AP 9 Muscular System Part 3 - Muscle Movement

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chapter 9 Muscular system part 3 - muscle movements

Excitation-Contraction Coupling

Mechanism where an action potential causes muscle fiber cotraction; Involves: Sarcolemma, Transverse (T): invaginations of sarcolemma, Terminal cisternae, Sarcoplasmic reticulum:smooth ER, Triad: Ttubule, 2 adjacent terminal cisternae, Ca2+, Troponin

Actions Potentials & Muscle Contractions

4 steps

1st Step in Muscle Contraction

action potential produced at neuromuscular junction is propaged along the sarcolemma of skel. muscle. Depolarization spreads along membrane of T tubules into interior of muscle fiber

2nd Step in Muscle Contraction

depolarization of the T tubule causes gated Ca2+ channesl in the sarcoplasmic reticulum to open, resulting in increase in permeability of sarcoplasmic reticulum to Ca2+, especially in termina cisternae. Calcium ions then diffuse from sarcoplasmic reticulum into sarcoplasm surrounding myofibrils

3rd Step in Mucle Contraction

Calcium ions released from sarcoplasimic reticulum bind to troponin molecules of actin. Troponin molecules bound to G actin molecules are released, causing tropomyosin to move, and to expose active sites on G actin

4th Step in Muscle Contraction

Once active sites on G actin molecules are exposed, heads of myosin myofilaments bind to them to form cross-bridges. Movement of cross-bridges result in contraction

Cross Bridge Movement

1) Exposure of active sites, 2) Cross-bridge formation, 3) Power stroke, 4) cross-bridge releases, 5) Hydrolysis of ATP, 6) Recovery stroke, then back to #2 - how actin moves across myosin and contracts

Exposure to Active Sites

Before cross-bridges cycle. Ca2+ bind to troponins and the tropomyosins move, exposing active sites on actin myfilaments

Cross-Bridge Formation

the myosin heads bind to the exposed active site on the actin myofilaments to form cross-bridges and phosphates are released from mysoin heads

Power Stroke

energy stored in myosin heads is used to move the myosin heads (green arrow), causing the actin myofilaments to slide past the myosin myofilaments (purple arrow), and ADP molecules are released from myosin heads (black arrow)

Cross-Bridge Releases

an ATP molecule binds to each of the mysoin heads, causing them to detach from actin

Hydrolysis of ATP

the myosin ATPase portion of myosin heads split ATP into ADP and phosphate (P), which remain attached to the myosin heads

Recovery Stroke

the heads of myosin molecules return to their resting position (greenarrow) and energy is stored in the heads of the myosin molecules. If Ca2+ are still attached to troponins, cross-bridge formation and movement are repeated (return to step 2). This cycle occurs many times during muscle contraction. Not all cross-bridges form and release simultaneously

Cross-Bridge Movement (Cycling)

heads of myosin move at hinged region (cross-bridge). This forces actin to slide over surface of myosin. After cross-bridge movement, each myosin head releases from actin and returns to original position. Can then form another cross-bridge at different site on actin, followed by movement then release. During single contraction, occurs many times.

How many ATP needed for 1 Cross-Bridge Cycle?

1 ATP needed for each cycle of crossbridge formation, movement and release. Before myosin head binds to active site, it is in resting postion and ADP & phosphate are bound to head of myosin

Muscle Relaxation

Ca2+ moves back into sarcoplasmic reticulum by active transport (Requires energy) Ca2+ decreases in sarcoplasm and ions diffuse from troponin.Ca2+ moves away from troponin-tropomyosin complex; Complex re-establishes its position and blocks binding sites

Muscle Twitch

Muscle contraction in response to a stimulus that causes action potential in one or more fibers; Single, brief contraction and relaxation cycle in (1) muscle fiber; 3 Phases: Lag/Latent, Contraction, Relaxation

Lag Phase

time between application of stimulus to motor neuron and beginning of contraction

Contraction Phase

depolarization; time during which contraction occurs; increase to max in graph

Relaxation Phase

repolarization; time during which relaxation occurs; decrease from max in graph

Muscle Twitch Chart

Motor Units

single motor neuron and all muscle fibers innervated by it; an action potential in motor neuron generates action potential in each muscle fiber of its motor unit; not all motor units are identicle: may vary in terms of # of muscle fibers, their sensitivity to stimuli for contraction

Motor Unit Numbers

Different muscles contain diff # of muscle fibers; Large muscles preforming more powerful but less precise contractions have motor units with many fibers; Small muscles that make delicate movements contain motor units with few muscle fibers; ex -eye less than 10 fibers per unit, thigh several 100 per unit; few fibers give greater control

How is Force of Contraction Increased?

