Biomedical subjects

H Burton

Publications and source records attributed to H Burton.

At least 73 records · Page 4Linked to original sources

PubMed1988

Responses from area 3b of somatosensory cortex to textured surfaces during active touch in primate.

(1) The purpose of this experiment was to characterize the responses of neurons in somatosensory cortex while the hand was actively moved (stroked) across a textured surface. Surfaces consisted of horizontal gratings that varied by spatial period or ridge-groove ratio (roughness). Surfaces were attached to rectangular blocks. TOP and BOTTOM halves of each block could contain surfaces of different roughness. (2) Velocity and force of the stroke were behaviorally constrained within certain limits and continuously measured and recorded during the stroke. (3) Response samples for each neuron were obtained for repeated presentations of each surface. Statistical analyses consisted of analysis of variance and t tests across surfaces on the data of each neuron, and summary statistics on groups of neurons with similar response characteristics. The interaction effects of behavioral variables (velocity and force) were examined and found not to be significant. (4) The sample mainly consisted of rapidly adapting neurons in area 3b of somatosensory area I (SI). Three main response types were found: (a) GRADED cells showed a monotonic increase in firing rate to increasingly rougher surfaces. This effect was seen in one-third of cells studied and is consistent with other reports. These cells seem to code roughness in the magnitude of their response. (b) In some cells, response to a BOTTOM surface depended on the roughness of the preceding TOP surface. This is analogous to contrast in the visual system. These CONTRAST cells are a novel finding in the somatosensory system. (c) Some cells only responded to surfaces that were completely smooth. These "OFF"-response-type cells were seen in proximity to other cells that responded in a reciprocal fashion to surfaces with ridges, but not to smooth surfaces. SMOOTH cells did not respond to punctate or passively applied stimuli, and therefore could not be classified by adaptation of the responses. (5) An increase in firing rate as spatial period (roughness) increases (with a constant ratio of ridge to groove) seems contrary to vibratory models of texture perception. As spatial period increases, temporal frequency decreases, and thus "tuned" cells should show a decreased response rate. Yet GRADED cells showed an increased response. In addition, response varied on surfaces with different groove size, where spatial period, and thus temporal period, was constant. This suggests that in rapidly adapting neurons, at least for these simple surfaces, texture is coded by the magnitude of the firing rates rather than by its temporal fidelity.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1988

Responses of neurons in somatosensory cortical area II of cats to high-frequency vibratory stimuli during iontophoresis of a GABA antagonist and glutamate.

Areas in the second somatic sensory cortex (SII) of cats that responded vigorously to low-amplitude, high-frequency vibratory stimulation were mapped with respect to the surrounding somatotopic organization. Neurons with these properties were found in the posterior and medial parts of the distal forelimb zone and were judged as receiving input from Pacinian mechanoreceptors. The responses of these neurons to sinusoidal vibrotactile stimulation were studied during iontophoretic administration of glutamate or bicuculline methiodide (BMI) to determine if the temporal fidelity of these cortical neurons was controlled by inhibitory circuits that used gamma-aminobutyric acid (GABA) as a neurotransmitter. The data from 19 Pacinian-sensitive neurons were analyzed for changes in the mean firing rate, the percentage of entrainment, and the pattern of periodicity as revealed by autocorrelograms and interval histograms. Iontophoresis of BMI or glutamate caused significant increases in mean firing rates during low- and high-frequency vibratory stimulation. The pattern of increased activity produced by BMI was characterized by a small, yet significant, reduction in the percentage of entrainment, whereas glutamate caused smaller and fewer significant changes in this measure. Analysis of autocorrelation and interval histograms suggested that BMI increased the probability of firing on consecutive stimulus cycles in small segments of the stimulus duration.

Afferent Pathways↗

PubMed1988

Recovery of tactile function after damage to primary or secondary somatic sensory cortex in infant Macaca mulatta.

These studies were designed to determine the basis for recovery of tactile function after the removal of primary (SI) or secondary (SII) cortex in infant Macaca mulatta. From previous studies we know that although removal of SI or SII in the adult macaque produces severe and irreversible impairment on a variety of tactile tasks, normal function can be obtained after partial or total SI lesions in the infant. From the present studies we have found that, as with SI, neither unilateral nor bilateral removals of SII in infants significantly affected the acquisition of or the performance on size tasks, but did cause a temporary delay in acquisition of texture tasks. Performance on texture threshold tasks was normal. The removal of the remaining SI in a juvenile animal that had received a unilateral SI lesion in infancy did not disrupt the recovered function, indicating that recovery is not mediated by the intact SI. However, when SI and SII were removed together from the same hemisphere in an infant, either sequentially or simultaneously, major impairment in the acquisition of texture tasks followed. These results suggest that although SI and SII are necessary for normal tactile function in the adult macaque, they show an equipotentiality for mediating normal tactile function after damage to either area in infants.

