Neuro - Chapter 17 (Olfaction & Gustation)

• Special senses include smell, taste, vision, hearing, and equilibrium, increasing sensitivity to the environment.
• Special sensory afferent pathways are similar to general sensory pathways.

Chemical Senses:

• Olfaction (smell) and gustation (taste) are considered chemical senses because of the interaction of molecules with receptor cells.
• The sensation of smell and taste has a strong connection to the limbic system, evoking emotional responses or memories.
• Chemoreceptors enable these senses.

Olfactory Epithelium:

• It is a 5 cm2 membrane holding 10-100 million receptors.
• It covers the superior nasal cavity and cribriform plate.
• The three types of cells it contains are olfactory receptors, supporting cells, and basal cells.

Olfactory Receptors:

• They are the first-order neurons of the olfactory pathway.
• Each is a bipolar neuron with an exposed knob-shaped dendrite and an axon projecting through the cribriform plate and ending in the olfactory bulb.
• Olfactory hairs respond to inhaled chemicals.
• Odorants are chemicals that have an odour and can stimulate the olfactory hairs.
• Olfactory receptors respond to odorant molecule stimulation by producing a generator potential, initiating the olfactory response.

Supporting Cells:

• They are columnar epithelial cells of the mucous membrane lining the nose.
• They provide physical and metabolic support and electrical insulation to the olfactory receptors.
• These cells also help detoxify chemicals coming into contact with the olfactory epithelium.

Basal Cells:

• They are stem cells between the bases of the supporting cells.
• These cells continually undergo cell division to produce new olfactory receptors and supporting cells, replacing the epithelium monthly.

Olfactory (Bowman's) Glands:

• They produce mucus that moistens the surface and dissolves odorants for transduction.
• They are innervated by the CN VII Facial nerve.
• Impulses in this nerve stimulate the lacrimal and nasal mucous glands, resulting in tears and a runny nose after inhaling substances like pepper and ammonia.

Olfaction Summary:

• Olfactory receptors are bipolar neurons with cilia or olfactory hairs.
• Supporting cells are columnar epithelium.
• Basal cells are stem cells that replace the epithelium monthly.
• Olfactory (Bowman's) glands produce mucus.
• The epithelium and glands are innervated by cranial nerve VII (Facial nerve).

Olfaction: How We Smell:

• Hundreds of primary scents exist.
• The ability to recognize 10,000+ odors depends on activity patterns in the brain arising from different combinations of activated olfactory receptors.
• Olfactory reception involves a generator potential that triggers nerve impulses.

Olfactory Transduction:

• An odorant binds to an olfactory transmembrane receptor protein (GPCR) in the plasma membrane of an olfactory hair.
• This is coupled to a G protein, which activates adenylate cyclase.
• Adenylate cyclase produces cAMP, which then opens sodium ion channels.
• Sodium inflow causes a depolarizing generator potential.
• The generation of a nerve impulse propagates along the axon of the olfactory receptor.

Olfactory Transduction Summary:

• Odorant binds to receptors.
• Na+ channels open.
• Depolarization occurs.
• A nerve impulse is triggered.

Adaptation:

• Adaptation to odors occurs quickly, and the threshold of smell is low.
• Methyl mercaptan, similar to ethyl mercaptan, is detectable at 1/25 billionth of a milligram per ml of air.
• This is added to odourless petroleum gas for warning of gas leaks.
• Threshold depends on the chemical, with high sensitivity to methyl mercaptan.
• Adaptation is rapid, with strong odors resulting in complete insensitivity in about a minute, involving CNS input.
• Olfactory receptors convey nerve impulses via olfactory nerves: olfactory bulbs ➞ olfactory tracts ➞ cerebral cortex ➞ limbic system

Pathway:

• Approximately 40 bundles of axons collectively form the right and left olfactory nerves.
• These nerves terminate in paired masses of gray matter in the olfactory bulb.
• Axon terminals form synapses with dendrites and cell bodies of olfactory bulb neurons.
• These extend posteriorly and form the olfactory tract.
• They project to the primary olfactory area of the cerebral cortex, located in the temporal lobe.
• Olfactory sensations are unique in that they reach the cerebral cortex without synapsing in the thalamus first.
• Collateral axons of the olfactory tract project to the limbic system and hypothalamus, which accounts for emotional and memory-evoked responses to odors, like sexual excitement or nausea.
• From the primary olfactory area, signals extend to the frontal lobe for odor identification and discrimination, which is the orbitofrontal area.

