The spinal cord serves as a central pathway for communication within the body, acting as a direct extension of the brain. This long, tubular structure of nervous tissue transmits signals between the brain and the rest of the body. It coordinates movement, processes sensory information, and regulates automatic bodily functions.
Overall Structure of the Spinal Cord
The spinal cord is a cylindrical structure, typically measuring about 45 centimeters in men and 43 centimeters in women, with a diameter ranging from 6.4 mm in the thoracic region to 13 mm in the cervical and lumbar areas. It begins at the base of the skull, connecting with the medulla oblongata, and extends downwards to approximately the first or second lumbar vertebra in adults. This structure is shorter than the vertebral column that encases it.
The spinal cord is organized into 31 segments, each giving rise to a pair of spinal nerves. These segments are grouped into four main regions: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. The cervical and lumbar regions are enlarged due to the increased number of nerves serving the upper and lower limbs. At its lower end, the spinal cord tapers into a cone-shaped structure called the conus medullaris. From this, a fibrous extension, the filum terminale, anchors it to the coccyx. Below the conus medullaris, a collection of nerve roots continues downward, resembling a horse’s tail, known as the cauda equina.
Internal Components of the Spinal Cord
A cross-section of the spinal cord reveals two distinct areas: an inner, H-shaped or butterfly-shaped region of gray matter surrounded by an outer layer of white matter. The gray matter primarily consists of neuronal cell bodies, dendrites, unmyelinated axons, glial cells, and synapses. Its characteristic shape is formed by projections known as horns.
The gray matter is divided into three main columns or horns. The dorsal (posterior) horns receive sensory information from the body. The ventral (anterior) horns contain motor neurons that send signals to skeletal muscles. Present only in the thoracic and upper lumbar regions, the lateral horns house neurons involved in autonomic functions. Connecting the two halves of the gray matter is the gray commissure, which surrounds the central canal, a small tube filled with cerebrospinal fluid that runs the length of the spinal cord.
The white matter is composed of myelinated nerve fibers organized into columns or funiculi. These funiculi are divided into three main regions: the anterior (ventral) funiculus, the posterior (dorsal) funiculus, and the lateral funiculus. These columns contain ascending tracts that carry sensory information towards the brain and descending tracts that carry motor commands from the brain to the body.
Key Functions of Spinal Cord Regions
The distinct regions within the spinal cord’s gray matter perform specific functions. The dorsal horns serve as the primary receiving centers for sensory input from various parts of the body, including pain, temperature, and touch sensations. This sensory information then travels through ascending pathways to the brain for further processing. The ventral horns are responsible for transmitting motor commands to skeletal muscles, initiating voluntary movements. The neurons within these horns extend their axons out of the spinal cord to directly innervate muscles.
The lateral horns, found in the thoracic and upper lumbar segments, regulate involuntary bodily functions, such as heart rate, digestion, and sweating, as they contain neurons of the autonomic nervous system.
A significant function mediated by the spinal cord, often without direct brain involvement, is the reflex arc. This neural pathway produces an automatic, involuntary response to a stimulus. When a sensory receptor detects a stimulus, such as touching something hot, the signal travels along a sensory neuron to the spinal cord. Here, this signal can directly activate a motor neuron, often with an interneuron acting as a bridge, causing an immediate muscle contraction to withdraw from the stimulus. This rapid processing at the spinal cord level allows for quicker protective actions, with the brain receiving the sensory information only after the reflex has occurred.
How the Spinal Cord is Protected
The delicate spinal cord is encased and shielded by several layers of protective structures. The outermost protection is provided by the vertebral column, which is a series of 33 bones called vertebrae. These bones form a strong bony canal that surrounds the spinal cord. Between each vertebra are intervertebral discs, which act as shock absorbers, cushioning the vertebrae and the spinal cord from impacts.
Beneath the vertebral column, three layers of membranes known as meninges further safeguard the spinal cord, similar to those protecting the brain. The dura mater is the tough, outermost layer, providing a durable protective coating. The arachnoid mater is the middle layer, characterized by its web-like appearance. The innermost layer, directly adhering to the surface of the spinal cord, is the pia mater.
Spaces exist between these meningeal layers. The epidural space, located between the dura mater and the vertebral bone, contains adipose tissue and a network of blood vessels. The subarachnoid space, situated between the arachnoid mater and the pia mater, is filled with cerebrospinal fluid (CSF). This fluid provides additional cushioning and buoyancy, protecting the spinal cord from sudden movements and impacts.