Animals↗

PubMed1987

Responses in the first or second somatosensory cortical area in cats during transient inactivation of the other ipsilateral area with lidocaine hydrochloride.

Simultaneous recordings were obtained from the primary and secondary somatosensory cortical areas (SI and SII) in cats anesthetized with ketamine or pentobarbital. A total of 40 individual neurons were studied (29 in SII and 11 in SI) before, during, and following injections of microliter quantities of lidocaine hydrochloride in the other ipsilateral cortical area. Activity in the cortex injected with the local anesthetic was monitored with single-neuron, multi-neuron, or evoked potential responses to determine the time course of inactivation within 0.5-2 mm of the injection sites. Recording sites in both cortical locations were in the representations of the distal forelimb. Responses were elicited by transcutaneous electrical stimulation across the receptive fields with needle electrodes. Short-latency responses were synchronously activated, and, in those circumstances where single neurons were isolated in both areas, no overall differences in latency were noted. Anesthetization of either cortical area never blocked access of somatosensory information to the intact area, even when the injected cortex was completely silenced in the vicinity of the injection mass. In 15 SII neurons and 7 SI neurons, changes were seen in short-latency evoked responses to stimulation of their receptive fields or in background activity following local anesthesia of the other area through several cycles of injection and recovery. In 7 of these 15 SII cells, changes were noted in the timing and/or firing rates of the short-latency responses; changes were noted in the short-latency responses of 2 of these 7 SI cells while SII was silenced. In 11 SII and 6 SI cells, "background" activity that was recorded during the interstimulus intervals either increased (most cases) or decreased during local anesthesia of the other area. The results are discussed in reference to the hypothesis that primary sensory cortical areas feed information forward to secondary areas, and these feed back modulatory controls to the primary regions.

Animals↗

PubMed1987

Mapping human somatosensory cortex with positron emission tomography.

Positron emission tomography measurements of regional cerebral blood flow were used to detect focal neuronal activation in the first somatosensory cortex (SI) of humans induced by cutaneous vibratory stimulation. Intravenously administered water labeled with oxygen-15 (H2(15)O) was used as a blood flow tracer to obtain five stimulated-state and two resting-state blood flow images in each of eight normal volunteers. Three cutaneous surfaces were tested: lips, fingers, and toes. Intense, highly focal SI responses were seen during all 39 stimulated-state trials. The SI responses from the three stimulation sites were anatomically distinct and formed a medial-to-lateral homonculus in every subject. Response magnitudes (increase in local blood flow) and response locales (expressed as proportionately measured bicommissural stereotaxic coordinates) were highly consistent among subjects and on repeated trials for each subject. These findings suggest that eliciting cerebral blood flow responses by cutaneous vibration provides a safe, rapid, and reproducible tool for locating and assessing the functional status of somatosensory cortex, and offers potential clinical and research utility. This study has established normative values for future applications of this experimental paradigm.

Brain Mapping↗

PubMed1986

Second somatic sensory cortical area (SII) in a prosimian primate, Galago crassicaudatus.

