Sensory Reception and Processing: Have you tried to see or locate things in the dark? Even though things are not clearly visible yet, we manage to locate many objects in the dark. It happens because of our perception. Although our eyes cannot see things clearly, our memory and vision allow us to spot our objects. Sometimes trees appear like big giants in the dark.  In the presence of light, we see the colourful world, whereas we perceive things differently in the absence of light. Isn’t it interesting that our senses do not give us 100% objective information, but we interpret what our senses tell us? Read further to learn more about sensory reception processing and perception.

Stimulus and Sensory Receptors

In scientific terminology, a stimulus is defined as anything that causes an organism or a part of an organism to react in any way. Internal and external stimuli are both possible.

Fig: Types of Receptors

Sensory receptors are nerve cells that can respond to a specific type of stimulus. It develops a nerve impulse to detect changes in the internal or external surroundings. The major types of  sensory receptors are following:

1. Mechanoreceptors: Mechanoreceptors are sensory organs that respond to forces and movements. They can be present in the skin to detect touch and in the ear to detect hearing and balance.
2. Thermoreceptors: Thermoreceptors are heat-sensitive sensors found in the skin—the signal fluctuations in temperature.
3. Photoreceptors: Photoreceptors are light-sensitive cells. These receptors are situated in the retina of the eye and are known as rods and cones.
4. Chemoreceptors: Chemoreceptors are chemical substance-responsive sensory receptors. Chemoreceptors include olfactory receptors in the nose that help with smell detection and gustatory receptors on the tongue that help with taste detection.

Sensory Organs

The sensory organs provide information about the body and its surroundings. Apart from five major sense organs: eye, ear, nose, tongue, and skin, we also have somatosensation, or general senses, which respond to stimuli such as temperature, pain, pressure, and vibration. Somato-sensation includes vestibular sensation and proprioception.

Vestibular System: The vestibular system is a sensory system in the human body that helps to provide a sense of balance and spatial orientation by coordinating movement and equilibrium. It comprises two parts: semicircular canals, which specify rotational movements, and otoliths, which determine linear accelerations. It also offers a sense of balance and information about body location, allowing quick movements.

Proprioceptive System: The proprioceptive system is a sensory system that allows a person to control force and pressure. Muscles and joints are where this system is found. It links sensory receptors and the neurological system, allowing information to flow between the two. The skin, joints, and muscles of the human body all have sensory receptors.

Sensory Pathway

The sensory pathway begins with the reception of the sensory signal, triggered in the presence of the stimulus. The pathway can be explained by the following steps:

Fig: Sensory Pathway

1. Sensory Reception: Sensory receptors present in sense organs respond to a stimulus. Although stimuli might be combined in processing areas of the brain, a specific receptor will only be triggered by its individual stimulus. The rate of action potentials produced by the sensory receptor is frequently used to encode the intensity of a stimulus.
2. Sensory Transduction: Sensory signals are translated to electrical impulses when sensory neuron membranes depolarise in response to a stimulus, causing gated ion channels to open, resulting in the membrane potential to cross its threshold. This entire process is called sensory transduction.
3. Sensory Transmission: Translated signals are transmitted to the related part of the brain via axons. Sensory neurons carry messages in the form of an electrical impulse to the brain.
4. Perception: Perception of the senses is a vital aspect of grasping the world around us. There are five stages of perception: stimulation, organisation, interpretation, memory, and recall. Sensory perception enables us to gather external data and, ideally, make sense of it. Although perception depends upon sensory reception and processing, various factors may affect it, such as motivations, emotions, biases, interests, expectations, cultural interpretation, etc. For example; in the case of the eyes, the purpose of perception is to determine the distal stimulus (an object kept at a distance) from information about the proximal stimuli (image formed on the retina). Perception tells us what the world out there “truly” looks like using one’s imagination.

Factors Affecting Sensory Reception and Processing

Sensory reception and processing are affected by four major components:

1. The type of stimulus,
2. The stimulus location within the receptive field,
3. The duration, and
4. The intensity of the stimulus.

Sensory adaptation: “Sensory adaptation” refers to a decrease in sensitivity to a stimulus following prolonged exposure to it. We are able to transfer our attention to other things in our environment rather than focusing on one overpowering sensation because frequent exposure to a sensory stimulation diminishes our sensitivity to it.

Example of Sensory Reception in Different Sensory Organs

1. Eyes: A photoreceptor organ.
Parts of Eyes: The wall of the eyeball is composed of three layers, i.e., scleroid, choroid and retina. The cornea is an exterior, transparent layer. The lens is a biconvex, a circular, living, multicellular, and non-vascularised organ with the power of accommodation. The retina is the sensory coating of the human eye with two kinds of cells: rods and cones. The macula lutea is a yellow patch on the retina’s visual axis. There are just cones in this (rods absent). The fovea centralis, or central concave point of the yellow spot, provides the clearest vision. The blind spot is the point where the optic nerve emerges. Since both rods and cones are missing at this point, there is no vision.

