Spinal Cord

The spinal cord is one of the two parts of the Central Nervous System. It is located in the vertebral canal, within the vertebral column, and surrounded by three protective membranes (Meninges). In concert with the associated spinal nerves, the spinal cord plays important roles in motor and sensory functions related to the neck, trunk, limbs and visceral organs.

Other Names

• Medulla spinalis
• Segmental nervous system

Description

The spinal cord is a long (44 cm in mean length), roughly cylindrical structure that generally narrows along its cranial-to-caudal extent, and has two swellings, the cervical and lumbosacral enlargements. At its superior (or cranial) end, the spinal cord is continuous with the brainstem. The inferior (or caudal) end of the spinal cord is called the conus medullaris because of its shape.

The basic plan of the internal anatomy of the spinal cord is gray matter in the central region, surrounded by white matter. The gray matter is often described as "H"-shaped, with three major zones (or gray columns) on each side: posterior and anterior horns, and an intermediate zone. The white matter of each side of the spinal cord is partially divided by the gray matter horns into three funiculi (or white columns): anterior, lateral and posterior. The gradual cranial-to-caudal tapering of the spinal cord is due to decreasing amounts of white matter. The two enlargements are due to focal increases in the amount of gray matter due to the association of those levels with the upper and lower limbs.

Although the spinal cord is not actually segmented, it is often referred to as the segmental part of the CNS because of the association of spinal cord "levels" with the 31 pairs of spinal nerves.

Role of Spinal Cord in the Body

The spinal cord has motor, sensory, reflex and integrative roles within the nervous system.

• Somatic motor: control of skeletal muscles of neck, trunk and limbs
• Visceral motor: component of the sympathetic and pelvic parasympathetic nervous systems
• Somatic sensory: exteroception and proprioception in neck, trunk and limbs
• Visceral sensory: interoception in thoracic, abdominal and pelvic visceral organs

How It Works

The principal cells of the spinal cord are the motor neurons and interneurons within its gray matter. The activity of these neurons is modulated by sensory input from the neck, trunk, limbs, and visceral organs, and descending fiber projections from the cerebral cortex and several brainstem nuclei.

The axons of spinal cord motor neurons are the motor fibers of spinal nerves. Somatic motor neurons in the anterior horn include large alpha-motor neurons that innervate skeletal muscle fibers, and smaller gamma-motor neurons that innervate intrafusal muscle fibers of muscle spindles. Visceral motor neurons in the intermediate zone innervate autonomic (sympathetic and parasympathetic) ganglia. Activity of these motor neurons controls muscle tone, posture and body movement and influences activity of smooth and cardiac muscle, and glands. The activity of lower motor neurons is influenced by spinal interneurons (central pattern generators, e.g., rhythmic movements in locomotion), sensory fibers (reflexes), and upper motor neurons in the cerebral cortex and brainstem.

Peripheral and central processes of neurons whose cell bodies reside in spinal (dorsal root) ganglia convey sensory information from the neck, trunk and limbs via spinal nerves and from visceral organs via sympathetic splanchnic nerves to the spinal cord. Some of these central processes make synaptic contacts with interneurons in the posterior horn or intermediate zone, and others ascend in the posterior funiculus to the gracile, cuneate and accessory cuneate nuclei of the medulla oblongata. Other ascending pathways that carry sensory information to the brain begin with projection interneurons in the posterior horn and intermediate zone. For example, projection interneurons in the dorsal horn (laminae I and V) give rise to the contralateral spinothalamic and spinoreticular tracts,and projection neurons in the intermediate zone (posterior thoracic nucleus of Clarke) give rise to the ipsilateral posterior spinocerebellar tract.

Diseases of the Spinal Cord

• Physical injury - e.g., Trauma, Compression by herniated intervertebral disc or by primary or metastatic extramedullary tumor
• Developmental anomalies and congenital disorders - e.g., Spinal bifida and related disorders, Hydromyelia and Syringomyelia
• Vascular disorders - e.g., Anterior spinal artery occlusion (anterior cord syndrome)
• Infectious diseases - e.g., Poliomyelitis
• Demyelinating Disorders - e.g., Multiple sclerosis
• Degenerative disorders - e.g., Amyotrophic lateral sclerosis, Spinocerebellar ataxias, Spinal muscle atrophy
• Toxic and deficiency (nutritional) diseases - e.g., Subacute combined degeneration (avitaminosis B12), Avitaminosis E
• Neoplasia - e.g., Ependymoma, Astrocytoma

Related Professions

• Neuroanatomy
• Neurophysiology
• Neurology
• Neurosurgery
• Neuropathology
• Neuroradiology