The Muscular System

The Muscular System Chapter 8

Structure of a Skeletal Muscle • Layers of fibrous connective tissue called fascia separate an individual skeletal muscle from adjacent muscles and hold it in position • Fascia may project beyond the end of the muscle forming a tendon

Cont. • Epimysium – layer of con. tissue that surrounds a skeletal muscle • Perimysium – extends inward from the epimysium and separates the muscle tissue into small compartments • Fascicles – bundles of skeletal fibers found in compartments • Endomysium – con. tissue that surrounds individual skeletal fibers within fascicles

Cont. • Skeletal muscle fiber • Thin, elongated cylinder with rounded ends • The cytoplasm (sarcoplasm) contains many small, oval nuclei, mitochondria, and myofibrils • Myofibrils contain two types of protein filaments • Myosin (thick) • Actin (thin)

Striation pattern • I bands (the light bands) are composed of thin actin filaments directly attached to Z lines • A bands (the dark bands) are composed of thick myosin filaments overlapping thin actin filaments • Sarcomere – segment of a myofibril that extends from one Z line to the next Z line

Sarcoplasmic reticulum – membranous channels that surrounds each myofibrl and runs parallel to it • Transverse tubules – extend inward as invaginations from the fiber’s membrane and passes all the way through the fiber; contains extracellular fluid • Both activate the muscle contraction mechanism when the fiber is stimulated

Neuromuscular Junction • Each skeletal muscle fiber connects to an axon from a nerve cell, called a motor neuron • The axon extends outward from the brain or spinal cord • The muscle fiber contracts only when the motor neuron stimulates it • The connection between the motor neuron and the muscle fiber is called a neuromuscular junction • Muscle fiber membrane forms a motor end plate

The end of the motor neuron branches and projects into recesses of the muscle fiber membrane • Cytoplasm at the distal ends of these motor neuron axons contains synaptic vesicles that store chemicals called neurotransmitters

Summary of Muscle Contraction • Nerve impulse travels from brain to the end of a motor neuron axon; synaptic vesicles release a neurotransmitter (acetylcholine) into the synaptic cleft between the neuron and the motor end plate of the muscle fiber stimulating the muscle fiber to contract.

Role of Myosin and Actin • Myosin – composed of two twisted protein strands with globular parts called cross-bridges projecting outward along their lengths • Actin – a globular structure with a binding site to which the myosin cross-bridges can attach • Composed of troponin and tropomyosin

Sliding filament model • Sarcomeres shorten as myosin cross-bridge attaches to an actin binding site that is exposed by the release of calcium ions and bends slightly, pulling the actin filament • Then the head can release, straighten, combine with another binding site further down the actin filament, and pull again

Globular portions of the myosin filaments contain an enzyme, ATPase • ATPase catalyzes the breakdown of ATP to ADP and phosphate, releasing energy • This energy puts the myosin cross-bridge in a “cocked” position • When a cocked cross-bridge binds to actin, it pulls on the thin filament • Filament moves toward the center of the sarcomere, and the sarcomere shortens • Cycle repeats as long as ATP is available and muscle fiber is stimulated to contract

Actin Myosin Cross bridges Binding sites Sarcomere Tropomysium Troponin Calcium Sarcoplasmic reticulum Propagation Action potential Permeability Channels Vocabulary Use the above vocabulary and pictures to explain the sliding filament model.

Stimulus for Contraction • Acetylcholine – neurotransmitter that stimulates skeletal muscle fibers • When nerve impulses cease, two events lead to muscle relaxation • Acetylcholine is decomposed by acetylcholinesterase • Calcium ions transported back into the sarcoplasmic reticulum

Energy Sources for Contraction • Existing ATP molecules • Once used up cells must regenerate ATP from ADP and phosphate • Creatine phosphate contains high energy phosphate bonds • An enzyme in the mitochondria catalyzes the synthesis of creatine phosphate • Stores excess energy released from the mitochondria • Cellular respiration of glucose

Oxygen Supply and Cellular Respiration • Glycolysis can take place in the absence of oxygen • More complete breakdown of glucose (cellular respiration) requires oxygen • Oxygen comes from lungs and is carried by rbcs and are attached to hemoglobin • Myoglobin is synthesized in muscle cells and imparts the reddish-brown color of skeletal muscle tissue; temporarily stores oxygen

