Chapter 9 – The Muscular System

Introduction There are 3 types of muscle tissue in the muscular system: Skeletal muscle: Attached to bones of skeleton Voluntary (consciously controlled) Cardiac muscle: Makes up most of the wall of the heart Involuntary (non-consciously- controlled) Responsible for pumping action of the heart Smooth muscle: Found in walls of internal organs, such as those of digestive tract

9.1: Structure of a Skeletal Muscle Skeletal Muscles: Attach to bones, and skin of face Under conscious control (voluntary) A skeletal muscle is an organ of the muscular system Skeletal muscles are composed of: Skeletal muscle tissue Nervous tissue Blood Connective tissues

9.2: Skeletal Muscle Contraction Contraction of a muscle fiber: Requires interaction from several chemical and cellular components Results from a movement within the myofibrils, in which the actin and myosin filaments slide past one another, shortening the sarcomeres Muscle fiber shortens and pulls on attachment points

Neuromuscular Junction A type of synapse Also called a myoneural junction Site where an axon of motor neuron and skeletal muscle fiber interact Skeletal muscle fibers contract only when stimulated by a motor neuron Parts of a NMJ: Motor neuron Motor end plate Synaptic cleft Synaptic vesicles Neurotransmitters

Oxygen Supply & Cellular Respiration Anaerobic Phase : In the absence of oxygen Glycolysis Occurs in cytoplasm Produces little ATP Aerobic Phase: In the presence of oxygen Occurs in the mitochondria Produces the most ATP

Oxygen Debt Anaerobic (Lactic Acid) Threshold: Oxygen debt: During rest or moderate exercise, respiratory & cardiovascular systems supply enough O2 to support aerobic respiration Anaerobic (Lactic Acid) Threshold: Shift in metabolism from aerobic to anaerobic, during strenuous muscle activity, when the above systems cannot supply the necessary O2. Lactic acid is produced. Oxygen debt: Amount of oxygen needed by liver cells to convert the accumulated lactic acid to glucose, and to restore muscle ATP and creatine phosphate concentrations.

Muscle Fatigue Muscle Fatigue: Muscle Cramp: Inability to contract muscle Common causes of muscle fatigue: Decreased blood flow Ion imbalances across the sarcolemma Loss of desire to continue exercise Accumulation of lactic acid Muscle Cramp: Sustained, involuntary muscle contraction May be caused by changes in electrolyte concentration in extracellular fluids in the area

Heat Production Heat is a by-product of cellular respiration in active cells Muscle cells are major source of body heat More than half the energy released in cellular respiration becomes heat; less than half is transferred to ATP Blood transports heat throughout body core

9.3: Muscular Responses Muscle contraction can be observed by removing a single skeletal muscle fiber and connecting it to a device that senses and records changes in the overall length of the muscle fiber. Electrical stimulator promotes the contractions

Threshold Stimulus Threshold Stimulus: Minimum strength of stimulation of a muscle fiber required to cause contraction When strength of stimulus reaches threshold, an action potential is generated Impulse spreads through muscle fiber, releasing Ca+2 which begins to contraction process One action potential from a motor neuron releases enough Ach (neurotransmitter) to produce threshold stimulus in muscle fiber, causing a muscle impulse

Recruitment of Motor Units Increase in the number of motor units activated, to produce more force Certain motor units are activated first, and others are activated only when the intensity of stimulus increases As intensity of stimulation increases, recruitment of motor units continues until all motor units are activated

Sustained Contractions Smaller motor units (smaller diameter axons) - recruited first Larger motor units (larger diameter axons) - recruited later Summation and recruitment can produce sustained contractions of increasing strength Whole muscle contractions are smooth movements Muscle tone (tonus): Continuous state of partial contraction in resting muscles

Types of Contractions Isotonic: muscle contracts and changes length; produces movement Walking Moving a body part Isometric: muscle contracts but does not change length; produces no movement Sitting up Standing Maintaining posture

9.4: Smooth Muscle Compared to skeletal muscle fibers, smooth muscle fibers are: Shorter Single, centrally located nucleus Elongated with tapering ends Myofilaments randomly organized Lack striations

Types of Smooth Muscle 2 types of smooth muscle: Multi-unit Smooth Muscle: Cells are less organized Function as separate units Fibers function independently Iris of eye, walls of blood vessels Stimulated by neurons, hormones Visceral Smooth Muscle: Single-unit smooth muscle; cells respond as a unit Sheets of spindle-shaped muscle fibers Fibers held together by gap junctions Exhibit rhythmicity Conduct peristalsis Walls of most hollow organs More common type of smooth muscle

Smooth Muscle Contraction Resembles skeletal muscle contraction in these ways: Interaction between actin and myosin (components of muscle fibers) Both use calcium and ATP Both are triggered by membrane impulses (motor neurons) Different from skeletal muscle contraction in these ways: Hormones can stimulate or inhibit smooth muscle Stretching can trigger smooth muscle contraction Smooth muscle slower to contract and relax Smooth muscle more resistant to fatigue

9.5: Cardiac Muscle Cardiac Muscle: Located only in the heart Striated muscle cells Muscle fibers joined together by intercalated discs Fibers branch, contain a single nucleus Network of fibers contracts as a unit (conduction system) Rhythmic contractions Longer refractory period (time where muscle is unresponsive) than skeletal muscle No sustained contractions

Characteristics of Muscle Tissues

9.6: Skeletal Muscle Actions Skeletal muscles generate a great variety of body movements The action of each muscle mostly depends upon - the type of joint it is associated with - the way the muscle is attached on either side of the joint

Body Movement When bones or body parts move, bones and muscles act as levers. 4 basic components of levers: Rigid bar or rod (bones) Fulcrum or pivot on which bar moves (joint) Object moved against resistance (weight) Force that supplies energy for movement (muscles)

Origin and Insertion One end of a skeletal muscle is more fixed, and the other end is more movable: Origin: less movable end Insertion: more movable end When a muscle contracts, insertion is pulled toward origin

Interaction of Skeletal Muscles Most skeletal muscle function in groups. Roles of muscles in performing certain actions: Agonist: muscle that causes an action Prime mover: agonist primarily responsible for movement (In some cases, the terms “agonist” and “prime mover” are used interchangeably) Synergists: muscles that assist agonist / prime mover Antagonist: muscles whose contraction causes movement in the opposite direction of the prime mover

9.7: Major Skeletal Muscles Anterior View of the Superficial Skeletal Muscles:

Major Skeletal Muscles Posterior View of the Superficial Skeletal Muscles:

9.8: Lifespan Changes Myoglobin, ATP, and creatine phosphate decline, starting in the 40s Connective tissue and adipose cells replace some muscle tissue By age 80, almost half of muscle mass has atrophied (wasted away) Muscle strength decreases, and reflexes become slower Exercise helps to maintain muscle mass and function