**6. End plate spikes are different from fibrillation potentials**

the primary and secondary endings to the length and changes in the length of the muscle [21]. Dynamic gamma neurons innervate the bag 1 fibre by a p2 plate ending. Static gamma neurons innervate the bag 2 fibre and chain fibres by the trail endings. Dynamic skeletofusimotor beta neurons innervate the bag 1 fibre and extrafusal slow oxidative type 1 muscle fibres by p1 plate endings. Static beta neurons innervate the long chain fibres and extrafusal fast oxidative type 2 muscle fibres by p1 plate endings [13]. Each spindle receives about 7 motor axons, mean 3.2 beta and 3.8 gamma axons. The bag 1 fibre is almost always separately innervated by dynamic beta and gamma axons. Static beta branches supply exclusively the long chain poles. The bag 2 and chain fibres may receive a completely or variously segregated input in

6 Anatomy, Posture, Prevalence, Pain, Treatment and Interventions of Musculoskeletal Disorders

Where is the origin of EPSs if they are not nerve potentials or postsynaptic muscle fibre action potentials, activated by peripheral nerve injury? Partanen and Nousiainen [22] suggested that EPSs are action potentials of intrafusal muscle fibres such as small nuclear bag and nuclear chain muscle fibres inside the muscle spindles. EPSs can also be observed in active sites after manoeuvres for activating the gamma and beta motor activity such as passive stretch of the muscle, voluntary effort and repetitive nerve stimulation [9]. If multichannel EMG recordings are used, there are also different propagation patterns of EPSs such as local junction potentials as those observed in nuclear bag fibres [23], propagation for a very short distance as in nuclear chain fibres and propagation like MUPS but with the EPS firing pattern, as in beta (skeletofusimotor) motor units [9, 24, 25]. EPSs were also conjectured to be confined to the end plate zone of a muscle [26]. In fact EPSs can be found far from the end plate zone [9, 27]. It is a misconception that MEPPs are observed solely at the end plate zone, where the extrafusal neuromuscular junctions are situated [26]. Actually, MEPPs which are found far from the end plate zone, are mostly intrafusal representing synaptic activity of motor p2, p1 and trail endings. These MEPPs are often associated with EPSs, that is, gamma and/or beta motor unit potentials. At the end plate zone, MEPPs representing an alpha motor nerve terminal are not associated with EPSs [27, 28]. However, there are also muscle spindles at the end plate zone

Each pole of the muscle spindle receives 4–5 different motor axons and each gamma or beta axon innervates several spindles, but in a selective manner [13]. Thus junction and action potentials arise in several different spindles, when gamma and beta motor units are activated. This can also be seen in multichannel needle EMG recording. Synchronously firing EPSs may be found in remote active sites of a muscle, if these sites are innervated with the same gamma motor unit [27]. If EPSs in different remote active sites of a muscle are not innervated by the same gamma motor units, EPS firing is asynchronous. Intramuscular EPSs are not seen in the surface EMG, but MUPs of surface EMG are seen in the intramuscular sites with EPSs [27]. EPSs cannot be activated voluntarily, but voluntarily stopping of this activity is possible [27, 29]. Active spots with EPSs can also be stimulated with the concentric needle electrode, using electric impulses. With such stimulation, a reflex response resembling a myotatic reflex can be recorded [27]. Stimulation of an active spot with very small electric stimuli yields a response

each pole [13].

**5. Origin of end plate spikes**

and thus, also MEPPs with EPSs may be found there.

In clinical EMG, EPSs may be confused with fibrillation potentials, which are spontaneous action potentials of muscle fibres, or pieces of muscle fibres, which have lost contact with their motor axons. The development of fibrillation potentials needs time and there may be both rhythmic and irregular fibrillation sequences [30]. However, fibrillation potentials are distinctly different from EPSs both by the wave form and by the firing properties [9]. There is also a rare type of fibrillation-like activity, 'myokymic' fibrillations, which are elicited by so-called 'giant miniature end plate potentials' [31, 32]. The essential difference between EPSs and fibrillation potentials is the fact that denervation causes prolongation of the refractory period of the muscle fibre and thus the fibrillation potential cannot recur as promptly as action potential in a normal muscle fibre [33]. This causes the relatively long minimum interpotential interval of both rhythmic and irregular fibrillation potentials [31]. On the contrary, EPSs have numerous short intervals less than 30 ms [9].
