The spinal cord serves as a fundamental pathway within the central nervous system. It functions as the primary communication link, relaying messages between the brain and the extensive network of nerves throughout the body. This intricate structure transmits information efficiently, coordinating various bodily functions and actions.
Anatomy and Appearance of the Spinal Cord
The spinal cord is a long, thin, tubular structure of nervous tissue. In an average adult, its length is approximately 43-45 centimeters (17-18 inches), extending from the brainstem down into the lower back. Its diameter varies along its length, being wider in the cervical and lumbar regions and narrower in the thoracic area.
This delicate structure is housed within the protective vertebral column, also known as the backbone. Surrounding the spinal cord are three layers of protective membranes called the meninges. The outermost layer is the dura mater, a tough, fibrous membrane.
Beneath the dura mater lies the arachnoid mater, a delicate, web-like layer. The innermost layer, the pia mater, adheres to the spinal cord’s surface. Between the arachnoid and pia mater, in the subarachnoid space, circulates cerebrospinal fluid (CSF). This fluid acts as a cushion, protecting the spinal cord from physical shocks.
Internal Structure and Composition
A cross-section of the spinal cord reveals two distinct regions: grey matter and white matter. The grey matter is centrally located and typically assumes a “butterfly” or “H” shape. This region is composed of neuron cell bodies, dendrites, and unmyelinated axons.
The grey matter serves as a processing center where neurons synapse and transmit information. Surrounding this central grey matter is the white matter. Its lighter appearance is attributed to the myelin sheath, a fatty insulating layer that encases nerve fibers (axons).
The white matter is organized into columns or funiculi, which include dorsal, lateral, and ventral regions. These columns consist mainly of myelinated axons, and their primary role is to conduct nerve signals up and down the spinal cord. While grey matter processes information, white matter ensures its rapid transmission throughout the nervous system.
How the Spinal Cord Functions
The spinal cord acts as a two-way communication pathway for nerve signals. Ascending tracts, also known as sensory or afferent tracts, carry information upwards from the body to the brain. This includes sensations such as pain, temperature, touch, and proprioception (the sense of the body’s position in space).
Conversely, descending tracts, or motor/efferent tracts, convey commands from the brain downwards to the body. These signals are responsible for voluntary movements, such as kicking a ball. For instance, a signal to move a muscle originates in the brain and travels down a descending tract to reach the appropriate spinal nerves.
The spinal cord also facilitates spinal reflex arcs. These are rapid, involuntary responses to stimuli that occur at the spinal cord level, often without immediate input from the brain. For example, if a hand touches a hot surface, sensory receptors send a signal to the spinal cord. The spinal cord then sends a motor command back to the muscles, causing the hand to withdraw before the brain fully registers the pain.
The Connection to the Nervous System
The spinal cord forms a continuous structure with the brainstem, specifically the medulla oblongata, at the base of the skull. This direct anatomical link ensures communication between the brain and the rest of the nervous system. As the spinal cord descends, it gives rise to 31 pairs of spinal nerves, which emerge at different levels from the vertebral column.
Each of these spinal nerves is “mixed,” meaning it contains both sensory fibers that transmit information from the body to the spinal cord and motor fibers that carry commands from the spinal cord to muscles and glands. These nerves branch out extensively, forming the peripheral nervous system, which innervates the trunk and limbs. Specific areas of skin, known as dermatomes, are supplied by sensory fibers from a single spinal nerve root.
At its lower end, around the first or second lumbar vertebra, the spinal cord tapers into a cone-shaped structure called the conus medullaris. Below this point, a collection of spinal nerves descends within the vertebral canal before exiting at their respective levels. This distinctive bundle of nerves is known as the cauda equina, a Latin term meaning “horse’s tail” due to its resemblance to one.
Consequences of Spinal Cord Damage
Damage to the spinal cord can disrupt the flow of information between the brain and the body. The specific symptoms and their severity depend on both the location and the extent of the injury. Injuries occurring higher up the spinal cord affect a larger portion of the body compared to those lower down.
Spinal cord injuries are categorized as either complete or incomplete. A complete injury results in a total loss of sensation and motor function below the level of the injury. In contrast, an incomplete injury allows some nerve communication to persist, leading to some preserved sensation or motor control below the injury site.
The level of the injury determines the extent of paralysis experienced. An injury affecting the lower spinal cord can result in paraplegia, which is paralysis affecting the lower body, including the legs and trunk. Damage to the upper spinal cord can lead to quadriplegia, also known as tetraplegia. This condition involves paralysis of all four limbs, the trunk, and pelvic organs. Additional symptoms can include numbness, tingling, weakness, pain, and loss of bladder or bowel control.