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Chapter 13 - THE SPINAL CORD AND SPINAL NERVES

(figures relate to Tortora/Grabowski 9^th edition of Principles of Anatomy and Physiology)

I. INTRODUCTION

A. The spinal cord and spinal nerves mediate reactions to environmental changes.

B. The spinal cord has several functions.

1 . It processes reflexes.

2. It is the site for integration of EPSPs and IPSPs that arise locally or are

triggered by nerve impulses from the periphery and brain.

3 . It is a conduction pathway for sensory and motor nerve impulses.

II. SPINAL CORD ANATOMY

A. The spinal cord is protected by two connective tissue coverings, the meninges and vertebra, and a cushion of cerebrospinal fluid.

1. Meninges

a. The meninges are three coverings that run continuously around the spinal cord and brain (Figures 13.1a, 14.4a). (Know order and spaces)

1) The outermost layer is the dura mater.

2) The middle layer is the arachnoid.

3) The innermost meninx is the pia mater, a thin, transparent
connective tissue layer that adheres to the surface of the
spinal cord and brain.

4) Inflammation of the meninges is known as meningitis.

2 The vertebral column provides a bony covering of the spinal cord (Figure 13.1b).
B External Anatomy of the Spinal Cord

1 . The spinal cord begins as a continuation of the medulla oblongata and terminates at about the second lumbar vertebra in an adult (Figure 13.2).

2. It contains cervical and lumbar enlargements that serve as points of origin for nerves to the extremities.

3 . Spinal nerves

a. The 3 1 pairs of spinal nerves are named and numbered according to the region and level of the spinal cord from which they emerge (Figure 13.2).

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b. There 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.

c. Spinal nerves are the paths of communication between the spinal
cord and most of the body.

d. Roots are the two points of attachment that connect each spinal
nerve to a segment of the spinal cord (Figure 13.3a).

1) The posterior or dorsal (sensory) root contains sensory
nerve fibers and conducts nerve impulses from the
periphery into the spinal cord; the posterior root ganglion
contains the cell bodies of the sensory neurons from the
periphery.

2) The anterior or ventral (motor) root contains motor neuron
axons and conducts impulses from the spinal cord to the
periphery; the cell bodies of motor neurons are located in
the gray matter of the cord.

5. Removal of cerebrospinal fluid from the subarachnoid space is called a spinal tap (lumbar puncture). This procedure is used to diagnose pathologies and to introduce antibiotics, contrast media, anesthetics, and chemotherapeutic drugs. (Clinical Application)

C. Internal Anatomy of the Spinal Cord

1. The anterior median fissure and the posterior median sulcus penetrate the white matter
of the spinal cord and divide it into right and left sides (Figure 13.3).

2. The gray matter of the spinal cord is shaped like the letter H or a butterfly
and is surround by white matter.

a. The gray matter consists primarily of cell bodies of neurons and
neuroglia and unmyelinated axons and dendrites of association and
motor neurons.

b. The white matter consists of bundles of myelinated axons of motor
and sensory neurons.

3. The gray commissure forms the cross bar of the H-shaped gray matter.

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4. In the center of the gray commissure is the central canal, which runs the
length of the spinal cord and contains cerebrospinal fluid.

5. Anterior to the gray commissure is the anterior white commissure, which
connects the white matter of the right and left sides of the spinal cord.

III. SPINAL CORD PHYSIOLOGY

A. The spinal cord has two principal functions.

1. The white matter tracts are highways for nerve impulse conduction to and
from the brain.

2. The gray matter receives and integrates incoming and outgoing information.

B. Sensory and Motor Tracts

1. Figure 13.4 shows the principal sensory and motor tracts in the spinal
cord. Exhibit 15.1 describes these tracts in detail.

2. The axons of various nerves and CNS tracts develop myelin sheaths at
different times, which explain the poor sensory and motor development of
newborns. (Clinical Application)

C. Reflexes

1. The spinal cord serves as an integrating center for spinal reflexes. This
occurs in the gray matter.

2. A reflex is a fast, predictable, automatic response to changes in the
environment that helps to maintain homeostasis.

3. Reflexes may be spinal, cranial, somatic, or autonomic.

D. Reflex Arc (Fig 13.5)

1. A reflex arc is the simplest type of pathway; pathways are specific
neuronal circuits and thus include at least one synapse.

2. The five functional components of a reflex arc are the receptor, sensory neuron,
integrating center neuron, motor neuron, and effector (Figure 13.5).

3. Reflexes help to maintain homeostasis by permitting the body to make exceedingly rapid adjustments to homeostatic imbalances.

4. Somatic spinal reflexes include the stretch reflex, tendon reflex, flexor (withdrawal) reflex, and crossed extensor reflex; all exhibit reciprocal innervation.

