R. H. Schor

Localization of proprioceptive reflexes in the splenius muscle of the cat.

Activity of the cat splenius muscle was modulated by sinusoidal rotation of the head around the C1-C2 joint in decerebrate cats with labyrinth intact or with all semicircular canals plugged, or, in one intact and alert cat, by rotation of the body with the head fixed in space. EMG modulation, recorded from the areas of splenius innervated by the C1-C4 nerves, was due to the cervicocollic reflex. Modulation was not uniform, but decreased with progressively more caudal recording locations; with stimuli of small amplitude it was often possible to obtain modulation of the rostral part of the muscle only. The results demonstrate localization of proprioceptive reflexes, including the stretch reflex, within the splenius muscle.

Functional organization of premotor neurons in the cat medial vestibular nucleus related to slow and fast phases of nystagmus

SummaryExtracellular spikes were recorded from secondary vestibular neurons in the cat medial vestibular nucleus (MVN) and were identified as type I or II neurons by horizontal rotation. Type I neurons were further classified as excitatory or inhibitory premotor neurons on the basis of their axonal termination in the contralateral or ipsilateral abducens nucleus, demonstrated by spike-triggered averaging of abducens nerve discharges, or by antidromic activation using systematic microstimulation within the abducens nucleus.Both excitatory and inhibitory premotor type I MVN neurons exhibited a rhythmic modulation of their firing rate in association with nystagmus elicited by rotation or electrical stimulation of the vestibular nerve. Their tonic activity during the slow phase was suppressed at the quick phase directed to the ipsilateral side.Excitatory type I MVN neurons terminating in the contralateral abducens nucleus sent collateral axons to the contralateral MVN. These commissural neurons also showed a nystagmus-related discharge pattern.Type II MVN neurons activated at short latency by stimulation of the contralateral vestibular nerve exhibited burst discharges when the activity of ipsilateral type I neurons was suppressed at the quick phase. These type II neurons made monosynaptic inhibitory connections with type I neurons as shown by the post-spike average of the membrane potential of secondary MVN neurons triggered from spikes of single type II neurons. Thus, the inhibitory action originating from burst activity of type II MVN neurons contributes to suppression of type I premotor MVN neurons during fast eye movements.

Responses of cat vestibular neurons to sinusoidal roll tilt

SummaryExtracellular recordings were made in the lateral and inferior vestibular nuclei of decerebrate, unanesthetized cats. The firing patterns of single units were studied using small amplitude sinusoidal roll tilts of from 0.01 Hz to 1.0 Hz. Three-fourths of the tilt-sensitive units showed greater modulation of their firing rates as the frequency of the sinusoidal tilt was increased. The responses of cells in both nuclei were similar. These responses were virtually unchanged in cats with chronically plugged semicircular canals, indicating a probable otolith origin for the dynamics of the tilt response.

Relationship of cat vestibular neurons to otolith-spinal reflexes

SummaryThe dynamics of neurons in the vestibular nuclei of canal-plugged, decerebrate cats were studied in response to lateral (roll) tilt. Forelimb and neck extensor reflexes recorded simultaneously develop a progressive phase lag above 0.1 Hz. Neurons which exhibited a muscle-like phase lag were excited during low frequency stimuli by ipsilateral side-up tilt (beta response). Neurons with alpha responses, excited during side-down tilt, exhibited a constant phase, without a high frequency lag. Vestibulospinal neurons were present in both of these response groups, as were units driven at monosynaptic latencies by electrical stimulation of the ipsilateral labyrinth. The phase-lagging beta responses are appropriate for contributing to the reflexes observed in the ipsilateral neck and contralateral forelimb.

Off-vertical axis rotation: a test of the otolith-ocular reflex

The vestibulo-ocular reflex was studied via off-vertical axis rotation (OVAR) in the dark. The axis of the turntable could be tilted from vertical by up to 30°. Eye movements were measured with electro-oculography. Results from healthy asymptomatic subjects indicated that 1) a reliable otolith-induced response could be obtained during constant velocity OVAR using a velocity of 60°/s with a tilt of 30°; 2) constant velocity OVAR rotation was nausea-producing and, especially if subjects were rotated in the dark about an earth-vertical axis prior to being tilted, disorienting; and 3) sinusoidal OVAR produced minimal nausea; the eye movement response appeared to be the result of a combination of semicircular canal and otolith components. We conclude that OVAR has the potential of becoming a useful method for clinically assessing both the otolith-ocular reflex and semicircular canal—otolith interaction.

