• All sensory organs have receptors that increase sensitivity to the environment.
• The relatively simple receptors and pathways for general senses of touch, pressure, vibration, temperature, pain and proprioception were considered in Chapter 16.
• Special senses include smell, taste, vision, hearing, and equilibrium.
• Special sensory afferent pathways generally resemble general sensory pathways.
• Special sense receptors are arranged in complex sensory organs.

Chemical Senses

• Olfaction (smell) and gustation (taste) are chemical senses via interaction of molecules with receptor cells.
• Smell and taste have strong connection to the limbic system.
• Certain odors or tastes can evoke strong emotional responses or memories.

Olfactory Epithelium

• One square inch of membrane holding 10-100 million receptors.

• Covers superior nasal cavity and cribriform plate.

• Total area of 5 cm2.

• Three cell types in olfactory epithelium;

  • Olfactory receptors,
  • Supporting cells,
  • Basal cells.

• Olfactory receptors are first-order neurons in the olfactory pathway.

• Each olfactory receptor is a bipolar neuron.

  • Exposed knob-shaped dendrite
  • Axon projecting through cribriform plate
  • Axon ending in olfactory bulb,

• Olfactory hairs on olfactory receptors respond to inhaled chemicals.

  • Olfactory hairs are cilia projecting from dendrite.
  • Transduction is the conversion of stimulus energy into a graded potential.

• Odorants are chemicals that have an odor and stimulate olfactory hairs

• Olfactory receptors respond to chemical stimulation of an odorant molecule by producing a generator potential.

• Olfactory (Bowman's) glands produce mucus, which moistens the surface and dissolves odorants for transduction.

  • Innervated by CN VII -- the same as supporting cells.
  • Impulses can stimulate lacrimal glands/nasal mucous result in tears and runny nose after inhaling pepper or ammonia.

• Columnar epithelial supporting cells of the mucous membrane lining the nose.

  • Provide physical and metabolic support, and electrical insulation
  • Help detoxify chemicals that contact the olfactory epithelium.

• Basal cells are stem cells between the bases of supporting cells – continually divide to produce new olfactory receptors and supporting cells.

  • Basal cells differentiate constantly, producing new olfactory epithelium about every 30 days.

Olfactory Transduction

• G protein and adenylate cyclase activation occurs when an odorant molecule binds to an olfactory receptor protein, resulting in cyclic AMP (cAMP) production.
• Sodium ion (Na+) channels open with cAMP.
  • Na+ ions enter the olfactory receptor,
  • Depolarization may generate an action potential, which propagates along the axon of the olfactory receptor.
• Odorants bind to receptors.
• The nerve impulse is triggered after Na+ channels open and depolarization occurs.
• Adaptation to odours occurs quickly and the threshold of smell is low.
  • Only a few molecules of certain substances need be present in air to be smelled.
  • Methyl mercaptan is detected at 1/25 billionth of a milligram per ml of air – similar to ethyl mercaptan.
    • Added to odourless petroleum gas for cooking to provide olfactory warning.
• Adaptation is rapid; 50% in the first second, slowing thereafter. Complete insensitivity to certain strong odours occurs about a minute after exposure (involves CNS input).
• Olfactory receptors convey nerve impulses through olfactory nerves.
  • To:
    • Olfactory bulbs,
    • Olfactory tracts,
    • Cerebral cortex,
    • Limbic system.
• Unmyelinated axons from olfactory receptors extend through 20 olfactory foramina in the cribriform plate of ethmoid bone.
• 40+ bundles of axons form right and left olfactory nn.
  • They terminate in paired masses of gray matter called the olfactory bulb.
    • Located inferior to the frontal lobes of the cerebrum.
    • Located lateral to the crista galli of the ethmoid bone
  • Axon terminals form synapses with dendrites and cell bodies of the olfactory bulb neurons
  • Posteriorly extend and form the olfactory tract – project to primary olfactory area in the cerebral cortex.
• There is debate on the olfactory cortex's location.
  • Generally accepted it is on medial aspect of temporal lobe.
  • Sometimes including the base of frontal lobe.
  • Primary olfactory area = start of conscious awareness of smell recognition.
• Olfactory sensations reach the cerebral cortex without first synapsing in the thalamus
• Collateral axons of the olfactory tract project to the limbic system and hypothalamus, accounting for emotional, memory-evoked responses to odours.
  • E.g. sexual excitement, nausea, odour-evoked memory.
• The primary olfactory area extends to frontal lobe – for odour identification and discrimination.
  • The orbitofrontal area R hemisphere exhibits greater activity during olfactory processing.

