bio ch 9

compare and contrast the 3 types of muscle tissue

Skeletal muscle is striated and under voluntary control, attached to bones or skin

cardiac muscle is striated and involuntary, found only in the heart

smooth muscle is non-striated and also involuntary, found in walls of hollow organs

4 functions of muscle

1. produce movement

2. maintain posture and body position

3. stabilize joints

4. generate heat as they contract

describe gross anatomy of a muscle cells nerve and blood supply

each muscle receives and nerve, artery and veins

skeletal muscle had nerves supplying every fibre to control activity

contracting muscle fibres require huge amounts of oxegyn and nutrients

describe gross anatomy of a muscle cell: connective tissue sheaths

support cells and reinforce muscle

epimysium: dense irregular connective tissue surrounding entire muscle

perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibres)

endomysium: fine areolar connective tissue surrounding each muscle fibre

describe gross anatomy of a muscle cell: attachments

muscles span joints and attach to bones in at least 2 places:

• origin: attachment to immovable or less movable bone

• insertion: attachment to moveable bone

attachments can be direct or indirect

• direct (fleshy): epimysium fused to periosteum of bone or perichondrium of cartilage (intercostal muscles)

• indirect: connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis (most common and most durable)

list the levels of organization of skeletal muscle

muscle: muscles cells, connective tissue wrappings, blood vessels and nerve fibres

fascicle: bundle of muscle cells

muscle fibre: elongated multinucleate cell

microscopic anatomy of skeletal muscle fibre

skeletal muscle fibres are long, cylindircal cells that contain multiple nuclei

sarcolemma: muscle fiber plasma membrane

sarcoplasm: muscle fiber cytoplasm

modified organelles: myofibrils, sarcoplasmic reticulum, T tubules

microscopic anatomy of skeletal muscle fibre: myofibrils

densely packed, rodlike elements accounting for 80% of muscle cell volume

features:

• striations: formed by alternating

  • A Bands (dark region which has H zone: lighter region in middle of dark A band, and M line: line of protien that bisects H zone vertically) and

  • I Bands (lighter regions with Z disc: coin shaped sheet of proteins on midline of light I band)

• sarcomeres: smallest functional unit of muscle fibre, contains A band with half of an I band at each end (consists of area between Z disc), individual sarcomeres line up end to end along myofibril)

• myofilaments: orderly arrangment of actin and myosin within sarcomere.

  • Actin are thin filaments that extend across I band and partway in A band, anchored to Z discs and

  • Myosin and thick filaments extending length or A band, connected at M line

myosin and actin: molecular composition

myosin: heads contain an actin binding site

actin: contains active sites for myosin head attachment, regulatory proteins control availability of active sites (tropomyosin and troponin)

cross bridge: linking of myosin head to actin

sliding filament model of contraction

when relaxed: thin and thick filaments overlap only slightly at ends of A band

during contraction: thin filaments slide past thick ones, causing actin and myosin to overlap more

when nervous system stimulates muscle fibre, myosin heads are allowed to bind to actin which forms cross bridges which cause contraction

cross bridge attachments form and break several times, each time pulling thin filaments closer toward centre of sarcomere in ratcheting action

1. fully relaxed sarcomere of muscle fibre

2. Z discs pulled toward M line, I bands shorten, Z discs become closer, H zones disappear, A bands move closer to each other

3. fully contracted sarcomere of a muscle fibre

microscopic anatomy of skeletal muscle fibre: sarcoplasmic reticulum

network of smooth ER tubules surrounding each myofibril

most run longitudinally, terminal cisterns form perpendicular cross channels at A-I band junction, SR functions in regulation of intracellular calcium levels, stores and releases calcium

microscopic anatomy of skeletal muscle fibre: T Tubules

tube formed by perpendicular protrusion of sarcolemma deep into cell interior

increase muscle fibre SA, lumen continuous with extracellular space, allow electrical nerve transmissions to reach deep into interior of each muscle fibre

tubules penetrate cells interior at each A-I band junction between terminal cisterns

electrical impulse in SR and T tubules

1. electrical impulse pass through sarcolemma

2. T tubule proteins change shape

3. SR proteins change shape

4. triggers release of calcium into muscle cell cytoplasm

neuromuscular junction

decision to move is activated by brain, electrical signal is transmitted down spinal cord to motor neurons which then chemically activate muscle fibres

