The Neuromuscular Junction and Cross Bridge Cycling

Question 1

_________ __________ activate muscle fibers.

Answer 1

Motor neurons

Question 2

Define resting membrane potential.

Answer 2

voltage across the plasma membrane

Question 3

Average resting membrane potential of a muscle cell is:

Answer 3

-70 to -90 millivolts (mV). This negative value indicates that the inside of the cell
is more negatively charged compared to the extracellular fluid.

Question 4

Define action potential.

Answer 4

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

Question 5

Action potentials are converted to _________
__________ to cross the _________ ___________.

Answer 5

chemical signals, synaptic cleft

Question 6

Define synaptic cleft.

Answer 6

the small gap between cells

Question 7

Define neurotransmitter. What neurotransmitter crosses from a motor
neuron to a muscle cell? Where specifically does this neurotransmitter bind on the
muscle cell?

Answer 7

Neurotransmitter: chemical substance that transmits signals or messages
between neurons

An action potential crosses from a neuron to a muscle cell via a neurotransmitter
– Acetylcholine ( Ach )

Binds to the sarcolemma or ACh Receptors

Question 8

Compare and contrast chemically-gated and
voltage-gated ion channels. Give examples of each!

Answer 8

Chemically Gated : opened by chemical messengers such as
neurotransmitters

Ex: Ach receptors on muscle cells

Voltage-Gated: open/close in response to voltage changes -underlie all
action potentials

Ex: Na+ Channels or K+ Channels

Question 9

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

Answer 9

In skeletal muscle fibers, chemically gated ion channels cause small
depolarizations. Small depolarizations trigger voltage-gated ion channels to
create action potentials.

Question 10

Define axon. What are the smallest branches of an axon called?

Answer 10

long, threadlike extensions of motor neurons; travel from central nervous system to skeletal muscle

axon terminal

Question 11

Define synaptic vesicle. What does it contain?

Answer 11

membrane-bound sacs stored within the axon terminal; contain Acetylcholine ( ACh )

Question 11

What are the 3 components of the neuromuscular junction?

Answer 11

The axon terminal and the muscle fiber are separated by a gel-filled space –
a synaptic cleft

Synaptic Vesicles - membrane-bound sacs stored within the axon terminal; contain Acetylcholine ( ACh )

On the muscle fiber side, infoldings of sarcolemma – called junctional folds
– contain millions of ACh receptors

Neuromuscular Junction = axon terminal + synaptic cleft + junctional folds

Question 12

Learn the 7 steps of the events at the neuromuscular junction.

Answer 12

1. Action potential arrives at the axon terminal
2. Voltage-Gated Ca2+ channels open; Ca2+ ions enter the motor neuron
3. Entry of Ca2+ ions causes the release of ACh neurotransmitter into the
   synaptic cleft via exocytosis
4. ACh diffuses across the synaptic cleft to ACh receptors in the junctional
   folds
5. ACh binding opens chemically gated ion channels
6. Na+ ions enter the muscle fiber and K+ ions exit the muscle fiber. Net
   movement of Na+ in creates local change in membrane potential/ end plate
   potential.
7. Once the membrane potential hits a threshold value (about -55mV), an
   unstoppable action potential propagates along the sarcolemma

ACh is degraded by acetylcholinesterase and diffuses away from the
junction. This stops neural transmission to the muscle fiber.

Question 13

Based on what you know about the events neuromuscular junction, explain BOTH myasthenia gravis and how Botox works.

Answer 13

myasthenia gravis involves the immune system attacking acetylcholine receptors at the neuromuscular junction, leading to muscle weakness.

Botox works by blocking acetylcholine release, causing temporary muscle paralysis and is used for various medical and cosmetic purposes.

Question 14

What ion influxes into a muscle cell in depolarization? What ion effluxes out of a
muscle cell in repolarization?

Answer 14

Large influx of Na+ triggers an AP

K+ effluxes out the cell; restores initial resting membrane voltage

Question 15

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

Answer 15

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

Called Propagation, like ripples from a pebble dropped in a pond

Question 16

Define refractory period.

Answer 16

muscle fiber cannot be stimulated for a specific amount of time – until repolarization is complete

Question 17

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

Answer 17

Ionic resting state is restored by Na+-K+ Pumps

Na+ is pumped back out, K+ is pumped back in

Question 18

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

Question 19

An action potential traveling along the sarcolemma dives down into the __________________. This action potential triggers the release of ____________ from the
_____________________.

Answer 19

T-tubules, Calcium Ions Ca2+, Sarcoplasmic Reticulum

Question 20

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

Answer 20

Tropomyosin blocks the active sites on actin

Myosin heads cannot attach to actin

Muscle fiber remains relaxed

Question 21

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

Answer 21

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

Question 22

What happens to calcium is when nervous stimulation ceases?

Answer 22

Ca2+ is pumped back into the SR and contraction ends

Question 23

Learn the 4 steps of cross bridge formation. I
recommend writing/drawing them out – many times! In what step does ATP bind? In what step is ATP hydrolyzed?

Answer 23

Cross Bridge Formation : high-energy myosin head attaches to the thin filament’s active site

Working (Power) Stroke : myosin head pivots and pulls the thin filament towards the M line

Cross Bridge Detachment : ATP attaches to myosin head, cross bridge detaches

Cocking of Myosin Head : energy from hydrolysis of ATP “cocks” myosin head back into high-energy state

Question 24

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

Answer 24

ATP is the primary energy currency of cells

If we don’t make ATP -
We have muscle weakness/fatigue, Organ Dysfunction, Cell Death
Metabolic Disorders, Hypoxia, Cancer