Gray matter of spinal cord

The gray matter of spinal cord is a critical component of the central nervous system (CNS) that houses neuronal cell bodies, and the intervening neuroglia, blood vessels, and a dense network of synapses. When viewed in a cross-section of the spinal cord, the gray matter has a distinctive H-shaped or butterfly-shaped appearance, with anterior and posterior gray columns (or horns) connected by a slim band called the gray commissure, which contains the small central canal. This canal runs the length of the spinal cord and is continuous with the ventricular system of the brain. The configuration and size of the gray matter of spinal cord vary throughout the length of the cord. It is most prominent in the cervical and lumbosacral enlargements, where it innervates the large muscle groups of the upper and lower limbs, respectively. By contrast, it is smaller in regions serving fewer muscles, such as the thoracic region.

Structural Organization:

The gray matter of spinal cord is organized into columns, which are further divided by function and location. These include the anterior gray column (horn), the posterior gray column (horn), and, only in certain segments, the lateral gray column (horn). Together, these regions mediate sensory input, motor output, and autonomic functions.

Anterior Gray Column (Horn):

The anterior gray column predominantly contains multipolar motor neurons, responsible for sending motor commands to skeletal muscles via the anterior roots of spinal nerves. The nerve cells within the anterior gray column are organized into three groups based on function:

• Medial Group: It is present throughout the spinal cord. It innervates the skeletal muscles of the neck and trunk, including the intercostal muscles involved in breathing and the abdominal muscles used in posture and movement.

• Central Group: It is found only in specific cervical and lumbosacral segments. Key nuclei include the a)-Phrenic Nucleus (C3-C5): It controls the diaphragm and is the main muscle of respiration. This explains why spinal cord injuries at or above C3 can lead to respiratory failure. b)-Accessory Nucleus (C1-C5): This nucleus gives rise to fibers forming the spinal part of the accessory nerve (cranial nerve XI), which supply the sternocleidomastoid and trapezius muscles.

• Lateral Group: It is found specifically in cervical and lumbosacral enlargements and is responsible for innervating the skeletal muscles of the upper and lower limbs, respectively.

Posterior Gray Column (Horn):

The posterior gray column processes sensory inputs from the body, such as touch, pain, temperature, and proprioception (sense of body position). It is contains four important functional nerve cell groups:

• Substantia Gelatinosa: The substantia gelatinosa is located at the tip (apex) of the posterior column and spans the entire spinal cord. It processes afferent inputs related to pain, temperature, and light touch. These signals can be modulated by information from descending pathways, which helps explain phenomena like pain suppression.

• Nucleus Proprius: The nucleus proprius is located anterior to the substantia gelatinosa and forms the bulk of the posterior horn. It processes information from the posterior white column, such as signals for proprioception, two-point tactile discrimination, and vibration sense.

• Nucleus Dorsalis (Clarke's Column): The Clarke's column is found at the base of the posterior column from C8 to L3. It primarily processes proprioceptive information from muscle spindles and tendon organs, which is then relayed to the cerebellum for coordination of movement. This forms part of the spinocerebellar tracts.

• Visceral Afferent Nucleus: It is located lateral to Clarke’s column, extending from T1 to L3. It receives and processes sensory input from the viscera (internal organs).

Lateral Gray Column (Horn):

The lateral gray column is present only in the thoracic (T1-L2/3) and sacral (S2-S4) regions of the spinal cord and plays a significant role in the autonomic nervous system.

• Thoracic and upper lumbar Region: Contains the intermediolateral cell column, which gives rise to preganglionic sympathetic fibers. These fibers ultimately control the "fight or flight" responses, such as increasing heart rate or dilating pupils.

• Sacral Region: Gives rise to preganglionic parasympathetic fibers, which are involved in "rest-and-digest" functions such as bladder control and intestinal motility.

The Gray Commissure and Central Canal:

The right and left halves of the spinal cord's gray matter are connected by the gray commissure, which runs transversely. The central canal, located at the center of the gray commissure, contains cerebrospinal fluid (CSF) and is lined by specialized ciliated cells called ependymal cells, which help circulate CSF.