Neuromuscular Junction and Cross Bridge Cycling Study Guide

Question 1

what activates muscle fibers

Answer 1

Motor Neurons

Question 1

Define resting membrane potential

Answer 1

voltage across the plasma membrane

Question 2

Average resting membrane potential of a muscle cell is: _______.

Answer 2

~70mV

Question 3

Define action potential.

Answer 3

A type of electrical signal - a large change in resting membrane potential

Question 4

Action potentials are converted to ______ to cross the ________.

Answer 4

• chemical signals
• synaptic cleft

Question 5

Define synaptic cleft.

Answer 5

The small gap between cells

Question 6

Define neurotransmitters

Answer 6

chemical messengers, an action potential crosses from a neuron to a muscle cell via a neurotransmitter

Question 7

What neurotransmitter crosses from a motor neuron to a muscle cell?

Answer 7

Acetylcholine (Ach) crosses from a motor neuron to a muscle cell

Question 8

Where specifically does this neurotransmitter bind on the muscle cell?

Answer 8

ach receptors embedded in the sarcolemma in the neuromuscular junction on the muscle cell

Question 9

Compare and contrast chemically-gated and voltage-gated ion channels

Answer 9

• Chemically-gated ion channels: opened by chemical messengers such as neurotransmitters
  Example: Ach receptors on muscle cells
• Voltage-gated ion channels:
  open/close in response to voltage changes - underlie all action potentials
  Example: t-tubules

Question 10

How do chemically-gated and voltage-gated ion channels work together in skeletal muscle fibers?

Answer 10

Chemically gated ion channels cause small depolarizations. Small depolarizations trigger voltage-gated ion channels to create small action potentials

Question 11

Define axon

Answer 11

Axon: long, threadlike extensions of motor neurons; travel from central nervous system to skeletal muscle (bundle of axons = nerves)

Question 12

What are the smallest branches of an axon called

Answer 12

axon terminal

Question 13

What are the 3 components of the neuromuscular junction?

Answer 13

Axon terminal
Synaptic cleft
Junctional folds

Question 14

Define synaptic vesicles. What does it contain?

Answer 14

• membrane-bound sacs stored within the axon terminal
• Contains millions of ACh receptors

Question 15

myasthenia gravis

Answer 15

immune system destroys ACh receptors, which are needed to open the chemically gated channels to allow movement of Na+ and K+ to change the membrane potential and eventually cause an action potential. This is caused by toxins, drugs, and diseases that interfere with events at the neuromuscular junction. Causes symptoms such as drooping eyelids, difficulty swallowing, difficulty talking, generalized muscular weakness

Question 16

Botox(botulinum toxin)

Answer 16

less ACh is diffused in the neuromuscular junction, which means there will be fewer action potentials, and therefore fewer muscle contractions. The decreased muscle contractions lead to less wrinkles.

Question 17

What ion influxes into a muscle cell in depolarization

Answer 17

Na+ influxes into a muscle cell in depolarization

Question 18

What ion effluxes out of a muscle cell in repolarization

Answer 18

K+ effluxes out of the cell

Question 19

After depolarization, how does an AP travel along the sarcolemma surface?

Answer 19

AP spreads across sarcolemma from one voltage-gated NA+ channel to the next causing additional depolarizations

Question 20

Define refractory period.

Answer 20

Muscle fiber cannot be stimulated for a specific amount of time - until repolarization is complete.

Question 21

How do Na+-K+ Pumps restore ionic resting state?

Answer 21

Restored by Na+-K+ pumps – Na+ is pumped back out, K+ is pumped back in

Question 22

Be ready to define the parts of an action potential from a graphical representation

Answer 22

Beginning of upwards slope: Na+ channels open
Top of upwards slope: depolarization due to NA+ entry
Top of downwards slope: NA+ channels close, K+ channels open
Middle of downwards slope: repolarization due to K+ exit
Flat line after slope: K+ channels closed

Question 23

An action potential traveling along the sarcolemma dives down into the ______. This action potential triggers the release of ____ from the ___________.

Answer 23

• t tubules
• ca2+
  sarcoplasmic reticulum (SR)

Question 24

In terms of cross bridge formation, what happens when intracellular levels of Calcium are low?

Answer 24

Tropomyosin blocks the active sites on actin
Myosin heads cannot attach to actin
Muscle fiber remains relaxed
In response to AP, voltage-sensitive proteins in the t-tubules change shape and cause sarcoplasmic reticulum to release Ca2+

Question 25

In terms of cross bridge formation, what happens when intracellular levels of Calcium are high?

Answer 25

Ca2+ binds to troponin
Troponin changes shape and moves tropomyosin away from myosin-binding sites
Myosin heads can bind to actin and form cross bridges
Cycling is initiated leading to sarcomere shortening and muscle fiber contraction

Question 26

What happens to calcium when nervous stimulation ceases?

Answer 26

Ca2+ is pumped back into the SR and contraction ends

Question 27

4 steps of cross bridge formation

Answer 27

1. Cross bridge formation: high-energy myosin head attaches to the thin filament’s active site
2. Working (power) stroke: myosin head pivots and pulls the thin filament towards the M line
3. Cross bridge detachment: ATP attaches to myosin head, cross bridge detaches
4. Cocking of myosin head: energy from hydrolysis of ATP “cocks” myosin head back into high-energy state

Question 28

In what step of cross bridge formation does ATP bind? In what step is ATP hydrolyzed?

Answer 28

ATP is hydrolyzed in the 4th step
ATP binds in the 3rd step

Question 29

What clinical impact(s) does the necessity of ATP have? Think about what happens when we’re no longer making ATP!

Answer 29

Without ATP, the cross bridges cannot detach, causing muscles to be stiff and in a constant state of contraction. (rigor mortis)