Electrically evoked fictive swimming in the low-spinal immobilized turtle.

Fictive swimming was elicited in low-spinal immobilized turtles by electrically stimulating the contralateral dorsolateral funiculus (cDLF) at the level of the third postcervical segment (D(3)). Fictive hindlimb motor output was recorded as electroneurograms (ENGs) from up to five peripheral nerves on the right side, including three knee extensors (KE; iliotibialis [IT]-KE, ambiens [AM]-KE, and femorotibialis [FT]-KE), a hip flexor (HF), and a hip extensor (HE). Quantitative analyses of burst amplitude, duty cycle and phase were used to demonstrate the close similarity of these cDLF-evoked fictive motor patterns with previous myographic recordings obtained from the corresponding hindlimb muscles during actual swimming. Fictive rostral scratching was elicited in the same animals by cutaneous stimulation of the shell bridge, anterior to the hindlimb. Fictive swim and rostral scratch motor patterns displayed similar phasing in hip and knee motor pools but differed in the relative amplitudes and durations of ENG bursts. Both motor patterns exhibited alternating HF and HE discharge, with monoarticular knee extensor (FT-KE) discharge during the late HF phase. The two motor patterns differed principally in the relative amplitudes and durations of HF and HE bursts. Swim cycles were dominated by large-amplitude, long-duration HE bursts, whereas rostral scratch cycles were dominated by large-amplitude, long-duration HF discharge. Small but significant differences were also observed during the two behaviors in the onset phase of biarticular knee extensor bursts (IT-KE and AM-KE) within each hip cycle. Finally, interactions between swim and scratch motor networks were investigated. Brief activation of the rostral scratch during an ongoing fictive swim episode could insert one or more scratch cycles into the swim motor pattern and permanently reset the burst rhythm. Similarly, brief swim stimulation could interrupt and reset an ongoing fictive rostral scratch. This shows that there are strong central interactions between swim and scratch neural networks and suggests that they may share key neural elements.

Fictive hindlimb motor patterns evoked by AMPA and NMDA in turtle spinal cord-hindlimb nerve preparations.

Application of the glutamate agonists alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA, 5-10 microM), or N-methyl-D-aspartate (NMDA, 50-100 microM) to the turtle spinal cord produced fictive hindlimb motor patterns in low-spinal immobilized animals (in vivo) and in isolated spinal cord-hindlimb nerve preparations (in vitro). For in vivo experiments, drugs were applied onto the dorsal surface of 2-4 adjacent spinal cord segments in and near the anterior hindlimb enlargement. Motor output was recorded unilaterally or bilaterally from hindlimb muscle nerves. AMPA elicited vigorous motor patterns in vivo that included strict hip flexor-extensor and right-left alternation. In most turtles, the monoarticular knee extensor nerve FT-KE was active during the HE phase of AMPA evoked burst cycles, similar to the timing of pocket scratch motor patterns. NMDA was less effective in vivo, typically producing only weak and irregular bursting from hip nerves and little or no knee extensor (KE) discharge. Sensory stimulation of a rostral scratch reflex in vivo could reset an ongoing AMPA-evoked motor rhythm, indicating that cutaneous reflex pathways interact centrally with the chemically activated rhythm generator. Most in vitro preparations consisted of six segments of spinal cord, including the entire 5-segment hindlimb enlargement (D8-S2) and the segment immediately anterior to the enlargement (D7), with attached hindlimb nerves. In contrast to in vivo experiments, in vitro preparations exhibited highly regular, long-lasting motor rhythms when NMDA was superfused over the spinal cord. AMPA also produced rhythmic motor patterns in vitro, but these lasted only a few minutes before they were replaced with tonic discharge. FT-KE timing during in vitro chemically elicited activity was similar to that of sensory-evoked pocket scratch motor patterns. Some NMDA-evoked rhythmicity persisted even in 3-segment (D6-D8) and 1-segment (D8) in vitro preparations, demonstrating that neural mechanisms for chemically activated rhythmogenesis reside even in a single segment of the hindlimb enlargement.

Reciprocal interactions in the turtle hindlimb enlargement contribute to scratch rhythmogenesis.

