Motor units

Motorized gear drives, which are commonly called gear motors or motor reducers, are used extensively throughout industry. Any of the basic gear drives can be manufactured as motorized units. 

Bigland, B. Lippold, O.C.J. (1954). Motor unit activity in the voluntary contractions of human muscle. J. Physiol. 125,322-335. 

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. 

Multiple motor unit summation involves recruitment of motor units. As the number of motor units stimulated at any given moment increases, the strength of contraction increases. Asynchronous motor unit summation refers to the condition in which motor unit activation within a muscle is alternated. In other words, at one moment, some of the motor units within the muscle are activated, while other motor units are relaxed. This is followed by the relaxation of previously activated motor units and activation of previously relaxed motor units. Consequently, only a fraction of the motor units within the muscle generate tension at any given moment. Therefore, this type of summation may generate submaximal contractions only. 

Many factors influence the contractile activity of smooth muscle. The strength of contraction of multiunit smooth muscle may be enhanced by stimulation of a greater number of cells, or contractile units. This mechanism is directly comparable to motor-unit recruitment employed by skeletal muscle. As the number of contracting muscle cells increases, so does the strength of contraction. However, this mechanism is of no value in single-unit smooth muscle. Due to the presence of gap junctions, all of the muscle cells in the tissue are activated at once. 

Paul But we recruit different motor units and they are going on and off. 

Figure 13.11 Motor units. The diagram shows two motor units. In reality, the fibres would be much more tightly packed. In large fast nerves (e.g. supplying limb muscles), a single motor unit will innervate many fibres (up to 1000) via a synapse on each fibre.

Dantrolene interferes with the release of activator calcium through this sarcoplasmic reticulum calcium channel by binding to the RyRl and blocking the opening of the channel. Motor units that contract rapidly are more sensitive to the drug s effects than are slower-responding units. Cardiac muscle and smooth muscle are minimally depressed because the release of calcium from their sarcoplasmic reticulum involves a different RyR channel (RyR2). 

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

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. 

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