Chapter 9 Muscular System.docx

Muscular System Types of Muscle Tissue *** See Histology (chap. 4) lecture outlines – Review*** Characteristics of Muscle Tissue Excitability (Responsiveness) - the ability to receive and respond to a stimulus. Contractility – the ability to forcibly shorten when adequately stimulated. Extensibil...

Muscular System Types of Muscle Tissue *** See Histology (chap. 4) lecture outlines – Review*** Characteristics of Muscle Tissue Excitability (Responsiveness) - the ability to receive and respond to a stimulus. Contractility – the ability to forcibly shorten when adequately stimulated. Extensibility – the ability to extend or stretch Elasticity – the ability of a muscle cell to recoil and resume its resting length. Muscular functions Produce movement - types of movement depends on muscle tissue type. Maintenance of posture Stabilizes joints Heat Production Structure of a skeletal muscle at different levels of organization (organ level / cell level / chemical level) Skeletal muscle are well vascularized organs. Why? Each muscle is supplied by a nerve ending. Attachments Skeletal muscles span joints and attach directly or indirectly to bones in at least two places. Both points of attachments are pulled on when the muscle shortens during a contraction. Types of Attachments Origin - The stationary / fixed attachment point while contracting. Insertion – The moveable attachment point while contracting. Skeletal muscles may have more than one origin and / or insertion but must have at least one of each. *** Please Review from Histology lecture how tendons physically attach to bone*** Connective tissue (CT) coverings: Endomysium – fine areolar CT that covers each individual muscle fiber. Perimysium – dense irregular CT that surrounds bundles of muscle fibers called fascicles. Epimysium – dense irregular CT that surrounds an entire skeletal muscle. Fascia – Fibrous CT which blends with the epimysium and/or hold groups of skeletal muscles together. Parts of skeletal muscle fibers: ** a muscle fiber = a muscle cell ** Sarcolemma – the cell membrane of a muscle cell. Sarcoplasm – the cytoplasm of a muscle cell. Contains numerous nuclei, mitochondria, glycosomes (glycogen storage granules), and myoglobin (oxygen storing pigmented molecule). Myofibrils – each muscle cell contains numerous cylindrical structures called myofibrils. Accounts for approx. 80% of the cell volume. Made up of two kinds of proteins filaments called myofilaments. *Discussed in detail below. Sarcoplasmic Reticulum – specialized form of smooth endoplasmic reticulum (SER). Stores calcium via active transported from the sarcoplasm. Calcium used to initiate a muscle cell contraction. Expanded perpendicular running tube regions called terminal cisternae connected by many longitudinally running tubules. Transverse Tubules – invagination tubes of sarcolemma which extend all the way through the muscle fiber. These tubules are continuous with the outside (interstitial fluid space). NOTE: Each transverse tubule is situated between two cisternae of sarcoplasmic reticulum. The arrangement of a T-tubule between two cisternae of SER is called a triad. Myofilament Structure: Two types; Thick Filaments – made up of the protein myosin. Thin Filaments – made up of the following proteins; Actin Tropomyosin Troponin ***Lecture presentation will give details of myofilament structure and relationships*** TAKE NOTES! KNOW! Neuromuscular Junction – the point where the end of the axon from a motor neuron communicates with the sarcolemma of a muscle cell. Motor End Plate – a highly folded area of the sarcolemma at the site of the neuromuscular junction. Contains millions of chemical gated ion integral protein channels with Ach receptors. Synaptic Cleft – the space between the axon terminal bulb and the motor end plate. Contains Acetylcholinesterase (enzyme that breaks down Ach. Synaptic Vesicles – membrane sacs located in the terminal bulbs. Contain the neurotransmitter (chemical messenger) Acetylcholine (Ach). Events of a skeletal muscle contraction: Nerve impulses traveling down a motor neuron reaches the terminal bulb. Membrane permeability for calcium increase as calcium voltage gated ion channels open. Calcium diffuses into the terminal bulb from the synaptic cleft. Calcium binds to synaptic vesicles containing the neurotransmitter, Acetylcholine (Ach). Those synaptic vesicles with Ach attached migrate and fuse with terminal bulb membrane releasing Ach into the synaptic cleft (exocytosis). Ach diffuses across the synaptic cleft and binds Ach receptors located on chemical gated ion channels in the motor end plate. If sufficient amounts of Ach binds (threshold amount), a muscle impulse is generated. The muscle impulse (electrical current) travels in all directions away from the motor end plate. Impulses traveling down the T-tubules stimulates the release of calcium from the terminal cisternae of the sarcoplasm reticulum by opening Ca++ channels. Calcium diffuses into the sarcoplasm where it binds to the troponin complex that is located on the thin myofilament, causing it to change shape. This change in shape pulls tropomyosin away from the myosin head binding sites located on the actin protein of the thin filament. Cross bridges form between the thick and thin filaments. (ADP & P attached to myosin head) Power Stroke – myosin heads bend pulling the thin filament across it. ADP & P released form myosin head. ATP now attaches to the exposed ATP binding site on myosin head. Change in shape of myosin causes the cross bridge detachment. ATP hydrolysis causes myosin heads to cock (high energy position). Cycle repeats many times from cross bridge attachment until sarcomere has reached maximum shortening. Muscle Relaxation 1. Nerve impulse ceases 2. Ach is broken down at the synapse by Acetylcholinesterase or reabsorbed back into synaptic bulb. This prevents a single impulse from causing a continued contraction. 