Lecture 10: Muscular Tissue
how a nerve impulse elicits a muscle AP
- Ca flows into synaptic end bulbs of somatic motor neuron which stimulates the vesicles to release ACh into synaptic cleft - the ACH receptors on the motor end plate are activated, which opens ion channels for Na - the inflow of Na makes inside of muscle fiber more positive, which triggers an action potential that propagates along the sarcolemma into system of t-tubules, causing SR to release Ca into sarcoplasm and muscle fiber contracts - termination of ACh activity
action potential
- actual action, takes place - all/nothing - a stronger stimuli will not cause a larger impulse, all the same - 2 phases: depolarization and repolarization
functions of muscular tissue
- create motions, muscles work with nerves, bones, and joints to produce body movements - stabilize body positions and maintain posture - store substances within the body using sphincters - move substances by peristaltic contractions - generate heat through thermogenesis
properties of muscular tissue
- electrical excitability (muscle action potentials) - contractible (shorten in length) - extensible (can extend or stretch) - elastic (can return to original shape)
imbalances of homeostasis
- exercise-induced muscle damage - intense exercises lead to torn sarcolemmas and disrupted z-discs, blood levels of proteins increase - spasm - a sudden involuntary contraction of a single muscle within a large group of muscles, painless - cramps - involuntary and painful muscle contractions caused by inadequate blood flow to muscles from dehydration, overuse and injury, abnormal blood electrolyte levels - diseases - fibrosis (myofibrosis) and myosclerosis - aging - due to decreased levels of physical activity, slow progressive loss of skeletal muscle mass that is replaced by fibrous connective and adipose tissue, slow oxidative fibers increase
diseases
- fibrosis (myofibrosis) - replacement of muscle fibers by excessive amounts of connective tissue (fibrous scar tissue) - myosclerosis - hardening of the muscle caused by calcification - both occur as result of trauma and various metabolic disorders
troponin and tropomyosin
- found on the thin filament - regulate actin-myosin interactions - move in response to Ca2+ - when Ca 2+ binds to troponin it changes the shape of the troponin-trypomyosin complex and uncovers binding sites on actin and muscle contraction begins as myosin binds to actin
types of muscle contraction
- isotonic contractions - result in movements (concentric and eccentric) - isometric contractions - result in no movement, contraction against a fixed resistance with no change in muscle length - isokinetic contraction - resistance = force applied, change in muscle length (full ROM)
tension in a muscle
- latent period - brief delay as calcium sweeps over the sarcolemma and calcium ions are released from the sarcoplasmic reticulum - next phase - fiber is actively contracting - relaxation - calcium ions are requestered into the SR and myosin binding sites are covered by tropomyosin - refractory period - temporary loss of excitability, all muscles fibers in a motor unit will not respond to a stimulus during this short time - recruitment - allows a muscle to accomplish increasing gradations of contractile strength
smooth muscle
- location: GI tract, uterus, eye, blood vessels - function: peristalsis, blood pressure, pupil size, erects hairs - appearance: no striations, one central nucleus - control: involuntary
cardiac muscle
- location: heart - function: pump blood continuously - appearance: striated, one central nucleus - control: involuntary
skeletal muscle
- location: skeleton - function: movement, heat, posture - controls voluntary movement (controlled by somatic nervous system) - appearance: striated, multi-nucleated (eccentric), fibers parallel - contains lower motor neurons
what muscle fiber make up most skeletal muscles
- most skeletal muscles are a mixture of all 3 types of skeletal muscle fibers - about half the fibers in a typical skeletal muscle are slow oxidative fibers - within a particular motor unit, all skeletal muscles are same type - different motor units in a muscle are recruited based on task performed
skeletal muscle fiber types by appearance
- red muscle fibers - endurance fibers, higher muscle-oxygen (myoglobin) content, more mitochondria & blood supply - white muscle fiebrs - less myoglobin, mitochondria, blood supply, low twitch muscle fiber
isokinetic contraction
- resistance = force applied - change in muscle length (full ROM) - negative = no matter how fast you try to go - speed is constant - positive = strength building, full ROM
anatomy of skeletal muscle fiber
- sarcolemma - plasma membrane - transverse (t) tubules - invaginations of sarcolemma tunnel towards center of each muscle fiber, open to outside, filled with interstitial fluid - sarcoplasm - cytoplasm of muscle fiber, contains myoglobin and glycogen - myofibrils - small structures, contractile organelles of skeletal muscle, entire length of muscle fiber, makes cell appear striated (striped) - sarcoplasmic reticulum (SR) - fluid-filled system of membraneous sacs encircling each myofibril - terminal cisterns - dilated end sacs of SR butt against T tubules from both sides - triad - formed by t-tubules and 2 terminal cisterns - filaments - within myofibrils, small protein structures, thin and thick, contractile process, don't extend the entire length of cell, arranged in sarcomeres
