long, thin process that carries impulses away from the cell body to another neuron or tissue. There is usually only one axon per neuron. Axons vary in length and diameter and are "jelly-like" in appearance.

Myelin Sheath (Schwann Cells). The myelin sheath is a white segmented covering made up of Schwann cells. The covering is around axons and dendrites of many peripheral neurons. This covering wraps around the entire axon in "jelly-roll" fashion, except at the point of termination and at the nodes of Ranvier. (The nodes of Ranvier are intermittent constrictors along the myelin sheath.) The myelin sheath is made up of a layer of protein, two layers of lipids or fats, and one more layer of protein.

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Figure 3. Sections of a myelinated fiber.

Neurilemma. The neurilemma is the nucleated cytoplasmic layer of the Schwann cell. The neurilemma allows damaged nerves to regenerate. Nerves in the brain and spinal cord DO NOT have a neurilemma and, therefore, DO NOT recover when damaged.

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Types of Neurons

Neurons in the body can be classified according to structure and function. Classification by structure is based on the number of processes (projections or protrusions) extending from the cell body. Neurons are classified by functions according to the direction in which the neurons transmit impulses.

Classification of Neurons by Structure. Classifications are as follows: multipolar neurons, bipolar neurons, and unipolar neurons. Multipolar neurons have one axon and several dendrites. Brain and spinal cord neurons are generally multipolar neurons. Bipolar neurons have one axon and one dendrite. Most of these neurons are found in the retina of the eye, the inner ear, and the olfactory area. Neurons with only one process extending from the cell body are termed unipolar neurons. The one process divides with one part acting as an axon and the other part functioning as a dendrite. Unipolar neurons are found in the posterior (sensory) root ganglia of the spinal nerves.

Classification of Neurons by Function. According to function, there are two types of neurons: sensory neurons (also called afferent neurons) and motor neurons (also called efferent neurons). Sensory neurons conduct impulses from the receptors in the skin, sense organs, and viscera (the large internal organs) to the brain and the spinal cord. These neurons conduct impulses from receptors to the central nervous system and are usually unipolar. Impulses transmitted from the brain and spinal cord to either muscles or glands are carried by motor neurons. These neurons conduct information away from the central nervous system to the skin, muscles, glands, and organs of the body.

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Reflex Arc

An impulse follows a conduction pathway from its origin in the dendrites or neuron cell body in one part of the body to the impulse's end somewhere else in the body. One pathway is called a reflex arc and is a functional unit of the nervous system. The basic parts of a reflex arc are a receptor, a sensory neuron, a center, a motor neuron, and an effector.

Function of Components of a Reflex Arc.

Functions are:

fig0104.jpg (83452 bytes)
Figure 4. The pathway of an impulse over the reflex arc.

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Nerve Impulse Transmission

A nerve impulse is an electro-negative wave that travels over the cell's membrane. To understand the way in which a nerve impulse travels, follow this example of a hand touching a match:

(1) The hand touches a lighter match -- stimulus.

(2) A dendrite receives the impulse from the skin. The impulse is then transmitted over each neuron as follows:

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Figure 5. The transmission of a nerve impulse.

(3) The impulse continues to the next dendrite, in a chain reaction.

(4) The hand jerks away -- response.

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Major Components of the Central Nervous System

Brain. The brain fills the cranium and weighs about three pounds in the average adult. The brain is shaped like a mushroom. The brain consists of four principal parts: the brain stem, the diencephalon, the cerebrum, and the cerebellum. The diencephalon, also known as the forebrainstem, includes the thalamus and hypothalamus.

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Figure 6. Principal parts of the brain.

Brain matter. There are two types of matter in the brain: gray matter and white matter. Gray matter is in the active portion of the brain. Gray matter receives and stores impulses. Answering impulses originate in the brain's gray matter. Cell bodies of neurons and neuroglia are in the gray matter. White matter in the brain carries impulses to and from gray matter. White matter is composed of nerve fibers (axons).

Cerebrum. The cerebrum forms the bulk of the brain and is supported on the brain stem. The cerebrum is divided into two hemispheres. Each hemisphere controls the activities of the side of the body opposite that hemisphere. Each hemisphere is further subdivided into four lobes:

Frontal lobe. This lobe is responsible for voluntary motor function (origin of pyramidal motor system) and higher mental functions such as judgment and foresight, affect, and personality.

Temporal lobes. These lobes are responsible for hearing, speech in the dominant hemisphere, vestibular sense, behavior, and emotion.

Parietal lobe. This lobe is responsible for sensory function, sensory association areas, higher level processing of general sensory modalities, e.g., stereognosis -- recognizing the size and shape of objects by the sense of touch.

