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Nervous System


Components

Central Nervous System (CNS) - Completely within the dorsal body cavity

o        Brain

o        Spinal Cord

Peripheral Nervous System (PNS)

o        Afferent Nervous System

§         Sensory receptors

§         Sensory neurons (traveling toward CNS)

o        Efferent Nervous System

§         Motor neurons

§         Autonomic Nervous System (ANS) - innervates smooth and cardiac muscle and glandular epithelium

§         Sympathetic Nervous System

§         Parasympathetic Nervous System

o        Cranial Nerves


Glial Cells (Neuroglia)

Approximately 90% of the cells in the brain are not neurons but glial cells. These important cells serve several functions and, as listed below, there are different forms of these cells.

 

Astrocytes

individual cells have several filamentous extensions that interconnect neurons, vessels, and other glial cells; viewed by some as a form of structural support

Oligodendrocytes

like astroycytes, these cells have extensions but far fewer; they ensheath or myelinate the axons of many neurons in the CNS giving rise to the white matter of the brain; myelination speeds transmission of impulses

Microglia

smallest of the glial cells; capable of phagocytosis; considered the "clean up" cell; extensions are normally absent


Neurons & Nerves

Refer to your text and lecture notes for a diagram of each neuron types. This area outlines the important components and characteristics. A reference diagram is essential.

 

Multipolar Neuron

This is the most common type of neuron. A cell body with a single nucleus normally possesses several dendrites and one or more axon cylinders.

Bipolar Neurons

This neurons has a cell body with one dendrite and one axon branch. It is typically found in the ear and eye and is strictly sensory.

Unipolar Neurons

Sensory neuron with a single process or extension that divides in a T manner forming peripheral and central pathways; associated with sensory receptors (touch, pressure, etc)

Dendrite

Extension of the neuron that transmits impulses to the cell body; often branched extensively.

Axon 

Process of the neuron that carries impulses away from the cell body; also called the axon cylinder; a multipolar neuron may possess more than one axon

Myelin Sheath

Fatty, insulating sheath that surrounds the axons of many neurons and nerve fibers; in the CNS this sheath is composed of layers of oligodendrocytes, while in the PNS it is composed of Schwann cells

Teliodendrion

Branched terminus of the axon ending at a synapse or myoneural junction; also called an axon branch.

Synapse

Junction between the end of a teliodendrion and a dendrite; it includes the synaptic cleft, a microscopic gap between the two structures

Synaptic Vesicle

vesicles that release neurotransmitters into the synaptic cleft or myoneural junction; contain either a form of acetylcholine or epinephrine

Myoneural Juntion

also called a neuromuscular junction; the meeting of a teliodendrion with a muscle fiber

Schwann Cells

cells of the PNS that myelinate axon cylinders

Node of Ranvier

locations along the axon cylinder immediately between adjacent Schwann cells or oligodendrocytes (of the myelin sheath)

Ganglion

group of neural cell bodies in the PNS

Nucleus

not to be confused with a cell nucleus, this refers to a group of neural cell bodies within the CNS

Sensory Nerve

nerve fiber composed of many neurons, all of which are sensory; part of the afferent nervous system

Motor nerve

nerve fiber composed of many neurons, all of which are motor; part of the efferent nervous system

Mixed Nerve

a nerve composed of both sensory and motor neurons; this is the most common nerve type

Chain Ganglia

ganglia running parallel and just lateral to each side of the thoracic spinal cord; associated with the sympathetic division of the autonomic nervous system


Transmission

Stimuli are received by the dendrites and generate an impulse that travels toward the neural cell body and the axon hillock. Depending on the type of neuron, input may come from one or more dendrites. If the axon hillock is sufficiently depolarized, an impulse is sent along the axon cylinder. This is an all-or-nothing response inasmuch as no impulse will be generated unless depolarization at the axon hillock is sufficiently intense.

