SENSORIMOTOR CORTICAL ACTIVITIES 



823 



cortex of the convulsixe activity without, however, 

 excluding the probable participation of subcortical 

 Structures (cf. 335), as occurs in photic epilepsy 

 induced with pentylenetetrazol (170). 



Somatosensory (5, 8, 287, 387) and also visual and 

 auditory (142) responses, often with characteristics 

 diflferent from those recorded in the respective areas 

 of specific projections, have been observed in records 

 from the motor area (cf. 15, 55). The data referring 

 to the activity of single units within the motor and 

 somatosensory cortex ha\e been presented in Chapter 

 XV'II by Rose & Mountcastle and in Chapter 

 XXXIV by Patton & Amassian in this work. We 

 shall, therefore, confine ourseKes to a few aspects of 

 this problem. Li (personal communication) has ob- 

 ser\ed that the firing of pyramidal neurons within 

 the motor area mav be influenced in various degrees 

 by sensory \olleys initiated in skin nerves. Activation 

 of these pyramidal neurons can sometimes occur as a 

 consequence of the synchronous sensory volley. 

 Moreover, cortical stimulation may produce a 

 sustained depolarization of pyramidal neurons, as 

 shown in the intracellular records of Phillips (361- 

 362). This observation may indicate that the ex- 

 citability of these neurons is largely determined by a 

 background synaptic impingement frorn other cortical 

 cells. Many of the units studied by Li within the motor 

 cortex, which could not be classified as pyramidal 

 neurons on the basis of antidromic stimulation of 

 the pyramidal tract, have shown either an increase 

 or a decrease of their previous acti\it\' as a con- 

 sequence of the sensory volley. 



From these observations it may be concluded that 

 sensory afferents initiate within the sensorimotor 

 cortex changes in unit activity which in turn may 

 influence the motor output. However, no recon- 

 struction of the processes of integration on the basis 

 of studies of single unit discharges is as yet possible. 

 Also relevant to this problem is the question of the 

 influences exerted by recurrent axon collaterals of 

 the pyramidal neurons on adjoining cortical cells. 

 The data presented in Chapter XXXIV by Patton & 

 Amassian and new findings by Phillips (363) seem to 

 indicate that the activity of the axon collaterals may 

 not result exclusively in either excitatory or inhibitory 

 processes. 



Turning now to the role of deep somatic sensibilities 

 in the regulation of motor acts, it has been demon- 

 strated that alteration in the tension of the muscle 

 may influence the mo\-ement elicited by stimulation 

 of the motor cortex (171, 172, 469). Although these 

 effects, due to the action exerted by propriocepti\e 



and kinesthetic inputs, certainly occur mostly through 

 spinal mechanisms as in the case of scratch reflex 

 (428), section of the dorsal roots seems to deprive 

 higher centers of a component necessary to their 

 normal activity (336). It appears that the animal 

 fails to use the deafferented limb, despite the re- 

 tention of motor capacity, except in stereotyped and 

 instinctive behaxior, such as is seen in defensive acts 

 (336). Older evidence suggests that the threshold of 

 the motor area to direct stimulation becomes higher 

 following this operation (336). Further observations 

 support the view that the pyramidal tract rises in 

 neurons which are not normalh' autonomous in the 

 initiation of motor activity but are dependent on 

 afferent volleys from the periphery in attaining their 

 usual levels of excitability. Walshe (464, 465) has 

 summarized the view that it is the afferent system, 

 through its different receptors, which is concerned 

 in this aspect of cortical integration (cf. 259). 



The question of the representation of deep somatic 

 sensibilities in the motor cortex has been previously 

 discussed. No answers are yet available to numerous 

 problems in this field. A puzzling observation is that 

 section of the L'-shaped Ijundle, between the pre- 

 central and postcentral gyri (357), or isolation of the 

 motor cortex (251, 355, 451) is without consequence 

 on motor performance. If the kinesthetic sensibilities, 

 as elaborated in the cortex, play a role in the regula- 

 tion of motor mechanisms, the further possibility 

 must be considered that they play their part at 

 subcortical levels, or that somatosensory inputs are 

 available at the motor cortex, either directly or 

 indirectly. There is disagreement about this second 

 possibility (see page 814), due in part to the singular 

 difficulties in the verification of hypotheses about the 

 way kinesthetic sensibilities might influence patterns 

 of motor activity at the cortical level. 



The special senses, including visual and auditory, 

 may likewise participate in the patterning of central 

 activities related to motor functions. It is commonly 

 held that acoustic stimuli are responsible for various 

 forms of orienting reflexes which may inv-olve not 

 only the musculature of the ear, such as may be seen 

 in certain instances in lower animals, but may also 

 involve most of the body. It would seem, however, 

 that while in these cases most of the movement is of 

 subcortical origin, cortical participation is required 

 when acoustic information is the only source of 

 afferent inputs on which motor activity might be 

 based in a sequence of skilled, purposive movements. 



Visual inputs usually participate to a great degree 

 in the regulation of motor performance. The rich- 