NMJ is a small space (synapse) between a motor neuron and a muscle fibre (motor end plate )

axons and axon terminal

axon: long, threadlike extension of motor neurons, travel from CNS to skeletal muscle

axon terminal: at end of axon, storage of membrane bound synaptic vesicles contain chemical neurotransmitter acetylcholine

action potential is the electrical signal that causes ACh release into the NMJ

events at the NMJ

1. AP arrives at axon terminal

2. Voltage gated calcium channels open and calcium enters motor neuron

3. calcium entry causes release of ACh neurotransmitter into NMJ synapse

4. ACh diffuses across to ACh receptors (sodium chemical gates) on sarcolemma

5. ACh binding to receptors, opens gates, allowing sodium to enter resulting in end plate potential

6. acetylcholinesterase degrades Ach, stopping the chemical signal from reaching muscle

excitable cells

excitable cells: capable of changing resting membrane potential voltages

polarized: voltage exists across a membrane

neurons and muscle cells are both types of excitable cells capable of action potentials

action potential causes massive changes in electrical charge across plasma membrane, action potential causes cell to get less negative (become polarized) and action potential ceases when cell repolarizes

motor end plate

muscle portion of NMJ which contains chemically gated ion channels (receptors) which open in presence of ACh (chemical)

open receptors allow for net influx of positive charge into muscle causing motor end plate to depolarize

if end plate potential causes enough of a change in the membrane potential then a new action potential will be generated in muscle fibre

the threshold is the minimal level of depolarizing change a membrane must undergo to generate new AP

if thershold is attained, then voltage gated Na channels in sarcolema will open which triggers a massive influx of Na and a huge depolarization in muscle fibre

this large depolarization is the generation of AP which will spread along sarcolemma

repolarization of the muscle fibre restores resting conditions

• Na voltage gated channels close and voltage gated K channels open

• K efflux out of cell rapidly brings cell back to inital resting membrane voltage

• refractory period: muscle fiber cannot be stimulated for a specific amount of time until repolarization is complete

• ionic conditions of resting state are resotred be Na K pumo

Excitation Contraction Coupling

sequence of events that transmit AP along sarcolemma (excitation) leads to (coupling) sliding of myofilaments (contraction)

AP is propagated along sarcolemma and down into T tubules, where voltage sensitive proteins in tubules stimulate calcium release from SR which leads to contraction

AP is brief and ends before contraction is seen

cross bridge cycling

1. cross bridge formation: high energy myosin head attaches to actin thin filament active site

2. working (power) stroke: myosin head pivots and pulls thin filament toward M line

3. cross bridge detachment: ATP attaches to myosin head, causing cross bridge to detach

4. cocking of myosin head: energy from hydrolysis of ATP “cocks” myosin head into high energy state, this energy will be used for power stroke in next cross bridge cycle

as long as calcium is available, myosin binding sites on actin will be exposed and contraction will continue, calcium remains available as long as ACh is present in NMJ

when stimulation ceases, calcium is pumped back into SR and contraction ends, moving calcium from low concentration to high is primary active transport so more ATP is needed

4 steps that must occur for skeletal muscle contraction

1. events at NMJ

2. muscle fibre excitation

3. excitation contraction coupling

4. cross bridge cycling

motor unit

the motor neuron and the muscle fibers it supplies

muscle fibers are spread throughout the whole muscle so stimulation of a single motor unit causes weak contraction

muscle twitch

simplest contraction resulting from a muscle fibers response to a single AP from motor neuron

muscle fibre contracts quickly, then relaxes

1. latent period (EC coupling)

2. contraction (cross bridge formation, tension increases)

3. relaxatin (calcium reuptake into SR, tension zeros)

differences in strength and duration are due to variations in metabolic properties and enxymes between muscles

graded muscle responses

vary strength of contraction for different demands

graded by frequency of stimulation and changing strength of stimulation

temporal (wave) summation

results if 2 stimuli are recieved by a muscle in rapid sucession

when muscle fibes do not have time to completely relax between stimuli

unfused (incomplete) tetanus

further increase in stimulus frequency causing muscle to progress to sustained quivering contraction

fused (complete) tetanus

further increase in stimuli frequency when muscle tension reaches maximum, contractions fuse into one smooth sustained contraction platesu

recruitment (multiple motor unit summation)

stimulus is sent to more muscle fibers, leading to more precise control

motor units in muscle usually contract asynchronously to help prevent fatigue

isotonic contraction

muscle changes in length (myosin head pivots and actin slides toward H zone) moving load

• concentric: muscle shortens and does work

  • biceps contract to pick up a book

• eccentric: muscle lengthens and generates force

  • laying book down

isometric contraction

load is greater than the max tension muscle can generate, muscle does not shorten or lengthen

croos bridges generate force but actin filaments do not move

summary of ATP use in contraction

ATP supplies energy needed for muscle fiber to:

• move and detach cross bridges

• pump calcium back into SR

• pump sodium out of and pump k back into cell after EC coupling

available store of ATP is depleted in 4-6 seconds

ATP is the only source of energy for contractile activities and must be regenerated quickly

ATP regenerated by 3 mechanisms

1. direct phosphorylation of ADP by creatine phosphate (CP)

• creatine phosphate donates a phosphate to ADP to form ATP

• creatine kinase transfers phosphate

2. anaerobic pathway: glycolysis and lactic acid formation

• ATP can be generated by breaking down glucose, broken down into 2 pyruvic acid molecules, 2 ATPs are generated for each glucose broken down

