EXAM 4: Muscle Contraction Flashcards
What is the neuromuscular junction
A synapse between a motor neuron and a muscle cell
- contains high density of chemically gated ion channels (ACh receptors)
What is the motor endplate
The sarcolemma region associated with the NMJ
What does ACh stand for
Acetylcholine (neurotransmitter)
What are the four major stages of muscle contraction
- Excitation
- Excitation-contraction coupling
- Contraction
- Relaxation
What happens during the excitation stage
Communications between the neuron and muscle cell
Leads to excitation of muscle cell (action potential)
What happens during the excitation-contraction coupling stage
Conversions of action potential in muscle cell to activation of proteins in the sarcomere
What happens during the contraction stage
Muscles develop tension and may shorten
Sliding filament theory
What happens during the relaxation stage
Return of muscle cells to resting length
Excitation ( in depth )
Action potential from motor neurons reaches end of axon
Voltage-gated calcium channels open
- Calcium enters the neuron and causes synaptic vesicles to release acetylcholine (ACh) into the synapse
ACh binds to chemical-gated ion channels in the motor end plate
- non-specific
Sodium rushes into the cell, some potassium exits the cell
- resulting in depolarization
This is called an end plate potential
Depolarization from EPPs cause muscle cell to reach threshold leading to action potential
The action potential involves same voltage-gated channels as neurons
The chemically-gated ion channels in the motor end plate that ACh binds to are properly called…
Cholinergic receptors
What does non-specific mean
Allows diffusion of both sodium and potassium cations
When more sodium enters than potassium leaves, the end result is…
Depolarization
Excitation-Contraction Coupling ( in depth )
Action potential travels across sarcolemma and DOWN the sarcolemma of T-tubules
Action potential triggers voltage-gated Ca channels in T-tubules to open, which are physically connected to mechanically-gated Ca channels in SR
Opening of mechanically-gated Ca channels in SR releases Ca into the sacroplasm
Calcium binds to troponin and causes it to move tropomyosin off the actin myosin-binding sites
Contraction Step 1
Myosin hydrolyzes an ATP molecule
- Produces ADP + P
Activates the myosin head in an extended position
Contraction Step 2
Myosin binds to the actin active site
- forms a cross-bridge between actin and myosin
- ADP + P are still bound to myosin
Contraction Step 3
Interaction of cross-bridge causes release of ADP and P
- causes myosin head to flex
- thin filament slides past the thick
Contraction Step 4
Another ATP molecule binds to myosin, breaking cross-bridge
ATP is immediately hydrolyzed, going back to step 1 of contraction
Some heads are always bound to prevent slippage
What is the sliding filament theory
Cycle of contraction will continue as long as there is enough Ca and ATP
Thick and thin filaments do not become shorter, just slide past each other
Relaxation step 1
Action potentials in axon cease
- No more ACh is released from the neuron
Relaxation step 2
Acetylcholinesterase (AchE, an enzyme) breaks down remaining acetylcholine
- breakdown products are transported back to neuron and recycled
No EPP ( or action potentials ) are produced in the muscle membrane
Relaxation Step 3
Active transport needed to pump calcium back into SR
- Calcium pumps use ATP to move calcium back into SR
Relaxation step 4
Troponin-tropomyosin complex covers active sites
- due to lack of calcium
- myosin cannot bind
- stops the production or maintenance of tension
Sarcomere returns to resting length
Muscle cells return to its resting length
Complete cycle is called a muscle twitch
