Biomedical subjects

R E Fyffe

Publications and source records attributed to R E Fyffe.

At least 37 records · Page 2Linked to original sources

PubMed1992

Involvement of GABA and glycine in recurrent inhibition of spinal motoneurons.

1. Recurrent inhibitory postsynaptic potentials (IPSPs) were recorded intracellularly from chloride-loaded motoneurons in the isolated lumbar spinal cord of neonatal rats (day 5-day 12). This in vitro preparation exhibited an intact and functional recurrent inhibitory pathway that displayed characteristics previously described for this pathway in other species. 2. Although strychnine (1-5 microM) depressed the chloride-dependent recurrent synaptic potentials evoked by ventral root stimulation by 48.2 +/- 2.7% (mean +/- SE, n = 13), confirming that part of the recurrent IPSP is mediated by a glycinergic mechanism, in every case a residual strychnine-resistant synaptic potential was observed. 3. The gamma-aminobutyric acid (GABA) antagonist bicuculline, in low concentrations (2-10 microM), depressed the recurrent synaptic potentials in a dose-dependent manner by 27.0 +/- 4.3% (range 0-49%, n = 19). Application of bicuculline almost eliminated the strychnine-resistant component of the IPSP. However, in some motoneurons, a small synaptic potential remained after combined application of strychnine and bicuculline. 4. The selective antagonists of GABA uptake, (+/-)-nipecotic acid (1 mM) and guvacine (1 mM), increased the amplitude of recurrent synaptic potentials in 12 of 16 motoneurons by 37.2 +/- 7.2% (range 12.6-84.2%). 5. The excitatory amino acid antagonists kynurenic acid (1 mM), 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX (10 microM)] and 6,7-dinitroquinoxaline-2,3-dione (10 microM) potentiated recurrent synaptic potentials in 5 of 7 motoneurons. However, CNQX (10-15 microM) in the presence of strychnine and bicuculline virtually abolished the synaptic potential remaining after application of the inhibitory amino acid antagonists. It is concluded that ventral root stimulation evokes a small excitatory amino acid-mediated synaptic potential in neonatal rat motoneurons. 6. An antidromic synaptic potential due to electrotonic coupling between motoneurons was unaffected by changes in membrane potential, chloride loading, or antagonists of glycine, GABA, excitatory amino acid, and acetylcholine receptors. 7. The results suggest that a major portion of the strychnine-resistant component of the IPSP is mediated by a GABAergic mechanism. It is concluded that both glycinergic and GABAergic mechanisms play a role in recurrent inhibition of motoneurons in the mammalian spinal cord. It is unknown whether these inhibitory amino acids are released by a single pool of Renshaw cells or by neurochemically distinct populations.

Animals↗

PubMed1991

The size and dendritic structure of HRP-labeled gamma motoneurons in the cat spinal cord.

We report quantitative data obtained from 60 fully reconstructed dendritic trees belonging to eight gamma-motoneurons (gamma-MNs) and six additional gamma-MNs that were not completely reconstructed. The cells were labeled intracellularly with horseradish peroxidase (HRP). These data are compared to measurements from 79 reconstructed dendrites belonging to seven documented alpha-motoneurons (alpha-MNs), supplemented by a larger sample of alpha-MNs labeled intracellularly or by retrograde transport with HRP. As expected from earlier studies, the soma dimensions and total membrane area of gamma-MNs were smaller than those of alpha-MNs. Although gamma-MN dendrites were, on average, slightly but significantly longer than those of alpha-MNs, the former had, on average, smaller diameter stem dendrites, less membrane area, and less profuse branching, and they tended to branch closer to the soma and to terminate farther from the soma. These differences were evident even when subsets of dendrites with similar stem diameters were compared. Some of the anatomical distinctions suggest that gamma-MNs are qualitatively as well as quantitatively different from alpha-MNs, even though the distributions of many of the morphological variables examined showed no abrupt discontinuities between the two motoneuron groups.

