Special senses

Author: Michelle Vishu, MBBS, MD (DVL) • Reviewer: Declan Tempany, BSc (Hons)
Last reviewed: November 04, 2024
Reading time: 10 minutes

The special senses are those which have specific complex organs, like the eyes, ears, nose and tongue, dedicated to detecting certain types of stimuli. There are five such special senses: vision, hearing, equilibrium, smell and taste.

The special sense organs are concentrated in the head region, and thus use cranial nerves to communicate information to the central nervous system. This is in contrast to general somatic senses, like touch, which employ receptors distributed all over the body, using both cranial and spinal nerves to convey information to the brain and spinal cord.

This article will provide you with an overview of the anatomy and physiology of the five special senses.

Key facts about special senses
Vision (sight)	Organ: Eye Receptors: Rod cells and cone cells Receptor type: Photoreceptor Receptor location: Retina Stimulus: Light Cranial nerve: Optic nerve (II)
Audition (hearing)	Organ: Ear Receptors: Cochlear hair cells Receptor type: Mechanoreceptor Receptor location: Cochlea Stimulus: Sound Cranial nerve: Vestibulocochlear nerve (VIII)
Equilibrium (balance)	Organ: Ear Receptors: Vestibular hair cells Receptor type: Mechanoreceptor Receptor location: Utricle, saccule and semicircular ducts Stimulus: Linear and rotational acceleration Cranial nerve: Vestibulocochlear nerve (VIII)
Olfaction (smell)	Organ: Nose Receptors: Olfactory sensory neurons Receptor type: Chemoreceptor Receptor location: Olfactory epithelium Stimulus: Odor Cranial nerve: Olfactory nerve (I)
Gustation (taste)	Organ: Tongue Receptors: Gustatory sensory epithelial cells (taste receptor cells) Receptor type: Chemoreceptor Receptor location: Taste bud Stimulus: Tastant Cranial nerves: Facial nerve (VII), glossopharyngeal nerve (IX) and vagus nerve (X)

Contents

Vision (sight)
Audition (hearing)
Equilibrium (balance)
Olfaction (smell)
Gustation (taste)
Sources
References
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Vision (sight)

Our eyes enable us to see the world around us by transducing light energy into electrical signals that can be interpreted by the brain.

Light rays that enter the eye get refracted by structures such as the cornea and the lens, in order to come to focus on the retina. For focussing on nearby objects, the eye is capable of adjusting the curvature of the lens by a process called accommodation. For clear vision, the rays must be brought to focus exactly on the retina.

On reaching the retina, light gets absorbed by photoreceptors. These are the rod cells and cone cells, which serve as the primary sensory receptors of the eye. The rod cells are sensitive to dim light, useful for night vision and peripheral vision. The cone cells, on the other hand, function better in bright light, providing high visual acuity and color vision. Cone cells are concentrated at the central fovea, making it the region of highest visual acuity.

Photoreceptors are responsible for phototransduction, converting light into electrical impulses. They synapse with bipolar cells, which in turn are capable of generating action potentials in the retinal ganglion cells. The axons of these cells form the optic nerve (CN II), which carries visual information along the visual pathway, involving the optic chiasm, the optic tract and optic radiation, to the primary visual cortex, located in the occipital lobe of the cerebrum.

Take a deeper look at the eye and optic nerve with these study units.

Audition (hearing)

Hearing is how we experience sound. Humans have a hearing range of 20 - 20,000 Hz; the frequency of sound waves is sensed as pitch. On the other hand, the amplitude (or size) of sound waves is perceived as loudness.

Sound undergoes multiple steps of transduction in order to be interpreted by the brain, starting at the external ear. The auricle of the external ear captures sound waves and conveys them through the external acoustic meatus to the tympanic membrane. The vibrations of the tympanic membrane are conducted through the ossicles of the middle ear: the malleus, incus and stapes. These three little bones amplify sound, such that when vibrations get transmitted from the footplate of the stapes to the vestibular (oval) window, they generate pressure waves in the fluid of the cochlea located in the internal ear.

The spiral organ (organ of Corti), containing cochlear hair cells, is situated within the cochlea. These hair cells are the auditory sensory receptors. The fluid waves in the cochlea can bend stereocilia of the hair cells; thus, they function as mechanoreceptors. They can release neurotransmitters, which generate action potentials in the afferent cochlear neurons, whose axons form the cochlear division of the vestibulocochlear nerve (CN VIII).

This nerve carries sound information along the auditory pathway to reach the primary auditory cortex located in the temporal lobe after multiple synapses along the way. It also has connections to the limbic system, building associations between sound and emotions.

Equilibrium (balance)

The internal ear is also concerned with maintaining balance, using the vestibular apparatus for this purpose. This system consists of the utricle, saccule and semicircular ducts.

