Pre Admission Biology

Chapter 8

The Nervous System

I. Introduction

A.               Functions of the nervous system:

1.     Control of the body

2.     Integration

3.     Communication

Note: Both the nervous system and the endocrine system exert control and integration through communication. The nervous system communicates by fast travelling nerve impulses. The endocrine system uses hormones to communicate. Hormones travel more slowly than nerve impulses. The action of nerve impulses and hormones is to increase or decrease the activities of human body structures.

 

B.               Divisions of the Nervous System

1.     Central Nervous System (CNS)

a. Structures of the CNS:

[1]  Brain

[2Spinal cord

b.    Function: 

The CNS coordinates and interprets information to determine the best response 

     2. Peripheral Nervous System (PNS)

a. Cranial nerves (12 pairs)

b. Spinal nerves: (31 pairs)

Carry nerve impulses to and from the spinal cord to body parts not served by the cranial nerves.

c. Somatic nervous system

Carries nerve impulses to the skeletal muscles, joints and skin    

d. Autonomic nervous system (ANS)

Carries nerve impulses to the smooth muscles of internal organs and to glands without conscious thought

  Two subdivisions of autonomic nervous system :

[1]  Sympathetic system

  Controls "fight or flight" responses

Neurotransmitter is norepinephrine (or noradrenaline).

 [2] Parasympathetic system

Controls those responses associated with a relaxed state.

Neurotransmitter is acetylcholine (Ach).

 

II.             Cells of the Nervous System

A.               Neurons

1. Composed of:

a.       Cell Body

- Part that contains the nucleus

b.      Dendrite(s)

Carries a nerve impulse towards the cell body

c.       Axon(s)

Carries a nerve impulse away from the cell body (and towards the dendrite of the next neuron)

d.   Axons are also called nerve fibers.

e.   A nerve is a bundle of axons outside the CNS

2.     They are the basic structural and functional unit of the nervous system.

      3.  They conduct nerve impulses.

Note: Definition of a  nerve impulse :

        A self-propagating wave of electrical disturbance that travels along the surface of a neuron's plasma membrane.

4.  Three types of neurons (by functional classification):

 

a. Sensory or Afferent Neuron 

 

  •   Transmit nerve impulses to the spinal cord

and brain from all over the body.

b. Motor or Efferent Neuron

Carries impulses away from the spinal cord and brain to muscles or glands

 c. Interneuron or Connecting Neuron

  • Transmits nerve impulses from one neuronal dendrite to the axion of another neuron.

  • All are found only in the gray matter of the brain or spinal cord.

     

B.               Glial or Neuroglial Cells

1.     Special types of connective tissue cells that help support and protect neurons.

 

2.  Types of glial cells:

 

a. Found in the CNS

 

[ i. ] Astrocytes:

Form the blood-brain barrier (by holding together neurons and blood vessel with a separation into a two-layered structure).

[ ii ] Oligodendrites:

Form the myelin sheath around axons of the CNS. Help to hold nerve fibers together

[iii.]   Microglia:

Phagocyte cells that migrate through the CNS removing foreign matter and degenerated brain tissue.

[iv.] Ependymal cells

Epithelial cells that line the brain and central canal of the spinal cord and form cerebrospinal fluid and aid in its circulation.

b. Found in PNS

[i.]       Schwann cells      

Form myelin sheaths around peripheral axons and are composed of:

[a]  Myelin:

A white fatty substanceProvides insulation Forms myelin sheath around some axons in the PNS

[b]  Nodes of Ranvier:

Nonmyelinated gaps on axons that lie between adjacent Schwann cells.

 [c] Neurilemma:

     Outer membrane of a Schwann cell

       Plays an important role in the regeneration of cut or injured axons.

 

Note:   Since Schwann cells are only found in the PNS, the CNS (brain & spinal cord) has far less potential for regeneration.

[ii.] Satellite cells

Support neurons in the PNS ganglia.

 

  C.                Other Aspects Associated with Neurons

1.      Receptors

a.   They are the beginnings of dendrites or sensory neurons.

2.      Ganglion

a.       A group of nerve-cell bodies located in the PNS

3.      Effector

a.   Muscles or gland that the motor neuron axons form a synapse withb.   They put the nerve impulse (signal) into effect.

4.      Nerve impulse (or Action Potential)

a.   An electrochemical charge that travels along a nerve fiberb.   Travels about 200 meters per second

5.      Synapse

   A gap between an axon of one neuron and the dendrite of another neuron

6.      Gliomas

One of the most common types of brain tumors

7.      Reflex arcs

A specialized type of neuron pathway that allows a nerve impulse to travel in only one directionExample:    knee jerk.

