Sensory Systems Overview

Overview

This lecture examines the anatomy and physiology of smell (olfaction) and taste (gustation), focusing on how sensory signals are transduced and interpreted by the nervous system.

Case Study: Anosmia in Olivia

• Olivia lost her sense of smell (anosmia) after head trauma, impacting her taste, safety, and emotional well-being.
• Anosmia can result from trauma, infections, or aging and causes loss of both smell and much of taste.

Sensory Transduction and Receptors

• Sensory cells translate stimuli (chemical, electromagnetic, mechanical) into action potentials in a process called transduction.
• Photoreceptors detect light (vision), mechanoreceptors detect sound/pressure (touch, hearing, balance), and chemoreceptors detect chemicals (smell, taste).

The Olfactory System (Smell)

• Olfactory system detects volatile (gaseous) molecules inhaled through the nose.
• Odorant molecules reach the olfactory epithelium, a tissue patch on the nasal cavity's roof containing olfactory sensory neurons.
• Each olfactory neuron has receptors for one specific smell; odorants bind to these, triggering action potentials.
• Signals converge in glomeruli, where olfactory axons synapse with mitral cells, then travel to the brain.
• The olfactory cortex processes identity; the limbic system processes emotional responses and memories tied to smells.
• Humans have ~40 million receptor neurons, capable of distinguishing ≈10,000 smells.

The Gustatory System (Taste)

• Taste and smell work together; ~80% of taste is due to olfaction.
• Taste buds, mainly on the tongue, contain 50-100 taste receptor epithelial cells each.
• The taste map of the tongue is incorrect; all taste sensations can be detected across all areas of the tongue.
• Gustatory cells detect tastants (food chemicals) dissolved in saliva, triggering action potentials.
• Salty tastes involve sodium ions; sour tastes involve hydrogen ions; both trigger specific channels in taste cells.
• Taste signals travel through cranial nerves VII, IX, and X to the cerebral cortex, prompting digestive responses.

Key Terms & Definitions

• Anosmia — complete or partial loss of the sense of smell.
• Transduction — conversion of stimuli into electrical signals by sensory receptors.
• Chemoreceptors — cells that detect chemical stimuli (smell, taste).
• Olfactory epithelium — tissue containing neurons for detecting odors.
• Glomerulus (olfactory) — cluster where olfactory neuron axons and mitral cells synapse.
• Mitral cell — neuron relaying olfactory info to the brain.
• Gustatory cell — epithelial taste receptor cell in taste buds.
• Tastant — chemical substance that stimulates taste.

Action Items / Next Steps

• Review the anatomy and signal pathways of olfactory and gustatory systems.
• Be able to explain why loss of smell severely affects taste and emotional experience.
• Test the claim that all parts of the tongue can detect all basic tastes.