The Effects of Zinc on Proprioceptive Sensory Function and Nerve Conduction

Abstract

Zinc (Zn²⁺) is an essential element that can promote proper organ function, cell growth, and immune response; it can also, however, be present in too great a quantity. Zinc toxicity caused by overexposure may result in both minor and major physiological effects, with chronic exposure at low levels and acute exposure at high levels being harmful or even toxic. This investigation examines the effects of acute exposure to relatively high concentrations of Zn²⁺ on sensory nerve function and nerve conduction. A proprioceptive nerve in marine crab (Callinectes sapidus) limbs was used as a model to assess the effects of Zn²⁺ on stretch-activated channels (SACs) and evoked nerve conduction. Exposure to Zn²⁺ slowed nerve condition rapidly; however, several minutes were required before the SACs in sensory endings were affected. A depression in conduction speed and an increase followed by a decrease in amplitude were observed for the evoked compound action potential, while the frequency of nerve activity upon joint movement and stretching of the chordotonal organ significantly decreased. These altered responses could be partially reversed via extensive flushing with fresh saline to remove the zinc. This indicates that subtle, long-term exposure to Zn²⁺ may alter an organism's SAC function for channels related to proprioception and nerve conduction.

Keywords: conduction; crustacean; proprioception; recruitment; sensory; zinc.

Figure 1

The walking leg of the crab, with identification of the PD organ’s location. (A) The segments of the overall leg are shown. (B) The last two segments are shown to house the PD organ. (C) The PD nerve is identified as the branch from the main leg nerve to the PD chordotonal strand.

Figure 2

The distal PD joint and displacement range while PD nerve activity was monitored. (A) The PD joint was held in the flexed position before being displaced to an extended position and held there to stimulate the dynamic and static position-sensitive neurons. (B) The PD nerve is depicted as isolated from the main leg nerve. (C) The isolated PD nerve is pulled into a suction electrode to record neural activity during displacement of the joint.

Figure 3

The experimental paradigm for joint displacement and analysis of the spikes. (A) The joint was displaced from a flexed position to an extended position during a single second, held in the extended position for at least ten, and then moved back to a flexed position. This was repeated thrice for each bathing solution. (B) The number of spikes observed from the beginning of the movement (the first second) through the next nine (when it was held static in an extended position) was used as an index of neural activity for the PD organ. The average number of measured spikes across each of the three trials was used as a metric of neural activity in that condition.

Figure 4

Recording compound action potentials (CAPs) of the PD nerve. The PD nerve was dissected along the length of the main leg nerve to the cut end of the meropodite. The nerve was placed in a saline bath for recording. Each end of the nerve was pulled into a suction electrode for induction and recording of the CAPs. The bathing medium was exchanged from saline to the compounds of interest and back to fresh saline over the course of the experiment.

Figure 5

The effect of ZnCl₂ on the PD activity with joint displacement. The responses in the nerve activity varied among preparations, with some increasing and others decreasing at both (A) 1 mM and (B) 10 mM concentrations (no significant differences were observed). (C) At 20 mM, there was a significant decrease in overall activity, which partially recovered with washout. Each line/shape on the above graph represents the results observed from an individual preparation.

Figure 6

The effects of ZnCl₂ exposure on the production of spontaneous nerve firing (*). The dotted vertical line is shown as a reference to help illustrate the shift in the conduction time when exposed to Zn²⁺. Note the arrow indicates the shift to the right of the main CAP before (A) and during the exposure to Zn²⁺ (B).The * indicates the recruitment of additional axons that were recruited by the zinc exposure.

Figure 7

The acute effects of ZnCl₂ exposure on CAPs, increasing the amplitude and slowing the conduction velocity. CAPs are shown as follows: (A) During saline-only exposure. (B) Immediately upon changing the bath to a 20 mM ZnCl₂ solution, illustrating the amplitude increase and conduction velocity decrease. The arrow shows the direction of the shift to the right over time. Each superimposed trace was 20 s apart to visualize the rapid effect of ZnCl₂ on the conduction speed. The colors were added to help visualize the separate traces. (C) After 20 min of ZnCl₂ incubation, with CAPs of reduced amplitude, increased width, and still-slowed conduction velocity. (D) Upon removal of the ZnCl₂, three washouts with fresh saline and the flushing of the nerve, with CAP shape and conduction velocity trending to a return to normal conditions.

Figure 8

Effects of acute Zn²⁺ exposure on PD nerve activity across 30 min, as observed during joint displacement/extension by nine different groups of researchers. Furthermore, fourteen students used seven recording set-ups (two individuals per set-up) to confirm that CAPs from isolated nerves in the crab leg experienced both reduced conduction velocity and decreased amplitudes across 30 min of Zn²⁺ (20 mM) exposure, as shown above (N = 7, p < 0.05 paired t-test). Each line/shape on the above graph represents the results observed from an individual preparation.