Class Notes (836,591)
Canada (509,862)
BIOB32H3 (80)

muscles and muscle tissues.docx

11 Pages
Unlock Document

Biological Sciences
Kenneth Welch

Muscles and Muscle Tissue Muscle Overview • The three types of muscle tissue are skeletal, cardiac, and smooth • These types differ in structure, location, function, and means of activation Muscle Similarities • Skeletal and smooth muscle cells are elongated and are called muscle fibers • Muscle contraction depends on two kinds of myofilaments – actin and myosin • Muscle terminology is similar • Sarcolemma – muscle plasma membrane • Sarcoplasm – cytoplasm of a muscle cell • Prefixes – myo, mys, and sarco all refer to muscle Skeletal Muscle Tissues • Packaged in skeletal muscles that attach to and cover the bony skeleton • Has obvious stripes called striations • Is controlled voluntarily (i.e., by conscious control) • Contracts rapidly but tires easily • Is responsible for overall body motility • Is extremely adaptable and can exert forces over a range from a fraction of an ounce to over 70 pounds Cardiac Muscle Tissue • Occurs only in the heart • Is striated like skeletal muscle but is not voluntary • Contracts at a fairly steady rate set by the heart’s pacemaker • Neural controls allow the heart to respond to changes in bodily needs Smooth Muscle Tissue • Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages • Forces food and other substances through internal body channels • It is not striated and is involuntary Muscle Function • Skeletal muscles are responsible for all locomotion • Cardiac muscle is responsible for coursing the blood through the body • Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs • Muscles also maintain posture, stabilize joints, and generate heat Functional Characteristics of Muscles • Excitability, or irritability – the ability to receive and respond to stimuli • Contractility – the ability to shorten forcibly • Extensibility – the ability to be stretched or extended • Elasticity – the ability to recoil and resume the original resting length Skeletal Muscle • Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue • The three connective tissue wrappings are: • Epimysium – an overcoat of dense regular CT that surrounds the entire muscle • Perimysium – fibrous CT that surrounds groups of muscle fibers called fascicles • Endomysium – fine sheath of CT composed of reticular fibers surrounding each muscle fiber Skeletal Muscle: Nerve and Blood Supply • Each muscle is served by one nerve, an artery, and one or more veins • Each skeletal muscle fiber is supplied with a nerve ending that controls contraction • Contracting fibers require continuous delivery of oxygen and nutrients via arteries • Wastes must be removed via veins Skeletal Muscle: Attachments • Muscles span joints and are attached to bone in at least two places • When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone – the muscle’s origin • Muscles attach: • Directly – epimysium of the muscle is fused to the periosteum of a bone • Indirectly – CT wrappings extend beyond the muscle as a tendon or aponeurosis Microscopic Anatomy of a Skeletal Muscle Fiber • Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma • Fibers are 10 to 100 m in diameter, and up to hundreds of centimeters long • Each cell is a syncytium produced by fusion of embryonic cells • Sarcoplasm has numerous glycosomes and a unique oxygen-binding protein called myoglobin • Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules Myofibrils • Myofibrils are densely packed, rodlike contractile elements • They make up most of the muscle volume • The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident Sarcomeres • The smallest contractile unit of a muscle • The region of a myofibril between two successive Z discs • Composed of myofilaments made up of contractile proteins • Myofilaments are of two types – thick and thin Myofilaments: Banding Pattern • Thick filaments – extend the entire length of an A band • Thin filaments – extend across the I band and partway into the A band • Z-disc – coin-shaped sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another • Thin filaments do not overlap thick filaments in the lighter H zone • M lines appear darker due to the presence of the protein desmin Ultrastructure of Myofilaments: Thick Filaments • Thick filaments are composed of the protein myosin • Each myosin molecule has a rodlike tail and two globular heads • Tails – two interwoven, heavy polypeptide chains • Heads – two smaller, light polypeptide chains called cross bridges Ultrastructure of Myofilaments: Thin Filaments • Thin filaments are chiefly composed of the protein actin • Each actin molecule is a helical polymer of globular subunits called G actin • The subunits contain the active sites to which myosin heads attach during contraction • Tropomyosin and troponin are regulatory subunits bound to actin Arrangement of the Filaments in a Sarcomere • Longitudinal section within one sarcomere Sarcoplasmic Reticulum (SR) • SR is an elaborate smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril • Paired terminal cisternae form perpendicular cross channels • Functions in the regulation of intracellular calcium levels • Elongated tubes called T tubules penetrate into the cell’s interior at each A band–I band junction • T tubules associate with the paired terminal cisternae to form triads T Tubules • T tubules are continuous with the sarcolemma • They conduct impulses to the deepest regions o2+the muscle • These impulses signal for the release of Ca from adjacent terminal cisternae Contraction of Skeletal Muscle Fibers • Contraction – refers to the activation of myosin’s cross bridges (force generating sites) • Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening • Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced Sliding Filament Mechanism of Contraction • Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree • In the relaxed