Sensory interaction with central 'generators' during respiration in the dogfish.

The activity in sensory and motor nerves of the gills was recorded from selected branches of the vagus nerve in decerebrate dogfish, Scyliorhinus canicula. Vagal motoneuronal activity was observed at the start of the rapid pharyngeal contraction and was followed by sensory nerve activity which preceded the slow expansion phase. Rhythmical vagal motoneuronal activity was still present after all movements had been prevented by curare paralysis although the frequency of the rhythm was higher than in the ventilating fish. Electrical stimulation of vagal sensory fibres had 3 effects on the ventilatory movements. (1) It evoked a reflex contraction of several gill muscles after a latency of about 11 ms. (2) It could reset the respiratory cycle because a stimulus given during expansion delayed the onset of the subsequent contraction. (3) The stimulus could entrain the rhythm if it was given continuously at a frequency close to that of ventilation. The vagal motor rhythm was disrupted by trigeminal nerve stimulation in the paralyzed fish but not if the motor rhythm was being entrained by vagal nerve stimulation. Vagal sensory activity may be important, therefore, in maintaining the stability of the generating circuits.

Mechanoreceptor activity in the gills of the carp. I. Gill filament and gill raker mechanoreceptors.

Physiological properties of gill filament and gill raker mechanoreceptors in the gills of spontaneously breathing carp, Cyprinus carpio L., were analysed. Stroking stimuli applied to gill filaments elicited a phasic mechanoreceptive response, which was recorded from neurons in the epibranchial ganglia. Sustained deflection resulted in a short on-off response. The same neurons were also activated by slight movements of lamellae on a gill filament. The receptive field extended over all the lamellae of one filament at most, but generally covered a small part of it, including both dorsal and ventral lamellae. Deflection of gill rakers also elicited a brief response in epibranchial ganglion neurons. The threshold of both filament-related and gill raker mechanoreceptors was relatively high. They did not respond during normal respiration. It was therefore argued that these receptors do not function in normal respiratory control, but rather serve against mechanical damage from excessive pressure or particles in the water.

Mechanoreceptor activity in the gills of the carp. II. Gill arch proprioceptors.

The presence of gill arch proprioceptors in the gills of a teleost, Cyprinus carpio L., is demonstrated and their firing characteristics are analysed. Spontaneous activity of gill arch proprioceptors was recorded from epibranchial ganglia. In paralysed fish the mean discharge rate for 16 receptors ranged from 11.3 impulses X sec-1 (SD 0.3) to 25.7 impulses X sec-1 (SD 0.4). The discharge rate could be influenced by displacement of the main elements of the respiratory pumping system. The receptors showed a tonic response. Their firing frequency was approximately linearly related to gill arch position and, hence, showed a respiratory modulation during ventilation. Gill arch adduction caused a decrease and abduction an increase in discharge rate. In actively breathing fish the mean firing frequency of 30 neurons ranged from 9.9 impulses X sec-1 (SD 0.7) to 40.1 impulses X sec-1 (SD 6.7). These gill arch proprioceptors are located in the cartilaginous strip between the epibranchial and the ceratobranchial of each gill arch and are innervated by the pretrematic branches of the vagal branchial nerves. The role these proprioceptors play in the regulation of gill movements during both feeding and ventilation is discussed.

A hierarchy of neural control of mastication in the rat.

