The architecture of the reflex arc

The identification of local detectors, command neurons, and modulator neurons has opened up the possibility of the structural identification of individual synaptic contacts supporting the functioning of the reflex arc. Nonassociative plasticity (habituation and sensitization) are [sic] realized at the level of the receptors and potential-dependent calcium channels through dephosphorylation-phosphorylation of receptor and channel proteins. Associative plasticity (the development and extinction of the conditioned reflex) includes two leles of regulation: short-term (through dephosphorylation-phosphorylation of receptor and channel proteins) and long-term (through the expression of genes coding structural and translocational genes). Its selectivity is an extremely important characteristic of associative plasticity. The mechanism of associative plasticity is based on the principles of the Hebb plastic synapse, supplemented by indication of the role of nonspecific (modulating) influences. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 42, No. 6, pp. 1064–1074, November–December, 1992.     

Electrophysiological characteristics of the interoceptive reflex arc

Summary Acute experiments were performed on cats. The changes of the efferent impulsations were studied in the sympathetic fibers innervating various vascular areas in vesicular mechanoceptor stimulation of different intensity.Low intensity stimulation is required to produce changes in the impulsation of the nerves innervating the blood vessels of the urinary bladder proper (hypogastric nerve). With the rise of the intensity of stimulation these changes begin to spread to the more remote (in functional condition) vascular areas. At first the change of the efferent impulsation occurs to the renal vessels, then to the blood vessels of the skin of posterior extremity and, finally, to the vessels innervated by the cervical sympathetic nerve. With the rise of intensity of stimulation of the vesicular mechanoceptors these changes in the efferent impulsations in the hypogastric nerve undergo 3 phases: the first phase corresponds to the inhibition of the efferent impulsation, the second to its intensification and the third—to inhibition. Only the first two phases are observed in the renal nerve and only one in the skin and cervical sympathetic nerves. The intensification of the impulsation always leads to constriction while its inhibition—to the dilatation of the vessels.Presented by Active Member of the AMN SSSR V. N. Chernigovskii     

The vestibulo-ocular reflex arc in the newborn kitten

Summary Field potentials and postsynaptic potentials were recorded in the vestibular and abducens nuclei and neurons following vestibular nerve stimulation in anesthetized newborn kittens (within 72 h after birth). Stimulation of the ipsilateral vestibular nerve evoked an initial P wave and an N₁ field potential in the vestibular nuclei. No N₂ potential was evoked. Latencies of the peak of the P wave, the onset and the peak of the N₁ potential were 0.99±0.16 ms, 1.66±0.18 ms, and 2.51±0.23 ms, respectively. Ipsilateral vestibular nerve stimulation evoked monosynaptic excitatory postsynaptic potentials (EPSPs) and polysynaptic inhibitory postsynaptic potentials (IPSPs) in vestibular nuclear neurons. Stimulation of the contralateral vestibular nerve evoked polysynaptic IPSPs in vestibular nuclear neurons. In abducens motoneurons, ipsilateral vestibular nerve stimulation evoked monosynaptic EPSPs and disynaptic IPSPs; contralateral vestibular nerve stimulation produced disynaptic EPSPs. We conclude that short circuit pathways of the vestibul-ovestibular and vestibulo-ocular reflex arc are present in the kitten already at birth.Supported by the Japanese Ministry of Education, Science, and Culture Grants-in-Aid for Scientific Research nos. 572 140 30 and 575 700 53     

Electrophysiologic characteristics of interoceptive reflex arc

Summary Afferent impulsation in the peripheral end of a pelvic nerve branch and efferent impulsation to the urethra in the central end of the pudendal nerve have been recorded in the course of short experiments on cats. Pressure in the urinary bladder, blood pressure and contractions of the urethra have been simultaneously registered. Slight distention of the urinary bladder to 10–20 mm Hg by inflation of 2–10 cc of air caused insignificant afferent impulsation in the pelvic nerve, increased the frequency and amplitude of efferent impulsation in the pudendal nerve, and a rise of the tonus and contractivity of the urethra. Distention of the urinary bladder to 20–30 mm Hg by inflation of 10–20 cc of air intensified afferent impulsation in the pelvic nerve, lowered efferent impulsation in the pudendal nerve down to complete cessation and relaxation of the urethra. Excitation of intermediate degree led to rhythmic bursts and depression, likewise to altemating contraction and relaxation of the urethra. The same three phases have been noted during natural urination.Submitted by Active Member AMS USSR Drofessor V. N. Chernigovsky     

Short-term adaptive changes in the human vestibulo-ocular reflex arc

1. Two sets of experiments have examined the vestibulo-ocular response (VOR) to repeated sinusoidal rotation (A) in the dark and (B) after attempting visual tracking of a mirror-reversed image of the visual surround.