The utricle and saccule are concerned with static equilibrium. They contain maculae, which are sensory receptors detecting changes in head position and linear acceleration (e.g., when a car speeds up or slows down, when an elevator goes up or down). The maculae consist of vestibular hair cells with stereocilia embedded in a gelatinous statoconial (otolith) membrane with calcium carbonate statoconia (otolith crystals). In response to gravitational forces, the stereocilia of the hair cells bend, functioning as mechanoreceptors.

The ampullary crests of the semicircular ducts (anterior, posterior and lateral) also have vestibular hair cells with stereocilia embedded in a gelatinous cupula, that work like mechanoreceptors. However, they are concerned with dynamic equilibrium, detecting angular or rotational acceleration (e.g., spinning around while dancing). The release of neurotransmitters by these hair cells changes the action potential firing frequency in the afferent neuron, which is part of the vestibular division of the vestibulocochlear nerve (CN VIII).

This nerve carries information regarding equilibrium to the vestibular nuclei in the medulla oblongata, which in turn has connections with the cerebral cortex (conscious awareness of head position), cerebellum, spinal cord (coordinate skeletal muscle movements) and cranial nerve nuclei (coordinate eye movements).

With the following study units, you can balance your physiology knowledge with some anatomy of the internal ear.

Olfaction (smell)

Olfaction is the ability of the nose to detect odorants in the air, as the sensation of smell. The chemicals in odorants are sensed by specialized neurons called olfactory sensory neurons, which serve as chemoreceptors. They are located in the olfactory epithelium in the superior region of the nasal cavity.

The olfactory sensory neurons are bipolar neurons having modified dendritic ends with non-motile olfactory cilia. Odorants dissolved in mucus of the nasal cavity bind to receptor proteins in the cilia. Depolarization of the cells can generate action potentials, which get conducted along axon bundles forming the olfactory nerve (CN I).

The olfactory nerves synapse in glomeruli of the olfactory bulb and second-order neurons travel along the olfactory tract. Eventually they reach the primary olfactory cortex located in the temporal lobe, where the odor can be identified.

The olfactory cortex also has connections with the limbic system and hypothalamus, which can help create associations with emotions and memory. This is how familiar smells can trigger fond childhood memories.

The olfactory nerve is the first of 12 cranial nerves. Refresh and solidify your knowledge of all 12 nerves with our cranial nerve quizzes and labeling exercises

Gustation (taste)

Gustation is the sense of taste, most commonly associated with the tongue. The tongue is covered with raised papillae containing taste buds. These are the vallate (circumvallate), fungiform and foliate papillae. Each taste bud contains gustatory sensory epithelial cells (taste receptor cells), which are specialized cells, functioning as chemoreceptors for specific chemicals in food.

There are five primary taste modalities: sour, salty ,sweet, bitter and umami. Depending on the modality, the gustatory sensory epithelial cells respond either using G proteins or ion channels, eventually releasing a neurotransmitter that can act on the afferent neuron.

The facial (CN VII), glossopharyngeal (CN IX) and vagus (CN X) nerves carry taste sensation from different regions of the tongue, and all three cranial nerves synapse in the solitary nucleus of the medulla oblongata. Gustatory information ultimately reaches the gustatory cortex, located in the insula. However, there are also connections with the amygdaloid body and hypothalamus, associating taste with emotions. Some fibers also project to the frontal lobe, integrating multiple senses like smell and taste.

Thus our sensory perception of the world is a combination of multiple special sensory modalities, cleverly processed and integrated by our central nervous system.

Got a taste for more learning? Try out this quiz!

Sources

All content published on Kenhub is reviewed by medical and anatomy experts. The information we provide is grounded on academic literature and peer-reviewed research. Kenhub does not provide medical advice. You can learn more about our content creation and review standards by reading our content quality guidelines.

References

Hall, J. E., and Hall, M. E. (2021). Guyton and Hall textbook of medical physiology (14th ed.). Elsevier
Boron, W.F., & Boulpaep, E. L. (2017). Medical physiology (3rd ed.). Elsevier.
Silverthorn, D. U. (2019). Human physiology (8th ed.). Pearson Education.
Amerman, E. C. (2016). Human anatomy and physiology. Pearson Education
Betts, J. G., Desaix, P., Johnson, E., Johnson, J. E., Korol, O., Kruse, D., Poe, B., Wise, J. A., Womble, M., & Young, K. A. (2022). Anatomy and physiology (2nd ed.). Rice University. https://openstax.org/details/b...

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Special senses

Vision (sight)

Audition (hearing)

Equilibrium (balance)

Olfaction (smell)

Gustation (taste)

Sources

References

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