Decorticate Cats Research Articles

Mechanisms of ventilatory control: who is really in charge?

Mechanisms of ventilatory control: who is really in charge?

Synchronization of somato-sympathetic outflows during exercise: role for a spinal rhythm generator.

The synchronization of somatic motor outflow and sympathetic efferent activity during exercise has important functional consequences. For example, a certain degree of sympathetic vasoconstriction to active skeletal muscle vascular beds may be required to prevent an excessive decrease in vascular resistance during intense dynamic exercise involving a large skeletal muscle mass. Moreover, the redistribution of blood flow away from non-exercising vascular beds to contracting skeletal muscles is necessary to meet their increased metabolic demands. Historically, the nervous system has been separated arbitrarily into a volitional (somatic) and a reflex (autonomic) control system that does not necessitate volitional input (Koizumi & Brooks, 1972). However, during exercise this partition of the somatic and autonomic control systems is replaced by an overlapping control system that influences both somatic and sympathetic outflows in a parallel fashion. Previous work has suggested that the integration and co-ordination of the somatic and sympathetic motor responses during exercise were mediated by a supraspinal neural mechanism referred to as central command (Waldrop et al. 1996). However, there is an accumulating body of evidence, including papers on the role of the spinal cord in the regulation of sympathetic outflow (Weaver & Stein, 1989), suggesting that rhythmic neural activity destined for somatomotor and sympathetic outflows may be under the control of intrinsic neuronal circuitry in the spinal cord. In this issue of The Journal of Physiology, the paper by Chizh et al. (1998) demonstrates for the first time that some components of somatic motor and sympathetic neural outflows are synchronized by a spinal mechanism. The authors studied the relationship between sympathetic and somatic motor outflows from the thoraco-sacral spinal cord using an isolated trunk-hindquarters preparation of the adult mouse. They found that some of the activity in both somatic and sympathetic outflows was coupled. However, since there was also sympathetic neural activity that showed burst discharges in the absence of somatic motor activity, the spinal mechanism responsible for generating sympathetic burst activity was not the same mechanism that was responsible for somato-sympathetic coupling. Therefore, there remained spinal mechanisms that were capable of generating bursting activity independently of somatic and sympathetic activities. When NMDA was added to the perfusate to induce rhythmic somatic motor activity, a dose-dependent bursting pattern in sympathetic efferent and somatic motor nerves was evoked that was coupled at similar frequencies. Correlational analyses found a strong coherence between somatic motor outflow and sympathetic efferent discharge when triggered from peaks in somatic motor activity. However, this synchronization was not found when triggered from the peaks of sympathetic discharge. This finding led Chizh et al. (1998) to hypothesize that the generating mechanism responsible for somatic locomotor activity was the same mechanism responsible for coupling somatic outflow to sympathetic activity. These findings support the notion that the spinal cord possesses the necessary neuronal ‘machinery’ for generating and coupling sympathetic discharge with somatic motor outflow independently of supraspinal structures (i.e. the spinal rhythm generator represents a spinal central command). Important adjustments occur in the heart and blood vessels during exercise in order to provide the appropriate oxygen delivery to match the metabolic demand of contracting skeletal muscle. The coupling of somatic motor and sympathetic outflows is mediated by supraspinal neural mechanisms (supraspinal central command) in humans and animals (Fig. 1). In humans, intense intermittent static exercise produced a one-to-one synchronization of somatic motor activity with muscle sympathetic nerve activity that was purportedly directed to skeletal muscle vasculature (Victor et al. 1995). This synchronization persisted in the presence of both partial neuromuscular blockade (i.e. to increase central command) and after local anaesthetic block of the arm nerves to eliminate afferent feedback from skeletal muscle. Likewise, electrical and chemical stimulation of the hypothalamic locomotor region in decorticate cats after neuromuscular blockade evokes fictive locomotion that produces increases in arterial blood pressure (Waldrop et al. 1996) and synchronization of sympathetic nerve activity with hindlimb motor nerve activity (Hajduczok et al. 1991). And, as seen in humans, the synchronization of somatic motor activity and sympathetic discharge was unaffected by peripheral feedback mechanisms. These findings suggest that central command signals of supraspinal origin can regulate and co-ordinate sympathetic and somatomotor outflows during exercise. Figure 1 Synchronization of somatic motor outflow and sympathetic efferent activity during exercise is mediated by supraspinal and spinal mechanisms. The important work of Chizh et al. (1998) extends our understanding of the synchronization of somatic motor activity and sympathetic outflow by highlighting the role of a spinal somato-sympathetic generator (Fig. 1). Also, the coupling of somato-sympathetic neural outflows in the spinal cord may represent an exquisite mechanism to finely tune sympathetic discharge to the segmental level of somatic motor outflow during exercise. Finally, {fontsize the degree to which supraspinal central command signals influence the somato-sympathetic spinal generator needs to be studied to further our understanding of somato-sympathetic coupling during volitional exercise.

