C2.2.15 Perception of pain by neurons with free nerve endings in the skin (HL) Notes

Free Nerve Endings Detect Harmful Stimuli and Trigger Pain Perception

1. Pain perception is a critical protective mechanism in the body, enabling it to detect harmful stimuli and respond accordingly.
2. Free nerve endings in the skin are specialized sensory neurons that play a vital role in this process.
3. These nerve endings have ion channels that detect specific types of stimuli, ultimately leading to the perception of pain.

What Are Free Nerve Endings?

Free nerve endings are bare, unmyelinated extensions of sensory neurons found in the skin and other tissues.

1. These nerve endings are equipped with ion channels that open in response to specific triggers, such as:
   1. High temperatures
   2. Acidic conditions
   3. Chemical irritants like capsaicin (the compound that makes chili peppers hot)

How Do Free Nerve Endings Detect Pain?

1. Stimulus Detection: When a harmful stimulus (e.g., heat or acid) is detected, ion channels in the free nerve endings open.
2. Ion Movement: Positively charged ions (such as sodium $\text{Na}^+$) flow into the neuron.
3. Threshold Potential: The influx of ions causes the membrane potential to rise, reaching a critical level known as the threshold potential.
4. Action Potential: Once the threshold is reached, an action potential is generated and travels along the neuron.

The Journey of a Pain Signal

1. Transmission to the Spinal Cord: The action potential travels along the sensory neuron to the spinal cord.
2. Relay to the Brain: Interneurons in the spinal cord transmit the signal to the brain.
3. Perception of Pain: The brain processes the signal, resulting in the conscious perception of pain.

Example: Capsaicin and Pain

1. Capsaicin, the compound in chili peppers, can activate the TRPV1 receptor found in free nerve endings.
2. This receptor is particularly sensitive to both heat and capsaicin.
3. When capsaicin binds to this receptor, it causes the ion channels to open, allowing positively charged ions to flow into the neuron.
4. This results in the depolarization of the nerve ending, and if the threshold potential is reached, an action potential is generated.
5. The brain then interprets this signal as the sensation of burning or pain.

Why Is Pain Important?

1. Pain is a protective mechanism that helps prevent injury and promotes healing.
2. By alerting us to harmful stimuli, it encourages behaviors that minimize damage, such as withdrawing from a hot surface or avoiding sharp objects.

Factors Influencing Pain Perception

1. Intensity of the Stimulus: Stronger stimuli cause more ion channels to open, increasing the likelihood of reaching the threshold potential.
2. Type of Stimulus: Different ion channels respond to specific stimuli, such as heat or chemicals.
3. Individual Sensitivity: Genetic and environmental factors can affect how sensitive a person is to pain.