Mechanism of Action: The photoreceptors-rods and cones on the retina receive this light. The pigments in the rods and cones are activated by the absorbed light. In the membranes of the vesicles, the light is then transformed into action potentials. The impulses travel from the bipolar nerve cells to the ganglions, then to the optic nerves, and finally to the brain. The information is inverted, processed, and we can perceive the image.

Fig: Sensory System of Eye

2. Ears: It is a statoacoustic organ
Parts of Ear: Human ear has three parts –
1. External ear – Pinna + Auditory meatus
2. Middle ear – Tympanic cavity + ear ossicles
3. Internal ear – Vestibular apparatus + cochlea.

The Vestibular Apparatus and the Cochlea are the two most prominent parts of the internal ear. Vestibular Apparatus is a sensory element of the body that helps in balance. Hearing is controlled by the cochlea, which is a sensory structure. The ‘hair cells’ of the cochlea are attached to the basilar membrane to constitute the ‘Organ of Corti,’ the final sensory element for hearing.

Fig: Sensory System of Ear

Mechanism of Action: Pinna catches and amplifies sound waves, which are subsequently transmitted to the eardrum via the auditory canal. The vibrations from the eardrum reach the middle ear’s ear ossicles, which then transmit to the cochlea. The fluid in the cochlear canals vibrates as a result of this. The movement of hair cells in the organ of Corti in the cochlea is triggered by vibrations of fluid in the cochlear canals. Hair cell movement is translated into a nerve signal. Hearing is the consequence of nerve signals being transferred to the brain via the auditory nerve.

3. Nose: The nose contains the olfactory receptors or smell receptors. 
Part of Nose: The receptor of smell is present in the olfactory epithelium of the inner lining of the nose. The olfactory epithelium contains bipolar sensory neurons with dendrites extended from the apical surface into the mucus lining. The sensory neurons constitute the olfactory tract which connects to the olfactory bulb. The olfactory bulb is an extension of the limbic system. 

Mechanism of Action: The receptor present in the olfactory epithelium receives a stimulus and carry it to the olfactory bulb. Axons split from the olfactory bulb to carry the electrical impulse to the cerebral cortex, where the information is perceived.

Fig: Sensory System of Nose

4. Tongue: It has taste receptors.
Parts of Tongue: The tongue epithelium contains papillae and taste buds. Taste buds aid in the perception of flavour. They have a large bottom and are lined with squamous epithelial tissue.

Mechanism of Action: Taste is controlled by the glossopharyngeal nerve and the chorda tympani branch of the facial nerve.

Fig: Sensory Part of Tongue

5. Skin: Skin contains various somatosensory receptors along with mechanoreceptors.  Skin can sense heat, cold, smoothness, roughness, pressure, tickle, itch, pain, vibrations, and much more. A person is capable of feeling all of these simultaneously due to coordination between sensory receptors and neurons. The impulse travels along sensory neurons that make up bundled fibres that connect to neurons in the spinal cord. Then signals move to the thalamus, which relays information to the rest of the brain.

Fig: Different Kind of Receptors Present in Skin

Summary

A stimulus, in scientific terms, is anything that causes an organism or a part of an organism to react in any way is referred to as a stimulus. Stimuli can be internal or external. Sensory receptors are nerve cells that can respond to a specific type of stimulus. It develops a nerve impulse to detect changes in the interior or exterior surroundings. Mechanoreceptors, thermoreceptors, photoreceptors, and chemoreceptors are major kinds of receptors. Apart from five major sense organs: eye, ear, nose, tongue, and skin, we also have somatosensation, or general senses, which respond to stimuli such as temperature, pain, pressure, and vibration.

Somato-sensation includes vestibular sensation and proprioception. The sensory pathway begins with the reception of the sensory signals, triggered in the presence of the stimulus. It starts with the reception of stimulus, then sensory transduction or translation of sensory signals into an electrical impulse, Sensory transmission occurs after sensory transduction, and finally, the brain perceives the information. Perception depends on various factors like emotion, culture etc., along with sensory signals.

Frequently Asked Questions (FAQs)

Q.1. Where is sensory information processed in the brain?
Ans: Thalamus is the main region of the brain where sensory processing occurs. 

Q.2. What are the five sensory processes?
Ans: Vision, audition, tactile stimulation, olfaction, and gustation.

Q.3. What are the three steps of processing the senses?
Ans: Reception, transduction and perception are the three steps of processing the senses.

Q.4. What factors influence perception?
Ans: Perception is influenced by various factors such as motivation, emotion, biases, contains, culture etc.

Q.5. What are sensory issues?
Ans: Sensory issues are clinical conditions where a person or a child faces difficulty receiving and responding to information from their senses. 

Learn Everything About Neuron Here

We hope this detailed article on Sensory Reception and Processing is helpful to you. If you have any queries, drop a comment below, and we will get back to you.