graded potentials
- short distance communication - can be inhibitory or excitatory - must first be produced to depolarize the cell to threshold (fleeting thought)
levels of organization of skeletal muscle
- skeletal muscle - organ made up of fasciles containing muscle fibers, blood vessels, nerves, wrapped in epimysium - fascicle - bundle of muscle fibers wrapped in permystium, within skeletal muscle - muscle fiber (cell) - cyndrical, long cells, covered by endomysium and sarcolemma and contains sarcoplasm, myofibriks - myofibril - threadlike contracticle elements within sarcoplasm of muscle fiber that extend entire length of fiber; composed of filaments - filaments (myofilaments) - contractile proteins within myofibril that are 2 types: thick or thin, sliding of thin past thick produces muscle shortage
skeletal muscle fibers by function
- slow oxidative (SO) fibers - small, appear dark red, endurance fibers, least powerful type, fatigue resistant, ex: running a marathon - fast oxidative-glycolytic fibers (FOG) - intermediate in size, appear dark red, and are moderately resistant to fatigue, ex: walking - fast glycolytic fibers (FG) - large, white, powerful, for intense anaerobic activity of short duration
sources of muscle energy
- stored ATP - 3 seconds, once depleted goes to creatine phosphate, immediate/explosive energy system, most intense exercise - creatine phosphate - 12 seconds, intense exercise - aerobic ATP production - no time limit, indefinite, as long as we're fueled and rested enough can continue - anaerobic glucose use - 30-40 seconds, aka glycolysis/lactic acid system
exercise to rest ratio for muscle energy sources
- stored ATP and creatine phosphate - 1:3 - anaerobic - 1:2 - aerobic - 1:1 or 1:1.5
thin and thick filaments
- thin - actin, has attachment sites, golf balls - thick - myosin, golf club - during contraction, these interact to produce tension and shorten the muscle - compose myofibrils
repolarization
- voltage = -80 mv - starts when sodium gates close and potassium channels open - potassium rushes outward and cell hyperpolarizes (more negative) until RMP is reached again - after hyperpolarization - out flow of potassium is large enough to -90 mv
all-or-none principle of muscle contraction
- when an individual fiber is stimulated to depolarization and an AP is propagated along its sarcolemma, it must contract to its full force - when a single motor unit is recruited to contract, the muscle fibers in that motor unit must all contract at same time
what is threshold that must be reached for an AP to start
-55 mv
summary of events of contraction and relaxation in a skeletal muscle fiber
1. nerve impulse arrives at axon terminal of motor neuron, triggers release of ACh 2. ACh diffuses across synaptic cleft, binds to receptors in motor end plate, triggers a muscle AP 3. ACh is destroy so another AP does not arise unless more ACh is released from motor neuron 4. muscle AP traveling along transverse tubule opens channels sarcoplasmic reticulum (SR) membrane which allows Ca ions to flood in sarcoplasm 5. Calcium binds to troponin on thin filament, exposing binding sites for myosin 6. Contraction: power strokes use ATP, myosin heads bind to actin, swivel, and release, thin filaments pulled towards center of sarcomere 7. Calcium release channels in SR close and calcium active transport pumps use ATP to restore low level of calcium in sarcoplasm 8. Troponin-tropomyosin slides back into position where it blocks the myosin binding sites on actin 9. Muscle relaxes
aponeurosis
a thick fascia connecting 2 muscle bellies - ex: the epicranial aponeurosis connects the muscle bellies of occipitalis and frontalis to form one muscle - occipitofrontalis
epimysium, perimysium, and endomysium
all are continous with the connective tissues that form tendons and ligaments and muscle fascia
oxygen debt or excess post exercise oxygen consumption (EPOC)
amount of O2 repayment required after exercise in skeletal muscle to: - replenish ATP stores - replenish creatine phosphate and myoglobin stores - convert lactic acid back into pyruvate so it can be used in krebs cycle to replenish ATP
3 groups of muscle proteins
build myofibrils - contractile proteins - generate force during contraction, myosin and actin - regulatory proteins - help switch contraction process on and off - structural proteins - keep the thick and thin filaments in proper alignment and link the myofibrils to the sarcolemma and extracellular matrix
muscle fascia
connect muscles to other muscles to form groups of muscles
motor unit
consists of a motor neuron plus all the muscle cells it intervates - the axon of a somatic motor neuron branche sout and forms NMJ with many different muscle fibers - high precision (fewer muscle fibers per neuron and laryngeal and extraocular muscles) - low precision (many muscle fibers per neuron, thigh muscles 2k-3k) - all or none principle of muscle contraction
sarcomeres
contractile units within myofibrils, the smallest contractile unit of a muscle
epimysium
dense irreg CT encircling entire muscle
perimysium
dense irregular CT surrounding muscle fibers into bundles called fascicles
muscle action potential
