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Electronic Nerve Stimulator circuit

2018-02-25 08:09  
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Bio-Electric Stimulation Therapy(BEST) is the technique in which verysmall currentis passed through theSkinof an affected region to relieve pain. This pulsed current will mimic the naturalAction potentialsof the nerves so that the nerve conduction will be activated. The most common method of bio electric nerve stimulation is theTranscutaneous Electrical Nerve Stimulationabbreviated as TENS.

TENSis used inPalliative careandPain medicineto relieve pain in the affected parts of the body. It is recommended in cases ofNeuropathic paincaused by nerve damage. The effect of TENS depends on the individual’s physiology and response to electrical stimuli and the pain threshold. The TENS unit consists of one or more electrodes, a pulse generator and power source battery. The circuit inside the TENS is programmable to generate pulses of electrical stimuli with variable current strength, pulse rate and pulse widths. The waveform from the TENS is biphasic so that electrolytic effects on the skin can be eliminated. The important parameters used in TENS for the patient’s comfort are

1.Amplitude of Signal– A comfortable strength of current of low intensity , just above the threshold level is applied to the skin.
2.Pulse rate– Frequency of 80 – 100 impulses per second are used.
3.Pulse width– Duration of electrical pulses between 10 – 1000 micro seconds.

TENS Unit

Patients are instructed to select the correct frequency and intensity of pulses at which they feel free from pains. This can be found out by trial and error. Usually the electrodes are placed on the skin of the painful area. TENS have three settings to use in different therapeutic methods. TheConventional TENSuses a frequency of 40 – 150 Hz with low intensity and the current between 10 – 30 mA. Pulse duration is set at the maximum of 50 micro seconds. InAcupuncture mode, the frequency is set at 1-10 Hz at high intensity close to the tolerance limit. InPulsed Mode,low intensity stimuli are applied as high frequency bursts around 1 – 2 Hz.

Physiological aspects

TENS is based on the electrical conductivity of the Skin and the electrical activities of the nerves. The current output from the TENS depends on the combined impedance of the electrodes, skin and muscle tissue. When repetitive discharges are applied to the same area of the skin, the impedance of the skin reduces which results in the flow of greater current through the skin. Studies have shown that TENS reduces pain through the inhibition of theDorsal Hornof the Spinal cord thus limiting its central transmission. The electrical stimuli applied to the skin activate the low threshold Myelinated nerve fibers. TheAfferent inputfrom these fibers inhibits the flow of impulses in nonMyelinated “C fibers”.This blocks the transmission of impulses to the“target cells”present in the dorsal horn. High frequency TENS activates the“Delta Opioid”receptors present in the Spinal cord and medulla while low frequency TENS activates the “Mu-Opioid”receptors in the spinal cord and medulla. High frequency TENS causes the inhibition of“Excitatory Neuro transmitters”likeGlutamateand increases the production of“Inhibitory Neurotransmitters”likeGABA( Gamma Amino Butyric Acid ) in the spinal cord. At the same time, the“Muscarinic receptors”present in the Central Nervous system is activated. This causes a temporary blocking of the“Pain Gate”leading toAnalgesia.

The low frequency TENS causes the release of the NeurotransmittersSerotoninand activatesSerotonin receptorsin the spinal cord. The production ofGABAis also activated.

Gate Control Theory

Melzack and Wallin 1965 proposed theGate control theoryto explain the action of TENS on the nervous system. The ion gate is usually closed and inhibits the transmission of impulses through the“C Fibers”from the periphery to the target cell (T cell) of the dorsal horn of the spinal cord. When the peripheral pain stimulation occurs, C Fibers carry the pain stimuli that reach the T cell and “Gate” opens. This allows the transmission of pain stimuli to the Thalamus (Relay centre) and Cortex of the brain. Brain will interpret the stimuli as pain.

The gate control theory explains the mechanism of‘closing of the gate’to prevent further transmission of pain stimuli. The closing of Gate is achieved by the inhibition of C Fibers by impulses in the activated Myelinated fibers.

Safety considerations

Since pulsed electrical signals are used in TENS, adequate safety measures should be adopted while using the TENS electrodes.
1. Electrodes should not be used on or near the eyes, mouth, front part of the neck, on the area of numb skin, on wounds, on the facial nerves etc.
2. TENS should not be used in pregnant woman or persons having artificial pace maker.
3. TENS should be used with caution if there is a history of Epilepsy.

D.Mohan kumar


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