Objective 8

Question 0

Describe how the motor neurons stimulate skeletal muscles

Answer 0

• Axons of motor neurons travel from the CNS to the muscle cells via cranial and spinal nerves.
• Axons of motor neurons form many branches as they enter muscles

Question 1

How are skeletal muscles stimulated?

Answer 1

Skeletal muscles are stimulated by motor neurons of the somatic nervous system.

Question 2

What happens where each axonal branch enters the muscle?

Answer 2

Each axonal branch forms a neuromuscular junction with a single muscle fiber

Question 3

Where are the neuromuscular junctions situated?

Answer 3

They are situated midway along the length of the muscle fiber.

Question 4

What is the neuromuscular junction composed of?

Answer 4

• Synaptic vesicles of the terminal axon
• Synaptic cleft
• The motor end plate of a muscle

Question 5

What do the synaptic vesicles of the terminal axon contain?

Answer 5

the neurotransmitter ACh

Question 6

What does the motor end plate of a muscle contain?

Answer 6

ACh receptors that help form the neuromuscular junction

Question 7

What happens when a nerve impulse reaches the end of the motor axon at the neuromuscular junction

Answer 7

• voltage-regulated calcium channels open and allow Ca2+ to enter the axon terminal
• Ca2+ inside the axon terminal causes axonal vesicles to fuse with the axonal membrane
• This fusion increases ACh into the synaptic cleft via exocytosis
• ACh diffuses across the synaptic cleft to nicotinic ACh receptors on the sarcolemma

Question 8

What happens when ACh binds to its receptors?

Answer 8

It opens chemically-gated ion channels which cause an end-plate potential (an EPSP at the motor end plate)

Question 9

What happens when end plate potentials have summed to threshold?

Answer 9

Once the end plate potentials have summed to threshold, an action potential is initiated

Question 10

What happens when ACh is bound to ACh receptors?

Answer 10

ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase
- prevents continued muscle fiber contraction the absence of additional stimuli