Spinal Neurons and Synapses

1. The pattern of synaptic activity in lateral gastrocnemius (l.g.) motoneurones in the lumbar spinal cord of chick embryos (Stage 44‐45, 19‐21 d of incubation) has been examined using intracellular recording. In the motoneurones of normal chick embryos, stimulation of different peripheral, sciatic nerve branches gave rise to characteristic synaptic responses. Stimulation of the lateral gastrocnemius nerve caused a monosynaptic e.p.s.p. which was graded by the intensity of nerve stimulation. Stimulation of synergistic muscle afferents also caused a brief latency e.p.s.p., followed by longer latency excitatory and inhibitory synaptic potentials. Stimulation of antagonistic muscle afferents or cutaneous afferents gave rise to longer latency inhibitory and excitatory synaptic potentials respectively. 2. The synaptic activity of l.g. motoneurones was also recorded in embryos in which short segments of the lumbar neural crest had been destroyed by microcautery at 3 d of incubation (Stage 18). The embryos developed without sensory ganglia and dorsal roots in the corresponding region. 3. At 19‐21 d of incubation, the amplitude of the l.g. e.p.s.p. of l.g. motoneurones in deafferented segments was on the average only a half to a third of the amplitude seen in motoneurones of intact spinal segments. However, both the l.g. and synergist e.p.s.p.s were larger than those seen in acutely deafferented segments of normal embryos. 4. In spite of the weak monosynaptic input from l.g. and synergistic afferents, the pattern of synaptic activity evoked by antagonistic muscle afferent or cutaneous afferent stimulation was not different from normal. This was even the case for gastrocnemius motoneurones in which no early e.p.s.p. could be evoked by stimulating the l.g. or synergistic muscle nerves. 5. No muscle spindles could be seen in sections of l.g. muscles from embryos with extensive lesions of the lumbosacral neural crest. Incomplete lesions of l.g. segments reduced the number of spindles in the muscle. 6. These results suggest that when motoneurones are deprived of part of their normal synaptic input before the formation of peripheral connexions, the identity of the motoneurones (in terms of the origin of their synaptic input) is preserved. Missing synaptic inputs are either replaced by appropriate afferent fibres, if they are available, or not at all. The chick sensory ganglion cells with monosynaptic connexions to motoneurones appear to be unable to compensate significantly for peripheral or central defects in the innervation of the hind limb. They behave as if their developmental possibilities were quite rigidly determined at an early embryonic stage.

Section: Discussionmentioning

confidence: 86%

The effect of lesions in the neural crest on the formation of synaptic connexions in the embryonic chick spinal cord

Eide

Jansen

Ribchester

1982

“…Thus it seems that motoneurone size per se is not the single important factor governing recruitment order. It has been repeatedly pointed out on theoretical grounds (Burke, 1968b(Burke, , 1973Zucker, 1973;Burke & Rudomin, 1977) that a recruitment order will not be determined by cell size alone but by the synaptic density of terminals from active pathways. Further, since it is established that the cell size of type FF and type FR units is identical (Cullheim & Kellerth, 1978b) the difference in individual e.p.s.p.…”

Section: Discussionmentioning

confidence: 99%

Individual excitatory post‐synaptic potentials due to muscle spindle Ia afferents in cat triceps surae motoneurones.

Harrison¹,

Taylor²

1981

148

3. The projection frequency of homonymous Ia afferents to medial gastrocnemius motoneurones followed the same order.4. Similar trends were evident in both respects for the heteronymous projections of triceps surae.5. The mechanical property of units which related most directly to e.p.s.p. size was muscle unit contraction strength, a single continuous relation existing throughout all three unit types.6. On the basis of motor axon conduction velocity measurements, it appeared that motoneurone size could provide a simple explanation for e.p.s.p. size in type S and type FR units, but not in type FF. The latter had unduly small e.p.s.p.s relative to their axon conduction velocity, indicating a qualitatively different presynaptic organization for FF motoneurones from the others.7. The data make it possible to predict that Ia afferent excitation alone would cause recruitment of units in order of increasing contraction strength and that this order would also obtain if a substantial part of the excitation came from other sources with uniform effect on all unit types.8. It is shown that a consequence of the observed organization is that the gain of the Ia mediated stretch reflex would be approximately proportional to the developed force.

“…The strength of such disynaptic pathways depends on the efficacy of transmission at excitatory synapses between pyramidal cells and inhibitory neurones. At other excitatory synapses it seems that many presynaptic cells must be synchronously activated to achieve a depolarization large enough to generate an action potential (Burke & Rudomin, 1977). The observation of apparently disynaptic i.p.s.p.s elicited by single action potentials in a pyramidal cell therefore suggests that excitatory synapses made onto inhibitory neurones may be extremely potent.…”

Section: Activation Of Inhibitory Neuronesmentioning

confidence: 99%

Unitary inhibitory synaptic potentials in the guinea‐pig hippocampus in vitro.

Miles¹,

Wong²

1984

200

115

Mechanisms involved in the generation of synaptic inhibition have been investigated by making simultaneous intracellular recordings from pairs of neurones in the CA3 pyramidal cell field of guinea‐pig hippocampal slices. Inhibitory post‐synaptic potentials dependent on single presynaptic action potentials (i.p.s.p.s) and mediated through monosynaptic and disynaptic connexions have been identified. The recurrent nature of some hippocampal inhibition has been demonstrated by showing that activity in a single cell may initiate feed‐back i.p.s.p.s onto itself. The observation of synchronous i.p.s.p.s in recordings from two cells illustrates the divergence of synaptic contacts made by inhibitory neurones. The peak conductance change associated with an i.p.s.p. was in the range 5‐9 nS and it reversed uniformly throughout its time course at membrane potentials between ‐73 and ‐80 mV. The shortest time‐to‐peak of synaptic potentials was approximately 3 ms and in this case the i.p.s.p. decayed with a time constant comparable to the passive membrane time constant of the post‐synaptic neurone. The peak amplitude of i.p.s.p.s fluctuated in a way consistent with the quantal release of inhibitory neurotransmitter. Inhibitory neurones could fire bursts of action potentials not unlike those generated by pyramidal cells in this area. A comparison of the conductance change associated with identified i.p.s.p.s with that associated with the maximal inhibitory post‐synaptic potential resulting from electrical stimulation of fibre pathways suggested that, in the slice, a pyramidal cell is innervated by up to fifteen inhibitory neurones.