Once an impulse travels along the axon cylinder it reaches the axon branches (teliodenrion if there is only one) and finally the synapse. At the synapse, synaptic vesicles are released which traverse the synaptic cleft to reach the next dendrite or a muscle. Synaptic vesicles may contain acetylcholine or a form of epinephrine. These substances are degraded upon crossing the synaptic cleft.



 


 

Cranial Nerves

Nerve

Name

Sensory Function

Motor Function

I

Olfactory

smell

-

II

Optic

vision

-

III

Oculomotor

-

eye movement

IV

Trochlear

-

eye movement

V

Trigeminal

general sensation of head and face

chewing

VI

Abducens

-

eye movement

VII

Facial

taste

facial movement

VIII

Vestibulocochlear

hearing and balance

-

IX

Glossopharyngeal

taste

swallowing

X

Vagus

taste

heart rate, breathing, digestive function

XI

Accessory

-

head movement

XII

Hypoglossal

-

tongue movement

 


Autonomic Nervous System (ANS)

This subdivision of the peripheral nervous system innervates the smooth and cardiac muscles and glandular epithelium. It is controlled by the hypothalamus and is composed of two opposing fractions: the Sympathetic Nervous System and the Parasympathetic Nervous System. While it is only a generalization, the sympathetic system usually primes the body for action (the so-called "fight or flight syndrome") and the parasympathetic system dominates in less stressful situations.

The autonomic pathways travel from the brain or spinal cord to the site of innervation via two connecting neurons. The first of these neurons is called the pre-ganglionic neuron and the second is the post-ganglionic neuron. The junction between the two is a synapse while the junction between the post-synaptic neuron and muscle is a myoneural junction (also referred to as a neuro-muscular junction).

 

Comparison of Sympathetic and Parasympathetic Systems

Sympathetic System

Parasympathetic System

Thoraco-Lumbar System: nerves originate from spinal cord in thoracic and lumbar regions

Craino-Sacral System: nerves originate from brain and sacrum

Generally excites and prepares for "fight-or-flight". Both systems are active at all times.

Dominates in less tense situations. Both systems are active at all times.

Pre-ganglionic neuron is short and terminates in chain ganglia; post-ganglionic neuron is comparatively long

Pre-ganglionic neuron is comparatively long and may terminate in a ganglion near the organ innervated

System is adrenergic; synaptic vesicles at ganglia contain acetylcholine;
neuro-muscular junction vesicles contain nor-epinepherine

System is entirely cholinergic; synaptic vesicles within the ganglia and at the neuro-muscular junctions all contain acetylcholine


The Brain

 

Selected Structures and Functions

Cerebrum

controls reasoning and thinking; responsible for conscious movement

Pineal Body

endocrine gland secreting melatonin; prepares the body for sleep

Corpora Quadrigemina

control some reflex responses, such as the response to sound

Thalamus

relay area for sensory impulses passing to the cerebral cortex

Hypothalamus

controls the ANS, regulation of body temperature, water balance, thirst, and fat and carbohydrate metabolism

Corpus Callosum

major commissure connecting the cerebral hemispheres

Lateral Ventricle

fluid-filled chamber protecting the brain from trauma

Olfactory Lobe

sense of smell

Pons

consists primarily of motor and sensory fiber tracts between the brain and lower CNS centers

Medulla Oblongata

contains autonomic centers involved in the control of heart rate, respiratory rhythm, blood pressure, swallowing and vomiting

Spinal Cord

tracts of CNS fibers within the spinal column

Fourth Ventricle

chamber within the cerebellum that protects the area from trauma

Cerebellum

unconscious coordination of skeletal muscle activity and control of balance and equilibrium

Pituitary Gland

"master" endocrine gland; see description under Endocrine System

 



Additional Material on Nerves - Good Preparation for Physiology

       Part 1 - Unabridged    Abbreviated
       Part 2 - Unabridged    Abbreviated
       Part 3 - Unabridged    Abbreviated
       Part 4 - Unabridged    Abbreviated