T. Drew, MOTOR CORTICAL ACTIVITY DURING VOLUNTARY GAIT MODIFICATIONS IN THE CAT .1. CELLS RELATED TO THE FORELIMBS, Journal of neurophysiology, 70(1), 1993, pp. 179-199
1. The discharge patterns of 91 identified pyramidal tract neurons (PT
Ns), located within the forelimb region of area 4 of the cat motor cor
tex, were recorded during the voluntary modifications of gait needed t
o step over obstacles attached to a moving treadmill belt. Recordings
were made simultaneously from flexor and extensor muscles acting aroun
d the shoulder, elbow, wrist, and digits of the forelimb contralateral
to the recording site. 2. Analysis of the changes in electromyographi
c (EMG) activity during the gait modification showed increases in the
activity of most flexor muscles of the shoulder and elbow, as well as
in the wrist and digit dorsiflexors, when the contralateral forelimb w
as the first to pass over the obstacle. This period of augmented activ
ity could be subdivided into two parts: one associated with the initia
l flexion of the limb that was needed to bring it above and over the o
bstacle (phase I), and the second associated with increased wrist dors
iflexor muscle activity before foot contact (phase II). 3. The dischar
ge frequency of a total of 57/91 (63%) of the recorded PTNs was signif
icantly increased during the gait modification when the limb contralat
eral to the recording site was the first to step over the obstacle; si
x of these neurons also showed a significant decrease in their dischar
ge in a different part of the step cycle. In a further 21/91 (23%) neu
rons, discharge frequency was only decreased, whereas the remaining 13
/91 (14%) PTNs showed similar patterns of activity both during control
walking and during the gait modifications. 4. Most of those neurons (
47/57) in which significant increases in firing frequency were observe
d, discharged maximally during the period of increased activity of the
physiological flexor muscles. Twenty-three of these cells (23/47) dis
charged maximally in phase I, and 12 (12/47) in phase II. A third popu
lation of PTNS (12/47) started to increase their discharge in the stan
ce phase of the step cycle immediately preceding the modified cycle. S
even (7/57) PTNs increased their discharge during the stance phase of
the modified cycle, and the remaining three could not be classified as
being preferentially related to any one part of the step cycle. 5. Th
e frequency modulation of 41/57 PTNs was less when the leg contralater
al to the recording site was the second to encounter the obstacle. In
many neurons there was also an appreciable change in the time in the s
tep cycle that peak discharge occurred. These changes in amplitude and
timing paralleled the changes observed in the temporal relationships
of the muscles. 6. Receptive fields could be determined for 71 of the
73 PTNs tested. Altogether 54/71 PTNs had receptive fields that includ
ed the forepaw, and 55/71 PTNs were activated by light brushing of the
skin surface (cutaneous receptive fields). In most cases neither the
discharge of the cell during control walking, nor its discharge during
the steps over the obstacles, could be explained, or predicted, on th
e basis of the receptive field. In the extreme case, PTNs with recepti
ve fields restricted to the plantar surface of the paw discharged duri
ng the swing phase of the gait modification. It is suggested that in m
ost cases the increase in cell discharge is due to visually triggered
central, rather than peripheral, inputs. 7. Comparison of the relative
time of activation of motor cortical neurons when the contralateral l
imb led, and when it trailed, suggested that these PTNs could be divid
ed into different populations of cells active at different times durin
g the pit modification. It is suggested that each of these populations
may regulate the activity of muscles active at different times in the
step cycle by modulating the activity of interneurons that either for
m part of, or that are influenced by, the central pattern generator fo
r locomotion.