Presynaptic inhibition: primary afferent depolarization in crayfish neurons

Inhibition of transmission between tactile sensory neurons and interneurons in the crayfish was investigated by intracellular recording int the presynaptic processes. Inhibition is correlated with a depolarization of the presynaptic process, as in the mammalian spinal cord; the depolarization is accompanied by a conductance increase, and is mediated by interneurons that can be excited by a variety of routes.

Neuronal circuit mediating escape responses in crayfish

The neuronal circuit underlying rapid abdominal flexion in response to phasic tactile stimulation comprises identified afferents, interneurons of two orders, a decision unit, and several motor neurons. The circuit is organized hierarchically as a " cascade" in which electrical synapses predominate at higher levels. Behavioral habituation results from lability at chemical junctions early in the pathway.

The Different Connections and Motor Outputs of Lateral and Medial Giant Fibres in the Crayfish

1. High-speed cinematography was used to analyse the abdominal movements of crayfish in response to separate stimulation of medial and lateral giant axons. These films showed that the medial giant fibres command complete abdominal flexions with little flaring of the tail appendages. The lateral giants, in contrast, evoked a relatively weak flexion of the middle abdominal segments, accompanied by promotion of the exopodites of the uropods.

2. An examination of the muscles activated by the two types of giant fibres shows that differences in the connexions between the giant fibres and specific motor neurones can account for the behavioural differences observed.

3. The output of the giant fibres was determined in the sixth abdominal ganglion, where their differential effects are most pronounced. The medial giants activate motor neurones whose axons emerge from root 6 of the sixth ganglion. The lateral giants activate motor neurones whose axons emerge via roots 2 and 3, as well as those emerging via root 6.

4. The larger motor neurones associated with the giant axons in the sixth root of the sixth ganglion have been mapped by Procion Yellow injection, and the terminations of the central giant axons in the sixth ganglion have also been determined. The connexions revealed by this technique are consistent with the physiological findings.

5. The evidence suggests that root 6 of the sixth ganglion is homologous with root 3 of the more anterior ganglia. However, the giant motor neurone of the sixth ganglion has not been identified.

6. The medial and lateral giant fibres, and perhaps other specific ‘command’ interneurones, can thus drive specific ensembles of phasic motor neurones to provide a range of stereotyped quick movements. In this respect the organization of the phasic system of interneurones and motor neurones resembles that in the tonic system.

Quantitative aspects of transmitter release

The opener-stretcher motor neuron in crayfish makes 50 endings upon each of 1200 muscle fibers. We have calculated the quantal content of junctional potentials produced by individual terminals and by the whole cell at various physiological frequencies. The results show that when the motor neuron is active at 20 impulses/second, it releases 50 quanta/impulse per muscle fiber, or a total of 4.5 x 10(9) quanta/hr. These figures are similar to those for vertebrate muscles per fiber, but larger for the entire neuron because the opener motor unit is so large. On the basis that the quanta correspond to synaptic vesicles each containing 10(3)-10(4) molecules of transmitter, the release rate must be around 10(-11) mole/hr. This value is within an order of magnitude of the release figures obtained for mammalian neurons by collecting transmitter in perfusates, but it is far lower than the value reported for a crustacean inhibitory neuron. If the membrane materials surrounding each vesicle were lost in the release process, the replacement synthesis would involve 24 mm(2) of membrane/hr. We conclude that the metabolic load in terms of transmitter synthesis is probably sustainable, but that the release mechanism must operate in such a way that vesicle membrane materials are neither lost nor incorporated into the terminal membrane.

Lateral giant fibers of cray fish: location of somata by dye injection

Using a new technique for injecting fluorescent dye, we have analyzed the intraganglionic architectutre of the lateral giant interneuron in crayfish. Each lateral giant interneuron comprises a giant axon, an ipsilateral dendritic arborization of fairly constant form, and a contralateral soma. Antidromic axon spikes produce only, small, electronic potentials in the soma; direct depolarization there fails to produce electrically excitable membrane responses and cannot discharge the main axon.

Discharge Patterns of Neurones Supplying Tonic Abdominal Flexor Muscles in the Crayfish

1. The discharge patterns of tonic flexor motoneurones in the crayfish abdomen have been investigated by simultaneous recording from several nerve roots. The five flexor motoneurones supplying each segment are serially homologous, smaller units have higher discharge frequencies, and the excitability of a given unit is generally higher in more caudal ganglia.

2. Even the smallest axons are capable of generating substantial muscle tension at their spontaneous discharge frequencies. Tension development is extremely tonic. Single motor impulses are without effect, latencies are long, and the frequency/tension relation is steep.

3. The inhibitory axon to each flexor discharges during ‘extension’ reflexes, but has no visible effect upon relaxation time, or upon the response to subsequent excitation. Inhibitory impulses do not relax previously unexcited muscle.

4. Phase histograms for the discharge of pairs of homologous or non-homologous efferent axons across a single ganglion, within the same root, and between adjacent ganglia have revealed several coupling mechanisms. One is mediated by short time-constant electrotonic junctions between bilaterally paired inhibitory axons. The second involves weaker, more generalized interactions of longer time-constant between non-homologous axons. The third is brought about by common presynaptic sources of excitation. Inputs tend to be common for homologues in adjacent ganglia rather than for non-homologues in a single ganglion, a finding consistent with the selectivity shown by central interneurones in their effects upon motoneurones.

Regeneration in crustacean motoneurons: evidence for axonal fusion

Crayfish motor axons remain excitable for over 100 days after severance from their central cell bodies, and continue to store and release normal amounts of transmitter substance. Evidence indicates that regeneration occurs by fusion of the central process with its surviving peripheral segment.