Decortication and visual deprivation do not affect two types of directional properties of LGNd relay cells in the cat

Direction sensitivity is an important receptive property of visual cortical cells.On thesubcortical visual pathways,recent studies have shown that about 32% of LGNd relay cellsand 25% of retinal ganglion cells presented direction sensitivity.This kind of directionsensitivity was also present in LGNd neurons of decorticate cats and dark-reared cats.Zhou et al.described two different types of direction sensitive cells in cat LGNd according to their response properties of different measures.

Alterations of visual climbing fiber response properties in cat cerebellar flocculus after cerebral cortical lesions.

1. In the anesthetized decorticate cat, we recorded complex spike (CS) activity together with concomitant simple spike (SS) activity of the cerebellar flocculus (FL) Purkinje cells that responded to large-field visual pattern movement. The cerebral cortical contributions to the visual response characteristics were investigated by comparing the present results with the previous results of the anesthetized normal cat. 2. The direction-selective characteristics of the CS responses at a low (2 degrees/s) stimulus velocity are well preserved after the decortication. As in the normal cat, the cells are divided into two major types: the horizontal type that prefers horizontal stimuli and the vertical type that prefers vertical stimuli. The CS rate of the former increases during stimuli directed contralaterally to the recording site and decreases during ipsiversive stimuli, whereas that of the latter increases during upward stimuli and decreases during downward stimuli. Reciprocal properties of the CS and SS responses also are preserved: when the CS rate increases, the SS rate decreases and vice versa. 3. For both types, the CS modulation at high stimulus velocities is reduced after the decortication. The half-reduction of the average modulation occurs at approximately 100 degrees/s stimulus velocity compared with the modulation at 2 degrees/s in the normal cat, whereas it occurs at a lower velocity approximately 50 degrees/s in the decorticate cat. 4. For both types, the CS responses usually are driven through either eye in the normal cat. After the decortication, the CS modulation through the contralateral eye is reduced. 5. For both types, the receptive field of the ipsilateral eye extends into both visual hemifields in the normal cat. The CS responses through the contralateral visual hemifield are abolished, whereas those through the ipsilateral visual hemifield are preserved after the decortication. The CS modulation during stimuli restricted within a small central visual field (15 degrees x 15 degrees) projecting to the area centralis is prominent (approximately 70% of the full-screen response) in the normal cat and is sharply reduced (30%) after the decortication.(ABSTRACT TRUNCATED AT 400 WORDS)

Initially Organized Arrangement of Orientation and Direction Sensitivities of LGNd Relay Cells in Decorticate Cats

Since Levick et al. found and studied the orientation biases of retinal ganglion cells of cats, there has been increasing interest in subcortical orientation sensitivity. Shou et al. studied the orientation sensitivity of dorsal geniculate nucleus (LGNd) relay cells of the cat in details. They found that cells with similar preferred orientation are clustered in the LGNd, which is apparent for all cell types (ON, OFF, X, Y) and cells with different receptive field locations in retina. It is suggested that the initial sorting of orientation functional columns in the visual cortex may begin in the LGNd.

Central command increases sympathetic nerve activity during spontaneous locomotion in cats.

