Command Fibers Research Articles

Descending control of nonspiking local interneurons in the terminal abdominal ganglion of the crayfish.

1. Electrical stimulation of afferents innervating an exopodite causes a closing pattern of activity in the uropod motor neurons. In this reflex two distinct types of nonspiking local interneurons, posterolateral (PL) and anterolateral (AL) types, classified by their gross morphology and somata location, receive sensory inputs and control the motor output to the uropod. 2. In the sensory-motor pathway, the PL and AL nonspiking local interneurons formed opposing and parallel connections with uropod motor neurons. For example, the PL interneurons that excited the closer, reductor motor neuron by injecting depolarizing current received depolarizing postsynaptic potentials (PSPs), whereas the AL interneurons of the same output received hyperpolarizing PSPs. The PL interneurons that inhibited the reductor motor neuron received hyperpolarizing PSPs, whereas the AL interneurons of the similar output received depolarizing PSPs. 3. During fictive abdominal extension, induced by electrical stimulation of extension-evoking command fibers in the second-third abdominal connective, the uropod motor neurons show an opening pattern of activity that is opposite to the pattern elicited by sensory stimulation. Furthermore, sensory stimulation during ongoing fictive abdominal extension has little effect on the uropod motor neurons. 4. Except for the nonspiking local interneurons, the majority of other local circuit neurons, i.e., spiking local interneurons and ascending interneurons, are not driven by the descending inputs during abdominal extension. 5. A comparison of the responses of the nonspiking local interneurons to both sensory and descending inputs reveals that the majority of nonspiking local interneurons receive both inputs, but the sign of response to each is frequently opposite. This study suggests that the degree of excitability of two distinct types of PL and AL nonspiking local interneurons induced by sensory inputs changes depending on whether the crayfish is in a resting posture or is active with full extension of the abdomen. Ongoing abdominal extension in swimming or defensive crayfish would shift the gain of reflex pathways through the PL and AL interneurons, and motor response resulting from sensory inputs would be modulated.

Serotonin-containing neurons in lobsters: their role as gain-setters in postural control mechanisms.

1. The electrophysiological properties of two pairs of identified serotonin-containing neurons in the fifth thoracic (T5) and first abdominal (A1) ganglia of the lobster, Homarus americanus, were studied with the use of intracellular recording methods. Intracellular dye injection combined with immunocytochemistry verified the neurochemical status of the recorded neurons. 2. The serotonin-containing neurons usually are spontaneously active at 0.5-1.0 Hz and produce large, overshooting action potentials with a prominent after-hyperpolarization. The action potentials appear to be generated by a pacemaking mechanism endogenous to the cells. Extracellular recordings from thoracic connectives and from second thoracic roots show that action potentials from the cells in A1 and T5 are propagated rostrally along their axons and invade axon collaterals that innervate neurohemal organs in the second thoracic roots and the pericardial organs. These observations suggest that these serotonin-containing cells may function in part as important neurosecretory cells in the lobster. 3. Members of the pairs of serotonin-containing cells are not synaptically connected. They receive prominent inhibitory inputs in the form of inhibitory postsynaptic potentials (IPSPs), which exhibit discrete size classes and probably arise from several sources. Most IPSPs are temporally synchronized among the two pairs of serotonin-containing cells. 4. The serotonin-containing cells respond to stimulation of postural command fibers, with flexion command fibers exciting and extension command fibers inhibiting the cells, suggesting that these cells are a part of the postural flexion circuitry. 5. Intracellular activation or inhibition of the serotonin-containing cells has no effect on the spontaneous readout of postural motor programs recorded from motor nerve roots. Coactivation of the serotonin-containing cells and command fibers, or inhibition of the serotonin-containing cells while activating command fibers, however, shows that the cells act as "gain-setters," modulating the interaction between command inputs and motoneuron outputs. 6. About 24% of the motor neuron units analyzed are influenced by the serotonin-containing cells. There is a bias toward facilitation of the readout of flexion motor programs, particularly with stimulation of strong and moderate flexion command fibers. 7. The serotonin-containing cells in T5 and A1 ganglia are hypothesized to serve two functions, one tonic and the other phasic, in modulating behavioral output in lobsters. Tonic firing of the cells should result in a sustained release of serotonin from central and peripheral sets of nerve terminals, which, in turn, could influence peripheral and central targets of the amine.(ABSTRACT TRUNCATED AT 400 WORDS)

From command fibers to command systems to consensus. Are these labels really useful anymore?

