Human Nervous System: Spinal Cord

Study Notes

The spinal cord is a long, slender cylinder of nerve tissue that extends from the brain stem, measuring about 45cm in length and 1-1.5cm in width.
It is protected by the vertebral column and descends through the vertebral canal, exiting through a large hole in the base of the skull.

There are 31 pairs of spinal nerves, named according to the region of the vertebral column from which they emerge.
The pairs include:
- Cervical nerves (8 pairs, C1-C8)
- Thoracic nerves (12 pairs, T1-T12)
- Lumbar nerves (5 pairs, L1-L5)
- Sacral nerves (5 pairs, S1-S5)
- Coccygeal nerves (1 pair)

During development, the vertebral column grows 25cm longer than the spinal cord.
As a result, the spinal cord segments giving rise to spinal nerves are not aligned with the corresponding intervertebral spaces.
Most spinal nerves descend before emerging from the vertebral column at the corresponding space.
The spinal cord extends only to the L1-L2 vertebra level (waist level).

The grey matter is centrally located, functionally organized, and forms an inner butterfly-shaped region surrounded by outer white matter.
The grey matter consists of neuronal cell bodies, dendrites, and glial cells.
The central canal lies in the center of the grey matter and is filled with cerebrospinal fluid.
Each half of the grey matter is divided into:
- Dorsal (posterior) horn: contains cell bodies of interneurons that receive sensory information
- Ventral (anterior) horn: contains cell bodies of efferent motor neurons supplying skeletal muscles
- Lateral horn: contains cell bodies of autonomic nerve fibers supplying smooth muscles, cardiac muscles, and exocrine glands

The white matter is organized into tracts, consisting of bundles of nerve fibers (axons of long interneurons) with similar functions.
Tracts are grouped into columns that extend the length of the spinal cord.
Tracts begin or end within a particular brain area and transmit specific types of information.
There are two main types of tracts:
- Ascending tracts (cord to brain): transmit signals derived from afferent input
- Descending tracts (brain to cord): relay messages from the brain to efferent neurons
Tracts are named based on their origin and termination.

Damage to particular areas of the spinal cord can interfere with some functions while leaving others intact.
Various types of signals are carried in different tracts within the spinal cord.