A controversial issue in exercise physiology is the relative contribution of central command versus afferent input from contracting muscles and baroreceptors in the regulation of sympathetic nerve activity (SNA) during exercise. Recent studies of exercising humans have suggested that central command increases cutaneous sympathetic sudomotor nerve activity but have challenged the concept that central command contributes importantly to increases in sympathetic vasoconstrictor nerve activity to skin and skeletal muscle. The purpose of this study was to examine the influence of central command on renal SNA and lumbar SNA during spontaneous locomotion in decorticate cats. Unanesthetized decorticate cats that developed locomotion spontaneously or during electrical stimulation of the subthalamic locomotor region were studied in the presence and absence of input from skeletal muscle and baroreceptor afferents. Spontaneous rhythmic locomotion in the unparalyzed state was associated with significant increases in mean arterial pressure (MAP) from 106 +/- 10 to 133 +/- 11 mm Hg (p less than 0.05) and increases in renal SNA of 301 +/- 100% (p less than 0.05). During spontaneous fictive rhythmic locomotion in paralyzed cats, there were also significant (p less than 0.05) increases in MAP (43 +/- 6%), renal SNA (183 +/- 32%), and lumbar SNA (223 +/- 83%). Baroreceptor denervation did not attenuate increases in MAP, renal SNA, and lumbar SNA during locomotion. During electrical stimulation of the subthalamic locomotor region in paralyzed cats, MAP increased by 43 +/- 17% (p less than 0.05), and renal SNA increased by 175 +/- 47% (p less than 0.05). These findings indicate that central command is capable of increasing sympathetic neural drive in unanesthetized decorticate cats. This increase in sympathetic drive occurs even in the absence of feedback from contracting muscles or from arterial and cardiopulmonary baroreceptors.

Analysis of vertebrate eye movements following intravitreal drug injections. IV. Drug-induced eye movements are unyoked in the turtle

1. Eye movements of awake turtles were measured from both eyes simultaneously using two search-coil contact lenses. Optokinetic nystagmus (OKN) was evoked by full field patterns moving horizontally at different stimulus velocities. Intravitreal injections of either bicuculline or 2-amino-4-phosphonobutyrate (APB) were then made into one eye, after which eye movements were again recorded under similar stimulus conditions. Several days later, eye movements were again recorded and recovery was observed. 2. The effects of these two synaptic drugs on the optokinetic responses of the injected eye were similar to those previously reported in turtles, rabbits, and decorticate cats. APB, which blocks the retinal ON pathways, completely blocked visually evoked responses to any stimulus direction or velocity presented to the injected eye. On the other hand, the uninjected eye was still responsive to optokinetic stimuli. This difference between the eyes is consistent with the nonconjugate nature of OKN in the turtle. 3. After bicuclline application, the injected eye displayed a spontaneous nystagmus with its slow phase in the temporal-to-nasal direction. The movements of the injected eye were independent of stimulus direction or a range of stimulus velocities. During that effect, the eye contralateral to the injection still responded to visual stimuli in a direction- and velocity-dependent manner. For example, if the uninjected eye was exposed to optokinetic stimuli moving temporal-to-nasal, both eyes would then move in their respective temporal-to-nasal directions. This nonconjugate ocular behavior is similar to that seen when each eye of a normal turtle was exposed to its temporal-to-nasal stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

An intracellular study of muscle primary afferents during fictive locomotion in the cat

1. Presynaptic activity of identified primary afferents from flexor, extensor, and bifunctional hindlimb muscles was studied with intra-axonal recordings during fictive locomotion. Fictive locomotion appeared spontaneously in decorticate cats (n = 9), with stimulation of the mesencephalic locomotor region (n = 4), and in spinal cats injected with clonidine or nialamide and L-DOPA (n = 4). Representative flexor and extensor muscle nerves, recorded to monitor the locomotor pattern and dorsal rootlets of the sixth and seventh lumbar segments, were recorded simultaneously to monitor dorsal root potentials (DRPs). 2. From responses to muscle stretches and, in some instances, twitch contractions of the parent muscle, 75% of the single units examined were putatively identified as spindle afferents (40/53). On the basis of conduction velocity and stimulation threshold, 73% of these were further classified as group I fibers (29/40), the rest as group II fibers. 3. All units (n = 53 with resting potential more negative than -45 mV) showed fluctuations of their membrane potential (up to 1.5 mV) at the rhythm of the fictive locomotion. Subsequent averaging of these fluctuations over several cycles revealed that 89% of all units displayed a predominant wave of depolarization during the flexor phase, followed by a trough of repolarization. In 79% of the units, there was also a second, usually smaller, depolarization during the extensor phase. The relative size of each wave of depolarization could vary with different episodes of fictive locomotion in the same unit and among various afferents from the same muscle in the same experiment. 4. The firing frequency of some afferents from the ankle flexor tibialis anterior (5/16) and the bifunctional muscle posterior biceps-semitendinosus (4/15) was phasically modulated along the fictive step cycle. The maximum frequency always occurred during the flexor phase, i.e., during the largest depolarization of the unit. Because of the absence of phasic sensory input in the curarized animal, we assume that the phasic discharges were generated within the spinal cord and antidromically propagated. Phasic firing was never encountered in afferents from extensor muscles such as triceps surae (0/15) and vastus lateralis (0/4). 5. The results demonstrate that the pattern of rhythmic depolarization accompanying fictive locomotion is similar for the majority of flexor, extensor, and bifunctional group I (and possibly group II) muscle spindle primary afferents. They further indicate that there is a specific phasic modulation of antidromic firing for some flexor and bifunctional muscle spindle afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Phase-dependent modulation of dorsal root potentials evoked by peripheral nerve stimulation during fictive locomotion in the cat

