John M. Brookhart

The relation between electro-cortical waves and responsiveness of the cortico-spinal system.

Abstract 1. 1. Studies of the responsiveness to electrical stimulation of the system of cortical neurones giving rise to evoked volleys in the medullary pyramid have been carried out in cats. 2. 2. Phasic variations in responsiveness and relayed activity have been correlated with phasic changes of cortical surface potential during the elicitation of augmenting waves and during spontaneous spindle waves. Relayed pyramidal volleys appear during the initial surface positivity of each of these cortical wave forms. During the positive and early negative phases of the surface potential waves, the responsiveness of the efferent neurone system is augmented. During the late phase of cortical negativity, and for 50–150 msec. thereafter, responsiveness of the efferent neurone system is depresses. These changes affect the indirect portion of the evoked pyramidal volley to a significantly greater extent than the direct portion of the volley. 3. 3. Responsiveness of this corticifugal neurone system has also been tested during the elicitation of that particular form of recruitment wave which develops upon stimulation of the midline nuclei of the diffuse thalamic projection system. During this type of recruitment response there is no relayed pyramidal volley. Neither is there any detectable alteration in responsiveness of the efferent neurone system. 4. 4. The marked differences in the effects associated with these electrically similar potential changes indicate the existence of at least two fundamentally different mechanisms subserving thalamo-cortical relationships.

Studies of the Integrative Function of the Motor Neurone

Publisher Summary This chapter presents studies of the integrative function of the motor neuron. The key role played by the motor neuron in the overt expression of the functions of the central nervous system is explained in the chapter. The stimulation of the lateral column input and the dorsal root input to the motor neurons gives rise to ipsilateral ventral root responses. In the case of the lateral column stimulation, the discharge occurs after a brief latency, is highly synchronized, and lasts for only a brief period. When the motor neuron responses are examined with microelectrodes recording from within the cell body, the excitatory postsynaptic potential (EPSP) evoked by dorsal root excitation is complex and multiple, whereas that evoked by lateral column stimulation is simple and unitary. The multisynaptic nature of dorsal root excitation may be avoided, and the EPSP can be simplified by the addition of pentobarbital to the bathing fluid. The initial portion of the response to lateral column stimulation decayed significantly more rapidly than the response to dorsal root excitation. All of these findings are consistent with the inference that the monosynaptic excitation of frog motor neurons occurs through axodendritic synapses from dorsal root fibers and over axosomatic synapses from lateral column fibers.

The effects of applied polarization on evoked electro-cortical waves in the cat☆

Abstract 1. 1. Observations on the responsiveness, wave-form, pattern of pyramidal relayed activity, and thalamic threshold stimulus intensity were made during polarization of the cortex giving rise to recruiting and augmenting waves. 2. 2. Augmenting waves were more sensitive to alteration by polarization than were recruiting waves. 3. 3. The latency of the peaks was unchanged by polarization with the exception of the augmenting negativity during surface positive polarization. In this case, the initial responses in the train had a decreased latency which disappeared with the growth of augmenting positivity in the later responses. 4. 4. The net effect of surface positive polarization was an increase in the amount of negativity recorded in each response. Surface negative polarization produced the opposite effect. These changes were explainable on the basis of increases and decreases respectively in the negative component common to both responses. The qualitatively similar reaction to polarization in both phenomena was considered evidence for the idea that the negative components represent similar forms of activity. 5. 5. Polarization produced no change in thalamic stimulus threshold, or the pattern of relayed pyramidal activity. 6. 6. The results were discussed in detail. It was concluded that the effects of polarization were best explained on the basis of alterations produced in the resting membrane potentials of graded-response structures located in the superficial layers of the cortex.

Single pyramidal fiber responses to electrical stimulation.

Summary 1. Responses of single pyramidal fibers to stimulation with rectangular electrical pulses of varying parameters have been studied in the cat and the macaque. 2. Those fibers conducting at velocities in excess of 35 m.p.s. do not respond repetitively to pulses of less than 4.0 msec, in duration. Some fibers have shown off-responses to pulses in excess of this duration. 3. All fibers studied have followed repetitive stimulation up to frequencies of 500 per second without response deficit.