Proprioceptive influences on somatosensory and motor cortex

Influence of plantar cutaneous afferents on early compensatory reactions to forward fall

The influence of cutaneous afferents in the compensatory reactions to a forward fall was investigated. Modification of cutaneous afferent activity was obtained in two different ways: first, by varying the initial pedal support conditions, secondly by anesthetizing the plantar foot sole. The initial pedal support conditions were: 1) bipedal posture, 2) unipedal posture, with contact and 3) unipedal posture, without contact. Nine healthy subjects participated in the control (without anesthesia) experimental session, of which three subjects participated in a session where the plantar sole was anesthetized. The compensatory reactions to a perturbation of balance of a subject initially with a bipedal stance, showed synchronized EMG activity in both Soleus muscles, starting on average at 59 ms, and a burst of EMG activity in the Tibialis Anterior of the starting foot after 200 ms. When the subject was in unipedal posture, the EMG responses on the side which was without support, showed several modifications: the EMG burst in Soleus was strikingly depressed, the response in Tibialis Anterior appeared earlier (mean latency 90 ms) and its magnitude was enhanced. When this foot was in contact with a rigid support, the Soleus showed a short burst of activity and the activity in Tibialis Anterior started at a mean latency of 150 ms.(ABSTRACT TRUNCATED AT 250 WORDS)

Interfering cutaneous stimulation and the muscle afferent contribution to cortical potentials

Cerebral potentials were recorded in response to selective stimulation using microelectrodes of muscle afferents in motor fascicles innervating the intrinsic muscles of the foot or at the motor point of abductor hallucis. The early components of these potentials (P40, N50 and P60) were consistently attenuated by continuous tactile stimulation of related skin areas and by electrical stimulation of digital nerves, timed so that the digital volley reached cortex approximately 5 msec before the muscle afferent volley. The same conditioning cutaneous inputs also attenuated the cerebral potentials evoked by selective stimulation of cutaneous afferents. These findings confirm that there are intermodality and intramodality interactions between low-threshold cutaneous and muscle afferents and between cutaneous afferents, respectively. The findings indicate that 'interference phenomena' (Kakigi and Jones 1986) can occur between different afferent modalities, and within any one modality, and cannot be used to determine the afferent species responsible for the test evoked potential.

A quantitative study of the distribution of neurons projecting to the precentral motor cortex in the monkey (M. fascicularis)

The relative numbers and locations of neurons projecting to the "forelimb" region of the precentral motor cortex were studied in three monkeys by using the retrograde transport of horseradish peroxidase. Within the forelimb area of the motor cortex itself, there are extensive and profuse interconnections. However, regions within this area receive afferents from very few neurons in other parts of the motor cortex representing hindlimb or head movements. Most of the motor cortical representation of the forelimb in the anterior bank of the central sulcus is devoid of callosal connections. In both the ipsilateral and contralateral hemispheres, the premotor (lateral area 6) and supplementary motor (medial area 6) areas dominate quantitatively the inputs to the motor cortical representation of the forelimb. The afferents from the premotor area are restricted and come from a region immediately behind the arcuate spur and adjacent parts of the superior and inferior limbs of the arcuate sulcus in the floor, caudal bank, and caudal lip of that sulcus. From the supplementary motor area (SMA), afferents originate from its whole rostrocaudal extent. Thalamic nuclear regions projecting to a restricted zone in the anterior bank of the central sulcus are recipients of cerebellar and somatosensory outputs. Involvement of more anterior parts of the motor cortex by the tracer labels thalamocortical cells, which are targets of pallidal output also. Within the first somatosensory cortex, cytoarchitectonic areas 1, 2, and 3a project to area 4. The projection from area 3a may provide one pathway by which short-latency peripheral inputs, especially from muscles, reach the motor cortex.

Responses of precentral cells during cooling of post-central cortex in conscious monkeys

A cooling plate was implanted over the forelimb representation in area 2 of the post-central region of cerebral cortex in two monkeys. Recordings were made of the discharges of thirty-seven movement-related neurones (thirty-four precentral and three post-central) in the forelimb motor representation of the cerebral cortex during active and passively imposed limb movements before, during and after cooling area 2 and local surrounding regions. Perfusion of the cooling plate with ice-cooled water for 3-5 min caused marked clumsiness of the conscious animal's forelimb movement and anaesthesia of the contralateral hand. Cooling of area 2 did not reduce the responses of area 4 cells to passive joint movements, nor did it alter the over-all pattern of activity of these cells during self-initiated lever pulling while that could still be performed. Cooling of area 2 did cause a significant increase in background cellular discharge in area 4 while the animal was at rest. Afferent impulses which are generated by passive joint movement and which have been shown to influence cells in area 4 of the conscious monkey at short latencies are probably not transmitted through cortico-cortical connexions from area 2.