Showing papers in "The Journal of Physiology in 1966"

The variation in isometric tension with sarcomere length in vertebrate muscle fibres.

Abstract: 1. The variation of isometric tetanus tension with sarcomere length in single fibres from frog striated muscle has been re-investigated with special precautions to ensure uniformity of sarcomere length within the part of the fibre being studied. 2. In most respects the results of Ramsey & Street (1940) were confirmed, but (a) the peak of the curve was found to consist of a plateau between sarcomere lengths of 2·05 and 2·2 μ, (b) the decline of tension above this plateau is steeper than found by Ramsey & Street, and (c) the decline of tension below the plateau becomes suddenly steeper at a sarcomere length of about 1·67 μ. 3. Many features of this length—tension relation are simply explained on the sliding-filament theory. 4. It is concluded that, in the plateau and at greater lengths, the tension on each thin filament is made up of equal contributions from each bridge which it overlaps on adjacent thick filaments. 5. Internal resistance to shortening is negligible in this range but becomes progressively more important with shortening below the plateau.

The contrast sensitivity of retinal ganglion cells of the cat.

Abstract: 1. Spatial summation within cat retinal receptive fields was studied by recording from optic-tract fibres the responses of ganglion cells to grating patterns whose luminance perpendicular to the bars varied sinusoidally about the mean level. 2. Summation over the receptive fields of some cells (X-cells) was found to be approximately linear, while for other cells (Y-cells) summation was very non-linear. 3. The mean discharge frequency of Y-cells (unlike that of X-cells) was greatly increased when grating patterns drifted across their receptive fields. 4. In twenty-one X-cells the relation between the contrast and spatial frequency of drifting sinusoidal gratings which evoked the same small response was measured. In every case it was found that the reciprocal of this relation, the contrast sensitivity function, could be satisfactorily described by the difference of two Gaussian functions. 5. This finding supports the hypothesis that the sensitivities of the antagonistic centre and surround summating regions of ganglion cell receptive fields fall off as Gaussian functions of the distance from the field centre. 6. The way in which the sensitivity of an X-cell for a contrast-edge pattern varied with the distance of the edge from the receptive field centre was determined and found to be consistent with the cell's measured contrast sensitivity function. 7. Reducing the retinal illumination produced changes in the contrast sensitivity function of an X-cell which suggested that the diameters of the summating regions of the receptive field increased while the surround region became relatively ineffective.

The excitatory synaptic action of climbing fibres on the Purkinje cells of the cerebellum.

Abstract: 1. A single climbing fibre makes an extraordinarily extensive synaptic contact with the dendrites of a Purkinje cell. Investigation of this synaptic mechanism in the cerebellum of the cat has been based on the discovery by Szentagothai & Rajkovits (1959) that the climbing fibres have their cells of origin in the contralateral inferior olive. 2. Stimulation in the accessory olive selectively excites fibres that have a powerful synaptic excitatory action on Purkinje cells in the contralateral vermis, evoking a repetitive spike discharge of 5-7 msec duration. Almost invariably this response had an all-or-nothing character. In every respect it corresponds with the synaptic action that is to be expected from climbing fibres. 3. Intracellular recording from Purkinje cells reveals that this climbing fibre stimulation evokes a large unitary depolarization with an initial spike and later partial spike responses superimposed on a sustained depolarization. 4. Typical climbing fibre responses can be excited, but in a much less selective manner, by stimulation of the olive-cerebellar pathway in the region of the fastigial nucleus, there being often a preceding antidromic spike potential of the Purkinje cell under observation. 5. Impaled Purkinje cells rapidly deteriorate with loss of all spike discharge, the climbing fibre response being then reduced to an excitatory post-synaptic potential. This potential shows that stimulation of the inferior olive may evoke two or more discharges at about 2 msec intervals in the same climbing fibre. The complexity of neuronal connexions in the inferior olive is also indicated by the considerable latency range in responses. 6. A further complication is that, with stimulation in the region of the fastigial nucleus, the initial direct climbing fibre response is often followed by a reflex discharge, presumably from the inferior olive, which resembles the responses produced by inferior olive stimulation in being often repetitive. 7. Typical climbing fibre responses have been evoked by peripheral nerve stimulation and frequently occur spontaneously. 8. An account is given of the way in which the responses evoked by climbing fibres in the individual Purkinje cells can account for the potential fields that an inferior olive stimulus evokes on the surface and through the depth of the cerebellar cortex. 9. By the application of currents through the recording intracellular electrode it has been possible to effect large changes in the excitatory post-synaptic potential produced by a climbing fibre, it being diminished and even reversed with depolarizing currents and greatly increased by hyperpolarizing currents.

Optical quality of the human eye.

Abstract: 1. Optical quality of the eye was measured at eight pupil sizes between 1.5 and 6.6 mm diameter by recording the faint light emerging from the eye; this light was reflected from the bright image of a thin line on the fundus.2. The nature of the fundus reflexion was examined; it was found that the fundus acts very much like a perfect diffuser while retaining polarization.3. Using the result that the fundus acts like a diffuser, the recorded line images were Fourier analysed to provide modulation transfer functions. These functions indicate an optical quality considerably higher than that found in previous physical studies.4. Linespread profiles were then derived from the modulation transfer functions. These profiles are 40% narrower than those of previous physical studies for a 3.0 mm pupil. The narrowest profile occurred with a 2.4 mm pupil.5. Our results demonstrate that physical and psychophysical studies can yield similar estimates of optical quality. The influence of optical factors not common to both techniques is discussed. Evidence for the existence of neural ;image sharpening' mechanisms is reviewed.

