Synaptic connexions between primary afferents and thoracic motoneurones in the frog

Summary Synaptic actions were evoked in thoracic motoneurones of the isolated frog spinal cord by 6th or 7th dorsal root volleys or impulses in single primary afferents. In a few cases the impaled motoneurones or the whole dorsal root were filled with horseradish peroxidase. Although HRP labelling has revealed that motoneurones have extensive dendritic arborizations extending into the dorsal horn, the zone in which thoracic dorsal root afferents terminate, there is no positive evidence for a direct link between them. Synaptic actions produced in thoracic motoneurones by dorsal root stimulation are mediated via interneurones and do not involve direct sensorimotor synapses. This conclusion is in agreement with results of simultaneous intracellular recording from motoneurones and individual dorsal root fibres afferent to them.Dr. A.I. Shapovalov deceased on September 28, 1983     

Substance P-immunoreactive and serotonin-containing neurones in the ventral brainstem of the cat

Substance P-like immunoreactive cell bodies have been described in the cat brainstem after local injections of colchicine. Substance P-containing neurons were found in the nucleus raphe pallidus, the nucleus raphe obscurus and at the caudal end of the nucleus raphe magnus. Two groups of substance P-positive (+) cells were also found in the ventrolateral medulla. Small substance P+ cells were present in a narrow band immediately ventral to the facial nucleus, and a second group of small and medium sized neurones was found in a strip ventrolateral to the rostral half of the inferior olive. Some of these cells lay in close proximity to the ventral surface of the brain. Double labelling studies indicated that within the raphe nuclei many substance P+ neurones also contained serotonin, while within ventrolateral areas of the medulla, the most rostral cell group stained only for substance P and the more caudal nucleus for both substance P and serotonin.     

Synaptic connexions of alpha extensor motoneurones with ipsilateral and contralateral cutaneous nerves

1. Synaptic responses of alpha extensor motoneurones to stimulation of cutaneous nerves of the hind limb were recorded from the lumbosacral region in decerebrate and decerebrate-spinal unanaesthetized cats. Responses to ipsi- and contralateral stimulation are compared.2. Usually a single volley in an ipsilateral cutaneous nerve led to an inhibitory post-synaptic potential while one in a contralateral cutaneous nerve led to an excitatory post-synaptic potential or to discharge of the cell, thus demonstrating the classical reciprocal innervation pattern. In some cells either ipsilateral or crossed spinal action was of opposite sign to normal.3. Tests using graded stimulation showed the range of myelinated afferents associated with crossed spinal actions to be narrower than that for ipsilateral actions; for the sural nerve the ranges are 1-6 mu and 1-9 mu respectively. A substantial proportion of the coarser myelinated afferents do not make connexion with alpha motoneurones on either side of the cord; in the sural nerve this proportion includes the range 10-17 mu.4. Central transmission times for ipsilateral and crossed-spinal actions were estimated after making systematic allowance for conduction time to the cord in the associated cutaneous afferents. Mean values for inhibitory pathways from ipsilateral cutaneous nerves were about 2 msec, while those for crossed spinal facilitatory pathways were notably greater, 3-5 msec.5. Crossed spinal actions were more profoundly depressed by anaesthesia than were corresponding ipsilateral actions.6. Interpretation of the results was aided by a separate study of the input to the spinal cord; stimulus strength is related to the diameters of the cutaneous fibres excited and to peripheral conduction time in those fibres.     

Monosynaptic connexions among Aplysia neurones examined by the intracellular application of tea.

1. Several assumptions underlying the use of intracellularly applied tetraethylammonium (TEA) for assessing monosynaptic connexions were evaluated in identified neurones of Aplysia. 2. In the R2 neurons, intrasomatic TEA application prolongs the duration of the intrasomatically recorded action potential. Subsequently, the action potential in the axon of R2, recorded extracellularly 4-7 mm from the soma, was also prolonged. 3. Intracellular application of TEA into the somata of the multi-action interneurone L10 enhances the duration of the L10 AP and results in larger and more prolonged post-synaptic potentials (p.s.p.s) recorded from neurones believed to be connected monosynaptically with L10. The action potential duration and wave form of p.s.p.s elicited by nerve stimulation in these same post-synaptic neurones were unaffected during the time L10-mediated p.s.p.s were potentiated. 4. Following TEA injection into L10 the p.s.p. recorded in neurone L7 changes wave form in a manner similar to that observed when L10 is tetanized. 5. It is concluded that TEA migrates from its intracellular site of application, does not leave the injected neurone in significant quantities, and alters the wave form of the p.s.p in only those neurones connected monosynaptically to the injected neurone.     