From command fibers to command systems to consensus. Are these labels really useful anymore?

Postural interneurons in the abdominal nervous system of lobster

Using intracellular recording and dye-filling techniques, a survey of postural interneurons was undertaken by impaling their somata in the 2nd abdominal ganglion of lobster. During the course of study approximately fourty different intersegmental interneurons in this ganglion were sampled. Of these, 8 evoked unique, patterned responses in the postural (superficial) motoneurons; each could be identified morphologically. Five of the 8 interneurons had caudally directed axons; 4 of these projected beyond the 4th abdominal ganglion. The remainder projected rostrally, beyond the 1st abdominal ganglion. The postural interneurons were classified according to the motor program they elicited. Five were flexion producing interneurons (FPIs), one was extension producing (EPI), and two generated only inhibitory motor outputs. All motor responses were bilateral and occurred in several segments, including A2. Two neurons, FPIs 201 and 301, produced the full motor reciprocity that typically is observed when flexion command fibers are stimulated. However, three of the FPIs and the single EPI did not express complete reciprocity in synergistic and antagonistic motoneurons. The results indicate that some interneurons displaying all of the properties of command neurons are located entirely within the abdominal nervous system. The overall organization of posture-evoking interneurons appears to be similar to that found in crayfish, suggesting an even more fundamental homology in the neuronal connectivities of these two species than has been established previously.

The organization of flexion-evoking interneurons in the abdominal nerve cord of the crayfish, Procambarus clarkii.

Intracellular recording and dye injection was used to investigate the flexion-evoking interneurons in the isolated abdominal nerve cord of the crayfish, Procambarus clarkii. The interneurons described in this study have all of the characteristics of cells called flexion command fibers in earlier works. These interneurons show cell bodies in all of the abdominal ganglia, all are interganglionic, and those with posteriorly directed axons enter the terminal caudal ganglion. Some cells were found in as many as eight different preparations, supporting the idea that these interneurons are identified cells. In addition to presenting evidence for identities, evidence was obtained for serial homology in this premotor system. A summary of the general organization of the flexion premotor apparatus is offered. It consists primarily of parallel elements (command interneurons) that are either bipolar or monopolar whose axonal projections course to the terminal caudal ganglion or rostrally into the thoracic region, perhaps into the cerebral ganglion. When activated singly, these parallel elements do not recruit additional interneurons but there are data to suggest that several of these elements could act in concert and interact in a low-gain fashion to produce coordinated positioning behavior.

Phase plane description of crayfish swimmeret oscillator

Crayfish swimmeret system shows rhythmic, coordinated behavior when the command fibers are stimulated chronically by electrical pulses, and the oscillating frequency becomes faster with increasing stimulus frequency. This behavior is organized by the distributed neural oscillators in the abdominal ganglia. We investigated the dynamics of the neural oscillators which are controlled by command fibers. Phase resetting experiment technique was used for this purpose; a temporary cessation of commanding pulses, which was regarded as suppressive perturbation for the neural oscillator, was applied to the chronically stimulated oscillator, and phase transition curves (PTCs) were measured. For the short cessation of command pulses, type 1 PTCs were obtained. With increasing cessation length, the PTC shifted downward, and finally changed into type 0. We also measured PTCs for temporarily increased stimulus frequency, which was an excitatory perturbation for the neural oscillator and increased the frequency of the oscillation transiently. For the short excitatory perturbation, the PTCs were also type 1 and shifted upward. PTCs changed their shapes from type 1 into type 0, as increasing the perturbation length. These shapes of the PTCs contain important information about the properties of the neural oscillator. Analyzing these PTCs, we present a phase plane diagram which describes the character of the command control of the neural oscillator.

Central organization of crustacean abdominal posture motoneurons: connectivity and command fiber inputs.

Intracellular recordings and Lucifer dye injections were used to locate five of the six tonic abdominal flexor motoneurons, as well as several of the extensor motoneurons in the fourth abdominal ganglion of the crayfish, Procambarus clarkii. Each motoneuron was identified by its morphology and root records, and each was characterized by its inputs from identified flexion- and extension-evoking interneurons (command fibers). Finally its connections to other motoneurons were examined by current injection experiments. As indicated by others, the somata of most of the tonic flexor motoneurons are electrically silent; recordings were, therefore, made from neuropilar branches, but at unknown locations. The majority of motoneurons were found to be polysynaptically connected to the command neurons, although several apparent monosynaptic excitatory and inhibitory connections were also seen. Coupling between motoneurons was confirmed, but the major functional input to the motoneurons was from premotor interneurons. Coupling between motoneurons and interneurons is also indicated by the data, and may be responsible for the inhibition of some motoneurons seen during depolarizing current injections into other motoneurons. Again, such coupling is shown to be a minor influence in the overall organization of the behavior. The data suggest that flexion and extension behavior is predominantly organized via interneurons including the command fibers rather than by connections among the motoneurons themselves.