S‐potentials from colour units in the retina of fish (Cyprinidae)

Abstract: 1. S-potentials recorded from the excised tench retina left undisturbed in the optic cup show colour cells of the two types originally described by Svaetichin & MacNichol (1958). 2. One type (green/blue, G/B) is depolarized by signals from green cones and hyperpolarized by blue cones. The other type (red/green, R/G) is depolarized by deep red and hyperpolarized by green cones. 3. By superposing spectral flashes upon steady adapting lights it is possible to find a spectral range in which only one kind of cone is effective. In this range the effect of any spectral light may be matched with that of any other provided the energies are linked in a fixed ratio that defines the action spectrum of the pigment. 4. The green pigment has an action spectrum with maximum at 540 nm and corresponds well with the pigment that Marks measured in `green' cones. The blue pigment has not been measured, but it probably corresponds with that found by Marks in `blue' cones. However, the red pigment whose action spectrum we measured had its maximum at 680 nm, whereas the difference spectrum of Marks's red cone pigment peaked at 620 nm. The 620 nm cones excite the luminosity S-units but not the R/G units. 5. In the range where only one type of cone is effective the relation between the light intensity, I0, and V0, the S-potential generated (both expressed in suitable units), is given by equation (1) p. 545. It is the relation that would be found if cone signals increased the conductance through a polarized `S-membrane' in proportion to the flux of caught quanta.

Orientational selectivity of the human visual system

Abstract: 1. It is known that an object is less detectable when it is viewed against a background containing structures similar to the object. The effect of changing the orientation between the object and background is investigated.2. Gratings of variable contrast were generated on two oscilloscopes; these were superimposed optically. The angle of orientation between them could be changed. The threshold of one grating, the test grating, was determined in the presence of the other, the masking grating.3. When the gratings were presented with the same orientation (and locked in phase) the increment threshold of the test grating was found to be proportional to the suprathreshold contrast of the masking grating.4. As the angle between the test and masking gratings was increased the masking effect fell exponentially.5. At 12 degrees on either side of a vertical test grating the masking effect was reduced by a factor of two with respect to its maximum value. This angle was independent of the contrast level of masking, the focus, and also the phase coherence of the masking grating.6. If the test grating was presented obliquely the effect of masking was slightly less.7. The narrow orientationally tuned channels found psychophysically by this masking technique are compared with the orientationally sensitive cells discovered electrophysiologically in the visual cortex of the cat.

S-potentials from luminosity units in the retina of fish (Cyprinidae)

Abstract: 1. S-potentials were recorded in fish from units which never responded by depolarization. These hyperpolarizing units are the L-units of Svaetichin & MacNichol (1958). 2. Figure 5 shows some sets of action spectra from a single unit. For each curve the criterion of action was hyperpolarization to a fixed level, by lights of various wave-lengths. When these lights fell upon zero background (circles) the curves show that two kinds of cone contribute to the action spectrum, one with the 620 nm pigment of Marks and one with the 680 nm pigment of Naka & Rushton (1966a). 3. When the lights fell upon (i) a fixed green background (triangles, Fig. 5), or (ii) a fixed red one (squares), the action spectra changed in a way that indicated greater prominence of (i) the 680 nm system (ii) the 540 nm green system that was not conspicuous without adaptation to red. 4. These observations (on the tench Tinca) are contrary to the conclusions of Svaetichin & McNichol (on Gerridae) that the action spectrum is unaltered in shape by adaptation to coloured lights. The contribution of the green cones, for example, was actually absolutely greater under deep red adaptation. 5. It is concluded that L-units receive signals from 680, 620, 540 nm and possibly also the blue cones, that the quantum catch in all these contribute to the hyperpolarization produced, but their interaction is more complicated than the simple addition of independent cone effects.

The effect of orientation on the visual resolution of gratings

Abstract: 1. Visual resolving power is known to be poorer for objects oriented obliquely as compared with horizontal and vertical orientations. Experiments were designed to evaluate the optical and neurophysiological factors involved. 2. Gratings with a sinusoidal light distribution were generated on the face of an oscilloscope. Spatial frequency and contrast could be varied while keeping the mean luminance of the grating constant. 3. Using a homatropinized eye with an artificial pupil and carefully corrected refraction, high resolution in the vertical and horizontal meridians as compared with the oblique meridians was found for gratings ranging in spatial frequency from 1 to 35 c/deg. 4. It is concluded from the similar behaviour of low and high frequency gratings that neither focus errors nor optical aniseikonia can account for these findings. 5. Additional proof that optical factors cannot significantly account for these preferred directions of resolution was obtained by forming interference fringes directly on the retina using a neon—helium laser as a coherent light source. 6. Similar orientational changes in resolution were found by by-passing the dioptrics with interference fringes. It is concluded that the effect is due to some orientational inequality in the visual nervous system.

Tension development in highly stretched vertebrate muscle fibres

Abstract: 1. An apparatus is described by which the length of a selected part of an isolated muscle fibre can be held constant, giving isometric conditions, or alternatively its length can be measured while tension is held constant (isotonic). Control can be switched between length and tension so as to carry out afterloaded contractions with a shortening stop. 2. When a part of a fibre with uniform striation spacing is stretched so far that there is presumably no overlap of filaments, the tension developed during an isometric tetanus with this apparatus is very small (not more than 3-5% of the tension developed at optimum length). 3. If the tendon ends are held stationary, a fibre with the same initial length develops a large amount of tension (order of 30-40% of tension at optimum length) with a slow time course. This additional tension is due to shortening of the end parts of the fibre, where the striation spacing is smaller and overlap of filaments still exists. 4. The resistance to elongation of a part of a fibre where there is no overlap is only slightly increased on stimulation. 5. To a first approximation, the results are in good agreement with expectations based on the sliding filament theory. The development of detectable amounts of tension, and of a slight increase of stiffness, on stimulation, are however not expected on the simplest form of this theory; possible explanations are discussed.

