Vitreal and intraretinal responses to contrast reversing patterns in the pigeon eye.

The pattern electroretinogram (PERG) has been recorded vitreally and intraretinally in the pigeon eye. The amplitude of the PERG increases monotonically as pattern contrast is increased, with saturation at high levels. The PERG of the central yellow field has band-pass spatial tuning, with a high frequency cut-off at 8 c/deg.

Multiple equilibria and exotic behaviour in excitable membranes.

The excitation equation for an excitable membrane dV/dt = F(V) may have multiple equilibria where F(V) = 0, and these may be stable or unstable. We demonstrate multiple equilibria in the Hodgkin-Huxley equations when either -gK or [Ca2+]0 is lowered in the presence of a hyperpolarizing current density. Under these conditions molluscan somata exhibit exotic behaviours -- endogenous paroxysmal depolarising shifts and complex multiple spikes reminiscent of the normal complex activity of some mammalian central neurones.

The distribution of centrifugal terminals in the pigeon retina.

The terminals of the centrifugal fibres of the pigeon retina have been labelled by anterograde axonal transport or diffusion of horseradish peroxidase, and their distribution studied in whole amounts of the retina. Centrifugal terminal arborisations are concentrated in a band near the projection of the horizontal meridian on the retina. Within this band there are high density areas next to the area centralis and in the mid-temporal retina.

Artefacts of intrusion in the pattern electroretinogram.

The pattern electroretinogram (PERG) was recorded intravitreally. Comparison of responses recorded in both eyes and at the optic tectum shows that the PERG does not have an origin in distant CNS generators, but rather, that it has an ocular origin.

The distribution of displaced ganglion cells in the retina of the pigeon.

Displaced ganglion cells in the pigeon's retina, at the inner margin of the inner nuclear layer, were labelled by retrograde axonal transport of horseradish peroxidase (HRP). Large HRP injections were made in order to fill all the retinal projection sites in the thalamus and midbrain. The distribution of labelled cells was studied in retinal whole mounts incubated with tetramethyl benzidine (TMB) substrate for HRP.

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The induction of periodic and chaotic activity in a molluscan neurone.

During prolonged exposure to extracellular 4-aminopyridine (4 AP) the periodic activity of the somatic membrane of an identified molluscan neurone passes from a repetitive regular discharge of greater than 90 mV amplitude action potentials, through double discharges to less than 50 mV amplitude oscillations. Return to standard saline causes the growth of parabolic amplitude-modulated oscillations that develop, through chaotic amplitude-modulated oscillations, into regular oscillations. These effects are interpreted in terms of the actions of 4 AP on the dynamics of the membrane excitation equations.

Repetitive activity of a molluscan neurone driven by maintained currents: a supercritical bifurcation.

The amplitude and rate of transient and maintained responses of an identified giant neurone of the pond-snail, Lymnaea stagnalis, to a maintained injected current were investigated. The responses of the isopotential soma to a maintained current density of less than 10 microamperemeter cm -2 includes repetitive discharges of action potentials, sawtooth oscillations and amplitude-modulated oscillations. The frequency of the maintained response increases as the injected current increases, while the amplitude decreases until the maintained response is a stable steady state at a membrane potential of about -20 mV.

Classifying and comparing retinal ganglion cells.

A simple numerical method is suggested for comparing the types of ganglion cell in the cat, monkey, rabbit and pigeion, using the W,X,Y classification as a basis.

Ionic channel density of excitable membranes can act a bifurcation parameter.

As the maximal K+-conductance (or K+-channel density) of the Hodgkin-Huxley equations is reduced, the stable resting membrane potential bifurcates at a subcritical Hopf bifurcation into small amplitude unstable oscillations. These small amplitude solutions jump to large amplitude periodic solutions that correspond to a repetitive discharge of action potentials. Thus the specific channel density can act as a bifurcation parameter, and can control the excitability and autorhythmicity of excitable membranes.

Autorhythmicity and entrainment in excitable membranes.

Low calcium increases the excitability of neurones and can induce autorhythmicity in excitable cells. Numerical solutions of the Hodgkin-Huxley membrane equations, and numerical evaluations of the small-signal impedance and admittance are used to illustrate the increase in resonance produced by low [Ca2+]0. The resonant frequency may be located either by the peak of the amplitude of the impedance, or by the frequency at which the phase angle is zero for 1 : 1 entrained action potentials.

Field potentials evoked in the avian optic tectum by diffuse and punctiform luminous stimuli.

An investigation was made of the slow field potentials evoked in the pigeon optic tectum by visual stimulation. Diffuse illumination of the retina produces a polyphasic response which is negative-going in the surface layers and reverses polarity at 200--400 micron. It is suggested that this response reflects activity in radially disposed nerve cells.

Unitary and field potential responses in the pigeon optic tectum evoked by luminous stimuli.

Receptive fields of retinotectal axons and of tectal cells were plotted, and the magnification factor at the tectal surface was measured. In the SGFS, receptive fields were often below 6 degrees in size and are probably derived from radially disposed pyriform cells. In the SGC, receptive fields were up to 40 degrees in size and are probably derived from the multipolar cells.

Size classes of ganglion cells in the central yellow field of the pigeon retina.

Ganglion cells in the central yellow field of the pigeon retina were identified by retrograde transport of horseradish peroxidase from the optic tectum. The soma size range was from 5 to 16 micrometers, with the mean at 7.7 micrometers.

Identification of endogenous and exogenous activity in a molluscan neurone by spike train analysis.

The statistical properties of background spike train activity recorded from a molluscan neurone are used to identify lengths of discharge which are produced by endogeneous pacemaker mechanisms. Such pacemaker discharge has an infinitely divisible interspike interval probability density function.