In rats anaesthetized with ketamine, rhythmic jaw-opening and jaw-closing movements were induced by palatal stimulation. The two masseter muscles (jaw-closing) and the four digastric muscles (jaw-opening) were fitted with electrodes, which could be used either for electrical stimulation or for recording electromyographic responses. Electrical stimulation of the masseters in the phase when the digastrics were the contracting muscles, caused responses in the digastrics. The amplitude of these responses was dependent on whether the stimulated masseters were active or not. The responses in digastric persisted when contraction of the masseters during stimulation was prevented by dantrolene sodium but they disappeared when the masseteric nerves were blocked with xylocaine. The responses in digastric are thus reflexes from stimulating afferent fibres in the masseteric nerves. Likewise, electrical stimulation of the four digastrics in the phase when the masseters were contracting, caused responses in the masseters. The amplitude of these responses, however, was independent of the state of activity of the stimulated digastrics. Furthermore, the responses in masseter disappeared when contraction of the digastrics was prevented by dantrolene sodium; but they persisted when the digastric nerves were blocked with xylocaine, provided the digastrics continued to twitch to the electric stimuli. The responses in masseter are thus reflexes in masseter caused by mechanical stretch transmitted from the digastric twitches. In the rhythmic preparation, prevention of contraction of the masseters of digastrics by dantrolene sodium or xylocaine leaves the overall frequency and amplitude of the evoked rhythmic activity unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Gill arch movements and the function of the dorsal gill arch muscles in the carp.

The activity coordination of the dorsal gill arch muscles in a teleost, the carp, is described and the effect of their contraction in combination with the respiratory pump movements is analysed. Based on their origin and insertion the dorsal branchial arch muscles can be divided into three groups: the external branchial arch levators, connecting the branchial arches to the neurocranium, the internal branchial arch levators, connecting the pharyngobranchials to the neurocranium and the dorsal oblique muscles, interconnecting the branchial arches and pharyngobranchials. Functionally, however, there are only two categories with the following properties. The first, which consists of the external branchial arch levators alone, is active during every respiratory cycle, including the cough. These muscles expand the branchial basket through gill arch abduction and, in combination with hyomandibular pumping movements, lower the floor of the buccal cavity. The results of these combined movements are: The gill arches remain evenly distributed within the expanding branchial cavities during inspiration, so that continuity of the gill curtain is maintained. Water flow resistance is reduced also. The volume of water flowing into the buccal cavity during inspiration is increased. The second category, comprising the internal branchial arch levators and the dorsal oblique muscles, contracts only during the cough and else is completely inactive. Contraction of these muscles reinforces the dorsal suspension of the gill arches by firmly anchoring the pharyngobranchials and epibranchials to the base of the skull. In this way strong, caudally directed forces which develop during the intermediate expansion of the cough can be prevented from dislocating the branchial basket.

The respiratory function of gill filament muscles in the carp.

The activity pattern of the adductor muscles of the gill filaments has been determined with E.M.G. techniques and analysed in relation to the activity of the respiratory pump muscles, the respiratory movements and the hydrostatic pressures in buccal and opercular cavities. The gill filament adductor muscles contract twice during a normal respiratory cycle. First during the transition from the contraction to the expansion phase and for a second time at the end of the expansion phase. These two contractions serve different purposes. The first 'primes' the opercular pump for the start of the next expansion phase in the following way. At the end of the contraction phase, the final adduction of the opercula results in a positive pressure in the opercular cavities. If this pressure persisted until the start of the expansion, it would make the opercular suction pump inoperative, because it would blow away the flexible opercular flap which, as a passive valve, seals the widening opercular slit during abduction. Filament adduction at the transition from contraction to expansion, however, by lowering the resistance of the gill curtain, allows water to escape from the opercular cavities through the mouth and so reduces opercular pressure to zero before expansion starts. The second contraction of the filament adductor muscles, at the end of the expansion phase, occurs when the opercular flap separates from the body of the fish, opening the opercular slit. At this moment, there is a considerable negative pressure in the opercular cavity. Nevertheless, inflow of water through the opercular slit is negligible, because flow reversal is counteracted by the kinetic energy of the normal water flow from the buccal to the opercular cavities. This process is significantly facilitated by a reduction in gill resistance through filament adduction. In the cough, a burst of filament adductor activity occurs during the intermediate expansion. It then increases water flow velocity over the gills by lowering the gill resistance and also brings the filaments in such a position that the water flows parallel to their surface, which facilitates the flushing off of foreign matter.

Peripheral influences on the central pattern-rhythm generator for tongue movements in the rat.

