Senses

Special Senses

Learning Outcome
  • Explain the structure and functions of the special senses.
Objectives
  1. Describe the different categories of sensory receptors and their function.
  2. Explain how chemicals are sensed by the human body.
  3. Describe the structure and function of the human eye and its accessory structures.
  4. Discuss the structure and function of the human ear.
Overview of Senses
  • Types of Senses:
    • General Senses: Touch (detected by skin)
    • Special Senses:
    • Sight (eyes)
    • Hearing & Balance (ears)
    • Taste (tongue)
    • Smell (nasal cavity)
  • Sensory receptors respond to stimuli and convey information to the Central Nervous System (CNS).
Sensory Receptors
  • Categories of Sensory Receptors:
    • Mechanoreceptors: Detect touch, pressure, vibration, and stretch.
    • Thermoreceptors: Detect temperature changes.
    • Nociceptors: Detect harmful stimuli and convert them to pain sensations.
    • Photoreceptors: Detect & respond to light.
    • Chemoreceptors: Detect & respond to certain chemicals (odors & flavors).
Touch
  • Receptors Involved:
    • Mechanoreceptors, Thermoreceptors, Nociceptors.
  • Sensation Types: Pain, temperature, vibration, stretch, and pressure.
  • Locations: Highly concentrated in tongue, lips, face, palms of hands, and soles of feet.
Pathway of Touch Stimulus
  1. Sensory receptors in skin
  2. Sensory neurons (spinal nerves)
  3. Spinal cord
  4. Medulla oblongata
  5. Pons
  6. Midbrain
  7. Thalamus
  8. Parietal lobe of Cerebrum
Taste (Gustation)
  • Receptors Involved: Chemoreceptors located within taste pores on tongue surface.
  • Nerves: Facial and Glossopharyngeal nerves.
  • Types of Tastes:
    • Sweet
    • Sour
    • Salty
    • Bitter
    • Umami (savoury)
Pathway of Taste Stimulus
  1. Chemoreceptors in tongue
  2. Facial & Glossopharyngeal nerves
  3. Brainstem
  4. Thalamus
  5. Parietal (taste) & Frontal (memory of taste) lobes of Cerebrum
Smell (Olfaction)
  • Odorants: Airborne chemicals detected by chemoreceptors in the nasal mucosa (roof of nasal cavity).
  • Nerve: Olfactory nerve (CN I).
Pathway of Smell Stimulus
  1. Chemoreceptors in nasal cavity
  2. Olfactory nerve fibers
  3. Olfactory bulb & tract
  4. Thalamus & Hypothalamus (memory of smells)
  5. Temporal lobes of Cerebrum (smell itself)
Vision
  • The Eye: Organ of vision, responsible for sensing light.
  • Brain Area: Occipital lobe contains primary and association visual areas.
Eyeball Structure
  • Components:
    • Sclera: Provides shape & muscle attachment site.
    • Cornea: Transparent, allowing light to enter.
    • Chambers: Anterior & Posterior chambers contain aqueous humor.
    • Iris: Contractile diaphragm controls light entry.
    • Pupil: Regulates light entry.
    • Lens: Bends light to focus on retina.
    • Vitreous Body: Fills vitreous chamber, maintaining eye shape.
    • Retina: Contains light-sensitive receptors (rods for dim light, cones for color).
    • Choroid: Pigmented vascular layer preventing light reflection.
Visual Pathway
  • Light Pathway:
    1. Cornea
    2. Anterior chamber
    3. Pupil
    4. Posterior chamber
    5. Lens
    6. Vitreous chamber
    7. Retina
  • Optic Nerve Pathway:
    1. Optic nerve
    2. Optic chiasm
    3. Optic tract
    4. Lateral geniculate nucleus of thalamus
    5. Optic radiation
    6. Primary visual cortex in occipital lobe of cerebrum
Extra-Ocular Muscles
  • Control eyelid movement and eye direction.
  • Muscle Groups:
    • Palpebral Muscles: Control eyelid closure.
    • Extra-Ocular Muscles: Superior, inferior, lateral, medial rectus; superior and inferior oblique.
  • Cranial Nerve Supply: Oculomotor, Trochlear, and Abducens nerves.
Lacrimal Apparatus
  • Function: Production and drainage of lacrimal fluid (tears).
  • Components:
    • Lacrimal Gland: Produces tears.
    • Lacrimal Ducts: Distribute tears over the eye.
    • Lacrimal Sac & Nasolacrimal Duct: Drain tears into the nasal cavity.
Vision Problems
  • Myopia (Nearsightedness): Image converges in front of retina; requires concave lens.
  • Hyperopia (Farsightedness): Image converges behind the retina; requires convex lens.
  • Presbyopia: Loss of elasticity in lens with age, affecting close-up focus.
Hearing & Balance
  • Hearing: Ability to respond to sound waves.
  • Balance: Maintains body position by monitoring head position/movement.
Ear Structure
  • Parts:
    • External Ear: Conducts sound to tympanic membrane.
    • Middle Ear: Contains ossicles (malleus, incus, stapes) and Eustachian tube.
    • Internal Ear: Contains cochlea and vestibular structures for hearing and balance.
Auditory Pathway
  1. Ear canal
  2. Eardrum (Tympanic membrane)
  3. Malleus
  4. Incus
  5. Stapes
  6. Oval window
  7. Vestibule
  8. Cochlea (Organ of Corti)
  9. Vestibulocochlear nerve
  10. Temporal lobe of cerebrum
Organs of Balance
  • Vestibular Apparatus: Detects head movements and orientation.
  • Components: Semicircular ducts (anterior, posterior, lateral), utricle, saccule.
  • Balance Functions:
    • Dynamic Equilibrium: Monitors moving head.
    • Static Equilibrium: Responds to static position and linear acceleration.