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E G Merrill

Publications and source records attributed to E G Merrill.

At least 19 recordsLinked to original sources
PubMed1987

Functional morphology of frog retinal ganglion cells and their central projections: the dimming detectors.

Intracellular recordings were made from frog retinal ganglion cell axons in the optic nerve. Following electrophysiological characterisation of receptive field properties, HRP was injected into the axon, and the brain and retina were subsequently stained. The morphologies of retinal ganglion cells, their dendritic domains, and their central projections were determined with light microscopy, and the optic nerve portion of the ganglion cell axon was examined with electron microscopy. This paper describes the structural and functional features of one ganglion cell class, the off units (class IV or dimming detectors) whose characteristic response is a preferential sensitivity to decreasing light intensity within the receptive field. Typical receptive field diameter of these units was about 16 degrees with a range of 3 degrees to more than 30 degrees. Examination of the spatial characteristics of their receptive field centers and surrounds showed that the class IV cells could be divided into two broad categories. Linear class IV cells did not respond to phase-reversal of a fine grating pattern. These linear cells also tended to have clear surround suppression: illumination of the surround diminished their response to light off at the center. The second group responded briskly to each reversal of the fine grating pattern, whatever its position within the receptor field center. These nonlinear class IV cells did not show surround suppression, but rather they had surround antagonism and they responded to light on in the surround. Nonlinear units were much more frequently recorded in frogs maintained in summer conditions (12-hour days, constant 20 degrees C temperature). In spite of this functional heterogeneity, all cells had similar morphology consisting of a large ganglion cell with a large dendritic arbor (400-1,000 microns) confined to a single stratum in the outer third of the inner plexiform layer, a medium-sized axon (2.4-microns diameter), a smallish pretectal arbor, and a large tectal arbor (300-700 microns) at layer 8.

Animals↗
PubMed1987

Inputs to intercostal motoneurons from ventrolateral medullary respiratory neurons in the cat.

The investigation examined the synaptic input from medullary respiratory neurons in the nucleus retroambigualis (NRA) to external (EIM) and internal (IIM) intercostal motoneurons. Antidromic mapping revealed that 112/117 (96%) tested NRA units had axons descending into thoracic spinal cord with extensive arborizations at many thoracic segments, mainly contralaterally. The conduction velocities ranged from 10 to 105 m X s-1. The descending projections did not appear to be somatotopically arranged. Cross-correlation of the spike trains of NRA inspiratory units with the discharge of external intercostal nerves (performed usually with 4 contralateral nerves) showed significant narrow peaks only in 5 out of 40 averages. Of the 25 trigger units tested for the thoracic projection in this series of experiments, 24 were antidromically activated. Intracellular recordings were made from 52 IIMs [mean membrane potential 65.3 mV, central respiratory drive potentials (CRDPs) greater than 1 mV present in 23/52] and 53 EIM (mean membrane potential 54.3 mV, CRDPs in 31/53). During the depolarizing phase of the CRDPs, synaptic noise with frequent and apparently unitary EPSPs with amplitudes in excess of 1 mV was observed. Spike-triggered averages of synaptic noise were computed for 153 pairings between 137 NRA neurons and 105 contralateral intercostal motoneurons. Only four PSPs were revealed: two monosynaptic EPSPs between expiratory NRA units and IIMs and two probably disynaptic EPSPs between inspiratory NRA units and EIMs. When advancing the microelectrode down to the motoneuron pools, frequent recordings were made from interneurons with spontaneous respiratory discharge (inspiratory or expiratory) located dorsal and medial to the motor nuclei. The interneurons could be excited following stimulation of segmental afferents. It is concluded that monosynaptic connections between respiratory NRA neurons and intercostal motoneurons are rare (connectivity no more than approximately 4%). Segmental interneurons, interposed between the majority of descending respiratory axons and intercostal motoneurons, are likely to produce large unitary EPSPs and, thus, short-term synchronization in the discharge of intercostal motoneurons as observed by others.

Animals↗
PubMed1984

Axonal projections from the rostral expiratory neurones of the Bötzinger complex to medulla and spinal cord in the cat.