Rhythmic activity in the tongue and other oral muscles was evoked by mechanical stimulation of the hard palate in ketamine-anaesthetized rats. The relation between neural discharges of single hypoglossal motoneurones and activity in the masseter and anterior digastric muscle, and stimulus parameters was analysed. By varying the stimulus parameters, hypoglossal-motoneurone activity was modulated from phasic reflex activity into rhythmic activity. The tongue and other recorded muscles are likely to be controlled by the same pattern-rhythm generator, when rhythmic activity is induced by palatal stimulation. Exteroceptive oral stimuli can activate the tongue pattern-rhythm generator independently of proprioceptive feedback. Proprioceptive input from oral structures influenced the burst duration of rhythmic hypoglossal motoneurone activity. The cycle duration and the number of discharges/burst, however, was not affected by proprioceptive input. The hypoglossal-motoneurone pool may be influenced by more than one pattern-rhythm generator because the burst characteristics of rhythmically-firing hypoglossal motoneurones depend upon stimulus conditions. These hypoglossal pattern-rhythm generators have possible hierarchic relations because a shift from one burst pattern into another can be evoked by changing stimulus conditions. The pattern-rhythm generators for oral movements may be composed of a neural-pattern generator and a neural-rhythm generator, which can be modulated separately by peripheral inputs.

Masseter, digastric and omohyoidal responses from weak mechanical oral stimuli in the chewing rat.

In rats anesthetized with ketamine, effects of jaw jerks (rise time 5 ms, strength 70 mN) applied in the open direction were studied both in the quiescent preparation and during rhythmic jaw-opening and jaw-closing movements (induced by mechanical stimulation of the palate). The left masseter, posterior and anterior digastric and omohyoid muscles were fitted with electrodes for recording electromyographic responses. In the quiescent state a jaw jerk causes responses in all recorded muscles. In the rhythmic preparation masseter responses persisted but digastric and omohyoidal responses were suppressed. This suppression was most prominent when jerks were applied during closing. However, responses in digastrics and omohyoid persisted when masseter nerves and periodontal afferents of the lower incisors were blocked with xylocaine. The suppression of digastric and omohyoidal responses therefore originates--at least partly - from masseter and periodontal afferents. When the periodontal afferents alone were blocked with xylocaine only the response in the opening phase persisted. The suppression of digastric and omohyoidal responses during opening are therefore reflexes from periodontal receptors. These observations support the concept that oral activity is controlled by two relatively independent systems. The first program-oriented system generates the basic movements whereas the second, environment-oriented, system has access to the program for correcting perturbations. There is evidence that muscles poorly supplied with muscle spindles (digastrics and omohyoid) act as pure effectors. Modulation of their activity is prompted by exteroceptors and muscle receptors in the masticatory muscles.

Interaction of respiration with coughing, feeding, vision and oculomotor control in fish.

Data on the interaction between respiration, coughing, feeding, vision and oculomotor control in fish have been analyzed. A self-contained respiratory rhythm generator in the brainstem reticular formation is, at times, interrupted by a coughing generator, which is partly autorhythmic and partly under vagal afferent control. A number of cranial muscles are shared by the respiratory and masticatory systems. Others are predominately allocated to one of these systems but are recruited by the other under heavy loading conditions. The effect of retinal image displacements caused by respiration-induced eye movements appears to be centrally corrected during normal respiration. In addition, excessive eye movements are reduced by extraocular muscle contraction during intense respiration and cough. Both the central visual and the oculomotor corrections are based on open-loop control through input from the respiratory center.

Respiratory responses to stimulation of branchial vagus nerve ganglia of a teleost fish.

