BIOL 2457 1st Edition Lecture 21Outline of Last Lecture Muscle tissues Outline of Current LectureMuscle tissues  Control of Muscle Tension Motor Units Consist of a motor neuron and the muscle fibers it stimulates The axon of a motor neuron branches out forming neuromuscular junctions with different muscle fibers A motor neuron makes contact with about 150 muscle fibers Control of precise movements consists of many small motor units Muscles that control voice production have 2 - 3 muscle fibers per motor unit Muscles controlling eye movements have 10 - 20 muscle fibers per motor unit Muscles in the arm and the leg have 2000 - 3000 muscle fibers per motor unit The total strength of a contraction depends on the size of the motor units and the number that are activated Control of Muscle Tension Twitch Contraction The brief contraction of the muscle fibers in a motor unit in response to an action potential Twitches last from 20 to 200 msec L Phases Latent Period Contraction Period Relaxation Period Refractory Period Control of Muscle Tension Latent period (2 msec) A brief delay between the stimulus and muscular contraction The action potential sweeps over the sarcolemma and Ca2+ is released from the SR Contraction period (10–100 msec) Ca2+ binds to troponin Myosin-binding sites on actin are exposed Cross-bridges form Control of Muscle Tension Relaxation period (10–100 msec) Ca2+ is transported into the SR Myosin-binding sites are covered by tropomyosin Myosin heads detach from actin Muscle fibers that move the eyes have contraction periods lasting 10 msec Muscle fibers that move the legs have contraction periods lasting 100 msec Control of Muscle Tension Refractory period When a muscle fiber contracts, it temporarily cannot respond to another action potential Skeletal muscle has a refractory period of 5 milliseconds Cardiac muscle has a refractory period of 300 milliseconds Muscle Tone A small amount of tension in the muscle due to weak contractions of motor units Small groups of motor units are alternatively active and inactive in a constantly shifting pattern to sustain muscle tone Muscle tone keeps skeletal muscles firm Keeps the head from slumping forward on the chest Types of Contractions Isotonic contraction The tension developed remains constant while the muscle changes its length Used for body movements and for moving objectslike picking a book up off a table Isometric contraction The tension generated is not enough for the object to be movedand the muscle does not change its length Holding a book steady using an outstretched arm Types of Skeletal Muscle Fibers Muscle fibers vary in their content of myoglobin Red muscle fibers Have a high myoglobin content Appear darker (dark meat in chicken legs and thighs) Contain more mitochondria Are supplied by more blood capillaries White muscle fibers Have a low content of myoglobin Appear lighter (white meat in chicken breasts) Muscle fibers contract at different speeds, and vary in how quickly they fatigue Muscle fibers are classified into three main types Slow oxidative fibers Fast oxidative-glycolytic fibers Fast glycolytic fibers Slow Oxidative Fibers (SO fibers) Smallest in diameter Least powerful type of muscle fibers Appear dark red (more myoglobin) Generate ATP mainly by aerobic cellular respiration Have a slow speed of contraction Twitch contractions last from 100 to 200 msec Very resistant to fatigue Capable of prolonged, sustained contractions for many hours Adapted for maintaining posture and for aerobic, endurance-type activities such as running a marathon Fast Oxidative–Glycolytic Fibers (FOG fibers) Intermediate in diameter between the other two types of fibers Contain large amounts of myoglobin and many blood capillaries Have a dark red appearance Generate considerable ATP by aerobic cellular respiration Moderately high resistance to fatigue Generate some ATP by anaerobic glycolysis Speed of contraction faster Twitch contractions last less than 100 msec Contribute to activities such as walking and sprinting Fast Glycolytic Fibers (FG fibers) Largest in diameter Generate the most powerful contractions Have low myoglobin content Relatively few blood capillaries Few mitochondria Appear white in color Generate ATP mainly by glycolysis Fibers contract strongly and quickly Fatigue quickly Adapted for intense anaerobic movements of short duration Weight lifting or throwing a ball Distribution and Recruitment of Different Types of Fibers Most muscles are a mixture of all three types of muscle fibers Proportions vary, depending on the action of the muscle, the person’s training regimen, and genetic factors Postural muscles of the neck, back, and legs have a high proportion of SO fibers Muscles of the shoulders and arms have a high proportion of FG fibers Leg muscles have large numbers of both SO and FOG