The BRAIN & SPINAL CORD consist of 2 types of NERVOUS TISSUE (see FIG. 16.8, page
510):
A. WHITE MATTER consists of regions of the brain and spinal cord that contain dense collections of MYELINATED AXONS. Lipids in the myelin sheath give white matter its white color.
B. GRAY MATTER consists of collections of NEURON CELL BODIES and associated DENDRITES, or collections of UNMYELINATED AXONS.
Recall that a collection of neuron cell bodies in the CNS is called a
NUCLEUS. The lack of myelinated fibers makes the tissue gray in color. NEUROGLIA are also found in the gray matter.
PROTECTION OF THE SPINAL CORD
The spinal cord is protected by the bones of the VERTEBRAL COLUMN, as well as CEREBROSPINAL FLUID and the MENINGES. The CEREBROSPINAL FLUID
(CSF) acts as a fluid cushion and absorbs toxins produced by the neurons in the brain and spinal cord.
CSF is produced by the CHOROID PLEXUSES of the brain, which are located in the
VENTRICLES (spaces) of the brain (see FIGS. 18.3 & 18.4, page
548-549).
The MENINGES (singular, MENINX) are three connective tissue membranes that surround the brain and spinal cord. The meninges that surround the spinal cord are called the SPINAL MENINGES. Inflammation of the meninges is called meningitis.
SPINAL MENINGES (FIG. 17.1):
1. DURA MATER is the outer meninx composed of tough, fibrous DENSE IRREGULAR CONNECTIVE TISSUE. Between the dura mater and the wall of the vertebral column is a space called the EPIDURAL SPACE, which is filled with adipose tissue and blood vessels. See also FIG. 17.7.
2. ARACHNOID is the middle meninx composed of a delicate connective tissue membrane that forms a tube inside the dura mater. The arachnoid consists of a "spider web" of collagen and elastic fibers (arachnoid is Latin for "spider"). Between the dura mater and arachnoid is the SUBDURAL SPACE, which contains tissue fluid.
3. PIA MATER is the inner meninx composed of a thin, transparent fibrous membrane attached to the surface of the spinal cord. It contains numerous blood vessels that supply oxygen and nutrients to the cells of the spinal cord. The space between the arachnoid layer and pia mater is the SUBARACHNOID SPACE, which contains CEREBROSPINAL FLUID. See also FIG. 17.7.
STRUCTURE OF THE SPINAL CORD: FIG. 17.2
The spinal cord is enclosed in the vertebral canal of the vertebral column. It extends from the foramen magnum of the skull to between the first and second lumbar vertebrae. Its length in an adult ranges from 42-45
cm (16-18 in.). The diameter of the spinal cord is about 2 cm (3/4 in.), except it is somewhat wider at the CERVICAL and LUMBAR ENLARGEMENTS.
Below the lumbar enlargement, the spinal cord tapers to a point (CONUS MEDULLARIS) between L1 and L2. Spinal nerves that arise from the lower end of the spinal cord travel downward through the vertebral canal before exiting the spinal cord. This collection of nerves at the lower end of the vertebral canal is called the CAUDA EQUINA.
STRUCTURE OF THE SPINAL CORD IN CROSS-SECTION: FIG 17.3
The spinal cord is divided into right and left sides by the ANTERIOR MEDIAN FISSURE and the POSTERIOR MEDIAN SULCUS.
The white matter of the spinal cord contains bundles of myelinated axons called TRACTS. The gray matter forms an H-shaped area within the white matter. The gray matter consists of neuron cell bodies and unmyelinated dendrites, unmyelinated axons, and the neuroglia.
The crossbar of the gray matter is called the GRAY COMMISSURE. In the center of the gray commissure is a small space called the CENTRAL CANAL. The central canal runs the length of the spinal cord and contains CEREBROSPINAL FLUID. The two posterior projections of the gray matter are the POSTERIOR GRAY HORNS. The AXONS OF SENSORY NEURONS enter the spinal cord through the posterior horns. The anterior pair of gray matter projections are the ANTERIOR GRAY HORNS. The anterior horns contain the CELL BODIES OF MOTOR NEURONS.