Synaptic actions of individual vestibular neurones on cat neck motoneurones

1. Unitary synaptic potentials evoked by the activity of single vestibulocollic neurones were recorded by means of spike-triggered signal averaging in neck extensor motoneurones of decerebrate cats. Properties of the vestibulocollic neurones which produced the potentials were examined.2. Vestibulocollic neurones were first identified as projecting to the C3 grey matter by antidromic microstimulation within the C3 extensor motoneurone pool. The spontaneous or glutamate-driven activity of the vestibulocollic neurones was then used to trigger the averaging computer. In this way ten inhibitory and two excitatory neurones were identified (20% of neurones tested).3. Action potentials in local branches of vestibulocollic neurones were usually recorded in the vicinity of motoneurones. Mean orthodromic conduction time from the foot of the extracellular spike, recorded in the vestibular nuclei, that triggered the averager was 0.72 msec. Mean synaptic delay was 0.4 msec.4. I.p.s.p.s had a mean time to peak of 0.81 msec and were readily reversed by injection of hyperpolarizing current. These data, together with the shape indices of i.p.s.p.s indicate that they are generated proximally on motoneurones.5. All vestibulocollic neurones making synapses with motoneurones were monosynaptically driven by stimulation of the ipsilateral vestibular nerve. Four out of seven tested were inhibited by stimulation of the contralateral vestibular nerve (commissural inhibition).6. Two excitatory neurones were located in Deiters' nucleus or on the Deiters'-descending border. Inhibitory neurones were found relatively medially in the vestibular complex in the medial, descending and Deiters' nuclei.7. Vestibulocollic neurones acting on motoneurones were tested for axon branching to more caudal levels of the spinal cord with electrodes placed at C5-7. Both of the excitatory and two out of nine inhibitory neurones branched.     

Synaptic excitation of red nucleus neurones by fibres from interpositus nucleus

Summary Electrophysiological properties of the interpositus-rubral transmission were studied in anaesthetized cats. The axons of interpositus neurones were stimulated either at their origin in the interpositus nucleus or at their terminal in the ventrolateral nucleus of the thalamus. Impulses of the interpositus axons produced in the red nucleus neurones excitatory postsynaptic potentials (EPSPs) monosynaptically. As a unique feature, these EPSPs exhibited very little facilitation or depression during double shock or tetanic stimulation. Correspondingly, the unitary EPSPs evoked by the threshold stimulation showed little failure during many successive trials. The number of the interpositus axons converging onto a single red nucleus cell was about 50, when calculated from the ratio of the maximum rising slopes between the unitary and maximal EPSPs evoked from the interpositus nucleus.     

Chemical and electrical synaptic connexions between cutaneous mechanoreceptor neurones in the central nervous system of the leech

Experiments have been made to study the synaptic connexions between sensory cells in the C.N.S. of the leech. Each segmental ganglion contains six neurones that respond specifically to light touch applied to the skin; each of these ;touch cells' innervates a discrete area on the surface of the body and has a characteristic set of properties by which it can be recognized. Using intracellular electrodes it has been shown that these sensory cells interact with one another through chemical and electrical synapses by way of a stereotyped set of pathways.1. Action potentials occurring in one touch cell gave rise to synaptic potentials in the five other touch cells in the same ganglion and also in the three ipsilateral touch cells in the adjacent ganglia. Thus, synaptic interactions took place between sensory cells whose receptive fields lay within the same segment and on the same side of adjacent segments.2. The post-synaptic potentials consisted of a short-latency coupling potential, followed by an excitatory potential and a number of inhibitory potentials. These delayed synaptic potentials occurred inconsistently and with a variable latency; they could also be recorded in the cell which had been stimulated. All of the touch cells appeared to be equally effective in initiating synaptic potentials.3. The short-latency coupling potential was shown to be mediated through an electrical synapse by observing a voltage change in one touch cell when current was injected into its neighbour. It was not abolished by high concentrations of Mg in the bathing fluid, which blocked chemical synapses in this ganglion. This electrical synapse displayed remarkable rectification; a depolarization could spread from cell to cell in both directions, while a hyperpolarization could spread in neither.4. The inhibitory potentials were reversed by injecting Cl into the cell. In Cl-free Ringer solution this effect was so marked that the reversed IPSPs caused long trains of impulses in touch cells, which tended to excite each other by a process of positive feed-back.5. Synaptic potentials evoked by activation of a touch cell did not usually reach threshold since excitation and inhibition tended to cancel. The connexions between touch cells that mediated the delayed excitatory and inhibitory potentials are polysynaptic; the interneurones have not yet been found but some of their connexions could be inferred from electrical recordings.6. Action potentials in sensory cells of a different modality (responding to pressure) also initiated synaptic potentials in the same family of touch cells.7. The possible significance for integration of these synaptic interactions between sensory cells is discussed.     