2. In both A and B a horizontal sinusoidal rotational stimulus of 1/6 Hz and 60°/sec angular velocity amplitude was employed, specifically chosen to lie within the presumed range of natural stimulation of the semicircular canals.

3. In A each of seven subjects underwent ten 2-min runs of the standard stimulus in the dark on each of three consecutive days, with 3-min rest periods between runs. Using d.c. electro-oculography (EOG) the VOR gain was measured throughout as eye velocity/head velocity. Mental arousal was maintained by competitive mental arithmetic. Constancy of EOG gain was assured by 50 min dark adaptation before experimentation.

4. The results of A showed no consistent change of VOR gain over the three times scales of a run, a day and the 3-day experiment.

5. In B the same subjects underwent a similar pattern of vestibular stimulation, but during eight of the 2-min daily runs they attempted the reversed visual tracking task. VOR gain was measured during the 1st, 6th and last runs which were conducted in the dark for this purpose. Constancy of EOG gain was maintained by using red light throughout.

6. The results of B showed a substantial (approx. 25%) and highly significant (P « 0·001) reduction of VOR gain attributable solely to the 16 min of reversed visual tracking attempted during the 50 min daily experiment. In addition the pre-test control gain was lower on day 3 than on day 1 (approx. 10% attenuation, P < 0·01) indicating a small cumulative effect from beginning to end of the 3-day experiment.

7. It is concluded (A) that the repeated vestibular stimulus did not itself cause significant attenuation of VOR gain, but (B) that superposition of a reversed visual tracking task did induce retained VOR attenuation which was solely due to the antagonistic visual stimulus.

8. In conjunction with other experimental evidence it is inferred that this attenuation probably represents an adaptive change in the VOR induced at least in part by retinal image slip.

     

Acute adaptation of the vestibuloocular reflex: signal processing by floccular and ventral parafloccular Purkinje cells

The gain of the vertical vestibuloocular reflex (VVOR), defined as eye velocity/head velocity was adapted in squirrel monkeys by employing visual-vestibular mismatch stimuli. VVOR gain, measured in the dark, could be trained to values between 0.4 and 1.5. Single-unit activity of vertical zone Purkinje cells was recorded from the flocculus and ventral paraflocculus in alert squirrel monkeys before and during the gain change training. Our goal was to evaluate the site(s) of learning of the gain change. To aid in the evaluation, a model of the vertical optokinetic reflex (VOKR) and VVOR was constructed consisting of floccular and nonfloccular systems divided into subsystems based on the known anatomy and input and output parameters. Three kinds of input to floccular Purkinje cells via mossy fibers were explicitly described, namely vestibular, visual (retinal slip), and efference copy of eye movement. The characteristics of each subsystem (gain and phase) were identified at different VOR gains by reconstructing single-unit activity of Purkinje cells during VOKR and VVOR with multiple linear regression models consisting of sensory input and motor output signals. Model adequacy was checked by evaluating the residual following the regressions and by predicting Purkinje cells' activity during visual-vestibular mismatch paradigms. As a result, parallel changes in identified characteristics with VVOR adaptation were found in the prefloccular/floccular subsystem that conveys vestibular signals and in the nonfloccular subsystem that conveys vestibular signals, while no change was found in other subsystems, namely prefloccular/floccular subsystems conveying efference copy or visual signals, nonfloccular subsystem conveying visual signals, and postfloccular subsystem transforming Purkinje cell activity to eye movements. The result suggests multiple sites for VVOR motor learning including both flocculus and nonflocculus pathways. The gain change in the nonfloccular vestibular subsystem was in the correct direction to cause VOR gain adaptation while the change in the prefloccular/floccular vestibular subsystem was incorrect (anti-compensatory). This apparent incorrect directional change might serve to prevent instability of the VOR caused by positive feedback via the efference copy pathway.     

