The spinal cord relays information between the brain and the peripheral nervous system. This cylindrical structure is organized into an outer layer of white matter tracts and a central core known as the gray matter. Within this central gray area lies the gray commissure, which integrates signals across the body’s central axis. This structure is necessary for coordinating basic functions and ensuring sensory data from one side of the body reaches the opposite side of the brain.
Defining the Midline Position
The gray commissure is a narrow bridge of tissue spanning the central axis of the spinal cord. When viewed in cross-section, the gray matter has a distinctive shape, often described as an ‘H’ or a butterfly. The structure connects the dorsal horns, which receive sensory information, with the ventral horns, which send out motor commands, on either side of the cord.
This tissue forms the crossbar of the ‘H’ shape, lying precisely along the midline. The gray commissure is sometimes referred to by its anatomical layer designation, Lamina X, in the Rexed classification system for spinal cord gray matter. Running through the exact center of this commissural bridge is the microscopic central canal, which is a narrow tube containing cerebrospinal fluid.
The gray commissure is functionally divided into anterior and posterior parts, based on their position relative to the central canal. The anterior gray commissure is located in front of the canal, while the posterior gray commissure is situated behind it. This precise, central location ensures that any information needing to cross from the left side of the spinal cord to the right, or vice versa, has a direct and dedicated pathway.
Specialized Cellular Components
Unlike the surrounding white matter, which is composed primarily of insulated, myelinated axons, the gray commissure is rich in the components typical of gray matter. The tissue consists of numerous neuron cell bodies, unmyelinated axons, and dendrites. The unmyelinated fibers contribute to the tissue’s darker, “gray” appearance compared to the white matter.
The most functionally specific components of this area are the commissural neurons, a class of interneurons whose axons are specifically designed to cross the midline. These cells receive input on one side of the spinal cord and project their output to the gray matter on the opposite side. The crossing fibers that make up the bulk of the commissure’s structure are the axons of these specialized interneurons.
The cell bodies of these commissural interneurons are concentrated within the gray matter of the horns, but their axons converge and travel together through the gray commissure. This dense packing of crossing fibers is the defining physical characteristic and purpose of the gray commissure. Glial cells are also present to maintain the chemical environment and structure necessary for these interneurons to function.
Facilitating Neural Crossover
The primary function of the gray commissure is to serve as the site for decussation, where nerve fibers cross from one side of the central nervous system to the other. This crossover is necessary for coordinating bilateral reflexes and integrating sensory information that will eventually be processed by the opposite hemisphere of the brain. The commissure ensures that signals originating from one side of the body can influence neural activity on the other side.
This function is exemplified by the pathways for pain and temperature sensation. Sensory neurons carrying information about painful or hot stimuli enter the spinal cord and synapse with interneurons. The axons of these secondary neurons then immediately cross the midline via the gray commissure before ascending to the brain. This crossing ensures that a painful sensation felt on the left foot, for instance, is conveyed up the right side of the spinal cord to the right side of the brain.
The commissure’s role in integrating signals is also evident in coordinated motor activities and reflexes involving both sides of the body. By allowing interneurons to communicate across the midline, the structure enables simultaneous or reciprocal actions, such as the synchronized movement of limbs. This allows for complex reflexes and the maintenance of posture and balance that rely on coordinating muscle activity on both the left and right sides.