Micro-electrophoretic studies of neurones in the cat hippocampus.

Abstract: 1. Drugs have been applied micro-electrophoretically to units in the hippocampal cortex of the anaesthetized cat, and their effects on cell firing were recorded simultaneously. 2. L-Glutamate rapidly and powerfully excited hippocampal units, an effect which was quickly reversed on stopping the expelling current. The local application of L-glutamate also excited a fast seizure discharge at 15-50/sec. Both these effects of L-glutamate were strongly depressed by fimbrial stimulation. 3. γ-Aminobutyric acid had a strong depressant action on all the units on which it was tested; the time course of this effect was rapid. 4. ACh excited half the units to which it was applied. Characteristically this excitation developed slowly over many seconds and persisted after stopping the expelling current. Most cholinoceptive units were found to be concentrated in the superficial layer of the cortex corresponding to the hippocampal pyramidal cells and their main dendritic processes. 5. Atropine selectively blocked the excitation of cholinoceptive units by ACh, but not the excitation by L-glutamate. No cholinoceptive units were blocked by dihydro-β-erythroidine, though several were selectively blocked by dimethyl (+)-tubocurarine. 6. The most usual effect seen with 5-HT was depression, though several units were found to be excited. Some of the units tested with 3-hydroxytyramine (dopamine) or noradrenaline were found to be depressed.

The diffusion of radiopotassium across intercalated disks of mammalian cardiac muscle

Abstract: 1. Experiments were performed to determine whether intercalated disks represent a high or a low resistance to the diffusion of K ions. 2. Bundles of sheep ventricular fibres, 0·6-0·8 mm thick and 7-11 mm long, were pulled through a hole in a partition. One half was charged by radiopotassium (K*); the other half was washed by inactive Tyrode solution. 3. A steady state with respect to the distribution of K* along the bundle was reached 6 hr after the exposure to radioactivity. 4. On the `washed' side [K*] fell along a relatively smooth curve and did not tend to the value zero towards the cut end of the bundle. The average space constant for the decrease of [K*] was 1·55 mm, corresponding to a distance of about 12 × the length of a cell. This indicates that the disk resistance is low and that the fibres are healed over at their cut ends. 5. Rhythmical contraction had no detectable effect on the space constant. This may mean either that shortening and thickening of a cell does not effectively mix its content, or that stirring goes on even in resting cells. 6. On the assumption of mixing between disks the average resistance of 1 cm2 of disk to the movement of K ions is 3 Ω. If there is no mixing this value would be even lower. 7. The assumption that K* distribution within the `washed' half of the bundle is a consequence of cell-to-cell rather than of extracellular movement is supported by the results of several control experiments. 8. It is concluded that the propagation of the cardiac action potential is possible by local circuit currents.

The inhibitory innervation of the taenia of the guinea-pig caecum.

Abstract: 1. The inhibitory innervation of the taenia of the guinea-pig caecum has been studied, after blocking the responses to stimulation of excitatory cholinergic nerves with atropine.2. Stimulation of the perivascular nerves supplying the taenia caused relaxations. These nerves had properties which were typical of sympathetic post-ganglionic adrenergic nerves. The relaxations caused by stimulation were maximal at frequencies of stimulation above 30 pulses/sec and they were abolished by bretylium, guanethidine and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP).3. The taenia is also innervated by intramural inhibitory nerves with their cell bodies in Auerbach's plexus. These nerves can be excited by electrical stimulation of the taenia or by the application of ganglion-stimulating drugs.4. The intramural inhibitory nerves have different properties from sympathetic adrenergic nerves. Relaxations in response to stimulation were maximal with frequencies of stimulation of about 5 pulses/sec and they were not blocked by bretylium, guanethidine or DMPP.5. Preganglionic cholinergic fibres in the caecal wall make synaptic connexions with the intramural inhibitory neurones.6. The role of the intramural inhibitory neurones in intestinal activity and their possible connexions with the central nervous system have been discussed.

The inhibitory action of noradrenaline and other monoamines on spinal neurones.

Abstract: 1. L-Noradrenaline (NA), 5-hydroxytryptamine (5-HT) and acetylcholine (ACh) were administered micro-electrophoretically to feline lumbar neurones while recording their spike potentials extracellularly. 2. There was no evidence to suggest that NA acts as an excitatory transmitter in the spinal cord. 3. NA had potent inhibitory effects on some interneurones as revealed by a depression of spontaneous and synaptic firing and on the firing to a local application of an excitant amino acid. The effects on Renshaw cells and motoneurones were less marked. 4. The depressant actions of 5-HT were less marked than those of NA. ACh and carbamylcholine had depressant effects on some NA-sensitive interneurones but were invariably far less potent and on other NA-sensitive cells were completely inactive. 5. NA had no detectable effect on the normal spike amplitude but when the action potentials were reduced by excessive depolarization then both NA and synaptic inhibition increased the spike amplitude; this effect could be due to a hyperpolarization of the cell membrane. 6. There was a correlation between the distribution of NA-sensitive cells and the relative densities of NA-containing terminals in various layers of the grey matter. 7. It was postulated that NA acts as an inhibitory transmitter released from the terminals of descending pathways in the spinal cord. Other possible mechanisms were discussed but lacked experimental support.