Occipital lobe. This lobe is responsible for vision.

Cerebellum. The cerebellum is located behind and below the cerebrum. Its functions include the following:

Midbrain. The midbrain is located above the pons, extending from the pons to the lower part of the diencephalon. The midbrain provides conduction pathways to and from higher and lower centers. The righting, postural, and audiovisual reflexes are reflex centers located in the midbrain. The righting reflex helps keep the head right-side up. Postural reflexes deal with positioning the head in relation to the trunk of the body. Visual and auditory reflexes cause you to respond by turning your head in the direction of a loud noise.

Pons. The pons is located anterior and slightly superior to the cerebellum and between the midbrain and the medulla. The pons acts as a pathway to higher structures. It contains conduction pathways between the medulla and higher brain centers. It also serves to connect the two halves of the cerebellum. There is a respiratory center in the pons which prolongs inspiration (breathing in). The beginnings of some cranial nerves are in the pons.

Medulla oblongata (brain stem). This part of the brain is an expanded continuation of the spinal cord. The brain stem is located between the pons and the spinal cord and is only about one inch long. Contained in the brain stem are the centers for the regulation of respirations, heartbeat, and basomotor activators. These centers are often called the vital centers because they are essential to life. Some nerves cross over at the medulla oblongata which explains why one side of the brain controls activities on the opposite side of the body.

Thalamus. The thalamus is located in the walls of the third ventricle of the brain and is the area of arousal and conscious recognition of crude sensations; for example, temperature and pain. Sensory and afferent impulses go to the thalamus and are sorted and grouped there. Next, these impulses are sent to the proper area of the cerebral cortex where the impulses are interpreted. According to the Law of Specific Nerve Energies, the place at which an impulse ends in the thalamus determines the sensation to be felt. For example, if the impulse ends in the heat area of the thalamus, the individual feels heat.

Hypothalamus. The hypothalamus is a small part of the diencephalon. This portion of the brain has several functions. It monitors the chemical composition of blood. The hypothalamus acts as a relay station between the cerebrum and the lower autonomic centers. It controls hormone secretion by the pituitary gland and also controls the appetite.

Spinal Cord. The spinal cord is a cylindrical structure which extends from the foramen magnum through the spinal foramina of the vertebral column to the upper portion of the lumbar region. Extension varies from the 12th thoracic vertebra to the 2nd lumbar vertebra. The length of the cord remains fairly constant in adults: 18 inches in males and 16 inches in females. The conus medullaris is the cone-shaped termination of the cord. This portion of the spinal cord weighs about one ounce and is approximately one and one-half inches wide. The spinal cord itself appears wider from right to left than from anterior to posterior. The size and shape, however, do vary depending on the vertebral region. For example, the spinal cord presents cervical and lumbar enlargements, which are areas of nerve origin in the upper and lower lumbar region. The spinal cord is composed of a series of 31 segments. A pair of spinal nerves comes out of each segment. The region of the spinal cord from which a pair of spinal nerves originates is called the spinal segment. Both motor and sensory nerves are located in the spinal cord.

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Figure 7. Spinal cord and spinal nerves.

Meninges. The meninges are the three membranes that envelop the brain and the spinal cord. The outermost layer is the dura mater. The middle layer is the arachnoid, and the innermost layer is the pia mater. These three spinal meninges cover the spinal nerves to the point where the spinal column goes through the intervertebral foramen. The meninges offer protection to the brain and the spinal cord by acting as a barrier against bacteria.

Cerebrospinal Fluid (CSF). The cerebrospinal fluid protects the brain against injury. CSF circulates through the subarachnoid space (the area between the arachnoid and pia mater), around the brain and spinal cord and through the ventricles of the brain. In addition to protecting the brain, this clear fluid nourishes the central nervous system and carries off wastes.

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Peripheral Nervous System

The peripheral nervous system (PNS) is composed of nerves that connect the central nervous system to remote parts of the body, relaying and receiving messages from these parts of the body. This system is a pathway to the brain for the five senses which help us respond to the world around us. The peripheral nervous system is divided into the cerebrospinal nervous system and the autonomic nervous system (ANS).

Cerebrospinal Nervous System. This system is composed of 12 pairs of cranial nerves. The cranial nerves are symmetrically arranged and attached to the brain. Each cranial nerve leaves the skull through a foramen (an opening) at its base. The nerves are numbered in the order in which they emerge from the opening and from front to back. The numbered nerves along with the function of each nerve are given in table 1.

NOTE: Neither the olfactory nor the optic nerves are really cranial nerves since these nerves are composed of fibers belonging to the central nervous system.

