Donald S. Rushmer

Climbing fiber responses of cerebellar Purkinje cells to passive movement of the cat forepaw

The activity of cerebellar Purkinje cells during controlled and passive movement of the forepaw was studied in the cat. Burst responses characteristic of activation by climbing fibers were observed in Purkinje cells in lobules Vb and Vc of the cerebellar vermis and paravermis. The climbing fiber responses followed the onset of a movement with a latency ranging from 20 to 60 msec depending upon movement type and amplitude. Responsive Purkinje cells were localized in a well defined parasagittal strip very near the paravermal vein in lobules Vb and Vc. Cells within the responsive strip responded with identical response probabilities and latencies for any particular type of movement presentation. Responses were independent of starting paw position and direction of movement. Climbing fiber responses could be evoked by extremely small movements with most cells responding to displacements of 50 mum. The latencies and probabilities for climbing fiber responses were inversely related to movement amplitude with latencies as long as 80 msec for very small displacements.

Human automatic postural responses: responses to horizontal perturbations of stance in multiple directions

SummaryThe effect of the direction of unexpected horizontal perturbations of stance on the organization of automatic postural responses was studied in human subjects. We recorded EMG activity from eight proximal and distal muscles acting on joints of the legs and hip known to be involved in postural corrections, while subjects stood on an hydraulic platform. Postural responses to horizontal motion of the platform in 16 different directions were recorded. The amplitude of the EMG responses of each muscle studied varied continuously as perturbation direction was changed. The directions for which an individual muscle showed measurable EMG activity were termed the muscles “angular range of activation”. There were several differences in the response characteristics of the proximo-axial muscles as opposed to the distal ones. Angular ranges of activity of the distal muscles were unipolar and encompassed a range of less than 180°. These muscles responded with relatively constant onset latencies when they were active. Proximo-axial muscles, acting on the upper leg and hip showed larger angular ranges of activation with bimodal amplitude distributions and/ or onset latency shifts as perturbation direction changed. While there were indications of constant temporal relationships between muscles involved in responses to perturbations around the sagittal plane, the onset latency relationships for other directions and the response amplitude relationships for all directions varied continuously as perturbation direction was changed. Responses were discrete in that for any particular perturbation direction there appeared to be a single unique response. Thus, while the present results do not refute the hypothesis that automatic postural responses may be composed of mixtures of a few elemental synergies, they suggest that composition of postural responses is a complex process that includes perturbation direction as a continuous variable.

Organization of climbing fiber input from mechanoreceptors to lobule V vermal cortex of the cat

SummaryThe somatotopic organization of the climbing fiber (CF) projections to the vermal cortex of lobule V of the cat was revealed by low threshold natural stimulation of mechanoreceptors. Extracellular single-unit recordings were made from 554 Purkinje cells in cats anesthetized with sodium pentobarbital. Forty-nine percent of the CF responses were elicited by cutaneous stimulation of the forelimb (62%), hindlimb (25%), or upper back and neck (13%). The topographical arrangement consisted of a 1 mm wide medial zone and a 1–1.5 mm wide lateral zone. In the medial zone, the CF responses were mainly nonresponsive to any cutaneous stimulation except in the caudomedial portion of the lobule where the upper back, neck or ears were represented in a narrow parasagittally oriented strip. The lateral zone contained a mixture of CF responses representing projections from different portions of the ipsilateral forelimb and hindlimb. Although CF responses connected with the forepaw or hindpaw predominated throughout all parts of the lateral zone, the more medial portions of this zone contained larger receptive fields involving the more proximal areas of the limb whereas the lateral part of the zone had smaller receptive fields representing the distal regions, particularly the ventral forepaw surface. Cells with similar receptive fields were often grouped together, but adjacent skin areas were not necessarily represented in adjacent cortical patches. Thus, the cutaneous projections to this lobule terminated in a patchy or mosaic fashion.

Postural responses in the cat to unexpected rotations of the supporting surface: evidence for a centrally generated synergic organization

