PHYS335: Lab 11

Skeletal muscle

type of muscle that is rod-like with regular interval of filaments

striated and multinucleate, somatic voluntary

Ach esterase

final step of NMJ; enzyme that degrades Ach in synaptic cleft to prevent AP from going forever

Cardiac muscle

type of muscle that is striated, branched, and uninucleate

has intercalated disks, autonomic involuntary

Intercalated disks

cardiac muscle has this to speed up communication between cells and ensure cells stick together with every beat of the heart

Smooth muscle

type of muscle that has NO striations (instead flattened squamous cells)

uninucleated and autonomic involuntary

skeletal muscles contribution to homeostasis

• Acquisition of nutrients/fuel

• processing food (chewing/swallowing)

• breathing

• escape from harm

• generation of heat for temperature control

• source of amino acids for times of famine

Whole skeletal muscle → fascicle → fiber → myofibril → sarcomere → thick/thin filaments

What is the arrangement of muscle (from largest unit to smallest)?

Myosin

thick filaments of the sarcomere that contain a heavy chain segment

Heavy chain segment

part of myosin that has long helix-like structures to form core thick filament

Globular cross bridge head

two heads extending toward overlying actin

has 2 binding sites (for actin and ATP for contraction)

Light chain

part of globular head that is relevant to smooth muscle ONLY

Actin

thin filaments of sarcomere that are twisted

G actin

globular “bead-like” molecule that is the binding site for myosin

F actin

filamentous structure of multiple beads that make up actin helix

Tropomyosin

spans 7 G actins with 1 troponin per unit; completely covers binding sites

Troponin

regulates the location of tropomyosin to open/close myosin binding sites

Z line

anchor point for sarcomere (actin attached here), allowing it to pull

A band

structural term for region of thick filament myosin (some actin overlap depending on presence of contraction) in sarcomere

SPACE TAKEN BY THICK FILAMENT DOES NOT CHANGE WHEN CONTRACTION OCCURS

I band

structural term for region of thin filament actin anchored to Z line in sarcomere (NO MYOSIN)

reduced in size when contraction occurs

H zone

tiny space in middle of sarcomere where we have only thick filament myosin anchored to M line

reduced in size when contraction occurs

M line

middle anchor point of sarcomere separating H zone in half

Titin

coil protein of sarcomere that stretches and contracts

generates passive tension

• Hexagonal actin around myosin (allowing for cross bridging)

• triangular myosin around actin

What does a cross section of a sarcomere look like?

2:1

There is a _:_ ratio of actin to myosin in muscle.

Sarcolemma

plasma membrane of myofibril (muscle cell)

T tubule

places where plasma membrane was sucked into myofibril structure because of the invagination in sarcolemma

Extracellular fluid

What is the fluid inside the T tubule qualified as?

Sarcoplasmic reticulum

part of myofibril that is equivalent to smooth ER, contains terminal cisternae

Terminal cisternae

enlarged ridge section next to T tubules that is the storage site for Ca, which allows for thin filament regulation

Relaxes

As Ca concentration decreases, muscle ___.

Ca binds to troponin, moving tropomyosin out of the way for contraction

What is the last action Ca for excitation-contraction coupling?

Contraction

this occurs when myosin cross bridges are activated to exert force on thin filaments

Electrical signal → chemical signal → mechanical signal

How is action potential transmitted through the neuromuscular junction (GENERALLY)?

Alpha motor neuron

stimulus for skeletal muscle excitation is an AP arriving along this which originates in brainstem or ventral horn of spinal cord

Reflexes

When would a skeletal muscle neuron originating in ventral horn be activated?

Motor unit

a single alpha motor neuron and all the muscle fibers that it innervates

Neuromuscular junction

where alpha motor fibers terminate (near center of sarcomere) at specialized synapse

Motor end plate

highly folded region of sarcolemma beneath alpha motor neuron terminal forming a dish

increased surface area for nAchR receptors (ligand gated ionotropic)

1. Action potential propagates down axon

2. At axon terminal, Ca voltage gated channels open and Ca floods in

3. Vesicles containing Ach bind Ca on SNARE proteins

4. Ach released via exocytosis

5. Ach binds nAchR, opening non-specific ligand gated channels, so Na floods in

6. End plate potential occurs, so local current inside muscle cell propagates along sarcolemma to open voltage gated Na channels resulting in a HUGE graded potential

7. Muscle fiber action potential occurs so it propagates further along sarcolemma

8. Ach esterase - degrades Ach in synaptic cleft to prevent AP from going forever

What is the mechanism of the neuromuscular junction?

Interneurons of CNS

How does inhibition of skeletal muscle occur?

30-50mV

EPP depolarizes motor end plate by ___.

Botox

bacterial toxin that breaks down SNARE proteins preventing Ach release (reducing activities of overactive muscles)

Organophosphates

(pesticides/nerve gas) that inhibit Ach esterase, preventing Ach breakdown and repolarization resulting in paralysis (from long lasting, ongoing contractions)

Curare

plant toxin that is an antagonist, binding nAchR and preventing Ach from binding resulting in paralysis (preventing muscle contraction)

used as the 1st paralytic anaesthetic

Succinylcholine

synthetic paralytic that is an agonist, binding nAchR and causing paralysis through constitutive contractions

Latent period

amount of time it takes Ca increase within skeletal muscle to connect excitation to contraction

Dihydropyridine receptor (DHPR)

first receptor action potential hits in T tubule membrane; voltage censor

Ryanodine receptor (RyR)

Ca channel that is physically connected to voltage sensor within terminal cisternae membrane of sarcoplasmic reticulum

Sarcoplasmic Endoplasmic Reticulum pump (SERCA)

primary active transporter of Ca within longitudinal portion of sarcoplasmic reticulum (middle)

1. Action potential propagation in T tubules

2. Action potential triggers DHPR to tug on RyR

3. Ca channel opens, Ca released

4. Ca activates thin filament, contraction occurs

5. SERCA pump actively transports Ca back into sarcoplasmic reticulum

What is the mechanism of excitation-coupling?

Shortens

The cross bridge ONLY rotates in the direction that ____ muscle, but length may not change.

Insertion; origin

Movable ____ moves towards fixed ___.

1. Ca levels increase moving troponin/tropomyosin so that myosin heads are available

2. POWERSTROKE - Myosin cross bridge head pulls actin toward M line, kicking ADP/Pi off 

3. ATP binds cross bridge head, causing it to detach from actin

4. Hydrolyzing ATP (removal of phosphate → ADP + Pi) energizes the cross bridge

What is the mechanism of the cross bridge cycle?

Rigor mortis

state of perpetual contraction when no ATP is being produced (in death)

• powering Na/K pump

• powering SERCA pump

• energizes cross bridge (ATP hydrolysis)

• dissociates cross bridge (ATP binding to XB head)

List the roles of ATP in skeletal muscle contraction.