Motor Unit Structure, Function, and Behavior

A motor unit consists of a motor nerve and the muscle fibers it innervates.
Motor nerve: Looks like what we can see on the right hand side of our screen.
Dendrites: Little tentacles that form synapses with other nerves, typically from the brain.
Axon: Long cord traveling down to the muscle
Axon hillock: Region where the action potential is generated.
Myelin: Fatty acid covering that insulates the cell membrane, decreasing ion flow.
- Axons are myelinated, with varying thickness depending on the motor unit size.
- Conditions like multiple sclerosis disrupt myelin, impairing signal transmission.
Nodes of Ranvier: Gaps in the myelin that increase capacitance and facilitate saltatory conduction.
Motor End Plate: Region where the motor neuron synapses with muscle fibers.
Muscle Fibers: The muscle cells innervated by the motor neuron.

A series of chemical and electrical events that occur when a nerve is stimulated.
Resting potential: Around $-70$ mV.
Weak stimulus: Causes a small shift in charge, but not enough to trigger an action potential.
Strong stimulus: Rapid influx of sodium into the cell, causing the charge to go from $-70$ to $+30$ mV rapidly.
After hyperpolarization phase: A refractory period of a couple of milliseconds where the motor neuron is unlikely to fire again.

Excitatory Postsynaptic Potentials (EPSP): Localized factors that make it more likely to reach the firing threshold.
Inhibitory Postsynaptic Potentials (IPSP): Factors that make it less likely to reach the firing threshold (more negative).
Spatial Summation: Nerves release enough neurotransmitter to cause an action potential.
Temporal Summation: Enough excitatory input in a short amount of time to cause an action potential.

Influx of sodium into the nerve cell makes the cell less negative.
At rest, the cell sits around $-70$ mV.
When the threshold of $-55$ mV is reached enough excitatory input triggers an all-or-nothing response.
Sodium floods into the cell, creating a positive charge and generating an action potential.
The process is rapidly reversed as sodium is pumped back out of the cell by sodium-potassium pumps, which requires ATP.

As the axon approaches the muscle fiber, the myelin sheath ends, and the axon divides into smaller terminal branches.
A motor unit may innervate around five muscle fibers (very small motor unit).

Initially, acetylcholine receptors are evenly distributed along the muscle fiber.
As the nerve innervates the muscle, acetylcholine receptors congregate around the motor endplate.
With nerve injury, there is a redistribution of acetylcholine receptors.
The neuromuscular system is highly plastic and adaptable.

Action potential travels down the axon.
Synaptic vesicles release acetylcholine into the synaptic cleft.
Acetylcholine interacts with the postsynaptic membrane.
This travels along the sarcolemma and interacts with the T-tubule sarcoplasmic reticulum.

The nervous system varies force production through:
- Motor unit recruitment: To produce more force, recruit more motor units.
- Rate coding: Increase the frequency of nerve impulses conducted by the motor neurons.

In the hand muscles, most motor units are recruited even during weak contractions (10-20%).
- Further force increase primarily relies on increased firing frequency.
In muscles like the quads, increasing force requires recruiting more and higher-threshold motor units.

The size of the motor neuron is related to the size of the motor unit and the diameter of the nerve axon.
Thick axons result in higher conduction velocity.
The soma (cell body) determines the size of the motor neuron and its critical firing level.
Small somas (smaller motor units) have lower critical firing levels, while large somas have higher critical firing levels.
Large motor units encounter more resistance and have a higher threshold for activation.
Smaller motor neurons offer less resistance and thus have a lower threshold.

Smaller motor units are generally recruited before larger motor units during voluntary movement.
When decreasing force, the larger motor units are shut off first.

As the intensity of muscular contraction increases, different types of muscle fibers (type one, type 2a, type 2x) and motor units are recruited.
As force increases, more motor units are recruited and fire more quickly (increased rate coding).

During voluntary contractions, motor units fire at slightly different times.
Stimulating a muscle or nerve at 5 Hz results in twitches, but no fused contraction.
A fused contraction is typically achieved at 60 Hz of stimulation.

Stimulating a large nerve (e.g., femoral nerve) results in a twitch force response.
Fatigue can influence the twitch response, indicating changes at the muscle level.
- Amplitude of force produced by the twitch may reduce with fatigue.
- The speed at which force is produced may also slow down.
- Elongation of the half relaxation time, generally related to calcium mechanics.

Motor neurons are highly receptive to the monoamines serotonin and noradrenaline, which can significantly influence their firing behavior.
PICs push the motor neuron toward firing threshold action potential generation allowing them to fire repeatedly more easily.
PICs amplify the signal from the brain.
PICs may be beneficial for endurance and maximum muscle strength.
Computer simulations suggest that without adequate PICs, only 40-50% of maximum force generating capacity would be possible.

Too little or too much serotonin.
Activating an antagonist muscle at the same time tends to shut off PICs.
Whole body relaxation reduces PIC activity.
Contracting arms and legs can boost PIC activity.
Increased age is associated with a reduction in PIC activity, contributing to muscle weakness in older adults.