1.2 | Stimuli and Responses in Humans

Humans face constant changes in surroundings. These changes are called stimuli.

Examples of stimuli include light, sound and chemical substances.

Humans use their sensory organs to detect stimuli.

Humans have five sensory organs: eyes, ears, nose, skin and tongue

Eye

The sensory organs of vision

Stimulus - Light

There are 2 photoreseptors: -

Rod cells are sensitive to different light intensities including faint light but are not sensitive to the colours of light.

Cone cells are sensitive to light colors in bright conditions. There are three different types of cone cells, where each is sensitive to red light, green light and blue light.

How do we see?

ear

  • Sensory organ for hearing

  • Stimulus - Sound (Vibration)

  • Structure of ear

Mechanism of Hearing

nose

  • Sensory organ for smell

  • Stimulus - Chemical Substances (present in the air)

  • Sensory cells for smell are tiny and covered with a layer of mucus.

  • Chemical substances in the air will dissolve in this layer of mucus and stimulate the cells to produce nerve impulses.

  • The nerve impulses are then sent to the brain to be interpreted to determine the type of smell.

tongue

  • Sensory organ for taste

  • Stimulus - Chemical Substances (food and drink)

  • Taste receptors can detect five types of basic tastes which are sweet, salty, sour, bitter and umami.

skin

  • Sensory organ for touch

  • Stimulus - Touch, pain, heat, cold and pressure

Sensitivity of the Skin on Different Parts of the Body towards Stimuli

  • The sensitivity of skin towards stimuli depends on:-

  1. the number of receptors

  2. the thickness of the skin epidermis.

  • Areas that are very sensitive to touch - tip of finger, tongue, nose and lips.

  • Areas that are less sensitive to touch - elbow, the sole of the foot and the back of the body.

Different Areas of the Tongue are More Sensitive to Specific Taste

  • Areas of the tongue are sensitive to all five tastes.

  • However, different areas of the tongue have different sensitivities towards specific taste.

Combination of the Sense of Taste and the Sense of Smell

  • chemical substances in food which dissolve in saliva and stimulate the taste buds, chemical substances in food also evaporate to form vapour which enters the nasal cavity and stimulates the smell sensory cells.

  • The combination of sense of taste and sense of smell causes food to taste better.

limitation of Senses, Defect in Sensory Organs

  • An optical illusion occurs when an object is viewed differently from the actual situation.

  • Blind spots are images that appear on blind spots.

  • Short-sightedness - image in front of retina, thick lens, long ball

how to fix → concave lens

  • Long-sightedness - images behind the retina, thin lenses, short balls

how to fix → convex lens

  • Astigmatism - cornea or uneven lens

how to fix → cylinder lenses

  • Human hearing limit → 20 Hz - 20 000 Hz.

  • Defects of Hearing - infection, injury, age and long exposure to loud sounds.

how to fix → hearing aids, implants & surgery.

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