In trying to understand pain and temperature, temperature is the easier sense. So, let's start there! Either way, though, these senses are handled by free nerve ending sensory receptors.
Sensation of temperature:
You have already learned (back in our discussion of skin) that free nerve endings detect temperature. But, it turns out that some free nerve endings can detect heat, while other free nerve endings detect cold. Heat receptors tend to detect temperatures over 25 °C (77 °F); they do not typically, however, continue to detect heat when it gets too hot... like extremely hot and harmful temperatures of 45 °C (113 °F) or hotter. Likewise, cold receptors respond to cool temperatures between 10 °C (50 °F) and 20 °C (68 °F), but they do not respond to really cold temperatures below 10 °C.
We do feel the extremely hot or extremely cold sensations, however... we just don't feel them as hot or cold-- just as pain! Our pain receptors kick in at these extreme temperatures to warn us of potential harm.
Our temperature receptors tend to adapt, so that you can get "used" to a temperature over time.
Sensation of pain:
We have many different stimuli that we consider painful, for example:
Some of the pain receptors that we have can detect all of these types of stimuli, while others are more specialized for just one type.
Pain can be sensed through the skin, like temperature, touch and pressure. However, pain can also be sensed in our viscera. Poor blood flow or digestion can trigger pain receptors in the viscera and then we feel deeper pains. This is called visceral pain.
The sensation of pain can be acute or chronic. Acute pain is a sharp, unsustained pain. Chronic pain doesn't seem to go away. Different pain sensory receptors cause acute pain from those that cause chronic pain.
A very interesting and clinically useful type of pain is called referred pain. You may have heard that before a heart attack, many patients experience a pain in their left chest, shoulder, or upper arm. A patient will tell you that the pain they experienced in their arm, for example, before a heart attack was a severely painful type, clearly felt within the arm, not the heart. It is a bizarre thing, but we can't feel pain within our hearts, we just can't. Yet our bodies relay (or refer) this pain to another location-- the arm in this case, even though the pain isn't really from the arm.
How does this work? It is thought that as the pain sensory neurons from viscera enter the spinal cord to synapse on their postsynaptic targets, they overlap with pain sensory neurons coming in from the skin. The postsynaptic targets of visceral and skin pain sensory neurons end up getting mixed input-- input from both viscera and skin. Therefore, when the heart pain sensory neurons enter the spinal cord, they, by accident, synapse on postsynaptic targets of the skin pain sensory neurons from the left arm. Your brain only knows that spinal cord neurons that carry arm pain information have been activated, so you feel arm pain. What a trick!
The good thing about this "mistake" is that it provides us with a clinical warning. If we feel a bizarre pain, we may stop a strenuous activity in time to not actually have a heart attack, but know enough to go to a doctor immediately.
Referred pain not only informs us about the heart, but also about other viscera, as shown in Figure 12.3.
Regulation of Pain
Our nervous systems have ways to regulate our pain normally, although the nervous system of a person with real chronic pain does not seem to be able to do this. Basically, when pain receptors are active for a while, their input affects higher brain centers (through the spinothalamic tracts). These brain areas not only tell our cortex to perceive pain in a certain region of the body, but they also send descending information to our spinal cord to shut off the pain input.
Our spinal cord neurons respond to certain chemicals to shut off their recognition of pain receptor input. These chemicals are neurotransmitters that are released in the spinal cord, and include: enkephalins, serotonin, and endorphins. Enkephalins are referred to as "opioids" because they work much the same way as opium and morphine do, but they are naturally occurring within our bodies.
© 2011 STCC Foundation Press