SummaryPostural reactions to disruptions of stance are rapid and automatic in both quadrupeds and bipeds. Current evidence suggests that these postural responses are generated by the central nervous system as patterns involving muscle synergies. This study attempted to test this hypothesis of a centrally generated postural mechanism by determining whether the same postural response could be evoked in the freely-standing cat under two different biomechanical conditions. The present work is an extension of previous experiments in which the stance of cats was perturbed by a horizontal translation of the supporting surface in the anterior and posterior directions (Rushmer et al. 1983). We now tested whether simple rotation of the metacarpo- and metatarsophalangeal (M-P) joints that mimics the digit rotation occurring during platform translation, was sufficient to evoke the translation postural response. The rotational perturbations were biomechanically different from translations in that the rotation did not cause displacement of the centre of mass of the animal, nor did it result in any significant movement about any but the M-P joints. Even so, rotational perturbations did evoke the appropriate translational muscle synergies in all four animals. Both plantar flexion rotation and headward translation activated the posterior hindlimb synergy (which included gluteus medius, semitendinosus and lateral gastrocnemius). Similarly, dorsiflexion rotation and tailward translation both activated the same anterior hindlimb synergy (iliopsoas, vastus lateralis and tibialis anterior) together with the forelimb synergy. The postural responses elicited by rotational perturbations were biomechanically inappropriate, and caused the animal to displace its own centre of mass away from the stable, control position. The most striking finding was that the group of muscles in which the medium latency postural response was evoked was different than the group from which short latency reflex responses were elicited. These data support the hypothesis that postural reactions are not merely reflex responses to local sensory inputs associated with the perturbation but, instead, represent a centrally generated response, with the muscle synergy being the controlled unit.

Automatic Postural Responses in the Cat: Responses to Headward and Tailward Translation*

SummaryEMG responses, vertical and A-P shear forces and kinematics of “automatic postural responses” to unexpected translational perturbations in the headward and tailward directions were studied in cats. Muscles acting on the major joints of the forelimbs and hindlimbs were studied. Movement of the animals in response to perturbation were highly stereotyped and consisted of two phases: (1) motion of the feet during platform movement while the trunk remained relatively stationary followed by (2) active correction of posture by movement of the trunk in the direction of perturbation.Vertical force changes occurred after the perturbation was well underway (latency 65 ms) and were related to the displacement of the center of mass and active correction of trunk position. Shear forces showed both passive (inertial) and active components and suggested that the majority of the torque necessary for po,stural correction was generated by the hindlimb.EMG responses in forelimb and shoulder muscles were most correlated with increase in vertical force, showing a generalized co-contraction in tailward translation (when these limbs were loaded) and little activity when the forelimbs were unloaded.EMG responses in hindlimb showed reciprocal activation of agonists and antagonists during perturbation with strong synergies of thigh and foot flexors in tailward translation and thigh and foot extensors in headward translation. The forelimb EMG patterns were most consistent with the conclusion that the forelimb is used primarily for vertical support during perturbation.It was concluded that hindlimb EMG responses were appropriate for both vertical support and performance of the postural correction. The hindlimb muscle synergies observed during translation are the “mirror image” of those observed in humans by other workers.

The activity of dentate neurons during an arm movement sequence

Abstract Extracellular unit recordings were obtained from 350 dentate nucleus cells in squirrel monkeys during a performance of a brief, stereotyped sequence of visually guided arm movements. The operant paradigm involves hypothalamic stimulation as a reward. Units which participate in the performance can be divided into three functional subsets based upon physiologic and anatomic properties. Burst units are characterized by periods of silence or slow, irregular firing punctuated by high frequency discharges of short duration occurring spontaneously and during the motor performance. During the performance, they produce extraordinary correlation to short epochs or parts of the performance. Such units exist in natural, anatomic clusters or ‘collectives’ of a maximum 800–1000 μm depth. Each member of the ‘collective’ gives a similar response during the same epoch in the performance. ‘Collectives’ correlating with different periods in the performance exist and appear to congregate in the same part of the nucleus. Non-burst cells are those which gradually modulate up or down discharge rates (including complete inhibition) during performance. Their anatomic organization appears less precise; they correlate with longer epochs in the performance. Non-participant burst and non-burst units comprise the third subset. Functional questions are raised by this organization of dentate with respect to the correlation and execution of eye-limb pattern tasks.

Automatic postural responses in the cat: responses of hindlimb muscles to horizontal perturbations of stance in multiple directions.

SummaryThe effect of the direction of unexpected horizontal perturbations of stance on the organization of automatic postural responses was studied in cats. We recorded EMG activity in eight proximal and distal muscles of the hindlimb along with vertical forces imposed by the limbs in awake behaving cats while they stood on an hydraulic platform. Postural responses to motion of the platform in 16 different horizontal directions were recorded. Vertical force changes were consistent with passive shifts of the center of mass and active correction of stance by the animals. When the perturbation was in the sagittal plane, vertical force changes began about 65 ms following initial platform movement. When the perturbation contained a component in the lateral direction, latency for vertical force changes was about 25 ms and an inflection in the vertical force trace was observed at 65 ms. No EMG responses were observed with latencies that were short enough to account for the early force component and it was concluded that this force change was due to passive shifts of the center of mass. The amplitude of the EMG responses of each muscle recorded varied systematically as perturbation direction changed. The directions for which an individual muscle showed measurable EMG activity were termed the muscles “angular range of activation.” No angular range of activation was oriented strictly in the A-P or lateral directions. Most muscles displayed angular ranges of activation that encompassed a range of less than 180°. Onset latencies of EMG responses also varied systematically with perturbation direction. The amplitude and latency relationships between muscles, which made up the organization of postural responses, also varied systematically as perturbation direction was changed. This result suggests that direction of perturbation determines organizational makeup of postural responses, and for displacements in the horizontal plane, is considered a continuous variable by the nervous system.