Adaptation & Threshold

• Adaptation to olfaction decreases sensitivity
• Olfactory adaptation occurs rapidly:
  • 50% in 1 second
  • Complete in 1 minute
  • Phasic then tonic
• Low threshold allows for detection of a few molecules.
  • Methyl mercaptan is added to natural gas as a warning.

Olfactory Tract:

• The olfactory bulb transmits smell information from the nose to the brain.
• Within the olfactory bulb, the glomerular layer receives direct input from olfactory nerves, made up of axons from approximately ten million olfactory receptor neurons in the olfactory mucosa.
• Axons cluster in glomeruli, which receive input primarily from olfactory receptor neurons expressing the same olfactory receptor.
• Glomeruli are permeated by dendrites from mitral cells, which output to the olfactory cortex.
• Numerous interneuron types, including periglomerular and granule cells, synapse in the olfactory bulb.
• It functions as a filter rather than an associative circuit.
• It has interneurons known as granule cells

• Granule cells produce lateral inhibition between mitral cells.
• Dendro-dendritic synapses involve both sides releasing neurotransmitters onto each other

• Granule cells release GABA as an inhibitory neurotransmitter.
• Mitral cells release glutamate as an excitatory neurotransmitter.
• Dendro-dendritic synapses cause mitral cells to exhibit auto-inhibition and lateral inhibition.

Axons and Neural Signaling:

• Olfactory receptor axons form the olfactory nerves (cranial nerve I).
• Second-order neurons in the olfactory bulb form the olfactory tract that synapses on the primary olfactory area of the temporal lobe.
• Conscious awareness of smell begins.
• Collaterals lead to the limbic system.

Pathologies:

• Head trauma, upper respiratory infections, tumors, and toxic chemicals can cause olfactory dysfunction.

Hyposmia:

• Hyposmia is a reduced ability to smell, affecting as many as 4 million people in the U.S., with the incidence increasing with age.
• It can also be caused by neurological changes, head injury, Alzheimer's disease, Parkinson's disease, and certain drugs.

Anosmia:

• Anosmia is the absence of smell sensation.

Dysosmia:

• Dysosmia is a distortion of smell sensation.
  • It has two subcategories: parosmia and phantosmia.

Parosmia:

• Parosmia is the perception of smell in the absence of appropriate stimuli.

  In cacosmia, an individual has a perception of a bad or foul smell.

Phantosmia:

• Phantosmia is the perception of smell in the absence of an odorant. It can be a smell hallucination.
• Smelling burnt toast signals a stroke.
• Olfactory dysfunction is associated with early Parkinson disease, Alzheimer disease, and Huntington's.
• It can help with diagnosis.
• There is an association between abnormal olfactory identification and obsessive-compulsive order.

Gustation:

• Taste, like olfaction, is a chemical sense that requires molecules to be dissolved in saliva.
• The five primary tastes are sour, sweet, bitter, salty, and umami (meaty or savory).
• Flavours combine two or more of the primary tastes, in addition to other somatic sensations, meaning a feeling of the body.
• Odours from food pass upward from the mouth into the nasal cavity (retronasal olfaction).
• Since olfaction is more sensitive than taste, tasting is actually smelling.
• Taste is a uniform distribution on the tongue. Different tastes are not specific to different areas.

Taste Receptors

• Sweet tastes are activated by sugars, alcohols, ketones, and aldehydes.
• Salt tastes are activated by metal ions like Na+.
• Sour tastes are activated by free hydrogen ions and therefore by acids; stronger acid means lower pH.
• Bitter tastes are activated by alkaloids such as caffeine and nicotine.
• Umami tastes are activated by L-glutamate.

Taste Buds

• Approximately 10,000 taste buds are found on the tongue, soft palate, epiglottis, & pharynx.
• Their number declines with age.
• They are found in papillae elevations on the tongue.

Three Types of Papillae:

• Circumvallate Papillae contain taste buds, with 12 very large structures forming an inverted V-shaped row at the back of the tongue, housing 100–300 taste buds each.
• Fungiform Papillae are mushroom-shaped elevations scattered over the surface of the tongue, with 5 taste buds each.
• Foliate Papillae are located in trenches on the lateral margins of the tongue, most of which degenerate in early childhood.