Axonal projections of eighty-four rostral medullary expiratory neurones of the Bötzinger complex were tested using antidromic mapping techniques in anaesthetized cats. A projection to the ventral respiratory neurones of the medulla (n.r.a.) was shown in eleven out of twelve tested neurones. Also a spinal projection to the C5-C6 cervical segments was evident in more than 72% of tested neurones; probably near 100% project to cervical cord. These axonal projections were found bilaterally in both brain stem and spinal cord. The majority of Bötzinger complex expiratory neurones were seen to have two to four axonal collaterals to the ventro-lateral (v.l.) nucleus of the solitary tract (n.t.s.) and/or the n.r.a. and/or the spinal cord. In eight out of twelve of the tested neurones, electrophysiological evidence of axonal arborization in more than one of n.r.a. inspiratory, n.r.a. expiratory or v.l. n.t.s. regions was obtained. Similar evidence for the terminal arborization was found for 26% of tested neurones in the phrenic motor nucleus. The descending spinal expiratory axons of the Bötzinger complex neurones are located in the dorsal and medial parts of the lateral funiculus in C4 and C5 segments. Conduction velocity measurements indicate that these are large myelinated axons. We propose that the Bötzinger complex expiratory neurones are a source of synaptic inhibition for n.r.a. inspiratory neurones and phrenic motoneurones.

Animals↗
PubMed1984

Monosynaptic inhibition of phrenic motoneurons: a long descending projection from Bötzinger neurons.

Synaptic connections between medullary expiratory neurons and phrenic motoneurons were studied in anesthetized cats using spike-triggered averaging of synaptic noise in phrenic motoneurons. Monosynaptic inhibition of phrenic motoneurons originating from the rostral medullary expiratory neurons of the Bötzinger complex was revealed. Neither mono- nor disynaptic inhibition from expiratory neurons of the nucleus retroambigualis was found. It was concluded that Bötzinger complex expiratory neurons make long descending inhibitory connections with phrenic motoneurons: the expiratory neurons of the nucleus retroambigualis do not appear to make synaptic connections with phrenic motoneurons.

Animals↗
PubMed1983

Two descending medullary inspiratory pathways to phrenic motoneurones.

Synaptic connections of the medullary inspiratory neurones of the nucleus tractus solitarius (NTS) and nucleus retroambigualis (NRA) with phrenic motoneurones were studied using spike triggered averaging of the synaptic noise of phrenic motoneurones. More than 60% of NTS inspiratory neurones made monosynaptic connections with phrenic motoneurones, while similar connections between NRA and phrenic motoneurones could be shown in less than 7% of studied neuronal pairs. Relations between cross-correlations and observed synaptic connectivity of the phrenic motoneurones are also discussed.

Animals↗
PubMed1983

Origin of the expiratory inhibition of nucleus tractus solitarius inspiratory neurones.

Respiratory neurones from retrofacial (NRF) and Bötzinger nuclei (BOT) were tested electrophysiologically for projection to the contralateral inspiratory region of the nucleus of the tractus solitarius (NTS). Antidromic activation was demonstrated for 30 expiratory neurons (out of a mixed population of 61). There was evidence of extensive branching within NTS for these axons. Histological localization of projecting BOT neurones was confined to the medial border of the NRF. NTS inspiratory neurones were impaled in the vicinity of BOT axonal arborizations, and averages of post-synaptic noise were made, using the simultaneously recorded BOT spike trains to trigger the averager. Unitary IPSPs were found in 5 out of 9 pairs so analyzed. The waveforms of these IPSPs were similar to previously recorded monosynaptic IPSPs. 25 expiratory neurones, recorded from the caudal part of nucleus retroambigualis (NRA), were tested for projection to NTS; no projections could be demonstrated. 8 NTS inspiratory neurones were tested, using spike-triggered averaging of post-synaptic noise, for functional connections from caudal NRA expiratory neurones; no PSPs were seen. It is concluded that the expiratory inhibition of NTS inspiratory neurones is due, in part at least, to inputs from BOT expiratory neurones. Caudal NRA expiratory neurones do not appear to contribute to this inhibition.

Animals↗
PubMed1981

Where are the real respiratory neurons?

Transection experiments establish that the mechanisms responsible for the generation of the basic breathing pattern are located in the medulla. Several populations of neurons with activity patterns related to this motor pattern are readily recorded in the medulla, and much information has been obtained in the past 10 years about the physiology of these medullary respiratory neurons and their possible interconnections, inputs, and interactions. This evidence does not support the hypothesis that the basic alternations between expiration and inspiration is the result of a stable oscillatory network containing only the presently known medullary respiratory neurons. It is proposed that conventional extracellular recording methods have missed important parts of the medullary respiratory mechanism.

Animals↗
PubMed1979

Responses of single units in laminae 2 and 3 of cat spinal cord.

