Table of Contents

What are Sensory Receptors?

How Sensory Systems Work?

[Types of Sensory Receptors Based on Structure](#Types of Sensory Receptors Based on Structure)

Sensory Receptors Based on Location of Stimuli

Sensory Receptors Based on Types of Stimuli

Examples of Mechanoreceptors

Frequently Asked Questions

What are Sensory Receptors?

Sensory receptors are specialized cells that detect changes in the environment and convert them into electrical signals that can be interpreted by the brain. They are located in the skin, eyes, ears, nose, and other organs.

Have you ever pondered how creatures like bats and dolphins can find objects from a distance? Or how sharks can sense the electric field that surrounds them? The ability to respond to stimuli is a key characteristic of living organisms. Each animal has its own unique way of responding to the stimuli in its environment.

Sensory receptors can be found both externally on the body and internally in the organs. These receptors detect changes in the external and internal environment.

Sensory receptors are composed of specialized cells that are located near neurons or neuron endings, which are part of the afferent neurons and transmit signals to the central nervous system and brain for processing and integration.

We are all familiar with our five main senses - hearing, taste, sight, touch and smell - but there are also other sensory signals that we can perceive, such as pain, pressure, temperature, balance and muscle tension. These signals are detected by sensory receptors and specialised cells, which together form sense organs.

How Do Sensory Systems Work?

Sensory receptors are specialized to detect a specific type of stimulus. Upon receiving the stimulus, they convert the energy from the stimulus into electrical signals. These electrical signals, known as action potentials or impulses, are then transmitted by sensory neurons or afferent neurons to the brain and spinal cord. Once the information is received in the central nervous system (CNS), it is processed and integrated to generate a response.

The change in ion distribution across the plasma membrane due to a stimulus causes a change in the potential or voltage across the membrane. This change is dependent on the energy of the stimulus. The receptor potential can then initiate a release of neurotransmitters at the synapse, which binds to sensory or afferent neurons. When the action potential passes a threshold level of the membrane potential, it transmits the information to the CNS.

When a stimulus reaches a certain level of stimulation, the response rate begins to decrease. This phenomenon is known as sensory adaptation. Some stimuli, such as those that are important and transient, will continue to trigger action potentials until they are gone. However, other stimuli that are unimportant and persistent will be ignored as the sensory receptors adapt to them and focus on new and important stimuli.

See Also: Process of Neural Communication

Types of Sensory Receptors Based on Structure

Sensory receptors can be classified based on structure, location, and kind of stimulus it perceives.

On the basis of structure, sensory receptors can be divided into three main types:

Free Nerve Endings or Dendrites - These are unmyelinated nerve endings found embedded in tissue, such as thermoreceptors (for temperature) and nociceptors (for any kind of damage) located in the dermis and epidermis.

Encapsulated Nerve Endings

Here, nerve endings are encapsulated by connective tissue, making them more specialised and more sensitive to a stimulus. Examples of encapsulated nerve endings include receptors for touch and pressure (mechanoreceptors), such as Meissner and Pacinian corpuscles.

Specialized Receptor Cells

• They form distinct structures associated with other tissues and respond to a specific stimulus, such as the rod cells of the eyes which are photoreceptors.

Sensory Receptors Based on Location of Stimuli

On the basis of the location of the stimuli, sensory receptors can be categorized into two types:

Exteroreceptors: They respond to external stimuli, enabling a living organism to transfer information and changes from the surrounding environment and adjust accordingly. This helps them in predation, defence, navigation, and reproduction.

Interceptors: These are present within our body and respond to any changes in the internal environment, such as changes in temperature, pH of blood, etc. We become aware of these changes through sensations such as thirst, hunger, pain, nausea, etc.

Sensory Receptors Based on Types of Stimuli

On the basis of the types of stimuli they respond to, sensory receptors can further be divided into various types such as:

Electroreceptors: They can sense any variation in the electric field. Sharks and rays, among other fish, can detect electric fields generated by moving water, which helps them with defense, catching prey, and navigation.

Some fishes possess electric organs, which can be used for defence as they can generate a high voltage shock. Additionally, electroreceptors are utilized to distinguish between sexes.

Electromagnetic Receptors: These receptors have the ability to detect the Earth’s magnetic field. Many animals, such as fishes, birds, and amphibians, utilize these receptors to orient themselves. Additionally, migratory birds and sea turtles rely on magnetic fields to guide their navigation.