 

III. Nerves

A.               Definition

A group of peripheral nerve fibers (axons) that are bundled together.

B.               Nerve Structure (Figure 7-4 on page 168 of the 11th edition)

1.       Each axon in a nerve is surrounded by a thin wrapping of fibrous connective tissue called endoneurium.

2   Fascicles are groups of the above-mentioned wrapped axons.

3.   Each fascicle is surrounded by a thin, fibrous sheath called the epineurium, which covers the whole nerve.

IV.   Synapse

A.               Definition

A place where nerve impulses are transmitted from one neuron to another (from presynaptic neuron to the postsynaptic neuron)

B.               Three structures of a synapse:

1.     Synaptic knob (on presynaptic neuron's axon)       

Contains vessicles filled with a chemical(s) called a neurotransmitter(s)

  2. Synaptic cleft

a.   Space between synaptic knob and plasma membrane of postsynaptic neuron

b.   Very small

 3. Plasma membrane (of postsynaptic neuron)

     Has receptor site (proteins) in its membrane for the neurotransmitter

C.               Neurotransmitters

1.     Acetylcholine

a. associated with nerve impulse conduction at synapse.

b. belongs to a chemical group called cholinergics.

2.  Norepinepherine                

a.  belong to a chemical group called adrenergics.

b. and to the chemical family known as catecholamines.

  b. associated with sleep, mood, motor function and pleasure recognition

3. Dopamine

a.   also a catecholamine.

b.   associated with sleep, mood, motor function and pleasure recognition

4.  Serotonin

a.   also a catecholamine.

b.   associated with sleep, mood, motor function and pleasure recognition

5. Endorphins

Inhibits pain- type nerve impulses.

6. Enkephalins

Inhibits pain- type nerve impulses

V.    Divisions of the Brain

A.               Brain Stem

1.     In general: consist of a two-way conduction pathwaybetween the brain and other body regions.

a.       Sensory fibers bring nerve impulse from the spinal cord to the brain.

b.      Motor fibers carry nerve impulses down from the brain to the spinal cord.

2.     Brain stem components:

a.       Medulla oblongata

The two-way conduction pathways associated with the cardiac, respiratory and vasomotor control centers

b.      Pons

The word, “pons” means bridge Its two-way conduction pathway connects the spinal cord with the brain and parts of the brain with each other.

c.       Midbrain

The two-way conduction pathway for relaying visual and auditory nerve impulses

B.               Cerebellum

1.     Functions in the:

a.       Muscular coordination required for normal movements

b.      Maintains equilibrium

c.       Sustain normal posture

C. Diencephalon

1.   Hypothalamus 

a.   The posterior pituitary gland is an extension of the hypothalamus. It produces many hormones(chemicals) that regulate or help control the functioning of every cell in the body.

b.   Neurons of hypothalamus produce many kinds of hormones. Such as:

ADH or Antidiuretic Hormone that maintains the body's water balance by affecting the volume of urine excreted

c.   Functions:

Helps produce sensations by acting as a relay between the cerebral cortex and the body's sense organs.   Associates sensations with emotions    Plays a role in the arousal or alerting  mechanism

2.      Thalamus

a.        A section of gray matter located superior to the        hypothalamus

b.        Functions of it:

        Helps produce sensations by acting as a relay between   the body’s sense organs and the cerebral cortex

                      It associates sensations with emotions

        It plays a part in the so-called arousal or alerting mechanism

D.             Cerebrum

1.     Largest part of the brain that has

       a.Ridges called convolutions 

b.   Grooves called:

   Sulci         Fissures (deepest grooves) -         Divide cerebrum into a right and left half -         These two halves are connected by the corpus callosum.

     2.   Cerebral cortex- a thin layer of gray matter that composes the               surface of the cerebrum

3.   Below the cerebral cortex is white matter that contains a few islands of gray matter known as cerebral nuclei or basal ganglia. Functions to produce automatic movement and postures.         

Note: Parkinson Disease- affects the basal gangli.

 

Special Note:  In your college anatomy course you will have to learn/memorize the names of the 12 cranial nerves and the function of each (p. 188, Table 7-2).  Your instructor will also study the autonomic conduction pathways and neurotransmitters in greater detail.  I recommend saving this text.  It can help you to understand these aforementioned areas in your anatomy course that will be studied in depth.