Chemical senses

Importance of Receptor Cells

  • Receptor cells react to stimuli & perform transduction.

  • Transduction: Conversion of stimuli into electrical events.

Comparison of Receptor Cells

Similarities

  • Structure: All receptor cells have specific structures to detect stimuli.

  • Location: All can be found on the neuron at the dendrite side for sensory input.

Differences

  • Light receptors: Detects light (Brightness, color).

  • Smell receptors: Detects odor molecules.

  • Sound receptors: Detects sound waves and vibrations.

  • Taste receptors: Detects chemical composition in food.

Types of Receptors

Chemoreceptors

  • Chemically gated ion channels that respond to odorant or food molecules.

  • Function: Respond to chemicals in solution.

Mechanoreceptors

  • Function: Respond to physical movement.

Photoreceptors

  • Function: Respond to light wavelengths.

Macroscopic Anatomy of Taste Receptors

Location of Taste Buds

  • Found in Papillae: Projections on the tongue, can vary by size and location.

  • Taste Buds: Microscopic bulb-like structures on papillae, house taste receptor cells.

Microscopic Anatomy of Taste Buds

Composition

  • Contain:**

    • Gustatory receptor cells

    • Supporting cells

    • Gustatory pore

    • Basal cells, synapsed to nerve fibers.

Gustatory Receptor Cells Structure

  • Have gustatory hairs (microvilli).

  • Contain 5 types of protein receptors that synapse with cranial sensory nerves.

Five Basic Tastes

Types of Tastes

  1. Sour: Detects H+ ions; protective against sour spoilage.

  2. Salty: Detects Na+ influx; signals salt/metal ions.

  3. Sweet: Responds to sugars, alcohols, amino acids.

  4. Bitter: Detects alkaloids, toxic substances (e.g., nicotine, caffeine).

  5. Umami: Detects glutamate; related to savory tastes.

Mechanism of Taste Transduction

    1. Food dissolves in saliva.

    1. Molecules bind to receptors on gustatory hairs.

    1. Binding leads to depolarization (via direct or second messenger).

    1. Ca2+ channels open, leading to neurotransmitter release.

    1. Neurotransmitters cross synapse, bind to cranial nerves, initiating action potentials to the brain.

Taste Reflexes and Sensory Pathways

  • Reflexes triggered include:

    • Salivation

    • Gastrointestinal motility & secretion

    • Elicitation of gagging/vomiting from bad tastes

  • Impulses carried by three cranial nerves:

    • Facial (VII)

    • Glossopharyngeal (IX)

    • Vagus (X)

  • Pathways through the Thalamus to the gustatory cortex.

Olfactory Receptors

Structure and Function

  • Olfactory receptors are specialized neurons with long cilia, covered in mucus.

  • Dendrite end: Odorant binding receptors in nasal mucus.

  • Axon end: Synapses with Cranial Nerve I (Olfactory bulb) with over 1,000 different receptors.

Smell Transduction

  • Binding odorant molecules activates a G-protein and second messenger (cAMP), leading to Na+ and Ca2+ channel opening and depolarization.

Olfactory Pathway

Brain Regions Involved

  • Anterior Olfactory Nucleus: Create and store olfactory gestalts.

  • Temporal Lobe: Responsible for conscious olfactory experience.

  • Piriform Cortex: Processes behavioral, cognitive, and contextual information.

  • Amygdala: Handles emotional processing of olfactory information.

  • Hippocampus: Deals with odor thresholds and memory retention.

Sensory Adaptation

  • Definition: Body adjusts to external stimuli over time, reducing sensitivity to a sustained stimulus.

  • Examples:

    • Taste: Prolonged exposure makes flavors seem blander.

    • Smell: Prolonged exposure to odors leads to decreased detection sensitivity.

Chemical Senses in Neuroanatomy

  • Discusses specific cranial nerves controlled by chemoreceptors.

  • Differentiates between sensory-only and mixed nerves, their functions, and implications.