Question

A motor unit is best described as (a) All the nerve fibres and muscle fibres in a single muscle bundle. (b) One muscle fibre and its single nerve fibre. (c) A single motor neuron and all the muscle fibres that it innervates. (d) As the neuron which carries the message from muscles to CNS.

Short answer

The correct answer is choice (c): A motor unit is best described as a single motor neuron and all the muscle fibres that it innervates.

Step-by-step solution

- Understanding the Terminology

Before selecting a choice, it's important to understand the terminology used in the given options. In this context, a 'motor neuron' refers to the nerve cell that controls muscle contraction. 'Innervates' means that it supplies the muscle with nerve impulses.

- Eliminate Incorrect Options

Option (d) can be eliminated because a motor neuron doesn’t carry the message from muscles to the central nervous system (CNS), it carries the signal to the muscles. It is an efferent neuron, carrying signals away from the CNS. Option (a) refers to the entire structure of a muscle which contains many motor units, not just one. So this option can be eliminated as well.

- Choose the Correct Option

The correct definition of a motor unit is a single motor neuron and all the muscle fibres it controls. Therefore, option (c) is correct.

Key concepts

Motor Neuron Function

The function of a motor neuron is pivotal in the realm of muscle movement and coordination. These specialized neurons are like the body's conductors, orchestrating the movements of muscles in response to a symphony of nervous signals.

Think of a motor neuron as a transmitter: its primary role is to carry electrical impulses from the central nervous system to the muscle fibers it 'innervates' - that is, supplies with neural power. This action triggers the muscles to contract, enabling the myriad motions that we perform daily. Without motor neurons, muscles would remain listless, unable to contract or contribute to movement.

Muscle Contraction

Muscle contraction can be likened to a dance between cells and molecules, driven by chemical signals and resulting in movement. It begins when a motor neuron fires an action potential, which travels down its axon and reaches the muscle fibers it innervates.

The action potential causes the release of the neurotransmitter acetylcholine into the neuromuscular junction, a specialized synapse between the neuron and muscle fiber. This chemical messenger binds to receptors on the muscle cell, initiating a cascade of events that lead to the sliding of actin and myosin filaments within the muscle cell. This microscopic interaction is the fundamental underpinning of muscle contraction - the basis for everything from blinking to sprinting.

Neuromuscular Junction

The neuromuscular junction is a crucial crossroads where the nervous system meets muscle tissue. Here, motor neurons relay signals to muscle fibers, using a chemical messenger, acetylcholine, as its medium. This is where the conversation between the brain and muscles crystallizes into physical movement.

This junction consists of the axon terminal of the motor neuron, the synaptic cleft - a small gap separating neuron and muscle - and the motor end plate of the muscle fiber. When the motor neuron fires, it releases acetylcholine into the synaptic cleft, which finds its way to receptors on the motor end plate. This interaction triggers the intricate process that leads to muscle contraction. Understanding this delicate interface helps to appreciate how the deliberate action of neurons can result in the powerful force of a muscle in action.