Animals↗

PubMed1991

Glycine-like immunoreactivity in synaptic boutons of identified inhibitory interneurons in the mammalian spinal cord.

Glycine is thought to be a major inhibitory neurotransmitter in the mammalian CNS. Two types of physiologically identified interneurons, Renshaw cells and Ia inhibitory interneurons, were intracellularly stained with horseradish peroxidase, and their axon terminals were studied at the electron microscopic level. Post-embedding immunogold procedures were used to reveal the presence of glycine-like immunoreactivity. The synaptic terminals of both types of interneuron were significantly enriched with glycine-like immunoreactivity, providing support for the idea that glycine is a mediator of synaptic transmission in the recurrent and reciprocal inhibitory pathways to motoneurons.

Animals↗

PubMed1991

Spatial distribution of recurrent inhibitory synapses on spinal motoneurons in the cat.

1. Intracellular staining of Renshaw cells and alpha motoneurons was used to determine the spatial distribution of recurrent inhibitory synapses on spinal motoneurons in the cat. In each experiment, a Renshaw cell and one or more possible target motoneurons were labeled with horseradish peroxidase after physiological identification. 2. Paris of labeled neurons were reconstructed and measured at the light microscopic level. As defined by light microscopy, presumed synaptic contacts between nine Renshaw cells and 10 postsynaptic motoneurons were observed. On average, each Renshaw cell made three synaptic contacts (range 1-9) on each motoneuron. 3. Electron microscopic confirmation of several presumed contacts provided evidence that the appositions identified by light microscopic criteria are genuine contacts between Renshaw cell boutons and the labeled motoneuron. 4. All of the identified synapses observed in these experiments were located on motoneuron dendrites, between 65 and 706 microns from the soma. Use of a simplified cable model indicated that the synapses are electrotonically close to the soma, the average location being approximately 0.25 length constants from the soma (range 0.04-0.82 lambda). 5. These observations provide direct evidence to support the hypothesis that Renshaw cell synapses on motoneurons are located on the dendrites and not on the cell body (whereas reciprocal inhibitory synapses, from Ia inhibitory interneurons, are predominantly located on the soma). The functional significance of the observed distribution of Renshaw inhibitory synapses is discussed. One possibility is that the recurrent inhibitory pathway selectively inhibits particular dendritic inputs.

Animals↗

PubMed1990

Evidence for separate morphological classes of Renshaw cells in the cat's spinal cord.

Although some functions of Renshaw cells are well defined, pharmacological evidence suggests that there may be more than one type of Renshaw cell involved in recurrent inhibition of motoneurons, or one type employing more than one inhibitory amino acid neurotransmitter. Identified Renshaw cells were intracellularly stained with horseradish peroxidase (HRP), revealing the existence of a distinctive group of fusiform neurons as well as the more common multipolar cells. The fusiform neurons may represent a subgroup of Renshaw cells having separate functions and/or synaptic mediators.

Animals↗

PubMed1989

Selective suppression of endogenous peroxidase activity: application for enhancing appearance of HRP-labeled neurons in vitro.

A simple procedure to suppress selectively the endogenous peroxidase activity of red blood cells in histological sections of the mammalian nervous system is described. Pretreatment of sections with a series of ethanol solutions results in selective abolition of red blood cell staining and also leads to enhanced visualization of neurons that have been injected with horseradish peroxidase (HRP). The method is particularly useful for processing in vitro brain slices that contain HRP-labeled neurons. It can also be used for processing unperfused neural tissue from in vivo HRP labeling experiments. The ethanol pretreatment is compatible with several standard histochemical techniques for the demonstration of HRP.

Animals↗

PubMed1988

Morphology of neurons in area 4 gamma of the cat's cortex studied with intracellular injection of HRP [corrected and issued with original paging in J Comp Neurol 1988 Nov 8;277(2)].