Velocity storage in the vestibulo-ocular reflex arc (VOR)

Summary Vestibular and optokinetic nystagmus (OKN) of monkeys were induced by platform and visual surround rotation. Vision prolonged per-rotatory nystagmus and cancelled or reduced post-rotatory nystagmus recorded in darkness. Presumably, activity stored during OKN summed with activity arising in the semicircular canals. The limit of summation was about 120 °/s, the level of saturation of optokinetic after-nystagmus (OKAN). OKN and vestibular nystagmus, induced in the same or in opposite directions diminished or enhanced post-rotatory nystagmus up to 120 °/s. We postulate that a common storage mechanism is used for producing vestibular nystagmus, OKN, and OKAN. Evidence for this is the similar time course of vestibular nystagmus and OKAN and their summation. In addition, stored activity is lost in a similar way by viewing a stationary surround during either OKAN or vestibular nystagmus (fixation suppression).These responses were modelled using direct pathways and a non-ideal integrator coupled to the visual and peripheral vestibular systems. The direct pathways are responsible for rapid changes in eye velocity while the integrator stores activity and mediates slower changes. The integrator stabilizes eye velocity during whole field rotation and extends the time over which the vestibulo-ocular reflex can compensate for head movement.     

Habituation of the nictitating membrane reflex response in the intact frog.

Habituation of the nictitating membrane reflex response was measured in the intact bullfrog. A stimulus map was created, and stimulating electrode pairs directly opposed across the eye proved to be the optimal loci for cutaneous electrical stimulation. The response was found to exhibit all of the parametric characteristics of habituation. To date, this represents what is perhaps the simplest intact vertebrate preparation demonstrating habituation. The response was also found to exhibit a surprising periodicity in response amplitude when unchanging stimuli were infrequently presented over long periods of time. The convenience, reliability and successful behavioral assessment strongly recommend this preparation as an ideal simple vertebrate model system for further examination of the neural basis of behavioral plasticity.     

Principles of linear and angular vestibuloocular reflex organization in the frog.

We compared the spatial organization patterns of linear and angular vestibuloocular reflexes in frogs by recording the multiunit spike activity from cranial nerve branches innervating the lateral rectus, the inferior rectus, or the inferior obliquus eye muscles. Responses were evoked by linear horizontal and/or vertical accelerations on a sled or by angular accelerations about an earth-vertical axis on a turntable. Before each sinusoidal oscillation test in darkness, the static head position was systematically altered to determine those directions of horizontal linear acceleration and those planes of angular head oscillation that were associated with minimal response amplitudes. Inhibitory response components during angular accelerations were clearly present, whereas inhibitory response components during linear accelerations were absent. Likewise was no contribution from the vertical otolith organs (i.e., lagena and saccule) observed during vertical linear acceleration. Horizontal linear acceleration evoked responses that originated from eye muscle-specific sectors on the contralateral utricular macula. The sectors of the inferior obliquus and lateral rectus muscles on the utricle had an opening angle of 45 and 60 degrees, respectively and overlapped to a large extent in the laterorostral part of the utricle. Both sectors were coplanar with the horizontal semicircular canals. The sector of the inferior rectus muscle was narrow (opening 5 degrees), laterocaudally oriented, and slightly pitched up by 6 degrees. Angular acceleration evoked maximal responses in the inferior obliquus muscle nerve that originated from the ipsilateral horizontal and the contralateral anterior vertical canals in a ratio of 50:50. Lateral rectus excitation originated from the contralateral horizontal and anterior vertical semicircular canals in a ratio of 80:20. The excitatory responses of the inferior rectus muscle nerve originated exclusively from the contralateral posterior vertical canal. Measured data and known semicircular canal plane vectors were used to calculate the spatial orientation of maximum sensitivity vectors for the investigated eye muscle nerves in semicircular canal coordinates. Comparison of the directions of maximal sensitivity vectors of responses evoked by linear or angular accelerations in a given eye muscle nerve showed that the two vector directions were oriented about orthogonally with respect to each other. With this arrangement the linear and the angular vestibuloocular reflex can support each other dynamically whenever they are co-activated without a change in the spatial response characteristics. The mutual adaptation of angular and linear vestibuloocular reflexes as well as the differences in their organization described here for frogs may represent a basic feature common for vertebrates in general.     

