Importance of Sensory System and Sensory Receptors

The human sensory system is one of the most complex and highly evolved structures, which processes a myriad of incoming messages. This well-coordinated system helps an organism or individual to respond to external stimuli, appropriately. The sensory receptors are an important part of the sensory system. These receptors are specialized epidermal cells that respond to external environmental stimuli. These receptors consist of structural and support cells that form the peripheral unit of the receptor and the neural dendrites which receive and detect the external stimuli.

Sensory Receptors and Computer Input Devices – A Comparison

As sensory receptors are the primary medium of interaction with the surrounding external environment for living organisms, it can be compared to the input device of a computer. Here the brain acts as the central processing unit (CPU) and the receptors gather information from outside and transfer it to the brain for processing. The brain subsequently accords a response and commands the concerned organ to respond as needed. So, the whole stimuli and response of the sensory system work identically to that of a computer system.

Classification of Sensory Receptors

Sensory receptors convert external environmental stimuli into electrochemical signals for the central nervous system to process and respond accordingly. Since it is a very elaborate system, the receptors take up many forms to counter the external stimuli. They can be classified based on:

• Their structural complexity.
• Their location.
• Their basic function.

Classification based on structural complexity

1. Free nerve endings are receptors with dendrites that are embedded in tissue, with little or no physical specialization. For example, pain and temperature receptors in the skin dermis.
2. Encapsulated nerve endings are receptors with nerve endings enclosed in a capsule of connective tissue that enhances their sensitivity. For example, touch and pressure receptors in the skin dermis.
3. Specialized receptors are the ones that are exclusively present in the sense organs such as eyes and ears.They have unique structural components that detect a specific type of stimuli, with the help of connective, epithelial or other tissues. For example, photoreceptor present in the retina which responds to light.

Classification based on location

1. Exteroceptors are sensory receptors that exist at or near the surface of the skin and are responsive to stimuli occurring outside or on the surface of the body. The receptors consist of those for tactile functions such as touch, pain, and temperature and also the ones for vision, hearing, smell, and taste.
2. Interoceptors, also known as visceroceptors, are the receptors that respond to stimuli from the visceral organs (organs present in the chest, abdomen, and pelvis) and blood vessels within the body. These receptors are sensory neurons associated with the autonomic nervous system.
3. Proprioceptors are receptors located near a part of the body that is responsible for movements, such as a muscle or joint capsule. They primarily respond to stimuli occurring in skeletal muscles, tendons, ligaments, and joints. Proprioceptors collect information regarding body movement and interpret the physical position of the tissues as they move. For example, the Golgi tendon organ.

Classification based on function

1. Chemoreceptors detect and respond to chemical stimuli such as chemicals acquired during taste and smell. Also, they are sensitive to changes in the levels of oxygen and carbon dioxide in the blood.
2. Mechanoreceptors respond to physical stimuli such as pressure, force or vibration. They also respond to the sensation of sound and body position to maintain balance.
3. Photoreceptors respond to light which acts as a stimulus.
4. Thermoreceptors sense the changes in the temperature and are sensitive to temperatures that are above or below the normal body temperature.
5. Nociceptors respond to pain, which can be activated by chemicals (that occurs due to tissue damage) or sometimes mechanical or thermal stimuli.

How do some Sensory Receptors Work?

The basic function of a sensory receptor is to respond and interpret a stimulus. They make us aware of our surrounding external environment. These stimuli can be categorized into three groups. They may be ions and macromolecules, physical variations in the environment or electromagnetic variation in visible light.

In the following examples, the full functioning of two types of receptors, namely the photoreceptors and nociceptors, can be understood.

Photoreceptor

As already mentioned, photoreceptors are specialized receptors found exclusively in the retina of the eye. They convert light into electrical signals that can trigger a response from the brain. The photoreceptor cells contain protein that absorbs the photons from the light source, thus stimulating a change in the cell membrane potential (difference in electric potential between the interior and the exterior of the cell). At present, there are three known photoreceptor cells in the mammalian eye. They are:

• Rods.
• Cones
• Photosensitive retinal ganglion cells.

The two well-known photoreceptor cells are the rods and cones. The rods primarily contribute to night-time vision and the cones help in day-time vision. However, the chemical process that helps them achieve phototransduction is the same. A recently discovered set of mammalian photoreceptor cells called the intrinsically photosensitive retinal ganglion cells are thought to play a role in the circadian rhythm and pupillary light reflex (regulating the pupil size depending on the light intensity).

The rods are extremely photosensitive and are easily triggered by a single photon. In the presence of a low level of light, the visual experience is completely based on the functioning of the rods. On the contrary, cones require significantly higher levels of lights to produce a signal. There are three types of cone cells (S-cones, M-cones, and L-cones), which are classified according to their ability to respond to light of different wavelengths. This may be the reason why colors cannot be seen in low lights when only the rods are active.

Nociceptors

Nociceptors are pain receptors that consist of sensory neuron which responds to harmful or potentially harmful stimuli by sending signals to the central nervous system, thereby alerting it of the possible threatening situation. If the brain reads the incoming signal as a credible threat, it creates a sensation of pain to divert attention to the area of the body, thus mitigating a possible danger. This whole process is known as nociception. The pain stimuli are detected by the peripheral terminal of the nociceptor cell, which is then transduced into electrical energy. When the electrical energy reaches a threshold value, an action potential is generated and the signal is conducted towards the brain. The nociceptors get triggered only when the physical, chemical or mechanical stimuli reaches the high threshold.

How Important are the Sensory Receptors?

Sensory receptors, with their numerous functions, mediate a limitless number of processes in our body. These processes include vision, hearing, taste, touch and much more. One of the most significant forms of its function is to create awareness about the environment of the living organism and also about the condition of its inner environment. Hence, it can be inferred from all the studies that the sensory receptors play a very crucial part in an organism’s overall survival and well-being.

Common Mistakes and Pitfalls

Since there are a lot of classifications of the receptor types, based on so many factors, the students may confuse one name for the other and that may be a problem. So, it is important to learn the proper names and their consequent functions of the receptor cells. 

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for: 

• Bachelors of Science in Life sciences
• Bachelors of Anatomy 
• Bachelors of Surgery (MBBS)