An analysis of the mode of digital-analogue conversion in the pyramidal tract cell

$79 AN ANALYSIS OF THE MODE OF DIGITAL-ANALOGUE CELL

CONVERSION IN THE PYRAMIDAL TRACT

YOUNGNAM KANG, KATSUAKI ENDO and TATSUNOSUKE ARAKI, Department of Physiology,

Faculty of Medicine, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606, Japan. The concept of frequency coding is well known. Digital information (D) involved in frequency-coded impulses is converted into analogue information (A) expressed in terms of postsynaptic potentials (PSPs) at excitatory or inhibitory synapses of a target neuron. The mode of digital-analogue (D-A) conversion is not well understood in comparison with A-D conversion. We studied D-A conversion in pyramidal tract (PT) cells of the motor cortex of the cat anesthetized with pentobarbital sodium, by analysing PSPs produced by repetitive stimulation of the parietal cortex (microstimulation) and the ventralis lateralis (VL) nucleus of the thalamus. It was found that not only the amplitude but also the duration of depolarizing PSPs was determined as a function of the input impulse frequency. Two opposite patterns of D-A conversion were observed following activation of the two input systems to PT cells; lower amplitude and longer duration of PSPs were changed into higher amplitude and shorter duration in proportion to increased input impulse frequency in the parieto-motor system or to decreased frequency in the VLmotor system. Inhibitory postsynaptic potentials (IPSPs) were found to play an important role in regulation of the duration of PSPs by curtailing excitatory postsynaptic potentials (EPSPs) depending on the input impulse frequency. With participation of IPSPs, each frequency-coded impulse can be converted into each nearly independent PSPs during repetitive synaptic activation, and accurate D-A conversion can be performed in PT cells.

RECRUITMENT OF RECIPROCAL VOLUNTARY MOVEMENT IN MAN. YASUHIRO

KAGAMIHARA*

Department of Neurosciences,

and

INHIBITION

REISAKU

UPON

INITIATION

OF

TANAKA

Neurobiology, Tokyo Metropolitan Institute for 2-6 Musashidai, Puchu-shi, Tokyo 183, Japan.

Phasic changes in reciprocal inhibition of the soleus motoneurons upon initiation of a c t i v e a n k l e d o r s i f l e x i o n were investigated in six healthy subjects in sitting position. H-reflex and r e a c t i o n time m e t h o d s w e r e used for t e s t i n g the motoneuron excitability and for controlling movement initiation, respectively. T h e size of the test H - r e f l e x e s and the s t r e n g t h of the v o l u n t a r y contraction force w e r e c h a n g e d systematically. Two phases of reciprocal inhibition were noticed. The f i r s t inhibition appeared almost simultaneously w i t h the o n s e t of a g o n i s t EMG a c t i v i t y and was weak. The s e c o n d inhibition developed approximately 1 0 0 m s a f t e r E M G onset, in the e a r l y stage of d y n a m i c c o n t r a c t i o n , and was strong. The e x t e n t of the i n h i b i t i o n in e a c h p h a s e d e c r e a s e d as the test s t i m u l u s was s t r e n g t h e n e d and the f i r s t i n h i b i t i o n t e n d e d to d i s a p p e a r at stronger stimuli. These inhibitions increased as v o l u n t a r y effort was strengthened. It was discussed that the first inhibition was g e n e r a t e d , at least partly, via Ia i n h i b i t o r y interneurons by directly descending motor commands and the second inhibition resulted from the converged facilitatory effect on Ia interneurons from descending commands and Ia inflows via a y-loop. The r e s u l t s s u p p o r t the h y p o t h e s i s of "e-y-linkage in r e c i p r o c a l inhibition" by Hongo, J a n k o w s k a and Lundberg (Exp. B r a i n Res., !: p365, 1969).