Abstract
Proprioception of all animals is important in being able to have coordinated locomotion. Stretch activated ion channels (SACs) transduce the mechanical force into electrical signals in the proprioceptive sensory endings. The types of SACs vary among sensory neurons in animals as defined by pharmacological, physiological and molecular identification. The chordotonal organs within insects and crustaceans offer a unique ability to investigate proprioceptive function. The effects of the extracellular environment on neuronal activity, as well as the function of associated SACs are easily accessible and viable in minimal saline for ease in experimentation. The effect of extracellular [Ca2+] on membrane properties which affect voltage-sensitivity of ion channels, threshold of action potentials and SACs can be readily addressed in the chordotonal organ in crab limbs. It is of interest to understand how low extracellular [Ca2+] enhances neural activity considering the SACs in the sensory endings could possibly be Ca2+ channels and that all neural activity is blocked with Mn2+. It is suggested that axonal excitability might be affected independent from the SAC activity due to potential presence of calcium activated potassium channels (K(Ca)) and the ability of Ca2+ to block voltage gated Na+ channels in the axons. Separating the role of Ca2+ on the function of the SACs and the excitability of the axons in the nerves associated with chordotonal organs is addressed. These experiments may aid in understanding the mechanisms of neuronal hyperexcitability during hypocalcemia within mammals.
Highlights
It is well established that free Ca2+ within the presynaptic terminal is key for chemical synaptic transmission [1,2] and is important for electrical transmission though gap junctions [3]
These signs have led to much speculation as to the mechanism of the responses due to low extracellular Ca2+ which appears to be counterintuitive due to the role of Ca2+ in enhancing synaptic transmission at the neuromuscular junction [1,2]
In developing neuronal models for addressing the effects of extracellular Ca2+ on the biophysical properties of neurons and on a given sensory modality, we used a chordotonal organ in the limb of the marine crab which monitors the proprioceptive status of the distal joint in walking legs [22]
Summary
It is well established that free Ca2+ within the presynaptic terminal is key for chemical synaptic transmission [1,2] and is important for electrical transmission though gap junctions [3]. These signs have led to much speculation as to the mechanism of the responses due to low extracellular Ca2+ which appears to be counterintuitive due to the role of Ca2+ in enhancing synaptic transmission at the neuromuscular junction [1,2] The mechanisms behind these observed phenomena are still debated as to the effects of excitability, conduction of electrical signals, the types of ion channels activated or inhibited by Ca2+ in neurons [14,15,16,17,18,19]. In developing neuronal models for addressing the effects of extracellular Ca2+ on the biophysical properties of neurons and on a given sensory modality, we used a chordotonal organ in the limb of the marine crab which monitors the proprioceptive status of the distal joint in walking legs [22]. Given there is still an active interest in how [Ca2+ ]o influences the function of nerves as well as understanding the function of SACs, in the vast variety of SAC subtypes, among organisms [29], the crab leg proprioceptive neurons serve as a viable model for such investigations [24,25,26,27,28,30]
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