Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 173 of 367 synapse on lower motor neurons in the motor nuclei of cranial nerves III, IV, V, VI, VII, IX, XI, and XII. Skeletal muscles of the eye, jaw, and face, and some muscles of the neck and pharynx are under conscious control by the corticobulbar tracts . Axons in the corticospinal tracts synapse on lower motor neurons in the anterior gray horns of the spinal cord. As in the main sensory cortex, the main motor cortex corresponds point by point with specific regions of the body. Amotor homunculus shows the cortical areas mapped out in diagram form. The motor homunculus differs in proportion from the actual body as its proportions are related to area of cortex and the number of motor units involved in the control of a particular region. The Medial and Lateral Pathways The components of the medial pathway help regulate gross movements of the trunk and proximal limb muscles, whereas those of the lateral pathway help control the distal limb muscles that perform more precise movements. The medial pathway controls the muscle tone and gross movements of the trunk, neck, and proximal limb muscles. The purpose is to maintain balance and posture. The descending fibers in the spinal cord constitute the vestibulospinal tracts. Axons of upper motor Chapter 15: Functional Divisions of the Nervous System: Somatic and Autonomic Nervous System 15.1 The Somatic Nervous System The somatic nervous system is an efferent branch that operates skeletal muscles. Motor commands issued by the CNS are supplied by the somatic nervous system (SNS) and the autonomic nervous system (ANS). The contractions of skeletal muscles are regulated by the somatic nervous system, also called the somatic motor system. At least two motor neurons are involved in each somatic motor pathway. It possesses an upper motor neuron with a cell body in a CNS processing center and a lower motor neuron, where the cell body lies in a nucleus of the brain stem or spinal cord. The upper motor neuron synapses on the lower motor neuron, which in turn innervates a single motor unit in a skeletal muscle. Voluntary and reflex control over the innervated motor unit are eliminated when the lower motor neuron is damaged. Conscious and subconscious motor commands control skeletal muscles by traveling over three integrated motor pathways: the corticospinal pathway, the medial pathway, and the lateral pathway. Activity within these motor pathways is monitored and adjusted by the basal nuclei and cerebellum. The output of these centers stimulates or inhibits the activity of either motor nuclei or the primary motor cortex. The Corticospinal Pathway The corticospinal pathway provides voluntary control over skeletal muscles. This system begins at the pyramidal cells of the primary motor cortex . These upper motor neurons have axons that descend into the brain stem and spinal cord and synapse on lower motor neurons that control skeletal muscles. The corticospinal pathway contains three pairs of descending tracts; the corticobulbar tracts, the lateral corticospinal tracts, and the anterior corticospinal tracts. Axons in the corticobulbar tracts