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5. Reflexes are often used for diagnosing disorders of the nervous system and locating injured tissue. (Clinical Application)

a. If a reflex is absent, or abnormal, the damage may be somewhere
along a particular conduction pathway.

b. Among the clinically important reflexes are the plantar flexion (>18 mos)
and Babinski reflexes (<18 mos).

IV. SPINAL NERVES

A. Spinal nerves connect the CNS to sensory receptors, muscles, and glands and are
part of the peripheral nervous system.

1. The 31 pairs of spinal nerves are named and numbered according to the region
and level of the spinal cord from which they emerge (Figure 13.2a).

2. Spinal nerves connect to the cord via an anterior and a posterior root
(Figure 13.3a). Since the posterior root contains sensory axons and the
anterior root contains motor axons, a spinal nerve is a mixed nerve, at least
at its origin.

B. Connective Tissue Covering of Spinal Nerves

1. Spinal nerve axons are grouped within connective tissue sheathes (Figure 13.10).

2. Numerous blood vessels are within the coverings.

C. Distribution of Spinal Nerves

1. Shortly after passing through its intervertebral foramen, a spinal nerve divides into
several branches; these branches are known as rami (Figure 13.11).

2. Branches of a spinal nerve include the dorsal ramus, ventral ramus,
meningeal branch, and rami communicantes.

3 The ventral (anterior) rami of spinal nerves, except for T2-T12, form networks of nerves called plexuses (Figure 13.2 and Exhibits 13.1-13.4).

a. Emerging from the plexuses are nerves bearing names that are
often descriptive of the general regions they supply or the course
they take.

b. The cervical plexus supplies the skin and muscles of the head, neck, and
upper part of the shoulders; connects with some cranial nerves; and
supplies the diaphragm (Figure 13.12, Exhibit 13.1).

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c. The brachial plexus constitutes the nerve supply for the upper
extremities and a number of neck and shoulder muscles (Figures
13.13 and 13.14, Exhibit 13.2).

1) A number of nerve disorders may result from injury to the
brachial plexus.

2) Among these injuries are Erb-Duchene palsy or waiter's tip
palsy, Klumphe's palsy, wrist drop, carpal tunnel
syndrome, claw hand, and winged scapula.

d. The lumbar plexus supplies the anteriolateral abdominal wall,
external genitals, and part of the lower extremities (Figure 13.15,
Exhibit 13.3).

1) The largest nerve arising from the lumbar plexus is the
femoral nerve.

2) Injury to the femoral nerve is indicated by an inability to
extend the leg and by loss of sensation in the skin over the
anteriomedial aspect of the thigh..

3) Obturator nerve injury is a common complication of
childbirth and results in paralysis of the adductor muscles
of the leg and loss of sensation over the medial aspect of
the thigh.

e The sacral plexus supplies the buttocks, perineum, and part of the lower extremities (Figure 13.16, Exhibit 13.4).

1) The largest nerve arising from the sacral plexus (and the
largest nerve in the body) is the sciatic nerve.

2) Injury to the sciatic nerve (common peroneal portion) and
its branches results in sciatica, pain that extends from the
buttock down the back of the leg.

3) Sciatic nerve injury can occur due to a herniated (slipped) disc, dislocated hip, osteoarthritis of the lumbosacral spine, pressure from the uterus during pregnancy, or an improperly administered gluteal injection.

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4. The anterior rami of spinal nerves T2-T12 do not enter into the formation of plexuses and are known as intercostal or thoracic nerves.

a. These nerves directly innervate structures they supply in the
intercostal spaces (Figure 13.20).

b. Their posterior rami supply the deep back muscles and skin of the
posterior aspect of the thorax.

D. Dermatomes

1. The skin over the entire body is supplies by spinal nerves that carry
somatic sensory nerves impulses into the spinal cord.

2. All spinal nerves except C1 innervate specific, constant segments of the
skin; the skin segments are called dermatomes (Figure 13.17).

3. Knowledge of dermatomes helps a physician to determine which segment
of the spinal cord or which spinal nerve is malfunctioning.

E. An injury that entirely severs the spinal cord is said to cause a complete
transection. (Clinical Application)

1. After the injury, there will be a permanent loss of sensations in
dermatomes below the injury.

2. Voluntary muscle contractions will also be lost below the transection.
V. DISORDERS; HOMEOSTATIC IMBALANCES

A. Inflammation of one or several nerves is known as neuritis; it can have many causes.

B. Shingles is an acute infection of the peripheral nerves by the herpes zoster virus;
the virus migrates down peripheral nerves, causing pain, skin discoloration, and a
characteristic line of skin blisters.

C. Poliomyelitis (infantile paralysis or polio) is a viral infection characterized by
fever, headache, stiff neck and back, deep pain and weakness, and loss of certain
somatic reflexes. Paralysis is produced when the virus destroys motor neuron cell
bodies.