Textbook Notes (368,795)
United States (206,114)
NSC 2201 (16)
Smith (14)
Chapter 13

NSC 2201 Chapter 13: CH 13 Spinal Control of Movement
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Department
Neuroscience
Course
NSC 2201
Professor
Smith
Semester
Spring

Description
CH 13 SPINAL CONTROL OF MOVEMENT INTRODUCTION • Motor system consists of all our muscles and the neurons that control them • Motor control divided into two parts o Spinal cord’s command and control of motor programs in spinal cord o Brain’s command and control of motor programs in spinal cord • Isotonic contraction: movement of muscle and joint angle • Isometric contraction: stabilizing the joint, no muscle or joint angle change THE SOMATIC MOTOR SYSTEM • Smooth muscle and striated muscle are the two muscle categories o Smooth muscle lines digestive tract, arteries, etc. and is innervated by ANS o Striated muscle can be either cardiac or skeletal ▪ Cardiac muscle contracts in absence of innervation; innervation from ANS serves to increase or decrease contraction rate ▪ Skeletal muscle moves bones around joints, moves eyes in heads, inhales and exhales, control facial expression, and to produce speech • Skeletal muscle o Each muscle is enclosed in protective connective tissue sheath that forms tendons at end of muscle o Each muscle has hundreds of muscle fibers, which are cells of skeletal muscle o Each fiber is innervated by single axon branch from the CNS o Smooth muscle and nervous system that control them are collectively called the somatic motor system because they come from the same embyrological 33 somites o Flexion decreases angle of joints o Extension increases angle of joints • Muscles that flex a joint are called flexors • If they all work together to achieve same flexion or extension, muscles are called synergists • Muscles that cause extension are called extensors • Flexors and extensors that pull on the joint in opposite directions are called antagonists o Flexion of elbow joint requires relaxation of antagonistic extensor muscles and coordinated contraction of synergistic flexor muscles • Axial muscles: responsible for movements of the trunk; important for posture • Proximal (girdle) muscles: move shoulder, elbow, pelvis, and knee; important for locomotion • Distal muscles: move hands, feet, and digits; important for specialized manipulation of objects THE LOWER MOTOR NEURON • Only lower motor neurons (somatic motor neurons in ventral horn of spinal cord) directly command muscle contraction • Upper motor neurons supply input to the spinal cord THE SEGMENTAL ORGANIZATION OF LOWER MOTOR NEURONS • Mixed spinal nerve: made of ventral root (full of lower motor neurons) and dorsal root (sensory and motor neurons) • 30 spinal nerves on each side • Skeletal muscles not evenly spread on body o C3-T1 is swollen because contains all nerves for arms, etc. and the spinal interneurons and motor neurons so these dorsal and ventral horns are swollen o L1-S3 have swollen dorsal and ventral horns because this is where leg musculature is controlled • Lower motor neurons also organized in ventral horn in predictable way – cells innervating axial muscles are medial to those innervating distal muscles and cells innervating flexors are dorsal to those innervating extensors ALPHA MOTOR NEURONS • Two kinds of lower motor neurons in spinal cord: alpha motor neuron and gamma motor neuron • Alpha motor neurons directly trigger the generation of force by muscles • Motor unit: one alpha motor unit and all the muscle fibers it innervates • Motor neuron pool: collection of alpha motor neurons that innervate a single muscle GRADED CONTROL OF MUSCLE CONTRACTION BY ALPHA MOTOR NEURONS • Can use several mechanism to control force of muscle contraction in graded fashion • Controls firing rate of motor neurons o ACh release causes reliable EPSP onto muscle fiber o Causes twitch • Can recruit additional synergistic motor units o Muscles with smaller muscle fiber to alpha motor neuron ratios are more finely controlled by CNS o Antigravity muscles of leg have ratio of 1000:1 but muscles of eyes are 3:1 • Size principle: orderly recruitment of small alpha motor units before larger ones o Small motor units have small alpha motor neurons and big motor units have big alpha motor neurons o Small neurons are more easily excited and are used up first INPUTS TO ALPHA MOTOR NEURONS • Three major sources of input to alpha motor neuron o Dorsal root ganglion cell with axons that innervate a muscle spindle (sensory apparatus embedded in muscle); this input provides feedback about muscle length o Upper motor neurons in brain stem and motor cortex also provides input to alpha motor neuron; important for voluntary movement o Largest input is through interneuron in spinal cord; can be excitatory or inhibitory TYPES OF MOTOR UNITS • Slow (S) fibers o Can sustain contraction for long time without fatigue o Slow to contract o Antigravity muscles of leg and torso • Fast (F) fibers o Contract rapidly and powerfully, but tire faster as well o Fatigue-resistant (FR) fibers have moderately strong and fast contractions and are resistant to fatigue o Fast fatigable (FF) fibers generate the strongest, fastest contractions but are quickly exhausted • All three types of muscle fibers (S, FR, and FF) can be in one muscle, but each motor unit contains muscle fibers of only one kind • One type of slow motor unit and two types of fast motor units with either FR and FF fibers • Motor neurons of FF units have biggest and largest axon diameter and cell • Motor neurons of FR units have intermediate axon and cell • Motor neurons of slow units have small-diameter NEUROMUSCULAR MATCHMAKING • Muscle phenotype changed to match what kind of nerve (or pattern of stimulation) was innervating it • Provides possibility that neurons switch phenotype as consequence of synaptic activity (experience) and that this may be the basis for learning and memory • Hypertrophy: exaggerated growth of muscle as long-term consequence of increased activity • Atrophy: degeneration of muscle fibers from prolonged inactivity EXCITATION-CONTRACTION COUPLING • Excitation-contraction coupling: AP trigger of release of Ca2+ from an organelle
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