We examined interactions between the spinal networks that generate right and left rostral scratch motor patterns in turtle hindlimb motoneurons before and after transecting the spinal cord within the anterior hindlimb enlargement. Our results provide evidence that reciprocal inhibition between hip circuit modules can generate hip rhythmicity during the rostral scratch reflex. "Module" refers here to the group of coactive motoneurons and interneurons that controls either flexion or extension of the hip on one side and coordinates that activity with synergist and antagonist motor pools in the same limb and in the contralateral limb. The "bilateral shared core" hypothesis states that hip flexor and extensor (HF and HE) circuit modules interact via crossed and uncrossed spinal pathways: HF modules make reciprocal inhibitory connections with contralateral HF and ipsilateral HE modules and mutual excitatory connections with contralateral HE modules. It is currently unclear how much reciprocal inhibition between modules contributes to scratch rhythmogenesis. To address this issue, fictive scratch motor patterns were recorded bilaterally as electroneurograms from HF, HE, knee extensor (KE), and respiratory (d.D8) muscle nerves in immobilized animals. D3-end (low-spinal) preparations had intact spinal cords posterior to a complete D2-D3 transection. Unilateral stimulation of rostral scratch in D3-end turtles elicited rhythmic alternation between ipsilateral HF and HE bursts in most cycles; consecutive HF bursts were separated by complete silent (HF-OFF ) periods. D3-D9 and D3-D8 preparations received a second spinal transection at the caudal end of segment D9 or D8, respectively, within the anterior hindlimb enlargement. This second transection disconnected most HE circuitry (located mainly in segments D10-S2 of the posterior enlargement) from the rostral scratch network and thereby reduced the HE-associated inhibition of HF circuitry. Unilateral stimulation of rostral scratch in most D3-D9 and D3-D8 preparations evoked rhythmic or weakly modulated ipsilateral HF discharge without HF-OFF periods between bursts and without ipsilateral HE activity in the majority of cycles. In contrast, bilateral stimulation in D3-D9 and D3-D8 preparations reconstructed the HF-OFF periods, increased HF rhythmicity (assessed by fast Fourier transform power spectra and autocorrelation analyses), and reestablished weak HE-phase motoneuron activity. We suggest that bilateral stimulation produced these effects by simultaneously activating reciprocally inhibitory hip modules on opposite sides (right and left HF) and the same side (HF and residual ipsilateral HE circuitry). Our data support the hypothesis that reciprocal inhibition can contribute to spinal rhythmogenesis during the scratch reflex.

Reconstruction of flexor/extensor alternation during fictive rostral scratching by two-site stimulation in the spinal turtle with a transverse spinal hemisection.

Analyses of fictive scratching motor patterns in the spinal turtle with transverse hemisection provided support for the concept of bilateral shared spinal cord circuitry among neurons responsible for generating left- and right-side rostral, pocket, and caudal fictive scratching. Rhythmic bursts of hip flexor activity, the hip extensor deletion variation of fictive rostral scratching, were elicited by ipsilateral stimulation in the rostral scratch receptive field of a spinal turtle [transection at the segmental border between the second (D2) and third (D3) postcervical spinal segments] with a contralateral transverse hemisection one segment anterior to the hindlimb enlargement (at the D6-D7 segmental border). In addition, other sites were stimulated in this preparation: (1) contralateral sites in a rostral, pocket, or caudal scratch receptive field or (2) ipsilateral sites in a caudal scratch receptive field. A reconstructed fictive rostral scratch motor pattern of rhythmic alternation between hip flexor and hip extensor activation was produced by simultaneous stimulation of one site in the ipsilateral rostral scratch receptive field and another site in one of the other scratch receptive fields. This reconstructed rostral scratch motor pattern resembled the normal rostral scratch motor pattern produced by one-site rostral scratch stimulation of a spinal turtle (D2-D3 transection) with no additional transections. The observation of a reconstructed rostral scratch motor pattern produced by two-site stimulation in the spinal turtle with transverse hemisection supports the concept that hip extensor circuitry activated by stimulation of other scratch receptive fields is shared with circuitry activated by ipsilateral rostral scratch receptive field stimulation.

Spinal motor patterns in the turtle.

