Week 1 bio 2 online notes

Chemical Senses

  • Two primary senses: 1) Smell 2) Taste

  • Relevant Information: Search for "tongue" and "smell" in Chapter 14.1 of OpenStax.

  • All diagrams and pictures © 2018 Pearson Education Inc unless otherwise indicated.

Generating Action Potentials

  • Sensory Neuron Stimulation:

    • Stimulation of receptor cells leads to depolarization of sensory neurons.

    • Smell: Receptors located on the dendrites of sensory neurons.

      • Odorants bind to dendrites, triggering action potentials to travel to the CNS.

    • Taste: Receptors are specialized cells that synapse with sensory neurons.

      • Membranes of taste receptor cells are unexcitable; neurotransmitters bridge the gap to sensory neurons, generating an action potential to the CNS.

Olfactory System Structure

  • Anatomy:

    1. Begins in the peripheral structures of the nasal cavity.

    2. Olfactory receptor neurons are within the olfactory epithelium.

    3. Axons project through the cribriform plate of the ethmoid bone and synapse with neurons of the olfactory bulb.

    • Micrograph provided by the Regents of University of Michigan Medical School © 2012.

Olfaction - Sense of Smell

  • Anatomy:

    • Olfactory organs straddle the nasal septum; penetrate the ethmoid bone.

Layers of Olfactory Organs

  1. Olfactory Epithelium:

    • Contains up to 20 million receptor cells, support cells, and basal cells (stem cells).

  2. Lamina Propria:

    • Mostly connective tissue; contains nerves/vessels and olfactory glands for mucus production.

Process of Olfaction

  • Odorants:

    • Dissolved chemicals that stimulate olfactory neurons.

    • Binding to membrane receptors activates adenylate cyclase -> ATP becomes cyclic AMP (cAMP) -> Opens Na+ channels (begins action potential).

  • If sufficient stimulation occurs, an action potential is generated.

Olfactory Pathway

  • Neurons:

    • First order neurons: Sensory neurons of olfactory organ travel through cribriform plate.

    • First synapse: Olfactory bulb.

    • Second order neurons: Olfactory tract upward synapse with olfactory cortex/hypothalamus/limbic system.

    • Unique aspect: The only sense that does not pass through the thalamus; strong connections to emotions.

Gustation - Sense of Taste

  • Lingual Papillae:

    • Epithelial projections on tongue where taste buds are found. There are four types of papillae:

    1. Circumvallate Papillae: Large, contain up to 100 taste buds, found at the back of the tongue in a V formation.

    2. Foliate Papillae: Located on the sides near the rear of the tongue.

    3. Fungiform Papillae: Scattered across the tongue surface, few taste buds.

    4. Filiform Papillae: No taste buds, used for friction to push food back (like a cat's tongue).

Taste Bud Anatomy

  • Gustatory Cells:

    • Receptor cells with taste hairs that extend through a taste pore to contact tastants in saliva. Live about 10 days, 40-100 in each taste bud.

  • Basal Cells:

    • Stem cells that regenerate gustatory cells.

Taste Sensations

  • Four primary sensations:

    1. Sweet

    2. Salty

    3. Sour

    4. Bitter

  • Fifth sensation: Umami, related to savory taste associated with protein.

  • Sensitivity varies; some receptors respond to multiple tastes. Taste perception changes throughout life as taste buds are lost.

Gustation Signal Transmission

  • Each taste has different thresholds for receptor stimulation; more sensitive to bitter and acidic tastes.

  • Taste reception involves dissolved chemicals causing neuron depolarization.

Cranial Nerves for Taste

  • Facial Nerve (CN VII): Carries taste from the anterior 2/3 of the tongue.

  • Glossopharyngeal Nerve (CN IX): From the posterior 1/3 of the tongue.

  • Vagus Nerve (CN X): From the throat, under the tongue, and roof of the mouth.