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One of the most noteworthy studies in the later pressure literature was that of Campenot (1975), who wished to define the neural mechanisms underlying the observed changes in behavior of shallow‐dwelling species in response to pressure. The changes in which he was most interested were the excitation caused by pressures <150 atm and the moribundity or depression caused by pressures >150 atm.

Two previous studies provided background relevant to the observed excitability of species at lower pressures. The first noted that hydrostatic pressure dropped the firing threshold for giant axons in squid (Spyropoulos 1957), and the second observed a similar response in frog nerve (Grundfest 1936). That is, less stimulation was required to elicit neural activity in isolated nerve preparations in both species when they were under low pressure. Their results failed to explain the observed moribundity at high pressures, nor did they give a mechanistic explanation for why neural stimulation occurred at modest pressures.

Campenot used a neuromuscular preparation of the walking legs of two Crustacea to evaluate the effects of pressure. The first preparation was from Homarus americanus, the New England lobster dwelling in water 520 m and shallower. The other was of Chaceon (formerly Geryon) quinquidens, a deeper‐dwelling red crab found from 300 to 1600 m on the continental slopes of coastal North America. Dr. Campenot’s technique was straightforward; he stimulated the excitatory neuron leading to the muscle with one electrode and recorded the response from the muscle with another.

Figure 2.17 The effect of pressure on Excitatory Junction Potentials (EJP) recorded from a lobster muscle fiber. Each bar represents the average of about 20 EJP's. The ordinate is an amplitude index with the first average at 1 atm for each frequency set at 1. Its value in millivolt is given in parenthesis.

Source: Adapted from Campenot (1975), figure 3 (p. 136). Reproduced with the permission of Elsevier.

Muscles respond to neural excitation with Excitatory Junction Potentials (EJPs) which then effect a muscle contraction. By examining the amplitude of the EJPs in response to a given fixed stimulus at different hydrostatic pressures, the effect of pressure on the neuromuscular response could be described. Dr. Campenot found that pressure caused an across‐the‐board depression of EJP amplitude in the lobster (Figure 2.17), but the EJP amplitude was independent of pressure in the deeper‐dwelling red crab. In fact, it is now known that independence from pressure effects in species that dwell under pressure is the most common adaptive strategy.

The postulated cause for EJP depression in lobster was a pressure‐induced interference of neurotransmitter release at the synapse. At virtually all junctions between nerve and muscle, the neural signal is propagated across the microscopic gap at the neuromuscular junction using a chemical, or neurotransmitter, the best known of which is acetylcholine. Both excitatory and inhibitory neurotransmitters are present at the neuromuscular junction. It was speculated that the observed stimulatory effects of modest pressure were caused by a differential inhibition of transmitter release at inhibitory synapses. In such a situation, excitatory neural activity would then greatly over‐ride the depressed inhibitory synapses, resulting in hyperactivity.