Synaptic potentials induced by postganglionic stimulations in cat bladder parasympathetic neurones

Intracellular recording techniques were used to examine and compare synaptic potentials evoked by stimulating pre- and postganglionic nerve trunks in cat bladder parasympathetic ganglia. In the 76 ganglion cells exammed, two types of responses were recorded on stimulating the postganglionic nerve: an antidromic action potential (type Post NS1;n=30) or a fast excitatory postsynaptic potential (f-EPSP; type PostNS2;n=46) which resulted in an orthodromic-like action potential. In some of the cells exhibiting a PostNS1 response (n=19), a fast depolarization was superimposed on the antidromic spike. This depolarization was due to the synaptic activation of nicotinic receptors. In many of the cells exhibiting either PostNS1 or PostNS2 responses, repetitive stimulation of the postganglionic nerve induced a slow hyperpolarization. Applying nicotinic (hexamethonium, methonium, 0.5–1 mM), muscarinic (atropine, 1 M), alpha-adrenergic (phentolamine, 1 M) and purinergic (caffeine, 0.5–1 mM) receptor antagonists completely inhibited the tetanus-induced slow hyperpolarization in some cells (n=5). In other cells (n=15), a slow hyperpolarization persisted in the presence of these antagonists. These results indicate that stimulation of the postganglionic nerve trunk of cat bladder parasympathetic ganglia can elicit not only an antidromic action potential, but also synaptic potentials which are mediated by the activation of cholinergic (nicotinic and muscarinic), noradrenergic and purinergic receptors, as well as a non-cholinergic, non-alpha-adrenergic and non-purinergic synaptic potential.     

Input-output organization of reticulospinal neurones,with special reference to connexions with dorsal neck motoneurones in the cat

Summary Dorsal neck motoneurones receive disynaptic tectal and pyramidal EPSPs via common reticulospinal neurones (RSNs). This study was aimed at identification of the RSNs projecting directly to neck motoneurones and mediating these EPSPs. 1. Stimulation of the tectum and the cerebral peduncle evoked monosynaptic descending volleys in the spinal cord, which were chiefly mediated by reticulospinal neurones in the pons and the medulla. Systematic tracking of the C3 and C7 segments was made to locate descending volleys in the spinal funiculi. The tectal monosynaptic volley was largest in the medial part of the ventral funiculus and decreased gradually as the recording electrode was moved to the lateral part of the ventral funiculus and the lateral funiculus. In contrast, the peduncle-evoked monosynaptic volley was distributed rather evenly in the ventral funiculus and the ventral half of the lateral funiculus. 2. Differences in funicular distribution of the two descending volleys suggest the existence of subgroups of RSNs which differed in strength of inputs from the two descending fibre systems and in the funicular location of descending axons. 3. The RSNs were classified into the following four groups; (1) mRSNs which descended in the medial part of the ventral funiculus, (2) in RSNs which descended in the ventrolateral funiculus, (3) 1RSNs which descended in the dorsal 2/3 of the lateral funiculus and (4) coRSNs which descended in the contralateral funiculi. The mRSNs were located in a fairly localized region corresponding to the nucleus reticularis pontis caudalis (N.r.p.c.), while inRSNs, 1RSNs and coRSNs were mainly in the nucleus reticularis gigantocellularis (N.r.g.), in the nucleus reticularis magnocellularis (N.r.m.) and in the nucleus reticularis ventralis (N.r.v.). RSNs were further divided into three types depending on the levels of projection. L-RSNs projected to the lumbar spinal segments. C-RSNs descended to the C6–C7 spinal segment but not to the lumbar segments. N-RSNs projected to the C3 but not to the C6–C7 segments. 4. Stimulation of the tectum and the cerebral peduncle produced monosynaptic negative field potentials in the medial two thirds of the reticular formation in the pons and medulla. Tectal field potentials were largest in the N.r.p.c. and the rostral part of the N.r.g., while pyramidal field potentials were largest in the N.r.g. Correspondingly, RSNs in the N.r.p.c. (mRSNs) received larger monosynaptic EPSPs from tectal than from pyramidal volleys, while RSNs in the N.r.g. (in-, 1- and coRSNs) received stronger input from the peduncle than from the tectum. 5. Stimulation of the C7 ventral but not the lateral funiculus evoked monosynaptic EPSPs on all the dorsal neck motoneurones tested. Stimulation of the L1 segment only produced monosynaptic EPSPs in 35% of the motoneurones. The L1 evoked EPSPs were much smaller than C7 evoked EPSPs. 6. The C7 evoked EPSPs (C7 EPSP) showed complete occlusion (collision) with the tectal or pyramidal disynaptic EPSPs. Similar results were obtained with L1 EPSPs. These results indicate that tectal and pyramidal disynaptic EPSPs in dorsal neck motoneurones were mediated chiefly by C-mRSNs and C-inRSNs and partly by L-RSNs.     

