Unit 10: Key Terms A and P

Types of muscle tissue

skeletal (voluntary), cardiac (involuntary), smooth (involuntary)

Structure of the skeletal muscle fiber

skeletal muscle tissue consists of many fibers and surrounding endomysium (Extracellular matrix)

Parts of muscle cell

muscle cell (myocyte), sarcolemma, sarcoplasm, myofibril, sarcoplasmic reticulum, mitochondrion, nucleus

Myofibrils

unique structures found in all muscle cells, made up of bundles of specialized proteins: allow for contraction

Cylindrical organelles

100s to 100s in each myocyte, 50-80% of cell volume

Sarcoplasmic reticulum (SR)

surrounds myofibrils: stores and releases calcium ions

Myofilaments

myofibril is made up of these, consist of one or more types of proteins

3 types of myofilaments

thick filaments, thin filaments, elastic filaments

Thick filaments

bundles of contractile protein myosin

Thin filaments

proteins actin, tropomyosin and troponin

Elastic filaments

single massive, spring-like structural protein (titin), stabilizes myofibril structure, resists excessive stretching

Thick filaments cont.

globular heads at each end linked by intertwining tails heads are connected to tails by hinge-like neck, each head has active site that bins with actin

Thin filaments cont.

actin, tropomyosin, and troponin, multiple actin subunits string together, form two intertwining strands in functional thin filament, each head-shaped actin has active site, binds with myosin heads

The sarcomere

microscopically, striations are alternating light and dark bands with specific regions

The sarcomere: I band

"i" in light mnemonic, only thin filaments

The sarcomere: Z disc

in the middle of I band, composed of structural proteins that anchor thin filaments in place to one another, serves as attachment points for elastic filaments

The sarcomere: A band

"a" in dark mnemonic, contains zone of overlap, both thick and thin filaments, generate tension during contraction

The sarcomere: H zone

HA mnemonic, H is in the A band, middle of A band where only thick filaments exist

The sarcomere: M line

M is in middle mnemonic, dark line in middle of A band, structural proteins hold thick filaments in place, serve as anchoring point for elastic filaments

The sliding-filament mechanism of contraction

sarcomere extends from on z-disc to the next, functional unit of contraction, explains tension generation during muscle contraction, both I band and H zone narrow, A band unchanged, Myosin heads attach to actin pulling thin filaments toward M line, bring Z-discs closer together, shorten whole muscle fiber

Innervated

includes all skeletal muscles, connected to a neuron

A single motor neuron

communicates with many muscle fibers

Synapse

each connection is called this

Neuromuscular Junction (NMJ)

synapse where a single motor neuron communicates with many muscle fibers, transmits signal (nerve impulse/action potential) from neuron to sarcolemma of muscle fiber

NMJ: Axon Terminal

contains synaptic vesicles filled with the neurotransmitter acetylcholine (ACh), neurotransmitters are chemicals that trigger changes in a target tissue, allow for cell to cell communication

Synaptic cleft

space b/w axon terminal and muscle fiber, filled with collagen fibers and gel that anchors neuron in place

Skeletal Muscle Contraction 1

In preparation for muscle contraction, 1. calcium ions released from terminal cisternae bind to troponin 2. tropomyosin moves and active sites of actin are exposed

Skeletal muscle contraction

a) at rest, tropomyosin blocks actin's active sites b)after stimulation calcium release cause the active sites of actin to be exposed

Contraction Phase

begins when actin's active site is exposed to initiate cross-bridge cycle

The first Step in the bridge cycle

Myosin head cocked once ATP is bound and energy is gathered by hydrolysis, ATP -> ADP+P

The second step in cross-bridge cycle

In high energy position (with ADP+P still attached), head is able to bind to active site of actin, cross-bridge is at 90 degree angle relative to thick filament

The third step in the cross-bridge cycle

Power stroke-released from head, myosin pulls actin toward M line as it pivots to relaxed (low energy) position, cross-bridge is at 45 degree angle relative to thick filament

The fourth step in the cross-bridge cycle

Myosin can bind to another ATP, breaks link with actin active site, detachment does not allow thin filaments to slide backward (some myosin heads still attached to action)

Contraction Cycle repetition

may repeat as long as stimulus to contract continues and ATP is available, myosin head recocked, binds to first actin molecule, and power stroke repeats, myosin binds to second action, and so on, over and over, for leverage contraction, process will repeat about 20-40 times for each myosin head in each sarcomere

Muscle relaxation

2 components: motor neuron action potentials stop signaling for release of acetylcholine from axon terminals, calcium ions are actively pumped back into SR terminal cisternae, in absence of calcium, tropin and tropomyosin black sites of actin, therefore muscle relaxes, myofilaments slide back into original positions

Rigor Mortis

progressive stiffening (contraction) of skeletal muscles, begins 3-4 hours after death, pumps that drive calcium back into SR no longer have ATP to fuel their activity, muscle fibers are unable to relax without ATP, myosin heads cannot detach from actin, muscles remain contracted until myofilaments proteins being to degenerate, about 48-72 hours after death

Muscle twitch

smallest muscle contraction, occurs in laboratory, not in whole muscle of body

3 phases of twitch on myogram

latent period, contraction period, relaxation period

Latent period

time for action potential to propagate across sarcolemma

Contraction period

repeated cross-bridge cycles generate tension

Relaxation period

calcium ion levels reduced in cytosol by SR pumps, tension diminishes

Wave summation

increase in tension caused by repeated stimulation of muscle fiber by motor neuron

Greater tension production

repeated stimulation results in progressively greater tension production pumps in SR membranes have inadequate time to pump all released calcium ions back into SR before the fiber is restimulated, concentration of calcium ions cytosol increases with each stimulation

Tension Production

depends on frequency of motor neuron stimulation, results in 2 possible myogram patterns: unfused tetanus, fused (complete) tetanus

Unfused tetanus

fibers are stimulated about 50 times per second, fiber partially relaxes between stimuli, tension pulsates (individual twitches remain visible)

Fused (complete) tetanus

fiber is stimulated at a rate of 80-100 stimuli per second, does not relax between stimuli, the tension stays constant at nearly 100% of maximum, increased availability of calcium allows more cross-bridges to form, contributing to increase in tension