The reflex excitation of the soleus muscle of the decerebrate cat caused by vibration applied to its tendon

Abstract: 1. Vibration was applied longitudinally to the fully innervated soleus muscle of the decerebrate cat by attaching its tendon to a vibrator. Vibration at frequencies of 50-500/sec with amplitudes of 10 mu upwards caused the muscle to contract reflexly for as long as the vibration was maintained. The response was recorded myographically by a myograph mounted upon the vibrator, and electromyographically by gross ;belly-tendon' leads. The reflex contraction produced several hundred g wt. of tension and involved too many motor units for their discharges to be separable. The maintained reflex was abolished by making the preparation spinal or by anaesthetizing it with pentobarbitone, but it persisted after removing the cerebellum.2. The minimum latency for the appearance of the reflex response at the beginning of a period of vibration was about 10 msec. The latency of cessation of the response at the end of vibration was similarly short.3. On increasing the amplitude of vibration at any particular frequency in the range 100-300/sec the resulting reflex tension increased to an approximate plateau for amplitudes of vibration of 100-200 mu. Further increase in the amplitude decreased the size of the contraction, though there was no such reduction in records of the ;integrated' electromyogram.4. Such large amplitudes of vibration also reduced the tension, and shortened the duration, of a twitch contraction of the muscle elicited by stimulating its nerve. The strength of a tetanic contraction was much less affected by vibration than was that of the twitch contraction, and the muscle action potential elicited by stimulation of the nerve was unaffected. Thus, large-amplitude vibration influenced the contractile mechanism of the muscle (cf. Buchtal & Kaiser, 1951).5. Increasing the frequency of vibration increased the value of the plateau tension reached on increasing the amplitude. The effect was, however, relatively small and the largest increase seen was 3 g wt. of contractile tension per c/s increase in vibration frequency.6. The primary afferent ending of the muscle spindle is considered to be the receptor whose excitation leads to the reflex response to vibration. The vibration reflex thus appears to be the well-known stretch reflex, elicited by a rather unusual form of stretching. The size of the vibration reflex and its variation with frequency are discussed in relation to the servo theory of muscular contraction.

Anomalous rectification in the metacerebral giant cells and its consequences for synaptic transmission

Abstract: 1. In the central neurones that have so far been examined the passive electrical properties of the extrasynaptic membrane has been shown to be relatively constant in the subthreshold range. Consequently, excitatory synaptic potentials produced by chemical transmission tend to vary in amplitude with changes in membrane potential, decreasing with depolarization and increasing with hyperpolarization. 2. In the two symmetrical giant cells of the ventral metacerebrum of the snail, the EPSPs failed to show the expected alterations in amplitude with changes in membrane potential. Near the resting level the EPSP increased slightly with membrane depolarization and decreased slightly with hyperpolarization. 3. These paradoxical results were not attributable to a change in transmitter release since similar results were obtained when ACh, the putative transmitter, was released iontophoretically on to the cell membrane by means of an extracellular pipette. 4. Measurement of the current—voltage relation of the extrasynaptic membrane revealed two types of rectifying conductance changes. The first, an increase in conductance with depolarization, was turned on at a depolarization of about 15 mV. Its conductance change was similar to the delayed rectification familiar from studies of peripheral nerve and muscle. The second occurred on either side of the resting level, from about 15 mV hyperpolarization to about 10 mV depolarization, and manifested itself as a decrease in conductance with depolarization and an increase with hyperpolarization. By analogy to a similar phenomenon known to occur in skeletal muscle this second rectification has been termed anomalous rectification. 5. The average resistance at 25 mV hyperpolarization was 2·3 × 106 Ω, while at 10 mV depolarization it was 2·1 × 107 Ω, yielding an average rectification ratio of 10 for the anomalous conductance change. 6. The anomalous rectifying conductance seems to account for the paradoxical behaviour of the EPSP and ACh response to changes in membrane potential. Moreover, the finding that the sharpest change in the anomalous rectification curve occurred on either side of the resting level suggests that this rectification is functionally important as a postsynaptic determinant of synaptic efficacy. Several additional lines of evidence in support of this suggestion have been obtained.