Synaptic excitation and inhibition resulting from direct action of acetylcholine on two types of chemoreceptors on individual amphibian parasympathetic neurones.

1. Synaptic transmission was studied in visually identified parasympathetic ganglion cells that modulate the heart beat of the mudpuppy Necturus maculosus).2. The brief pulse of acetylcholine (ACh) released from terminals of the vagus nerve after each impulse can produce two distinct post-synaptic responses in individual principal cells of the ganglion: (i) within a milli-second of release, ACh generates a rapid and strong excitatory post-synaptic potential (e.p.s.p.) that normally initiates a post-synaptic impulse; (ii) this excitation is usually followed by a slow hyperpolarizing inhibitory post-synaptic potential (i.p.s.p.) that lasts for several seconds. The magnitude and time course of the i.p.s.p. depends on the frequency and number of vagal stimuli. When the hydrolysis of ACh is inhibited by prostigmine, a train of nerve stimuli may be followed by an i.p.s.p. lasting half a minute or longer.3. The rapid e.p.s.p. and slow i.p.s.p. result from the direct action of ACh on two different types of chemoreceptors in the post-synaptic membrane of the principal cell. The e.p.s.p. can be preferentially blocked by the nicotinic antagonist dihydro-beta-erythroidine (5 x 10(-7)M), while the i.p.s.p. is selectively blocked by the muscarinic antagonist atropine (5 x 10(-9)M).4. Potentials resembling nerve-evoked e.p.s.p.s and i.p.s.p.s can be produced by iontophoretic release of ACh from micropipettes onto the post-synaptic membrane. Application of the muscarinic agonist bethanechol generates exclusively inhibitory responses.5. The reversal potential for the i.p.s.p. is about -105 mV, which is approximately the equilibrium potential for potassium (E(K)). When the external K(+) concentration is altered, the reversal potential for inhibition is shifted to the new value of E(K) as expected from the Nernst equation. Changes in the external Na(+) and Cl(-) concentrations have no appreciable effect on the reversal potential. Thus, the i.p.s.p. is the result of a conductance increase for K(+).6. The conductance change producing the i.p.s.p. is voltage sensitive. When the membrane potential is shifted from -40 to -60 mV, the i.p.s.p becomes larger and longer. Beyond -60 mV the inhibitory response decreases in proportion to the driving force on K(+) without any further change in time course.7. The inhibitory response produced by an iontophoretically applied pulse of bethanechol has a delayed onset of about 150 msec at 24 degrees C. The early portion of this response, including the delay, is proportional to t(3), where t is time. The proportionality factor (the apparent rate constant) decreases elevenfold when the temperature is lowered by 10 degrees C. This suggests that a multi-step process is involved in the activation of the conductance increase that leads to the inhibitory response. Inhibitory responses with similar kinetics were produced in heart muscles of the mudpuppy upon application of ACh.     

