To study the effects of millimeter waves (42.25 GHz) on electrical activity of the murine sural nerve (one of the main branches of the sciatic nerve). The aim of the study was to find out whether millimeter wave exposure was able to activate cutaneous sensory receptors in the murine hind paw.
Millimeter waves are almost totally absorbed within the superficial layers of the skin. Therefore, the initial effect of millimeter waves is limited to structures located in the outer layers of the skin (there is no scientific theory based on a known physiology that would explain how local millimeter wave exposure of the skin produces a systemic effect).
All experiments were conducted under anesthesia. The paw temperature was 35.8°C (n=17). In some experiments the paw skin temperature was lowered to examine different temperature influences on nerve responses.
Different branches of the sciatic nerve were dissected. The nerve was cut 3-5 mm from the spinal cord.
To reproduce the thermal effects of millimeter waves, a radiant heat source was used (the same mice were used).
Capsaicin (a TRPV1 (transient receptor potential ion channel) agonist used for activation of polymodal nociceptor fibers) was used to examine the role of capsaicin-sensitive receptors. Compound 48/80 was used for depletion of mast cells.
| Exposure | Parameters |
|---|---|
|
Exposure 1:
42.25 GHz
Modulation type:
CW
Exposure duration:
continuous for 20 s to 10 min
|
|
|
Exposure 2:
42.25 GHz
Modulation type:
pulsed
Exposure duration:
continuous for 20 s to 10 min
|
|
| Frequency | 42.25 GHz |
|---|---|
| Type | |
| Exposure duration | continuous for 20 s to 10 min |
| Modulation type | CW |
|---|
| Exposure source | |
|---|---|
| Setup | rectangular horn antenna or open-ended waveguide directed the exposure beam to the ipsilateral hind paw |
| Measurand | Value | Type | Method | Mass | Remarks |
|---|---|---|---|---|---|
| power | 40 mW | maximum | - | - | - |
| power density | 260 mW/cm² | maximum | - | - | - |
| Modulation type | pulsed |
|---|---|
| Repetition frequency | 1 kHz |
| Pulse type | rectangular |
| Exposure source |
|
|---|
| Measurand | Value | Type | Method | Mass | Remarks |
|---|---|---|---|---|---|
| power | 40 mW | maximum | - | - | - |
| power density | 260 mW/cm² | maximum | - | - | - |
Two types of responses of the sural nerve to millimeter wave exposure were found. First, millimeter wave exposure at the incident power density >/= 45 mW/cm² inhibited the spontaneous electrical activity of the sural nerve. Exposure with lower intensities (10-30 mW/cm²) produced no detectable changes in the firing rate. Second, the nerve responded to the cessation of exposure with a transient increase in the firing rate (this effect lasted 20-40 s). The threshold intensity for this effect was 160 mW/cm².
Radiant heat exposure reproduced only the inhibitory effect of millimeter wave exposure, but not the transient excitatory response after the cessation of the heat exposure.
Depletion of mast cells by compound 48/80 eliminated the transient response of the nerve. It was suggested that the cold sensitive fibers were responsible for the inhibitory effect of millimeter wave and radiant heat exposures. However, the receptors and mechanisms involved in inducing the transient response to millimeter wave exposure are not clear. The hypothesis of mast cell involvement was discussed.
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