Muscular System: Histology and Physiology Chapter 9 Muscular System: Histology and Physiology

Functions of the Muscular System Body movement (skeletal muscles attached to bones) Maintenance of posture Respiration (skeletal muscles of thorax are responsible for the movement necessary for respiration) Production of body heat (when skeletal muscles contact, heat is given off as a by-product) Communication (speaking, writing) Constriction of organs and vessels (contraction of smooth muscle) Heart beat (contraction of cardiac muscle)

General Functional Characteristics of Muscle Contractility: ability of a muscle to shorten with force Excitability: capacity of muscle to respond to a stimulus (by nerve or hormone) Extensibility: muscle can be stretched to its normal resting length and beyond to a limited degree Elasticity: ability of muscle to recoil to original resting length after stretched

Types of Muscle Tissue Skeletal Smooth Cardiac Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement Voluntary Smooth Walls of hollow organs, blood vessels, eye, glands, skin Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow In some locations, autorhythmic Controlled involuntarily by endocrine and autonomic nervous systems Cardiac Heart: major source of movement of blood Autorhythmic

Skeletal Muscle Structure Composed of muscle cells (fibers), connective tissue, blood vessels, nerves Fibers are long, cylindrical, multinucleated Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length Develop from myoblasts (they are converted to muscle fibers as contractile proteins accumulate within their cytoplasm); numbers remain constant (# of muscle fibers remain constant after birth----so, enlargement of muscles is an increase in size rather than #) Striated appearance due to light and dark banding

Connective Tissue Coverings of Muscle Layers External lamina. Delicate, reticular fibers. Surrounds sarcolemma (P.M.) Endomysium. Loose C.T. with reticular fibers. Perimysium. Denser C.T. surrounding a group of muscle fibers. Each group called a fasciculus Epimysium. C.T. that surrounds a whole muscle (many fascicles) Fascia: connective tissue sheet Forms layer under the skin Holds muscles together and separates them into functional groups. Allows free movements of muscles. Carries nerves (motor neurons, sensory neurons), blood vessels, and lymphatics. Continuous with connective tissue of tendons and periosteum.

Nerves and Blood Vessel Supply Motor neurons: stimulate muscle fibers to contract. Nerve cells with cell bodies in brain or spinal cord; axons extend to skeletal muscle fibers through nerves Axons branch so that each muscle fiber is innervated Capillary beds surround muscle fibers

Muscle Fibers Nuclei just inside sarcolemma Cell packed with myofibrils within cytoplasm (sarcoplasm = cytoplasm without myofibrils) Threadlike (extends from one end of muscle fiber to the other) Composed of protein threads called myofilaments: thin (actin 8nm) and thick (myosin 12nm) Sarcomeres: actin & myosin myofilaments form highly ordered units called sarcomeres. They are joined end to end to form the myofibrils.

Parts of a Muscle

Actin and Myosin Myofilaments

Actin (Thin) Myofilaments 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 an active site 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: one that binds to actin, a second that binds to tropomyosin, and a third that binds to calcium ions. Spaced between the ends of the tropomyosin molecules in the groove between the F actin strands. The tropomyosin/troponin complex regulates the interaction between active sites on G actin and myosin.

Myosin (Thick) Myofilament Many elongated myosin molecules shaped like golf clubs. 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 Can bind to active sites on the actin molecules to form cross-bridges. Attached to the rod portion by a hinge region that can bend and straighten during contraction. 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

Sarcomeres: Z Disk to Z Disk Z disk: filamentous network of protein. Serves as attachment for actin myofilaments Striated appearance I bands: from Z disks to ends of thick filaments A bands: length of thick filaments H zone: region in A band where actin and myosin do not overlap M line: middle of H zone; delicate filaments holding myosin in place In muscle fibers, A and I bands of parallel myofibrils are aligned. Titin filaments: elastic chains of amino acids; make muscles extensible and elastic

Sliding Filament Model Actin myofilaments sliding over myosin to shorten sarcomeres Actin and myosin do not change length Shortening sarcomeres responsible for skeletal muscle contraction During relaxation, sarcomeres lengthen because of some external force, like forces produced by other muscles (contraction of antagonistic muscles) or by gravity. - agonist = muscle that accomplishes a certain movement, such as flexion. - antagonist = muscle acting in opposition to agonist.

Sarcomere Shortening

Physiology of Skeletal Muscle Fibers Nervous system controls muscle contractions through action potentials Resting membrane potentials Membrane voltage difference across membranes (polarized) Inside cell more negative due to accumulation of large protein molecules. More K+ on inside than outside. K+ leaks out (through leak channels) but not completely because negative molecules hold some back. Outside cell more positive and more Na+ on outside than inside. Na+ /K+ pump maintains this situation. Must exist for action potential to occur

Ion Channels Types Each is specific for one type of ion Ligand-gated. Ligands are molecules that bind to receptors. Receptor: protein or glycoprotein with a receptor site Example: neurotransmitters Gate is closed until neurotransmitter attaches to receptor molecule. When Ach (acetylcholine) attaches to receptor on muscle cell, Na gate opens. Na moves into cell due to concentration gradient Voltage-gated Open and close in response to small voltage changes across plasma membrane Each is specific for one type of ion

Action Potentials Phases Depolarization: Inside of plasma membrane becomes less negative. If change reaches threshold, depolarization occurs Repolarization: return of resting membrane potential. Note that during repolarization, the membrane potential drops lower than its original resting potential, then rebounds. This is because Na plus K together are higher, but then Na/K pump restores the resting potential All-or-none principle: like camera flash system Propagate: Spread from one location to another. Action potential does not move along the membrane: new action potential at each successive location. Frequency: number of action potential produced per unit of time

Gated Ion Channels and the Action Potential

Action Potential Propagation

Neuromuscular Junction Synapse: axon terminal resting in an invagination of the sarcolemma Neuromuscular junction (NMJ): Presynaptic terminal: axon terminal with synaptic vesicles Synaptic cleft: space Postsynaptic membrane or motor end-plate

Function of Neuromuscular Junction Synaptic vesicles Neurotransmitter: substance released from a presynaptic membrane that diffuses across the synaptic cleft and stimulates (or inhibits) the production of an action potential in the postsynaptic membrane. Acetylcholine Acetylcholinesterase: A degrading enzyme in synaptic cleft. Prevents accumulation of ACh

Excitation-Contraction Coupling Mechanism by which an action potential causes muscle fiber contraction Involves Sarcolemma Transverse (T) tubules: invaginations of sarcolemma Terminal cisternae Sarcoplasmic reticulum: smooth ER Triad: T tubule, two adjacent terminal cisternae Ca2+ Troponin

Action Potentials and Muscle Contraction

Cross-Bridge Movement