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Anatomy of the Spine and PNS
The spinal cord is a continuation of the brain stem. It's long, cylindrical, and passes through a tunnel in the vertebrae called the vertebral canal. The spinal cord has many spinal segments, which are spinal cord regions from which pairs (one per segment) of spinal nerves arise. Like the cerebrum and cerebellum, the spinal cord has gray and white matter, although here the white matter is on the outside. The spinal cord carries messages between the CNS and the rest of the body, and mediates numerous spinal reflexes such as the knee-jerk reflex.

The Vertebra, Discs & What Keeps Them Together

The spinal cord lies inside the spinal column, which is made up of 33 bones called vertebra. The vertebra, which are stacked on top of one another are divided into four regions:
7 cervical or neck vertebra (labeled C1-C7)
12 thoracic or upper back vertebra (labeled T1-T12) and attached to the ribcage
5 lumbar or lower back vertebra (labeled L1-L5)
5 sacrum (labeled S1-S5) and 4 coccyx (labeled as coccyx)

In regards to the lowest region, the 5 vertebra are fused together to form the sacrum (which is part of the pelvis), and the 4 small vertebra are fused together to form the coccyx or tailbone.

An individual vertebra is made up of several parts. The body of the vertebra is the primary area of weight bearing and provides a resting place for the fibrous discs which separate each of the vertebra. The lamina covers the spinal canal, the large hole in the center of the vertebra through which the spinal nerves pass. The spinous process is the bone you can feel when running your hands down your back. The paired transverse processes are oriented 90 degrees to the spinous process and provide attachment for back muscles. There are four facet joints associated with each vertebra. A pair that face upward and another pair that face downward.

Between the vertebral bodies (except C1 and C2) are discs that serve as supporting structure for the spine. These discs are oval-shaped, with a tough outer layer or annulus that surrounds a softer material called the nucleus pulposus. These discs act as shock absorbers for the spinal bones. Ligaments attached to the vertebra also serve as supporting structures.

These interlock with the adjacent vertebra and provide stability to the spine. The vertebra are separated by intervertebral discs which act as cushions between the bones.

Each disc is made up of two parts. The hard, tough outer layer called the annulus surrounds a mushy, moist center termed the nucleus. When a disc herniates or ruptures, the soft nucleus spurts out through a tear in the annulus, and can compress a nerve root. The nucleus can squirt out on either side of the disc or in some cases both sides.

In many cases, degeneration or pressure from overexertion can cause a disc to shift or protrude and bulge, causing pressure on a nerve and resultant pain. When this happens, the condition is called a slipped, bulging, herniated, or ruptured disc, and it sometimes results in permanent nerve damage.

The vertebra are linked by ligaments, tendons, and muscles. Back pain can occur when, for example, someone lifts something too heavy, causing a sprain, pull, strain, or spasm in one of these muscles or ligaments in the back.

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The Spinal Cord and PNS

The spinal cord is an extension of the central nervous system (CNS). Anatomically, it begins at the bottom of the brain stem at the area called the medulla oblongata which is where it joins the C1 vertebra (the highest neck bone), and it ends approximately at the level of the L1 vertebra, which is the highest bone of the lower back.

The spinal cord is about 18 inches (45 centimeters) in length and is basically a cylindrical mass of nervous tissue, oval or rounded in transverse section. It has two areas of enlargement which are found in the cervical (neck) and lumbar (lower back) segments. It occupies the upper two-thirds of the vertebral canal.

In contrast to the cerebral hemispheres, gray matter is found in the interior, surrounded by white matter. The white matter contains ascending and descending tracts. Some ascend to or descend from the brain, whereas others connect cells at various levels of the cord.

The neurons of the spinal cord include: (1) somatic motor cells, the axons of which leave by way of ventral roots and supply skeletal muscles; (2) autonomic motor cells, the axons of which leave by way of ventral roots and go to autonomic ganglia; (3) transmission neurons that give rise to ascending projections to the brain and to connections with other spinal cord levels; and (4) interneurons, which connect with other neurons at the spinal level and are concerned with sensory and reflex mechanisms.

The spinal cord is divided into segments similar to the corresponding vertebra: cervical, thoracic, lumbar, sacral, and coccygeal. The cord also has nerve roots and rootlets which form branch-like appendages leading from its ventral side (the front of the body) and from its dorsal side (the back of the body). Along the dorsal root are the cells of the dorsal root ganglia, which are critical in the transmission of "pain" messages from the cord to the brain. It's here where injury, damage, and trauma become pain.

Attached to the spinal cord on each side is a series of spinal roots, termed dorsal and ventral according to their position. Generally there are 31 pairs, which comprise 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Corresponding dorsal and ventral roots join to form a spinal nerve. Each spinal nerve divides into a dorsal and a ventral ramus, and these are distributed to various parts of the body.

The 31 pairs of spinal nerves from between the vertebra, each emerge in two short branches (roots):
One at the front (motor or anterior root) of the spinal cord
One at the back (sensory or posterior root) of the spinal cord

The motor roots carry commands from the brain and spinal cord to other parts of the body, particularly to skeletal muscles. The sensory roots carry information to the brain from other parts of the body.

At the bottom of the spinal cord, called the conus medullaris, is a collection of nerves known as the cauda equina, which is Latin for "horse's tail". Early anatomists thought the collection of nerves looked like a horse's tail.

The spinal cord is bathed in cerebrospinal fluid (CSF) and surrounded by three protective layers called the meninges (dura, arachnoid, and pia mater) just as the brain is.

The spinal cord carries out sensory, integrative, and motor functions, which can be categorized as reflex, reciprocal activity (as one activity starts, another stops), monitoring and modulation of sensory and motor mechanisms, and transmission of impulses to the brain.

Nerve impulses travel to and from the brain through the spinal cord to a specific location by way of the peripheral nervous system (PNS). The PNS is a complex system of nerves that branch off from the spinal nerve roots. These nerves travel outside of the spinal canal to the upper extremities (arms, hands and fingers), to the muscles of the trunk, to the lower extremities (legs, feet and toes), and to the organs of the body.

Any interruption of spinal cord function by disease or injury at a particular level may result in a loss of sensation and motor function below that level. Depending on the severity of the disease or injury, the loss of function may be permanent.

Multiple sclerosis (MS) attacks on the spine has a predilection for the cervical spinal cord (67% of cases), with preferential, eccentric involvement of the dorsal and lateral areas of the spinal cord abutting the subarachnoid space around the cord. Approximately 55-75% of patients with MS have spinal lesions at some point during the course of the disease.