The cortex adjacent to and along the upper bank of the lateral sulcus (UB-LS) of a prosimian primate, Galago crassicaudatus, was explored to determine the topographical representation of low-threshold cutaneous inputs to this region. The somatic sensory projections to this cortex were considered homologous to those defined in other species as the second somatosensory cortical area (SII). Multiple and single neuron recordings were obtained with tungsten microelectrodes in animals anesthetized with sodium pentobarbital or ketamine hydrochloride; receptive fields were determined by means of manually applied tactile stimuli. The area of SII was located approximately 1-1.5 mm rostral to the posterior limit of LS, extended rostrally approximately 4 mm, and occupied nearly all of the upper bank of the sulcus throughout this region. Receptive fields (RFs) in SII were primarily contralateral except for some bilateral input in the cortex representing portions of the trunk, head, and face. The boundaries of RFs were well defined, especially where recordings were located in the middle layers of the cortex. The distribution of RFs across SII was somatotopically organized into a single, relatively erect representation of the body that involved inputs from the face rostral and medial (superficially along the UB-LS) surrounding an enlarged forelimb area; the latter, in turn, lies rostral and medial to the hindlimb zone. Projections from the tail and sacrum are located furthest caudal and lateral (deeper along UB-LS). Separate regions that were devoted to the glabrous skin surfaces of the distal limbs formed the rostral and lateral boundaries of the distal fore- and hindlimb representations, respectively. In the zone for the glabrous surfaces of the forelimb digits, individual digits dominated discrete components of the SII map, especially medially where digit 1 was represented. The glabrous tip of digit 5 was represented caudal and lateral to the tip of digit 1. A similar radial to ulnar medial to lateral sequence was noted in the area representing the palm. Except for a possible medially located toe 1 zone in the hindlimb representation, separated representations for the glabrous skin of individual toes were not noted. The dorsal hairy surfaces of the digits and toes were, respectively, amalgamated within the representations for the dorsal surfaces of the hand and foot. In these regions, which were found superficial and slightly caudal to their respective glabrous zones, some RFs were found that were devoted only to the distal extremities, but most RFs included more proximal portions of the hand or foot dorsum.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1986

The doctor-nurse relationship: an historical perspective.

The purpose of this historical research was to explore the evolution of the doctor-nurse relationship. Specifically, older nurses were interviewed regarding their nursing interactions with physicians approximately 50 years ago. A grounded theory approach was employed to analyse the data. Inherent to the difficulties nurses experienced was the dominant power position assumed by doctors in the health profession. The data give added insights into the development of this relationship. It was found that because nurses were educated primarily by doctors and because they were hired by doctors if they were considered to be 'good' nurses, a sex role stereotype of the nurse emerged. Historically these roles have influenced and continued to influence the nursing profession.

Canada↗

PubMed1986

Bicuculline-induced alterations in neuronal responses to controlled tactile stimuli in the second somatosensory cortex of the cat: a microiontophoretic study.

Single-neuron activity (n = 29) was recorded from the second somatosensory cortex of cats, and the effect of glutamate, gamma-amino-butyric acid (GABA), and bicuculline methiodide (BMI) on spontaneous and stimulus-induced responses were analyzed. Iontophoresis of glutamate produced dose-dependent increases in spontaneous activity, whereas GABA suppressed both spontaneous and glutamate-induced activity. Neuronal responses elicited by cutaneous stimuli were also inhibited by GABA in a dose-dependent fashion; current levels needed to produce at least a 25-50% decrease in stimulus-evoked activity ranged from 5 to 100 nA, with a mean of about 45 nA. Iontophoresis of BMI (10-75 nA) effectively antagonized GABA-induced inhibition of stimulus-evoked responses without altering spontaneous activity. Furthermore, BMI increased the magnitude of responses produced by ramp stimuli and caused a several-fold increase in receptive field size. For neurons responsive to sinusoidal stimulation, BMI caused an increase in the frequency-following probability at preferred frequencies, but failed to alter the response to nonpreferred frequencies. These results suggest that GABA-ergic circuits may limit response magnitude but not the submodality properties of somatosensory cortical neurons.

Animals↗

PubMed1985

The distribution and topographical organization in the thalamus of anterogradely-transported horseradish peroxidase after spinal injections in cat and raccoon.

The distribution of anterogradely-transported horseradish peroxidase (HRP) was examined in the rostral mesencephalon and thalamus of cats and raccoons that had received injections of HRP in the cervical and/or lumbosacral enlargements of the spinal cord. Labeling was consistently observed in a large number of loci. All regions previously identified as targets of spinomesencephalic or spinothalamic fibers were included. Evidence of topographical organization was obtained in several regions. Adjacent fields of labeling were often separable on the basis of the distribution, appearance and topographical organization of the labeling. Subject to the methodological constraints imposed by the possibilities of transneuronal and/or collateral labeling, we conclude that a wide variety of loci in the thalamus receive direct spinal input. The organization of these projections suggests that each terminal region may be associated with different aspects of spinal cord function.

Animals↗

PubMed1985

Submodality and columnar organization of the second somatic sensory area in cats.

