Matching focal and non-focal magnetic coil stimulation to properties of human nervous system: mapping motor unit fields in motor cortex contrasted with altering sequential digit movements by premotor-SMA stimulation

Abstract

Possible classifications of effects of magnetic coil (MC) stimulation are discussed, with the conclusion that the most useful is focal versus non-focal excitation. The mode of excitation of peripheral motor axons by the longitudinal-orthogonally orientated round MC is deduced from the insignificant latency shift in motor unit response when the current direction is reversed either by rotating the coil or by a switching device. A hypothesis is advanced of how peak membrane current entry and exit could occur only 1-3 nodes apart. The mode of excitation of cerebral cortex is more complex. Related to the orientation of the round MC, corticospinal neurons are: (1) directly excited and; or (2) indirectly excited through stimulation of corticocortical and other presynaptic inputs. Although the round MC can directly excite monkey corticospinal neurons at two sites, i.e. the initial segment and the node, the node is believed to be the main target in humans. The meaning of 'focality' of excitation is discussed as applied to MC stimulation of peripheral nerve and cerebral cortex. A potential conflict exists between focality and magnitude of response to MC excitation. However, by appropriately orientating the round MC, activation of all motor axons in one nerve (e.g., the median nerve at the wrist) can be achieved without coactivation of another (e.g., the ulnar nerve). By contrast, no orientation of the round MC, or use of a specially designed MC (e.g., double coil) over motor cortex permits all members of a defined set to be activated in isolation. Nevertheless, some members of the set can be activated in isolation with the MC over motor cortex. Response properties of individual motor units in the extensors of the digits when focally stimulating motor cortex with the figure '8' MC include: (1) Responses are variable to a given stimulus a little above threshold. Comparing responses by individual motor units with that of the population, or with other simultaneously recorded units revealed both coherent and independent sources of variability. (2) The scalp field from which the motor units could be driven by suprathreshold stimulation was of the order of 4-6 cm2. The fields were elongated in the antero-posterior axis, possibly related to the similar orientation of the junction region of the figure '8' MC. (3) Motor units initially excited by threshold MC stimulation were typically recruited early during voluntary contraction (confirming Hess et al. 1987).(ABSTRACT TRUNCATED AT 400 WORDS)