Rhythmic alternation between ipsilateral hip flexors and extensors occurs during the normal pattern of fictive rostral scratching in response to unilateral midbody stimulation in D3-end turtles (complete spinal transection posterior to the forelimb enlargement). Unilateral midbody stimulation evokes rhythmic bursts of ipsilateral hip flexor activity with no hip extensor activity in D3-end turtles with D6-D7 contralateral hemisection (transverse hemisection anterior to the hindlimb enlargement). Bilateral midbody stimulation in these turtles evokes reconstruction of rhythmic alternation between intact side hip flexors and extensors. These normal motor patterns in response to two-site stimulation are reconstructed because one-site stimulation in this preparation activates only hip flexor rhythms (J. Neurosci. 18: 467). Hip flexor rhythms can therefore occur without hip extensor activation. This supports the concept that reciprocal inhibition between flexor and extensor interneurons is not required for flexor motor rhythm generation. Reciprocal inhibition, when present, also contributes to rhythmicity (J. Neurophysiol. 78: 3479; see also Currie and Gonsalves, this volume). Both mechanisms for rhythmicity are included in the Grillner unit burst generator model: hip flexor unit burst generators may be rhythmogenic in the absence of hip extensor activity and reciprocal inhibition contributes to rhythmogenesis. Contralateral midbody stimulation assisted in the activation of ipsilateral hip extensor rhythmicity during reconstructed rostral scratching. This result provides additional support for the hypothesis that a bilateral shared core of hip interneuronal circuitry plays a critical role in the generation of the normal pattern of fictive rostral scratching (J. Neurosci. 15: 4343).

Glycinergic inhibition contributes to the generation of rostral scratch motor patterns in the turtle spinal cord.

Cutaneous stimulation within the rostral scratch receptive field in a low spinal-immobilized turtle elicits a fictive rostral scratch reflex characterized by robust rhythmic motor output from ipsilateral hindlimb muscle nerves and weaker, alternating motor discharge in contralateral nerves. Simultaneous bilateral stimulation elicits bilateral rostral scratch motor patterns in which activity on the right and left sides alternates. We investigated the role of glycinergic inhibition in the generation and coordination of fictive rostral scratch motor patterns. Glycine (2 or 5 mM) and strychnine (5-50 microM), a glycine antagonist, were superfused over the anterior spinal hindlimb enlargement while fictive rostral scratch motor output was recorded bilaterally from hindlimb muscle nerves in the form of electroneurograms (ENGs). Although glycine reduced rostral scratch burst frequencies, strychnine tended to increase burst frequency. Strychnine also changed the shape of hip flexor ENG bursts, resulting in more abrupt burst onsets, indicating an earlier recruitment of motor neurons with large ENG spikes. During bilateral stimulation, strychnine increased the variability of interlimb phase values (left vs right hip flexor bursts) but did not abolish right-left alternation. These results indicate that glycinergic neurons in or near the anterior hindlimb enlargement contribute to the overall timing of the rostral scratch rhythm and to the recruitment timing of individual hip flexor motor neurons within each scratch burst. Our data also indicate that glycinergic mechanisms contribute to, but are not critically important for, maintaining an alternating interlimb coordination during bilateral scratch motor patterns.

Right-left interactions between rostral scratch networks generate rhythmicity in the preenlargement spinal cord of the turtle.

We examined the rhythmogenic capacity of the midbody D3-D7 spinal cord during stimulation of the rostral scratch reflex in turtles. Fictive scratching was recorded bilaterally as electroneurograms (ENGs) from prehindlimb enlargement nerves [transverse D7 (TD7) and oblique D7 (OD7)] and hip flexor nerves (HF). TD7 and OD7 innervate transverse- and oblique-abdominus muscles, respectively. D3-end preparations had intact spinal cords caudal to a D2-D3 transection site. Unilateral stimulation of the rostral receptive field in D3-end preparations evoked rhythmic bursting in the ipsilateral (ipsi) HF nerve and bilateral rhythmic discharge in the TD7 and OD7 nerves. Right HF bursts were coactive with right TD7 and left OD7 bursts and alternated with left TD7 and right OD7 bursts. D3-D7 preparations received a second spinal transection at the caudal end of segment D7, thus resulting in activation of strictly preenlargement circuitry in response to rostral scratch stimulation and preventing activation of hindlimb enlargement circuitry in segments D8-S2. D3-D7 preparations responded to unilateral stimulation with modulated or tonic discharge in the ipsi TD7 and contralateral (contra) OD7 nerves. In contrast, bilateral stimulation reestablished robust bursting in which coactive right TD7-left OD7 bursts alternated with coactive left TD7-right OD7 bursts. These data imply that TD7 circuit modules make 1) crossed excitatory connections with contra OD7 circuitry, 2) crossed inhibitory connections with contra TD7 circuitry, and 3) uncrossed inhibitory connections with ipsi OD7 circuitry. Our results also suggest that bilateral stimulation evokes rhythmic alternation in the preenlargment cord by simultaneously exciting reciprocally inhibitory circuit modules.

