Final answer:
Motor units are categorized into large and small types. Large motor units control gross movements with fewer neurons connected to multiple muscle fibers, while small motor units allow fine muscle control by connecting single neurons to fewer muscle fibers. This system permits a vast range of muscular activities, from delicate to powerful motions.
Step-by-step explanation:
Types of motor units include large and small motor units, both playing key roles in muscle function. Large motor units are characterized by a single motor neuron innervating a large number of muscle fibers, corresponding to gross movements such as thigh muscle extension, with one neuron potentially controlling thousands of fibers.
Small motor units, conversely, involve a single motor neuron innervating a much smaller number of muscle fibers, allowing for finer control in muscles like the extraocular muscles of the eye. This disparity in size and control allows for the nervous system's wide range of influence over muscular contraction, from delicate movements to powerful actions.
Through a process known as recruitment, the nervous system can progressively increase muscle contraction strength by sequentially activating these motor units from the smallest to largest size as more force is needed. The smallest motor units, with more excitable, lower-threshold motor neurons and smaller muscle fibers, are activated first, generating minimal contractile strength. Larger motor units are enrolled for greater muscle force requirements.