The role of dendritic events in the initiation of monosynaptic spikes in frog motoneurons

Simulation of the Effect of Synapses: the Significance of the Dendritic Diameter in Impulse Propagation

The effectiveness of synapses at various sites of the dendritic tree was studied using a segmental cable model with a program developed by Hines (Int. J. Biomed. Comput., 24, 55 - 68, 1989). The model rendered possible a high-fidelity simulation of the dendritic geometry of a frog motoneuron described in the accompanying paper (Birinyi et al., Eur. J. Neurosci., 1003 - 1012, 1992). The model was used in the passive membrane mode and the synaptic activity was simulated with current injections into large and small diameter dendrites at proximal and distal locations. Synaptic efficiency was defined by the charge transfer ratio expressed as the proportion of the injected current which appeared at the soma. The charge transfer ratio was determined with uniform and non-uniform distribution of specific membrane resistance over the soma - dendrite surface while the diameter of selected dendrite segments changed. The best charge transfer ratio was found with the largest dendrite membrane resistance, and the maximum efficiency of synaptic activity appeared at the original size of the dendrite segment stimulated. The amount of current that flowed in the proximal and distal directions from the segment stimulated depended on the diameter of that segment. The increase in diameter of proximal dendrites increased synaptic efficiency on distal dendrites, whereas the reverse caused a decline in synaptic efficiency on proximal dendrites. In addition to the diameter of dendrites, the arborization pattern also played a significant role in this mechanism. It is concluded that the cellulipetal increase in dendrite diameter greatly increases synaptic efficiency.

The Extent of the Dendritic Tree and the Number of Synapses in the Frog Motoneuron

Frog motoneurons were intracellularly labelled with cobaltic lysine in the brachial and the lumbar segments of the spinal cord, and the material was processed for light microscopy in serial sections. With the aid of the neuron reconstruction system NEUTRACE, the dendritic tree of neurons was reconstructed and the length and surface area of dendrites measured. The surface of somata was determined with the prolate - oblate average ellipsoid calculation. Corrections were made for shrinkage and for optical distortion. The mean surface area of somata was 6710 microm2; lumbar motoneurons were slightly larger than brachial motoneurons. The mean length of the combined dendritic tree of brachial neurons was 29 408 microm and that of lumbar neurons 46 806 microm. The mean surface area was 127 335 microm2 in brachial neurons, and 168 063 microm2 in lumbar neurons. The soma - dendrite surface area ratio was 3 - 5% in most cases. Dendrites with a diameter of </= 1.0 microm constituted approximately 75% of the combined dendritic length in most of the neurons. Unlike in the cat, there was no correlation between the size of stem dendrites and the extent of daughter branches. From the synaptic density estimated in earlier electron microscope investigations of frog motoneuron dendrites (Antal et al., J. Neurocytol., 15, 303 - 310, 1986; 21, 34 - 49, 1992), and from the present data, the number of synapses on the dendritic tree was calculated. The calculations indicated 26 949 synapses on the smallest and 61 519 synapses on the largest neuron if the synaptic density was multiplied by the length of the dendritic tree. If the synaptic density was multiplied by the surface area of the dendritic tree the calculation yielded 23 337 synapses for the smallest and 60 682 synapses for the largest neuron. More than 60% of the combined surface area of dendrites was >600 microm from the soma. This suggests that about two-thirds of the synapses impinged upon distant dendrites >600 microm from the soma. The efficacy of synapses at these large distances is investigated on model neurons in the accompanying paper (Wolf et al., Eur. J. Neurosci., 4 1013 - 1021, 1992).

Synapses on motoneuron dendrites in the brachial section of the frog spinal cord: a computer-aided electron microscopic study of cobalt-filled cells

