Activity of motoneurons during fictitious scratch reflex in the cat

(1) Intracellular recording of motoneurons of different hindlimb muscles: tibialis anterior (TA), gastrocnemius and soleus (GS), vastus crureus (Vast), posterior biceps and semitendinosus (PBSt), was carried out during the fictitious scratch reflex in decerebrate cats. (2) During the postural stage of the reflex, a depolarizaiton (3.8 mV on average) was observed in TA motoneurons accompanied by tonic discharge. No change of the membrane potential (MP) and no discharge were observed during this stage in GS, Vast and PBSt motoneurons. (3) In the rhythmical stage of the reflex, the MP of TA motoneurons changed only slightly during the 'long' (L) phase of the scratch cycle and remained at approximately the same level as during the postural stage. In this phase, motoneurons discharged at frequencies of 20-100 pps. In the 'short' (S) phase of the scratch cycle a strong repolarization occurred, the MP reached the same level as observed during resting conditons (MP0), and the discharge discontinued. (4) GS motoneurons were gradually depolarized during the second half of the L-phase. The depolarization reached its maximum (5.5 mV on average in relation to the MP0) on average in relation to the MP0) in the S-phase, and several action potentials were generated with intervals of 5-10 msec. Then, at the beginning of the L-phase, the motoneurons were repolarized and the MP reached the level of the MP0. The behavior of Vast motoneurons was essentially similar to that of GS motoneurons. (5) The PBSt motoneurons usually had two peaks of depolarization per cycle--in the S-phase and at the beginning of the L-phase. The maximal depolarization was 3.5 mV (on average). The motoneurons generated action potentials at one or both peaks of depolarization. (6) The possible organization of the central influences upon motoneurons of different muscles during scratching is discussed.

Generation of scratching. I. Activity of spinal interneurons during scratching

1. In decerebrate, curarized cats, stimulation of the cervical spinal cord evoked fictitious scratching (9), i.e., periodical activity of the hindlimb motoneurons with a discharge pattern typical of actual scratching (cycle duration about 250 ms, flexor phase about 200 ms, extensor phase about 50 ms). During fictitious scratching, extra-cellular records were obtained from 182 spinal neurons located in different regions of the gray matter cross section (except for the motor nuclei), from segments L4 and L5. 2. The firing rate of 73% of neurons was rhythmically modulated in relation with the scratch cycle. Most of the modulated neurons fired in bursts and were silent between bursts. They were located mainly in Rexed's (22) layer VII. 3. Burst onsets ("switchings on" of the neurons) were distributed rather evenly throughout the scratch cycle except for a small maximum at the very beginning of the cycle (the cycle was assumed to start with the termination of the extensor phase). Burst terminations ("switchings off") in the overwhelming majority of the neurons were distributed over the last-third part of the cycle. As a result, those neurons which began to fire earlier in the cycle usually had longer bursts, compared to the neurons which began to fire later. Besides, since there were very few switchings off in the first half of the cycle, the number of simultaneously active neurons increased during the first half of the cycle, reached the maximum somewhat later than the middle of the cycle, and considerably decreased by the end of the cycle. 4. With more intensive scratching, the firing rate in the bursts considerably increased in all neurons tested, while the duration of the scratch cycle changed only slightly. 5. A correlation between the burst position in the cycle and the behavior during the latent period of scratching (when stimulation of the cervical spinal cord had already been started but rhythmical oscillations had not yet appeared) was found in many neurons. Most of the neurons which began to fire at the beginning of the scratch cycle and had long bursts were tonically activated during the latent period. On the contrary, most of the neurons which fired in short bursts at the end of the cycle were either inhibited or not affected during this period. 6. A correlation betwen the burst position in the cycle and the frequency pattern was found in many neurons. In most of the neurons which began to fire in the first half of the cycle (except for the very beginning), the discharge rate increased in the course of the burst. In the remaining neurons, the discharge rate changed only slightly during the burst. 7. Hypotheses concerning organization of the spinal mechanism of scratching are discussed.

