Senses

The Senses

Sensory Receptors

  • Activation: Sensory receptors are activated by external stimuli (e.g., touch, pressure).
  • Types of Receptors:
    • Exteroceptors: Detect stimuli from outside the body.
    • Interoceptors: Detect stimuli from within the body (internal conditions).
    • Proprioceptors: Detect changes in stretch in joint tissues.

Classification of Sensory Receptors

  • Mechanoreceptors: Respond to touch, pressure, vibration, itch; involved in hearing and equilibrium.
  • Thermoreceptors: Respond to changes in temperature.
  • Photoreceptors: Respond to light stimuli; critical for vision.
  • Chemoreceptors: Respond to changes in blood chemistry (e.g., CO2 levels, pH, Ca2+ concentration).
  • Nociceptors: Respond to painful stimuli (e.g., extreme temperatures, excessive pressure).

Key Concepts

  • Sensation: Awareness of changes in the external environment.
  • Perception: Conscious interpretation of sensory stimuli.
  • Adaptation: Decreased sensitivity to constant stimuli (e.g., not noticing a smell after prolonged exposure).

Olfaction (Sense of Smell)

  • Mechanism: Detects chemicals dissolved in nasal mucus via Olfactory Nerve (C.N. I).
  • Chemoreceptors: Activated by odorants in the air.

Gustation (Sense of Taste)

  • Mechanism: Chemoreceptors in taste buds activated by chemicals in saliva.
  • Taste Qualities: Sweet, salty, bitter, sour, umami (savory).

Vision

  • Receptors: 70% of all sensory receptors are in the eye.
  • Retinal Structure:
    • Pigmented Layer: Absorbs light, prevents scattering.
    • Neural Layer: Contains photoreceptors that transduce light into nerve signals.
Photoreceptors in Vision
  • Rods: Activated by dim light, provide low acuity, non-color peripheral vision.
  • Cones: Activated by bright light, provide high acuity color vision.
Retinal Structures
  • Optic Disc: Blind spot due to lack of photoreceptors; site where optic nerve arises.
  • Macula Lutea: Contains fovea centralis, location of most acute vision (almost entirely cones).
  • Peripheral Retina: Mostly rods, activated in dim light, less acute vision.

Pathway of Light

  • Light Path: Light enters cornea → aqueous humor → lens → vitreous humor → neural layer of retina → photoreceptors.
  • Refraction: Light is refracted at the cornea and lens.
  • Pupil Control:
    • Sympathetic Division: Dilates pupil, flattens lens for distant vision.
    • Parasympathetic Division: Constricts pupil, relaxes lens for near vision.

Hearing

  • Ear Structures:
    • Outer Ear: Ear canal to tympanic membrane.
    • Middle Ear: Contains ossicles that conduct vibrations.
    • Inner Ear: Contains cochlea for hearing, vestibule and semicircular canals for equilibrium.

Cochlea and Sound Transduction

  • Organ of Corti: Site for transduction of sound waves to sensory signals.
  • Hearing Process:
    1. Sound waves vibrate tympanic membrane.
    2. Vibrations carried by ossicles to oval window.
    3. Oval window movements create fluid waves in cochlear duct.
    4. Waves move tectorial membrane, activating mechanoreceptors.

Equilibrium

  • Vestibular Apparatus: Composed of utricle, saccule, and semicircular canals.
  • Utricle and Saccule: Detect static equilibrium and linear acceleration.
  • Semicircular Canals: Involved with dynamic equilibrium (angular acceleration).
  • Mechanosignal Transduction: Movement of hair cells activates mechanoreceptors, sending signals through the Auditory Nerve (C.N. VIII).