The effects of electrical stimulation of epibranchial vagus ganglia upon respiration of the carp were investigated. Single shocks evoked fast twitch responses in a number of respiratory muscles with latencies around 18 msec to the beginning and 30-35 msec to the peak of activity. Shocks given during abduction decreased the respiratory cycle duration by shortening abduction and accelerating adduction. Stimuli given throughout most of adduction also shortened the respiratory cycle, accelerating the adduction only. These responses are similar to vagally mediated lung receptor reflexes of mammals. Stimulation with short trains of pulses produced a rapid expansion-contraction movement. This movement resembles in all respects (shape, time in the respiratory cycle, muscle coordination) the intermediate expansion of a normal coughing movement. Continual stimulation at frequencies close to the normal respiratory rate had a synchronising influence upon respiration, speeding up or slowing down its rate. HRP applied to the third vagal ganglion showed that there is a small projection of this ganglion to the nucleus intermedius facialis, although the majority of sensory fibres terminate in the vagal lobe. The nucleus intermedius facialis is already known to connect directly with the respiratory motor centres and thus might provide a pathway for the fast twitch response. A projection was also found to the nucleus ambiguus; in mammals this nucleus plays an important role in the regulation of respiratory movements.

Tegmental neurons controlling medullary respiratory centre activity in the carp.

The dorsal mesencephalic tegmentum of the carp was explored for respiratory rhythmic neurons. The properties of two types of neurons which could be distinguished on the grounds of their firing pattern and their behaviour during paralysis and electrical stimulation were analysed. One group is involved in the control of a special type of respiration called bout-respiration and besides influences the frequency of the respiratory rhythm. The other group can modulate the output of the motor neurons of the cranial respiratory muscles via a multiple central route. HRP injections revealed extensive connections between these tegmental respiratory neurons and the respiratory centres in the medulla oblongata.

Identification of respiratory motor neurons in the carp and determination of their firing characteristics and interconnections.

Motor neurons of the respiratory muscles of the carp, located in the medulla oblongata, are identified with a signal averaging technique. Analysis of the firing characteristics of these neurons provides data on their activity patterns under different circumstances. The results show that, on the one hand, the number of spikes a motor neuron fires per respiratory cycle depends on the respiratory intensity, and can even be as low as one action potential per respiration. On the other hand, with constant respiratory intensity, the activity of individual motor neurons can stop and be resumed again in periods of several minutes. During mechanical loading of respiration, action potential synchronization between specific respiratory muscles takes place. Possible neuronal circuits causing this synchronization, which is thought to increase the coordination of the muscular effort, are discussed.

Neural control and proprioceptive load matching in reflex respiratory movements of fishes.

A brief anatomical description of topographical arrangements of respiratory neurons in the fish respiratory medulla introduces a discussion centered on neural controls of reflex breathing movements during rhythmic and ram gill ventilation, especially the role of proprioception in "load matching." Paralysis experiments show that proprioceptive signals are processed in medullary respiratory areas of the brain. Some neurons depend completely on proprioceptive input for their activity, while others "mix" proprioceptive and rhythmic information. The existence of direct proprioceptive control loops is revealed by fast compensating changes in electrical activity of respiratory muscles following mechanical interference with respiratory movements. Responses of respiratory neurons to short artificial twitches of respiratory muscles show that some neurons process proprioceptive information at higher levels of integration, often with a long latency and after effect. Other neurons react immediately to the stimulus and often show properties of elements in proprioceptive control systems in which length receptors facilitate and tension receptors inhibit motor neuron activity. Stimulus-response averaging applied to neuron activity and respiratory electromyograms, and to electrical stimuli and neurograms, leads to the conclusion that motor neurons and neurons mediating sensory signals from the respiratory muscles are situated close together in the medullary respiratory areas. Neuroanatomical studies are discussed which indicate that the sensory projection fields of some trigeminal fibers terminate adjacent to the cranial motor nuclei. These presumably project onto motor neurons forming fast disynaptic reflex arcs relaying sensory trigeminal information.

Proprioceptive motor control in fish respiration.

The response of single respiratory neurones in the medulla oblongata of carp to short twitches of individual respiratory muscles were analysed. The muscle contractions were obtained through automatic electrical stimulation and could be consistently elicited in a predetermined phase relation to the ventilatory cycle. The results show that, apart from nerve cells which take part in long-term processing of proprioceptive information from several sources, neurones also exist which possess the properties of elements of a peripheral proprioceptive control loop such as tension receptor neurones, length or stretch receptor neurones and motor neurones.