Part Five: The Neuro-Muscular Junction or Synapse

Question 1

The entire structure “A” represents the knobby end of a nerve cell.
What TWO names is this structure known as?

Answer 1

A= presynaptic terminal or synaptic knob (part of nerve cell)

Question 2

The entire structure “I” represents the surface of a muscle cell.
What is it called?

Answer 2

I= postsynaptic terminal (part of the sarcolemma, or covering of
the muscle cell)

Question 3

What is “G” called?

Answer 3

G= synaptic cleft or gap (small space between nerve & muscle)

Question 4

What is substance “C,” the neurotransmitter released by the nerve
cell’s presynaptic vesicles (“D”)?

Answer 4

C= acetylcholine (ACh)

Question 5

What must happen first in the nerve cell to allow “C” to be released?

(Hint: See “B.”)

Answer 5

Calcium enters the synaptic knob of the nerve through the calcium
channels (B), which is what triggers the release of the ACh.

Question 6

Substance “C” binds to a specific receptor (“E”), causing it to open.
What substance, shown at “F,” enters the muscle cell through this
receptor?

Answer 6

The ACh receptor is a sodium (Na+) channel that allows sodium to
enter the muscle.

Question 7

Once substance “F” enters the muscle cell, what has been started
in the muscle cell?

Answer 7

Once sodium has entered the muscle, an action potential
(electrical current) has now been started that flows into the muscle and causes it to contract.

Question 8

What happens if substance “C” remains in the synaptic cleft for too
long?

Answer 8

If ACh remains in the synapse for too long, action potentials
will continue to be made, causing a constant contraction in the
muscle (spastic paralysis).

Question 9

What enzyme, shown at “H,” prevents substance “C” from remaining in
the synapse for too long?

Answer 9

The enzyme Cholinesterase (AChE) breaks apart ACh as soon as it
is used and sends the pieces of ACh back to the presynaptic
terminal where they are reused to make more ACh.

Question 10

What two types of poisons prevent enzyme “H” from working? What
is the result in the muscle?

Answer 10

Insecticides and military nerve gases kill by preventing AChE
(the enzyme) from breaking apart ACh quickly enough. The result
is that ACh stays in the synapse, causing spastic paralysis of
the muscle. The muscle is “stuck” in a contraction and cannot
relax.

Question 11

What specific part of the synapse (choose from A-I) is affected by
the poison curare and in the disease Myasthenia Gravis?

Answer 11

Both the poison curare and the disease Myasthenia Gravis affect
E, the acetylcholine receptor.

Question 12

With curare poisoning and with Myasthenia Gravis, what type of
paralysis results?

Answer 12

Flaccid paralysis results. The muscle cannot contract.

Question 13

Explain exactly what happens in the synapse with Myasthenia Gravis
and with curare poisoning.

Answer 13

In Myasthenia Gravis, white blood cells slowly destroy the ACh
receptors over time. Curare binds to ACh receptors and causes
them to close. The result in both situations is that, with the
ACh receptors impaired, no sodium can enter the muscle cells and
thus no electrical currents (action potentials) can be started in
the muscle. The muscle is then unable to contract.