The spinal cord serves as the primary pathway for communication between the brain and the body, encased protectively within the vertebral column. It is functionally organized into distinct segments, each transmitting nerve signals that govern movement and sensation.
The Five Regions of the Spinal Cord
The human spinal cord is approximately 45 cm long in men and 43 cm in women and is divided into five distinct regions. The uppermost region is the cervical spine, containing eight segments (C1–C8). Below this, the thoracic region consists of twelve segments (T1–T12), followed by the lumbar region with five segments (L1–L5). The sacral region also contains five segments (S1–S5), and the coccygeal region has one segment. 31 pairs of spinal nerves emerge from these segments.
The spinal cord segments do not perfectly align with the bony vertebrae that protect them. The spinal cord itself ends around the first or second lumbar vertebra in a structure known as the conus medullaris. Below this point, a bundle of nerve roots, called the cauda equina, extends downward through the remainder of the vertebral canal. This means lower spinal nerves must travel a significant distance from their origin in the spinal cord to their exit point from the vertebral column.
Segmental Function and Body Control
Each segment of the spinal cord is responsible for innervating specific parts of the body. The cervical segments are primarily in control of the head, neck, and diaphragm, which is the muscle that controls breathing. Nerves from this region also extend to the shoulders, arms, wrists, and hands.
The thoracic segments innervate the torso, including the muscles of the chest and abdomen, and are also involved in the function of internal organs. The lumbar segments control the hips, thighs, and knees, as well as parts of the feet. The sacral segments control the remaining muscles in the feet and play a significant role in controlling bowel, bladder, and sexual functions.
Dermatomes and Myotomes
The organization of the spinal cord is clinically mapped using dermatomes and myotomes. A dermatome is a specific area of skin where sensory information, such as touch, pain, and temperature, is supplied by a single spinal nerve root. For instance, feelings of numbness or tingling in the thumb and index finger can indicate an issue with the C6 spinal nerve.
Similarly, a myotome refers to a group of muscles innervated by a single spinal nerve root. By testing the strength of different muscle groups, clinicians can identify weakness and pinpoint which motor nerve is affected. For example, difficulty extending the knee could suggest a problem with the nerves originating from the lumbar region. This systematic approach of testing both sensation and muscle function provides a detailed picture of the spinal cord’s health.
Impact of Segmental Damage
Damage to the spinal cord affects body function, with the severity largely dependent on the location of the injury. The higher the injury on the spinal cord, the more extensive the loss of function. An injury to the cervical region, such as at the C4 level, can lead to quadriplegia, which is paralysis of all four limbs, and may also impair the ability to breathe independently.
In contrast, an injury lower down, for example in the lumbar region at the L2 level, typically results in paraplegia, where function is lost in the lower body, but the arms and upper body remain unaffected. This is because the nerve signals from the brain cannot travel past the point of injury to reach the lower parts of the body.