• in absence of oxygen, (anaerobic glycolysis) pyruvic acid is converted to lactic acid

3. aerobic pathway

• produces 95% of ATP during rest and light to moderate exercise

• consists of series of chemical reactions that occur in mitochondria and require oxegyn

• fuels used include glucose from glycogen stored in muscle fibre, then bloodborne glucose and free fatty acids

muscle fatigue

physiological inability to contract respite continued stimulation

causes:

• ionic imbalance disrupting membrane potential of muscle cell

• increased inorganic phosphage from CP and ATP breakdown may interfere with calcium release from SR or hamper power

• decreased ATP and increased magnesium can interfere with voltage sensitive T tubule proteins

• decreased glycogen

muscle fatigue serves to prevent complete depletion of ATP which would result in muscle cell death (rigor mortis)

excess postexercise oxygen consumption

for a muscle to return to its pre exercise state:

• oxygen reserves are replenished

• lactic acid is reconverted to pyruvic acid

• glycogen stores are replaced

• ATP and creatine phosphate reserves are resynthesized

force of muscle contraction

depends on number of cross bridges attatched which is effected by 4 factors:

1. number of muscle fibers stimulated: the more motor units recruited, the greater the force

2. relative size of fibers: the bulkier the muscle, the more tension it can develop

3. frequency of stimulation: the higher the frequency, the greater the force

4. degree of muscle stretch: muscle fibers with sarcomeres that are 80-120% their normal resting length generate more force

• sarcomere less than 80% resting length, filaments overlap too much and force decreases

• sarcomere greater than 120% of resting length, filaments do not overlap enough so force decreases

velocity and duration of contraction

how fast a muscle contracts and how long it can stay contracted is influenced by:

• muscle fiber type

• load

• recruitment

muscle fiber type

• classified according to 2 characteristics:

  • speed of contraction (slow or fast) according to: speed at which myosin ATPases split ATP and pattern of electrical activity of motor neurons

  • metabolic pathways used for ATP syntheis: oxidative fibers (use aerobic pathways) and glycolytic fibers (use anaerobic glycolysis)

• different muscle types are better suited for different jobs

  • slow oxidative fibers: low intensity endurance activities

  • fast oxidative fibers: medium intensity activites

  • fast glycolytic fibers: short term intense or powerful movements

  • glycolytic fibers may convert to oxidative fiber or vice versa with prolonged usage

smooth muscle

found in walls of hollow organs

alternating contractions/relaxations of smooth muscle layers mix and squeeze substances through the lumen of hollow organs

lacks connective tissue sheaths (endomysium only)

contains varicosities (store and release neurotransmitters into wide synaptic cleft which is a diffuse junction, innervated by ANS) or nerve fibers instead of NMJs

smooth muscle P2

less elaborate ER and no T tubules

sarcolemma contains pouch like infoldings that contain numerous calcium channels that open to allow rapid influx of extracellular calcium

electrically connected via gap junctions

no striations or sarcomeres

smooth muscle filaments

thick filaments are fewer and have myosin heads along entire length, lower ratio of thick to thin filaments then in skeletal muscle

no troponin complex

thick and thin filaments arranged diagnolly

intermediate filament dense body netowork:

• contain lattice like arrangment of non contractile intermediate filaments that resist tension

• dense bodies: proteins that anchor filaments to sarcolemma at regular intervals

• during contraction, areas of sarcolemma between dense bodies bulge outward

smooth muscle mechanism of contraction

slow synchronized contractions

cells electrically couples by gap junctions (AP transmitted fibre to fibre)

some cells are self excitatory (depolarize without external stimuli)

similair to skeletal muscle in the following ways:

• actin and myosin interact by sliding filament mechanism

• final trigger is increased intracellular calcium level

• ATP energizes sliding process

• contraction stops when calcium is no longer available

different to skeletal muscle in the following ways:

• some calcium still obtained from SR, mostly from extracellular space

• calcium binds to calmodulin, not troponin

• activated calmodulin then activates myosin kinase

• activated myosin kinase phosphorylates myosin head, activating it

stop smooth muscle contraction:

• calcium detachment from calmodulin

• active transport of calcium into SR and extracellularly

• dephosphorylation of myosin to inactive myosin

response to stretch

retains ability to contract on demand

enables organs such as stomach and bladder to temporarily store contents

unitary and multiunit categories of smooth muscle

unitary: all hollow organs except heart

multiunit: located in large airways in lungs, larger arteries, pili muscles of skin, iris of eye

• similair to muscle cell in some features: independent muscle fibers, motor unit recruitment, graded contractions in response to neural stimuli, very few gap junctions with rare spontaneous depolarization

• like unitary smooth muscle: controlled by autonomic nervous system and hormones