Electrophysiological responses of 519 single and 405 multiple neurons located in the distal forelimb zone of the second somatic sensory cortex (SII) of 11 intact cats were characterized according to their submodality and receptive field properties. In 4 of these animals, 46 single and 134 multiple neuronal responses were studied after transection of the dorsal columns contralateral to the cortical recording sites. Receptive field positions overlapped considerably in SII during orthogonal electrode penetrations, but were shifted during tangential penetrations. Analysis of the receptive field positions for neurons encountered in tangential penetrations indicated that receptive fields rarely overlapped when the neurons were separated by more than 750 microns. Using a variety of hand-held stimuli, neuronal responses were assessed according to several criteria including: velocity, adaptation, following rate, spontaneous activity, and whether the response was elicited by stimulating hairs, skin, claws, or deep tissue. Based on these parameters, it was possible to discern several types of neuronal responses in SII. Among these, over 60% of the neurons in our sample responded best to movement of hairs. A smaller number of neurons responded as though they received inputs from Pacinian receptors or rapidly adapting receptors in the glabrous skin. In about 20% of the single neuron sample, it was not possible to identify a selective adequate stimulus, however, these cells responded to somatic stimuli, such as taps. Approximately 5% of the neurons could not be driven with somatic sensory stimuli. Following dorsal column lesions, some neurons in SII still responded to cutaneous stimulation, primarily hair movement. Most SII neurons were more difficult to drive, the responses were more sluggish and receptive fields were less well-defined. A greater proportion of single neuron responses (greater than 60%) could not be activated by any type of somatic sensory stimulus. These results indicate that the dorsal columns provide a potent, but not exclusive, source of afferent input to SII.

Afferent Pathways↗

PubMed1985

Projections from the paratrigeminal nucleus and the medullary and spinal dorsal horns to the peribrachial area in the cat.

The projections from the medullary and spinal dorsal horns to the dorsolateral pons were investigated in the cat utilizing both the retrograde and anterograde transport of a wheat germ agglutinin-horseradish peroxidase complex and the retrograde transport of the fluorescent dyes Fast Blue and Nuclear Yellow. After injections of wheat germ agglutinin-horseradish peroxidase into the area surrounding the brachium conjunctivum, numerous neurons were labeled ipsilaterally near levels of the obex in the paratrigeminal nucleus. Such neurons were located in connected pockets of neuropil located within the spinal trigeminal tract and along its medial edge. Most of the neurons labeled in the dorsal horns after such injections were found in lamina I. Those found in the medullary dorsal horn were mostly ipsilateral to the injection while those in the spinal dorsal horn were found bilaterally. Some labeled neurons were also found in lamina V of both the medullary and spinal dorsal horns bilaterally. When the injection was centered in either the medial parabrachial nucleus or the Kolliker-Fuse nucleus, a greater number of neurons were labeled ipsilaterally in lamina V of the medullary dorsal horn. Since neurons in lamina I of the medullary dorsal horn also project to the medial thalamus, fluorescent dyes were used to determine if the same neuron might project to both targets. Fast Blue was first injected into either the peribrachial area or the medial thalamus. After an appropriate period, Nuclear Yellow was injected into that target not injected first with Fast Blue. The injection of Nuclear Yellow was always placed on the side of the brain opposite to the first injection. Both dyes were transported retrogradely and were found in neurons located in lamina I of the medullary dorsal horn. However, no double-labeled neurons were seen. In general those labeled after injections of the medial thalamus were more superficial than those labeled after injections of the dorsolateral pons. The anterograde transport of wheat germ agglutinin-horseradish peroxidase was used to determine the termination of the projections from neurons in the medulary dorsal horn and the cervical spinal cord to the peribrachial area. After injections into these areas a moderate to sparse labeling of the lateral parabrachial nucleus and the Kolliker-Fuse nucleus was seen. It was mostly ipsilateral in cases with injections of the medullary dorsal horn but was bilateral following injections into the cervical enlargement.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1985

Homotypical ipsilateral cortical projections between somatosensory areas I and II in the cat.

In 11 cats, small quantities of horseradish peroxidase conjugated to wheat germ agglutinin were placed into cortical zones of somatosensory area I representing the distal digits (n = 3), distal toes (n = 2), toes and digits (n = 1), proximal forelimb (n = 1), proximal hindlimb (n = 1), trunk (n = 2), and the face and nose (n = 1). Reconstruction of the pattern of retrograde labeling in somatosensory area II revealed dense, heavily labeled patches of cells in regions that were precisely homotypical to the injection site as determined by electrophysiological recordings. This dense, homotypical patch of labeled cells was usually surrounded by a less densely populated fringe of labeled cells that bordered, but did not appear to enter, heterotypical zones. In two animals, however, some retrogradely labeled cells were found in the cortex representing somatotopic zones adjacent to the sites injected with horseradish peroxidase. These results indicated that somatosensory area II primarily sends homotypical projections to somatosensory area I. In a few cases, however, some retrogradely labeled cells may represent either homo-or heterotypical projections depending on how receptive field sizes and the areal extent of labeling in somatosensory areas I and II are interpreted.

