Spinal cord

As part of the central nervous system (CNS), the spinal cord is the gateway for information transfer between body and brain, as well as a center for neuronal circuits that integrate and coordinate complex sensory, motor, and autonomic functions. This primer provides an overview of the organization and function of the mammalian spinal cord. With millions of neurons arising from eleven distinct progenitor domains, and differentiating into perhaps more than fifty separable populations of neurons, the spinal cord is clearly a multifaceted CNS structure. The spinal cord is housed within the spinal (vertebral) column surrounded by meninges. It is divided into segments along its length. Each segment has a spinal nerve that exits the CNS containing the sensory, motor and autonomic nerve fibers involved in body function. In humans, there are eight cervical, twelve thoracic, five lumbar, and five Primer sacral segments. Two portions of the spinal cord are enlarged to house the greater number of neurons required for limb function: the cervical (forelimbs) and lumbar (hindlimbs) enlargements. The rostrocaudal organization of the spinal cord is associated with regional innervation patterns and its operation includes the integrative processing of cutaneous, musculoskeletal and autonomic function. The segmental organization within the spinal cord is shown in Figure 1. Sensory neurons (primary afferents) synapse onto spinal neurons largely within the dorsal horn, and also project rostrocaudally via axon tracts. White matter surrounds the cord and comprises the axon tracts that relay signals to and from brain and between spinal segments. The spinal cord gray matter consists of the dorsal and ventral horns and contains: sites of termination of primary afferent neurons and neurons descending from the brain; interneurons; and ascending tract cells projecting to higher CNS levels. The several hundred thousand neurons per segment are housed within cytoarchitectonically defined anatomical layers called laminae (I–X). These can broadly be divided into: the sensory dorsal horn (laminae I–VI); the intermediate gray (lamina VII); and the ventral horn (VII–IX). Motor neurons are located in lamina IX. An intermediolateral cell column contains sympathetic (T1–L2) and parasympathetic (S2–S4) preganglionic neurons. The axons of motoneurons and preganglionic neurons exit via ventral (anterior) roots to innervate skeletal muscle and postganglionic neurons, respectively.