Lecture Notes: Stimuli and Responses in Humans

Introduction to Stimuli

• Stimuli: Changes in the surroundings; plural of stimulus.
• Examples: Light, sound, and chemical substances.
• Sensory organs: Detect stimuli using eyes, ears, nose, skin, and tongue.
  • Eyes: Sense of sight
  • Ears: Sense of hearing
  • Nose: Sense of smell
  • Tongue: Sense of taste
  • Skin: Sense of touch

The Eye

• External structures: Sclera, iris, pupil.
• Main function: Converts light into nerve impulses via the retina.
  • Photoreceptors:
    • Rod cells: Sensitive to light intensity, not color.
    • Cone cells: Sensitive to color under bright conditions; types include red, green, and blue.
• Mechanism of sight:
  • Light passes through cornea, aqueous humor, lens, vitreous humor, and hits the retina.
  • Light stimulates photoreceptors, producing nerve impulses sent to the brain.

The Ear

• Divided into three sections: Outer ear, middle ear, inner ear.
  • Outer ear: Pinna, ear canal.
  • Middle ear: Eardrum, ossicles, oval window, Eustachian tube.
  • Inner ear: Semicircular canals, cochlea, auditory nerves.
• Mechanism of Hearing:
  • Sound waves collected by pinna, channeled to ear canal, transformed to vibrations at eardrum.
  • Vibrations amplified by ossicles, transferred to cochlea, converted to nerve impulses.
  • Nerve impulses carried to brain by auditory nerves.

The Skin

• Largest sensory organ, divided into epidermis, dermis, and fat layer.
  • Receptors: Cold, pain, heat, touch, pressure.
• Sensitivity:
  • Depends on number of receptors and skin thickness.
  • Sensitive areas: Finger tips, tongue, nose, lips.
  • Less sensitive areas: Elbow, sole of foot, back.

The Nose

• Sensory organ for smell.
  • Contains 10 million sensory cells in nasal cavity.
  • Chemical substances dissolve in mucus, stimulate cells to produce nerve impulses sent to brain.

The Tongue

• Sensory organ for taste, covered in papillae with taste buds.
  • Detects: Sweet, salty, sour, bitter, umami.
  • Chemical substances dissolve in saliva, stimulate taste receptors, produce nerve impulses.
• Sensitivity:
  • Front: Sweet
  • Sides: Sour and salty
  • Back: Bitter
  • Center: Umami

Interaction Between Smell and Taste

• Taste and smell are interconnected, enhancing flavor perception.
• Illness (e.g., flu) can reduce flavor perception due to mucus blockage.

Limitations and Corrections

• Sight Limitations:
  • Cannot see tiny or distant objects, optical illusions, and blind spots.
  • Tools: Microscopes, binoculars, telescopes, ultrasound, x-ray machines.
• Sight Defects:
  • Short-sightedness (corrected with concave lens)
  • Long-sightedness (corrected with convex lens)
  • Astigmatism (corrected with cylindrical lenses)
• Hearing Limitations:
  • Frequency range: 20 to 20,000 Hz; varies with age.
  • Tools: Stethoscopes, loudspeakers.
• Hearing Defects:
  • Caused by infection, injury, aging, loud noise exposure.
  • Hearing aids and other technologies assist correction.

Conclusion

• Five senses are a gift; maintain healthy lifestyles and take precautions in risky environments.
• Engage with educational content for further understanding.