Neurons in laminae II, III, V, and VI of area 4 gamma of the cat motor cortex were studied following intracellular penetration with an HRP-filled micro-electrode. Antidromic and synaptic responses produced by stimulation of the cerebral peduncles and/or of the ventrolateral nucleus of the thalamus were investigated. Horseradish peroxidase was then iontophoresed into the same neurons to allow examination of their detailed morphology. The morphology of pyramidal neurons whose somata were located in a particular lamina was similar but differed from that of pyramidal neurons in other laminae. The modified pyramidal neurons of lamina II had a truncated apical dendrite or did not possess an obvious apical dendrite, even though the ascending dendritic branches were longer and more extensive than the "basal" branches. As was the case for the pyramidal cells in other laminae, the axons of these lamina II modified pyramidal cells descended toward the white matter; their somata were generally pyramidal in shape, and their dendrites were spiny. All pyramidal neurons except some of lamina VI had ascending dendrites which terminated in a tuft in lamina I, subpially. No intracortical collaterals were seen originating from the axons of lamina II or of lamina VI pyramidal neurons. Lamina III pyramidal neurons had extensive short and long axon collaterals which contributed synaptic boutons to all laminae of the cortex. Pyramidal neurons of lamina V had fewer axon collaterals whose synaptic boutons were restricted to laminae V and VI. All somata of pyramidal tract neurons (PRNs), identified by antidromic responses from peduncular stimulation, were located in lamina V, except for one which was located in lamina VI. Recurrent collaterals of pyramidal neurons were activated by peduncular stimulation. Recurrent excitatory postsynaptic potentials (epsps) could be evoked in fast PTNs, slow PTNs, other pyramidal neurons of lamina V, and pyramidal neurons of lamina VI at latencies between 1.3 and 6.25 msec. In some slow PTNs, a recurrent inhibitory postsynaptic potential of long duration was the predominant response. Stimulation of the ventrolateral nucleus of the thalamus resulted in epsps in pyramidal neurons of lamina III, V, and VI at latencies between 1.0 and 5.0 msec.

Animals↗

PubMed1986

Intracellular staining study of the feline cuneate nucleus. I. Terminal patterns of primary afferent fibers.

The terminal arborizations of single identified cutaneous hair follicle and slowly adapting type I receptors and muscle (Ia) afferents have been studied in the cuneate nucleus of cats after intra-axonal injection of horseradish peroxidase. Penetrations were mainly at the middle and caudal levels of the nucleus--i.e., from obex to approximately 7 mm caudal to it. Following histochemical processing, the injected axons, along with their collateral branches and synaptic terminals, were visualized and examined with light and electron microscopy. Cutaneous afferents in middle cuneate (from obex to approximately 4 mm caudal to it) issued collateral branches, along the rostrocaudal axis of the nucleus, at intervals between 100 and 1,000 microns. The terminal field of each collateral's branches encompassed an area elongated largely rostrocaudally and virtually confined to the dorsal part of the middle cuneate. Although adjacent collaterals had nonoverlapping terminal arborizations, each one could give rise to separate foci of terminations. Muscle afferents differed, on the whole, from cutaneous afferents in the location and extent of collateral branching and terminal arborizations. However, because muscle fibers terminated primarily in the ventral region of the cuneate, but nevertheless exhibited sparser terminations in the dorsal part of the middle cuneate, there was some spatial overlap between zones of muscle and cutaneous projection. Synaptic boutons of cutaneous afferent fibers contained round clear vesicles, contacted dendritic profiles (sometimes more than one), and were postsynaptic to small boutons containing polymorphic vesicles. In contrast, boutons of muscle afferent fibers contacted somatic and dendritic profiles and were not postsynaptic to other boutons. The results are in general agreement with previous anatomical and electrophysiological work; however, the extent of the terminal field of single collateral branches may provide for a greater convergence of different receptor classes and of receptive fields on neurons in the middle cuneate than estimated by previous electrophysiological investigations.

Afferent Pathways↗

PubMed1986

Intracellular staining study of the feline cuneate nucleus. II. Thalamic projecting neurons.

