Stimulus and Response in Humans

1.2  Stimulus and Response in Humans



Sensory organs are organs that can detect stimuli

Humans have five sensory organs:

  • Skin (touch)

  • Nose (smell)

  • Tongue (taste)

  • Ears (hearing)

  • Eyes (Sight)

The function of eyes:

  • Focusing on objects near and far

  • Produces continuous images that are sent directly to the brain

  • As a sense of sight

The structure of eyes:


The function of each eyes structure:



The mechanism of vision:

  • Light from an object enters the eye through the pupil
  • Light is refracted by the cornea, aqueous humor, eye lens, and vitreous humor

  • Inverted and small images are formed on the retina

  • The retina produces nerve impulses and the optic nerve transmits them to the brain

  • The brain translates impulses and produces upright images

The function of ears:

  • To keep our balance and hearing

  • As a sense of smell

The structure of ears:


The function of each ears structure:


The structure of ears



Amplifies the vibration of sound and sends it to the oval window


Receives sound waves

Ear canal

Transmits sound waves to the eardrum

The eardrum

Vibrates when sound waves hit it

Semicircular canal

Controls body balance

The auditory nerve

Sends impulses from the cochlea to the brain


Converts sound vibrations into impulses

Oval window

Transmits sound vibrations from the middle ear to the inner ear

Eustachian tube

Balances the air pressure on both sides of the eardrum

The mechanism of hearing:

  • Sound waves enter the ear canal

  • The eardrum receives sound waves and vibrates

  • The vibrations are amplified by the ossicle bone and transmitted to the oval window

  • The oval windows vibrated

  • The cochlea converts vibrations into impulses

  • Impulses are carried by the auditory nerve to the brain

  • The brain interprets it as sound

The function of the nose:

  • Provides air for breathing

  • Filters and cleans foreign debris during respiration

  • As a sense of smell

The structure and mechanism of the nose:


The function of the tongue:

  • As a digestive organ with ease of movement of food during chewing and swallowing

  • As a sense of taste

The structure and mechanism of the tongue:

  • The surface of the tongue is covered by taste buds that contain many taste receptors and are scattered on the surface of the tongue

  • Taste receptors are sensitive to chemicals in food

  • The tongue has 5 types of taste receptors, namely sweet, sour, bitter, and umami (savory taste).

The function of the skin:

  • Protect us from microbes

  • Allows sensations of touch, temperature, and pressure

  • As a sense of touch

The structure of skin:


Skin sensitivity:

  • The concentration of the skin depends on the number of receptors present and the thickness of the epidermis

  • The more receptors, the more sensitive that part of the skin is

  • The thinner the epidermis, the more sensitive the skin is to stimuli

  • Blind people read Braille using their fingertips

Very sensitive parts of the skin

Part of the skin that is less sensitive

Fingertips, back of neck, lips, and earlobes

Elbows and knees

  • The sensory organs are interconnected with each other and can balance the human body on a single daily basis

The limit of sensory:

  • Limits our sensory ability to detect a stimulus

  • Caused by sensory organ defects or the aging process

The visual sensory limitations:

  • The eye cannot detect an image that falls on a blind spot

  • Human vision can also be affected due to farsightedness, nearsightedness, astigmatism, and presbyopia

The structure of the normal eye, farsightedness, and nearsightedness:


  • Farsightedness cannot see distant objects clearly because light from the object is focused in front of the retina

  • Requires a concave lens

  • Nearsightedness cannot see near objects clearly because light from the object focuses behind the retina

  • Requires a convex lens

The structure of the astigmatism eye:


  • Images are blurred and unclear because light from objects is separated and focused at different points on the retina

  • Requires a cylindrical lens or undergoes surgery

The structure of the presbyopia eye:


  • Not being able to see distant objects and close objects clearly is common among the elderly

  • Requires a dual focus lens to see near and far objects clearly

Hearing sensory limitations:

  • Humans can only hear between 20Hz to 20,000Hz

  • Deafness is caused by the ossicle bones joining together due to infection, the aging process or exposure to loud noise for a long period of time

  • Only surgery or hearing aids can help deaf people

Technology to enhance the ability of sensory organs:



Sensory organ capacity

Limitation of vision

Magnifying glass & microscope

Can see art/small objects

Binoculars & telescopes

Can see distant objects

Limitation of hearing


Amplifies the sound of the heartbeat


Amplifies the sound