Thermoreceptors

• They respond to changes in temperature such as heat and cold.
• External thermoreceptors are present in the skin, tongue, etc. and detect any changes in the external temperature.
• Internal thermoreceptors are present in the hypothalamus and respond to internal changes, helping to maintain homeostasis.

Explore NEET MCQs on Homeostasis Here

Blood-sucking insects use thermoreception to detect their host; the pit organ of the viper, containing thermoreceptors, helps them locate their prey.

Nociceptors (pain receptors): These receptors detect any extreme thermal or mechanical stimuli that could be damaging, and they trigger a pain response.

Mechanoreceptors: They are sensory receptors that respond to mechanical stimuli such as touch, movement, stretching, gravity, etc. When they are pushed or pulled, they change shape and transduce mechanical energy to electrical energy. These receptors are important for maintaining body posture with respect to gravity, hearing, and maintaining body balance. Tactile receptors for touch, pressure, vibration, etc. are also mechanoreceptors.

Examples of Mechanoreceptors

Tactile Receptors

Merkel cells are specialized cells present in the epidermis which sense light touch and softness.

Meissner corpuscles - An encapsulated nerve ending found in the upper part of the dermis, that is sensitive to light touch and vibrations.

Ruffini endings - These are also encapsulated, and can be found in the dermis. They are able to detect heavy and continuous pressure.

Pacinian corpuscles - They are encapsulated by connective tissue and are located deep in the dermis of the skin. They are sensitive to deep pressure.

Proprioceptors

These receptors sense body movement and position, helping to maintain posture which is necessary for locomotion and motor skills.

Muscle spindles sense muscular movement.

Golgi Tendon Organs sense tension in the tendons during muscle contractions.

Joint Receptors - Sense movement in the ligaments.

Lateral Line Organs - Present in fishes, they detect vibrations and help in navigation.

The vestibular apparatus in humans is located in the inner ear and helps with maintaining balance and equilibrium. Sensory hair cells detect the position with respect to gravity. The vestibular apparatus is made up of the saccule, utricle and three semicircular canals, and is responsible for both static and dynamic equilibrium.

The cochlea is where auditory receptors are located. The organ of Corti, which is situated in the cochlea, contains many hair cells present on the basilar membrane. These hair cells receive an auditory signal and then transmit the signal to the brain via the cochlear nerve.

Learn more about the structure of ear

• Chemoreceptors - They respond to chemical stimuli, such as gustatory receptors (present in the taste buds of the tongue) and olfactory receptors (present in the olfactory epithelium of the nasal roof). Each olfactory receptor is associated with an axon, which together forms the olfactory nerve, which extends to the olfactory bulb in the brain and to the olfactory cortex, which is part of the limbic system of the brain.

Many animals use pheromones to find a mating partner, such as female moths, and to mark their territory, such as dogs and wolves.

Some insects, such as ants and bees, have olfactory sensory hairs or sensilla present in their antennae. Additionally, taste sensilla can be found in the mouthparts, legs, or on the antennae.

Photoreceptors: They respond to light and are found in the retina of the eyes, contained in both rods and cones. Rhodopsin, a photopigment, is present in the rods.

Eyespots (or ocelli) are found in cnidarians and flatworms.

Ommatidia are the individual light-sensing units in the compound eyes of insects.

Frequently Asked Questions

What is a Statocyst?

A statocyst is a sensory organ found in some aquatic animals, such as certain molluscs and crustaceans, that helps them maintain balance and orient themselves. It contains a mineralized mass and several hairs that, when stimulated, send signals to the animal’s nervous system to help it maintain balance.

Many invertebrates have statocysts as their proprioceptors, which help to orient the organism with respect to gravity and maintain equilibrium. This is accomplished by sensory hair cells present on the epidermis, which are stimulated by calcium carbonate crystals present in the statolith. For example, crayfish have this capability.

What is the importance of sensory receptors?

The sensory system works in a coordinated way to help with defence, predation, navigation and better adaptation to changes in the external and internal environment. This is done by sensory receptors transmitting signals to the Central Nervous System (CNS) through afferent neurons for integration. The brain then directs the effector tissue to respond by sending signals through motor neurons.