Muscle Physiology PDF

MUSCLE PHYSIOLOGY  Properties of Muscular Tissue Like nervous tissue, muscles are excitable or "irritable” they have the ability to respond to a stimulus Unlike nerves, however, muscles are also: Contractible (they can shorten in length) Extensible (they can extend or stretc...

MUSCLE PHYSIOLOGY  Properties of Muscular Tissue Like nervous tissue, muscles are excitable or "irritable” they have the ability to respond to a stimulus Unlike nerves, however, muscles are also: Contractible (they can shorten in length) Extensible (they can extend or stretch) Elastic (they can return to their original shape)  Seventeen muscles are used when you smile, and 43 of them when you frown  Functions of the Muscular System 1. Movement of the body. Most skeletal muscles are attached to bones and are responsible for the majority of body movements, including walking, running, chewing, and manipulating objects with the hands. 2. Maintenance of posture. Skeletal muscles constantly maintain tone, which keeps us sitting or standing erect. 3. Respiration. Skeletal muscles of the thorax carry out the movements necessary for respiration. 4. Production of body heat. When skeletal muscles contract, heat is given off as a by-product. This released heat is critical for maintaining body temperature. 5. Communication. Skeletal muscles are involved in all aspects of communication, including speaking, writing, typing, gesturing and smiling or frowning. 6. Constriction of organs and vessels. The contraction of smooth muscle within the walls of internal organs and vessels causes those structures to constrict. This constriction can help propel and mix food and water in the digestive tract; remove materials from organs, such as the urinary bladder or sweat glands; and regulate blood flow through vessels. 7. Contraction of the heart. The contraction of cardiac muscle causes the heart to beat, propelling blood to all parts of the body  Connective Tissue Coverings Fascia Surrounds an individual skeletal muscle, separating it from other muscles Fascia may extend beyond the ends of the muscle to become a tendon Fascia may connect muscle to muscle and is called an aponeurosis  Muscles and Muscle Fibers Muscles are composed of many fibers that are arranged in bundles called FASCICLES Individual muscles are separated by FASCIA, which also forms tendons and aponeuroses Many large muscle groups are encased in both a superficial and a deep fascia Coverings of Muscle Layers  ENERGY Fibers contain multiple mitochondria for energy Most fibers have multiple nuclei Sarcomere Arrangement  Myofibrils are striated Striations due to arrangement of thick and thin filaments Seen as alternating areas of light and dark bands The length of each myofibril is divided into repeating units called sarcomeres A sarcomere is the functional unit of skeletal muscle  Sarcomere Structure Sarcomere exists from Z-line to Z-line A-Band is dark middle band Overlapping think and thin filaments I-Band – ends of A-Band, thin filaments only Z-line is in the middle if the I-Band Myosin filaments are held to the Z-line by titin proteins  Myofibril Contains protein filaments – ACTIN (thin) and MYOSIN (thick) These filaments overlap to form alternating dark and light bands on the muscle fiber A band = dArk thick (myosin) I band = lIght thIn (actin) In the middle of each I band are Z lines. A sarcomere is one Z line to another  RMP  charge difference across the plasma membrane of cells  Depolarization  results from an increase in the permeability of the plasma membrane to Na+.  