Animals↗

PubMed1984

Corticothalamic connections from the second somatosensory area and neighboring regions in the lateral sulcus of macaque monkeys.

Corticothalamic connections were shown between the second somatosensory area in primates and the ventroposterior nuclei of the thalamus. These projections were topographically arranged with those from the hindlimb portions of SII traced to the most lateral and posterior parts of the ventroposterior lateral nucleus (VPLc) and those from the forelimb located medially within VPLc. The densest labeling was found ventrally in VPLc and dorsally within ventroposterior inferior n. (VPI) only after injections of the forelimb. A more scattered, dorsal distribution of labeling was seen in the rest of VPLc from injections involving more proximal parts of the body representation in SII.

Animals↗

PubMed1984

Ipsilateral cortical connections from the second and fourth somatic sensory areas in the cat.

The ipsilateral corticocortical connections of the second and fourth somatic sensory areas (SII and SIV) were traced with the aid of anterograde or retrograde axonal transport techniques involving horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA) or tritiated amino acids. The injections were placed into physiological defined components of the body representation in SII or SIV. The results from cases with localized injections into SII showed precise topographically organized, reciprocal connections with SI and motor cortex area 4. The distribution of connections in SI included areas 3a, 3b, and 1-2. A uniform pattern of cell and fiber labeling was seen across area 3b and 1 within the zones that were homotypical to the injection site in SII as though only a single representation of the cutaneous surface of the body existed in SI. Intrinsic connections within SII were also topographically arranged. Additional areas found to be interconnected with SII included, in decreasing order of density: area 5, insula, perirhinal cortex (area 36), and ventrolateral orbital cortex. SII connections with area 6 were seen only in the region of the lateral bank of the presylvian sulcus. There may be interconnections between SII and SIV but these were from possible local intrinsic connections in the AEG. The results from injections involving SIV showed reciprocal connections with area 5, the suprasylvian fringe, insula, dorsolateral orbital area, and area 6. The densest connections for SIV were with area 5. No topography was noted in the connections for SIV.

Animals↗

PubMed1984

Fetal rat sympathetic neurons maintained in a serum-free medium retain induced cholinergic characteristics.

When maintained in some serum-containing media, fetal rat sympathetic neurons acquire substantial choline acetyltransferase activity and form cholinergic synapses in vitro. However, when they are maintained in a serum-free, defined culture medium, choline acetyltransferase activity is not detected and cholinergic synapses are not observed. In this study, we have examined the effects of various times of exposure to a medium inducing cholinergic function on the properties of neurons subsequently maintained in defined medium. We report that 2-day, but not 2-h, exposure to this inducing medium causes a long-lasting (greater than 6 weeks) increase (7-10-fold) in the activity of choline acetyltransferase and that, under these conditions, sympathetic neurons in vitro form cholinergic, electrical and mixed function cholinergic and electrical synapses. We conclude that a relatively brief exposure to media inducing cholinergic function can cause long-lasting changes in the functional properties of sympathetic neurons in vitro.

Acetylcholine↗

PubMed1982

Second somatic sensory area in the cerebral cortex of cats: somatotopic organization and cytoarchitecture.

