The spinal cord serves as a central component of the nervous system, extending from the brainstem down the back. It relays messages between the brain and the rest of the body. This structure coordinates actions, processes sensory information, and enables involuntary responses, allowing for movement, sensation, and automatic functions.
External Anatomy and Protection
The spinal cord is a cylindrical structure, approximately 45 centimeters long. It begins at the brainstem, the medulla oblongata, and extends downwards, usually terminating between the L1-L2 vertebrae. Its diameter varies, being wider in the cervical and lumbar regions (approximately 13 mm) and narrower in the thoracic area (around 6.4 mm).
Two enlargements exist: the cervical enlargement (C3-T1) and the lumbar enlargement (L1-S2). These wider sections accommodate the increased number of neurons needed to innervate the upper and lower limbs. At its lower end, the spinal cord tapers into a cone-shaped structure called the conus medullaris. Extending from the conus medullaris is a thin, fibrous strand, the filum terminale, which anchors the spinal cord to the coccyx. Below the conus medullaris, a bundle of nerve roots resembling a horse’s tail forms the cauda equina.
The spinal cord is housed within the vertebral column, consisting of 33 vertebrae that provide bony protection. Further protection comes from three layers of connective tissue membranes, known as the meninges. These include the outermost dura mater (a tough, fibrous membrane), the delicate, web-like arachnoid mater beneath it, and the innermost pia mater, which adheres directly to the spinal cord surface. Cerebrospinal fluid (CSF) circulates between the arachnoid and pia mater, providing buoyancy and cushioning to protect the spinal cord from physical shocks.
Internal Organization
A cross-section of the spinal cord reveals an internal organization characterized by a central H-shaped region of gray matter surrounded by white matter. This arrangement is consistent along the spinal cord’s length, though the relative proportions of gray and white matter vary by region.
The gray matter is composed of neuron cell bodies, dendrites, unmyelinated axons, and glial cells. Its H-shape is formed by projections called horns. The anterior (ventral) horns contain motor neuron cell bodies, which send signals to muscles. The posterior (dorsal) horns receive sensory information.
Small lateral horns are also present in certain segments, specifically from T1 to L2, housing autonomic neurons involved in involuntary bodily functions. At the center of the gray matter is the central canal, a narrow channel that runs the length of the spinal cord and contains cerebrospinal fluid. The gray commissure connects the two sides of the H-shaped gray matter, surrounding the central canal.
Surrounding the gray matter is the white matter, which consists of myelinated axons organized into bundles called tracts. These tracts are grouped into three main columns: the anterior, posterior, and lateral columns. These columns serve as pathways for nerve impulses traveling to and from the brain. Ascending tracts carry sensory information from the body to the brain, while descending tracts transmit motor commands from the brain to muscles and glands. These pathways ensure continuous communication throughout the central nervous system.
Spinal Cord Segments and Nerves
The spinal cord is longitudinally divided into segments, with each segment giving rise to a pair of spinal nerves. This segmentation allows specific body areas to be innervated. There are five main regions of the spinal cord, aligning with the vertebral column’s divisions.
The five main regions and their associated spinal nerves are:
Cervical region (C): Eight pairs of cervical spinal nerves (C1-C8) in the neck.
Thoracic region (T): Twelve pairs of thoracic spinal nerves (T1-T12) in the upper back.
Lumbar region (L): Five pairs of lumbar spinal nerves (L1-L5) in the lower back.
Sacral region (S): Five pairs of sacral spinal nerves (S1-S5) in the pelvis.
Coccygeal region (Co): A single pair of coccygeal nerves (Co1) at the spinal cord’s end.
In total, 31 pairs of spinal nerves emerge from the spinal cord.
Each spinal nerve forms from the fusion of two roots: a dorsal (posterior) root and a ventral (anterior) root. The dorsal roots carry sensory information towards the spinal cord, containing sensory neuron cell bodies in the dorsal root ganglia. The ventral roots carry motor commands from the spinal cord to muscles and glands. These roots combine to form a mixed spinal nerve, which then exits the vertebral column through intervertebral foramina.
Essential Functions
The spinal cord performs two primary roles: acting as a conduit for information and serving as a reflex center. These functions enable coordinated movements, sensation, and protection from harm.
The spinal cord acts as a two-way communication pathway, transmitting nerve signals between the brain and the rest of the body. Sensory (afferent) information, such as touch, temperature, pain, and proprioception (awareness of body position), travels from sensory receptors, ascending through tracts in the white matter to the brain. Conversely, motor (efferent) commands originating from the brain travel downwards via descending tracts to reach muscles and glands. This continuous exchange allows the brain to perceive the environment and control bodily responses.
Beyond relaying signals, the spinal cord also mediates rapid, involuntary responses known as reflexes. These reflexes occur without direct brain input, providing immediate reactions to stimuli. A reflex arc is the neural pathway that mediates a reflex, typically involving a sensory neuron, an interneuron within the spinal cord, and a motor neuron. A common example is the withdrawal reflex, where touching something hot immediately causes the hand to pull away before the sensation of pain is consciously registered. This protective mechanism highlights the spinal cord’s ability to process and respond to stimuli independently.