electrical signal that excites the muscle fiber to contract - resting potential (polarized): Na and K gates are closed, voltage = -70 mv - depolarization - K gate closed, Na gates open, become less negative, voltage = +40 mv - repolarization - Na gate closed, K gates open and K rushes outward, making cell more negative until RMP is reached again, voltage = -90 mv - hyperpolarization - becoming more negative
quad muscles
extend the knee, flex the hip - rectus femoris (unique) - originates anterior, only quad muscle crossing hip joint - vastus medialis - vastus intermedius - vastus lateralis
hamstring
flex the knee, extend the hip - biceps femoris (unique) - inserts to the fibula - semi-tendonosis - most lateral, attatched to tibia - semi-membranosous - most medial, attached to tibia
generating an action potential
involves transfer of information from an electrical signal (down a neuron) to a chemical signal (at the NMJ) back to an electrical signal (depolarization of the sarcolemma)
skeletal muscle metabolism
muscle use of O2 and nutrients is balanced by production of manageable levels of waste products like CO2, heat (70-80% of energy lost by muscles is heat) and lactic acid (anaerobic)
what are the structural proteins
muscles contain about a dozen structural proteins contributing to alignment, stability, elasticity, and extensibility of myofibrils - titan - 3rd most plentiful protein, extends from Z disc and accounts for much of elasticity of myofibrils - dystrophin - relates to muscular dystrophy
relaxed muscle
myosin is blocked from binding to actin because strands of tropomyosin cover myosin binding sites on actin
isometric contraction
no movement, contraction against a fixed resistance with no change in muscle length - muscle force = resistance - supporting objects in a fixed position and posture - negative effects: increase blood pressure and specific to joint angle - positive effects: don't need equipment, can be done anywhere
endomysium
penetrates the interior of each fascicle and separates individual muscle fibers from one another
contracted muscle
presence of calcium ions bind to troponin, tropomyosin moves away from myosin binding sites on actin and muscle contraction begins as myosin binds to actin
sliding filament mechanism
process of muscle contraction in which shortening occurs by thick and thin filaments sliding past each other - step 1: atp hydrolyze - creates energy (atp and water creates adp and p) which energizes myosin head - step 2: attachment - myosin head binds to actin and forms cross bridges - step 3: power stroke - myosin crossbridges rotate toward center (swing), sliding thin filament passes thick - step 4: detachment - make atp again, crossbridges detach from actin
what is the major hip flexor in trunk
psoas muscle (attaches to lumbar disc)
why is it bad to do a sit-up with straight legs
pull on psoas muscles, pulling lumbar discs
isotonic contractions
results in movement - concentric isotonic - muscles shorten while generating force (ex: lifting weight towards biceps) - eccentric isotonic - muscle tension is less than the resistance (muscle lengthens)
where does muscle contraction take place
sarcomere
3 types of muscle tissue
skeletal, cardiac, smooth
neuromuscular junction (NMJ)
synapse between a somatic motor neuron and a skeletal muscle fiber, where muscle action potentials arise - electrical signal (AP) cannot travel from one cell to another, must be a chemical event with neurotransmitters - the chemical events at the NMJ transmit the electrical events of a neuronal action potential into the electrical events of a muscle action potential - presynaptic membrane on sensory on neuron - post synaptic membrane is the motor end plate on the muscle cell - synaptic cleft separates pre and post membrane - conscious thought to move a muscle results in activation of motor neuron and release of ACh at NMJ, binding to receptors of motor neuron and produces muscle action potential
motor end plate
the flattened end of a motor neuron that has gated sodium channels that respond to ACh
excitation-contraction coupling
the steps that connect excitation (a muscle AP propagating along the sarcolemma into the T tubule) to contraction (sliding of the filaments - the thought process going on in the brain - action potential arriving at neuromuscular junction - regeneration of an action potential on the muscle membrane, propagates along sarcolemma and t-tubules - SR opens Ca release channels and release Ca2+ from the sarcoplasmic reticulum - Ca combines with troponin, moves tropomyosin so that myosin binding sites on actin are uncovered - sliding of thick on thin filaments in sarcomeres - generation of muscle tension (work)
what are the regulatory proteins
troponin and tropomyosin
upper and lower motor neurons
upper: gives us breakpoint to reassess situation, begins at cerebral cortex lower: neuron that directly excites a muscle, in direct contact with sarcolemma, begins at spinal cord
cross bridges
when myosin heads attach to actin during contraction
refractory periods
- absolute refractory period - even a very strong stimulus could not initiate a 2nd AP, during depolarization and beginning of repolarization (as Na gates inactivate) -relative refractory period - neuron will respond to new, but only very strong stimulation, later stage of repolarization and after hyperpolarization
what are the contractile proteins
- actin - compose thin filaments, has binding sites for myosin - myosin - compose thick filaments - most plentiful proteins