Cobalt-labelled motoneuron dendrites of the frog spinal cord at the level of the second spinal nerve were photographed in the electron microscope from long series of ultrathin sections. Three-dimensional computer reconstructions of 120 dendrite segments were analysed. The samples were taken from two locations: proximal to cell body and distal, as defined in a transverse plane of the spinal cord. The dendrites showed highly irregular outlines with many 1-2 microns-long 'thorns' (on average 8.5 thorns per 100 microns 2 of dendritic area). Taken together, the reconstructed dendrite segments from the proximal sites had a total length of about 250 microns; those from the distal locations, 180 microns. On all segments together there were 699 synapses. Nine percent of the synapses were on thorns, and many more close to their base on the dendritic shaft. The synapses were classified in four groups. One third of the synapses were asymmetric with spherical vesicles; one half were symmetric with spherical vesicles; and one tenth were symmetric with flattened vesicles. A fourth, small class of asymmetric synapses had dense-core vesicles. The area of the active zones was large for the asymmetric synapses (median value 0.20 microns 2), and small for the symmetric ones (median value 0.10 microns 2), and the difference was significant. On average, the areas of the active zones of the synapses on thin dendrites were larger than those of synapses on large calibre dendrites. About every 4 microns 2 of dendritic area received one contact. There was a significant difference between the areas of the active zones of the synapses at the two locations. Moreover, the number per unit dendritic length was correlated with dendrite calibre. On average, the active zones covered more than 4% of the dendritic area; this value for thin dendrites was about twice as large as that of large calibre dendrites. We suggest that the larger active zones and the larger synaptic coverage of the thin dendrites compensate for the longer electrotonic distance of these synapses from the soma.

Distal dendrites of frog motor neurons: a computer-aided electron microscopic study of cobalt-filled cells

With the aid of the cobalt labelling technique, frog spinal cord motor neuron dendrites of the subpial dendritic plexus have been identified in serial electron micrographs. Computer reconstructions of various lengths (2.5-9.8 micron) of dendritic segments showed the contours of these dendrites to be highly irregular, and to present many thorn-like projections 0.4-1.8 micron long. Number, size and distribution of synaptic contacts were also determined. Almost half of the synapses occurred at the origins of the thorns and these synapses had the largest contact areas. Only 8 out of 54 synapses analysed were found on thorns and these were the smallest. For the total length of reconstructed dendrites there was, on average, one synapse per 1.2 micron, while 4.4% of the total dendritic surface was covered with synaptic contacts. The functional significance of these distal dendrites and their capacity to influence the soma membrane potential is discussed.

Intrasomatically recorded action potentials in snail neurons (Helix pomatia): different shapes with different sites of origin in the neuronal arborization. A combined morphological and electrophysiological study

Fibres of the identified neurons B1 to B3 in the buccal ganglia of Helix pomatia can be divided into three types according to their diameters. Electrical stimulation of nerves containing the different fibres induces typical fast depolarizations in the somata of neurons B1 to B3. The appearance of these depolarizations is strictly correlated to the fibre types. The depolarizations are interpreted as axonal action potentials.

Acute ethanol treatment modifies response properties and habituation of the DR-VR reflex in the isolated frog spinal cord

Ethanol modification of habituation, a fundamental form of behavioral plasticity, was examined in the isolated frog spinal cord preparation. The polysynaptic dorsal root to ventral root (DR-VR) reflex response was assessed in normal Ringer's and at one of four ethanol concentrations. The reflex itself was facilitated at lower levels (0.025%) and depressed at higher levels (0.05-0.5%) of ethanol. Habituation, decrement of the polysynaptic ventral root response to repeated dorsal root stimulation, was reduced at all ethanol concentrations. Understanding the mechanisms of ethanol action involved in the disruption of simple forms of plasticity will help us to explain its actions on more complex forms of associational processes.

Separation of temperature sensitive and temperature insensitive components of the postsynaptic potentials in the frog motoneurons

Intracellular measurements were made in the in situ spinal cord of the frog at temperatures below 5 degrees C. Responses to volleys in the sciatic nerve, in the descending fibres and in the motor axons were studied. About 30% of the motoneurons responded to sciatic volleys with 1-3 ms segmental latency, which was short enough to assume electrotonic mediation of these responses. Another group of motoneurons responded with 6-8 ms latency, i.e. with the expected delay at chemical synapses at low temperature. Latency distribution of the sciatic-evoked postsynaptic potentials was clearly bimodal in contrast with that found at higher temperatures. Postsynaptic discharges occurred with rather long latency and they were attributed to chemically-mediated excitation. Some of the postsynaptic potentials to descending volleys also occurred with quite short latency, indicating possible electrotonic transmission from supraspinal centres to motoneurons. Latency distribution of the action potentials evoked from the motor axons was bimodal, corresponding to the different, i.e. antidromic and recurrent facilitatory, mechanism of these spikes. Calculated Q10 ratios for the sciatic-evoked reflex discharges and the afferent fibre volleys were about 2.3 and 1.8, respectively. We concluded that cooling helps to separate postsynaptic potentials according to their electrotonic and chemical mediation and that electrotonic excitation does not seem to have a primary role in the generation of postsynaptic discharges initiated by dorsal root volleys in the frog.

