EXAM 4: Muscle Contraction

Question 1

What is the neuromuscular junction

Answer 1

A synapse between a motor neuron and a muscle cell
- contains high density of chemically gated ion channels (ACh receptors)

Question 2

What is the motor endplate

Answer 2

The sarcolemma region associated with the NMJ

Question 3

What does ACh stand for

Answer 3

Acetylcholine (neurotransmitter)

Question 4

What are the four major stages of muscle contraction

Answer 4

1. Excitation
2. Excitation-contraction coupling
3. Contraction
4. Relaxation

Question 5

What happens during the excitation stage

Answer 5

Communications between the neuron and muscle cell
Leads to excitation of muscle cell (action potential)

Question 6

What happens during the excitation-contraction coupling stage

Answer 6

Conversions of action potential in muscle cell to activation of proteins in the sarcomere

Question 7

What happens during the contraction stage

Answer 7

Muscles develop tension and may shorten
Sliding filament theory

Question 8

What happens during the relaxation stage

Answer 8

Return of muscle cells to resting length

Question 9

Excitation ( in depth )

Answer 9

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

Question 10

The chemically-gated ion channels in the motor end plate that ACh binds to are properly called…

Answer 10

Cholinergic receptors

Question 11

What does non-specific mean

Answer 11

Allows diffusion of both sodium and potassium cations

Question 12

When more sodium enters than potassium leaves, the end result is…

Answer 12

Depolarization

Question 13

Excitation-Contraction Coupling ( in depth )

Answer 13

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

Question 14

Contraction Step 1

Answer 14

Myosin hydrolyzes an ATP molecule
- Produces ADP + P
Activates the myosin head in an extended position

Question 15

Contraction Step 2

Answer 15

Myosin binds to the actin active site
- forms a cross-bridge between actin and myosin
- ADP + P are still bound to myosin

Question 16

Contraction Step 3

Answer 16

Interaction of cross-bridge causes release of ADP and P
- causes myosin head to flex
- thin filament slides past the thick

Question 17

Contraction Step 4

Answer 17

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

Question 18

What is the sliding filament theory

Answer 18

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

Question 19

Relaxation step 1

Answer 19

Action potentials in axon cease
- No more ACh is released from the neuron

Question 20

Relaxation step 2

Answer 20

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

Question 21

Relaxation Step 3

Answer 21

Active transport needed to pump calcium back into SR
- Calcium pumps use ATP to move calcium back into SR

Question 22

Relaxation step 4

Answer 22

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