Oculomotor reflexes after semicircular canal plugging in cats

The horizontal ocular reflexes of cats were studied before and after plugging all 6 semicircular canals (2 cats) or the horizontal canals only (2 cats). After plugging, the vestibulo-ocular reflex (VOR) was virtually absent in the cats with all canals plugged; a small phase-advanced VOR was detectable only at high velocities of rotation. There was no VOR recovery over several months. A compensatory cervico-ocular reflex (COR) appeared within a few days of plugging in all cats, and increased in gain slowly over time. The cats with horizontal canals plugged had little VOR during yaw rotation with the head pitched near 30 degrees down from the stereotaxic plane. When the head was pitched nose up from this null plane, a horizontal VOR appeared and increased as the sine of pitch angle, indicating that the cats relied on normal or increased coupling of vertical canals to horizontal eye movers for generation of compensatory eye movements. Periods of compensatory and anticompensatory eye movements were recorded during downward pitched yaw.

The algebra of neural response vectors

The concept of a “polarization vector” was introduced in two papers on the coding properties of otolith neurons in order to describe the spatial coding properties of these afferents.’.’ The polarization vector was a spatial vector of unit length oriented in the direction of the stimulus which produced the maximal excitatory response in the afferent. This vector could be used to predict the response to a (constant) stimulus in an arbitrary direction by considering that only the stimulus component in the direction of the polarization vector was effective; this operation of taking a vector component can be realized by taking the scalar, or “dot,” product between the polarization vector and a vector describing the stimulus orientation. Indeed, working backwards to the hair cell, which has long been known to have a morphological polarization vector given by the asymmetrically placed kinocilium, the depolarization of the hair cell is directly proportional to the component of bending of the hair bundle in the direction of the kin~cilium.~ We will develop the concept of a neural response vector as a mathematical descriptor which will allow us to predict the response of a neuron or a reflex system to complex time-varying stimuli. We will also develop rules for combining these response vectors to enable predictions when, for example, multiple inputs are considered, or to describe responses expected from converging outputs. In the descriptions that follow, the main assumption is that we are dealing with linear systems; this allows us to describe the output to several inputs by considering the response to the inputs separately, then adding the responses. The other well-known consequence of linearity is that if we stimulate with a sinusoid, the response will be a sinusoid at the same frequency. The concept of a “spatial response vector” arises directly out of the polarizatio? vector concepts described above. For an afferent, A, we use the polarization vector A to characterize the orientation in space, and use the sensitivity (or gain) g,, the response when stimulated in this optimal orientation, as” the vector length (FIGURE 1A). The response to an arbitrary constant stimulus S can be expressed as the comeonent of the stimulus along A, which is computed by taking the dot product of A and S. If we consider the plane that contains both the A and S vectors, and express the orientations of A and S by 5 and 8, the response, as a function of stimulus orientation 8, can be written as

Compartmentalization of the cervicocollic reflex in cat splenius muscle

SummaryWe have examined the cervicocollic reflex (CCR), evoked by horizontal rotation of the head of decerebrate cats, in the dorsal neck extensor muscle splenius. This muscle is divided into compartments which are innervated by three or four spinal segments; an analogous Compartmentalization may be observed in the CCR.When the CCR is evoked by rotation of the head about a vertical axis centered over C1–C2, the modulation of EMG activity is higher in the rostral than in the caudal compartments; in some cases, the rostral compartments can be modulated selectively. The rostrocaudal gradient of modulation is absent if the axis of rotation is shifted caudally to C4–C5.In muscles which had been completely detached from their origin and insertion, the pattern of activation of the CCR was similar to that observed in intact muscle, although the gain of the reflex fell by two thirds. This suggests that significant inputs to this reflex arise both from within splenius itself and from receptors outside this muscle.The typical CCR disappears if the C1–C4 dorsal roots ipsilateral to splenius are cut; furthermore, the reflex appears normal in animals with spinal transections above C1. A significant component of the CCR in splenius appears to be a segmental stretch reflex, originating partly in splenius and partly from receptors outside the muscle.

Raesponses to head tilt in cat eighth nerve afferents

SummaryResponses to head tilt were recorded from eighth nerve axons in barbiturate anesthetized cats. The maximally excitatory head tilt (polarization vector), a zero-force discharge rate, and tilt sensitivity were measured for each cell. In one population of afferents, the maximum discharge frequency was obtained by aligning the saccular plane with gravity. The response properties of these saccular afferents were compared with a second population arising from the utriculus. Both the resting discharge rate and the response sensitivity were lower for saccular than utricular afferents in the cat. The average resting discharge was about 20% lower and the sensitivity about 15% higher in the cat than in the squirrel monkey.

The neural substrate of the vestibulocollic reflex

Abstract The purpose of this review is to assess the role of short-latency pathways in the vestibulocollic reflex (VCR). First the current knowledge about the disynaptic and trisynaptic pathways linking semicircular canal and otolith afferents with cat neck motoneurons is summarized. We then discuss whether these pathways are sufficient or necessary to produce the responses observed in neck muscles by natural vestibular stimulation and conclude that they are neither. Finally, alternate pathways are considered, most likely involving reticulospinal fibers, which are an important part of the neural substrate of the VCR.