* Muscular System Functions * Body movement (Locomotion) * Maintenance of posture * Respiration * Diaphragm and intercostals contractions * Communication (Verbal and Facial) * Constriction of organs and vessels * Peristalsis of intestinal tract * Vasoconstriction of blood vessels & other structures (pupils) * Heart beat * Production of body heat (Thermogenesis)

* Properties of Muscle * Excitability: capacity of muscle to respond to a stimulus * Contractility: ability of a muscle to shorten and generate pulling force * Extensibility: muscle can be stretches when pulled * Elasticity: tends to return to original shape & length after contraction or extension

* Types of Muscle * Skeletal * Attached to bones * Makes up 40% of body weight (Women’s skeletal muscle makes up 36% of their body mass, Men’s skeletal muscle makes up 42% of their body mass) * Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement * Voluntary in action; controlled by somatic motor neurons * Smooth * In the walls of hollow organs, blood vessels, eye, glands, uterus, skin * Functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow, * Controlled involuntarily by endocrine and autonomic nervous systems * Cardiac * Heart: major source of movement of blood * Autorhythmic * Controlled involuntarily by endocrine and autonomic nervous systems

* Connective Tissue Sheaths * Connective Tissue (CT) of a Muscle * Epimysium: * Epimysium: Dense regular CT surrounding entire muscle * Separates muscle from surrounding tissues and organs * Perimysium: * Perimysium: Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle * Contains blood vessels and nerves * Endomysium: * Endomysium: Loose CT that surrounds individual muscle fibers * Also contains blood vessels and nerves * Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

* Nerve and Blood Vessel Supply * Motor neurons * stimulate muscle fibers to contract * Neuron axons branch so that each muscle fiber (muscle cell) is innervated * Form a neuromuscular junction (= myoneural junction) * Capillary beds surround muscle fibers * Muscles require large amount of energy * Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

* Basic Features of a Skeletal Muscle * Muscle attachments * Most skeletal muscles run from one bone to another * One bone will move – other bone remains fixed * Origin – less movable attachment * Insertion – more movable attachment

* Basic Features of a Skeletal Muscle * Muscle attachments (continued) * Muscles attach to origins and insertions by connective tissue * Fleshy attachments – connective tissue fibers are short * Indirect attachments – connective tissue forms a tendon or aponeurosis * Bone markings present where tendons meet bones

* Skeletal Muscle Structure * Composed of muscle cells (fibers), connective tissue, blood vessels, nerves * Fibers are long, cylindrical, and multinucleated * Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length * Develop from myoblasts; numbers remain constant * Striated appearance * Nuclei are peripherally located

* Muscle Fiber Anatomy * Sarcolemma - cell membrane * Surrounds the sarcoplasm (cytoplasm of fiber) * Punctuated by openings called the transverse tubules (T- tubules) * Myofibrils -cylindrical structures within muscle fiber * Are bundles of protein filaments (=myofilaments) * Two types of myofilaments 1. Actin filaments (thin filaments) 2. Myosin filaments (thick filaments) – When myofibril shortens, muscle shortens (contracts)

* Sarcoplasmic Reticulum (SR) * SR is an elaborate, smooth endoplasmic reticulum * runs longitudinally and surrounds each myofibril * SR stores Ca ++ when muscle not contracting * When stimulated, calcium released into sarcoplasm * SR membrane has Ca ++ pumps that function to pump Ca ++ out of the sarcoplasm back into the SR after contraction

* Sarcoplasmic Reticulum (SR)

* Parts of a Muscle

Z line

* Sarcomeres: Z Disk to Z Disk * Sarcomere - repeating functional units of a myofibril * About 10,000 sarcomeres per myofibril, end to end * Each is about 2 µm long * Differences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance. * A bands: * A bands: a dark band; full length of thick (myosin) filament * M line * M line - protein to which myosins attach * H zone * H zone - thick but NO thin filaments * I bands: * I bands: a light band; from Z disks to ends of thick filaments * Thin but NO thick filaments * Extends from A band of one sarcomere to A band of the next sarcomere * Z disk: * Z disk: filamentous network of protein. Serves as attachment for actin myofilaments * Titin filaments: * Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment

* Structure of Actin and Myosin

* Myosin (Thick) Myofilament * Many elongated myosin molecules shaped like golf clubs. * Single filament contains roughly 300 myosin molecules * Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally. * Myosin heads 1. Can bind to active sites on the actin molecules to form cross- bridges. (Actin binding site) 2. Attached to the rod portion by a hinge region that can bend and straighten during contraction. 3. Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

