Special Senses - Olfaction & Gustation

Overview of Sensations

• Sensation: Conscious or subconscious awareness of changes in the external or internal environment.

• Four elements required for sensation:

  • Stimulus: Change in the environment capable of activating sensory neurons (light, heat, pressure, mechanical or chemical energy).

  • Sensory Receptor: Converts stimulus into electrical signal, producing nerve impulses.

  • Conduction: Nerve impulses conducted along a neural pathway from receptor to the brain.

  • Interpretation: Brain region receives and interprets nerve impulses into sensation.

• Stimulation of sensory receptors doesn't immediately result in perception.

• Sensory receptors respond to stimuli by generating action potentials propagated to the spinal cord and brain.

• Perception results when action potentials reach the cerebral cortex.

Classification of Sensory Receptors

• By Structure:

  • Free Nerve Endings: Bare dendrites associated with pain, thermal, tickle, itch, and some touch sensations.

  • Encapsulated Nerve Endings: Dendrites enclosed in connective tissue capsule for pressure, vibration, and some touch sensations.

  • Separate Cells: Receptor cell that synapses with first-order neuron; located in the retina of the eye (photoreceptors), inner ear (hair cells), and taste buds of the tongue (gustatory receptor cells).

• By Function:

  • Mechanoreceptors: Detect mechanical pressure; sensations of touch, pressure, vibration, proprioception, hearing, and equilibrium; monitor stretching of blood vessels and internal organs.

  • Thermoreceptors: Detect changes in temperature.

  • Nociceptors: Respond to damaging or potentially harmful stimuli (large mechanical, thermal, or chemical stimuli).

  • Photoreceptors: Detect light striking the retina of the eye.

  • Chemoreceptors: Detect chemicals in the mouth (taste), nose (smell), and body fluids.

  • Osmoreceptors: Sense osmotic pressure of body fluids.

General Classification of Senses

• Two basic groups: general and special.

  • General Senses: Receptors distributed over a large part of the body; divided into somatic and visceral senses.

    • Somatic Senses: Sensory information about the body and environment.

    • Visceral Senses: Information about internal organs; primarily involving pain and pressure.

  • Special Senses: More specialized in structure and localized to specific parts of the body.

    • Smell, taste, sight, hearing, balance.

Olfaction (Smell)

• Olfactory epithelium occupies the upper portion of the nasal cavity.

  • Contains olfactory receptor cells (olfactory neurons), which are first-order neurons of the olfactory pathway.

  • Olfactory receptor cells are bipolar neurons.

  • Distal processes (dendrites) have cilia, olfactory hairs, that project onto the olfactory epithelium and get stimulated by inhaled chemicals.

  • Proximal processes (axons) of olfactory sensory neurons extend from olfactory epithelium to the olfactory bulb.

  • Approximately 40 bundles of unmyelinated axons course through around 20 holes in the cribriform plate of the ethmoid bone, collectively forming the left and right olfactory nerves (Cranial Nerve I, CN I).

• Olfactory nerves terminate in the brain in paired masses of grey matter called olfactory bulbs, located below the frontal lobes of the cerebrum.

• Within olfactory bulbs, axons of olfactory neurons form synapses with second-order neurons of the olfactory bulb; axons of olfactory bulb neurons form the olfactory tract.

• Some axons of olfactory tract project to the primary olfactory area in the temporal lobe of the cerebral cortex, beginning conscious awareness of smell.

• Other axons project to the limbic system of the hypothalamus, accounting for emotion and memory-evoked responses to odors.

Olfaction (Continued)

• Primary olfactory cortex processes olfactory information, allowing us to perceive the odor.

  • Information also sent to the orbitofrontal cortex in the frontal lobe where smells can be analyzed and compared to other smells for identification.

  • Path to the limbic system elicits emotional responses to smells and evokes memories.

Gustation (Taste)

• Allows us to feel tastes.

• Odors from the mouth pass into the nasal cavity where they stimulate olfactory receptors as well.

• Receptors for taste sensations are located in taste buds, mostly on the tongue, but some on the roof of the mouth, pharynx, and epiglottis (larynx).

• On the tongue, taste buds are located in elevations called papillae, which provide a rough texture to the upper surface of the tongue.

  • Several types of papillae: vallate (wall-like), fungiform (mushroom-like), filiform (thread-like), and foliate (leaf-like).

• Taste buds contain gustatory receptor cells that have gustatory hair processes projecting to the external surface through the taste pore (an opening in the taste bud).

• Unlike olfactory receptor cells (special neuronal cells), gustatory receptor cells are separate cells; they do not have axons but synapse with dendrites of first-order neurons.

• Gustatory Pathway:

  1. Primary neurons extend from gustatory cells of the tongue through the cranial nerves CN VII & CN IX, and synapse in the nucleus solitarius located in the medulla oblongata.

  2. Secondary nerves extend from the nucleus in the medulla oblongata and synapse in the thalamus.

  3. Tertiary neurons extend from the thalamus and terminate in the primary gustatory cortex in the insula of the cerebrum.