Repolarization  occurs when the Na+ channels close and the K+ channels open briefly  The Action Potential  If the action potential (nerve impulse) starts, it is propagated over the entire axon  Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane  The sodium-potassium pump restores the original configuration  This action requires ATP The Sodium Potassium Pump  Starting a Nerve Impulse  Depolarization – a stimulus depolarizes the neuron’s membrane  A depolarized membrane allows sodium (Na+) to flow inside the membrane  The exchange of ions initiates an action potential in the neuron  Nerve Impulse Propagation  The impulse continues to move toward the cell body  Impulses travel faster when fibers have a myelin sheath  Continuation of the Nerve Impulse between Neurons  Impulses are able to cross the synapse to another nerve  Neurotransmitter is released from a nerve’s axon terminal  The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter  An action potential is started in the dendrite Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 7.21  NERVE IMPULSES Na + Na + Na + Na + +_ +_ +_ +_ +_ +_ +_ K+ K+ K+ K+ K+ _ _ _ _ _ _ _ + + + + + + + Resting membrane electrical conditions. The external face is slightly positive while the internal face is slightly negative  NERVE IMPULSES Na + Na + Na + Na + +_ +_ +_ +_ +_ +_ +_ K+ K+ K+ K+ _Na +_ _ _ _ _ _ + + + + + + + Stimulus initiates DEPOLARIZATION. Change in the permeability allows Na ions to diffused rapidly into the cell.  NERVE IMPULSES _ _ +_ +_ +_ +_ +_ + + + + + + _ _ _ _ _ _ _ + + + + + DEPOLARIZATION & generation of ACTION POTENTIAL Reversal of polarity and initiation of action potential.  NERVE IMPULSES _ _ _ _ + + + _ _ _ + + + + + + + + _ _ _ _ _ _ _ + + + PROPAGATION of Action Potential Changes in the polarity continue along the entire length of the membrane.  NERVE IMPULSES +_ +_ +_ +_ + _ _ + + K+ _ _ _ _ + + _ _ + + + + + REPOLARIZATION K+ diffuse out of the cell & permeability changes again Restoration of (-) charge on the outside & (+) charge on the inside. An all-or-none action potential is produced if depolarization reaches threshold Muscles & Nervous System Motor end-plate (Neuromusclar Junction) Sarcolemma of muscle fiber directly beneath motor nerve ending contains an abundance of mitochondria and nuclei  SARCOLEMMA Sarcolemma = muscle fiber membrane Sarcoplasm = inner material surrounding fibers (like cytoplasm) Myofibrils = individual muscle fibers --> made of myofilaments  SLIDING FILAMENT THEORY (MODEL) The theory of how muscle contracts is the sliding filament theory. The contraction of a muscle occurs as the thin filament slide past the thick filaments. The sliding filament theory involves five different molecules plus calcium ions. The five molecules involved in muscle contraction are: myosin actin tropomyosin troponin ATP The Sliding Filament Theory Step 1: ATP hydrolysis Step 2: Attachment Step 3: Power Stroke Step 4: Detachment Relaxation occurs when 1. calcium is taken up by the sarcoplasmic Incomplete tetanus is reticulum partial relaxation 2. ATP binds to myosin between contractions 3. tropomyosin moves back so that active complete tetanus is no sites on actin are no longer relaxation between exposed to myosin contractions The Sliding-Filament Mechanism  Energy Source Provided by ATP from cellular respiration (mitochondria) Creatine phosphate increases regeneration of ATP Much of the energy forms heat, which keeps our bodies warm  Events during contraction in the Sarcomere A band stays the same I band gets smaller H zone gets smaller Sarcomere shortens A stimulus of increasing frequency increases the force of contraction (multiple-wave summation).  Energy for Contraction ATP initially supplied from cellular respiration If ATP is abundant, is converted to creatine phosphate and stored in skeletal muscles When ATP is low, creatine phosphate supplies phosphate to ADP making ATP CP & ATP stores only good for about a 10 second maximal contraction ATP must then come from cellular respiration or glycolysis  Methods of regenerating ATP during muscle activity Glucose (from O2 Glucose (from glycogen breakdown or glycogen breakdown or delivered from blood) delivered from blood) CP ADP O2 Glycolysis Pyruvic acid in cytosol Fatty O2 acids O2 Aerobic respiration ATP 2 ATP Amino Creatine in mitochondria Pyruvic acid acids net gain 38 ATP Released CO2 Lactic acid H2O to blood net gain per glucose (a) Direct phosphorylation (b) Anaerobic mechanism (glycolysis (c) Aerobic mechanism (aerobic cellular [coupled reaction of creatine and lactic acid formation) respiration) phosphate (CP) and ADP] Energy source: glucose; pyruvic acid; free Energy source: CP Energy source: glucose fatty acids from adipose tissue; amino acids from protein