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D KENNEDY

Publications and source records attributed to D KENNEDY.

At least 19 recordsLinked to original sources
PubMed1965

THE MECHANISM OF DISCHARGE PATTERN FORMATION IN CRAYFISH INTERNEURONS.

Excitatory and inhibitory processes which result in the generation of output impulses were analyzed in single crayfish interneurons by using intracellular recording and membrane polarizing techniques. Individual spikes which are initiated orthodromically in axon branches summate temporally and spatially to generate a main axon spike; temporally dispersed branch spikes often pace repetitive discharge of the main axon. Hyperpolarizing IPSP's sometimes suppress axonal discharge to most of these inputs, but in other cases may interact selectively with some of them. The IPSP's reverse their polarity at a hyperpolarized level of membrane potential; they sometimes exhibit two discrete time courses indicating two different input sources. Outward direct current at the main axon near branches causes repetitive discharges which may last, with optimal current intensities, for 1 to 15 seconds. The relation of discharge frequency to current intensity is linear for an early spike interval, but above 100 to 200 impulses/sec. it begins to show saturation. In one unit the current-frequency curve exhibited two linear portions, suggesting the presence of two spike-generating sites in the axon. Current threshold measurements, using test stimuli of different durations, showed that both accommodation and "early" or "residual" refractoriness contribute to the determination of discharge rate at different frequencies.

Animals↗
PubMed1964

IMPULSE ORIGIN AND PROPAGATION IN A BIPOLAR SENSORY NEURON.

Intracellular recording techniques were used to study electrical activity in bipolar sensory cells associated with crayfish tactile receptors. Several lines of evidence indicate that spikes evoked by natural stimulation of the receptor originate at a dendritic locus. Although overshooting spikes are recorded in the soma in response to both natural and antidromic stimulation receptor potentials are observed only rarely, and, when present, their amplitude is less than 5 mv. Impulses propagating centrifugally into the soma following antidromic stimulation always exhibit an inflection in the rising phase of the spike; however, orthodromic spikes are usually uninflected. Occasionally, orthodromic responses (in the soma) exhibit rather unusual wave forms. Such spikes evoked by natural stimuli are indistinguishable from those elicited electrically in the dendrite, but they do not resemble antidromic impulses. Because the axonal and dendritic boundaries of the soma have a low safety factor for spike transmission, at high frequencies invasion of the soma by dendritic spikes is impeded and often blocked. The soma region can thus act as a low-pass filter. The significance of this self-limiting mechanism for the behavior of the animal is not known; it is suggested, however, that this impediment is a potentially critical one, and may, in other situations, have encouraged the evolution of alternative arrangements.

Animals↗
PubMed1963

Physiology of photoreceptor neurons in the abdominal nerve cord of the crayfish.

Nerve fibers which respond to illumination of the sixth abdominal ganglion were isolated by fine dissection from connectives at different levels in the abdominal nerve cord of the crayfish. Only a single photosensitive neuron is found in each connective; its morphological position and pattern of peripheral connections are quite constant from preparation to preparation. These cells are "primary" photoreceptor elements by the following criteria: (1) production of a graded depolarization upon illumination and (2) resetting of the sensory rhythm by interpolated antidromic impulses. They are also secondary interneurons integrating mechanical stimuli which originate from appendages of the tail. Volleys in ipsilateral afferent nerves produce short-latency graded excitatory postsynaptic potentials which initiate discharge of one or two impulses; there is also a higher threshold inhibitory pathway of longer latency and duration. Contralateral afferents mediate only inhibition. Both inhibitory pathways are effective against both spontaneous and evoked discharges. In the dark, spontaneous impulses arise at frequencies between 5 and 15 per second with fairly constant intervals if afferent roots are cut. Since this discharge rhythm is reset by antidromic or orthodromic impulses, it is concluded that an endogenous pacemaker potential is involved. It is postulated that the increase in discharge frequency caused by illumination increases the probability that an inhibitory signal of peripheral origin will be detected.

Animals↗
PubMed1962

Spontaneous activity in crustacean neurons.

Single units which discharged with regular spontaneous rhythms without intentional stimulation were observed in the ventral nerve cord by intracellular recording close to the sixth abdominal ganglion. These units were divided into two groups: group A units in which interspike intervals varied less than 10 msec.; group B units in which interspike intervals varied within a range of 10 to 30 msec. Group A units maintained "constant" interspike intervals and could not be discharged by sensory inputs, while the majority of group B units could be discharged by appropriate sensory nerve stimulation. Both group A and B units discharged to direct stimulation when the stimulating and recording electrodes were placed in the same ganglionic intersegment, and directly evoked single spikes reset the spontaneous rhythm. In group B units, presynaptic volleys reset the spontaneous rhythm of some units; but in others, synaptically evoked spikes were interpolated within the spontaneous rhythm without resetting. The phenomenon of enhancement could also be demonstrated in spontaneously active units as a result of repetitive stimulation. It is concluded that endogenous pacemaker activity is responsible for much of the regular spontaneous firing observed in crayfish central neurons, and that interaction of evoked responses with such pacemaker sites can produce a variety of effects dependent upon the anatomical relationships between pacemaker and synaptic regions.

Neurons↗
PubMed1961

The spectral sensitivity of crayfish and lobster vision.