Morphological and physiological features of thalamic projecting neurons in the middle region of the cuneate nucleus of cats (from obex to 4 mm below it) have been studied, using intracellular recording and iontophoresis of horseradish peroxidase. All cuneothalamic neurons in the present sample) responded to movement of hairs on wrist, paw, or digits. However, approximately 50% of the neurons could be activated by other types of stimulation (e.g., light or maintained pressure on the skin, movement of claws, etc.). No clear differences were apparent in the physiological responses correlated with the varied dendritic pattern of stained neurons. Dendritic arborizations of most cuneothalamic neurons were more extensive than assumed previously, from Golgi impregnated material. As a consequence, only a few neurons have dendrites ramifying within a restricted region--i.e., corresponding to a typical cluster of the middle cuneate nucleus. Dendrites extending in various directions and spanning a distance up to 500 microns provide cuneothalamic neurons with the ability to receive input from relatively widespread areas. Collateral branches of axons of cuneothalamic neurons were observed in 50% of the stained neurons. Most of these collaterals terminated ventrally within the cuneate nucleus. Extensive collateral arborizations were observed in the dorsal as well as the ventral cuneate. These results, together with those reported in the previous paper, suggest complex interactions of afferent inputs on cuneothalamic neurons. In particular, such neurons are likely to be influenced by convergent input from different receptor classes and, because of their axonal collaterals, probably affect the excitability of other neurons, projecting or intrinsic, in their immediate vicinity or in other nuclear regions.

Afferent Pathways↗

PubMed1985

Recurrent inhibition of cat phrenic motoneurons.

These experiments were performed to re-examine the presence of a recurrent inhibitory pathway to phrenic motoneurons in cats. Following intracellular staining of 49 phrenic motoneurons with horseradish peroxidase, initial axon collaterals were demonstrated in six axons (12%). Intracellular recording and averaging of the synaptic responses evoked by stimulation of the phrenic roots with an intensity subthreshold for axons of the examined motoneurons revealed (in spinalized cats with dorsal roots cut) inhibitory postsynaptic potentials (i.p.s.p.s) in 11/28 (40%) motoneurons (amplitude, 115 +/- 69 (SD) microV; latency, 3.0 +/- 0.6 msec; duration from 12 to 25 msec). In the vicinity of the phrenic nucleus, Renshaw-like units were extracellularly recorded, which fired high-frequency bursts of action potentials following the stimulation of the phrenic roots (burst duration, 8 to 21 msec). Antidromic activation of the C5 phrenic root reduced the firing probability of fibers in the C6 root. The duration of the reduced firing probability corresponded to the duration of both the i.p.s.p.s and the Renshaw units' firing period. It is concluded that Renshaw inhibition is present in phrenic motoneurons. The inhibitory effects of this recurrent pathway may be greater than judged from the small amplitude of the i.p.s.p.s recorded in individual motoneurons, particularly when the whole phrenic pool is activated and when motoneurons are near threshold. It is suggested that one of the functions of the pathway is to control phrenic motoneurons supplying the dorsal part of the diaphragm through the C6 root when the more ventral part of the diaphragm is strongly activated.

Animals↗

PubMed1984

The ultrastructure of group Ia afferent fiber synapses in the lumbosacral spinal cord of the cat.

Ia synapses in laminae VI and IX of the cat's spinal cord were examined in the electron microscope following iontophoretic injection of horseradish peroxidase (HRP) into single, identified, Ia afferent fibers from gastrocnemius muscles. Ia boutons contacting motoneuron dendrites in lamina IX contained spherical synaptic vesicles and generally contacted only one postsynaptic profile. The Ia boutons were often postsynaptic to smaller P-type axonal terminals. Consequently Ia boutons may be classified as S-boutons with axo-axonic contacts.

Animals↗

PubMed1984

Fine structure of normal and degenerating primary afferent boutons associated with characterized spinocervical tract neurons in the cat.

