8.1

The Five Special Senses

  • The five special senses include:
    • Vision
    • Smell
    • Taste
    • Hearing
    • Equilibrium (balance)
  • Specialized organs exist for these senses, containing receptor cells that carry impulses via visceral afferents.
  • Touch is a somatic sense and does not have a specialized organ; it uses general receptors made of modified dendrites of sensory neurons.
  • Touch includes pressure, vibration, pain, and heat, which are carried by general somatic afferents and general visceral afferents.

Function of Special Senses

  • What:
    • The special senses respond to stimuli involved in vision, hearing, balance (equilibrium), and taste.
  • How:
    • Specialized sense organs like the eyes, ears, and nose house a variety of receptors that detect stimuli in the environment.
  • Why:
    • Without the special senses, activities such as smelling favorite foods, appreciating colors, or hearing music would be impossible.

Olfactory Epithelium and Sense of Smell

Basic Overview of Smell (Olfaction)

  • Smell is classified as a special sense and one of the chemical senses.
  • Olfactory receptors: Activated by small airborne organic molecules known as odorant molecules, which dissolve in fluid on nasal mucous membranes.
  • Binding Process: Odorant molecules bind to G protein-coupled receptors in the olfactory dendrite's cell membrane.
    • This binding activates a G protein which converts ATP to cyclic AMP (cAMP).
    • cAMP opens cation channels, allowing sodium and calcium ions to rush into the cell, causing depolarization.
    • If depolarization reaches the threshold, an action potential is generated in the axon.

Steps in Olfactory Reception

  • The key steps in olfactory reception include:
    1. Binding of odorant to receptor protein activates adenylate cyclase, converting ATP to cAMP.
    2. cAMP opens cation channels in the membrane causing depolarization.
    3. If sufficient depolarization occurs, an action potential is triggered, relaying information to the central nervous system (CNS).

Anatomy of Olfactory Sensory Neurons

  • Olfactory nerves (CN 1):
    • Bipolar receptor cells located in the olfactory epithelium at the roof of the nasal cavity.
    • Dendrites detect odorant molecules in nasal mucosa.
    • Basal epithelial cells divide to replace worn-out olfactory sensory neurons.
    • Olfactory nerves penetrate the ethmoid bone via the cribriform plate.
    • Their axons synapse in the olfactory bulbs (paired masses of gray matter).
  • Pathway of smell:
    • From olfactory bulbs, axons form the olfactory tracts that relay information to the temporal lobes in the primary olfactory area.
    • Signals are also directed to the hypothalamus, amygdala, and limbic system, indicating the impact of smell on behavior, mood, and memory.

Nature of Smell

  • Smell is defined as a combination of primary odors detectable by the brain.
    • Any discernible odor arises from a combination of several chemical substances.
  • Animal comparisons: Animals like dogs have a significantly wider range of detectable smells and allocate larger brain areas for smell interpretation.
  • Human capabilities: Research in 2014 suggests humans may detect up to 1 trillion scents.
  • Seven primary odors have been identified:
    • Floral (e.g., roses)
    • Ethereal (e.g., dry-cleaning fluid)
    • Camphorates (e.g., mothballs)
    • Musky (e.g., perfumes)
    • Peppermint (e.g., mint gum)
    • Pungent (e.g., vinegar)
    • Putrid (e.g., rotten eggs)

Taste Buds and Sense of Taste

Overview of Gustation

  • The gustatory system is responsible for the special sense of taste (gustation).
  • Chemical activation: Taste receptors are activated by chemical substances dissolved in saliva.
  • Location of taste receptors:
    • Primarily within taste buds on the tongue and throughout the mouth/throat.
  • Tongue anatomy:
    • Contains papillae: raised bumps aiding in food grip.
    • Four main types of papillae:
    • Circumvallate (vallate)
    • Fungiform
    • Foliate
    • Filiform (does not contain taste buds)
  • Taste bud structure:
    • Composed of gustatory cells and supporting cells.
    • Gustatory epithelial cells are chemoreceptors that end in gustatory hairs (microvilli) that project into saliva to detect dissolved chemicals.
    • Taste pores surround taste hairs for dissolved chemical detection.

Taste Reception

Main Types of Taste
  • There are five primary tastes:
    • Sweet
    • Salty
    • Sour
    • Bitter
    • Savory (Umami)
Mechanisms of Taste Detection
  • Salt and Sour Tastes:
    • Salt taste arises from sodium ions (e.g., from NaCl), while sour taste is from hydrogen ions from acidic compounds (e.g., acetic acid in vinegar).
    • For salty taste:
    • Sodium ions diffuse into gustatory cells, generating depolarization and action potential.
    • For sour taste:
    • Hydrogen ions enter cells causing a similar depolarization.
  • Sweet, Bitter, and Umami Receptors:
    • These tastes involve food molecules binding to G protein-coupled receptors (gustducins) leading to depolarization via secondary messengers.
    • Bitter taste often produced by drugs (e.g., aspirin), while umami comes from amino acids (e.g., found in fish, ripe tomatoes).
Pathway of Taste
  • The taste pathway involves food chemicals contacting gustatory hairs.
  • Chemoreceptors generate signals detected through dendrites entwined around gustatory cells.
  • Cranial Nerve Involvement:
    • Facial nerve: Carries information from the anterior two-thirds of the tongue.
    • Glossopharyngeal nerve: Carries information from the posterior one-third of the tongue.
    • Vagus nerve: Carries information from taste buds on the epiglottis.
  • Taste Processing:
    • Information travels through the thalamus to the gustatory cortex located in the insular and frontal cortices where taste processing occurs.
    • The trigeminal nerve provides general sensory information regarding texture and temperature of food.

Homeostatic Imbalances of Olfaction/Gustation

Anosmia

  • Anosmia is the loss of smell arising from various causes including:
    • Brain damage
    • Viral infections
    • Trauma
    • Chemical exposure
  • Commonly occurs after traumatic brain injuries (TBI) causing temporary or permanent loss depending on severity and regions affected.
  • Gradual anosmia may occur from obstructions such as brain tumors.
  • Symptoms/Effects:
    • Temporary anosmia might arise from respiratory infections/allergies causing nasal blockage, leading to smell loss.
    • Permanent anosmia can result from chemical exposure damaging olfactory neurons, impairing regeneration.
    • Loss of smell affects taste perception, requiring stronger spices for flavor.
    • Age-related anosmia may lead to increased sodium intake in the elderly, posing cardiovascular risks.

Ageusia

  • Ageusia is the loss of taste sensation closely linked to olfactory function.
    • Receptors activation by odors enhances taste; obstruction of olfactory receptors dulls/suppresses taste sensation.
  • Causes include:
    • TBI
    • Tumors
    • Tongue burns from hot substances impacting taste bud function.
  • Lack of salivary production (dry mouth) can impede taste perception due to undissolved ions.
  • Further Changes with Age:
    • Children have over 10,000 taste receptors, decreasing significantly by age 50.
    • Age-related anosmia and ageusia contribute to increased salt usage in elder individuals.
    • Elevated sodium intake correlates with cardiovascular disease risks and may lead to other health issues.