VMED 5163 - Neuromuscular Junctions (NMJ) / Excitation Coupling

One axon from one somatic motor neuron

One muscle cell receives how many axons? How many neurons contribute to this innervation?

The axon produces many branches to innervate individual fibrils/fibers

How is it possible for one single axon to innervate an entire muscle cell and its components?

One motor neuron and all of the muscle fibers it innervates

What are the components of a motor unit?

How many muscle fibers are innervated by one motor neuron (the size of the motor unit)

The strength of a muscle contraction is based on what factor?

Synapses which are located between motor neurons and skeletal muscle cells

What are neuromuscular junctions and where are they located?

Motor end plate

The postsynaptic portion of the neuromuscular junction is known as the ___

Presynaptic terminals

Specialized endings of an axon within a neuromuscular junction are known as?

ACh receptors and ACh esterase

Motor end plates contain what substances?

Synaptic cleft

Between the pre- and post-synaptic terminals of a neuromuscular junction is what structure?

Acetylcholinesterase (AChE)

What molecules are present within the synaptic cleft of a NMJ?

Depolarization

The opening of a sodium channel and sodium influx into a muscle cell will cause what change in the neuromuscular junction?

Voltage-gated sodium channels will open and create an action potential

Once a certain threshold of depolarization is met within the muscle cell...

Excitation-contraction coupling

The generation of an action potential via sodium channels is a part of the initiation of what process?

Found on the post-synaptic membrane or within the synaptic cleft, functions to terminate ACh when it is no longer needed

Where is AChE found, and what is its function?

RYR1 channel

Which type of calcium channel will allow calcium to be released in skeletal muscle cells?

Acetylcholine (ACh)

When an action potential reaches a motor neuron, the release of what substance occurs?

Opens ion channels which allow sodium and calcium to flow in, and potassium to flow out

How does ACh function once released within a neuromuscular junction?

To T tubules; stimulation of calcium ion release via SR

Once an action potential is generated within a muscle cell, it is carried where? What happens at these structures?

It is pumped back into the sarcoplasmic reticulum for storage

After muscle contraction, what is the fate of calcium bound to troponin?

Matrix-bound

ACh is present in the synaptic cleft in what form?

Slow- and fast-twitch fibers

There are two types of muscle fiber based on reaction to stimuli. What are they?

Red

Slow-twitch, or ___ muscle fibers, have a high concentration of myoglobin

Prolonged, aerobic

Red muscle fibers are used for what kind of energy generation?

They have less extensive blood supply and low concentration of myoglobins

Why do white muscle fibers appear as such?

White muscle fibers / fast-twitch muscle fibers; they are adapted to anaerobic energy generation

Which muscle fiber type is seen during intensive exercise and why is this?