The Nervous System - Saddleback College



The Nervous System

Organization

Central Nervous System (CNS)= Brain + Spinal Cord

Peripheral Nervous System (PNS) = All the nerve fibers and nerve cells located outside the central nervous system; the bundles of afferent and efferent nerve fibers and their associated ganglia which make up 1) the Cranial Nerves, 2) the spinal nerves and 3) the Autonomic Nervous System.

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Definitions

Neurology- the study of the nervous system

Nerve- a bundle of nerve fibers (axons) held together by connective tissue. (Be sure that you can distinguish between a nerve and a nerve fiber. A nerve can be compared to a cable made up of individual wires. Each wire corresponds to a nerve fiber.) Nerves are located outside the central nervous system. Bundles of nerve fibers within the central nervous system are called TRACTS.

Nerve Fiber - an axon (a process of a nerve cell) and its sheath.

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Afferent Fibers- nerve fibers that conduct impulses toward the CNS.

Efferent Fibers- nerve fibers that conduct impulses away from the CNS.

SOMATIC - on or relating to the wall of the body or the framework of the body and not to the viscera; i.e., relating to the skin and the skeletal muscles.

Visceral - relating to the organs of the ventral body cavities (the viscera) and also to smooth muscle and glands outside these cavities.

Autonomic Nervous System- the system of nerves and ganglia concerned with the distribution of nerve impulses to 1) Cardiac Muscle (located only in the heart), 2) Smooth Muscle (located in essentially all of the viscera, in the walls of blood vessels, and in the skin [arrector pili muscles]} and 3) Glands.

Neuron - a nerve cell consisting of a cell body and all its processes.

Axon- a single, usually elongated process that conducts impulses away from the nerve cell body.

Dendrites - multiple, shorter, branched processes that conduct impulses toward the nerve cell body.

Perikaryon (soma)- the nerve cell body; the part containing the nucleus.

Peripheral process - the part of the axon of a sensory ganglion cell that conducts nerve impulses from the receptor to the perikaryon. Some textbooks refer to this as a dendrite, although morphologically it is an axon.

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Central process - the part of the axon of a sensory ganglion cell that conducts impulses from the perikaryon to the central nervous system.

Receptor - any structure which detects a change in its environment and causes the transmission of a nerve impulse in an afferent fiber.

Effector - a muscle or gland.

Synapses - functional contacts between neurons.

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Gray Matter- that part of the nervous tissue composed mainly of nerve cell bodies and dendrites, unmyelinated axons, and neuroglia.

White Matter - aggregations of myelinated axons.

Nucleus- a collection of nerve cell bodies within the CNS

Ganglion - a collection of nerve cell bodies outside the CNS, i.e., within the PNS.

Autonomic Ganglion- a ganglion made up of the cell bodies of efferent fibers to smooth muscle, to cardiac muscle, or to glands (postganglionic efferent fibers).

Cranial Nerves- There are 12 pairs of cranial nerves numbered and named as follows: They are primarily sensory neurons (but some do provide innervation to muscles).

|I Olfactory |VII Facial |

|II Optic |VIII Vestibulocochlear |

|III Oculomotor |IX Glosspharyngeal |

|IV Trochlear |X Vagus |

|V Trigeminal |XI Accessory |

|VI Abducens |XII Hypoglossal |

Gross Features of Cranial Nerves

Unlike the spinal nerves, cranial nerves do not have a regular pattern of dorsal and ventral roots and primary rami.

Spinal Nerves

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Special Branches Found Only in Thoracic (T1-T12), Upper Lumbar (L1,L2) or Mid-Sacral (S2-S4) Nerves

In addition to the roots and rami listed above, thoracic and upper lumbar nerves have

1. White rami (T1-L2) (think thoraco-lumbar)

Mid-sacral nerves have

2. Pelvic Splanchnic nerves (S2-S4)

Note: White and Gray rami together are called COMMUNICATING RAMI because they are the structures through which the spinal nerve communicates with the sympathetic trunk. Both white and gray rami connect with sympathetic ganglia.

SPLANCHNIC NERVES, on the other hand, are visceral nerves (nerves to viscera) which contain preganglionic fibers. Since the pelvic splancnic nerves do not connect to the sympathetic trunk, they are not communicating rami.

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Plexus Formation

Plexus: a network of interlacing nerves. Nerves tend to interchange fibers by branching of the fiber bundles and subsequent rejoining, branching, and joining again so that peripheral nerves contain fibers from more than one spinal nerve. There are four main plexuses:

1) the cervical plexus

2) the brachial plexus

3) the lumbar plexus

4) the sacral plexus.

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Overlap

Redundancy or “overlap” lessens the possibility of sensory loss due to injury. Because of the phenomenon of plexus formation, the fibers entering any single nerve to skin or to muscle generally come from more than one spinal nerve. Also adjacent cutaneous nerves tend to overlap areas supplied by each other. Therefore, sectioning of a single dorsal root will not result in total loss of skin sensation, nor will sectioning of a single ventral root result in paralysis of a muscle.

Dermatomes- The area of skin supplied by the afferent fibers in the dorsal root of a single spinal nerve.

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Referred pain

There are two types of pain: somatic and visceral. Somatic pain is caused by stimulation of receptors in skin (superficial somatic pain) or in skeletal muscles, joints, tendons (deep somatic pain). Visceral pain is caused by stimulation of receptors in viscera. Our awareness of the location of the painful stimulus is generally accurate and precise for somatic pain. In many instances of visceral pain, however, the pain is experienced in some site other than where the stimulus occurred. Frequently, it will be experienced as a diffuse, poorly localized pain in some part of the body surface innervated by afferent fibers from the same spinal level that supplies the viscus that is the origin of the pain. This phenomenon is called “referred pain.”

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There are 31 pairs of spinal nerves divided regionally as follows:

Cervical- 8 pairs (C1-C8)

Thoracic- 12 pairs (T1-T12)

Lumbar- 5 pairs (L1-L5)

Sacral- 5 pairs (S1-S5)

Coccygeal- 1 pair (Cc1)

Gross Features Common to all Spinal Nerves

All spinal nerves have the following roots and branches (rami):

1. Dorsal root

2. Ventral root

3. Posterior (dorsal) primary ramus (branch)

4. Anterior (ventral) primary ramus

5. Gray ramus

Special: splanchnic nerve (#6) – Only for S2-S4

S2, S3 or S4

Pelvic splanchnic nerve

T1-L2

White Ramus

C1-C5- Cervical plexus

C5-T1- Brachial plexus

L1-L4- Lumbar Plexus

L4-S4- Sacral Plexus

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