Sensory-evoked pocket scratch motor patterns in the in vitro turtle spinal cord: reduction of excitability by an N-methyl-D-aspartate antagonist.

1. In intact turtles, tactile stimulation of the body surface in the "shell pocket" region surrounding the hindlimb elicits a pocket scratch reflex, in which the hindlimb reaches toward and rhythmically rubs the stimulated site. In the present study, we utilized reduced in vitro preparations of the turtle spinal cord with attached peripheral nerves to investigate the time course and pharmacology of sensory-evoked excitability in the pocket scratch neural network. Fictive pocket scratch motor output was elicited by electrically stimulating either the ventral-posterior pocket (VPP) cutaneous nerve or the distal D8 (d.D8) nerve. Both nerves contain afferents innervating part of the pocket scratch receptive field. 2. Six-segment (D7-S2) preparations of the spinal cord, which included the entire hindlimb enlargement, produced fictive pocket scratch motor output in response to VPP nerve stimulation (n = 6). We recorded fictive motor output as electroneurograms from up to five peripheral nerves in D7-S2 preparations, including three knee extensor muscle nerves (IT-KE, which innervates triceps femoris pars iliotibialis; AM-KE, which innervates pars ambiens; and FT-KE, which innervates pars femorotibialis), a hip flexor (protractor) muscle nerve (VP-HP, which innervates puboischiofemoralis internus, pars anteroventralis), and a mixed cutaneous-muscle nerve that exhibits hip-extensor-correlated motor output during the scratch (d.D8). The timing characteristics of activity in these nerves during in vitro motor patterns were similar to what has been observed during the in vivo pocket scratch. 3. Even a single segment of spinal cord from the anterior hindlimb enlargement (D8) contained sufficient neural circuitry to generate rhythmic motor patterns in AM-KE, VP-HP, and d.D8 nerves during repeated stimulation of VPP (n = 5) or d.D8 (n = 1). Stimulus trains delivered at 3-5 Hz for > or = 6 s elicited one or more VP-HP bursts with clear burst terminations; in some cases, these were followed by distinct hip-extensor-correlated d.D8 bursts. AM-KE timing was characteristic of a pocket scratch synergy, beginning during the VP-HP burst and continuing after VP-HP offset. Thus even isolated D8 segments were capable of expressing rhythmic alternation between hip-flexor- and hip-extensor-correlated motor bursts as well as a pocket-scratch-specific knee-hip synergy. 4. A single electrical pulse delivered to the VPP or d.D8 nerve increased the excitability of the pocket scratch network in D7-S2 and D8 preparations for > or = 5-10 s. We estimated the time course of increased excitability by observing the temporal summation of scratch motor output in response to single pulses applied to cutaneous afferents at multisecond intervals. Stimulus parameters were adjusted so that a single pulse delivered to a "rested" preparation (rested = no stimulation for > 2 min) was at or just below threshold for evoking motor output. Single pulses delivered at 5- to 10-s intervals evoked strongly summating scratch motor output in D7-S2 and D8 preparations. These results show that neural mechanisms that store sensory-evoked excitation in the pocket scratch circuit exist within the spinal hindlimb enlargement and even within the isolated D8 segment. 5. With the use of in vitro preparations, we have begun to examine the pharmacology of sensorimotor processing in the pocket scratch network. Application of the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovaleric acid (APV) (50-100 microM) to the spinal cord greatly reduced pocket scratch excitability. APV lowered the motor burst frequency of pocket scratch responses in D7-S2 preparations elicited by 3-Hz stimulation; it also reduced the amplitude of summating motor output in D7-S2 and D8 preparations in response to single electrical stimuli delivered at 5-s intervals. These results indicate that NMDA receptors have a key role in synaptic processing and sustained excitation within the pocket scratch neura

Glycinergic inhibition in the turtle spinal cord regulates the intensity and pattern of fictive flexion reflex motor output.