(1) The spectral sensitivity function for the compound eye of the crayfish has been determined by recording the retinal action potentials elicited by monochromatic stimuli. Its peak lies at approximately 570 mmicro. (2) Similar measurements made on lobster eyes yield functions with maxima in the region of 520 to 525 mmicro, which agree well with the absorption spectrum of lobster rhodopsin if minor allowances are made for distortion by known screening pigments. (3) The crayfish sensitivity function, since it is unaffected by selective monochromatic light adaptation, must be determined by a single photosensitive pigment. The absorption maximum of this pigment may be inferred with reasonable accuracy from the sensitivity data. (4) The visual pigment of the crayfish thus has its maximum absorption displaced by 50 to 60 mmicro towards the red end of the spectrum from that of the lobster and other marine crustacea. This shift parallels that found in both rod and cone pigments between fresh water and marine vertebrates. In the crayfish, however, an altered protein is responsible for the shift and not a new carotenoid chromophore as in the vertebrates. (5) The existence of this situation in a new group of animals (with photoreceptors which have been evolved independently from those of vertebrates) strengthens the view that there may be strong selection for long wavelength visual sensitivity in fresh water.

Adaptation, Ocular↗
PubMed1960

Activity patterns of interneurons in the caudal ganglion of the crayfish.

Responses of ascending interneurons from the caudal ganglion of crayfish have been recorded from single units isolated by dissection from the ventral nerve cord; in addition, post-synaptic activity within the ganglionic neuropile has been studied with intracellular micropipettes. The following classes of interneurons have been found: (1) Large fibers which responded to tactile stimuli with single spikes or phasic bursts. These units usually showed broad receptive fields; and spontaneous activity, when present, showed transitory depressions following responses to natural stimuli. (2) A group of fibers, including many small ones, which responded to proprioceptive stimuli with tonic discharges of varying adaptation rate. (3) Interneurons which showed responses both to tactile stimuli and to activation of the sixth ganglion photoreceptor; and (4) units with constant frequency discharges which were unmodifiable by any of the above afferent inputs. Intracellular recording of post-synaptic activity has shown (1) that widely graded excitatory post-synaptic potentials occur; (2) that multiple firing from single synaptic potentials is usual; (3) that the post-synaptic responses to phasic natural stimuli and to electrical stimulation of ganglionic roots are similar. The existence of widely graded post-synaptic potentials and of extensive receptive fields suggests a high degree of convergence from primary afferents to interneurons. The activation of such post-synaptic units involves integrative synaptic transfer, without 1:1 correspondence between pre- and post-fiber activity.

Animals↗
PubMed1960

Integrative synaptic mechanisms in the caudal ganglion of the crayfish.

A study of activity recorded with intracellular micropipettes was undertaken in the caudal abdominal ganglion of the crayfish in order to gain information about central fiber to fiber synaptic mechanisms. This synaptic system has well developed integrative properties. Excitatory post-synaptic potentials can be graded, and synaptic potentials from different inputs can sum to initiate spike discharge. In most impaled units, the spike discharge fails to destroy the synaptic potential, thereby allowing sustained depolarization and multiple spike discharge following single pulse stimulation to an afferent input. Some units had characteristics which suggest a graded threshold for spike generation along the post-synaptic fiber membrane. Other impaled units responded to afferent stimulation with spike discharges of two distinct amplitudes. The smaller or "abortive" spikes in such units may represent non-invading activity in branches of the post-synaptic axon. On a few occasions one afferent input was shown to inhibit the spike discharge initiated by another presynaptic input.

Animals↗
PubMed1960

Neural photoreception in a lamellibranch mollusc.

The pallial nerves of Spisula solidissima each contain a single afferent nerve fiber which responds directly to illumination of the nerve, and apparently mediates the "shadow" response of siphon retraction. These units show constant-frequency spontaneous activity in the dark; illumination abruptly inhibits this discharge, and cessation of the light stimulus then evokes a prolonged burst of impulses at high frequency (the off-response). Impulses are initiated at a point near the visceral ganglion, and propagated unidirectionally toward it. Stimulation with monochromatic light has revealed that more than one photoreceptor pigment is involved, since the discharge patterns evoked are wavelength-specific. Inhibition is relatively prominent at short wavelengths, excitation at long wavelengths. Following selective adaptation with blue light, "on" responses can be produced with red stimuli, demonstrating the unmasking of an excitatory event which takes place during illumination. The two photoreceptor pigments may be segregated in two or more cells presynaptic to the recorded unit, or,-more likely-may both be contained in the same cell. The spectral sensitivity function for inhibition shows a single maximum at 540 mmicro, and is probably dependent upon a carotenoid pigment. No photoreceptor function has been demonstrated for a hemoprotein, apparently identical with cytochrome h, which occurs in high concentration in Spisula nerve.

Animals↗
PubMed1957

The visual system of the alligator.

The eye tissues and liver of the alligator contain vitamin A(1) alone. The retina contains rhodopsin, typical in absorption spectrum (lambda(max) 500 mmicro); but synthesized in solution from neo-b retinene and opsin much more rapidly than are the frog, mammalian, or chicken rhodopsins previously examined. In this regard alligator rhodopsin resembles the rhodopsins and porphyropsins of fishes, all of which so far investigated are synthesized rapidly in solution. The rates of synthesis in vitro of frog and alligator rhodopsins are matched closely by the rates of rod dark adaptation in living frogs and alligators, measured electrophysiologically at the same temperature. Alligator rods dark-adapt, and alligator rhodopsin is synthesized in solution, at rates characteristically associated with cones and cone pigments in frogs, mammals, and birds.

Alligators and Crocodiles↗