Three-dimensional kinematic analysis of frog hindlimb movement in reflex wiping

The three-dimensional kinematics of the hindlimb back-wipe were examined in spinal frogs. The component movements were identified and the relationship between stimulus position and hindlimb configuration was assessed. The planes of motion of the hindlimb were examined throughout the movement. The backwipe comprises three essential phases: a placing phase (I), in which the foot is drawn over the back of the frog and placed in a position near to the stimulus; a pre-whisk phase (II), in which the endpoint of the foot moves away from the stimulus; and a whisk/extension phase (III), in which the stimulus is removed. The pre-whisk phase contributes to force production for the whisk/extension (III). In the placing phase a systematic relationship was found between limb endpoint position and stimulus position in the rostro-caudal direction. The hip, knee and metatarsal joint angles were related to the position of the endpoint in the rostro-caudal direction. However, different frogs tended to adopt different strategies to remove the stimulus. In one strategy, when the knee angle was strongly related to the rostro-caudal stimulus position, the metatarsal angle was weakly related and vice versa. Other strategies were observed as well. There was no adjustment in limb endpoint position for stimulus placement in the medial-lateral direction. Consistent with this finding, the point on the foot at which stimulus contact occurred changed systematically as a function of medial-lateral stimulus placement. Thus, in order to remove the stimulus in different medial-lateral positions, the frog used a different part of the foot rather than moving the foot in the direction of the stimulus. In two frogs a relationship was observed between the elevation of the femur and the medial-lateral stimulus position. The motion planes of the hindlimb were studied by examining the instantaneous plane of motion of the endpoint and the planes of motion of adjacent limb segments. The motion of the endpoint was found not to be planar in any phase of the wipe. In contrast, planar motion of the femur and tibia was observed for all phases. Systematic changes in the orientation of these planes characterized the different phases. The position of the hindlimb was found to be variable prior to the placing phase. This variability was not related to stimulus position. However, in trials with multiple wipes, once an initial limb configuration was assumed, the limb returned to this configuration before each wipe in the sequence. Evidence for motor equivalence was sought in two ways. The pattern of hindlimb joint angles corresponding to a fixed position of the limb endpoint was examined, and the variability of the endpoint positions was examined for fixed stimulus positions. It was found that for a given endpoint position there was little variation in joint angles. However, for a fixed stimulus position there was greater variation in the endpoint position at the end of the placing phase.     

Gustatory processing differences in PTC tasters and non-tasters: a reaction time analysis

The present study examined differences in gustatory processing for tasters and non-tasters of phenylthiocarbamide (PTC) by assessing intensity judgment reaction times in these two groups. After prescreening for sensitivity to PTC, subjects were asked to judge whether solutions of sucrose, sodium chloride, quinine sulfate, hydrochloric acid and PTC were weak, medium or strong as quickly as they were able. Based on the reaction time and PTC sensitivity data, three groups of subjects were identified; tasters, sensitive non-tasters and less sensitive non-tasters. The taster population was characterized by their high sensitivity to PTC and a typical, negatively skewed distribution of reaction times. Sensitive non-tasters demonstrated a distribution of reaction times that was similar to that observed with tasters. However, these subjects were insensitive to PTC. The less sensitive non-tasters were insensitive to PTC and produced reaction times that were significantly slower than those produced for the other two groups across all of the substances tested. It is hypothesized that two groups of PTC non-tasters exist. One group has a specific deficit involving PTC and related compounds. The other group demonstrates sensitivity deficits for a wide range of gustatory stimuli.     

The influence of novocaine on pessimal inhibition in various links of the reflex arc

Summary The mechanism of the action of novocaine was studied experimentally. It was demonstrated that in resorptive action of novocaine it increases the pessimal inhibition in the reflex centers of the spinal cord the vegetative ganglia and the neuro-muscular apparatus. Experiments were performed according to the teaching of Vedensky on the functional mobility of the excitable formations.Presented by Active Member AMS USSR V. V. Zakusov     

The neurochemical basis of backward inhibition in a defense reaction reflex arc

An investigation was carried out on a preparation of the isolated nervous system of the edible snail of the mechanism of habituation in the network of defense behavior neurons. It was established that the intracellular activation of one of the systems of the defense behavior command neurons, as in the case of the application of the neuropeptide FMRF-amide, leads to a decrease in the amplitude of the postsynaptic potentials in the remaining neurons of this class. An inference is reached, when account is taken of the presence of endogenous FMRF-amide in the defense behavior command neurons, regarding the possibility of active inhibition of the behavioral response to an appropriate stimulus through this neurochemical mechanism of the backward connection. The possible interrelationships with other modulating systems and the significance of such an association in the organization of behavior are discussed.Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 41, No. 5, pp. 1033–1038, September–October, 1991.