GCSE level School Biology revision notes: Sense organs - with particular reference to humans

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GCSE level Biology exam revision notes on our senses

Function of sense organs - with particular reference to humans

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[Key points and learning objectives for this page, after the main body of notes]

See also detailed notes on the nervous system  and brain function

Introduction

What do we mean by a sensory organ?

What are your five principal sense organs?

You have five 'traditional' different sense organs, namely the ears, eyes, nose, skin and tongue which contain receptors (groups of specialised cells) that are sensitive to particular stimuli.

Sense organs as groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals.

detect signals, brain process - response

The ear sense organs detect sound

The human ear detects sound waves that enter the ear canal and cause the eardrum to vibrate.

Three small bones transmit these vibrations to the cochlea which generate nerve signals.

These nerve impulses (electrical signals) pass along the auditory nerve to the brain, where they are interpreted as sound.

Your ear organ can detect and distinguish a wide range of frequencies e.g. 20 to 20 000 kHz and a wide range of amplitude (loudness).

Sound waves properties, uses of sound, ultrasound, infrasound (GCSE physics notes, no ear diagram)

The eye sense organs detect visible light

The human eye detects visible light frequencies/wavelengths - that's why these are referred to a s 'visible' light or the 'visible spectrum'. The eye and associated optics are described in detail on the following two pages.

The eye - structure and function - correction of vision defects (GCSE biology notes)

Optics - types of lens and uses, experiments, ray diagrams, correction of eye defects (GCSE physics notes)

The nose sense organ detects chemical odours ('smells')

The inner roof of nose houses the olfactory system whose receptor cells detect chemicals in the air - 'your sense of smell'.

These olfactory cells respond to different chemicals and you perceive the different generated nerve signals as a different smells.

The odours may be aesthetically pleasing or unpleasant to your sense of smell.

Animals can emit and detect very small quantities of molecules called pheromones - often for sexual attraction strategies.

The jury is out on whether this applies to human beings other than specially designed perfumes, which perform the same role sometimes!

A pheromone is defined as a secreted or excreted chemical substance that triggers a social response in members of the same species.

Pheromone molecules are capable of acting like hormones outside the body of the secreting individual, to affect the behaviour of the receiving individuals of the same species.

You skin senses can detect physical contact (touch), temperature ('hot and cold' surfaces) and chemicals

Your skin has a variety of receptor cells that respond to:

(i) A physical contact with the skin, from a weak physically touching effect to detecting some high pressure causing pain.

(ii) A change in temperature on the skin surface e.g. detecting whether the object is touched is cold or hot including cold air or warm air and even the warming effect of infrared radiation from a fire or the sun.

(iii) The skin responds to irritating chemicals that cause inflammation e.g. acids or alkalis disrupt the bodies skin chemistry and the immune system responds accordingly.

The tongue - detects the taste of different substances (different chemical)

Receptor cells on your tongue surface respond to different chemical substances and the nerve signals generated are perceived as 'taste', which may be pleasant or unpleasant.

How many senses do we really have?

When you are taught about the human body in school, you are usually told that we have five senses: vision, hearing, touch, smell, and taste described above. While these are the obvious ‘main’ senses, some neuroscientists have suggested that we have between 22 and 33 different senses - there is a lot more to senses than you think!

As the World Economic Forum explains, our lesser known senses include:

Equilibrioception – a sense of balance, important not to topple over!

Proprioception – a sense of body position and movement, e.g. while doing physical tasks.

Kinaesthesia – a sense of movement - by your volition or being 'pushed' in some way.

Thermoception – a sense and perception of temperature (see above), this could be in contact with a cold/hot object or feeling the effects of infrared radiation absorbed by the skin.

Nociception – the ability to feel pain (see above), part of your complex nervous system sending 'warning' messages to the brain.

Chronoception – time perception, e.g. the passage of time and how long did an event take.

For more on our senses see

https://learn.genetics.utah.edu/content/senses/twentysenses/

and

https://learn.genetics.utah.edu/content/senses/

See also detailed notes on the nervous system

and brain function

Key points - our sense organs

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10 biology).

Human Sense Organs – Biology Revision Notes

Humans have five key sense organs that allow us to detect and respond to stimuli in our environment. Each sense organ contains receptor cells that are sensitive to specific types of stimuli.

1. Eye (Sight)

Stimulus: Light
Receptors: Light-sensitive cells (rods and cones) in the retina

Key Structures & Functions:

  • Cornea: Transparent front part; refracts (bends) light.

  • Iris: Pigmented; controls the size of the pupil to regulate light entry.

  • Lens: Changes shape (accommodation) to focus light onto the retina.

  • Retina: Contains photoreceptors:

    • Rods – sensitive to dim light, black and white vision

    • Cones – detect color, require bright light

  • Optic Nerve: Carries electrical impulses from retina to the brain.

Important Concept:

  • Pupil Reflex: In bright light, the pupil constricts; in dim light, it dilates. This protects the retina.

2. Ear (Hearing and Balance)

Stimuli: Sound waves and changes in position

Parts for Hearing:

  • Pinna: Collects sound waves.

  • Ear canal: Directs sound to the eardrum.

  • Eardrum: Vibrates when sound hits it.

  • Ossicles (tiny bones): Amplify vibrations.

  • Cochlea: Converts sound vibrations into electrical impulses (contains sound receptors).

  • Auditory nerve: Sends impulses to brain.

Parts for Balance:

  • Semicircular canals: Detect movement and rotation of the head.

3. Nose (Smell)

Stimulus: Chemical molecules in the air
Receptors: Olfactory receptors in the nasal cavity

  • Located in the lining of the nose.

  • Information is sent to the olfactory bulb and then to the brain.

  • Closely linked to the sense of taste.

4. Tongue (Taste)

Stimulus: Chemicals in food
Receptors: Taste buds on the tongue

There are five basic tastes:

  • Sweet

  • Salty

  • Sour

  • Bitter

  • Umami (savoury)

Each taste bud contains chemoreceptors that send signals to the brain.

5. Skin (Touch, Temperature, Pain, Pressure)

Stimuli: Temperature, pressure, pain, touch
Receptors: Various receptors located in the dermis of the skin

Types of receptors:

  • Mechanoreceptors: Detect pressure and touch

  • Thermoreceptors: Detect changes in temperature

  • Nociceptors: Detect pain

Integration with the Nervous System:

All sense organs work closely with the central nervous system.

Once receptors detect a stimulus:

  1. They generate an electrical impulse.

  2. This travels along sensory neurons to the brain or spinal cord.

  3. The brain interprets the information, and a response may follow via motor neurons.

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