Interneurons involved in abdominal posture in crayfish: Structure, function and command fiber responses

1. An intracellular search was undertaken in an effort to locate and characterize the elements involved in command-driven positional behavior in the isolated abdominal nerve cord of the crayfish,Procambarus clarkii. Small bundles containing flexion or extension-evoking command fibers were isolated in the anterior connectives; these bundles are considered here to be equivalent to identified command fibers. A microelectrode filled with Lucifer Yellow CH was used to find elements within the fourth abdominal ganglion that showed synaptic responses to either flexion or extension command fibers; and these elements were further characterized by current and dye injections. 2. The interneurons thus encountered have been classified primarily by the form of motor output they evoked when strongly depolarized, and secondarily by their morphologies. We present data in this paper from 12 types of flexion-evoking cells, 4 types of extension-evoking cells and one inhibitory type that we term ‘flexion-antagonistic’. 3. With the exception of this latter group, many of the interneurons evoked a fully or nearly fully organized motor output. That is, flexion-evoking interneurons activated the flexor motoneurons and the peripheral inhibitor to the extensor muscles, and usually inhibited much of the activity in the extensor motoneurons. Conversely, the extension-evoking interneurons activated the extensor motoneurons and the peripheral inhibitor to the flexor muscles and usually inhibited the flexor motoneurons. The flexion-antagonists inhibited the flexor motoneurons and the peripheral inhibitor to the extensor muscles, but did not activate any motoneurons. 4. Because of the search strategy used, the majority of the cells that were studied showed synaptic connections to one or more of the three flexion and one extension command fiber bundle stimulated. Both apparently monosynaptic and polysynaptic connections were evident, but very few inhibitory connections were found. Thus, while flexion-evoking interneurons were in general depolarized by the flexion CFs, they were not hyperpolarized by the extension CF. Furthermore, we have demonstrated that several flexion-evoking interneurons share inputs from more than one flexion CF, whereas others have inputs from only one of three flexion CFs. Finally, at least two interneurons were found (type 5 and 6) that were apparently independent of the command neurons used in this study. 5. Six of the flexion-evoking types were seen to have their somata within ganglion 4 and all had axonal processes extending either anteriorly or posteriorly. The remaining 6 flexor interneurons were seen only as an axonal process running through ganglion 4. We cannot determine from the present data their origins or the extent of their axons in the nerve cord. In several preparations, both forms of flexion-evoking cells were seen to produce motor output in the ganglia adjacent to G4, suggesting that these cells may be either command neurons or driver interneurons. Different experimental approaches will be required to resolve this question. 6. While we cannot at this point propose the organization of CF-driven postural behavior, we have encountered several potentially important elements involved in the control of abdominal flexion. The motor output appears to be achieved by a combination of direct synaptic connections from the CF to the motoneurons and through connections with several other interneurons acting in parallel. A combination of these connections may provide for the strong reciprocity between flexor and extensor motoneurons, since individual driver cells are not always capable of this reciprocal output. Finally, redundancy of driver cells is indicated since we have not been able to identify any critical elements that, when functionally removed by hyperpolarization, eliminate a significant portion of the CF driven motor output.

Command fiber activation of superficial flexor motoneurons in the lobster abdomen

1. In the first four abdominal segments of the lobster,Homarus americanus, the superficial flexor muscle on each side is innervated by five motoneurons and a peripheral inhibitory neuron. The somata of these neurons are approximately twice the diameter of the homologous neurons in the crayfishProcambarus clarkii. 2. In the fifth abdominal segment each superficial flexor muscle is innervated by four motoneurons and a peripheral inhibitory neuron. 3. Spontaneously occurring EPSPs and IPSPs were recorded from the somata of the largest motoneuron (f6) and the peripheral inhibitory neuron (f5) which innervate the superficial flexor muscles in the third abdominal segment of the lobsterHomarus americanus. 4. The two f5 neurons and the two f6 neurons of the third abdominal ganglion were electrically coupled. 5. Stimulation of some axons in the abdominal connectives (Category I) produced EPSPs in the f5 and f6 neurons which were phase locked with the stimulus pulses. These effects were characterized by comparatively short latency (mean 40 ms), absence of inhibition and restriction of effects to neurons located in the ipsilateral half of the ganglion. 6. Other connective fibers (Category II), when stimulated electrically, produced synaptic potentials in f5 and f6 which were not phase locked with the stimulus pulses. These responses were characterized by their long latencies (mean 125 ms), the simultaneous synaptic activation of both ipsilateral and contralateral neurons and the reciprocity of their synaptic effects. 7. Category II fibers correspond to the flexion, extension and suppression “command fibers” which have been extensively studied in the crayfish. 8. The results of this paper support the hypothesis that command fibers exert their motor effects by activating one or more ‘driver interneurons’.

