How senses work: an overview
Our focus of this lesson will be on vertebrates and we'll use human anatomy in our examples.
A sensory receptor is a part of a sensory neuron (or associated cell) that receives information from the internal or external environment and relates it to your nervous system. Basically, some "outside" signal has to have a way to act on dendrites. Some sensory receptors are activated when they are bent, squished, or disturbed in some way. Others are activated by chemicals or temperature or light. Each type of sensory receptor only responds (or only responds well) to one of these types of signals (i.e., receptors that are activated by chemicals cannot be activated by light). The signal is called a stimulus for the receptor.
sensory reception from sunlight to sensory receptor cell to neuron to brain
Sensory perception
Our senses can be classified as general or special. General senses are those that are distributed throughout the body rather than having a specific organ dedicated to them. An example of a general sense is touch (pressure). Pressure sensory receptors are distributed throughout the body in the layers of skin, muscles, and blood vessels. A special sense has a particular organ dedicated to it. An example of a special sense is vision and the eye is the organ dedicated to vision.
Whether general or special, the way the senses function is generally the same. A mechanical deformation (i.e., pressure) or a chemical or light stimulus, etc. can activate certain sensory receptors. How exactly does that happen? How does that lead to an action potential in the sensory neuron itself so that it can give the information to the central nervous system (CNS)?
Whatever the appropriate stimulus is (like enough pressure to cause pain, as depicted in the picture below), that will cause a depolarization to occur in the sensory neuron. If the depolarization is "large" enough to reach the threshold potential, then an action potential in the sensory neuron will result. This starts the transfer of information from the sensory neuron toward the CNS.
pain sensory reception from stepping on a nail through the leg to the spinal cord
The main types of sensory receptors are:
1. Mechanoreceptors - these receptors detect a physical change caused by a mechanical force. For example, if you press down on your skin, the pressure is detected by mechanoreceptors. When the hair cells in your ear are moved, that movement is detected by mechanoreceptors.
2. Chemoreceptors - these receptors detect chemicals in the body. For example, chemoreceptors in the blood detect oxygen and carbon dioxide levels there.
3. Electromagnetic receptors - these receptors detect electromagnetic energy like light, electricity and magnetism. For example, the photoreceptors in the retina of your eye detect light entering your eye.
4. Thermoreceptors - these receptors detect temperature. For example, thermoreceptors on your skin allow you to detect the warm or cold things you touch.
5. Nociceptors (aka pain receptors) - these receptors detect extreme temperature or pressure or chemicals. For example, if the thing you touch is too hot or the pressure on your skin is too much so that it hurts, this sensation is detected by nociceptors.
After a sensory stimulus is received, when do we stop feeling (seeing, smelling, hearing, or tasting) it? One of two ways:
1. When the stimulus stops being applied (e.g., when the lights are turned off), or
2. Sensory adaptation: After a while, we stop noticing the stimulus. Some of our sensory receptors adapt (or habituate). That means that they stop having a receptor potential after awhile, even when the stimulus is still present. Example: You put on your shirt in the morning and feel the material against your body. Then, after a little while, you forget about your shirt because you don't feel it anymore. The sensory receptors in your skin adapted to the shirt touch stimulus because it went on too long. We will do a lab experiment on this concept (the one with the coins).
Additional examples of sensory adaptation:
* Gradually no longer hearing the ticking of a clock or the sounds of passing cars outside of a city apartment
* Forgetting the radio or TV is on during daily activities
* Walking out of a dark movie theater on a matinee day, the sunlight outside seems painfully bright. Within minutes, the light level feels comfortable and normal.
* Walking into a stinky (or yummy smelling!) room. The first few minutes inside are often very uncomfortable but over time the smell seems to diminish.
* When the hot bathtub water seems comfortable after a few minutes
sensory adaptation joke with a neuron saying I learned to ignore my fan
We probably experience sensory adaptation to avoid "sensory overload." We want the important signals to stand out from the rest.
Interesting fact: Nociceptors (the sensory receptors that detect pain) typically do not adapt to the pain stimulus. This is due to their role in alerting the body of danger. Adaptation to pain would result in an individual getting used to the pain and therefore not responding to it. This could result in serious bodily harm or even death.
We will not be investigating all of the senses in depth here but instead we will focus on a few: hearing, vision, smell and taste, and skeletal muscle contraction.