Neurophysiological characteristics of the motor evoked potential in rats, cats, and dogs
Abstract
The motor evoked potential (MEP) test is a recently developed and increasingly popular neurodiagnostic technique for evaluating motor system function in man and animals. Fundamental information is lacking on proper methods of performing and interpreting the MEP test. This thesis describes the quantitative and qualitative characteristics of MEPs in three animal models (rats, cats, and dogs). In practice, the motor system is stimulated to evoke a response which can be recorded at several points, particularly the spinal cord, peripheral nerves, and muscles. Six methods exist for stimulating the brain. The neurophysiological mechanisms which generate the spinal cord MEP from each of these methods is unclear. Multiphasic spinal cord MEPs can be recorded in rats, cats and dogs in response to two types of electrical brain stimulation. The amplitude of the average MEP in all three species ranges from 1-100 $\mu$V. In all three species, a change in the spinal cord MEP occurs with increasing stimulus intensity. MEPs with long latencies occur with low intensity stimuli; short latency peaks occur with higher intensity stimuli. Spinal cord MEPs in cats, generated with transcranial stimulation, contain individual peaks representing statistically unique populations which are significantly (p $<$ 0.05) different in latency from other peaks. This is true independent of the effect of cat weight (range: 2.2-4.2 kg) and proximity of the cranial stimulating anode (scalp, dura, and cortex). Furthermore, for each of the earliest two peaks in feline MEPs, a strength-duration curve can be derived. The mean chronaxie values range from 190-337 $\mu$sec and rheobase values range from 8.5-10.9 mA, which fall within the expected range for stimulation of central neurons. Qualitatively, the spinal cord MEP reflects activation of both pyramidal and extrapyramidal pathways. In cats, short latency peaks are mediated in ventral cord pathways and probably represent vestibulospinal and/or reticulospinal activation. Long latency peaks (generated with lower stimulus intensities) reportedly travel in the pyramidal tract. Thus, it is feasible to differentially monitor lateral versus ventral cord function, depending upon the stimulus and monitoring parameters. Additionally, canine spinal cord MEPs are more resistant to global ischemia than peripheral nerve MEPs.
Degree
Ph.D.
Advisors
Tacker, Purdue University.
Subject Area
Neurology
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