The cortex of the anterior ectosylvian gyrus and adjoining ectosylvian and suprasylvian sulci was explored with tungsten microelectrodes to determine the distribution of responses to light cutaneous stimulation in barbiturate-anesthetized cats. Recordings were spaced between 125 and 250 micrometers and, in several cases, nearly all of the somatic areas in this cortex were explored in the same brain. Four somatic sensory areas were identified on the basis of responses properties, sequences of receptive fields, and cytoarchitecture. The largest area, which occupied the rostral and medial two-thirds to three-fourths of the exposed, relatively flat portion of the anterior ectosylvian gyrus, was called the second somatic sensory area (SII). Receptive fields in SII were primarily from the contralateral side of the body; they were well defined and somatotopically organized into an erect representation of the body. The top of the head was located next to a similar representation of the periphery in a portion of the first somatic sensory area (SI). Individual distal digits and toes occupied discrete components of the SII map. Another representation for the distal forelimb and hindlimb was noted medially along the lateral bank of the anterior suprasylvian sulcus. Receptive fields and response properties in this region were equivalent to those seen in SII proper. However, only a crude anteroposterior, fore- to hindlimb topographical organization was noted, but with more distal parts of the limbs generally located closer to the fundus of the sulcus in this medial representation. As the cytoarchitecture in this medial region was similar to the rest of SII it was considered a medial subdivision of SII. A third, topographically organized zone was located lateral to SII largely within the upper bank of the anterior ectosylvian sulcus and adjoining lateral crest of the anterior ectosylvian gyrus. Large, stockinglike, contralateral receptive fields were common; ipsilateral components to the receptive fields were present. Some individual digit receptive fields were located in the rostral part of the forelimb zone within the anterior ectosylvian sulcus. This lateral somatic area is probably equivalent to a fourth somatic sensory area (SIV) recently identified by Clemo and Stein ('82). Posterior to the hindlimb zones of SII and medial to SIV was another region that responded to cutaneous plus auditory stimulation. There was no detectable topography in this area; nearly all of the receptive fields were large, frequently bilateral, and often involved the whole body or all four extremities. This area's cytoarchitecture was comparable to previous descriptions of the suprasylvian fringe (Rose, '49). The location and physiology of these four areas were discussed in reference to previous controversies regarding the topography of the body representation in SII and the location of an acallosal zone in this region of cortex.

Animals↗

PubMed1982

Origin of ascending intratrigeminal pathways in the cat.

The retrograde horseradish peroxidase technique was used to locate neurons projecting to rostral trigeminal areas via ascending intranuclear pathways. After rostral trigeminal injections, labeled neurons were found in the subnucleus interpolaris and in all laminae of the medullary dorsal horn. Most neurons were labeled ipsilaterally in laminae III and IV, but some small neurons were labeled in lamina II in the medullary dorsal horn. The distribution of labeled cells suggested that these projections are topographically arranged. Labeled neurons were found in lamina I and lamina V, bilaterally; this was especially true if the parabrachial complex was included in the injection.

Animals↗

PubMed1982

Electrotonic synapses are formed by fetal rat sympathetic neurons maintained in a chemically-defined culture medium.

Principal neurons from the superior cervical ganglia of rat fetuses were maintained for up to 101 days in dissociated cell cultures in a serum-free, chemically-defined medium; non-neuronal cells were killed by the periodic addition of fluorodeoxyuridine to the medium. Intracellular recordings, obtained at various times between 16th and 98th day in vitro, showed that these neurons could generate substantial (up to 90 mV) action potentials in response to depolarizing current injections; these responses were dependent on tetrodotoxin-sensitive Na+ channels, cobalt-sensitive Ca++ channels, and tetraethylammonium-sensitive K+ channels. Action potentials were often followed by prominent, long hyperpolarizing after-potentials (10-15 mV, greater than 150 ms); the duration of these after-potentials was reduced by the addition of Co++ (2-5 mM) to the perfusate. Acetylcholine depolarized these neurons by a hexamethonium-sensitive mechanism. To determine whether sympathetic neurons formed synapses in a defined medium, intracellular recordings were obtained from pairs of neighboring neurons. Synaptic interactions were frequently observed at all times in vitro (up to 60% of all pairs tested). At many synapses, both hyperpolarizing and depolarizing DC potential changes spread from one neuron to another. At other synapses, the spread of DC potential changes could not be directly demonstrated; however, interactions at such synapses were not inhibited by antagonists of several neurotransmitters, by elevation of the Mg++/Ca++ ratio, or by the addition of Co++. Thus most, if not all, of the synaptic interactions among sympathetic neuron were electronic; such electrical synapses were not observed among dorsal root ganglion neurons maintained in the same medium. These data indicate that, when maintained in a chemically-defined culture medium, sympathetic neurons of rat fetuses express many of the basic membrane properties observed in neurons of superior cervical ganglia recently removed from adult rats. However, fetal sympathetic neurons maintained in this defined medium also differ from their counterparts in vivo; they adopt a mode of synaptic transmission (electrical) that has not been observed in the sympathetic ganglia of the adult rat. Thus, as late as the 21st embryonic day, not only the choice of neurotransmitter, but also the mode of transmission has not been irrevocably determined in sympathetic neurons.

Acetylcholine↗