3. A Ca++ pump in the sarcoplasm reticulum (SR) quickly pumps Ca back into the SR. 4. The troponin-tropomyosin complex moves back in between the actin and myosin myofilaments. 5. The cross bridges are broken. 6. Muscle relaxation occurs. Motor Unit – is composed of a single motor neuron and all the muscle fibers it forms junctions with (controls). NOTE: some areas of the body where a great degree of coordination is required will have only a few muscle fibers per motor neuron. Conversely, a muscle which is involved in powerful movements will have numerous muscle fibers per motor neuron. Muscular Responses Threshold stimulus – the minimal strength of an electrical stimulus which will result in a muscular contraction. All-or-None response – when a muscle cell (fiber) is exposed to a threshold or greater stimulus it contracts completely. There are no partial contractions of muscle cells. Twitch Contraction – a brief muscle contraction which is followed immediately by relaxation. Demonstrated in the lab by removing a muscle from an animal and attaching it to an apparatus that produces a myogram, a recording contractile activity. Stages of a muscle twitch: Latent period – time period from stimulation to beginning of cross bridge cycling but no measurable muscle tension seen. Period of contraction – cross bridges cycling and tension produced. If tension overcomes resistance of the muscle load, muscle shortens. Period of relaxation - calcium transported back into SR, active cross bridges decline, and tension returns back to baseline. Muscle resumes resting length. Muscular Response to change in stimulus strength Treppe (staircase effect) – an increase in the strength of a contraction of a muscle when a series of identical stimuli are applied. Relaxation period complete before next stimulus applied. It is the result of more calcium being available in the cytoplasm from the previous twitch. Muscle is “warmed up”. By recruiting more Motor units we can smoothly increase force of muscle contraction as more muscle fibers contract. Muscular Response to decrease in stimulus frequency Wave summation - The adding together of force of contractions due to successive stimuli applied to a muscle forming a stronger contraction. If a muscle is stimulated at an increasingly faster rate: the relaxation period between twitches becomes shorter and shorter the concentration of Ca++ in the cytosol rises higher and higher the degree of wave summation becomes greater and greater Tetanus – the fusion of contractions to produce a continuous contraction. Unfused (incomplete) tetanus - a sustained but quivering contraction due to wave summation. Fused (complete) tetanus – a sustained muscle contraction with no relaxation. Muscle Tone Although skeletal muscles are voluntary, they are almost always slightly contracted. This is known as muscle tone. It is due involuntary signals from the nervous system in response to activated stretch receptors in muscles. Types of Contractions Isotonic contraction threshold stimulus applied to muscle causing muscle tension. peak tension levels great enough to overcomes load resistance. muscle shortens and load moves Isometric contraction threshold stimulus applied to muscle causing muscle tension peak tension levels to not high enough levels to overcome load resistance. Muscle does not shorten so load remains stationary. Providing energy for muscle contraction: Most energy for muscle contractile activity’s come from Adenosine triphosphate (ATP). Myosin filaments contain (ATPase), which is an enzyme which causes the reaction: ATP ADP + P + energy released (used for muscle contraction) Muscles ATP reserves are limited to about 4-6 seconds of activity. As a result we must regenerate it quickly in order to continue muscle contractions. Mechanisms of ATP regeneration Direct phosphorylation of ADP by Creatine phosphate (CP) creatine kinase Creatine phosphate + ADP Creatine + ATP Duration of energy use: 15 seconds Anerobic Respiration - break down of glucose from bloodstream by glycolysis. Glucose Pyruvic Acid Lactic acid 2 ATP regenerated Duration of energy use: 30 - 40 seconds Aerobic Respiration - metabolic pathways requiring oxygen. Glucose + Oxygen Carbon Dioxide + Water + Heat + 32 ATP regenerated Duration of energy use: Hours Glycogen – storage form for glucose in liver and muscle cells. Hemoglobin – the molecule which transports O2 in the blood. Myoglobin – molecule in muscle cells which stores O2. Oxygen Debt: after strenuous exercise accumulated lactic acid is converted to glucose in the liver, muscle cells need to resupply themselves with ATP and CP. The O2 debt is the amount of O2 which is needed after a strenuous activity to perform these functions. Muscle Fatigue: the state of physiological inability to contract even though a stimulus is provided. Can result from of an inadequate supply of O2, a depletion of glycogen, an accumulation of lactic acid or inadequate calcium release from SR. Excess Postexercise Oxygen Consumption (EPOC) (formerly called oxygen debt) * The extra amount of energy the body must take in to recover after vigorous exercise has occurred. The EPOC represents the amount of oxygen need for total aerobic activity and the amount used during that strenuous activity. Steps needed for complete recovery back to pre-activity state Rebuild myoglobin reserves Reconvert lactic acid to pyruvic acid Rebuild glycogen reserves Rebuild stored ATP and CP reserves Factors effecting Force of Muscle Contraction (READ ABOUT THEM) Number of muscle fibers recruited – Size of muscle fibers – Frequency of stimulation – Degree of stretch -