To help elucidate the role of presynaptic mechanisms in the control of locomotor movements, the transmission of PAD pathways was investigated by recording dorsal root potentials (DRPs) evoked by electrical stimulation of cutaneous and muscle nerves of both hindlimbs at various phases of the fictive step cycle. Fictive locomotion occurred spontaneously in decorticate cats or by stimulating the mesencephalic locomotor region (MLR) as well as in low spinal cats injected with nialamide and l-DOPA. Evoked DRPs were superimposed on a fluctuating DRP accompanying the fictive locomotor rhythm (locomotor DRP) which typically consisted of two peaks of depolarization per cycle, the largest peak occuring during the flexor phase. The amplitude of evoked DRPs was substantially modulated throughout the locomotor cycle and followed a similar modulation pattern for all stimulated nerves whether ipsilateral (i-) or contralateral (co-). The amplitude of evoked DRPs decreased at the beginning of the flexor phase, dropped to a minimum later in the flexor phase and then increased during the extensor phase where it became maximum. Results were comparable in decorticate and spinal preparations and for L 6 and L 7 rootlets with cutaneous and muscle nerve stimulation. It is noteworthy that the modulation pattern for a given rootlet was similar for i- and co- stimulation, even though the bilateral locomotor DRPs fluctuate out-of-phase with each other, subjecting the stimulated fibres to opposite presynaptic polarization changes. This suggests that the modulation may depend more on the presynaptic mechanisms of the receiving fibres than on those of the stimulated fibres. These results demonstrate that the transmission in spinal pathways involved in primary afferent depolarization (PAD) is phasically modulated by the activity in the spinal locomotor network. It is further suggested that the presynaptic inhibition associated with PAD evoked by movement-related sensory feedback during real locomotion could be modulated in a similar way.

Phase-dependent modulation of primary afferent depolarization in single cutaneous primary afferents evoked by peripheral stimulation during fictive locomotion in the cat

Previous results from our laboratory have shown with intra-axonal recordings that hindfoot cutaneous primary afferents are subjected to rhythmic depolarizations during fictive locomotion (L-PAD) suggesting that cutaneous presynaptic mechanisms are activated by the central locomotor program. In this study, we examined the transmission in pathways responsible for primary afferent depolarizations (PAD) of cutaneous fibres during spontaneous fictive locomotion in decorticate cats and in spinal cats injected with nialamide and l-DOPA. PADs were evoked (E-PADs) by electrical stimulation of peripheral nerves and recorded intra-axonally with micropipettes in identified superficialis peroneal (SP; n = 7) and tibialis posterior (TP; n = 17) cutaneous primary afferents. Results showed that the amplitude of E-PADs, which were superimposed on the L-PAD, was deeply modulated throughout the locomotor cycle; decreasing to reach a minimum during the flexor phase and increasing to a maximum during the extensor phase. The results were not statistically different in fibres of the two nerves and in both types of preparation. The amplitude of E-PADs was always maximum during the extensor phase whether there was a large L-PAD or not during that phase. This suggests that the presynaptic mechanisms activated by central locomotor networks (L-PAD) and those activated by peripheral inputs (E-PAD) may in part be controlled differently. The results thus show that the transmission in PAD pathways activated by cutaneous inputs is phasically modulated by the central pattern generator for locomotion. This strongly suggests that the presynaptic inhibition in cutaneous fibres evoked by the movement-related feedback during real locomotion could be similarly modulated.

