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

8 Pages
Unlock Document

NSC 2201

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
More Less

Related notes for NSC 2201

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.