General Senses, Olfactory, and Gustation Powerpoint

Fundamentals of Sensory Receptors

• Definition of Sensory Receptors: Specialized structures designed to detect sensory stimuli and generate a resulting electrical response.

• Core Characteristics of Sensory Receptors:
    - Stimulus Detection: Capable of detecting a stimulus, which is any change in the internal or external environment.
    - Neural Association: Connected to a sensory neuron that conveys information to the Central Nervous System (CNS).
    - Action Potential Production: Most operate to produce action potentials in sensory neurons.
    - Stimulus Specificity: Optimally respond to one specific type of stimulus (e.g., photoreceptors for light).

Classification of Sensory Receptors by Location

• Exteroceptors:
    - Function: Sensitive to external stimuli.
    - Location: Near the body surface, often in the skin.
    - Sensory Types: Detect touch, pressure, pain, and temperature.

• Interoceptors:
    - Function: Receive internal stimuli.
    - Location: In or near internal organs.
    - Stimuli Monitored: Changes in chemical concentrations, taste stimuli, stretching of tissues, and internal temperature.

• Proprioceptors:
    - Function: Respond to stimuli in joints and muscles.
    - Location: Found in skeletal muscles, tendons, joints, and ligaments.   - Role: Monitor stretch and movement at joints.

Classification of Sensory Receptors by Stimulus Type

• Nociceptors:
    - Structure: Free nerve endings.
    - Function: Detect harmful stimuli, resulting in pain.

• Thermoreceptors:
    - Structure: Free nerve endings.
    - Function: Respond to temperature changes.
    - Location: Found in skeletal muscle, dermis, and others.

• Mechanoreceptors:
    - Function: Respond to mechanical forces (touch, pressure, stretch, vibration, itch).
    - Trigger: Stimulated by distortion of the plasma membrane.
    - Subclasses:
      1. Tactile Receptors: For touch and pressure.
      2. Baroreceptors: Monitor stretch in the cardiovascular system.
      3. Proprioceptors: Detect stretch in joints.

• Chemoreceptors:
    - Function: Respond to chemicals in fluids.
    - Location: In the medulla oblongata (respiratory centers) and carotid/aortic bodies.

Classification of Sensory Receptors by Structure

• Simple Receptors (General Senses): General sense receptors structured as naked dendrites or encased in connective tissue.

• Complex Receptors (Special Senses): Specialized receptor cells that communicate stimuli to sensory neurons.

Olfactory Anatomy and Physiology

• Olfactory Receptor Cells:
    - Nature: Modified sensory neurons responding to chemicals in mucus.
    - Location: In the olfactory epithelium of the nasal cavity.
    - Regeneration: Replaced approximately every $60$ days.

• Cells of the Olfactory Epithelium:
    - Olfactory Receptor Cells: Primary detectors.
    - Supporting Cells: Provide support to receptor cells.
    - Basal Cells: Stem cells that regenerate olfactory receptor cells.

• Afferent Pathway for Olfaction:
    1. Chemicals dissolve in mucus.
    2. Bind to olfactory receptor cilia.
    3. Transmit nerve impulses through the olfactory nerve.
    4. Synapse with mitral cells in the olfactory bulbs.
    5. Impulses travel along olfactory tracts to the cerebral cortex (interpret odors) and limbic system (emotional response).

Gustatory Anatomy and Physiology

• Gustatory Receptor Cells:
    - Location: In taste buds on the tongue's papillae.
    - Structure: Possess microvilli; classified as chemoreceptors.
    - Regeneration: Replaced every $7$ to $10$ days.

• Tongue Papillae Types:
    - Fungiform Papillae: Mushroom-shaped projections.
    - Vallate Papillae: Dome-shaped projections.

• Taste Bud Structure:
    - Collectively form $50$ to $100$ epithelial cells.   - Gustatory Receptor Cells: Main sensing cells.
    - Basal Cells: Stem cells for regeneration.

• Afferent Pathway for Gustation:
    1. Chemicals from food detected by gustatory receptor cells.
    2. Impulses transmitted to the CNS via cranial nerves.
    3. Synapse in the medulla oblongata, then thalamus, and finally gustatory cortex.

Sensory Physiology: Transduction and Adaptation

• Sensory Transduction:
    - Converts stimulus energy into an electrical signal (receptor potential) that can trigger action potentials if large enough.

• Sensory Adaptation:
    - Definition: Receptors cease responding to constant stimuli.
    - Types:
      - Tonic Receptors: Constant firing, slow adaptation.
      - Phasic Receptors: Burst of activity; quick adaptation.

Sensory Physiology: Acuity

• Definition of Acuity: The sensitivity of a sensory system to distinguish details in stimuli.

• Factors Affecting Acuity:
    1. Receptive Field Size: Smaller fields have higher acuity.
    2. Convergence in Afferent Pathway: Higher convergence leads to lower acuity.