Rhythmic antidromic discharges of single primary afferents recorded in cut dorsal root filaments during locomotion in the cat

Single units recorded in the proximal stump of cut dorsal root filaments were found to antidromically discharge rhythmically during fictive locomotion in decorticate and paralyzed cats. Some units fired throughout the period of flexor or extensor nerve activity, whereas other units discharged near the transitional phases. Similar findings were made in acutely spinalized and paralyzed cats injected with L-DOPA, as well as in non-paralyzed decorticate cats walking on a treadmill. These results suggest that different types of primary afferents may be depolarized cyclically at different specific time in the step cycle by the central pattern generator for locomotion, and that this central control of the primary afferents may be involved in the modulation of the reflex transmission observed during locomotion.

Discharges of forelimb spindle primary afferents during locomotor activity in the decorticate cat

In the acute decorticate cat, the discharge patterns of single spindle primary afferents from various forelimb muscles during locomotion have been examined in relation to the extrafusal activity. In a given muscle under isometric conditions, the activation of spindle primary endings was closely related to that of extrafusal muscle fibers. since this increase in spindle discharge could occur in the absence of detectable electromyographic activity in the parent muscle, it resulted, at least to some extent from an enhanced activity in gamma efferents. The data are discussed in relation to previous studies of forelimb efferent activity and hindlimb fusimotor control in the same preparation.

Alterations in response properties in the lateral and dorsal terminal nuclei of the cat accessory optic system following visual cortex lesions.

The response properties of cells in the lateral (LTN) and dorsal (DTN) terminal nuclei of the accessory optic system (AOS) were examined in 14 cats which underwent unilateral visual cortex ablation. Following decortication, single units in the LTN and DTN no longer showed the high degree of binocular convergence characteristic of the intact animal, but instead LTN and DTN units became almost completely dominated by the contralateral eye. In addition, responsivity of LTN and DTN cells to high stimulus velocities was abolished by removal of cortical input. This decrement in high velocity response was observed in both the excitatory and the inhibitory components of the velocity response profile. While the incidence of direction selective neurons in both the LTN or the DTN was not affected by decortication, the distribution of preferred and nonpreferred directions was dramatically altered in the LTN, and to a lesser extent in the DTN. In the LTN, there was a severe reduction in the number of cells which displayed maximal excitation for upward stimulus motion. Instead, most LTN units in the decorticate cat preferred downward directed stimulus motion. In the DTN, most units still preferred horizontal stimulus motion as in the intact animal, but the overall distribution of preferred directions displayed a clear downward vertical vector component. In other respects, such as receptive field size and position in visual space, on/off responses, and resting discharge rate, LTN and DTN units appeared unaffected by cortical lesions. These experiments demonstrate that the cortical input to the LTN and DTN plays a highly significant role in the formation of response properties of cells located in these nuclei. The results presented in this report indicate that the visual cortex is a major source of ipsilateral eye input, high velocity responses, and upward direction selectivity for the AOS units examined in these experiments.

Static and dynamic fusimotor controls in various hindlimb muscles during locomotor activity in the decorticate cat

During stereotyped locomotor movements of the acute decorticate (thalamic) cat, the static and dynamic fusimotor actions upon spindles of various hindlimb muscles were studied in relation to extrafusal activities. For this purpose, the rhythmic alterations in the responses of spindle endings to appropriate repetitive stretches were analyzed. Co-activation of alpha motoneurones with static and dynamic fusimotor neurones throughout the locomotor cycle was demonstrated. Quantitative differences were found between the fusimotor influences on spindles of various muscle groups. The static action was stronger than the dynamic one in pure flexor muscles; a predominantly dynamic action occurred in pure extensor ones while both actions were balanced in proximal bifunctional muscles. These results are discussed in relation to the known characteristics of fusimotor innervation and to the control of locomotor movement.

Exercise hyperpnea and locomotion: parallel activation from the hypothalamus.

Unanesthetized decorticate cats walked or ran normally on a treadmill either spontaneously or during electrical stimulation of the subthalamic "locomotor" region. The respiratory response usually preceded the locomotor response and increased in proportion to locomotor activity despite control or ablation of respiratory feedback mechanisms. Respiration increased similarly in paralyzed animals during fictive locomotion despite the absence of muscular contraction or movement. Hypothalamic command signals are thus primarily responsible for the proportional driving of locomotion and respiration during exercise.

Is there localized cerebral cortical influence on hypoxic ventilatory response?