Mechanical transmission in a Pacinian corpuscle. An analysis and a theory

Abstract: 1. An analysis is made of the transmission of mechanical forces through the Pacinian corpuscle. In particular, forces are analysed which produce pressure differences at the centre of the corpuscle and lead to excitation of the sensory nerve ending.2. The main structural elements in force transmission through the corpuscle are the lamellae, their interconnexions, and the interlamellar fluid. The two former provide the elastic elements and constraint for the fluid; and the latter, the viscous elements. The mechanical equivalent incorporating these elements is a system of dashpots (the lamellar surfaces and the interlamellar fluid) and springs (the lamellae and their interconnexions); it is a mechanical filter which suppresses low frequencies. The dynamic and static patterns of lamella displacements in the equivalent are in close agreement with those observed in Pacinian corpuscles.3. Steady-state and transient pressure fields were determined for the equivalent. Under static compression, only elastic forces exist in the corpuscle. Analysis shows that such forces are transmitted poorly from periphery to centre through the lamellated structure. The compliance of the lamellar interconnexions is so high in relation to that of the lamellae themselves, that most of the pressure load is carried by the outer lamellae. As a result, only a small fraction of the steady-state pressure at the outer surface reaches the centre of the corpuscle where the sensory ending is located. This is the mechanical basis of receptor adaptation.4. Under dynamic compression, viscous forces develop in the corpuscle; and these account for most of the pressure at times too early for development of elastic deformations. Analysis shows that such forces are transmitted well. For example, if a typical corpuscle of 500mu diameter is compressed by 20mu linearly during 2 msec, the pressure differences near to the centre of the corpuscle are initially as high as at the periphery, and stay within the same order throughout the process of compression. In general, pressure at the centre increases steeply with velocity of compression. This explains the marked velocity dependence of the generator response of the sensory ending.If, in the foregoing example, the 20mu compression is held fixed after 2 msec, the pressure differences at the centre fall abruptly to near zero with the onset of the static phase. The duration of pressure transients at the centre approximates that of the ;active' phase of the generator current of the sensory ending derived from experiments, as expected in a causal relationship: pressure difference --> generator current. Taken together with the earlier experimental finding of marked prolongation of generator response in corpuscles partially stripped of lamellae (Loewenstein & Mendelson, 1965), this result warrants the conclusion that the mechanical filter action of the corpuscle is the rate-limiting factor in generator response adaptation.5. When the corpuscle is released from compression, energy stored in the elastic elements during compression is released and consumed in viscous flow. Thus, viscous pressure is produced anew. The magnitude of this pressure depends on the velocity of release. The pressure distribution is rotated by 90 degrees with respect to that in compression; i.e. during release, compression occurs once again, but this time at right angles to the direction of initial compression. Experiments show that the sensory ending does not discriminate such a rotation; the polarity and order of magnitude of the generator response to compression in one plane are the same as in another. Analysis shows that considerable pressure differences may be developed at the centre of the corpuscle during releases at physiological velocities. For instance, in a passive return from a compression of 20mu, the pressure difference at the centre (and the generator current) is of the same order of magnitude as that in a compression of the velocity in 4. This accounts for the ;off'-response of the sensory ending in purely mechanical terms.

Rod and cone interaction in dark-adapted monkey ganglion cells

Abstract: 1. Rod—cone interaction has been studied by analysing the response latency of large ganglion cells in the perifovea of dark-adapted Rhesus monkey retina. 2. Both rod and cone signals have been found to converge on such cells. The cone system is less sensitive but much faster than that of the rods so that the cones determine latency whenever stimuli become suprathreshold for them. Responses to dimmer stimuli are determined entirely by the rods. 3. The earliest signals to excite the ganglion cell leave a transitory refractoriness in their wake. Therefore when both rods and cones are stimulated simultaneously, the earlier cone signal, arriving at the ganglion cell, has a greater chance of producing excitation than the later rod signals.

An attempt to analyse colour reception by electrophysiology.

Abstract: 1. The problem of colour reception is that we do not know the action spectra of the visual pigments involved, the nature of the signals generated nor the interaction between these signals. We only know the incident light and the electric results of interaction. 2. In Part 1 we show that S-potentials from red/green (R/G) units saturated with deep red light show this property: added green light pulls down the ceiling of depolarization, but more added red had no power to raise it again. Thus lights that depress the deep red ceiling equally stimulate the green pigment equally. From this the action spectrum of the green pigment can be obtained. 3. If we assume that only two visual pigments are involved in the R/G unit, and that lights which do not pull down the deep red ceiling are below the threshold for green cones, then in this range only the red pigment is excited and we may obtain its action spectrum. Its maximum is at 680 nm where no visual pigment so far has been found. 4. In Part 2 we consider the following mathematical problem: `Is it possible that two pigments of given action spectra could combine their outputs in such a way that the resultant would be identical with the output of a third pigment of given action spectrum, for every intensity of every monochromatic light?' The solution shows that this is always mathematically possible, and the necessary interaction function is deduced. 5. It is shown further that if the log action spectra are the `visual parabolas' that resemble Dartnall's nomogram, then the interaction function is simply a linear transform such as Hartline & Ratliff (1957) have found with lateral inhibition in Limulus and Donner & Rushton (1959) with silent substitution in the frog. 6. An interaction that matches a single pigment to perfection for all monochromatic lights will not match it for certain mixtures. By this criterion the 680 nm excitability is a pigment and not the resultant of two other pigments, i.e. pigments more excitable in other spectral regions. 7. In Part 3 monochromatic lights are matched by red+green mixtures that give identical responses. From this the action spectrum of the red pigment may be obtained without involving nerve organization (except as a null detector). The result, which has one arbitrary constant, is given by the curves of Fig. 10, the continuous curve R or one of the dotted curves. Of these only curve R is acceptable. 8. Knowing the action spectra for red and green cones we may consider what signals are generated and how they interact to give the records. Figure 11 suggests a model that will account for the size and sign of S-potentials as function of the quantum catch by the two pigments. It does not embrace the time or space parameters which can be very complex.