Spinocervical tract neurons in the dorsal horn of the cat spinal cord were intracellularly stained with horseradish peroxidase. The neurons came from one intact animal and from animals with dorsal rhizotomies (L3-S2) 3, 5, 10, 28 and 42 days previously. The morphology of terminals associated with spinocervical tract neurons was examined in a combined light and electron microscopical study. Some terminals containing agranular, circular vesicles degenerated as a result of deafferentation; these are therefore the terminals forming monosynaptic inputs to the neurons from primary afferent fibres. Other terminals containing agranular circular vesicles and terminals containing ovoid agranular vesicles survived deafferentation; these boutons therefore do not originate from primary afferent fibres.

Animals↗

PubMed1984

Is ATP a central synaptic mediator for certain primary afferent fibers from mammalian skin?

The possibility that ATP acts as a synaptic mediator at the central terminals of primary afferent fibers was examined by applying it iontophoretically to neurons of the outer layers of the cat spinal cord in vivo. ATP proved to be selectively excitatory for a limited subset of spinal neurons. Those units consistently excited by ATP iontophoresis with very small currents (2-15 nA) responded to gentle mechanical stimulation of the skin and usually evidenced excitatory input from unmyelinated primary afferent fibers. Most units excited by ATP were specifically mechanoreceptive; a few neurons receiving excitatory input from both low-threshold mechanoreceptors and nociceptors also responded to ATP. Selectively nocireceptive neurons were unresponsive. Generally, the mechanoreceptive neurons excited by ATP were located in the deeper substantia gelatinosa or in the immediately adjacent nucleus proprius of the dorsal horn. The results suggest the presence of a purinergic excitatory receptor on central neurons receiving excitatory projection from tactile mechanoreceptors with fine-diameter afferent fibers and are consistent with the possibility that an ATP-like agent may mediate central synaptic excitation for this set of sense organs.

Adenosine Triphosphate↗

PubMed1984

Fine structure of primary afferent axon terminals projecting from rapidly adapting mechanoreceptors of the toe and foot pads of the cat.

Two Pacinian corpuscle afferents and two rapidly adapting afferents from Krause corpuscles were intra-axonally labelled with horseradish peroxidase in the lumbosacral enlargement of the cat's spinal cord. Tissue was prepared for combined light and electron microscopical analysis. Boutons from both classes of afferent had similar ultrastructural appearances. They both formed from one to three synaptic junctions with dendritic shafts and spines and received axo-axonic synapses. In addition, both categories of bouton were seen to be presynaptic to structures interpreted as vesicle-containing dendrites. It is concluded that both types of afferent fibre are subject to presynaptic control and that they synapse with dorsal horn neurones which are possibly interneurones involved in primary afferent depolarization and post-synaptic dorsal column neurones.

Adaptation, Physiological↗

PubMed1984

Fine structure of primary afferent axon terminals of slowly adapting cutaneous receptors in the cat.

Three slowly adapting type I and two slowly adapting type II afferent fibres from the lumbosacral cord of the cat were intra-axonally labelled with horseradish peroxidase and processed for light and electron microscopy. Terminals from both types of afferent exhibited similar ultrastructural features in that both formed contacts with one to five post-synaptic profiles, including dendritic shafts and spine heads, some of which contained vesicles. The stained axons were themselves post-synaptic in axo-axonic synapses. Maximum diameters of slowly adapting boutons and the dendritic shafts on which they terminated were measured. The present results indicate that there is considerable overlap in the morphological characteristics studied for all large myelinated cutaneous afferent boutons. It is not possible therefore to distinguish between these on ultrastructural grounds alone.

Adaptation, Physiological↗

PubMed1984

Effects of hind limb nerve section on lumbosacral dorsal horn neurones in the cat.