A cortical inhibitory influence on net ventilatory response to hypoxia is inferred from the hyperexcitable response of decorticate cats. Electrical stimulus exploration of the cortex localizes the regions inhibitory to breathing predominantly on the orbitals gyrus of the frontal lobes. The present experiments were designed to determine whether ablations of those regions of the orbital gyrus would replicate the effect of total decortication of hypoxic ventilatory response. Steady state hypoxic ventilatory response curves were determined in 8 cats under both hypocapnic and eucapnic conditions, first in the control state and second at various times after regional ablations of the orbital gyrus. Only 3 of the 8 cats showed a hyperexcitable response under hypoxia after lesioning, and they all recovered to the control state by 10 days. The isocapnic and hypocapnic responses were affected similarly. No unique contribution of this region of the brain to hypoxic ventilatory control can be documented.

Main characteristics of the hindlimb locomotor cycle in the decorticate cat with special reference to bifunctional muscles

In acute decorticate (thalamic) cats, efferent activities to various hindlimb muscles during the locomotor cycle were studied, using several complementary methods: muscle and motor nerve recordings, monosynaptic testing, intracellular recording from motoneurones, recording of fusimotor actions on single Ia afferents. The limb was fixed and peripheral influences could be either increased by exteroceptive stimulations or decreased by curarization and/or deafferentation. Our main results were the following: in all studied muscles, there is an alpha-gamma coactivation; there are pure flexor and pure extensor muscles with simple alternating activities; bifunctional pluriarticular muscles show complex activations which allow the division of the locomotor cycle into a flexion, an extension and two transition phases; alpha motoneurones of these muscles receive both flexor and extensor commands; the relative importance of the corresponding excitation depends on interactions between these central commands and peripheral inflow, especially through afferents acting according to the flexor reflex pattern. Based on these results, an attempt is made to explain how the complex and variable efferent activity can arise from an initially simple rhythmic command and produce the biochemically adapted locomotor movement of the hindlimb.

Effects of thioproperazine and sulpiride on the locomotor rhythms in the decorticate cat

The decorticate cat develops sequences of locomotor movements, especially in the two posterior limbs: those appear either spontaneously, or following a single shock applied to L7 dorsal root. Using this preparation, we tested the effects of two neuroleptic agents, Thioproperazine (TZ) and Sulpiride (S), through either systemic administration or local injection into the lateral-posterior hypothalamus and into the lumbar spinal cord. TZ administered i.v. (0.5 mg/kg) suppressed all locomotor rhythms, while S induced unclear effects. The tested drugs clearly acted at the hypothalamic level, but the effects of the drugs were opposite; TZ (70 microgram) suppressed rhythms, while S (350 microgram) increased or even induced them. T2 (50 microgram) injected at the L7 cord-level abolished rhythms, and S (350 microgram) reduced their amplitude but increased the duration of locomotor sequences. The flexion reflex was never affected by these drugs. The two drugs seem to act at the spinal and the hypothalamic levels. The possible mechanisms involved in their action are discussed.

The activity of cells of nucleus reticularis tegmenti pontis during spontaneous locomotion in the decorticate cat

The activity pattern of nucleus reticularis tegmenti pontis (NRTP) cells was analyzed during sequences of spontaneous locomotion in decorticated, unanaesthetized cats. The majority of NRTP cells, half of them projecting to the cerebellum, were phasically modulated during locomotion, while a smaller group was tonically activated or inhibited. For some cells, both kinds of modulation started before the stepping movements.

Electrophysiological studies on interpositus neurones in the normal and Lurcher mutant mouse

The response characteristics of interpositus neurones (IP) to sciatic nerve stimulation were studied in normal and Lurcher mutant mice under pentobarbitone anaesthesia. The response of IP neurones in the normal mouse was a short latency bimodal excitation (E1-I1-E2) followed by a depression of the firing rate (I2) and ending with a longer latency excitation (E3) which was completed within 225 msec. The response of the majority of IP neurones in the Lurcher mouse was a short latency unimodal excitation (LE1) which corresponded in time to the E1-I1-E2 phase in the normal. This was followed by a pause in the excitation. The response ended with a longer latency excitation (LE2) corresponding in time to the E3 phase in the normal mouse but which persisted for a considerably greater period of time. The response of IP neurones in normal and Lurcher mice appear to be similar to those observed in the normal and experimentally cerebellar decorticate cat, respectively.