Monosynaptic excitatory connexions of reticulospinal neurones in the nucleus reticularis pontis caudalis with dorsal neck motoneurones in the cat

Summary 1. Projections of reticulospinal neurones (RSNs) in the nucleus reticularis pontis caudalis (N.r.p.c.) to dorsal neck motoneurones supplying splenius (SPL, lateral head flexor) and biventer cervicis and complexus (BCC, head elevator) muscles were studied in the cat anaesthetized with pentobarbiturate or -chloralose. 2. Threshold mapping for evoking antidromic spikes revealed that most of RSNs tested projecting down to brachial segments but not to lumbar segments (C-RSNs) gave off collaterals to the gray matter of the upper spinal cord in C2–C3 segments. 3. Spike triggered averaging showed that negative field potentials were evoked after firing of a single C-RSN (single fibre focal synaptic potentials, FSPs) in the region of C2–C3 where large antidromic field potentials from nerves supplying SPL or BCC muscles were evoked. The single fibre FSPs ranged between 1 and 10 V in amplitude and had latencies between 0.7 and 1.2 ms from the onset of the triggering spike. In most cases, a presynaptic spike preceded the negative potential by 0.3 ms. These results indicated that C-RSNs project to the SPL or BCC motor nucleus. 4. Spike triggered averaging of postsynaptic potentials revealed EPSPs (single fibre EPSPs) in 36 dorsal neck motoneurones, predominantly in SPL (25) and less in BCC (11) motoneurones, evoked from 15 C-RSNs. The amplitude of the single fibre EPSPs ranged from 5 to 310 V, and had latencies of 0.8–2.0 ms from the onset of the triggering spikes of C-RSNs, or 0.3–0.5 ms from the presynaptic spike when recorded. The results indicated monosynaptic excitatory connexions of C-RSNs to dorsal neck motoneurones. 5. Single fibre EPSPs from a C-RSN were usually recorded from either BCC or SPL motoneurones but not from both types of motoneurones, when tested in many motoneurones. This showed that connexions of C-RSNs with dorsal neck motoneurones were muscle specific. 6. RSNs projecting down to the lumbar segment (L-RSN) also showed branching in C2–C3 segments. Excitatory monosynaptic connexion of L-RSNs with neck motoneurones were demonstrated by recording single fibre postsynaptic population potentials (p.s.p.p.s.) from the C2 ventral root perfused with sucrose. The probability of evoking monosynaptic single fibre p.s.p.p.s. was less (19%) than for C-RSNs (59%).     

Synaptic action on Clarke's column neurones in relation to afferent terminal size

1. Excitatory post-synaptic potentials (e.p.s.p.s) were recorded intracellularly from Clarke's column neurones (DSCT neurones) of the cat in response to adequate stimuli applied to a variety of sensory receptors.2. The amplitude of e.p.s.p.s so produced varied from less than 0.2 mV to more than 2-3 mV. The amplitude distribution of e.p.s.p.s suggested that the mean number of ;quanta' of transmitter released by one impulse varied widely from one fibre to another arising from a given type of sensory receptor.3. The average amplitude of e.p.s.p.s evoked by single afferent impulses was significantly smaller for cutaneous inputs than for muscle or joint inputs. However, synaptic action on DSCT neurones produced by different sensory inputs was equally greater, on the average, than that on spinal motoneurones.4. Both small and large e.p.s.p.s in DSCT neurones failed to increase in amplitude during post-synaptic hyperpolarization applied through the cell body. This failure could not be attributed to possible anomalous rectification in the post-synaptic membrane.5. Small and large e.p.s.p.s were comparable in half-decay time, but there was a positive correlation between amplitude and time-to-peak of e.p.s.p.s. It is suggested that the locations of synapses responsible for small and large e.p.s.p.s are intermingled on the dendrites and that large e.p.s.p.s are associated with a longer duration of transmitter action than small e.p.s.p.s.6. Degenerating terminals of primary afferent fibres on DSCT neurones and motoneurones were examined with the electron microscope after chronic section of the dorsal roots.7. Dendritic degenerating terminals showed no significant difference in size between motoneurones and DSCT neurones. Degenerating ;giant' terminals were found on DSCT neurones, but they were located only on or very close to the cell body.8. It is concluded that the major factor responsible for a large number of ;quanta' of transmitter released at synapses on DSCT neurones is the number of multiple synaptic contacts formed by one afferent fibre rather than the size of individual synapses.     

Synaptic potentials recorded from neurones of the submucous plexus of guinea-pig small intestine.

1. Intracellular recordings have been made from neurones lying in the submucous plexus of guinea-pig mid small intestine. 2. Most neurones in this plexus receive an extensive excitatory input which could be abolished by tubocurarine. 3. A proportion of neurones also received a single inhibitory input which was activated by transmural stimulation. 4. Some of the characteristics of the inhibitory potentials evoked by transmural stimulation are described. 5. The observations are discussed in relation to the concept of descending excitation (Hirst, Holman & McKirdy, 1975).