Impulse conduction in a sponge.

Area of Science:

• Marine Biology
• Neuroscience
• Cellular Physiology

Background:

• Sponges, as primitive multicellular organisms, lack nervous systems.
• Understanding non-neuronal electrical signaling provides insights into the evolution of excitability.
• Previous research has hinted at electrical activity in sponges but lacked detailed characterization.

Purpose of the Study:

• To record and characterize all-or-none propagated electrical impulses in the hexactinellid sponge Rhabdocalyptus dawsoni.
• To investigate the physiological properties of these impulses, including propagation velocity and refractory periods.
• To identify the cellular structures involved in impulse conduction and the ionic mechanisms underlying excitability.

Main Methods:

• Suction electrodes were used to record electrical impulses from grafted sponge tissue.
• Electrical shocks, tactile stimuli, and particulate matter were used to evoke impulses.
• AC-coupled amplifiers, integration techniques, and ion substitution/blockade were employed for analysis.

Main Results:

• All-or-none electrical impulses were successfully recorded, exhibiting triphasic and integrated diphasic waveforms.
• Impulse propagation velocity was measured at 0.27 cm/s, with absolute and relative refractory periods of 29s and ~150s, respectively.
• Impulse passage temporarily arrested water flow, and conduction was blocked by Co2+, Mn2+, nimodipine, and tetraethylammonium ions.

Conclusions:

• The syncytial trabecular reticulum and pinacoderm layers are likely the conducting tissues in sponges.
• Impulse conduction and effector functions (water flow arrest) are independent systems.
• Sponge electrical excitability shares some characteristics with protozoa and plants, and offers insights into non-nervous conduction.