Have you ever wondered how bats, dolphins, etc. can locate distant objects? How can sharks detect the electric field surrounding them? Responding to stimuli is a distinguishing feature of living organisms. All the animals respond differently to the surrounding stimulus.
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 receptors 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 receptors 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.
How Sensory Systems Work?
Sensory receptors are specialised to perceive a certain kind of stimulus. When they receive the stimulus, they convert the energy from stimulus to electric signals. The electric signal, action potential or impulses thus generated are transmitted by sensory neurons or afferent neurons to the brain and spinal cord. In the CNS the information is processed and integrated to generate a response.
The change in ion distribution across the plasma membrane due to stimulus causes a change in the potential or voltage across the membrane. This change depends on the energy of the stimulus. The receptor potential can initiate a release of neurotransmitters at the synapse, which bind to sensory or afferent neurons. This can generate an action potential, when it passes a threshold level of the membrane potential, it then transmits the information to CNS.
The stimulus generates graded potential and if it is high enough, it generates an action potential. When a stimulus reaches the maximum level of stimulation, they cannot further increase the stimulation and there is a drop-off. So even if the stimulus is applied continuously with the same intensity, the response rate differs. This is called sensory adaptation. Some stimulus continues to trigger action potentials till the time they exist as they adapt slowly, whereas others, which are unimportant and persistent are ignored, as the sensory receptors adapt to it and focus on a new and important stimulus.
Also see: Process of Neural Communication
Types of Sensory Receptors
Sensory receptors 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).
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.
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 change in the 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.
Examples of mechanoreceptors:
Merkel cells- Specialised cells present in the epidermis, sense light touch and softness.
Meissner corpuscles- An encapsulated nerve ending, present at the upper part of the dermis. They can sense light touch and vibrations.
Ruffini endings- These are also encapsulated, present in the dermis. They sense heavy and continuous pressure.
Pacinian corpuscles- They are encapsulated by connective tissue, located deep in the dermis of the skin. They respond to deep pressure.
These receptors sense body movement and position and help in maintaining the posture. It is required for locomotion and motor skills.
Muscle spindles- sense muscular movement.
Golgi tendons- sense tension in the tendons during muscle contractions.
Joint receptors- sense movement in the ligaments.
Statocysts- Many invertebrates have statocysts, which help in the orientation with respect to gravity and help in maintaining equilibrium. Sensory hair cells are present on the epidermis, which are stimulated by calcium carbonate crystals present in the statolith. E.g. crayfish.
Lateral line organs- Present in fishes, they detect vibration and help in the navigation
Vestibular apparatus- In humans, the inner ear has vestibular apparatus, which helps in maintaining balance and equilibrium. Sensory hair cells detect the position with respect to gravity. The saccule, utricle and three semicircular canals together form vestibular apparatus, which is responsible for the static and dynamic equilibrium.
Auditory receptors are located in the cochlea. The organ of Corti contains many hair cells present on the basilar membrane, which on receiving an auditory signal, transmit the signal to the brain by the cochlear nerve.
Also read: Structure of ear
- Chemoreceptors- They respond to chemical stimuli. E.g. receptors for taste, i.e. gustatory receptors and smell, i.e. olfactory receptors are examples of chemoreceptors. Gustatory receptors are present in the taste buds present over the tongue. Olfactory receptors are present in the olfactory epithelium of the nasal roof. Each olfactory receptor is associated with an axon, which together forms the olfactory nerve. The olfactory nerve extends to the olfactory bulb in the brain and to the olfactory cortex, which is the part of the limbic system of the brain.
Pheromones- Many animals use pheromones to find a mating partner, e.g. female moths and some use it to mark their territory, e.g. dogs and wolves.
Olfactory sensory hairs or sensilla are present in the antennae of some insects, e.g. ants, bees, etc. Taste sensilla are present in the mouthparts, legs or on antennae.
- Photoreceptors- They respond to light. Rods and cones contain photoreceptors in the retina of the eyes. Rhodopsin is a photopigment present in the rods.
Eyespots or ocelli- cnidarians and flatworms.
Ommatidia- in the compound eyes of insects
Sense perception is a characteristic feature of living beings. A simple sensory system is present in lower organisms and a highly complex and evolved in the higher organisms. Sensory receptors transmit signals to the CNS through the afferent neurons for integration and the brain directs the effector tissue to respond by sending signals through motor neurons. The sensory system helps in defence, predation, navigation and better adaptation to any changes in the external and internal environment.
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