Post‐tetanic hyperpolarization and electrogenic Na pump in stretch receptor neurone of crayfish

Abstract: 1. Two types of after-potentials in the stretch receptor neurone of crayfish are described. 2. A short-duration after-hyperpolarization associated with a single spike or a few spikes is diminished and reversed on applying hyperpolarizing currents. However, a much longer-lasting post-tetanic hyperpolarization (PTH) is enhanced by conditioning hyperpolarization; thus, no reversal potential can be obtained. 3. No changes in membrane conductance occur during PTH. 4. Reducing K concentration in the bathing fluid diminishes PTH, while it shifts the reversal potential of the short after-potential toward greater negativity. 5. Replacement of Na with Li, or addition of 2,4-dinitrophenol in the bathing fluid suppresses PTH in a reversible manner. 6. Electrophoretic injection of Na into the cell induces a long-lasting hyperpolarization. 7. No change in K-equilibrium potential, as indicated by the reversal point of the short after-potential, is detected during PTH. 8. It is concluded that the short after-potential is caused by a permeability increase for potassium ion, whereas PTH is produced by an electrogenic Na-pump.

The time and voltage dependence of the slow outward current in cardiac Purkinje fibres

Abstract: 1. In normal Tyrode solution the initial inward sodium current which is produced in Purkinje fibres in response to a sudden depolarization is followed by a very slow change in current in an outward direction. The magnitude and speed of onset of this slow change both increase with the strength of the depolarization. 2. On repolarization, a transient outward current is observed (Deck & Trautwein, 1964). The initial magnitude of this outward current also increases with the strength and duration of the preceding depolarization, with a time course similar to the time course of the slow change in current during depolarization. 3. Evidence is presented for the view that the slow change in current during depolarization represents the onset of delayed potassium rectification and that the decline in outward current following repolarization represents its decay. 4. Removal of sodium ions greatly increases the threshold of the slow outward current. In the presence of sodium ions 15 mV depolarization is sufficient to produce an appreciable current. In the absence of sodium ions at least 50 mV depolarization is required and, in some fibres, no delayed rectification is observed even when the membrane potential is made positive. 5. Prolonged depolarization of the membrane by outward current changes the quiescent membrane potential in a positive direction. This change in potential is attributed to an accumulation of potassium ions in a space immediately outside the cell membrane, which equilibrates slowly with the extracellular fluid. It is shown that this effect is very small during depolarizations of the magnitude and duration of the normal action potential. 6. It is concluded that the results are consistent with the view that repolarization of the Purkinje fibre action potential is initiated by a slow increase in K conductance similar to, but much smaller and slower than, that observed in nerve fibres.

Unstirred layers in frog skin.

Abstract: 1. Estimates of the magnitudes of the unstirred regions associated with isolated frog skin in sulphate Ringer's solution have been made under different stirring conditions. 2. The method of investigation was an analysis of the time course of the p.d. transients which occurred when external sodium concentration and internal potassium concentration changes were made in the bathing solution. 3. Making an arbitrary but reasonable assumption about the diffusional coefficient of Na2SO4 in the outer unstirred region, the magnitudes of the outer unstirred layers were found to lie within the ranges 40-60 μ, 30-50 μ and 30-40 μ under stirring conditions of 120, 300 and 500 rev/min, respectively. 4. Making an arbitrary but reasonable assumption about the diffusion coefficient of K2SO4 in the inner unstirred region, the magnitudes of the inner unstirred layers were found to lie within the ranges 150-230 μ, 120-200 μ and 100-170 μ under stirring conditions of 120, 300 and 500 rev/min, respectively.

The dual effect of calcium on the action potential of the frog's heart

Abstract: 1. Using ventricle strips of the frog's heart stimulated at the low rate of about 1 shock/min intracellular action potentials were recorded under conditions of varying calcium concentrations.2. Overshoots of action potentials were increased by about 18.3 mV as a result of a 10-fold increase, within the range of 0.1-5 mM, of the calcium concentration.3. A similar effect was obtained by strontium, but magnesium was ineffective.4. The increase of the overshoot by high calcium was associated with an increased rate of rise of the potential during the later part of its ascending phase. The initial fast upstroke remained unaltered.5. Another effect, a depression of the overshoot, developed during periods of repetitive stimulation, at the rate of 20/min, and this was followed by a gradual recovery during subsequent periods of rest.6. The depression of the overshoot increased with increasing calcium concentrations reaching values of over 40 mV.7. High concentrations of strontium and low concentrations of sodium also induced depression of the overshoot, but high magnesium was ineffective.8. A tentative hypothesis has been proposed attributing these two effects: (a) to an entry of calcium through the excitable membrane thus contributing to the ionic inward current, and (b) to a resulting accumulation of calcium in some cellular store.

The action of calcium on neuronal synapses in the squid.

Abstract: 1. The isolated stellate ganglion of the squid (L. pealii) was studied with intracellular and extracellular micro-electrodes. Three or four nerve fibres in the preganglionic nerve establish synaptic relations with the giant axon in the last stellar nerve. Accordingly, 1-3 small presynaptic spikes (< 1 mV) could be recorded from within the post-synaptic axon.2. A micro-electrode was inserted in the presynaptic fibre and used to polarize and record simultaneously. In the distal (giant) synapse, hyperpolarization of the ending produced an increase in the size of the presynaptic action potential and post-synaptic potential (PSP). Depolarization had the opposite effect. These effects of polarization took more than 10 sec to develop fully, and declined with a similar time course at the end of polarization. Analogous results were obtained with two other preganglionic fibres, which make contacts in the proximal synaptic region.3. The second of a pair of preganglionic impulses evoked a PSP larger than the first. This facilitation of PSP was sometimes accompanied by a small increase in the size of the second action potential in the presynaptic axon. At some shorter intervals, the second presynaptic action potential was reduced in amplitude, but the PSP was still increased. Hyperpolarization of the presynaptic terminal increased the size of both PSPs in a pair and abolished the facilitation. With stronger hyperpolarization the second PSP was even smaller than the first.4. Removing or reducing the Ca in the bathing fluid reversibly abolished the post-synaptic response. The small presynaptic spikes remained practically unaffected. In these conditions a nerve impulse still invaded the ending and normal action potentials could be recorded from the pre-synaptic terminal. This shows that electrical coupling between pre- and post-synaptic axons is insufficient to account for synaptic transmission.5. In low-Ca solution synaptic transmission could be restored locally by extracellular ionophoretic application of Ca to a small portion of the synapse. At sensitive spots a post-synaptic current (recorded with the Ca pipette) and PSP could be detected earlier than 1 sec after commencing the application of Ca.6. Ca was ineffective when injected intracellularly into the presynaptic fibre at a spot where extracellular ionophoresis of Ca restored the PSP.7. The results indicate that synaptic transmission in the squid stellate ganglion is not electrical but due to the release of an unidentified transmitter. Release of this transmitter by the presynaptic nerve impulse requires the presence of Ca in the external medium. During the impulse Ca would combine with a ;Ca-receptor' in the membrane and initiate the reactions which lead to transmitter release. It appears that the ;Ca-receptor' is only accessible from the outside of the membrane.