 

I Olfactory Smell
II Optic Sight
III Oculomotor Movement of the eyeball, lens, and pupillary sphincter
IV Trochlear Superior oblique muscle of the eye
V Trigeminal Affects ophthalmic, maxillary, and mandible areas; controls muscles of mastication
VI Abducens Moves the eye outward
VII Facial Controls muscles of the face, scalp, ears; controls salivary glands; receives taste sensation from the anterior 2/3 of the tongue
VIII  Acoustic Hearing and equilibrium
IX Glossopharyngeal Sensations from the pharynx and the posterior 1/3 of the tongue
X Vagus Sensory and motor to thoracic and abdominal viscera
XI Accessory Permits movement of head and shoulders
XII Hypoglossal Controls muscles of tongue

Table 1. 12 pairs of cranial nerves and their functions.

Spinal Nerves. There are 31 pairs of spinal nerves. They are named for the region from which they leave the spinal canal through the intravertebral foramina. Included are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves. Fibers from the spinal nerves extend to all muscles of the trunk of the body and to the extremities. These fibers carry impulses between the spinal cord and the skin, muscles, and other structures. Each spinal nerve is derived from the spinal cord by two roots: a sensory (dorsal) root and a motor (ventral) root.

Autonomic Nervous System (ANS). This system regulates the action of secretory activity of glands and the involuntary contraction of smooth muscle, as in the blood vessels, the skin, the heart, and the bronchial tubes. The autonomic nervous system usually operates without conscious control. Originally, the system was named automatic because physiologists thought the system functioned completely automatically --that is, without any control from the central nervous system. Currently, it is believed that the ANS is structurally and functionally dependent on the central nervous system. The autonomic nervous system is subdivided into two systems that counteract each other: the sympathetic system and the parasympathetic system.

Sympathetic system. This system generally increases the activity of the body. The sympathetic system prepares the body to meet danger or undergo strenuous physical activity. The system responds to stimuli by releasing adrenalin (epinephrine) or acetylcholine into the body, thus, making the individual feel that he has extra energy to flee or fight.

Parasympathetic system. This system acts in opposition to the sympathetic system and maintains homeostasis (maintenance of a relatively constant body internal environment). The parasympathetic system exercises nonstress situational controls on the internal organs of the body allowing the body to rest and repair. To this end, cholinesterase, a substance which counteracts the effects of acetylcholine, is replaced.

NOTE: The two systems, the sympathetic system and the parasympathetic system, work in opposition to each other to give the body protection.

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Exercises

1. List two roles of the central nervous system.

a. __________
b. __________

2. List the two major parts of the central nervous system.

a. __________
b. __________

3. A major function of the________ is to control behavior.

4. The two types of cells in the nervous system are_________ and cel ls.

5. The nervous system cells which carry impulse from one body part to another body part are cells.

6. What is the function of a dendrite?______________ .

7. The function of an axon is to_____________ .

8. The white segmented covering around axons and dendrites is called __________.

9. Multipolar neurons, bipolar neurons, and unipolar neurons are neurons which are classified by__________.

10. A __________. activates a nerve impulse in a sensory neuron in response to a change in the body's environment.

11. List the four principal parts of the brain.

a. __________
b. __________
c. .__________
d. .__________

12._____ matter, the active portion of the brain, receives and stores impulses.

13. The portion of the brain which sorts out sensory and afferent impulses is the _____.

14. The eight pairs of cervical nerves, the 12 pairs of thoracic nerves, the 5 pairs of lumbar nerves, and the 5 pairs of sacral nerves are all part of the _____.

15. List three functions of cerebrospinal fluid (CSF).

a. .__________
b. .__________
c. .__________

16. List three functions of the autonomic nervous system.

a. .__________
b. .__________
c. .__________

17. The sympathetic nervous system generally increases the activity of the body, and the__________ system acts in opposition to the sympathetic system, allowing the body to rest and repair itself.

 18. The twelfth nerve in the cerebrospinal nervous system (the hypoglossal nerve) controls the _________.

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Solutions to Exercises

1. The body's communication center. The body's control center.
2. The brain. The spinal cord.
3. Central nervous system.
4. Neurons. Neuroglia.
5. Neuron.
6. Dendrites carry impulses away from the cells.
7. Carry impulses away from the cell body to another neuron or tissue.
8. Myelin sheath.
9. Structure.
10. Reflex arc.
11. The brain stem.
      The diencephalon.
      The cerebrum.
      The cerebellum.  
12. Gray. 
13. Thalamus.
14. Spinal cord
15. Protects the brain against injury. Nourishes the central nervous system. Carries off wastes.
16. Regulates cardiac muscle activities.
     Regulates smooth muscle activities.
     Regulates glandular secretions.
17. Parasympathetic.
18. Muscles of the tongue.

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