Motor units, fibers

A nerve axon activating a particular muscle fiber is known as a motor nerve or motor neuron (Fig. 2A). This axon can in fact be just one branch of a large nerve that also interacts with many other fibers in the muscle. The set of fibers activated by a single nerve is known as a motor unit. Fibers in a motor unit need not be adjacent to each other, but may be distributed through a muscle. 

Describe the factors that influence the strength of skeletal muscle contraction including multiple motor unit summation, asynchronous motor unit summation, frequency of nerve stimulation, length-tension relationship, and diameter of the muscle fiber... 

Muscle fibers are incapable of mitosis. In fact, the number of muscle fibers per muscle is likely determined by the second trimester of fetal development. Therefore, enlargement of a whole muscle is not due to an increase in the number of fibers in the muscle, but rather to the hypertrophy of existing fibers. Because muscle fibers have no gap junctions between them, electrical activity cannot spread from one cell to the next. Therefore, each muscle fiber is innervated by a branch of an alpha motor neuron. A motor unit is defined as an alpha motor neuron and all of the muscle fibers that it innervates. 

A motor unit is defined as an alpha motor neuron and all of the skeletal muscle fibers it innervates. The number of muscle fibers innervated by an alpha motor neuron varies considerably, depending upon the function of the muscle. For example, the muscles of the eyes and hands have very small motor units. In other words, each alpha motor neuron associated with these muscles synapses with only a few muscle fibers. As a result, each of these muscles is innervated by a comparatively large number of alpha motor neurons. Densely innervated muscles are capable of carrying out more precise, complex motor activities. On the other hand, antigravity muscles have very large motor units. For example, the gastrocnemius muscle of the calf has about 2000 muscle fibers in each motor unit. Muscles with large motor units tend to be more powerful and more coarsely controlled. 

Fig. 2. (A) A set of fibers in a motor unit, activated by a single nerve axon also known...

Degenerating axons in the peripheral nerves can also be examined as described below, and electrophysiological estimates of motor unit number could be informative. Motor units are defined as a single motor neuron (axon) and the muscle fibers its terminals innervate. In motor neuron diseases, the number of motor units is anticipated to decrease as motor neurons die, but the size of motor units may increase with compensatory sprouting and reinnervation (e.g., (1)). Interpretation can be further confounded by factors such as a change in muscle fiber number or innervation of muscle fibers by multiple motor axons. Therefore, the best interpretation results from corroborating evidence from the spinal cord, nerve, synapse, and the muscle. 

In summary, all mammals possess a large fraction of hlgh-oxl-datlve muscle. The ordered pattern of motor unit recruitment Involves these high oxidative muscle fibers before the low-oxldatlve fibers, as exercise Intensity progresses from mild, to moderate, to severe. This progression favors an enhanced exercise performance at submaxlmal exercise Intensities, since the slow and fast—twitch red fibers are capable of repeated contractions for long periods of time. 

The motor unit consists of the motor neuron in the anterior horn of the spinal cord and the muscle fibers supplied by its branched axon closely applied at the motor end-plates. In this way a single motor neuron may supply a very large number of fibers, which tend to be grouped together but may be dispersed between different fasciculi. 

Sustained contraction of muscle fibers produces repetitive electrical potentials in them that increase in rapidity with strength of contraction and in number as more motor units take part. These potentials may be conducted from a coaxial needle electrode in the muscle, amplified, and their amplitude and frequency analyzed and recorded on a cathode-ray oscilloscope as the very typical full interference pattern of normal voluntary contraction. Characteristic departures from this normal electro-myogram are of very great importance in the diagnosis of muscle disease (B21a). 

In established neurogenic weakness and wasting due to lower motor neuron damage the characteristic appearances are those of groups of atrophic fibers, lately supplied by the damaged neuron, lying beside healthy fibers with intact innervation. This is in full accord with the structure of the motor unit, described above. In peripheral nerve disease, clinical evidence of sensory involvement, such as paraesthesias, may be present as weU. 

The motor unit has four components a motor neuron in the brain or spinal cord, its axon and related axons that comprise the peripheral nerve, the neuromuscular junction, and all the muscle fibers activated by the neuron. Like other cells, nerve and muscle cells have an external membrane that separates the inner fluids from those on the outside. The fluid on the inside is rich in potassium (K), magnesium (Mg), and phosphorus (P), whereas the fluid on the outside contains sodium (Na), calcium (Ca), and chloride (Cl). When all is quiet, the internal chemical composition of both nerve and muscle cells is remarkably constant and is called resting membrane potential. A primary reason for this constancy lies in the cells ability to regulate the flow of sodium— thanks to an enzyme in the membrane called Na+/K+ ATP-ase. Because the inside of the cell has less sodium than the outside, there is a negative potential (like a microscopic battery) of 70-90 mV. Under ordinary circumstances, the interior of the cell is 30 times richer in potassium than the extracellular fluid and the sodium concentration is 10-12 times greater on the outside of the cell. At rest, sodium tends to flow into cells and potassium oozes out. 

A constant need for ready energy exists because muscles must be able to respond on demand. Compounds such as creatine, phosphate, adenosine triphosphate (ATP), myoglobin, creatine kinase (CK), calcium, and a host of oxidative enzymes are all involved. Red musculature is usually more efficient than pale muscle because it contains more myoglobin and more oxidative enzymes. In any one motor unit, however, all the muscle fibers are the same type. 

Fig. 3. A motor unit of the peroneal nerve is seen branching and innervating several EDL muscle fibers. 220x (from Fri-scher and Strand, 1988).

The muscle fibers are organized into motor units, each innervated by one motor neuron (Fig. 

Nerve regrowth in neonatal rats appears to be even more sensitive to melanocortin administration than regrowth in the adult rat. Endplate morphology, muscle fiber diameter, muscle metabolism, motor unit size and electrophysiological parameters all respond positively to these peptides by 15 days of age, although there are variations in response during the first week of postnatal life. 

The cell bodies of somatic motor neurons reside in the ventral hom of the spinal cord (see Figure 6-1) the axon divides into many branches, each of which innervates a single muscle fiber, so more than 100 muscle fibers may be supplied by one motor neuron to form a motor unit. At each neuromuscular junction, the axonal terminal loses its myehn sheath and forms a terminal arborization that lies in apposition to a specialized surface of the muscle membrane, the motor end plate (see Figure 9-2). 

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