Generation of scratching. II. Nonregular regimes of generation

1. The activity of muscle nerves and that of spinal interneurons from the L4 and L5 segments was recorded during fictitious scratching (5), which was evoked in decerebrate curarized cats by stimulation of the cervical spinal cord. In some experiments, rhythmical generation was disturbed by stimulation of the fifth lumbar dorsal root (DL5). 2. Excluding the very beginning of scratching, rhythmical generation was usually rather regular: fluctuations of the cycle duration were less than +/-5%. But changes in the stimulation strength, in the stimulating electrode position, and in the hindlimb position led to changes of the generation regime. In different regimes, the mean value of the cycle duration could differ by 20-30%. No correlation was found between mean durations of flexor and extensor phases for different regimes. 3. Rhythmical generation was possible only if the hindlimb was put to "scratch posture," i.e., deflected forward. Generation immediately stopped when the limb was deflected backward, and immediately started when it was returned to scratch posture. 4. In some experiments, stimulation of the cervical spinal cord first resulted in generation of slow oscillations with the temporal pattern typical of stepping (cycle duration about 500 ms, flexor and extensor phases being almost equal to each other). Then, during 5-20 cycles, gradual transition to a normal scratch cycle (about 250 ms) occurred mainly due to considerable shortening (5-10 times) of the extensor phase. In some experiments, considerable spontaneous variations of the flexor phase were observed, while the extensor phase was constant. 5. A single stimulus applied to DL5 considerably affected the cycle duration. Repetitive DL5 stimulation,with a rhythm close to that of scratching, resulted in synchronization of the spinal generator by the stimuli. 6. Spinal interneurons recorded during transition from slow oscillations to a normal scratch cycle only slightly changed phases of their activity in relation to the activity of motoneurons. 7. A hypothesis is advanced that generation of different kinds of limb movements is produced by one and the same central spinal mechanism which can operate in different regimes. The role of sensory input for operation of this mechanism is discussed.

On the role of central program and afferent inflow in the control of scratching movements in the cat

Rhythmical scratching movements of the hindlimb were evoked in decerebrate and decapitate cats by stimulation of C1-C2 segments of the spinal cord. Movements of the limb and electrical activity of its muscles were recorded. All muscles were divided into two groups according to their activity. Muscles of the first group supported the limb in a propriate position; they were active during most of the cycle and relaxed during small intervals when muscles of the second group contracted. A deafferented limb was also capable of rhythmical scratching movements with approximately the same cycle duration. However, after deafferentation, the mean position of the limb changed and the amplitude of oscillations increased. This is due mainly to decreased activity of the first group muscles. In curarized preparations, stimulation of C1-C2 segments evoked a rhythmical process within the lumbosacral spinal cord ('fictive' scratching) with a cycle duration nearly the same as in normal scratching. Electrical activity of the muscle nerves during 'fictive' scratching resembled that of corresponding muscles during normal movements. 'Fictive' scratching could be easily elicited provided that the limb was put in a position similar to that of normal scratching.

Activity of interneurons mediating reciprocal 1a inhibition during locomotion

The activity of interneurons monosynaptically excited by 1a afferents and mediating the reciprocal inhibition of motoneurons of the antagonistic muscles was recorded in mesecephalic cats. The activity was resulted during locomotion evoked by stimulation of the "locomotor region" of the mesencephalon. The neurons turned out to be active in a definite phase of the step simultaneously with that muscle which supplied them by 1a afferent input. In preparations with de-efferent hindlimbs, stimulation of the "locomotor region" evoked a periodic process in the spinal cord ("fictive" locomotion); in this case bursts of activity in the interneurons also coincided with the activity of motoneurons of corresponding muscles. Under such conditions excitation of 1a afferents by the passive stretch of the muscle or by electric stimulation of the muscle nerve studied in an increase of the activity of interneurons, which depended on the phase of the cycle. The data obtained show that during locomotion there are at least two sources of inhibition of motoneurons of antagonistic muscles; (i) the activity of 1a afferents of the active muscle, mediating by corresponding interneurons; and (ii) signals coming through the same interneurons from the central mechanisms of generating stepping movements.