Abnormal Electric Activity
Insertional Activity
--Normal is 100-200 msec, >400 is prolonged
--Prolonged: normal variants, denervated muscle, myotonic discharges
--Reduced: periodic paralysis, replacement of muscle by CT or fat
--Normal variants: short trains of regular positive waves (familial or subclinical
myotonia); short, recurrent irregular bursts (muscular calves)
Endplate Potentials
--Spontaneous miniature endplate potentials occur randomly as quanta are released
--endplate spikes are rapid and irregular
--endplate spikes: “sputtering fat in a frying pan”
--endplate noise is monophasic negative waves
--endplate noise: “sea shell sound”
Fibrillation Potentials
--APs of single muscle fibers that are twitching spont without innervation
--fire in regular pattern
--Rate of 0.5 to 15/sec
--brief spike: triphasic or biphasic, 1-5ms duration, initial positivity, recorded
extracellularly
--positive wave: long duration, biphasic, initial sharp pos followed by long negative,
recorded from injured portion of muscle fiber
--can sometimes be mistaken for endplate noise or spikes
--“ticking of clock”
--LMN diseases: anterior horn cell diseases, polyradiculopathies, radiculopathies,
plexopathies, peripheral neuropathies, mononeuropahties
--Neuromuscular junction diseases: myasthenia gravis, botulism
--Muscle diseases: myositis, Duchenne dystrophy, myotonic dystrophy, myotubular
myopathy, late-onset rod myopathy, toxic myopathy, hyperkalemic periodic paralysis,
acid maltase deficiency, rhabdomyolysis, trichinosis, muscle trauma
Myotonic Discharges
--APs of muscle fibers firing in a prolonged fashion after external excitation
--potentials wax and wane in freq and amplitude
--can also have brief spikes or positive waves
--“Dive bomber”
--seen in: myotonic dystrophy, myotonic congenital, paramyotonia, hyperkalemic
periodic paralysis, polymyositis, acid maltase deficiency
Complex Repetitive Discharges
--APs of muscle fibers discharging in near synchrony at high rates
--may result from ephaptic activation of groups of adjacent muscle fibers
--abrupt onset and cessation
--uniform frequency, typically polyphasic
--“motor boat” that misfires occasionally
--seen in many chronic disorders
--Neuropathies: polio, ALS, spinal muscular atrophy, chronic radiculopathies, CharcotMarie-Tooth, chronic neuropathies
--Myopathies: polymyositis, Duchenne, Limb-girdle dystrophy, myxedema, SchwartzJampel syndrome
Fasciculation Potentials
--APs of a group of muscle fibers innervated by an anterior horn cell that discharges in a
random, irregular fashion
--may vary from a few per sec to less than 1/min
--“raindrops on a roof”
--most commonly seen in chronic neurogenic disorders in an overworked muscle
--normal: benign (fatigue), benign with cramps
--LMN: ALS, root compression, peripheral neuropathy, Jakob-Creutzfeldt
--metabolic: tetany, thyrotoxicosis, anticholinesterase medication
Myokymic Discharges
--Spontaneous muscle potential associated with fine, worm-like quivering of facial
myokymia
--Fire at 40-60Hz, regular intervals bursts of 0.1-10 sec
--“marching soldiers”
--facial disorders: MS, brainstem neoplasm, polyradiculopathy, facial palsy
--extremity: radiation plexopathy, chronic nerve compression (eg CTS)
Neurotonic Discharge
--MUPs associated with continuous muscle fiber activity at freq of 100-300 Hz
--may be continuous or recur in bursts
--Seen in: syndrome of continuous muscle fiber activity (Isaac’s syndrome or familial),
anticholinesterase poisoning, tetany, chronic spinal muscular atrophies
Cramp Potentials
--potentials fire rapidly at 40-60 Hz
--abrupt onset and cessation
--may fire irregularly in a sputtering fashion
--occur when muscle is activated strongly in a shortened position
--seen in: salt depletion, chronic neurogenic atrophy, benign nocturnal cramps,
myxedema, pregnancy, uremia
MUP Characterization
--Appearance
--Phases
--Amplitude (peak to peak)
--Turns
Short-Duration MUPs
--commonly low in amplitude with rapid recruitment
--occurs when there is a physiologic or anatomic loss of muscle fibers from the motor
unit or atrophy of component muscle fibers
--seen in: myasthenia gravis, botulism, early reinnervation after nerve damage, late stage
neurogenic atrophy, muscular dystrophies, periodic paralysis, polymyositis, toxic
myopathies, congenital myopathies
Long-Duration MUPs
--commonly high amplitude with poor recruitment
--occur when there is an increase in number of fibers in a unit or loss of synchrony of
firing fibers
--seen in: motor neuron diseases, axonal neuropathies with collateral sprouting, chronic
radiculopathies, chronic mononeuropathies, residual of neuropathy, chronic myositis,
inclusion body myositis, chronic polymyositis
Polyphasic Motor Unit Potentials
--five or more phases
--most commonly seen in myopathies in which there is a regeneration of fibers and
increased fiber density
--can also be from regeneration of axons in neurogenic diseases
Variation in MUPs
--caused by blocking of the AP d/c of individual muscle fibers within the motor unit
--seen in: myasthenia gravis, botulism, myositis, muscle trauma, reinnervation after nerve
injury, ALS
Recruitment
--in normal muscle: incr voluntary effort  incr rate of firing of indiv MUP  initiation
of discharges of additional MUP
--recruitment pattern: relationship of rate of firing of individual potentials to number of
potentials firing  constant for a particular muscle
--reduced recruitment: loss of MUPs, rate of firing too high compared to number of
potentials  seen in all neurogenic disorders or if whole motor unit is lost
--rapid recruitment: more motor units activated than expected, normal ratio of rate of
firing in relationship to number
Synkinesis
--caused by aberrant regeneration of axons following nerve injury  two different
muscles innervated by the same axon
--can result in voluntary potentials being mistaken for spontaneous activity