Spinal Cord

Spinal Cord

The spinal cord is a thin column of nerves leading from the brain to the vertebral canal. It starts at the point where nervous tissue exits the cranial cavity near the foramen magnum and eventually tapers off to terminate near the point where the first and second­ lumbar are located (FIGURE 12-13). The spinal cord is approximately 42 cm in length and 1.8 cm thick. It appears as a shiny white structure, protected by bone, meninges, and CSF. The spinal cord provides two ways of communication, to and from the brain, and contains the spinal reflex centers. The spinal cord is continuous throughout its length, with slight internal structure changes. The spinal cord is divided into right and left halves by a deep anterior median fissure and a shallow posterior median sulcus. It is slightly flat from front to back.

The spinal meninges are continuous with those of the brain. In the spinal cord, the dura mater has a ­single layer and is not attached to the vertebral col-umn. The inner surface of the dura mater contacts the outer surface of the arachnoid mater, which is the middle meningeal layer of the spine. An epidural space exists between the dura mater and vertebrae, which is padded by fat and a vein network. The CSF fills the subarachnoid space, which lies between the arachnoid and pia mater meninges. The dural and arachnoid membranes extend inferiorly to the S₂ level, which is far below the end of the spinal cord that ends between the L₁ and L₂ levels. The subarach-noid space inside the meningeal sac inferior to the lumbar region is an excellent spot for the removal of CSF. This procedure is called a lumbar puncture or spinal tap.

The spinal cord terminates inferiorly in a tapered, cone-shaped structure called the conus medullaris. A fibrous extension of the conus medullaris called the filum terminale extends inferiorly to the coccyx to anchor the spinal cord. Sawtooth- shaped sections of pia mater are called denticulate ligaments and bind the spinal cord to the dura mater meninx for its entire length. Components of the filum terminale blend with a dense cord of collagen fibers continu-ous with the spinal dura mater to form the coccygeal ligament. The spinal cord is only about as wide as a human thumb over most of its length. It has prom-inent enlargements at points where the nerves arise that serve the upper and lower limbs. These are called the cervical and lumbar­ enlargements.

Cross-Section of the Spinal Cord

The inner core of the spinal cord is made of gray mat-ter surrounded by white matter. Motor fibers pass out of portions of the gray matter through spinal nerves to skeletal muscles; however, most of the gray mat-ter neurons are interneurons. Each segment of the spinal cord is designated by its paired spinal nerves. Each spinal nerve emerges from the vertebral column superiorly to its related vertebra via the intervertebral foramen. Each nerve travels to the body region that it specifically serves.

The sensory fibers that enter the spinal cord usu-ally end on interneurons, which receive input from the sensory neurons. Impulses may be transmitted by the interneurons to adjacent multipolar motor neurons. The axons of these motor neurons leave in the spinal nerve’s ventral root and nerve impulses are transmitted to muscles and glands. The arrangement of these neurons enables information to enter and leave the spinal cord very quickly. The spinal cord conducts not only nerve impulses, but also spinal reflexes.

The gray matter of the spinal cord appears as a butterfly shape, also described as similar to the letter “H,” within the spinal cord’s white matter. Major nerve pathways called nerve tracts are made up of long bundles of myelinated nerve fibers. A horizontal bar of gray matter in the very middle of the spinal cord surrounds its central canal and contains CSF. This is known as the gray commissure. Its two dorsal gray matter projections are called the dorsal (posterior) horns, whereas the ventral pair is called the ventral (anterior) horns (FIGURE 12-14). They run the entire length of the spinal cord. The dorsal horns contain somatic and visceral sensory inputs, which receive and relay sensory information for peripheral receptors. An additional pair of gray matter columns called the lateral horns exists in the thoracic and superior lum-bar segments (sympathetic neurons).

All neurons that have cell bodies in the gray matter of the spinal cord are multipolar. Interneurons com-pletely make up the dorsal horns, whereas the ventral horns have mostly somatic motor neurons with lower amounts of interneurons. The motor neurons send their axons out to the skeletal muscles, which are their effector organs, via ventral rootlets. These rootlets fuse together, becoming the spinal cord’s ventral roots. Sensory and motor roots are bound together into one spinal nerve. This occurs distal to each dorsal root ganglion.

Cell bodies of autonomic (sympathetic division) motor neurons, which serve visceral organs, mostly make up the lateral horns. Their axons leave the spinal cord with the ventral root alongside those from the somatic motor neurons. Ventral roots serve both PNS motor divisions because they contain somatic and autonomic efferent fibers.

The dorsal roots are formed by afferent fibers, which carry impulses from peripheral sensory recep-tors. They fan out as the dorsal rootlets before entering the spinal cord. Associated sensory neuron cell bodies lie in an enlarged region of each dorsal root, which is known as the dorsal root ganglion (spinal ganglion).

The white matter of the spinal cord is made up of myelinated and nonmyelinated nerve fibers. These allow communication between sections of the spi-nal cord and between the spinal cord and brain. The nerve fiber tracts run in three different directions. The tracts that carry information to the brain are called ascending tracts and the tracts that carry infor-mation to the muscles and glands are called descend-ing tracts. The ascending tracts run up to higher centers for sensory input and the descending tracts run down from the brain to the spinal cord or inside the spinal cord to its lower levels for motor output.

The transverse tracts run across the spinal cord from one side to the other with commissural fibers. Most white matter is made up of ascending and descend-ing tracts.

There are three white matter columns called funiculi on each side of the spinal cord. They are named by their positions as the dorsal (posterior) funiculi, the lateral funiculi, and the ventral (anterior) funiculi. Each spinal tract contains several fiber tracts made up of axons that have similar functions and destinations. The names of the spinal tracts describe their destinations as well as origins (FIGURE 12- 15). The anterior white columns are interconnected by the anterior white commissure, which is where axons cross from either side of the spinal cord. The lateral white column is made up by the white matter between the anterior and posterior columns on each side.

The roots of the lumbar and sacral spinal nerves angle sharply downward. They travel inferi-orly through the vertebral canal for a long distance, finally reaching their intervertebral foramina. At the inferior end of the vertebral canal is a collection of nerve roots called the cauda equina, since it looks like a horse’s tail.

1. List the layers of the spinal cord.

2. What are the two major functions of the spinal cord?

3. Explain the cervical and lumbar enlargements of the spinal cord.