Anatomy of the Spine & Peripheral Nervous System

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 cylindrical in shape. It has two areas of enlargement which are found in the cervical (neck) and lumbar (lower back) segments.

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 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.

There are 31 pairs of spinal nerves and roots. Eight pairs of cervical nerves (called C1-C8) exit from the cervical cord at each vertebral level. Each pair of nerves then splits and one exits on the right side, and the other exits on the left. The first cervical root exits above the C1 vertebra. The second cervical root exits between the C1-C2 segment, and the remaining roots exit just below the correspondingly numbered vertebra. The C8 nerve root exits between the C7 and T1 vertebra. There are 12 thoracic nerve pairs (T1-T12). The T1 nerve root exits between the T1 and T2 vertebrae. There are five lumbar nerve pairs (L1-L5). The L1 nerve root exits between L1 and L2. There are five sacral nerve pairs (S1-S5). The S1 nerve root exits between S1 and S2. One pair of coccygeal (Co1) nerves meets in the area of the tailbone.

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.