caudal (i.e., towards the bottom) of the brainstem structures. Spinal cord In the spinal cord, α-MNs are located within the gray matter that forms the ventral horn. These α-MNs provide the motor component of the spinal nerves that innervate muscles of the body. As in the brainstem, higher segments of the spinal cord contain α-MNs that innervate muscles higher on the body. For example, the biceps brachii muscle, a muscle of the arm, is innervated by α-MNs in spinal cord segments C5, C6, and C7, which are found rostrally in the spinal cord. On the other hand, the gastrocnemius muscle, one of the muscles of the leg, is innervated by α-MNs within segments S1 and S2, which are found caudally in the spinal cord. Alpha motor neurons are located in a specific region of the spinal cord's gray matter. This region is designated lamina IX in the Rexed lamina system, which classifies regions of gray matter based on their cytoarchitecture. Lamina IX is located predominantly in the medial aspect of the ventral horn, although there is some contribution to lamina IX from a collection of motor neurons located more laterally. Like other regions of the spinal cord, cells in this lamina are somatotopically organized, meaning that the position of neurons within the spinal cord is associated with what muscles they innervate. In particular, α-MNs in the medial zone of lamina IX tend to innervate proximal muscles of the body, while those in the lateral zone tend to innervate more distal muscles. There is similar somatotopy associated with α-MNs that innervate flexor and extensor muscles: α-MNs that innervate flexors tend to be located in the dorsal portion of lamina IX; those that innervate extensors tend to be located more ventrally. Development Alpha motor neurons originate in the basal plate, the ventral portion of the neural tube in the developing embryo. Sonic hedgehog (Shh) is secreted by the nearby notochord and other ventral structures (e.g., the floor plate), establishing a gradient of highly concentrated Shh in the basal plate and less concentrated Shh in the alar plate. Under the influence of Shh and other factors, some neurons of the basal plate differentiate into α-MNs. Like other neurons, α-MNs send axonal projections to reach their target extrafusal muscle fibers via axon guidance, a process regulated in part by neurotrophic factors released by target muscle fibers. Neurotrophic factors also ensure that each muscle fiber