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What is the Pain Modulation System?

Endogenous system that can increase or decrease perception of pain.

Gate Theory

A neural circuit in the dorsal horn of the cord.

Name the components involved in the Gate Theory?

Mechanoreceptive sensory n. (most are "A betas" which means fast, myelinated axons) Entering the dorsal horns are mechanoreceptive and send axons straight up ipsilaterally of the dorsal column.
Axons also send axons that terminate on the G cell.

What are pain neurons entering the gate?

"A delta" or "C" pain fibers entering the dorsal root to the dorsal horn. The enter and synapse on the T cell (this is the 2nd order neuron in the spinothalamic tract). Goes up the lateral spinothalamic tract
Also sends a branch that synapses on the G cell.

What does the GATE cell do?

The Gate cell is stimulated by the A beta mechanoreceptive n. There is a synapse (+) sign by g cell means it will stimulate EPSPs .
There is also a synapse which is inhibitory (-) sign IPSP. It is inhibited by the pain neurons. When they are active they will drive the membrane potential away from theshold.

What does the G cell do with the T cell?

The G cell presynaptically inhibits synaptic transmission from the pain n. to the T cell. If the G cell is active, it will reduce the amt. of N.T. released.

Explain the A beta sensory neuron when it is activated?

Active n. releases an excitatory neurotransmitter.

What is the T cell?

Transmission cell. It is a 2nd order sensory n. in the spinothalamic tract. If this generates A.P. (going up the spinothalamic tract and pain will be received). If quiet, no pain perceived.

Where does all this occur?

In the dorsal horn

How does it work if NO peripheral stimulation?

If no peripheral stimulation-->no A.P. will be conducted up the dorsal column system or spinothalamic system--> no stimulation (pain, pressure, vibration).

What happens if pressure is felt along these sensory neurons?

Low intensity mechanico stimulation -->illicit A.P. in the A betas. (threshold for pain is much higher) so no A.P. in pain fibers-->only perceive pressure.

What happens if a painful stimulus applied?

Pain will initiate an A.P. -->2 things will happen:
1). The G cell will stay quiet. A.P. coming down so G cell will be bombarded with inhibitory N.T. so membrane potential will be driven away form threshold--> G cell stays quiet-->little or no presynaptic inhibition.
2). The T cell will be stimulated and lead A.P. up the spinothalamic system --> perception of pain.

What happens if you have a painful stimulus and you rub the area?

Rubbing stimulates A betas. (you can feel rubbing--going up the dorsal column) -->The stimulation of A beta stimulates the G cells-->(it sums all of its input--temporal summation, spacial summation, grand synaptic potential) its membrane potential starts moving up and if it exceed threshold it starts firing A.P.--> then it travels down its axon--> reduces the amt. of Ca that enters the axon terminal-->and reduces the amt. of glutamate or Substance P released--> decreases the firing the frequency of A. P. by that T cell.-->results in a decrease in pain intensity.

Compare Active and Quiet activity

Active--gate closed--blocking information coming though. Quite--gate open and A.P. free to come in.

How does a TENS unit work?

Adjust the stimulation parameters to selectively stimulate the "A beta" fibers--> close the gate to T cell-->decreasing the pain.

How does electrical stimulation of the ipsilateral dorsal column relieve pain?

Stimulator needs to be above where pain is and ipsilateral-->1st order sensory n. comes into the cord dorsally-->dorsal horn sends a branch that goes up the dorsal column on the same side-->if stimulate the column in the middle of axon-->will get antidromic (goes back down) conduction of A.P.--->stimulates the G cell--> closes the gate-->decreases stimulation of the T cell--> decrease in pain.

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