The ability to perceive is the key to learn and to survive. The ability to the perception of our environment whether it is external or internal helps us to learn about it. The central nervous system perceives the information of the surroundings to address the body according to the need. And how you are able to perceive things and surroundings? The simple answer is “senses”. Your eyes create a visual image of everything on which they focus. The ocular support that eyes provide helps us differentiate among things based on their colors and shapes. We can call it the sense of sight.
Similarly, there are ears that help us sense or hear sounds that are in our environment. Ears also help to differentiate between things and their nature-based on how they sound, so that the body may act accordingly. Just like that, there are senses of smell, touch, and taste. All these senses reach the brain through nerve impulses, traveling across and through the vast complex network of neurons.
On one end there is the brain which takes in all the information and processes it and gives orders to organs, tissues, and cells to carry out the required task, whereas on the outer end there are the Sensory receptors which act as information reception system that continuously receives information from the environment in which it is located. This information comes in form of a stimulus, which activates a nervous response as a result of its reception by the receptor and the information goes to the Central Nervous System which then commands the effector to carry out the necessary action for that particular response.
Structure of Sensory Receptors
As a matter of fact, the sensory receptors are the ends of dendritic sensory neurons. The sensory neurons make up sensory nerve bundles which are characterized by their ability to send or transmit a message to the brain. The structure of sensory receptors can vary according to their location or function. That is why we broadly classify and categorize sensory receptors into many types. Because they are not just simple neurosensory endings, they are modified to perform necessary and specific tasks of reception of specific location.
Classification of Sensory Receptors by their cell type or Structure
This classification also loosely covers the structural basis and differences which are found in a different type of sensory receptors.
- They can exist in such structural form that they are simply open or free nerve endings. These can be observed in tissues in embedded form, to simply receive sensations
- They can be found as free endings but encapsulated. These encapsulated sensory receptors are usually associated with connective tissues and the encapsulation enhances their sensitivity.
- The most suitable example of this third type can be the rod cells which act as functional units for the ocular support the eye provides by sensing light. These have very specific cell receptors that are designed to release neurotransmitters which move to the optic nerve by the process of synapses, through the help of bipolar cells. These receptors are modified types of receptors, these particular modifications, unlike the encapsulated or simple free nerve endings, allow them to detect specific signals like chemoreceptors which detect certain chemical or photoreceptors as briefed above they are located in the retina and are sensitive towards the light.
Classification on the basis of location
This is another way of classifying the sensory receptors by understanding the fact that they are specialized for and are sensitive to specific stimuli. And these stimuli are not anywhere but on certain locations or are perceived by certain locations and that is where the receptors are supposed to be.
- Exteroceptors are those which specialize in sensing or perceiving the stimuli which are originated from the outside or external environment. For example, the receptors which are present on the skin and they specialize in the sense of touch and touch associated sensation. These are known as Somatosensory Receptors. Another worthy example is that of photoreceptors present in the eye. They sense light and that is indeed an external stimulus, to produce images and assist the brain in producing an image in the visual cortex.
- Interoceptors are located in the inside of the body. They are specialized in detecting the stimuli originating from the internal environment. The most worth mentioning and appropriate example is the chemoreceptors. These are responsible for detecting pH, Oxygen, Carbon dioxide, and many other chemicals, their levels, and also chemical products which are produced by organs and tissues to inform the brain so that certain procedures could be measured to counter the excess or depression of levels chemicals e.g., hormones.
- Proprioceptors are those sensory receptors that are located near or on the parts of the body which are designed for motion or their function involves movements. Proprioceptors can be observed near joint capsules and skeletal muscles, they assist in reading the motion of those muscles and joints.
Function and Physiology of Sensory Receptors
Before heading to the functional classification of the sensory receptors it would be very suitable if we understand the basic physiology and functioning of sensory receptors. It is a very basic concept that the function of a receptor is to receive information. In order to understand the functioning of a receptor, we need to understand how it receives its information? What that information actually is.
Stimulus and Sense Modalities
As we said earlier, it is the basic work philosophy of a receptor to receive information. This information always comes from the environment in which it is located. An important characteristic is that this information should be strong enough to be registered by the receptor, so it may lead to a noticeable response. This information is called stimulus. The nature of a stimulus is also an interesting thing to understand. The nature of a stimulus can be understood by the way it is perceived and encoded by the sensory system. For example, a chemical is perceived by the sense of smell and taste. Another example is the color of a wall that is observed through eyes using the sense of sight with the help of the photoreceptors in the eyes. However, the texture of the wall can only be felt by touch which is assisted by the somatosensory receptors present in the fingertips and also embedded all over in your skin. This nature of the stimulus and how it is perceived and encoded by our sensory system are modalities of senses. In brief, these can be overgeneralized as senses. Because, if we start counting the modalities of senses (sensory modalities) we can reach up to 17 of them. In them, there are sub modalities too. Which are divisions of the 5 major modalities.
