Define the three primary types of muscle fibers in humans by number. Which is in

Question

Define the three primary types of muscle fibers in humans by number. Which is intermediate in nature?

Are motor units composed of different fiber types? Are muscles composed of different fiber types?

What type of fiber (I or IIb) has the greatest [myoglobin]? What is the purpose of myoglobin?

What type of fiber (I or IIb) has the greatest [lactate dehydrogenase]? What is the purpose of lactate dehydrogenase?

Which fiber type (I or IIb) contains the myosin ATPase with the fastest rate of cycling?

Explanation / Answer

A motor unit is made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron's axonal terminals. Groups of motor units often work together to coordinate the contractions of a single muscle; all of the motor units within a muscle are considered a motor pool. The concept was proposed by Charles Scott Sherrington.

All muscle fibres in a motor unit are of the same fibre type. When a motor unit is activated, all of its fibres contract. In vertebrates, the force of a muscle contraction is controlled by the number of activated motor units.

The number of muscle fibers within each unit can vary within a particular muscle and even more from muscle to muscle; the muscles that act on the largest body masses have motor units that contain more muscle fibers, whereas smaller muscles contain fewer muscle fibers in each motor unit. For instance, thigh muscles can have a thousand fibers in each unit, while extraocular muscles might have ten. Muscles which possess more motor units (and thus have greater individual motor neuron innervation) are able to control force output more finely.

Motor units are organized slightly differently in invertebrates; each muscle has few motor units (typically less than 10), and each muscle fiber is innervated by multiple neurons, including excitatory and inhibitory neurons. Thus, while in vertebrates the force of contraction of muscles is regulated by how many motor units are activated, in invertebrates it is controlled by regulating the balance between excitatory and inhibitory signals.

To test motor unit stimulation, electrodes are placed extracellularly on the skin and an intramuscular stimulation is applied. After the motor unit is stimulated, its pulse is then recorded by the electrode and displayed as an action potential, known as a motor unit action potential (MUAP). When multiple MUAP’s are recorded within a short time interval, a motor unit action potential train (MUAPT) is then noted. The time in between these pulses is known as the inter-pulse interval (IPI). In medical electrodiagnostic testing for a patient with weakness, careful analysis of the MUAP size, shape, and recruitment pattern can help in distinguishing a myopathy from a neuropathy.

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