Application of strychnine sulfate (10-50 mu M) to the anterior hindlimb enlargement of the turtle spinal cord increased the amplitude of the ipsilateral fictive flexion reflex and revealed a contralateral (crossed) fictive flexion reflex response to cutaneous stimulation of the foot. Strychnine abolished the crossed inhibition of fictive flexion reflex that was normally evoked by contralateral foot stimulation, unmasking the crossed excitation. Our observations are consistent with the hypothesis that intraspinal inhibition mediated by strychnine-sensitive glycine receptors regulates the amplitude of fictive flexion reflex motor output and confines the response to the appropriate neural pathways.

Bilateral control of hindlimb scratching in the spinal turtle: contralateral spinal circuitry contributes to the normal ipsilateral motor pattern of fictive rostral scratching.

In a spinal turtle, unilateral stimulation in the rostral scratch receptive field elicited rhythmic fictive rostral scratching in ipsilateral hindlimb motor neurons; contralateral hip motor activity was also rhythmic and out-of-phase with ipsilateral hip motor activity. When left and right rostral scratch receptive fields were stimulated simultaneously, bilateral rhythmic fictive rostral scratching was produced; left hindlimb scratching was out-of-phase with right hindlimb scratching. Thus, spinal circuits coordinate interlimb phase during bilateral fictive scratching. We examined the contributions of contralateral spinal circuitry to the normal pattern of right hindlimb fictive rostral scratching by removing the left halves of the D7 segment and the hindlimb enlargement (D8-S2 segments). After left-hemicord removal, stimulation in the right rostral scratch receptive field usually elicited a variation of rostral scratching with rhythmic right hip flexor activity and no right hip extensor activity; thus, right hip flexor rhythm generation does not require left hindlimb enlargement circuitry. Normal right hindlimb rostral scratching with rhythmic alternation between hip flexor and extensor activities was rarely observed; thus, contralateral spinal circuitry contributes to the production of normal ipsilateral fictive rostral scratching. After left-hemicord removal, stimulation in the left rostral scratch receptive field elicited rhythmic right hip extensor activity; thus, contralateral spinal circuitry can generate a hip extensor rhythm during ipsilateral rostral scratch receptive field stimulation. Our observations and those of Berkowitz and Stein (1994a,b) support the concept that an ipsilateral hindlimb's normal rostral scratch motor pattern is generated by a modular central pattern generator that is bilaterally distributed in the spinal cord.

NMDA receptors in layers II and III of rat cerebral cortex.

Glutamate receptors are found in all layers of the cerebral cortex, but NMDA receptors are concentrated in layers II and III in the adult. We investigated the location of these receptors, and their contribution to the responses of cells in layers V and VI, by iontophoresing NMDA at various distances from the cell body along the apical dendrite of the cells, first in artificial CSF, then in TTX to abolish action potentials. Comparison of responses at various distances along the apical dendrite showed that the response generally increases as distance from the cell body decreases. Comparison of responses in layers II and III, before and after TTX, showed that TTX reduced the response considerably. We conclude first that NMDA receptors in layers II and III are located primarily on cells in layers II and III, rather than on the apical dendrites of cells in layers V and VI, and second that the contribution of NMDA receptors to the response of cells in layers V and VI comes primarily from receptors close to the cell body.

Glutamate antagonists applied to midbody spinal cord segments reduce the excitability of the fictive rostral scratch reflex in the turtle.

Glutamate antagonists applied to the cutaneous-processing region of the rostral scratch circuit in turtles reduced the excitability of the rostral scratch reflex. Segments D3-D6 (D3 = 3rd postcervical) of the midbody spinal cord receive cutaneous afferents from the rostral scratch receptive field and perform the initial integration of this cutaneous sensory input. These cutaneous-processing segments are located anterior to the rostral scratch motor pattern generator that resides mainly in segments D7-D10 located in and near the hindlimb enlargement. We prepared 1 or 2 of the midbody segments for bath application of glutamate antagonists in preparations with a complete transection of the spinal cord anterior to segment D3. Each preparation was immobilized by neuromuscular blockade and fictive scratch motor output was recorded from hindlimb muscle nerves. Application of the NMDA N-methyl-D-aspartate) antagonist APV (D-2-amino-5-phosphonovaleric acid, 50 microM) to a midbody segment significantly reduced the motor burst frequency of rostral scratch responses evoked by 3-Hz electrical stimulation of a site in that segment's dermatome. These data suggest that NMDA receptors contribute to cutaneous processing in the rostral scratch circuit. Application of APV to a midbody segment also reduced the magnitude of temporal summation in the scratch circuit in response to electrical stimuli delivered to the shell at 4- to 5-s intervals. Temporal summation was monitored at the level of hindlimb motor output as well as at the level of unit activity from 'long-afterdischarge' neurons in the midbody segments. Our observations are consistent with the hypothesis that NMDA receptors contribute to the prolonged activation of 'long-afterdischarge' neurons and the multisecond storage of excitation in the scratch reflex pathway.