state, thin and thick filaments overlap only slightly • Upon stimulation, myosin heads bind to actin and sliding begins • 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 Role of Ionic Calcium (Ca ) in the Contraction Mechanism • At low intracellular Ca concentration: • Tropomyosin blocks the binding sites on actin • Myosin cross bridges cannot attach to binding sites on actin • The relaxed state of the muscle is enforced • At higher intracellular Ca concentrations: • Additional calcium binds to troponin (inactive troponin binds two Ca ) 2+ • Calcium-activated troponin binds an additional two Ca at a separate regulatory site • Calcium-activated troponin undergoes a conformational change • This change moves tropomyosin away from actin’s binding sites • Myosin head can now bind and cycle • This permits contraction (sliding of the thin filaments by the myosin cross bridges) to begin Sequential Events of Contraction • Cross bridge attachment – myosin cross bridge attaches to actin filament • Working (power) stroke – myosin head pivots and pulls actin filament toward M line • Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches • “Cocking” of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high energy state Regulation of Contraction • In order to contract, a skeletal muscle must: • Be stimulated by a nerve ending • Propagate an electrical current, or action potential, along its sarcolemma • Have a rise in intracellular Ca levels, the final trigger for contraction • Linking the electrical signal to the contraction is excitation-contraction coupling Nerve Stimulus of Skeletal Muscle • Skeletal muscles are stimulated by motor neurons of the somatic nervous system • Axons of these neurons travel in nerves to muscle cells • Axons of motor neurons branch profusely as they enter muscles • Each axonal branch forms a neuromuscular junction with a single muscle fiber Neuromuscular Junction • The neuromuscular junction is formed from: • Axonal endings, which have small membranous sacs (synaptic vesicles) that contain the neurotransmitteracetylcholine (ACh) • The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors that helps form the neuromuscular junction • Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft • When a nerve impulse reaches the end of an axon at the neuromuscular junction: • Voltage-regulated calcium channels open and allow Ca to enter the axon 2+ • Ca inside the axon terminal causes axonal vesicles to fuse with the axonal membrane • This fusion releases ACh into the synaptic cleft via exocytosis • ACh diffuses across the synaptic cleft to ACh receptors on the sarcolemma • Binding of ACh to its receptors initiates an action potential in the muscle Action Potential • A transient depolarization event that includes polarity reversal of a sarcolemma (or nerve cell membrane) and the propagation of an action potential along the membrane Action Potential: Electrical Conditions of a Polarized Sarcolemma • The outside (extracellular) face is positive, while the inside face is negative • This difference in charge is the resting membrane potential • The predominant extracellular ion is Na + + • The predominant intracellular ion is K • The sarcolemma is relatively impermeable to both ions Action Potential: Depolarization and Generation of the Action Potential • An axonal terminal of a motor neuron releases ACh and causes a patch of the sarcolemma to become permeable to Na (sodium channels open) + • Na enters the cell, and the resting potential is decreased (depolarization occurs) • If the stimulus is strong enough, an action potential is initiated Action Potential: Propagation of the Action Potential • Polarity reversal of the initial patch of sarcolemma changes the permeability of the adjacent patch • Voltage-regulated Na channels now open in the adjacent patch causing it to depolarize • Thus, the action potential travels rapidly along the sarcolemma • Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a muscle Action Potential: Repolarization • Immediately after the depolarization wave passes, the sarcolemma permeability changes • Na channels close and K channels open • K diffuses from the cell, restoring the electrical polarity of the sarcolemma • Repolarization occurs in the same direction as depolarization, and must occur before the muscle can be stimulated again (refractory period) + + • The ionic concentration of the resting state is restored by the Na -K pump Destruction of Acetylcholine • ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase (AChE) • AChE activity prevents continued muscle fiber contraction in the absence of additional stimuli Excitation-Contraction Coupling • Once generated, the action potential: • Is propagated along the sarcolemma • Travels down the T tubules 2+ • Tr2+gers Ca release from terminal cisternae • Ca binds to troponin and causes: • The blocking action of tropomyosin to cease • Actin active binding sites to be exposed • Myosin cross bridges alternately attach and detach • Thin filaments move toward the center of the sarcomere • Hyd2+lysis of ATP powers this cycling process • Ca is removed into the SR, tropomyosin blockage is restored, and the muscle fiber relaxes Contraction of Skeletal Muscle (Organ Level) • Contraction of muscle fibers (cells) and muscles (organs) is similar • The two types of muscle contractions are: • Isometric contraction – increasing muscle tension (muscle does not shorten) • Isotonic contraction – decreasing muscle length (muscle shortens during contraction) Motor Unit: The Nerve-Muscle Functional Unit • A motor unit is a motor neuron and all the muscle fibers it supplies • The number of muscle fibers per motor unit can vary from four to several hundred • Muscles that control fine movements (fingers, eyes) have small motor units • Large weight-bearing muscles (thighs, hips)
More Less

Related notes for BIOB32H3

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.