Automatic postural responses in the cat: responses of proximal and distal hindlimb muscles to drop of support from a single hind- or forelimb

SummaryCats respond to drop of the support from beneath a single limb with the “diagonal stance response” (Coulmance et al. 1979). They load the limbs on the diagonal opposite to the one containing the dropped limb and unload the third supporting limb in the diagonal containing the dropped limb. Characteristic biomechanical delays in limb motion and in vertical force changes imposed upon the limbs are observed. These delays range from 30 to 45 ms, depending upon the location of the dropped limb. This study describes the kinematics of the “diagonal stance response” and the activation of selected agonist-antagonist muscle pairs acting on the joints of the hindlimb during the response. Proximal and distal hindlimb muscles respond to perturbations in groups that are appropriate to the vertical forces imposed upon the limb. When the hindlimb containing the recording electrodes is loaded by drop of the contralateral hindlimb or the ipsilateral forelimb medium latency (25–45 ms) EMG responses occur in the extensors. This response serves to stiffen the limb against the increased vertical force of loading. A similar response is observed when the hindlimb is reloaded after being dropped. In this case, however, short latency responses precede the medium latency responses in muscles that are passively stretched by the limb drop. When drop of the diagonal forelimb unloads the hindlimb containing the electrodes, medium latency responses are observed in the distal hindlimb flexors, which indicates that the unloading is evoked in part by active lifting of the limb. In most cases, the medium latency responses precede or are coincident with the changes in force imposed on the limb, suggesting that the observed responses are centrally programmed.

Classical Conditioning of Kindled Seizures

Summary: As a model of reflex epilepsy, rats were classically conditioned to show paroxysmal spike activity to an auditory stimulus. A tone stimulus was paired with seizures induced by kindling. After repeated pairings, paroxysmal discharges were elicited at tone onset. The duration of the tone, proximity to the stimulation, and the number of pairings appeared to be important in the conditioning process.

Automatic postural responses in the cat: responses of distal hindlimb muscles to paired vertical perturbations of stance*

SummaryThe active components of the quadrupedal diagonal stance response to rapid removal of the support from beneath a single limb were studied in cats to further define the mechanisms that trigger and generate the response. We recorded EMG activity from lateral gastrocnemius and tibialis anterior muscles in awake, behaving cats while they stood on an hydraulic posture platform. By dropping the support from beneath a single limb, we evoked the diagonal stance response, with its characteristic changes in vertical force and EMG patterns. As the animal responded to this drop, a second perturbation of posture was then presented at intervals of 10 to 100 ms following the first. This second perturbation, which consisted of dropping the support from beneath the two limbs that were loaded as a result of the initial limb drop, made the first response biomechanically inappropriate. The EMG responses observed in both muscles during paired perturbations were triggered by the somatosensory events related to the perturbations. Muscle responses that were appropriate for the first perturbation always occurred with amplitudes and latencies similar to control trials. This was true even when the second perturbation occurred 10–20 ms after the first, that is, when this perturbation either preceded or was coincident with the response to the initial limb drop. The EMG responses that were normally associated with the second perturbation were delayed and/or reduced in amplitude when the time interval between perturbations was short. As the inter-perturbation interval was lengthened beyond 60–100 ms, however, EMG responses to the second perturbation were unaffected by the occurrence of the first perturbation. When the hindlimb containing the recording electrodes was dropped as part of the second perturbation, a myotatic latency response was observed in tibialis anterior. The amplitude of this response to the second perturbation was greater than controls when this displacement was presented during the period between initiation of the first perturbation and execution of the response to it. When the second displacement was presented after execution of the first response began, the amplitude of the myotatic response was reduced below control levels. While the results do not preclude the possibility that these “automatic” postural responses are segmental or suprasegmental reflexes, they support the hypothesis that the active component of the response to drop of the support beneath a single limb is centrally programmed and that the appropriate response can be riggered very rapidly by the somatosensory information signalling the perturbation.