Chapter 8 Movement OUTLINE Principles of Muscular Contraction Neural Control of Muscles Motor Systems of the Brain Movement Disorders Types of Muscle Smooth muscle Digestive tract Arteries Reproductive system Striated muscle Skeletal muscle Cardiac muscles of heart Muscle Fibers Muscle fiber Individual muscle cell Fiber similar to the membrane of neurons Interior made of myofibrils Membrane contains nicotinic ACh receptors Muscle Fibers Nerve fiber releases Acetylcholine into neuromuscular junction Each AP produces single contraction of muscle fiber known as a twitch Muscle Fibers Myofibrils Long strands of protein that run the length of the muscle Sacromere single segment of myofibril Z line boundary of each sarcomere Thin filaments are made of actin blue Thick filaments are made of myosin red Muscle Fiber Contraction In the resting muscle troponin prevents interactions between actin and myosin Action potential and Na influx leads to internal release of calcium in muscle fiber Calcium binds with troponin and allows actin and myosin to interact Muscle Fiber Contraction Three Types of Myosin Filament Type I fibers slow twitch Predominate in muscles of back neck and legs Use aerobic metabolism Appear dark red because of myoglobin Participate in endurance movements Type IIa and IIb fibers fast twitch Predominate in arms and shoulders Use anaerobic metabolism Appear white Participate in brief powerful movements Quadriceps muscle Fast and slowtwitch fibers Muscle Interactions at Joints Antagonistic pairs of muscles allow movement Additional muscles allow joints to rotate OUTLINE Principles of Muscular Contraction Neural Control of Muscles Motor Systems of the Brain Movement Disorders Alpha Motor Neurons Skeletal muscle contraction controlled by motor neurons in the spinal cord or in nuclei of the cranial nerves Spinal motor neurons responsible for contracting muscle fibers known as alpha motor neurons The Motor Unit A motor unit consists of an alpha motor neuron and all the muscle fibers it controls Size of motor unit corresponds to its function Neurons serving slow twitch fibers have small cell bodies innervate few fibers and produce little force Neurons serving fast twitch fibers have larger cell bodies innervate more fibers and produce greater force Control of Alpha Motor Neurons How is muscle movement sensed and controlled Muscle stretch monitored and controlled by muscle spindles Alpha motor neurons innervate extrafusal fibers at NMJ responsible for muscle contraction Spindles provide info about muscle length Intrafusal fibers stretch same as extrafusal fibers 1a sensory fibers generate Aps when stretched which activates alpha motor neurons Gamma motor neurons activate intrafusal fibers and cause contraction Control of Alpha Motor Neurons How is muscle movement sensed and controlled Muscle stretch monitored and controlled by muscle spindles Muscular response regulated by Varying the firing rate of alpha motor neurons Recruitment activating more motor units as more load is placed on a muscle Control of Alpha Motor Neurons How is muscle movement sensed and controlled Muscle stretch monitored and controlled by muscle spindles Muscle contraction or force monitored and controlled by the Golgi tendon organs Golgi tendon organs located at junction between muscle and its tendon forms a feedback loop that limits contraction Golgi Tendon Organs Sense Force OUTLINE Principles of Muscular Contraction Neural Control of Muscles Motor Systems of the Brain Movement Disorders Descending Control of Movement Spinal cord Spinal Motor Pathways to Alpha MNs Lateral pathway serves long distance structures including hands and feet Ventromedial pathway serves proximal structures including neck and torso Ratio of motor neurons to innervated muscle fibers is proportional to the degree of fine motor control Spinal Motor Pathways Lateral pathway connects the primary motor cortex with the spinal motor neurons and is responsible for voluntary movements of the lower arms lower legs hands and feet Spinal Motor Pathways Lateral pathway connects the primary motor cortex with the spinal motor neurons and is responsible for voluntary movements of the lower arms lower legs hands and feet Ventromedial pathway originates in the brainstem and is responsible for automatic movements posture muscle tone of the neck torso upper arms upper legs