Motor unit action potentials

Clozapine-induced myokymia (weakness and reduced muscle tone, with undulating movements of the muscle and skin, accompanied by involuntary repetitive firing of grouped motor unit action potentials) has been reported (196). 

Motor imit potential (MUP) Spatial and temporal summation of aU single-fiber potentials innervated by the same motor neuron. Also referred to as the motor unit action potential (MUAP). 

FIGURE 6.16 Representative motor unit action potentials (MUAPs). (a) Change in di ies due to increase in stimulation, (b) Sequence of MUAPs during voluntary contraction. [Source Fang et al., 1997.]... 

Motor unit action potential (MUAP) Surface EMG (SEMG) Skeletal muscle Smooth muscle Needle electrode Surface electrodes 5 Hz-10 kHz 2-500 Hz 0.01-1 Hz 100 pV-2 mV 50 pV-5 mV ... 

Engel WK. (1975) Brief, small, abundant motor-unit action potentials. Neurology 25, 173-176. 

EMG did not detect recent denervation, and motor-unit action potentials were of normal size (note that routine EMG does not allow quantitation of the size of the later-activated type-2-fiher fast-twitch motor units, but ordy of the earher-activated type-1-fiber slow-twitch motor units) [9], Nerve conductions were normal. Muscle biopsy histochemistry showed moderate-prominent type-2 fiber atrophy, slight recent denervation, and shght- moderate established reinnervation. 

Concentric needle electromyography demonstrates a pattern of early-recruiting, short-duration, low-amplitude polyphasic motor-unit action potentials (MUAPs) in affected muscles. In addition, larger long-duration MUAPs are often also present, as well as fibrillation potentials, positive waves, and in-... 

When an action potential traveling down the axon of a motoneuron reaches the myoneural endplate, a process occurs that releases acetylcholine into the synaptic cleft and consequently depolarizes the postsynaptic membrane. A similar process probably occurs at cholinergic synapses in the central nervous system. In 1950 Fatt and Katz discovered a spontaneous subthreshold activity (MEPP) of motor nerve endings and were thereby led to the concept that acetylcholine is released in definite units (quanta) of 10 to 10 molecules. Electron microscopy subsequently revealed characteristic vesicles about 40 nm in diameter, clustered near presynaptic membranes. Subcellular fractionation procedures were devised by Whittaker and de Robertis for the isolation of these vesicles from brain homogenates in sucrose density gradients, and it was soon demonstrated that they were indeed concentrated reservoirs of acetylcholine. The hypothesis that the vesicles discharge the quanta of transmitter became irresistible. 

Nonelectrogenic functions. During the resting state, there is a continual slow release of isolated quanta of the transmitter that produces electrical responses at the postjunctional membrane [miniature end-plate potentials (mepps)] that are associated with the maintenance of physiological responsiveness of the effector organ. A low level of spontaneous activity within the motor units of skeletal muscle is particularly important because skeletal muscle lacks inherent tone. The activity and turnover of enzymes involved in the synthesis and inactivation of neurotransmitters, the density of presynaptic and postsynaptic receptors, and other characteristics of synapses probably are controlled by trophic actions of neurotransmitters or other trophic factors released by the neuron or the target cells. 

A few studies have examined the effect of hypothyroidism on the conduction velocity of action potential propagation in mixed nerves. In both hypothyroid rats and humans, nerve conduction velocity was either unchanged (Quattrini et al., 1993) or modestly reduced (Dyck and Lambert, 1970 Rao et al., 1980). Since these studies examined mixed nerves, a selective effect on motor axons can be obscured. Indeed, as mentioned above, we found that hypothyroidism reduced axon diameters only in type S motor units in the rat diaphragm muscle (Prakash et al., 1996a). GhnicaUy, hypothyroidism results in prolongation of the deep tendon teflex (Swanson et al., 1981), likely related to the slowing of conduction velocities in sensory and/or motor axons. 

As the net current source propagates toward the tendon, an extracellular potential field arises and follows the action potential complex. At a given field point, this phenomenon is observed as a temporal potential waveform (Figure 25.4b) however, a more complete picture of the phenomenon is obtained by considering the spatial distribution of potentials in the cross section of the motor unit. Figure 25.5 shows schematically how the concentric isopotential lines of individual fibers overlap with those of other fibers in the same motor unit. In a typical healthy motor unit (Figure 14.5a), the mean interfiber distance... 

Jitter Mean variation in interpotential interval between single-fiber action potentials of the same motor unit. 

FIGURE 17.14 Illustration of the general form of an action potential event of a single motor unit as recorded by a needle electrode. 

When the innervation of muscles is severed by accident or disease, spontaneous action events known as fibrillation waves appear several weeks after the trauma. These waves result from weak random contraction and relaxation of individual motor units and produce no muscle tension. Fibrillation potentials persist as long as denervation persists and as long as the muscles are present. The... 

EMG is a method to record the action potential along the muscle fiber s surface as a result of electrical activities of many motor units [4, 5], Application of EMG involved in medical research, sport training, rehabilitation, and ergonomics. Maximal Voluntary Contraction (MVC) is defined as a method to normalize the recorded data in percentage in order to standardize them. It solves the problem of how effective is a muscle in achieving a required task and what capacity level of muscle did the task [6]. 

Fig. 9.1 Muscle fibre action potentials from a motor unit displayed sequentially. The firing rate is 15 Hz...

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