A motor neuron, along with the muscle fibres connected to it, constitutes a 

A motor neuron along with the muscle fibres connected to it constitute a motor unit.

• Motor unit: A motor unit consists of a motor neuron and all the muscle fibers it innervates (controls). Motor units vary in size, with some motor neurons controlling only a few muscle fibers (fine control, e.g., in the eye muscles), while others control many muscle fibers (coarse control, e.g., in large leg muscles). When a motor neuron fires an action potential, all the muscle fibers in its motor unit contract simultaneously.
• Motor end plate: The motor end plate, also known as the neuromuscular junction, is a specialized region where a motor neuron's axon terminal meets a muscle fiber. It is the point of communication between the nervous system and muscle tissue. When an action potential reaches the motor end plate, it triggers the release of neurotransmitters (usually acetylcholine) into the synaptic cleft, leading to muscle fiber contraction.
• Sarcomere: A sarcomere is the fundamental contractile unit of a muscle fiber. It is a highly organized segment of the myofibril, located between two Z-lines (also known as Z-discs). Sarcomeres are composed of actin and myosin filaments, and they are responsible for muscle contraction. During contraction, sarcomeres shorten as actin and myosin filaments slide past each other.
• Muscle Spindle: Muscle spindles are sensory receptors located within skeletal muscles. They are responsible for monitoring muscle length and changes in muscle length (stretch). Muscle spindles help the body sense changes in muscle position and are involved in the stretch reflex. When a muscle is stretched quickly, the muscle spindle signals a reflexive contraction to prevent overstretching or injury.