11.3 Motor units

What is a motor neuron?

A nerve cell that stimulates skeletal muscle fibers; its cell body is in the brainstem or spinal cord.

What are somatic motor fibers?

Axons of somatic motor neurons that lead to skeletal muscles.

What is a motor unit?

One motor neuron and all the muscle fibers it innervates.

Why do all fibers in a motor unit contract together?

Because the nerve signal spreads across all terminal branches, stimulating all fibers at once.

How many muscle fibers does a typical motor unit control?

About 200, though the number varies depending on the muscle’s function.

What is the advantage of having multiple motor units in a muscle?

It allows variation in strength and endurance; motor units can take turns to prevent fatigue.

What is the difference between small and large motor units?

Small motor units allow fine control; large motor units generate greater strength.

Give an example of muscles with small motor units.

Eye and hand muscles, which require precise control.

Give an example of muscles with large motor units.

Thigh or calf muscles, which require more strength than precision.

How does the nervous system prevent fatigue in postural muscles?

By alternating the activation of motor units so some rest while others contract.

What is a synapse?

The point where a nerve fiber meets any target cell.

What is a neuromuscular junction (NMJ)?

The synapse between a motor neuron and a muscle fiber; also called the motor end plate.

What is the function of the axon terminal at the NMJ?

It contains synaptic vesicles filled with acetylcholine (ACh) to communicate with the muscle fiber.

What is the synaptic cleft?

The narrow space between the axon terminal and the muscle fiber where neurotransmission occurs.

What does acetylcholine (ACh) do?

It acts as a chemical messenger that transmits the nerve signal to the muscle fiber.

What are ACh receptors and where are they found?

Proteins in the sarcolemma that bind ACh; found opposite the axon terminals in junctional folds.

What increases the sensitivity of the NMJ to ACh?

Junctional folds in the sarcolemma that increase surface area for ACh receptors.

What enzyme breaks down ACh in the synaptic cleft?

Acetylcholinesterase (AChE).

What is the purpose of acetylcholinesterase?

It stops muscle contraction by degrading ACh, allowing the muscle to relax.

What is the basal lamina and what is its function?

A collagen-glycoprotein layer that encloses the NMJ and contains AChE to help terminate signals.

What is denervation atrophy?

Shrinkage of a muscle that occurs when its nerve connection is lost.

What is spastic paralysis?

A state of continual muscle contraction caused by overstimulation.

What causes spastic paralysis?

Cholinesterase inhibitors (e.g., organophosphates) or tetanus toxin that prevent relaxation.

What is flaccid paralysis?

A state in which muscles are limp and cannot contract.

What causes flaccid paralysis?

Agents such as curare (blocks ACh receptors) or botulinum toxin (blocks ACh release).

What is botulinum toxin (Botox) and its effect?

A toxin that prevents ACh release, causing muscle relaxation; used cosmetically to reduce wrinkles.

What are electrically excitable cells?

Cells, like neurons and muscle fibers, that respond to stimulation with voltage changes across their membranes.

What is electrophysiology?

The study of electrical activity in cells, key to understanding nerve and muscle function.

What creates the resting membrane potential (RMP)?

Differences in ion concentrations across the plasma membrane—more K⁺ inside, more Na⁺ outside.

What is the typical RMP of a muscle cell?

About -90 millivolts (mV).

What maintains the RMP?

The sodium-potassium pump (Na⁺/K⁺ pump).

What happens when a muscle or nerve cell is stimulated?

Ion channels open, Na⁺ flows in (depolarization), then K⁺ flows out (repolarization).

What is depolarization?

When Na⁺ enters the cell, making the inside temporarily positive.

What is repolarization?

When K⁺ exits the cell, restoring the negative internal charge.

What is an action potential?

A quick voltage change from negative to positive and back again during cell excitation.

How do action potentials propagate along a cell membrane?

Each action potential triggers another ahead of it, forming a wave of excitation.

What is a nerve impulse or signal?

A traveling wave of action potentials along a nerve fiber or muscle sarcolemma.