A motor neuron and all the muscle fibres it innervates are known as what?

A motor neuron and all the muscle fibres it innervates are collectively referred to as a motor unit. This unit is fundamental to muscle contraction because the motor neuron sends signals to its associated muscle fibres, causing them to contract and perform movement. The strength of a contraction is modulated by the number of motor units recruited; more units lead to a stronger contraction, while fewer units lead to a weaker contraction. Understanding the motor unit is essential in kinesiology, as it provides insight into how muscular control is achieved at the nervous system level.

The other options, while related to muscle anatomy and function, do not accurately describe this specific concept. The neuromuscular junction refers to the specific synapse where the motor neuron communicates with the muscle fibre, but it does not encompass the entire motor unit. The muscle belly is the central part of a muscle, consisting of the muscle tissue itself but not including the motor neuron. A fascicle refers to a bundle of muscle fibres within a muscle, and again, does not include the motor neuron connections. Thus, the term motor unit is the most precise and relevant answer to the question.