What are Sensory Organs?

Sensory organs can be labeled as special sensory structures that permit sight, hearing, odor, and flavor. Sensory structures permitting proprioception, touch, thermal, and pain perception can be classified as more advanced sensory organs. The sensory neurons are trained to find out modifications in the external and internal conditions so that a person's body can react to that change. A stimulus is the first signal that is recognized by any sensory receptor of the body. Stimulus is an impulse generated when there is a change in the surroundings of a person. For example, a heated environment will alert the brain through the thermal sensory organs and generate a reflex accordingly.

Functions of Special Senses

The abilities of the five particular senses embody the capability to encounter certain stimuli and transduce that energy into an electrical impulse that the central nervous system can recognize.

Sight

It is the vision capacity of the eye(s) to interpret the environment by using light's visible spectrum, which is reflected by both living and non-living objects. The perception generated is called visual perception or vision. The visual system includes a series of actions- cornea and lens focusing the light onto the retina at the back of the eye.

Hearing

The auditory perception is the capability to detect a sound vibration in the surrounding. The change of pressure in the surrounding medium generates vibrations that the ears can perceive. The human ear has three main parts: the inner ear, middle ear, and outer ear. Sound can be perceived in all three states of matter that is gas, liquid and solid.

Smell

The olfaction is the chemoreception that perceives the smell and detects pheromones, food, and hazards. The nose is the sensory organ for this purpose. The odorants which produce smell bind to certain sites in the nasal cavity's olfactory receptors. Land animals usually have two separate systems for smell and taste, while water animals usually have a single system for smell and taste. 

Taste

When a substance in the mouth reacts chemically with the receptor cells for taste, the sensation of taste is produced. The taste buds are located in the oral cavity, that is, the tongue. Taste with olfaction determines the flavors of the food. The gustatory cortex is specifically responsible for the perception of taste.

Parts of the Eye

Orbit

The eye socket of the skull is called the orbit. The eye is cushioned into this orbit by certain pads of fat. The orbit has vessels, nerves, and certain muscles which enable the movement of the eye.

Conjunctiva

It is the thin transparent layer of tissues in front of the eye. It includes the sclera. The conjunctiva helps in keeping bacteria or any foreign material away from getting into the eye. It has blood vessels against the sclera.

Sclera

It is the white part of the eye. The sclera is leather-like tough tissue giving the eye its shape. The muscles surrounding the sclera cause the eye to look in different directions.

Cornea

The transparent clear layer in the front and center of the eye is the cornea. The cornea is in front of the iris. Its function is to focus the light when it enters the eye. The contact lenses rest on the cornea performing the same work.

Iris and pupil

The iris represents the colored portion of the eye. The iris regulates the amount of light that enters the retina of the eye. It has a central opening called the pupil. The pupil dilates and contracts when the iris tries to control the amount of light entering the eye.

Lens

They are located behind the iris and pupil. The lens and ciliary body adjust the fine focusing of light when it passes through the iris and pupil. They together focus the light on the retina.

Retina

It is the part that creates the image of the object one is seeing. The chemical reactions occur in the retina within layers of cells, generating electrical signals to be carried to the brain for perception.

Parts of the Ear

Anatomically, the ear is split into three crucial regions: external, middle, and internal.

External (outer) ear

The outer ear consists of the auricle, external auditory tube, and tympanic membrane.

• Auricle: The auricle, or pinna, is what most people call the "ear"- the shell surrounding the beginning of the auditory canal. It is the outer part of the ear.
• External auditory tube: The tube that connects the outer ear to the middle ear is the auditory tube.
• Tympanic membrane: This membrane is called the ear-drum. It separates the middle ear from the outer ear and enhances the ear's efficiency.

Middle ear

It has ossicles and Eustachian tube

• Ossicles: The three bones in the ossicle transmit the sound waves to the inner ear. These are the malleus, stapes and incus.
• Eustachian tube: It links the back of the throat to the middle ear. It helps in equalizing the pressure in the middle ear. The proper transfer of sound waves is possible at correct pressure conditions. This Eustachian tube is linked to the mucus in the throat and nose.

An oval window is present before the inner ear and at the end of the middle ear. It opens into the cochlea. The vibrations from the middle ear are transmitted to the oval window, which activates the hearing receptors due to fluid movement within the cochlea.

Inner ear

It has three parts:

• Cochlea: The nerves for hearing are present in the cochlea. The basilar membrane containing the auditory hair cells is also part of the cochlea. The hair cells are the sensory receptors for both the vestibular and auditory systems. The hair cell is another name for the cochlear sensory cell due to the hair-like cilia structure that projects from the apical end of the sensory cell. 
• Vestibule: The receptors for balance are present in the vestibule.
• Semicircular canals: The semicircular canal has receptors for balance. It is filled with fluid endolymph, which has motion sensors. The liquid inside this canal moves the tiny hairs lining each canal with the movement of the head, thereby maintaining the balance.

Olfactory Bulb

The sense of smell is carried out by the neural structure called the olfactory bulb. This structure is present in the forebrain of the vertebrate. The olfactory information is sent to the amygdala, then to the orbitofrontal cortex, and ultimately to the hippocampus. Thus, the different odors are processed and detected in the nasal cavity by this olfactory organ.

Cranial Nerves

Sensory cranial nerves send the smell, sight, sound signals, and vestibular signals. The 12 pairs of cranial nerves transmit different signals. Some of them are associated with special senses:

• The cranial nerve I is the olfactory nerve.
• The cranial nerve II is the optic nerve.
• The cranial nerves V, VII, VIII, IX, and X are also responsible for diverse functions like balance, hearing, and taste sensations.

Common Mistakes

• The parts of the ear are usually confused by the students.
• The taste perception needs both olfactory and taste receptors.

Context and Applications

Applications for the human anatomy of special sensory organs are:

• Bachelors of Science/Master of Science in Human Biology
• Bachelor of Medicine and Bachelor of Surgery (MBBS)

Related Concepts

1. The optic nerves
2. Taste buds
3. The nervous system