The white matter is divided into 3 regions: ANTERIOR, POSTERIOR and LATERAL WHITE COLUMNS. Each column contains several bundles of myelinated nerve fibers (TRACTS). The tracts run up and down the spine. See FIG. 17.4. The ASCENDING TRACTS consist of myelinated sensory axons that conduct nerve impulses that enter the spinal cord up to the brain. The DESCENDING TRACTS consist of myelinated motor axons that conduct nerve impulses from the brain down the spinal cord. In the spinal cord they synapse with motor neurons that transmit nerve impulses from the spinal cord to muscles or glands.
SPINAL NERVES
The spinal cord gives rise to 31 PAIRS OF SPINAL NERVES. See FIG. 17.2. The first cervical pair emerges between the atlas (1st cervical) vertebra and the foramen magnum of the occipital bone. The other spinal nerves leave the vertebral column through the intervertebral foramina between the vertebrae.
There are 8 pairs of CERVICAL nerves (C1-C8), 12 pairs of THORACIC nerves (T1-T12), 5 pairs of LUMBAR nerves (L1-L5), 5 pairs of SACRAL nerves (S1-S5), 1 pair of COCCYGEAL nerves. The lumbar, sacral and coccygeal nerves descend the vertebral column as the CAUDA EQUINA before emerging from the vertebral column.
See FIG. 17.3. Each SPINAL NERVE is connected to the spinal cord by the ANTERIOR and POSTERIOR ROOTS. The POSTERIOR (DORSAL) ROOT contains SENSORY NERVE FIBERS. The posterior root also contains the POSTERIOR (DORSAL) ROOT GANGLION, which consists of clusters of sensory neuron cell bodies. The ANTERIOR (VENTRAL) ROOT contains MOTOR AXONS. Notice that the spinal nerves are MIXED NERVES, consisting of both sensory and motor nerve fibers. See also FIG. 17.5 (reflex arc).
After a spinal nerve leaves the intervertebral foramen, it divides into several branches, called RAMI (singular, RAMUS). See FIG. 17.7. The DORSAL RAMI innervate the deep muscles and skin of the back. Except for spinal nerves T2-T11, the VENTRAL RAMI form networks on both sides of the spinal cord called PLEXUSES (singular, PLEXUS). See FIG. 17.2.
The CERVICAL PLEXUS (See FIG. 17.2 & FIG. 17.8) is formed by the ventral rami of nerves C1-C5. The cervical plexus supplies the skin and muscles of the head, neck and upper shoulders. The PHRENIC NERVES are a major pair of nerves that supply motor nerve fibers to the DIAPHRAGM.
The BRACHIAL PLEXUS (see FIG. 17.2 & FIG. 17.9) is formed by the ventral rami of spinal nerves C5-C8 and T1. It supplies the arms and shoulders.
The ventral rami of spinal nerves T2-T11 do not form plexuses and are called the INTERCOSTAL (THORACIC) NERVES (see FIG. 17.2). These nerves supply the intercostal muscles, skin of the chest wall, abdominal muscles and skin, and deep back muscles.
The LUMBAR PLEXUS (see FIG. 17.2 & FIG. 17.11) is formed by the ventral rami of spinal nerves L1-L4. It supplies the abdominal wall, external genitalia, and some skin and muscles of the leg. Its largest nerve is the FEMORAL NERVE.
The SACRAL PLEXUS (see FIG. 17.2, FIG. 17.11b & 17.12) is formed by the ventral rami of spinal nerves L4-L5 & S1-S4. It supplies the buttocks, perineum, external genitals, and legs. The largest nerve in the body is the SCIATIC NERVE, which supplies the muscles of the leg and foot.
DERMATOMES (FIG. 17.13):
All spinal nerves (except C1) supply SOMATIC SENSORY NEURONS to the skin. The skin segment supplied by a spinal nerve is called a DERMATOME.