333 units were recorded in laminae 2 and 3 of lumbar cord in decerebrate cats. Recording of small-amplitude spikes was made possible by the use of platinum-surfaced tungsten microelectrodes, continuously variable filters and an analogue delay line display. Stimulation of the lateral Lissauer tract showed that a sample of the units sent axons into this tract. Using iron-plated electrodes, recording sites were marked and shown to be within laminae 2 and 3. Axons of peripheral afferent axons were excluded from the sample, as were long-range descending axons. By using one electrode placed close to a cell body in lamina 4 and a roving electrode in the dendritic region dorsal to the cell body, it was possible to show that the recorded units were not field spread of deeper cells. Of the units 94% had peripheral receptive fields, RF; 30% had small RFs less than 2 sq.cm, intermediate in size between RFs of peripheral axons and RFs of large cells in dorsal horn. These small RF cells occurred in clusters and their RFs constituted a fraction of the larger RF of nearby large cells. Of the units 56% responded to brush and touch, 19% to brush, touch and pressure, while 19% required pressure on skin to excite them. Latency of response to electrical stimulation showed that all cells were excited by myelinated afferents. While no cells were detected exclusively by C afferents, many may have been excited by both A and C afferents. Eighteen per cent of the cells showed a prolonged discharge lasting more than 5 sec after a single stimulus. Some of these long discharge cells continued firing for minutes. Another unusual class, 14% of all cells, habituated very powerfully to intermittant natural or electrical stimuli, and remained unresponsive for many seconds after responding to the first stimulus.

Afferent Pathways↗
PubMed1978

Properties of two unmyelinated fibre tracts of the central nervous system: lateral Lissauer tract, and parallel fibres of the cerebellum.

1. Monoplar tungsten micro-electrodes were used to stimulate and platinun plated tungsten micro-electrodes to record from single, unmyelinated cerebellar parallel fibres and lateral Lissauer tract axons in cats. 2. Stimulation of the lateral Lissauer tract resulted in the activation of a narrow, longitudinal 'beam', much as on the cerebellar surface. 3. Following impulse conduction, parallel and Lissauer tract fibres showed a supernormal conduction velocity (up to 25% increase) and increased excitability (up to 40% increase). No subnormality was encountered following supernormality. Some Lissauer tract fibres had prolonged relative refractory periods and no supernormal periods. 4. Chronaxies ranged from 155 to 380 microseconds. 5. Single fibres exhibited a remarkable increase in conduction velocity (up to 18% and excitability (up to 40%) following a single subthreshold stimulus. The duration of this effect (up to 20 msec) was much longer than expected from membrane time constant estimates.

Action Potentials↗
PubMed1977

Dorsal horn cells that respond to stimulation of distant dorsal roots.

Experiments were carried out to find if there were post-synaptic effects produced by impulses in the long ranging primary afferents, which had been shown by Wall & Werman (1976) to extend from upper lumbar dorsal roots to the sacral segments. Dorsal rootlets were stimulated in decerebrate low-spinal adult cats.1. The dorsal root potential and ventral root reflex were recorded on S1 root filaments, in response to stimulation of dorsal rootlets extending from L1 to S1. With increasing distance between stimulating and recording segments, these potentials became smaller and more delayed. In two animals, there was no response at S1 to stimulation of L1 and L2 dorsal roots.2. In all animals, stimulation of L3 or L4 dorsal roots produced cell responses in dorsal horn segments L7 or S1. The density of such cells was variable, from animal to animal. Responding cells were mainly concentrated laterally in the dorsal horn.3. The latency and response variability of L7-S1, dorsal horn cells to L3-L4 stimulation was consistent with at least some of them being fired monosynaptically.4. Cells that respond to stimulation of one distant rootlet respond to many closer rootlets as well.5. The receptive fields of L7-S1 dorsal horn cells, responsive to stimulation of L3-L4 rootlets, were typical of those generally found in the L7-S1 segments, and were at some distance from the L4 dermatome. Only twenty cells had receptive fields which extended into the dermatome of the rootlets stimulated.6. It was established that some L4 cells respond to S1 dorsal root stimulation, just as the main study had shown that S1 responds to L4.7. It is concluded that substantial numbers of dorsal horn cells, including cells with many types of cutaneous receptive field, respond to two classes of synaptic in-put: one effective in firing the cell upon natural cutaneous stimulation, and one relatively ineffective, capable of driving the cell only when stimulated electrically and thus carrying a synchronous volley from a number of highly convergent axons. The contribution of this secondary afferent channel to normal and pathological cord physiology has now to be determined.

Afferent Pathways↗