The spinal cord has a cylindrical shape in which the white matter is located superficially and the gray matter is found deep to the white matter shell. The gray matter forms a continuous column that runs the length of the cord. However, the nerve roots that enter and exit the spinal cord bundle into discrete nerves, which form the basis for naming the •Fig. 9.1 Musculotopic Organization of Motor Neurons in the Ventral Horn of the Spinal Cord. A, Schematicviewofthecervicothoracicspinalcordandassociatedcross-sections,showingtheloca-tionsofmotorneuronsthatinnervateaflexor(blue dots) andanextensor(red dots). B, Spinalcordcross-section,withlocationsofdifferentmusclesrepresentedbyadrawingofthearm.(RedrawnfromPurvesD,etal[eds].Neuroscience. 3rded.Sunderland,MA:Sinauer;2004.)B Distal muscles Proximal muscles specific levels (“segments”) of the spinal cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal). When viewed in cross-section, the gray matter column typically has an “H” or butterfly shape. The “butterfly wings” are divided into dorsal and ventral horns that are separated by an intermediate zone (
Fig. 9.1 ) (at some spinal cord levels, a small lateral horn is also present; see ). The ventral horn is where motor neurons reside, and thus it has primarily a motor function. Correspondingly, it is the main target of descending motor pathways from the brain. In contrast, the dorsal horn is the major recipient of incoming sensory information and the main source of ascending sensory pathways (e.g., the spinothalamic tract; see
Chapter 7).
The description of the motor function of the spinal cord therefore begins with the properties of motor neurons and their organization in the ventral horn. Similar organizational principles hold for cranial nerve nuclei that are involved in controlling the musculature of the head and face (e.g., the facial and motor trigeminal nuclei).
A motor neuron is a neuron that projects to muscle cells. Because motor neurons represent the only route for CNS activity to control muscle activity, motor neurons have been termed the final common pathway. There are both somatic motor neurons and autonomic motor neurons. However, in this chapter, the term motor neuron refers only to somatic motor neurons; autonomic motor neurons are discussed in . Somatic motor neurons innervate the skeletal (striated) muscles of the body. Two main classes are distinguished on the basis of their axonal diameters: α and γ motor neurons.
The α motor neurons are large, multipolar neurons that range in size up to 70 µm in diameter (see
Fig. 4.10A ). Their axons leave the spinal cord through the ventral roots and the brainstem via several cranial nerves and are distributed to the appropriate skeletal muscles via peripheral nerves. The α motor neuron axon also projects to other neurons by giving off collateral axons before leaving the CNS. The main axon terminates by synapsing onto the extrafusal muscle fibers. These synapses are called neuromuscular junctions or end plates. Extrafusal fibers are large muscle fibers that make up the bulk of a skeletal muscle and essentially generate its contractile force (a muscle also contains intrafusal fibers whose functions are detailed later in this chapter; also see