Command fibers: only strategic points in neuronal communication systems

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Load compensation in the crayfish abdomen

1. Unit discharge of the superficial extensor motor neurons (SEMNs) and the tonic sensory neuron (SRI) of the muscle receptor organ (MRO) was monitored during extensions of the loaded crayfish abdomen. Extensions were evoked by stimulation of command fibers in the circumesophageal connectives. 2. The excitatory SEMNs increased their discharge when extending against a load. 3. During most extensions of the loaded abdomen the SR1 was silent. When the SR1 was active, its activity was accompanied by excitation of SEMNs No. 4 and No. 2 as proposed by Fields (1966). 4. In 6 of the 7 preparations in which the load was restricted to a single joint (A2–A3) the SR1 which spanned that joint was silent although the SEMNs increased their discharge in response to the load. 5. It is concluded that in extensions driven by single command fibers the principle load sensors in the abdomen are not the MROs. The load compensating increase in SEMN discharge is the result of excitation of an unidentified load sensitive sensory system.

Abdominal abductor muscle in crayfish: Physiological properties and neural control. I-twitch system

1. 1. Abdominal abductor dorsal head ( aad) muscle of the crayfish, Procambarus clarki (Girard) were investigated electrophysiologically. 2. 2. The aad muscle consisted of “fast” contracting muscle fibers of 3 μm in sarcomere length. They receive complex, polyneuronal innervation from 5 excitors and 1 inhibitor. 3. 3. Stimulation of 4 excitors produced fast contraction of the aad muscle, whereas 1 showed slow contraction. 4. 4. Unilateral stimulation of either of the command fibers, 286 or 318 in the circumoesophageal connective evoked the discharges in the 6 axons of different combination and asymmetrical activation of the aad muscles and led to the rolling of the abdomen relative to the thorax.

Command fiber control of crayfish abdominal movement

1. The control of abdominal extension and flexion movements in the crayfishOrconectes rusticus, O. virilis andProcambarus clarkii was examined by monitoring the afferent and efferent unit activities in the dorsal nerve of the second abdominal segment during unrestrained abdominal extensions and flexions which were evoked by stimulation of command fibers (CFs) in the circumesophageal connective (CC). 2. CFs which when stimulated generate coordinated extensions and flexions are present in the CC ofP. clarkii. 3. Two classes of extension CFs are present in the CC ofP. clarkii: 1. those which excite the motor neuron (#4) which innervates the tonic receptor muscle (RM1) of the muscle receptor organ; 2. those which excite one or more of the excitatory motorneurons which do not innervate RM1 (#2, #3 and #6). 4. The extensor motorneurons are activated in reciprocal sets by CF stimulation. The inhibitory motor neuron (#5) is silent during CF evoked extensions (inP. clarkii.) 5. During flexion CF stimulation the inhibitory extensor motorneuron is excited while activity in the excitatory extensor motorneurons is suppressed. 6. The strong reciprocity observed in the superficial extensor motor neuron (SEMN) activities evoked by CF stimulation contrasts with the weak reciprocity which is characteristic of extensor motorneuron activities generated by the intact animal which moves its abdomen in response to a strong sensory stimulus (Sokolove, 1973). This difference in reciprocity supports the hypothesis that an intact animal initiates a postural movement of its abdomen by the conjoint excitation of several CFs. 7. Few if any extension CFs are present in the CC ofO. rusticus orO. virilis.