The sciatic and saphenous nerves of one hind limb were sectioned in young adult cats anaesthetized with halothane. Between 19 and 55 days later, under chloralose anaesthesia, dorsal horn neurones in the L6 and L7 segments were recorded and their receptive field properties examined. In seven animals recordings were made from identified spinocervical tract, post-synaptic dorsal column and dorsolateral funicular neurones as well as from neurones that did not project through these pathways. Thirty-one neurones were intracellularly stained with horseradish peroxidase, and fifty-three were recorded extracellularly and located by reference to stained cells. In two animals (both 31 days after nerve section) no attempt was made to identify axonal projections of the dorsal horn neurones in order to avoid any effects of cervical cord search stimuli on the cells' properties, but all isolated extracellularly recorded units were examined. On the side ipsilateral to the nerve sections 143 units were recorded. In all experiments, neurones in the medial three-quarters of the dorsal horn had no discernible cutaneous, mechanosensitive receptive fields between 19 and 55 days after nerve section. There were only two exceptions to this generalization, one neurone being one of the most rostral cells in the sample (in caudal L5) and the other being one of the most caudal cells (in caudal L7). We present evidence to show that neither of these two neurones had inappropriate receptive fields in terms of the somatotopic organization of the dorsal horn. All other neurones with receptive fields on the skin were appropriately located in the somatotopic map laid out in the dorsal horn. There was no evidence for gross anatomical changes in the dendritic trees of dorsal horn neurones following sciatic and saphenous nerve sections. We have been unable to confirm that, following loss of cutaneous receptive fields by peripheral nerve section, dorsal horn neurones in adult cats acquire 'inappropriate' receptive fields. Possible reasons for this are discussed.

Animals↗

PubMed1983

Morphological properties of physiologically characterized lamina III neurones in the cat spinal cord.

Six lamina III interneurones of the cat spinal cord were impaled and stained with intracellular injections of horseradish peroxidase. The responses of these neurones varied considerably: 1 neurone responded to light brushing of its receptive field, whilst 4 cells were excited by strong pressure. Morphologically, they were also a heterogeneous population. Two neurones had rostro-caudally orientated dendritic arbors that were confined to the lamina, while four of the cells were vertically orientated and possessed dendrites that crossed lamina boundaries. There was no correlation between the physiological characteristics of a neurone and its morphology. Three of the vertically orientated neurones were examined ultrastructurally. The first of these cells received several types of synaptic terminal which were distributed in an organised pattern over the entire dendritic tree. This neurone possessed recurrent dendrites which participated in a variety of complex synaptic arrangements. The second neurone also participated in a variety of synaptic arrangements, including glomeruli in lamina II, and received contacts from vesicle-containing dendrites. It gave rise to collateral axons which arborized in lamina II and formed boutons which formed synapses with dendrites. The third cell possessed varicose dendrites which were probably artifactual. It is concluded that lamina III interneurones are a heterogeneous population by electrophysiological, morphological and ultrastructural criteria. They differ in many respects from lamina II neurones and from the cells of origin of ascending systems. The diversity of their inputs and their variation in morphology suggests that they receive input from a variety of primary afferent fibres and dorsal horn neurones and hence may integrate information from these sources.

Animals↗

PubMed1983

Light and electron microscopy of dorsal spinocerebellar tract neurones in the cat: an intracellular horseradish peroxidase study.

Intracellular injections of horseradish peroxidase were made into dorsal spinocerebellar (d.s.c.t.) neurones in Clarke's column of the cat. All the d.s.c.t. neurones were excited from Group I muscle afferent fibres. The stained neurones were examined at the light and electron microscope level, and light microscope material was subjected to computer-aided reconstruction and quantitative analysis. The dendritic trees of d.s.c.t. neurons extended about 3 mm in the long axis of the spinal cord and were confined, in the transverse plane, within or very close to Clarke's column. The dendrites branched extensively and carried various irregularities: complex clusters of fine branches, spine-like protuberances and bead-like varicosities. None of the axons of the cells showed any sign that they gave off collaterals. Computer-aided reconstruction allowed the dendritic trees to be viewed from any angle and enabled measurements of their total dendritic lengths, surface area and volume. The dendritic diameters at branch points were tested against Rall's 3/2 power rule. Electron microscopical analysis confirmed and extended previous studies. Of particular significance was the observation that boutons containing flattened vesicles could be presynaptic to both a d.s.c.t. neurone and to a giant synaptic terminal that made contact with the same d.s.c.t. dendrite.

Animals↗