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.

Release of coordinated behavior in crayfish by single central neurons

By stimulating and recording from the same interneuron at two separate points, we have shown that coordinated output to the postural abdominal muscles of crayfish can be produced by electrical stimulation of a single cell. Several central neurons can individually initiate one type of movement(for example, flexion),each producing a unique abdominal geometry.

Visceral Afferent Signals in the Crayfish Stomatogastric Ganglion

1. The crayfish stomatogastric ganglion (SG) contains about twenty-five neurone somata; it supplies motor innervation for the anterior gut, and receives afferent input from mechanoreceptors associated with the stomach. Its proximal branches respond to normal stomach contractions with a complex, patterned, centrally directed discharge involving several units.

2. Discharges from the largest of these can be recorded nearly simultaneously in the two superior oesophageal (proximal) and the two lateral (distal) nerves from SG. In preparations with these lateral nerves connected to the stomach the cell shows constant-frequency spontaneous activity, which can be reset by direct stimulation of any one of the four branches. Mechanical stimulation of the cardiac ossicles evokes a burst discharge.

3. Simultaneous recording from the two lateral nerves shows that impulses in one of them consistently occur earlier; this temporal relationship is preserved when the leading side is stimulated, but reverses (usually accompanied by polarity changes) when the other is stimulated. Each lateral branch continues to respond independently after being cut at its point of entry into SG.

4. It is concluded that the cell is a new type of receptor neurone, with a bifurcating axon and two dendrites, each autogenically active at its receptor terminal. The higher-frequency input always determines the output rhythm. A cell with the appropriate configuration was figured by Orlov (1927) from methylene blue-stained SG preparations.

The distribution of pre- and postsynaptic inhibition at crustacean neuromuscular junctions

The relative contribution of pre- and postsynaptic mechanisms to peripheral inhibition has been analyzed in the abdominal slow flexor muscles of crayfish and lobsters. The conductance of the muscle fiber membrane may be increased to five or more times its resting value by repetitive stimulation of the peripheral inhibitory axon, and this effect accounts for all of the attenuation exerted by the inhibitor against excitatory junctional potentials. No "critical interval" has been found at which an inhibitory nerve impulse produces anomalously large reduction of a following depolarizing junctional potential; electrotonic depolarizations and junctional potentials are identically affected under all phase conditions. The presynaptic inhibitory mechanism is, therefore, absent in this system. In the dactyl opener muscle, on the contrary, most of the attenuation of excitatory junctional potentials is achieved presynaptically, though equally large postjunctional conductance changes are also seen (Dudel and Kuffler, 1961). The difference is correlated with a difference in the reflex operation of the two muscles. Reflex inhibition in the abdominal slow flexors is primarily central, whereas in the dactyl opener, inhibition is brought about by an increase in inhibitory nerve discharge frequency without central suppression of the single excitatory axon. The function of peripheral inhibition in the abdominal flexors is presumably to terminate residual depolarization by reducing the long time-constant of the muscle fibers.