Central control of cardiac and scaphognathite pacemakers in the crab,Cancer magister

Command fibers located in the circumesophageal connectives which modify scaphognathite and heart rhythms have been mapped and characterized in the crab,Cancer magister. Behavior: Crabs show a variety of responses to external stimuli often including simultaneous cessation of cardiac and scaphognathite “pumping”. Habituation and a return to prestimulus rhythms results from continued stimulation. The response to short stimulus durations, on the other hand, generally outlasts the stimulus indicating the playing-out of a motor program. Neurophysiology: Small bundles of fibers have been isolated from desheathed connectives. Activity in these fibers resulting from stimulation of various anterior sensory receptors was recordeden passant with suction electrodes. When sensory stimulation produced both electrical activity in the nerves under examination and a cardiac and/or scaphognathite response it was assumed such units were involved in inducing this response. This was tested by electrical stimulation delivered through the same electrode. Those units which produced similar responses to natural and artificial stimulation were deemed “command fibers”. It was invariably found that the minimum stimulating frequency needed to mimic naturally induced responses was much greater than the frequency at which the units discharged in response to those stimuli. During mapping experiments, command fibers were characterized with respect to their positions in the connectives and by the responses they produced at different frequencies of stimulation. 68% of the fibers identified affected both cardiac and scaphognathite systems, 29% the scaphognathites alone and 3% the heart alone. The frequency-response profiles of single bivalent command fibers were often different from the heart and scaphognathites. These findings help explain the responses of both systems to natural stimuli and also indicate that the circulatory and respiratory systems not only perform in concert, but are often under common control.

The central nervous organization underlying control of antagonistic muscles in the crayfish. I. types of command fibers

Abstract1. A limited number of “command” interneurons mediate the release of coordinated ganglionic output to antagonistic postural muscles in the crayfish abdomen. Graded contractions of these muscles are regulated by the frequency of discharge in several motoneurons, whose own discharge is in turn controlled by the level of activity in command fibers.2. Different command fibers having a similar effect (i.e., flexion or extension) vary in the relative effectiveness with which they excite specific motoneurons. This selectivity is responsible for different rates of tension development.3. Command fibers connect with the postural motor system in each segment in a bilaterally symmetrical fashion. Many of the same command elements also have unilateral or reciprocal effects on other motor systems, especially on those which control the swimmerets and uropods.4. The reciprocity between flexors and extensors is inherent in the ganglionic organization. Activation or modulation of this ganglionic center is accomplished by the command interneurons, which in turn are activated by appropriate combinations of sensory inputs. A wide variety of postural adjustments may thus be controlled by a limited number of central neurons that make complex but specific output connections.

The central nervous organization underlying control of antagonistic muscles in the crayfish. II. Coding of position by command fibers

AbstractSingle central interneurons that produce flexion or extension in the crayfish abdomen act in a coordinated fashion upon several ganglia. Each of several elements evoking a similar type of movement has a unique distribution of output to ganglionic centers: thus one of them may produce primarily rostral extension or flexion, another a primarily caudal movement, still another a more general one. Each motor command therefore appears to code for a specific abdominal geometry. Some units produce complex, cyclic motor outflow to postural muscles of the abdomen, or to the appendages.

Interneurons commanding swimmeret movements in the crayfish, Procambarus clarki (girard)

1. 1. Five interneurons causing bilateral rhytmic swimmeret movements when stimulated are described with their localizations in the cross-section of the cord between thorax and abdomen. 2. 2. At about 30 stimuli per sec the resulting rhythm is equivalent to that present during natural beats. Below 20 and above 100 stimuli per sec no rhythmic movements are obtained. 3. 3. As during endogenously caused rhythmicity, the swimmerets of the fifth ganglion are the first to respond. When the fifth ganglion is removed, discharges are first elicited in the first roots of the fourth ganglion, and similarly in those of the third, when the fourth is absent. No rhythmic discharges are caused in the second and first ganglion, when no posterior ganglia are present. 4. 4. The rhythmicity elicited by three of the five command fibers is indistinguishable. The other two give slightly different responses, one a more sustained one, the other a somewhat slower rhythm. 5. 5. At frequencies of stimulation away from the “optimal” (30/sec), a longer delay is observed before the first burst occurs when below, a shorter delay when above this frequency. The conduction time in the anterior direction between ganglia is not influenced, but the repetition rate of the bursts is higher with increased frequencies. 6. 6. Hemisections of the cord, involving one connective, have no influence on the ensuing bursts in any of the roots, when made in the anterior part of the cord. 7. 7. Hemisections between the third, fourth and fifth ganglia result in a dependency on the laterality of the cut. When ipsilateral to the command fiber, such cuts prevent beating in the roots posterior to the cut, but not when made contralateral. 8. 8. At least three different interneurons which inhibit the rhythmic motor discharges of both sides were shown to be present.