An inhibitory process in the cerebral cortex.

Abstract: 1. In cats, rabbits and monkeys, single cortical shocks can reduce the excitability of cortical neurones for 100-300 msec; the inhibitory effect is readily demonstrated, even in previously quiescent cells, against a background of activity evoked with small amounts of L-glutamate, released from an extracellular recording micropipette by iontophoresis.2. Other forms of cortical activity are also inhibited in a similar way by direct or indirect cortical stimulation; they include single unit discharges produced by iontophoretic applications of ACh or by a cathodal current, spontaneous discharges, and slow wave activity, both spontaneous and evoked.3. Most stimuli which elicit cortical activity also evoke some inhibition in the cortex, for instance, transcallosal volleys, and thalamic or peripheral shocks. In each case, a characteristic, prolonged depression is produced by single shocks.4. The most effective stimuli are direct cortical shocks, especially when applied within the cortex, below a depth of 0.6 mm; surface cathodal shocks are more effective than anodal shocks. These stimuli do not first excite the cells which are inhibited and they are not strong enough to cause appreciable local injury.5. Because of its long duration, the inhibition is often readily maintained by repetitive stimulation at frequencies of 5-7/sec. A cumulative effect leads to a further silent period after the end of stimulation; this increases with the strength, frequency and duration of the tetanus, so that after stimulation at 50-100/sec, the silent period may last for over 1 min. During this time, a stronger depolarizing stimulus can initiate firing.6. The inhibitory effect is often preceded and followed by phases of increased excitability; these may also show cumulative enhancement during repetitive stimulation, and a high frequency tetanus often leads to a short after-discharge, which is then followed by a long silent period, as above. Comparable changes take place in rabbits during spreading depression.7. The inhibitory effect of a direct shock can spread over an area covering 1 cm of cortical surface, affecting the cells through all cortical layers; but the spread is uneven in different directions, being particularly poor under most sulci.8. This type of inhibition can be elicited in all areas of the neocortex, and it is evident in kittens within a week of birth.9. Antidromic pyramidal stimulation is very much less effective in evoking inhibition of Betz cells, and other cortical neurones, than direct cortical stimulation; the inhibition by direct shocks is therefore not likely to be mediated through pyramidal excitation.

Discharge patterns of principal cells and interneurones in lateral geniculate nucleus of rat

Abstract: 1. The cells of the lateral geniculate nucleus of the rat are classified into one of two distinct groups on the basis of their responses to a single shock to optic nerve or visual cortex. The distinctive difference between these two groups is that the P cells (87% of all cells) responded with short bursts of 1-5 spikes, whereas the I cells (13%) responded with long bursts of about 10 spikes. Both groups give recurring bursts at intervals of 100 msec or more for a total duration of up to about 5 sec. 2. The majority of P cells (75%) have a lower threshold for late firing than for early firing. The majority of I cells (62%) have about the same threshold and all I cells have very low thresholds. 3. To stimulation of the optic nerve the mean latency of the first spikes of all P cells is 4·4 msec. The mean latency of the first spikes of all I cells is 5·8 msec. The latency of the earliest spike in an I cell is 0·9 msec longer than the earliest spike in a P cell. 4. To stimulation of the visual cortex the latency of the earliest spike in an I cell is 1·0 msec longer than the earliest spike in a P cell. Most P cells (78%) respond with either very short latency (less than 1·7 msec) or with a wave form characteristic of antidromic invasion or both. No I cell has either of these characterstics. 5. It is concluded that P cells are principal cells projecting to visual cortex and that I cells are interneurones.

Short‐latency projections to the cat cerebral cortex from skin and muscle afferents in the contralateral forelimb

Abstract: 1. The potentials evoked in the first sensorimotor area on stimulation of muscle and skin nerves in the contralateral forelimb were recorded in preparations with either the dorsal funiculus (DF) or the spinocervical tract (SCT) interrupted. 2. The short-latency, surface-positive potentials in these preparations are mediated by the remaining path, either the DF or SCT. 3. Cutaneous afferents project through both paths to two discrete areas which correspond to the classical sensory and motor cortices (Fig. 10 A and B). The projection areas are not identical: the DF path seems to activate most effectively the sensory cortex; and the SCT path, most effectively the motor cortex. 4. The potentials evoked from cutaneous nerves have a similar latency in the two areas. On stimulation of the superficial radial nerve the latency was about 4·5 msec in preparations with intact DF, and about 5·3 msec in preparations with intact SCT. 5. High threshold muscle afferents project to the same areas as the cutaneous afferents. 6. Group I muscle afferents project, exclusively through the DF path, to an area distinct from the two cutaneous projection areas (Fig. 10C). It occupies a caudal part of the motor cortex and an intermediate zone between the sensory and motor cortices. 7. The projection areas are compared with the recent cytoarchitectonic map of Hassler & Muhs-Clement (1964) (Fig. 10D). 8. It is suggested that the afferent projections to the motor cortex and the intermediate zone are used in the integration of movements elicited from the cortex. The general similarity in the organization of afferent paths to the motor cortex and the cerebellum is pointed out.