* Actin (Thin) Myofilaments * Thin Filament: composed of 3 major proteins 1. F (fibrous) actin 2. Tropomyosin 3. Troponin * Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere. * Composed of G actin monomers each of which has a myosin- binding site (see yellow dot) * Actin site can bind myosin during muscle contraction. * Tropomyosin: an elongated protein winds along the groove of the F actin double helix. * Troponin is composed of three subunits: * Tn-A : binds to actin * Tn-T :binds to tropomyosin, * Tn-C :binds to calcium ions.

H Band

* Sliding Filament Model of Contraction * In the relaxed state, thin and thick filaments overlap only slightly * Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree * Upon stimulation, myosin heads bind to actin and sliding begins

* Sliding Filament Model of Contraction * Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere * As this event occurs throughout the sarcomeres, the muscle shortens

I-band - actin filaments, A-band - myosin filaments which may overlap with actin filaments, H-band - zone of myosin filaments only (no overlap with actin filaments) within the A-band, Z-line - zone of apposition of actin filaments belonging to two neighbouring sarcomeres (mediated by a protein called alpha-actinin), M-line - band of connections between myosin filaments (mediated by proteins, e.g. myomesin, M-protein).

* Neuromuscular Junction * Region where the motor neuron stimulates the muscle fiber * The neuromuscular junction is formed by : 1. End of motor neuron axon (axon terminal) * Terminals have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh) 2. The motor end plate of a muscle A specific part of the sarcolemma that contains ACh receptors * Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

* Neuromuscular Junction Figure 9.7 (a-c)

* Nerve impulse reaches myoneural junction * Acetylcholine is released from motor neuron * Ach binds with receptors in the muscle membrane to allow sodium to enter * Sodium influx will generate an action potential in the sarcolemma * Action potential travels down T tubule * Sarcoplamic reticulum releases calcium * Calcium binds with troponin to move the troponin, tropomyosin complex * Binding sites in the actin filament are exposed

* Myosin head attach to binding sites and create a power stroke * ATP detaches myosin heads and energizes them for another contaction * When action potentials cease the muscle stop contracting

* Smooth Muscle * Cells are not striated * Fibers smaller than those in skeletal muscle * Spindle-shaped; single, central nucleus * More actin than myosin * No sarcomeres * Not arranged as symmetrically as in skeletal muscle, thus no striations. * Dense bodies instead of Z disks

* Smooth Muscle Grouped into sheets in walls of hollow organs parallel to organ’s long axis Longitudinal layer: muscle fibers run parallel to organ’s long axis around circumference of the organ Circular layer: muscle fibers run around circumference of the organ Both layers participate in peristalsis

* Cardiac Muscle * Found only in heart where it forms a thick layer called the myocardium * Striated fibers that branch * Each cell usually has one centrally-located nucleus * Fibers joined by intercalated disks * Under control of the ANS (involuntary) and endocrine system (hormones) * Some cells are autorhythmic * Fibers spontaneously contract (Pacemaker cells)

* Cardiac Muscle Tissue

* Disorders of Muscle Tissue * Muscle Fatique * Lack of oxygen causes ATP deficit * Lactic acid builds up from anaerobic respiration * Muscle Atrophy * a decrease in the mass of the muscle * Weakening and shrinking of a muscle * May be caused * Immobilization * Loss of neural stimulation

* Disorders of Muscle Tissue * Muscle Hypertrophy * Enlargement of a muscle * More capillaries * More mitochondria * Caused by * Strenuous exercise * Steroid hormones * Muscle Tonus * Tightness of a muscle * Some fibers always contracted

* Isometric Contraction * Produces no movement * Used in * Standing * Sitting * Posture * Isotonic Contraction * Produces movement * Used in * Walking * Moving any part of the body

* Developmental Aspects: Male and Female * These differences are due primarily to the male sex hormone testosterone * With more muscle mass, men are generally stronger than women * Body strength per unit muscle mass, however, is the same in both sexes

* Developmental Aspects: Age Related * With age, connective tissue increases and muscle fibers decrease * By age 80, 50% of muscle mass is lost (sarcopenia) * Decreased density of capillaries in muscle * Reduced stamina * Increased recovery time * Regular exercise reverses sarcopenia