Vibration-evoked startle behavior in larval lampreys.

Larval lampreys (ammocoetes) exhibit a rapid vibration-evoked startle response involving a bilateral activation of musculature along the length of the body. The resulting movement is variable, contingent on the animal's prestimulus posture: lateral curves along the trunk and tail contract more on the inner side of the curve than on the outer side. Thus, the startle response increases preexisting body curvature. Because ammocoetes are burrowing filter feeders, this startle behavior results in rapid withdrawal of the head into the burrow. A vibratory pulse to the otic capsules in a semi-intact preparation evokes simultaneous action potentials in both primary Mauthner neurons. Vibration also excites several Müller cells. Intracellular stimulation of one primary Mauthner axon (eliciting one action potential) produces bilateral trunk electromyographic potentials that are smaller than those evoked by vibration; simultaneous stimulation of both Mauthner axons (one action potential each) reproduces the vibration-evoked electromyographic amplitudes. The Mauthner cell's sensitivity to vestibular input is centrally modulated during changes in behavioral state. Mauthner action potentials are most easily elicited by vibratory or electrical stimulation of vestibular afferents while an intact animal is at rest; the same stimuli become subthreshold for Mauthner activity while the animal is swimming. A similar depression of Mauthner excitability is observed in semi-intact preparations during arousal. 'Arousal' was defined by the occurrence of tonic, descending spinal cord discharge. Mauthner cells are tonically depolarized during arousal and exhibit an increased membrane conductance; excitatory postsynaptic potentials evoked by vibratory or electrical stimulation of vestibular afferents are greatly attenuated. Modulated sensory transmission to the Mauthner cell may help to prevent inappropriate activation of the startle circuit.

Cutaneous stimulation evokes long-lasting excitation of spinal interneurons in the turtle.

1. We demonstrated multisecond increases in the excitability of the rostral-scratch reflex in the turtle by electrically stimulating the shell at sites within the rostral-scratch receptive field. To examine the cellular mechanisms for these multisecond increases in scratch excitability, we recorded from single cutaneous afferents and sensory interneurons that responded to stimulation of the shell within the rostral-scratch receptive field. A single segment of the midbody spinal cord (D4, the 4th postcervical segment) was isolated in situ by transecting the spinal cord at the segment's anterior and posterior borders. The isolated segment was left attached to its peripheral nerve that innervates part of the rostral-scratch receptive field. A microsuction electrode (4-5 microns ID) was used to record extracellularly from the descending axons of cutaneous afferents and interneurons in the spinal white matter at the posterior end of the D4 segment. 2. The turtle shell is innervated by slowly and rapidly adapting cutaneous afferents. All cutaneous afferents responded to a single electrical stimulus to the shell with a single action potential. Maintained mechanical stimulation applied to the receptive field of some slowly adapting afferents produced several seconds of afterdischarge at stimulus offset. We refer to the cutaneous afferent afterdischarge caused by mechanical stimulation of the shell as "peripheral afterdischarge." 3. Within the D4 spinal segment there were some interneurons that responded to a brief mechanical stimulus within their receptive fields on the shell with short afterdischarge and others that responded with long afterdischarge. Short-afterdischarge interneurons responded to a single electrical pulse to a site in their receptive fields either with a brief train of action potentials or with a single action potential. Long-afterdischarge interneurons responded to a single electrical shell stimulus with up to 30 s of afterdischarge. Long-afterdischarge interneurons also exhibited strong temporal summation in response to a pair of electrical shell stimuli delivered up to several seconds apart. Because all cutaneous afferents responded to an electrical shell stimulus with a single action potential, we conclude that electrically evoked afterdischarge in interneurons was produced by neural mechanisms in the spinal cord; we refer to this type of afterdischarge as "central afterdischarge." 4. These results demonstrate that neural mechanisms for long-lasting excitability changes in response to cutaneous stimulation reside in a single segment of the spinal cord. Cutaneous interneurons with long afterdischarge may serve as cellular loci for multise

Interruptions of fictive scratch motor rhythms by activation of cutaneous flexion reflex afferents in the turtle.

