Human Sensory Reception: Sensory receptors are present throughout the body externally as well as in the internal organs. Sensory receptors detect changes in the internal as well as external environment. Sensory Reception comprise specialised cells close to neurons or neuron endings, which are a part of the afferent neurons and send signals to the central nervous system and brain for processing and integration.

Sensory Reception along with certain specialised cells form sense organs. We all are familiar with our five main senses, i.e. hearing, taste, sight, touch and smell but we also perceive many other signals such as pain, pressure, temperature, balance, muscle tension, etc.

Types of Sensory Reception based on Structure

Sensory Reception can be classified based on its 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 nerve endings, the unmyelinated part, found embedded in the tissue, e.g. receptors for temperature (thermoreceptors), receptors for any kind of damage (nociceptors) present in dermis and epidermis.
• Encapsulated nerve endings- Here nerve endings are encapsulated. These are more specialised and more sensitive to a stimulus. The nerve endings are covered by connective tissue. E.g. receptors for touch and pressure (mechanoreceptors), which are lamellated such as Meissner and Pacinian corpuscles.
• Specialised receptor cells- They form distinct structures associated with other tissues. They respond to a specific stimulus. E.g. rod cells of eyes (photoreceptors).

Sensory Reception based on Location of Stimuli

On the basis of the location of the stimuli, sensory receptors are of two types:

• Exteroreceptors- They respond to external stimuli. They enable a living organism to transfer information and changes from the surrounding environment and adjust accordingly. It helps them in predation, defence, navigation and reproduction.
• Interoceptors- These are present within our body and respond to any changes in the internal environment, e.g. changes in temperature, pH of blood, etc. We get to know about these changes when we feel thirsty, hungry, pain, nausea, etc.

Sensory Reception 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 change in the electric field. Some fishes such as sharks and rays can detect electric field generated by moving water, it helps them in their defence as well as to catch prey and navigate.

Some of the fishes have electric organs, which are useful in defence as they can create a high voltage shock. Electroreceptors are also used to identify opposite sexes.

• Electromagnetic receptors- These receptors can detect Earth’s magnetic field. Many fishes, birds, amphibians, etc. use electromagnetic receptors to orient themselves. Migratory birds and sea turtles use magnetic fields to navigate.
• Thermoreceptors- They respond to changes in temperature such as heat and cold. External thermoreceptors are present in the skin, tongue, etc. They detect any changes in the external temperature. Internal thermoreceptors are present in the hypothalamus, which respond to internal changes and maintain homeostasis.

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

• Nociceptors (pain receptors)- These receptors identify any extreme thermal or mechanical stimuli, which can be damaging. They induce pain.
• Mechanoreceptors- They respond to any mechanical stimuli such as touch, movement, stretching, gravity, etc. They change shape when they are pushed or pulled. They transduce mechanical energy to electrical energy. They help in maintaining body posture with respect to gravity, hearing, maintaining body balance. The tactile receptors for touch, pressure, vibration, etc. are mechanoreceptors.

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By Team Learning Mantras