Olfactory Tract

• Olfactory bulb transmits smell information from the nose to the brain.
• Glomerular layer receives direct input from olfactory nerves;
  • Made up of axons from 10 million olfactory receptor neurons.
• Ends of axons cluster to form spherical structures called glomeruli -Receive input primarily from olfactory receptor neurons that express the same olfactory receptor.
• Glomeruli also permeated by dendrites from mitral cells, which output to the olfactory cortex.
• Numerous interneuron types exist in the olfactory bulb, periglomerular cells, synapse within and between glomeruli, and granule cells, synapse with mitral cells.
• As a neural circuit, the olfactory bulb has one source of sensory input from axons from olfactory receptor neurons of the olfactory epithelium, and one output (mitral cell axons).
• Assumed function: a filter, not an associative circuit.
• The basal dendrites of mitral cells are connected to interneurons known as granule cells, which some theories produce lateral inhibition between mitral cells.
• The synapse between mitral and granule cells is rare; "dendro-dendritic" where both cells release a neurotransmitter.
• Mitral cells release glutamate, an excitatory neurotransmitter.
• Granule cells release the inhibitory neurotransmitter Gamma-aminobutyric acid (GABA).
• Resulting bi-directionality: dendro-dendritic synapse causes mitral cells to inhibit themselves (auto-inhibition) and neighbouring mitral cells (lateral inhibition).
• Axons from the olfactory receptor form olfactory nerve (cranial nerve I) that synapse in the olfactory bulb, passing in 40 foramina within the cribriform plate.
• Second-order olfactory bulb neurons form the olfactory tract, that synapses on the primary olfactory area of the mainly temporal lobe - point of conscious awareness of smell begins.
• Other collaterals lead to the limbic system.

Pathologies of Olfactory

• Many people in the US experience olfactory dysfunction; head trauma, upper respiratory infections, anterior cranial fossa tumours and exposure to toxic chemicals
• Many as 4 million people in the US are affected by Hyposmia, a reduced ability to smell, with incidence increasing with age. Hyposmia also caused by: neurological changes (head injury, Alzheimer/Parkinson's disease); drugs (antihistamines, analgesics/steroids); smoking.
• Following terms are used to describe degree of smell aberration:
  • Anosmia -Absence of smell sensation.
  • Hyposmia - Decreased sensation.
  • Dysosmia- Distortion of smell sensation.
    • Parosmia - perception of smell with appropriate stimulus.
    • Cacosmia- perception of a bad (foul) smell.
    • Phantosmia- perception of smell with no odorant.
• The typical description of partial complex epilepsy.
  • With a short-term focus, is foul odors known as 'uncinate fits) is caused by temporal lobe affecting; hippocampal uncus with seizure onset.
• The olfactory disorders can be an indicative sign of Parkinson illness and Alzheimer's.
• An association that exists between olfactory deficit and obsessive-compulsive disorder-can help with diagnosis

Gustation (Taste)

• Taste, like olfaction, is a chemical sense.
  • To be detected, molecules must be dissolved in a medium.
  • Primary tastes include sour, sweet, bitter, salty & umami (meaty or savory).
  • All other flavours are a combination of 2 or more of the 5 primary tastes, in addition to other somatic sensations
• Odours of food can pass upward from the mouth into the nasal cavity.
  • This is called retronasal olfaction, and olfaction is more sensitive than taste, so much of what you're tasting is actually what you're smelling.
  • Plugging nose while eating diminishes sense of taste.

Taste Receptors

• Sweet receptors activated by sugars, alcohol, ketones, aldehydes, etc.
• Salt receptors activated by metal ions, most commonly Na+.
• Sour receptors activated by free hydrogen ions, and therefore acids. -Stronger acid = lower pH and more sour taste.
• Bitter receptors are activated by alkaloids such as caffeine, nicotine, etc.
• Umami receptors are activated by 1-glutamate.
• Taste receptors detect tastes that help the body to determine what it needs. -Sweet tastes indicate energy-rich foods & salty: electrolyte-rich -Bitter & sour: considered negative, some toxic/gone bad -Umami- high in amino acids. -Short answer to whether taste detection helps assess body needs; no
• The 5 taste sensations have been localized to differing regions of the tongue - not right.
  • There are more uniform distributions of bitterness.
• 10,000 taste buds found on tongue, soft palate, epiglottis and pharynx. #Taste buds have been found all the way down in the lungs.

• of taste buds decline with age.

• Found in elevations on the tongue called papillae.
• Three types of papillae w/ taste buds:
  • Circumvallate papillae 12 big inverted V-shaped structures located back tongue each has 100-300 taste buds
  • Fungiform papillae Mushroom shaped scattered all over w/ 5 each.
  • Foliate papillae Small trenches in sides- mostly degenerate in early childhood,.
• Each taste bud is oval w/ 3 epithelial cell types
  • Gustatory receptor cells Sensory cells project with hairs/cilia though external taste pore.
  • Supporting cells ~20 gustatory each - can go past
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