The spinal cord

The lowest level of the central nervous system (CNS), anatomically and functionally, is the spinal cord . Continuous with the brainstem, it exits the skull through the foramen magnum . The spinal cord then passes through the vertebral canal of the vertebral column to the level of the first or second lumbar vertebrae.

The spinal cord is divided into four anatomical regions: cervical, thoracic, lumbar, and sacral. These regions are named according to the vertebrae adjacent to them during embryonic development. Each region is subdivided into functional segments. A pair of spinal nerves extends from each segment (one nerve from the left side of the spinal cord and one nerve from the right ) and exits the CNS through the intervertebral foramina , or openings between adjacent vertebrae. There are a total of 31 pairs of spinal nerves:
• 8 Cervical
• 12 Thoracic
• 5 Lumbar
• 5 Sacral
• 1 Coccygeal

There are a total of 31 pairs of spinal nerves:
• 8 Cervical
• 12 Thoracic
• 5 Lumbar
• 5 Sacral
• 1 Coccygeal

Spinal nerves arising from the cervical level of the cord are involved with sensory perception and motor function of the back of the head, neck, and arms.

Nerves arising from the thoracic level innervate the upper trunk. Spinal nerves from the lumbar and sacral regions of the cord innervate the lower trunk, back, and legs. Lesions of the spinal cord interrupt sensation and motor function.

The gray matter is composed of nerve cell bodies and unmyelinated interneuron fibers. The location of the gray matter in the spinal cord is opposite to that of the brain. In the brain, the gray matter of the cerebrum and the cerebellum is found externally forming a cortex, or covering, over the internally located white matter. In the spinal cord, the gray matter is found internally and is surrounded by the white matter

The spinal cord is responsible for two vital CNS functions. The cord:
• Conducts nerve impulses to and from the brain
• Processes sensory input from the skin, joints, and muscles of the trunk and limbs and initiates reflex responses to this input

As such, on each side of the spinal cord the gray matter is divided into three regions:
• Dorsal horn (posterior, toward the back)
• Ventral horn (anterior)
• Lateral horn

Each spinal segment contains millions of neurons within the gray matter.Functionally, four types of neurons exist:
• Second-order sensory neurons
• Somatic motor neurons
• Visceral motor neurons
• Interneurons

The cell bodies of second-order sensory neurons are found in the dorsal horn.