Define a motor unit. What roles do motor units play in graded contractions in skeletal muscles? Solution Motor unit is made of Motor Neuron, and its branches extended to the myofibrils of muscle fibres. Signal is passed through the neuron to the muscle fibres. Motor neuron reaches to the muscle fibre at neuromuscular junctions. Motor neurons work in group to assist the contraction of single muscle in body. Force of Muscle contraction is controlled by the number of activated motor neurons..