fasciculus

Fasciculus, plural fasciculi, is a term from Latin meaning a small bundle. In anatomy it denotes a bundle or tract of fibers that share a common origin, destination, or function.

In the nervous system, a fasciculus is a tract of nerve fibers within white matter. Axons within a fasciculus typically run together toward a common target or along a common route. Classic examples include the fasciculus gracilis and fasciculus cuneatus, located in the dorsal columns, which convey fine touch, vibration, and proprioceptive information from the lower and upper limbs to the gracile and cuneate nuclei in the medulla. From there the information continues to the thalamus and cortex via the dorsal column–medial lemniscus pathway. Other named fasciculi include the fasciculus proprius, a short local tract within the spinal cord, and the medial longitudinal fasciculus, a brainstem tract that coordinates eye and head movements.

In skeletal muscle, a fasciculus (often called a fascicle) is a bundle of muscle fibers surrounded by perimysium. Several fasciculi form the muscle body and are innervated by motor nerve fibers. The arrangement and size of fasciculi influence a muscle’s force production, speed, and metabolic properties.

Clinically, disruption of specific fasciculi in the CNS produces characteristic sensory or motor deficits corresponding to their origins and destinations. The term fasciculus is also used more broadly to describe small bundles of fibers within tissues in histology.