What if you felt no pain?
It might sound good to you to have a condition where you feel no pain. But actually this type of condition would be very dangerous. What would happen if you strained your back but felt no pain? Instead of resting your back, you might injure it further.
When you look at the prickly cactus pictured below (Figure below), does the word "ouch" come to mind? Touching the cactus would be painful. Touch is the sense of pain, pressure, or temperature. Touch depends on sensory neurons, or nerve cells, in the skin. The skin on the palms of the hands, soles of the feet, and face has the most sensory neurons and is especially sensitive to touch. The tongue and lips are very sensitive to touch as well. Neurons that sense pain are also found inside the body in muscles, joints, and organs. If you have a stomach ache or pain from a sprained ankle, it’s because of these sensory neurons found inside of your body.
The spines on this cactus are like needles; they help keep away animals that might want to eat the cactus.
The following example shows how messages about touch travel from sensory neurons to the brain, as well as how the brain responds to the messages. Suppose you wanted to test the temperature of the water in a lake before jumping in. You might stick one bare foot in the water. Neurons in the skin on your foot would sense the temperature of the water and send a message about it to your central nervous system. The frontal lobe of the cerebrum would process the information. It might decide that the water is really cold and send a message to your muscles to pull your foot out of the water.
In some cases, messages about pain or temperature don’t travel all the way to and from the brain. Instead, they travel only as far as the spinal cord, and the spinal cord responds to the messages by giving orders to the muscles. This allows you to respond to pain more quickly. When messages avoid the brain in this way, it forms a reflex arc, like the one shown below (Figure below).
Reflex Arc: When a reflex hammer taps your knee, you may immediately kick your leg—without even thinking about it. The nerve impulse from your knee travels to the spinal cord, and the spinal cord sends a message to your muscles to kick your leg.
Our sense of touch is controlled by a huge network of nerve endings and touch receptors. This system is responsible for all the sensations we feel, including cold, hot, smooth, rough, pressure, tickle, itch, pain, vibrations, and more. There are four main types of receptors: mechanoreceptors, thermoreceptors, pain receptors, and proprioceptors.
- Mechanoreceptors perceive sensations such as pressure, vibrations, and texture. Your brain gets an enormous amount of information about the texture of objects through your fingertips because the ridges that make up your fingerprints are full of these sensitive receptors.
- Thermoreceptors perceive sensations related to the temperature of objects. There are two basic categories of thermoreceptors: hot receptors and cold receptors. The highest concentration of thermoreceptors can be found in the face and ears.
- Pain receptors, or nociceptor detect pain or stimuli that can or does cause damage to the skin and other tissues of the body. There are over three million pain receptors throughout the body, found in skin, muscles, bones, blood vessels, and some organs.
- Proprioceptors detect the position of different parts of the body in relation to each other and the surrounding environment. These receptors are found in joints, tendons and muscles, and allow us to do fundamental things such as feeding or clothing ourselves.
- Sensory neurons in the skin sense pain, pressure, and temperature.
- When sensory messages only travel as far as the spinal cord, and skip the brain, this is called a reflex arc.
Use the resources below to answer the questions that follow.
- Human Senses at http://www.youtube.com/watch?v=Ajw_WJYxAAI (1:05)
- What do touch receptors detect?
- How do receptors communicate with the brain?
- What happens in the somatic sensory system after a stimulus triggers a receptor?
- What is touch?
- Where are touch neurons found?
- Imagine you touch a smooth stone. How is this sensation transmitted to your brain?
- How and why do reflex arcs occur?
- What is the role of mechanoreceptors? Where are these receptors found?