The Special Senses

Chapter 15: The Special Senses

1. Classification of Sensory Receptors

  • Types of Sensory Receptors
      - General Sensory Receptors: Related to general senses (discussed in Chapter 13).
      - Special Sensory Receptors: Related to special senses (discussed in Chapter 15).

  • Classification of ALL Receptors
      1. Special vs. General Receptors:
         - Special receptors are confined to the head; general receptors are distributed throughout the body.
      2. Exteroceptors vs. Interoceptors:
         - Exteroceptors are sensitive to stimuli outside the body; interoceptors monitor internal conditions.
      3. Classification by Type of Stimuli:
         - These classifications categorize receptors based on the types of stimuli they respond to.

2. Differences Between Special and General Sensory Receptors

  • Special Sensory Receptors
      - Located specifically in the head region.
      - Function through cranial nerves only.
      - Characterized by complex structures.
      - Responsible senses include smell, taste, sight, hearing, and equilibrium.

  • General Sensory Receptors
      - Found throughout the body including skin and internal organs.
      - Signals can travel through both cranial and spinal nerves.
      - Generally have simpler structures.

3. The Sense of Smell (Olfaction)

A. Olfactory Epithelium

  • Location: Found in the roof of the nasal cavity.

  • Function: Contains specialized epithelial cells for detecting chemicals (odorants) in the air.

  • Composition:
      - Contains three types of cells:
        - Basal (Stem) Cells: Regenerate into olfactory neurons.
        - Supporting Cells: Surround and support olfactory neurons.
        - Olfactory Neurons: Bipolar neurons with a lifespan of 30-60 days, possessing receptors on their dendrites located in olfactory cilia.

B. Olfactory Pathway

  • Olfactory Nerve (CN I):
       - Comprised of the axons of olfactory neurons.

  • Olfactory Bulb:
       - The synapse site for the first and second-order olfactory neurons in the olfactory pathway.

  • Olfactory Tract: Carries signals from the olfactory bulb to the brain.

  • Note: The olfactory pathway does NOT involve a synapse in the thalamus, which differentiates it from other sensory pathways.

4. The Sense of Taste (Gustation)

A. Anatomical Structures in Taste

  • Components:
      - Papillae: Types of small bumps on the tongue housing taste buds.
      - Types of Lingual Papillae:
        1. Fungiform Papillae:
           - Numerous with about 5 taste buds each; mushroom-shaped.
        2. Circumvallate Papillae:
           - Few in number (7-12) but each contains around 100 taste buds; arranged in an inverted V-shape.
        3. Filiform Papillae:
           - Most numerous, lacks taste buds; detects food texture and temperature.
        4. Foliate Papillae:
           - Located on the sides of the tongue; more prominent in children; contains taste buds that decline with age.

B. Gustatory Epithelium Composition

  • Taste Buds: Composed of three types of cells:
      1. Basal Cells: Stem cells that regenerate into gustatory cells.
      2. Supporting Cells: Support surrounding gustatory cells.
      3. Gustatory Cells: Specialized with a lifespan of 10-14 days, having taste receptors on microvilli/taste hairs.

C. Gustatory Microvilli and Sensory Neurons

  • Gustatory Microvilli:
       - Tiny projections on gustatory cells containing chemoreceptors activated by tastants dissolved in saliva.

  • Unipolar Sensory Neuron:
       - First-order neurons in the gustatory pathway located peripheral to the taste buds.

  • Cranial Nerves Involved:
      - VII (Facial), IX (Glossopharyngeal), and X (Vagus) carry taste fibers.

D. Taste Sensations

  • Five basic tastes:
      1. Sour
      2. Salty
      3. Bitter
      4. Sweet
      5. Umami

  • Flavor: Comprises taste (~20%) and smell (~80%).

  • Effect of Cold: Loss of smell can dampen the perception of taste.

5. The Sense of Sight (Vision)

A. Eye Structure

  • Accessory Structures:
      - Include eyebrows, eyelashes, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles.

  • Internal Structures:
      - The eyeball includes the lens, two cavities, and three tunics which aid in vision.

B. Functions of Accessory Structures

  • Eyebrows and Eyelashes: Prevent perspiration and particles from entering the eye.

  • Eyelids (Palpebrae): Protect from dryness and foreign matter.

  • Conjunctiva: Thin membrane covering the sclera and lining the eyelids. Its inflammation is known as conjunctivitis (pink eye).

C. Lacrimal Apparatus

  • Produces, distributes, and removes tears through the components:
      - Lacrimal Gland: Produces tears.
      - Lacrimal Ducts: Carry tears to the surface of the eye.
      - Nasolacrimal Duct: Drains tears into the nasal cavity.

D. Eye's Internal Structure - Cavities and Chambers

  1. Anterior Cavity: Contains aqueous humor and is subdivided into:
       - Anterior Chamber: Between the cornea and the iris.
       - Posterior Chamber: Between the iris and lens.

  2. Posterior Cavity: Contains vitreous humor, which maintains the eye's shape.

E. Vision and Light Refraction

  • Lens: Transparent structure that focuses light onto the retina.

  • Accommodation: Ciliary muscles alter the shape of the lens for focus based on proximity.

  • Cataracts: Clouding of the lens, treated surgically through lens replacement.

6. The Sense of Hearing (Audition) and Equilibrium

A. Ear Divisions

  • Consists of three main regions:
      1. Outer Ear: Includes the auricle, external auditory canal, and tympanic membrane (eardrum).
      2. Middle Ear: Contains auditory ossicles (malleus, incus, stapes) and connects to the pharynx via the Eustachian tube.
      3. Inner Ear: Contains the cochlea and vestibular apparatus, communicating with the brain through CN VIII (Vestibulocochlear Nerve).

B. Middle Ear Components

  • Auditory Ossicles: Amplify sound waves.
       - Malleus: Attached to the tympanic membrane.
       - Incus: Between malleus and stapes.
       - Stapes: Attached to the oval window of the cochlea.

C. Inner Ear Components and Functions

  • Bony and Membranous Labyrinth:
       - Contains structures for equilibrium and hearing.
       - Cochlea: Contains sensory receptors in the spiral organ (or organ of Corti) responsible for hearing.
       - Vestibular Complex: Utilizes maculae and cristae for balance.

D. Sound Wave Transmission Process

  1. Sound waves enter the external auditory meatus and reach the tympanic membrane.

  2. The tympanic membrane vibrates, moving the auditory ossicles.

  3. Vibration of the stapes at the oval window creates pressure waves in perilymph of the cochlea, resulting in distortion of the basilar membrane.

  4. Distortion leads to activation of hair cells within the cochlea, sending signals through the cochlear nerve to the brain.