Summation & Recruitment; it takes greater force to lift 25 lb weight than feather

Summation

increasing force of contraction of muscle fibers within muscle; when muscle fiber demonstrates summation, usually because conditions w/in fiber have changed; ex when stimulated in rapid succession, contracts w/greater force each time (treppe)

Recruitment

increasing number of muscle fibers contracting

All-or-None Law for Muscle Fibers

contraction of equal force in response to each action potential; Sub-threshold stimulus, Threshold stimulus, Stronger than Threshold

Sub-Threshold Stimulus

stimulus not strong enough; no action potential; no contraction

Threshold Stimulus

stimulus strength increases until strong enough to produce action potential; contraction

Stronger than Threshold

action potential; contraction equal to that with threshold stimulus - once max motor units are recruited, does not matter how much more stimulus you use, can not get any more than max

Stimulus Strength & Motor Unit Response

strength of contraction is graded: ranges from weak to strong depending on stimulus strength; Multiple motor unit summation (submaximal, maximal, supermaximal)

Multiple Motor Unit Summation

relationship between increased stimulation strength and increased number of contracting motor units; force of contraction increases as more and more motor units are stimualted; A muscle has many motor units; Submaximal stimuli, Maximal Stimuli, Supramaximal Stimuli

Submaximal Stimuli

progressively stronger stimuli, produce actin potentials in axons of additional motor units

Maximal Stimuli

produces action potentials in axons of all motor units of muscle

Supramaximal Stimuli

greater stimulus than maximal; has not additional effect because all motor units are already recruited

Treppe

Graded response; Occurs in muscle rested for prolonged period; When fiber stimulated (maximally at low frequency)in rapid succession, each fiber contraction is stronger than previous until all equal after few stimuli; Possible explanation: more and more Ca2+ remains in sarcoplasm and is not all taken up into the sarcoplasmic reticulum

Stimulus Frequency & Whole Muscle Contraction

As the frequency of action potentials increase, the frequency of contraction increases; Tetanus (Incomplete tetanus, Complete tetanus), Multiple-wave summation

Tetanus

as frequency of action potential increases, frequency of contraction increases until a period of sustained contraction is achieved

Incomplete Tetanus

muscle fibers partially relax between contraction

Complete Tetanus

No relaxation between contractions

Difference Between Trepee and Tetanus

Treppe - stimulus is maximal but delivered at low frequency; Tetanus - stimulus is at threshold but delivered at high frequency

Multiple-wave Summation

muscle tension increases as contraction frequencies increase

Muscle Length vs. Tension

Active tension; Stretched muscle; Crumpled muscle; Passive tension;Total tension

Active Tension

force applied to an object to be lifted when a muscle contracts; as length of muscle increases, its active tension also increases (to a point)

Passive Tension

tension applied to load when a muscle is stretched but not stimulated; similar to tension produced if musce replaced w/elastic band

Total Tension

sum of active plus passive tension

Stretched muscle

Not enough cross-bridging

Crumpled Muscle

myofilaments crumpled, cross-bridges can't contract; muscle damage and needs repair

Muscle Sarcomere Length

at the normal resting length of a muscle, the sarcomeres are also at an optimal length. The muscle produces maximum tension in response to a maximal stimulus at this length; Muscle length 1-3

Muscle Length 1

atrophy;the muscle is not stretched, and the tension produced when the muscle contracts is small because there is too much overlap between actin and myosin myofilaments. The myosin myofilaments run into the Z disks, and the actin myofilaments interfere with each other at the center of the sarcomere, reducing the number of cross-bridges that can form.

Muscle Length 2

healthy muscle; the muscle is optimally stretched, and the tension produced when the muscle contracts s maximal because there is optimal overlap of actin and myosin myofilaments, sot hte number of cross-bridges that can form is maximal

Muscle Length 3

damaged to point of bruising; the muscle is stretched severely, and the tension produced when the muscle contracts is small because there is little overlap between actin and mosin myofilaments, and few cross-bridges can form

Muscle Contractions

Isometric, Isotonic (concentric, eccentric), Muscle tone

Isometric Muscle Contraction

no change in length but tension increases; postural muscles of body

Isotonic Muscle Contraction

change in length but tension constant; ex - movements of upper limbs or fingers, as in waving or using computer keyboards; Concentric/Eccentric