Furthermore, the senses can also be differentiated and categorized by considering their viability to be detected, perceived, and encoded. These types are termed as General sense and Special Sense. General sense can be simply understood by taking the example of the sense of touch. Because it is easy to understand that you can feel touch sensation all over the body, this means the receptors for registration of the stimulus of touch are present all over the body and they are in excess. This makes it a very general sense. In addition to the sense of touch, there are some other general senses like proprioception which is related to the perception of the position in which the body is. Furthermore, there is kinesthesia and that is the sense of body movement. There is also a visceral sense which plays its role in registering stimulus for the autonomic organs or simply organs of smooth muscles. However, a special sense can be understood by simply considering it in this sense that there is a specific organ of the body which is devoted to it. This can be understood, by comparison, the skin has many functions like producing sebum, protection, providing support, and also a foundation of the sense of touch and other receptors which detect temperature and pressure. This creates a general sense of example. Unlike the previous, special sense means to have specific organs associated with it exclusively like eyes and ears, their major and exclusive functions are to provides sight and audibility, respectively. Similarly, nose and tongue. The nose is exclusive for smell and the tongue for taste.
Basic Functioning and General Physiology of Sensory Receptors
In the free nerve ending and encapsulated nerve ending sensory receptor types, there is a graded potential which is also termed as generated potential, when this potential becomes strong enough so that it may reach the threshold ( which is the lowest quantitative level of information or stimulus which can invoke a response by triggering the receptor) it excites and generates an action potential in the axon of the associated sensory neuron. However, there is also the existence of graded potential in other sensory neuron types like the “receptor cell having” sensory neurons e.g., sensory neurons of the present in the retina. Such graded potential in that situation is called receptor potential. The purpose of the existence of this receptor or generated potential is that it is necessary for the neurotransmitters to create graded postsynaptic potential by getting released in a sensory neuron. And this graded postsynaptic potential is qualified enough (in sense of quantitative electrical strength) to touch the threshold this will then raise an action potential in the sensory neuron it entered which conveys it to the brain, where it will be processed, and a suitable effector response will be made.
Classification of Sensory Receptor on Basis of their Functioning
- Chemoreceptor is very crucial interoceptors which detect several types of chemicals which are entering the body (through the nose), present in the blood, or present in the cerebrospinal fluid. They are specialized in the detection of chemicals and chemical products by binding them and producing signals (graded potential) in response which will lead to the effector response. Chemoreceptors can be sensitive towards certain chemical agents in a certain location. For example, in our respiratory system, they keep tabs on the level of Carbon dioxide and oxygen concentration levels so that body can manage them without undergoing stressful conditions like alkalosis or acidosis.
- Mechanoreceptors are responsible for the reception of physical force related stimuli like touch and pressure. They are present in muscles where they detect the stretching of that particular muscle tissue in which they are located. Moreover, mechanoreceptors are also involved in the perception of vibrations, vibrations coming from a source like when you are holding your phone that is vibrating or swiping your finger gently on a rough-textured wall. The mechanoreceptors specifically involved in these scenarios are explicitly known as Merkel Cells. These are to be located in stratum basale. Similarly, there are other associated structures that are known as mechanoreceptors like Meissner’s Corpuscles which are responsible for registering soft touches.
- Nociceptors have the ability to detect mechanical trauma as well as trauma which manifests itself in chemical form. These receptors are specialized in the reception of damage, they detect certain chemicals that may be released when there is damage dealt in the body or to the body. Those chemicals are most like cytokines. However, this phenomenon can be exploited for therapeutic means. Nociceptors are chemical sensitive when the temperature is above 37 Degree Celsius. That is why the sensation of hot and spicy exists after eating hot and spicy food that contains capsaicin. And this chemical agent has the ability to bind the nociceptor and not allowing other agents like cytokines to exhibit that kind of pain. That is why it can also be used as an analgesic.
- Thermoreceptors are specialized for sensing the ambient temperature. They get activated when the temperature falls below or rises above a certain level of temperature. They are crucial as they produce effector response that results in piloerection, thermogenesis, vasoconstriction, or maybe the opposite according to the situation.
- Photoreceptors are specialized for visualization by capturing light. They are associated with rod cells that absorb several frequencies of light to produce colors.
They are many types of receptors that can be differentiated by their functioning, a few of them are Hydroreceptors (Humidity), Baroreceptors, Electroreceptors, etc.
- Saladin, Kenneth S. (2011). Anatomy & Physiology: The Unity of Form and Function. New York: McGraw-Hill. ISBN 978-0-07-337825-1. OCLC 799004854
- Boulpaep, Emile L.; Boron, Walter F. (2005). Medical physiology: a cellular and molecular approach. St. Louis, Mo: Elsevier Saunders. ISBN 978-1-4160-2328-9. OCLC 56963726