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Jon Houseman

THE SOMATIC MOTOR SYSTEM The muscles in the body can be described according to two broad categories:  Smooth o Lines the digestive tract, arteries and related structures o Innervated by nerve fibers from the ANS o Plays a role in peristalsis and the control of blood pressure and blood flow  Striated – there are two types of striated muscles o Cardiac muscle  is the heart muscle  contracts rhythmically  innervations of the heart from the ANS functions to accelerate or slow down the heart rate o Skeletal muscle  Constitutes the bulk of muscle mass in the body  Functions to move bones around joints  Is enclosed in a connective tissue sheath that forms tendons  Contains hundreds of muscle fibers, each innervated by a single axon branch from the CNS (see Fig. 13.3)  Derived from 33 paired somites  Controlled by the somatic motor system  Under voluntary control and it generates behaviour Consider elbow joint (see Fig. 13.2)  Movement in the direction that closes the knife is called flexion o The muscles that cause flexion are called flexors. Because they work together, they are called synergists.  Movement in the direction that opens the knife is called extension o The muscles that cause extension are called extensors.  Flexors and extensors pull on a joint in opposite directions and are antagonists. Muscles that are responsible for movements:  of the trunk are called axial muscles o important for maintaining posture  of the shoulder, elbow, pelvis, and knee are called the proximal muscles o important for locomotion  of the hands, feet, and digits are called the distal muscles o important for specialized manipulation of objects THE LOWER MOTOR NEURON Somatic musculature is innervated by the somatic motor neurons in the ventral horn of the spinal cord (see Fig. 13.3).  These cells are sometimes called lower motor neurons to distinguish them from the upper motor neurons of the brain that supply input to the spinal cord.  Only the lower motor neurons directly command muscle contraction. The Segmental Organization of Lower Motor Neurons The axons of lower motor neurons bundle together to form ventral roots. Each ventral root joins with a dorsal root to form a spinal nerve that exits the cord through the notches between the vertebrae. Skeletal muscles are not distributed evenly throughout the body, nor are lower motor neurons distributed evenly within the spinal cord. For example, innervations of the more than 50 muscles of the arm originate entirely from spinal segments C3-T1. The ventral horns in this region of the spinal cord appear swollen to accommodate the large number of motor neurons that control the arm musculature (see Fig. 13.4). The lower motor neurons are also distributed within the ventral horn at each spinal segment in a predictable way, depending on their function (see Fig. 13.5). Alpha Motor Neurons There are two categories of lower motor neurons of the spinal cord: alpha and gamma motor neurons. Alpha motor neurons trigger the generation of force by muscles.  One alpha motor neuron and all the muscle fibers it innervates collectively make up the motor unit (the elementary component of control).  Muscle contraction results from the individual and combined actions of these motor units. The collection of alpha motor neurons that innervates a single muscle is called a motor neuron pool. Graded Control of Muscle Contraction by Alpha Motor Neurons The nervous system uses several mechanisms to control the force of muscle contraction in a fine
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