One type of Papillae

• Filiform Papillae cover the entire surface of the tongue, are pointed and conical structures that act as tactile receptors and don’t contains taste buds.
• Increases friction between tongue and food, making it easier to manipulate.
• Allows for general sensory sensation of food, such as touch and pain.

Do Taste Receptors

• Do taste receptors detect "taste" that help determine what our body needs/craves?
• NO
• Sweet taste indicates energy-rich foods.
• Salty tastes indicate electrolyte-rich foods.
• Bitter and sour tastes are considered aversive tastes, and many sour/bitter foods are toxic or "gone bad."
• Umami indicates amino acids for building proteins.

Epithelial Cells:

• Each taste bud consists of three kinds of epithelial cells.
  • Gustatory Receptor Cells are specialized sensory receptors with hairs (cilia) projecting to the external surface through a taste pore.
  • Supporting cells surround the ~ 20 gustatory receptor cells in each taste bud.
  • Basal cells are stem cells near the connective tissue that differentiate into supporting and receptor cells and develop into new receptor cells every 10 days.
• Gustatory receptor cells synapse with dendrites of first-order neurons that form part of the gustatory pathway, and branch profusely and in contact with gustatory receptor cells in several taste buds

Tastants:

• Chemicals that stimulate gustatory receptor cells are known as tastants.
• These are dissolved in saliva and contact the gustatory hair plasma membrane.
• Receptor potentials develop in gustatory hairs, causing neurotransmitter release(ATP from plasma membrabe)
• Thresholds for tastes vary among the 5 primary tastes.
• Most sensitive to bitter, least sensitive to salty and sweet.

Receptor Potentials:

• Na+ enters gustatory receptor cells via Na+ channels in the plasma membrane to cause depolarization.
• H+ may flow into gustatory receptor cells via H+ channels or influence other ion channels.
• Sweet, bitter, and umami tastes bind to G-protein receptors (GPCR) in the plasma membrane, activate G proteins and second messengers --> cause neurotransmitter release
• Different tastes arise from activation of different combinations of groups of taste neurons.
• Each individual gustatory receptor cell responds more strongly to some tastants than to others.

Taste Threshold:

• The threshold for taste varies for each primary taste.
• Threshold for bitter is lowest because poisonous substances are often bitter, providing a protective function.
• Threshold for sour is higher.
• Threshold for salty and sweet are similar and higher than those for bitter or sour (variable).

Taste Adaptation:

• With continuous stimulation, adaptation occurs in 1-5 minutes.
• Taste adaptation is due to changes in taste receptors, olfactory receptors, and neurons of the gustatory pathway in the CNS.

"Tasting":

• Involves temperature, texture, pain, sight, colour, sound, expectation, memory, satiety, and smell.
• Smell plays a part in the taste.
• Loss of smell can lead to depression.

Gustatory Pathway:

• First-order gustatory fibers are found in cranial nerves.
  • Is V Trigeminal gustatory
  • VII Facial serves the anterior two-thirds of the tongue.
  • IX Glossopharyngeal serves the posterior one-third of the tongue.
  • X Vagus serves the palate, epiglottis, and oesophagus.

Gustatory Pathway Signals:

• Signals travel to the thalamus and then to the cortex, limbic system, and hypothalamus.
• Taste fibers extend from the thalamus to the primary gustatory area on the insular lobe of the cerebral cortex.
• Provides conscious perception of taste.
• The cranial nerves carrying taste sensation make it to the gustatory nucleus in the medulla: - Info goes to thalamus (VPN)
  • Routed to limbic system (Emotions) and hypothalamus (satiety) for appreciation of taste
• Info routed to cortex for perception.

Limbic and Cortex System:

• Info goes to other cranial nerve nuclei for salivation reflexes.
• The primary gustatory cortex is located on the insula in the temporal lobe.

Olfactory Summary:

• Adaptation occurs rapidly, with a low threshold for methyl mercaptan, using free nerve endings, with CN1 and tracts for the pathways for the temporal and Frontal lobe

Gustation Summary:

• Adaptation is slow, with variable threshold for bitter, sour, unami, salty and sweets
• Separate cell, and various pathways exist CNVII, IX, X for medualla, salavatory reflezes, thalamus and temporal lobe