Interactions among lumbar motoneurons on opposite sides of the frog spinal cord: morphological and electrophysiological studies

Light and electron microscopy have been used to study the projections of dendrites from motoneurons in lumbar segments of the spinal cord of the frog following administration of horseradish peroxidase to cut ventral roots. Processes originating from motoneurons crossed to the opposite side of the spinal cord via the anterior commissure and made contact with dendrites and motoneuronal somata. Typically, in segments 6 to 8 the crossing dendrites showed irregular enlargements in diameter. Electrophysiological recordings were obtained both extracellularly from ventral roots and intracellularly from motoneuronal somata. In Ringer's solution containing 1 mM calcium, stimulation of a lumbar ventral root, elicited population responses with early and late components in the ventral root of the opposite side of the same segment. Only the early, short latency component remained in calcium-deficient Ringer's solution. In calcium-containing Ringer's solution, intracellular recording from an antidromically activated motoneuron showed an action potential with a short latency; this response was followed by excitatory postsynaptic potentials (epsps) from which action potentials could be generated. Contralateral ventral root stimulation also elicited in the same motoneuron a short latency action potential that was rarely followed by epsps. The short latency responses, that were elicited by stimulation of ventral roots of either side persisted in calcium-deficient Ringer's solution, but the epsps were abolished. Contralaterally elicited short latency responses were eliminated by section of the anterior commissure. We believe that electrically mediated crossed interactions among lumbar motoneurons may serve as a means of coordinating muscle groups of opposite sides that are used in movements that require bilateral synchronization, such as jumping and swimming.

Direct excitatory interactions between spinal motoneurones of the cat

1. Ninety-seven spinal motoneurones were identified by their antidromic invasion following stimulation of the muscle nerve and submitted to a series of four tests to reveal a possible direct excitation between motoneurones. 2. Threshold differentiation, refractoriness, hyperpolarization and collision revealed antidromically induced depolarizations in fourteen of the ninety-seven tested motoneurones. 3. The parameters of the antidromically induced depolarizations indicate a short latency, a low amplitude and independence with regard to the membrane polarization. 4. It is concluded that the antidromically induced depolarizations reached the impaled motoneurone via a route other than its own axon. 5. The mechanism may involve either electrotonic interactions between neighbouring motoneurones or excitatory recurrent collaterals between synergist motoneurones.

Dendritic responses of frog motoneurons produced by antidromic activation

Responses to ventral root stimulation were studied in spinal cords in situ in unanesthetized frogs. Extracellular as well as intracellular recordings from motoneurons indicated that considerable depolarization of the dendrites occurred in the response to ventral root volley. Active components of this dendritic depolarization could also be observed. Extracellularly, negative field potentials were recorded both in the vicinity of motoneuron cell bodies and in areas occupied mostly by motoneuron dendrites. The refractory period of the negativity recorded at the vicinity of dendrites was longer than in the vicinity of somata. Changes in antidromic excitability were studied by the double volley technique. Augmentation of the field potential to a test volley was observed during the period of dendritic depolarization, followed by a longer-lasting period of depression of the test response. It is concluded that an action potential in frog motoneurons induces depolarization of the dendrites. The depolarized dendrites can generate local action potentials and can produce negative field potentials remotely from the somatic pool. The response of dendrites to stimulation of the ventral root has particular importance in the recurrent facilitation of the frog motoneurons.

The morphology of motoneurons and dorsal root fibers in the frog's spinal cord

Ventral and dorsal roots of the frog's spinal cord were filled with cobaltous chloride, and the resulting cobaltous sulfide precipitate, following treatment with H2S-buffer solutions, was intensified with physical developers. A ventromedial and a dorsolateral motoneuron group could be discerned in the ventral horn. The ventromedial, motoneurons gave origin to a strong dendrite crossing to the contralateral side. In the dendritic arborization pattern of the dorsolateral motoneurons a dorsomedial, a dorsal and a lateral dendritic array were distinguished. They were regarded as representing three different input channels to the motoneurons. Intramedullary branching of motor axons and recurrent axon collaterals were never observed. The dorsal root could be divided into a medial and lateral division carrying small and large caliber fibers, respectively. The end-branches of the small caliber fibers were seen to terminate in the substantia gelatinosa. Fine collaterals of the large caliber fibers also terminated in the substantia gelatinosa; coarser collaterals penetrated deeper and terminated in a triangular-shaped area in the base of the dorsal horn and in the intermediate gray matter. From this area a tail was followed into the ventral horn and several synapses were seen on the proximal dendrites and on the somata of motoneurons. A few dorsal root fibers could be seen crossing to the contralateral side.