Concentric Isotonic Muscle Contraction

overcomes opposing resistance and muscle shortens; ex - lifting loaded backpack from floor to table

Eccentric Isotonic Muscle Contraction

tension maintained but muscle lengthens; ex - person slowly lowers a heavy wieght

Muscle Tone

constant tension by muscles for long periods of time; responsible for keeping back and lower limbs straight, head upright, abdomen flat

Muscle Responses Chart

Muscle Fatigue

Decreased capacity to work and reduced efficiency of performance that normally follows a period of activity; 3 poss sites for fatigue: nervous system, muscles, NMJ; Types: psycological, muscular, synaptic

Psycological Muscle Fatigue

most common; involves central nervous system; muscles capable of functioning but person "perceives" that additional work is not poss; depends on emotional state of individual; ex - chronic pain and depression, tired athlete increases due to encouragement from crowd

Muscular Muscle Fatigue

2nd most common; occurs in muscle fibers; results from Ca2+ ion imbalance from ATP depletion; ex - fatigue in lower limbs of marathon runners or upper limbs of swimmers

Synaptic Muscle Fatigue

least common; occurs in NMJ due to lack of acetylcholine in synaptic vesicles due to amount released greater than amount synthesized

Physiologcial Contracture

state of fatigue where due to lack of ATP neither contraction nor relaxation can occur

Rigor Mortis

development of rigid muscles several hours after death; Ca2+ leaks into sarcoplasm and attaches to myosin heads and crossbridges form but to little ATP avail so bridges can not release. Rigor ends as tissues start to deteriorate

Energy Sources

ATP provides immediate energy for muscle contractions. Energy required to produce ATP comes from 3 sources: creatine phosphate, anaerobic respiration, aerobic respiration; Oxygen debt

Creatine Phosphate

during resting conditions, energy from aerobic resp is used to synthesise creatine phospate which accumulates in muscle fibers and stores energy to synthesize ATP

Anaerobic Respiration

occurs in absence of oxygen and results in breakdown of glucose to yield ATP and lactic acid; for each glucose metabolized, 2 ATP & 2 lactic acid produced; less effecient than aerobic but much faster; ex - short periods of intense exercise such as sprinting

Aerobic Respiration

requires oxygen and breaks down glucose to produce ATP, carbon dioxide and water; More efficient than anaerobic; produces up to 36 ATP for each glucose; uses fatty acids and amino acids as energy source; ex - long term exercise or long-distance running

Oxygen Debt

insufficient oxygen consumption relative to increased activity at onset of exercise; deficit must be repaid during and after exercise once oxygen consumption catches up with increased activity level; oxygen taken in by the body, above that required for resting meabolism after exercise (recovery oxygen consumption). ATP produced from anaerobic sources contributes

Slow & Fast Fibers

composition of muscle fibers can contain slightly different forms of myosin which cause them act differently; Slow-twitch and Fast-twitch

Slow-Twitch Oxydative (type 1 fibers)

contract more slowly, smaller in diameter, better blood supply, more mitochondria, more fatigue-resistant than fast twitch, large amount of myoglobin; respond relatively slowly to nervous stimulation; aerobic respiration; postural muscles, more in lower than upper limbs. Dark meat of chicken

Fast Twitch (type 2 fibers)

respond rapidly to nervous stimulation, contain myosin that can break down ATP more rapidly than that in Type 1, less blood supply, fewer and smaller mitochondria than slow twitch; anaerobic resp; lower limbs in sprinter, upper limbs of must people. Whit meat in chicken; Comes in oxidative and glycolytic forms

Distribution of Slow/Fast Twitch

most muscles have both but varies for each muscle; large postureal muscles contain more slow twitch, upper lims contain more fast-twitch

Effects of Exercise on Slow/Fast Twitch

change in size of muscle fibers; Hypertrophy, Atrophy

Hypertrophy of Muscle Fibers

increase in muscle size; increases in myofibrils, increase in nuclei due to fusion of satellite cells, increase in strength due to better coordination of muscles, increase in production of metabolic enzymes, better circulation, less restriction by fat

Atrophy of Muscle Fibers

decrease in muscle size; reversable except in severe situations where cells die

Muscle Fiber Type Chart

Heat Production

Exercise:metabolic rate and heat production increase; Post- exercise; metabolic rate stays high due to oxygen debt; Excess heat loss because of vasodialation and sweating; Shivering: uncoordinated contraction of muscle fibers resulting in shaking and heat production

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