Nervous System and Spinal Cord

Structure and Function of the Spinal Cord

Spinal nerves are parts of the peripheral nervous system and carry motor, sensory, and autonomic signals between the spinal cord and the rest of the body.
The spinal cord transmits information to and from the brain, integrating information, locomotion, and reflexes. The regions of the spinal cord are named based on their relative location to the vertebral column. The spinal cord is made of ascending and descending tracts where motor signals are sent toward and away from the brain. These tracts are gray matter that is surrounded by white matter as well as the roots of spinal nerves that branch out from the spinal cord. White matter is made up of axons covered with myelin. It appears white because of the high lipid content of myelin. Gray matter consists of cell bodies that have no myelin covering and appears gray. The gray matter has several projections: the dorsal (or posterior) horn, lateral horn, and ventral (or anterior) horn. Each horn has a unique function. The dorsal horn contains neurons that transmit sensory information to the brain. The ventral horn contains the cell bodies of neurons that connect to skeletal muscle. The lateral horn includes cell bodies of neurons involved in controlling cardiac and smooth muscle.

Spinal Cord Cross Section

A cross-sectional view shows that the spinal cord consists of gray matter made of cell bodies not covered in myelin and is surrounded by white matter made of axons covered in myelin. The gray matter is divided bilaterally into the dorsal (or posterior) horn, lateral horn, and ventral (or anterior) horn, each of which has unique functions. The dorsal horn receives input from the brain, while the ventral horn sends motor signals out to the muscles.
Like the brain, the spinal cord has layers of protective tissue covering it called meninges. The dura mater is the outermost layer, sitting between the spinal cord and the bones of the vertebral column. There is a small amount of fat surrounding the dura mater to insulate it from the rigid bones. Cerebrospinal fluid fills the space between the arachnoid and the pia mater and the spinal cord, providing a cushioning layer. The dura mater and the arachnoid reach all the way down to the sacral vertebrae.

Inside a nerve, neurons are bundled together. In the central nervous system (CNS), this bundle arrangement is called a tract. The bundle is referred to as a fascicle in the peripheral nervous system (PNS). Fascicles are surrounded by connective tissue called the perineurium. Several fascicles and blood vessels are then wrapped in connective tissue called the epineurium to form a nerve. Nerves can comprise sensory neurons, motor neurons, or a combination of the two.

Spinal nerves are nerves that branch off from the spinal cord. There are 31 pairs of spinal nerves in humans: 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), and 1 pair of coccygeal nerves (Co1). Each nerve attaches to the spinal cord at two roots: a ventral (anterior) root and dorsal (posterior) root. The dorsal root is the afferent sensory root, which carries sensory information from the body to the brain and spinal cord. The ventral root is the efferent motor root and carries motor information from the brain to the rest of the body.

Spinal Nerves

A spinal nerve attaches to the spinal cord via a ventral (anterior) root and dorsal (posterior) root. The dorsal root contains sensory (afferent) neurons, which carry information from the body to the brain and spinal cord. This neuron attaches to an interneuron, which bridges a connection between the sensory and motor neurons. The ventral root carries motor information, with help from motor (efferent) neurons, from the brain to the rest of the body.