Stimuli and Receptors
In every manner of life, we experience the world through our senses, and as we are wrapped up in our own lives it is easy to take the process of our experiences for granted. Stimuli and receptors are explicative of how our brain comes to know the world.
Transduction or conversion, of one form of energy to another within all living organisms is how this comes to fruition. Energy from stimuli in our environment is converted to electrochemical energy so our brain can experience our surroundings and the events that are occurring around us. These receptors tirelessly provide us with sensory experiences and information.
Primary receptors that respond to stimuli;
Receptors are sensory neurons or also known as nerve endings that respond to various forms of sensory stimuli, or sensory modality. These are receptors then convert the energy from the stimulus into an action potential (or electrical message to the brain). These receptors differentiate by the various stimuli they respond to. An adequate stimulus is the type of stimulus the sensory receptor is sensitive to and basically tuned to that stimulus, waiting to accept it and provide the transduction to the brain. Location of the receptor is also vital to its classification, however regardless of its classification, the common ground for all sensory receptors is the process of registering stimuli internally, or externally and without sensory receptors, sensation and perception of taste, touch, sight and smell would not be possible.
The somatosensory system perceives touch, pressure, pain, temperature as well as body position and movement, and this system consists of three interactive sub-systems;
- Exteroreceptive System which senses external stimuli of the skin
- Proprioceptive System which provides information on the position of the body
- Interoreceptive System responsible for monitoring blood pressure and body temperature
This system is involved in touch and pain. We have receptors on our skin called cutaneous receptors;
- Free nerve endings are close to the surface and sensitive to pain, tickle and itch.
- Merckel (tactile) disc is a slow adapting receptor also known as tactile disc found in the hands, finger tips, lips and external genitalia.
- Ruffini corpuscles are in a deeper layer of the skin relevant in the sensation of stretching of muscles, as well as in the palm of the hand and soles of the feet, also found in tendons and ligaments.
- Meissner corpuscles is a slow adapting receptor found throughout the body and epidermal layers, tissue and membranes, as well as the pancreas and urinary bladder
- Pacinian corpuscle is very similar to the meissner. Both meissner and pacinian detect vibrations, with meissner corpuscles detecting the lower frequency vibrations and the pacinian detecting the higher frequency vibrations.
- Hair root plexuses are free nerve endings around hair follicles of the skin and can feel the brush of soft touch as well.
Free nerve endings that are activated by thermal sensations such as heat and cold with separate receptors for both. However, once exposed to temperature of below 10 degrees or higher than 48 degrees then nociceptors are activated which is the production of pain sensations.
Nociceptors and Pain
Free nerve endings associated with painful stimuli and found in every tissue of the human body other than the head. Nociceptors perceive pain in two manners, fast pain is acute and immediate sharp pain, while slow pain takes a slightly longer duration to be felt. The slow pain increases in intensity and can be associated with cutaneous receptors, as well as deep tissue and organs. The pain intensifies to a throbbing sensation.
Moving on from pain…brings us to proprioceptive sensations that are vital in enabling us to know the location of our own body such as arms, legs and hands without looking at them. Kinesthesia is the perception of one’s own body movements. This is vital in the awareness of your body relative to the environment around you. Proprioceptors are located in muscles, tendons, joints and even the inner ear which enables the perception of the head relative to the ground.
Proprioceptors continually send nerve impulses to the brain. These proprioceptors along with nociceptors are slow adapting to ensure that the neural messages firing to the brain are continuous, and non-stop. This is important for both perceptions of location of the head and body as well as the monitoring of damaged areas through pain signals. These messages cannot stop, or problems would occur.
Distant chemoreceptors such as olfaction are the sense of smell which derives from millions of receptors containing cilia called olfactory hairs. These receptors are located in the upper part of the nasal passage and respond to odorants or chemical molecules in the air. Messages of scent are sent to the brains olfactory bulb and onto other regions for processing. Interestingly, over 1000 different receptors exist in the olfactory system.
When a certain odor such as perfume, fresh cut grass, summer or apple pie triggers a fond memory, it results from certain olfactory senses that are in connection with parts of the brain responsible for memory which are the limbic system and amygdala.
Direct chemoreceptors such as taste are closely connected to smell, and have thousands of receptors in the tongue, cheek and roof of the mouth. Taste buds which are groups of receptor cells are nestled within the papillae and are miniscule bumps found on the tongue containing the taste buds. There can be anywhere between 15-50 receptor cells within one taste bud, and as many as 500-10,000 taste buds depending on genetics and age. Sadly taste buds begin to gradually decline after the age of approximately 40.
A set of five tastes have been defined by researchers as a result of the taste buds, which are sweet, salty, sour, bitter and umami (meaty / msg type flavor). Smell is also relevant in the taste senses as gases are released when food enters the mouth. If one lost their sense of scent, taste would be greatly affected.
These sensory receptors that detect stimuli from within the body, providing information on internal systems such as blood pressure, temperature and condition of internal organs are vital to survival. These receptors are found in blood vessels, visceral organs and muscles, and continuously monitor the conditions and sends feedback to the brain.
Respond to light and consist of receptors called rods and cones, found within the retina of the eye. Rods and cones are both involved in phototransduction, while rods are visual receptors that respond to dim light, cones are visual receptors that respond to wavelength and colour.
Rods are concentrated within the periphery of the retina with high sensitivity to light and are incapable of perceiving colour. They are photoreceptors that are active when in a dark room, and all items seem black, and color is unperceived, however rods also offer peripheral vision capability.
Cones however are highly sensitive to colour, and are mostly concentrated in the centre of the retina. They enable color vision because they have a high acuity to specific and varied wavelengths responsible for colour.
Dark adaptation is the process of chemical changes within the rods and cones during adaptation to a decrease in light, and the rods come in useful for seeing in dim light. This visual information is received by photoreceptors gathered by ganglian cells and forwarded to the optic nerve of the brain.
These neurons are capable of detecting and responding to changes in pressure located within the walls of certain blood vessels. Very important in visceral conditions such as high blood pressure as information is sent to the brain which can then respond accordingly to bring the internal system back to homeostasis. It was once believed that baroreceptors only controlled short term changes in blood pressure, but new studies have found that baroreceptors are a part of kidney function which has a standing correlation to chronic hypertension which is good news for sufferers of chronic high blood pressure.
Cellular Receptors and Viral Tropism
These receptor cells are the culprit of viral infection. Viruses need specific cell receptors to bind to in order to gain entrance into a cell. Tissue tropism is the affinity that specific tissue and cells have for viral bacteria and these certain cells and tissue support the bacteria. Tissue tropism is determined by the cell receptors recognition factors for the virus, as well as replication factors and if there is a hospitable environment for the virus or an enzymatic presence to dissolve the virus.
In conclusion, stimuli and receptors of all forms are absolutely vital to how we sense and perceive the world and our environment around us. And in addition our somatosensory system and the receptors involved are necessary to our very survival because without nociceptors and pain, or acknowledgement of heat or cold with thermoreceptors, and the vital importance of interoreceptors our existence would endangered as we would not have the capability or knowledge of the danger around or within us.
- Pinel, J. P. (2011). Biopsychology (8th ed.). Boston, MA: Pearson.
- Tortora, Gerard J. Derrickson, Bryan H.(2011). Introduction to the Human Body (9TH ed.), Wiley.
- Wade, Carole; Tavris, Carol; Saucier, Deborah; Elias, Lorin; (2007). Psychology, Second Canadian Edition. Pearsons