catabolism Oxygen use: None Oxygen use: None Oxygen use: Required Products: 1 ATP per CP, creatine Products: 2 ATP per glucose, lactic acid Products: 38 ATP per glucose, CO2, H2O Duration of energy provision: 15s Duration of energy provision: 30–60 s. Duration of energy provision: Hours  Anaerobic Respiration vs Aerobic Respiration Anaerobic Respiration Aerobic Respiration The ATP synthesized provides The ATP synthesized produces energy for a short time during energy for muscle contractions intense exercise. under resting conditions or during Produces ATP less efficiently but exercises such as long distance more rapidly running. Lactic acid levels increase because Although ATP is produced more of anaerobic respiration slowly After anaerobic respiration, aerobic respiration is higher than normal, as the imbalances of homeostasis that occurred during exercise become rectified  Muscle Terms Muscle twitch is a single, brief contraction and relaxation cycle in a muscle fiber - does not last long enough or generate enough tension to perform any work lag phase (latent phase) – is the application of the stimulus to the motor neuron and the beginning of contraction contraction phase - the time during which contraction occurs relaxation phase - the time during which relaxation occurs  Action Potential vs Contraction AP Contraction electrochemical event mechanical event measured in millivolts measured as a force also called completed in less than 2 tension milliseconds. reported as the number of grams lifted or the distance the muscle shortens, and requires up to 1 second to occur  Muscle contracts with less than maximum force if its initial length is shorter or longer than optimum  Motor Units One motor neuron and all the muscle fibers it controls Precise movements use INPUT small motor units. Gross Sensory Reception movements use large motor (Receptors) units Treppe is an increase in the INTEGRATION force of contraction during OUTPUT the first few contractions of Motor Response a rested muscle (Effector)  The force of contraction of a whole muscle increases with increased frequency of stimulation because of an increasing concentration of Ca2+ around the myofibrils and because of complete stretching of muscle elastic elements Isometric contractions cause a change in muscle tension but no change in muscle length Isotonic contractions cause a change in muscle length but no change in muscle tension Concentric contractions cause muscles to shorten and tension to increase. Eccentric contractions cause muscle to lengthen and tension to decrease gradually Muscle tone is the maintenance of steady tension for long periods Asynchronous contractions of motor units produce smooth, steady muscle contractions  Muscle Fatigue Fatigue, the decreased ability to do work, can be caused by the central nervous system, depletion of ATP in muscles or depletion of acetylcholine in the neuromuscular junction Physiological contracture (the inability of muscles to contract or relax) and rigor mortis (stiff muscles after death) result from inadequate amounts of ATP  Fast-Twitch Muscle Fibers vs Slow-Twitch Muscle Fibers Fast-Twitch Muscle Fibers Slow-Twitch Muscle Fibers split ATP rapidly split ATP slowly have a well-developed blood supply, many mitochondria and myoglobin. Type IIa fibers have a well-developed blood supply, more mitochondria and more myoglobin Type IIb fibers have large amounts of glycogen, a poor blood supply, fewer mitochondria and little myoglobin People who are good sprinters have a greater percentage of fast-twitch muscle fibers in their leg muscles, and people who are good long-distance runners have a higher percentage of slow-twitch muscle fibers  Muscles increase (hypertrophy) or decrease (atrophy) in size because of a change in the size of muscle fibers Anaerobic exercise develops type IIb fibers Aerobic exercise develops type I fibers and changes type IIb fibers into type IIa fast-twitch fibers  Effects of Aging on Skeletal Muscle Aging skeletal muscle is associated with reduced muscle mass increased time that muscle takes to contract in response to nervous stimuli, less precise muscle control longer recovery period FINISH