Oxygen Debt • When skeletal muscles are used even for a minute or two, the muscle fibers must increasingly use anaerobic respiration to obtain energy • Pyruvic acid used to regenerate glycolysis, but produces lactic acid • Lactic acid accumulates in the muscles, diffuses into the bloodstream, and reaches the liver • In the liver, reactions occur requiring ATP, synthesize glucose

Oxygen debt has to be repaid • May take several hours • Metabolic capacity may change with training (Table 8.2)

Muscle Fatigue • A muscle exercised strenuously for a prolonged period may lose its ability to contract, a condition called fatigue • Interruption in the muscle’s blood supply or lack of acetylcholine in motor neuron axons may also cause fatigue • Most often caused from the accumulation of lactic acid which lowers pH and fibers no longer respond to stimulation • Cramps occur when a muscle undergoes a sustained involuntary contraction

Heat Production • More than half of the energy released in cellular respiration is heat • Blood transports heat to other tissues, which helps maintain body temperature

Muscular Responses • When an isolated muscle fiber is exposed to a series of stimuli of increasing strength, the fiber remains unresponsive until a certain strength of stimulation is applied • Minimal strength is called the threshold stimulus • Skeletal muscle fiber does not respond partially – all-or-none response • Extent of shortening depends on resistance

Skeletal muscle removed from an animal can be used to show how a whole muscle responds to stimulation • Muscle is mounted in a device and then stimulated electrically • When the muscle contracts, it pulls on a lever, and its movement is recorded • Resulting pattern is a myogram

Twitch - single contraction that lasts only a fraction of a second • Latent period – delay between the time the stimulus was applied and the time the muscle fiber responded • Shorter than 0.01 sec. in humans • Period of contraction – muscle pulls at attachment • Period of relaxation – muscle returns to its former length

Summation – process that combines the force of individual twitches • When the resulting forceful, sustained contraction lacks even partial relaxation, it is called a tetanic contraction, or tetanus

Electromyography • EMG measurements record the electrical activity associated with muscle contraction, so it is not a direct measure of force

Muscle Tone • Def. – muscle undergoes some sustained contraction when it appears to be at rest • Response to nerve impulses that originate repeatedly from the spinal cord and stimulate a few muscle fibers • Important in maintaining posture

Smooth Muscle • Lack striations • Can change length without changing tautness • Can maintain contraction longer with a given amt. of ATP • Multiunit smooth muscle • Muscle fibers are separate • Found in the irises of the eyes and in walls of blood vessels • Contracts only in response to stimulation by motor nerve impulses or certain hormones

Visceral smooth muscle • Composed of sheets of spindle-shaped cells in close contact with one another • Found in the walls of hollow organs • Fibers can stimulate each other • Display rhythmic pattern of repeated contractions • Peristalsis – wavelike motion that occurs in tubular organs and helps force the contents of these organs along their lengths

Cardiac Muscle • Found only in the heart • Composed of branching, striated cells interconnected • Twitches are longer than skeletal twitches • Intercalated discs allow muscle impulses to pass freely from cell to cell • Self-exciting and rhythmic

Skeletal Muscle Actions • Origin – immovable end of a muscle • Insertion – movable end of a muscle • When a muscle contracts, its insertion is moved toward its origin

Interaction of Skeletal Muscles • Function in groups • Prime mover (agonist) – muscle responsible for a particular body movement • Synergists – muscles that contract and assist the prime mover • Antagonists – muscles that resist a prime mover’s action and cause movement in the opposite direction

Muscle disorders • Muscular dystrophies – inherited diseases; progressive weakness and deterioration; caused by mutation of gene which codes for dystrophin • Charcot-Marie-Tooth disease - comprises a group of disorders that affect peripheral nerves which lie outside the brain and spinal cord and supply the muscles in the limbs • Myotonic dystrophy – form of muscular dystrophy; gradual reduction in strength and control • Hereditary idiopathic dilated cardiomyopathy - the heart becomes weakened and enlarged, and cannot pump blood efficiently

Major Skeletal Muscles

Doc, what’s wrong with me? • Create a doctor/patient interview about a given muscular disorder • Include symptoms, description, cause, how diagnosis is made, risk factors, treatment, and alternate names • Must have two copies of interview and one prop

The Muscular System