exam 4 skeletal muscle

• movement

• support

• posture

• temperature

• communication

functions of skeletal muscle

contractility

the ability of a muscle to shorten and produce tension

extensibility

capacity to stretch to its normal resting length and beyond after contracting

elasticity

ability to return to original resting length after stretch

plasticity

ability to constantly adapt

epimysium

wraps together bundles of muscle fasicles

perimysium

surrounds an individual muscle fasicle

endomysium

surrounds groups of muscle fibers within a fasicle

sarcoplasm

cytoplasm of a muscle cell and contains myofibrils

sarcoplasmic reticulum

equivalent to the endoplasmic reticulum, calcium is stored here

sarcolemma

cell membrane of a muscle fiber

actin and myosin

2 major contractile proteins

thin filament

actin

thick filament

myosin

filamentous actin (f-actin)

used for cell motility and muscle contraction

globular actin (g-actin)

the precursor to f-actin, calcium dependent

sarcomeres

the smallest contractile unit of a muscle

I-band

consist of only actin

Z-disc

forms the left and right boundaries of each sarcomere

A-band

extends through the entire length of the myosin

H-zone

second light zone in the center with only myosin

M-line

connected to the Z-line by titin and stays in the center of the sarcomere

motor unit

1 alpha neuron and all of the muscle fibers it innervates

through the epimysium

where do neurons innervate skeletal muscle

more muscle fibers

muscles that have to perform heavy duty actions usually have…

fewer muscle fibers

muscles that have to perform very precise movements usually have…

false

t/f : the fibers in a motor unit contract at different times

the neuromuscular junction

a chemical synapse formed between an alpha motor neuron and muscle fiber

acetylcholine

what are the vesicles of the nmj filled with?

nicotinic Ach receptors

what type of receptors lie on the muscle sarcolemma?

acetylecholine

what molecule stimulates contraction

acetylcholinesterase

what degrades ACh

excitation

the generation of an action potential due to signaling from the alpha motor neuron

1st step of excitation

ACh diffuses across the synaptic cleft and binds to ACh receptors on the motor endplate (EXCITE)

2nd step of excitation

ACh receptors allow extracellular Na⁺ to enter and K⁺ to exit, causing a depolarization and end plate potential (EXCITE)

3rd step of excitation

EPP triggers an AP, which propagates along the sarcolemma (EXCITE)

4th step of excitation

the action potential then triggers the release of Ca^{2+ (EXCITE)}

1st step of ECC

excitation (ECC)

2nd step of ECC

AChR opening allows Na⁺ and K⁺ to move down their concentration gradients (ECC)

3rd step of ECC

Na⁺ causes a depolarization and EPP (ECC)

4th step of ECC

depolarization travels down T-tubules, carrying AP deep into the muscles (ECC)

5th step of ECC

Ca²⁺ release occurs (ECC)

6th step of ECC

influx of Ca²⁺ initiates muscle contraction (ECC)

7th step of ECC

Ca²⁺ binds to tropinin, causing a conformation change, causing the troponin to move the tropomyosin (ECC)

ryanodine receptors (RyRs) and dihydropyridine receptors (DHRs)

which 2 receptors mediate calcium release?

T-tubule depolarization

what causes DHR channels to open?

RyR channels

which channels release calcium onto the actin and myosin filaments?

when sarcoplasmic Ca²⁺ levels rise

when does filament sliding occur?

1st step of cross bridge cycle

Ca²⁺ binds to to troponin and causes a change in shape, causing the troponin to move tropomyosin (CBC)

2nd step of cross bridge cycle

a power stroke occurs as ADP dissociates (CBC)

3rd step of cross bridge cycle

the cross bridge detaches as another ATP molecule binds to myosin, causing the myosin-actin interaction to weaken

4th step of cross bridge cycle

the myosin head reactivates through a recovery stroke, rehydrolyzing ATP into ADP and P_i (CBC)

ATP

what molecule binds to myosin and turns into ADP and P_i?

the high energy state

which energy state is the myosin head in when ADP is attached to it?

strengthens the interactions of myosin and actin

what role does P_i play in the cross bridge cycle?

when the myosin head pivots and slides the actin fiber towards the center of the sarcomere

when does myosin enter a low energy state?

Sarcoplasmic/ Endoplasmic Reticulum Ca²⁺ ATPase pump

SERCA pump stands for…

returning Ca²⁺ to the the sarcoplasmic reticulum

what are SERCA pumps used for?

a decrease in sarcoplasmic Ca²⁺

how does relaxation occur?

twitch

a single contraction and a relaxation cycle

latent phase

the action potential depolarizes the sarcolemma and the sarcoplasmic reticulum releases Ca²⁺ (phase)

contraction phase

tension in the muscle fiber peaks, causing cross bridge interactions

relaxation phase

Ca²⁺ levels decrease causing a decrease in cross bridges (phase)

wave summation

the addition of one twitch to another for more powerful muscle contractions

tetanic contraction

a sustained muscle contraction caused by a motor neuron firing APs at a high rate

fused tetanus

occurs with a VERY high rate of stimulation and relaxation does not occur

unfused tetanus

muscle fibers only partially relax before the next stimulus

recruitment

if stimulation increases, a motor neuron can stimulate more motor units (like picking up a heavy object)

muscle tone

ability of a muscle to maintain a continuous and passive partial contraction

isotonic contraction

the generation of muscle force with constant muscle tension and a change in muscle length

concentric

muscle shortens

contractile force > external load

eccentric

muscle lengthens

contractile force < external load

isometric

muscle does not change in length

contractile force = external load