A low-spinal immobilized turtle displays a fictive scratch reflex in hindlimb muscle nerves in response to mechanical stimulation of specific regions of the shell (Robertson et al., 1985). There are 3 forms of the scratch reflex: the rostral, the pocket, and the caudal; each exhibits rhythmic activation of hindlimb motor neurons. Cutaneous stimulation of the distal hindlimb elicits a fictive flexion reflex that exhibits tonic excitation of hip protractor (flexor) motor neurons and tonic inhibition of knee extensor motor neurons (Stein et al., 1982). In the present study, we describe the motor pattern blends that resulted from transient activation of either the ipsilateral or the contralateral flexion reflex pathway during ongoing scratch motor patterns. Two types of blends were observed: (1) insertions of a flexion reflex synergy into an interrupted scratch cycle and (2) deletions of parts of a scratch cycle. Associated with each type of motor pattern blend was a permanent reset of the ongoing scratch rhythm. The sign of the reset (phase-advance or phase-delay) could be predicted for all forms of the scratch based on the location of the foot stimulus (ipsi- or contralateral) and its timing relative to the hip protractor/retractor cycle. The timing of knee extensor activity within the hip cycle is different for each form of the scratch (Robertson et al., 1985); thus, the sign of the reset cannot be predicted from the timing of the stimulus relative to the knee extensor cycle. These results indicate the importance of the hip rhythm in determining the overall timing of the scratch reflex.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical activation of the pocket scratch central pattern generator in the turtle.

1. A low-spinal, immobilized turtle displays a fictive scratch reflex in hindlimb motor neurons in response to tactile stimulation of the shell (17, 19). Turtles exhibit three forms of the scratch reflex: rostral, pocket, and caudal. Each form is elicited by tactile stimulation of a different receptive field on the body surface. The ventral-posterior pocket (VPP) cutaneous nerve innervates the ventral-posterior portion of the pocket scratch receptive field (Fig. 1). Natural stimulation within the VPP nerve's receptive field evoked a pocket scratch reflex (Fig. 2A). Electrical stimulation of this nerve elicited robust pocket scratch reflexes (Fig. 2, B and C). 2. A single electrical pulse to the VPP nerve delivered at a voltage (greater than 5 V, 0.1 ms) that activated all the axons in the nerve was termed a "maximal" pulse. A single maximal pulse did not evoke a scratch motor response. It raised the excitability of the pocket scratch central pattern generator for several seconds, however. We revealed such excitability changes by applying maximal pulses to the VPP nerve at multisecond intervals (Figs. 5 and 6). When we delivered maximal pulses with interpulse intervals of less than or equal to 5 s, the first pulse produced no motor response and the second pulse evoked one or more cycles of pocket scratch. 3. A stimulus pulse applied to the VPP nerve was used as a probe for studying changes in the excitability of the pocket scratch CPG following scratch motor patterns. In a rested preparation, the stimulus pulse did not activate motor output. In contrast, the stimulus pulse evoked one or two cycles of pocket scratch activity if delivered within 2.5 s after the cessation of rhythmic pocket scratch motor activity (Figs. 7-9). These results are consistent with the hypothesis that the pocket scratch CPG has elevated excitability for seconds following the cessation of pocket scratch motor output. A single pulse applied to the VPP nerve evoked no response if delivered after the cessation of rostral scratch motor activity, however (Fig. 9D). 4. We used a train of maximal pulses to the VPP nerve to probe the form-specificity of the changes in the excitability following a rostral scratch motor pattern (Fig. 10). We set the stimulus parameters so that the train evoked one or two cycles of a pocket scratch motor pattern in a preparation that had rested for over 1 min.(ABSTRACT TRUNCATED AT 400 WORDS)

Cranial components of startle behavior in larval and adult lampreys.