Membrane potential and conductance during transport of sodium, potassium and rubidium in frog muscle

Abstract: 1. Muscles with high intracellular sodium concentrations can extrude sodium into solutions which contain 10 m-equiv/l. of either potassium or rubidium. Potassium or rubidium replaces the extruded intracellular sodium. These cation movements take place equally well when the external anion is chloride or sulphate, though muscles deteriorate if left for long periods in sulphate solutions. 2. Measurements of intracellular potentials during extrusion of sodium into solutions containing potassium show: (a) an internal potential more negative than the potassium equilibrium potential (EK); at 20° C the difference is nearly 20 mV. (b) that a difference between the membrane potential and EK is dependent on temperature and is abolished by 10−5 M ouabain. (c) an internal potential which becomes more negative in the presence of 0·1% cocaine, a concentration of cocaine which substantially increases the membrane resistance to potassium movement. In the absence of potassium or rubidium no such hyperpolarization occurs. 3. When muscles extrude into solutions which contain rubidium they have internal potentials which are 10-20 mV more negative than when extruding sodium into corresponding solutions containing potassium. 4. Measurements of electrical conductance in the potassium solution suggest that the electrochemical potential difference for potassium ions may be large enough to account for the measured inward potassium movements during sodium extrusion. The reliability of the measurements does not, however, exclude the possibility that some part of the inward potassium movement is chemically linked to outward movement. 5. Measurements of membrane conductance in solutions containing rubidium, and of net movements of rubidium in the presence and absence of ouabain, lead to the conclusion that at least 90% of the inward rubidium movement during sodium extrusion must be chemically linked to the sodium movement. 6. The hyperpolarization during extrusion of sodium could be explained on the basis of a fall of the potassium or rubidium concentration in a region of the extracellular space immediately external to the membrane. It is argued that certain characteristics of the hyperpolarization make it difficult to explain the hyperpolarization on this basis alone, though some part of it may be due to extracellular depletion of either potassium or rubidium. The main conclusion is that the sodium pump is capable of transferring electric charge across the membrane in which it is operating, but that, in a given time, the net charge transferred is less than the charge on the sodium ions that the pump has transported, by an amount that corresponds to the charge on the potassium or rubidium ions chemically transported by the pump.

Location of sites of 5‐hydroxytryptamine storage and metabolism by radioautography

Abstract: 1. A technique for the radioautographic identification, localization, and study of the turnover of cellular depots of 5-hydroxytryptamine (5-HT) has been evaluated. A light microscopic survey was made of the uptake and turnover of 5-HT, synthesized in vivo from its administered tritium-labelled precursor, 5-hydroxytryptophan (5-HTP).2. 5-HT was taken up rapidly and retained for long periods of time by adrenal medullary chromaffin cells, gastric enterochromaffin cells, blood platelets, thyroid parafollicular cells, beta cells of pancreatic islets, mast cells, and septal cells of the lung. Reticulo-endothelial cells of liver and spleen took up radioactive 5-HT more slowly but also retained it for days.3. Specific uptake, and a rapid turnover of radioactive 5-HT, was found in pancreatic exocrine cells, neurones of the superior cervical ganglion, terminal axons of gastro-intestinal myenteric plexus, carotid body cells, and pinealocytes.4. A supranuclear localization of radioactive 5-HT in renal proximal tubular cells may represent tubular secretion of 5-HT.5. The localization of 5-HT by radioautography generally corresponds to that found by formaldehyde-induced fluorescence. It makes possible the study of 5-HT turnover and is capable of greater resolution than the fluorescence technique. Freeze-drying may also be avoided.

Pharmacological studies on feline Betz cells.

Abstract: 1. A study has been made of the sensitivity of single neurones in the pericruciate cortex of anaesthetized and unanaesthetized cats to cholinomimetics administered electrophoretically from multibarrel micropipettes. 2. A high proportion of deep pyramidal cells, including Betz cells, were excited by these substances, and the receptors involved have muscarinic characteristics. 3. Atropine specifically reduced the sensitivity of cortical neurones to acetylcholine, but no such action could be demonstrated for dihydro-β-erythroidine, gallamine or general anaesthetics. 4. The significance of these results is discussed in relation to the possible synaptic or non-synaptic action of acetylcholine upon cortical neurones.

The flicker fusion frequency of the blue‐sensitive mechanism of colour vision

Abstract: 1. The flicker fusion frequency for blue stimuli on zero background increases steadily with increasing luminance up to 50 c/s, but if a strong yellow background is added it increases steadily only to about 18 c/s, and then remains nearly independent of luminance over a range of a factor of 20.2. It is argued that the maximum flicker fusion frequency is about three times lower for the blue-sensitive mechanism of colour vision than for the red- or green-sensitive.