Larval lampreys (Petromyzon marinus) exhibit a combination of cranial reflexes during their vibration-evoked startle response, including strong contractions of the gill chamber, velum and oral hood. These reflexes were confirmed by applying brief vibratory stimuli to an otic capsule and recording movement and electromyograms in moving preparations and efferent cranial nerve activity in curarized preparations. Vibration elicited efferent discharge in cranial nerves V, IX and X on both sides. The responses were lost following labyrinthectomy. The larval startle response results in water from the contracting gill chamber being expelled through the mouth and temporarily reduces head width. Reduced head width may facilitate the rapid withdrawal which is observed during startle behavior in burrowed larvae [S. Currie (1985) Neurosci. Abstr. 11, 268; S. Currie and R. C. Carlsen J. exp. Biol. (in Press)]. Adult lampreys (Entosphenus tridentata) attached to the wall of an aquarium by their suctorial disc, exhibited a brief but intense suction increase following a vibratory stimulus initiated by a tap to the aquarium wall. Oral suction (negative pressure) ranged from -0.6 to -10 cm H2O at rest and increased to values as high as -160 cm H2O during the vibration response. Suction intensity increased in direct proportion to the amplitude of the vibratory stimulus. Most of the suction response was lost following labyrinthectomy. Electromyographic recordings from the pharyngeal dilator m. basilaris and the lingual retractor m. cardioapicalis revealed stimulus-locked activity which preceded increased suction in adults, however, no vibration-evoked electromyogram responses were noted while recording from the gill chamber musculature or funnel. Stimulus-locked efferent activity was observed in the V-basilaris and V-apicalis branches of both trigeminal nerves following vibration of an otic capsule. Efferent vibration-evoked activity was lost in the trigeminal nerve after labyrinthectomy. No vibration-evoked activity was observed in nerves IX or X. Sudden vibration evoked dramatically different responses in larval and adult lampreys. Larvae contracted their gill chambers and expelled water from their mouths while adults exhibited a powerful suction reflex and no gill contraction. The trigeminal components of these behaviors (including velum and oral hood movement in larvae, pharynx and apicalis movement in adults) are difficult to compare. All of the larval trigeminal muscles degenerate during metamorphosis and are replaced by new adult muscles [M. W. Hardisty and C. M. Rovainen (1982) In The Biology of Lampreys, Vol. 4A. Academic Press, London].(ABSTRACT TRUNCATED AT 400 WORDS)

Functional significance and neural basis of larval lamprey startle behaviour.

1. The vibration-evoked startle response mediates rapid withdrawal in burrowed larval lampreys (ammocoetes). Ammocoetes withdraw in response to vibration by contracting pre-existing lateral bends in the trunk and tail, thus pulling their heads deeper into the burrow. 2. The motor effects of an ammocoete startle response are dependent on pre-existing posture. Areas of lateral body curvature contract more and exhibit larger electromyogram (EMG) amplitudes on their inner sides than on their outer sides. 3. Both of the anterior Mth and posterior Mth' (Mauthner) cells and both of the B1 and B2 (bulbar) Müller cells fired action potentials in response to vibration of the otic capsules. Both B3 and B4 Müller cells were inhibited by vibration, while M (mesencephalic) and I1 (isthmic) Müller cells were inhibited by ipsilateral vibration and excited by contralateral vibration. 4. Simultaneous action potentials in both of the anterior Mth cells were appropriate and sufficient for initiating the startle response EMG in a semi-intact preparation. 5. This study demonstrates a Mauthner-initiated startle response which activates musculature on both sides of the body to produce a rapid withdrawal movement and is thus adapted to the eel-like form and burrowed lifestyle of larval lampreys.

Plasticity of fin command system function following spinal transection in larval sea lamprey.

The descending control of dorsal fin posture by a reidentifiable reticulospinal command neuron (I1), was examined in the sea lamprey, Petromyzon marinus. Intracellular stimulation of I1 controls the posture of both dorsal fins. Spinal transection between the fins results not only in loss of control of posterior dorsal fin posture, but after 2 h, the control of the anterior dorsal fin as well. Anterior dorsal fin responses remained absent at 4, 5 and 6 days after transection. Stimulation of I1 could control anterior dorsal fin posture in specimens which had recovered the ability to right during swimming (77 and 100 days after transection), although higher than normal stimulus frequencies were required. I1 control of posterior dorsal fin posture did not recover.