Special Senses: Smell and Taste

• Special senses include smell, taste, vision, hearing, and equilibrium.
• Special sense receptors are arranged in complex sensory organs, specifically the nose, tongue, eyes, and ears.
• Olfaction (smell) and gustation (taste) are chemical senses because their receptor cells interact with molecules.
• Smell and taste have a strong connection to the limbic system, so odors and tastes can evoke strong emotional responses or memories.

Olfactory Epithelium

• One square inch of membrane holds 10-100 million receptors.
• The total area is 5 cm² (a little less than 1 in.²).
• This epithelium covers the superior nasal cavity and cribriform plate.
• The nasal cavity's turbinates or nasal conchae direct inspired air toward the olfactory epithelium in the upper posterior region.
• Olfactory receptors are the first-order neurons of the olfactory pathway.
• Each olfactory receptor is a bipolar neuron and has an axon projecting through the cribriform plate and ending in the olfactory bulb.
• Olfactory hairs, or cilia projecting from the dendrite, respond to inhaled chemicals.
• Supporting cells are columnar epithelial cells of the mucous membrane lining the nose, and they provide physical and metabolic support and electrical insulation of the olfactory receptors.
• Supporting cells aid in detoxifying chemicals that contact the olfactory epithelium.
• Basal cells are stem cells located between the bases of the supporting cells and continually undergo cell division to produce new olfactory receptors and supporting cells.
• Basal cells differentiate constantly, and produce new olfactory epithelium about every 30 days.
• Olfactory (Bowman's) glands produce mucus and moisten surfaces to dissolve odorants for transduction, and the glands are innervated by CN VII.
• Impulses in CN VII can stimulate the lacrimal glands in the eyes and nasal mucous, leading to tears and a runny nose after inhaling substances like pepper and ammonia.
• Chemicals with an odor that can stimulate the olfactory hairs are called odorants.
• Olfactory receptors respond to odorant molecule chemical stimulation by producing a generator potential, which initiates the olfactory response.

Olfactory Tract

• Genetic evidence suggests hundreds of primary scents exist.
• Ability to recognize 10,000 different odors depends on patterns of activity in the brain arising from activation of different combinations of olfactory receptors.
• An odorant binding to an olfactory transmembrane receptor protein (GPCR) in the plasma membrane is the first step of olfactory reception.
• GPCRs are coupled to a G protein, then activate Adenylate cyclase, followed by cAMP production, which opens sodium ion channels, and sodium inflow.
• Sodium inflow causes a depolarizing generator potential, leading to nn impulse generation and propagation along the axon of olfactory receptor
• Adaptation to odors occurs quickly, with a low smell threshold where needing only a few molecules of some substances in the air are needed to smell them.
• Adaptation occurs rapidly, with a 50% reduction in the first second, with complete insensitivity to strong odors within minutes.
• Olfactory receptors send nerve impulses through olfactory nerves to olfactory bulbs, olfactory tracts, the cerebral cortex, and the limbic system.
• Unmyelinated axons of olfactory receptors extend through ~20 olfactory foramina in the cribriform plate of the ethmoid bone.
• Approximately 40 axon bundles form the R and L olfactory nerves terminating in the olfactory bulb in the brain and below the frontal lobes of the cerebrum and lateral to the crista galli of the ethmoid bone
• It is thought that the primary olfactory area, where conscious awareness of smell begins, is on the medial aspect of the temporal lobe, which sometimes includes the frontal lobe base.
• The 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 and memory evoked responses to odors.
• The primary olfactory area extends to the frontal lobe for odor identification and discrimination in the orbitofrontal area and the R hemisphere exhibits greater activity during olfactory processing.
• 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 and approximately ten million olfactory receptor neurons in the olfactory mucosa
• Odorant receptors are located on the olfactory receptors cells occupy a small area in the upper part of epithelium and detect inhaled odorant molecules.
• The ends of the axons cluster in spherical structures called glomeruli such that each glomerulus receives input primarily from olfactory receptor neurons that express the same olfactory receptor.
• Glomeruli are permeated by dendrites from neurons called mitral cells, which output to the olfactory cortex.
• Interneuron types exist in the olfactory bulb including periglomerular cells, which synapse within and between glomeruli, and granule cells which synapse with mitral cells.
• The olfactory bulb has many inputs from receptor neurons of the olfactory epithelium and many outputs with mitral cell axons and functions as a filter.
• Mitral cell basal dendrites connect to interneurons known as granule cells, theorized to produce lateral inhibition, and the synapse between mitral and granule cells is dendro-dendritic.
• Mitral cells release glutamate and granule cells release Gamma-aminobutyric acid (GABA)
• A dendro-dendritic synapse can cause mitral and neighbors to inhibit themselves.
• Axons from olfactory receptors (Cranial nerve I) synapse in the olfactory bulb by passing through 40 foramina in the cribriform plate.
• Second-order neurons within the olfactory bulb form the olfactory tract that synapse on the primary olfactory area mainly of the temporal lobe where conscious awareness of smell begins..
• Other collaterals lead to the limbic system

Olfaction - Adaptation and Threshold

• Adaptation results in decreasing sensitivity because olfactory adaptation is rapid in that 50% happens in 1 second and it completes in one minute.
• The threshold required to smell something is low where only a few molecules are required to be present and methyl mercaptan is added to natural gas as warning
• The trigeminal nerve innervates the posterior nasal cavity to detect noxious stimuli, and is involved in adaptation and threshold

Pathologies of Olfaction

• Olfactory dysfunction may stem from head trauma, respiratory infections, tumors, or toxic chemicals.
• Hyposmia, a reduced ability to smell, affects as many as 4 million people in the US and can be caused by neurological changes, antihistamines, and drugs.
• Anosmia is the absence of the sense of smell
• Hyposmia is a decreased sensitivity to odors
• Dysomia is a distortion of the sense of smell
  • Parosmia is a perceiving an odor when appropriate stimulus is not present
  • Cacosmia is perceiving a bad or foul smell
  • Phantosmia is the perceiving of smell in the absence of an odorant
• Early Parkinson disease and Huntington’s disease are associated with olfactory dysfunction.
• Uncinate fits (foul odors) of temporal lobe are known effect of seizures

Questions to Test Your Knowledge of the Olfactory System

• Smell receptors are in the superior portion of the nasal cavity.
• Receptors consist of bipolar neurons located between columnar epithelium cells.
• The distal end of each olfactory receptor cell consists of dendrites that have cilia known as olfactory hairs.
• Basal stem cells continually undergo cell division to produce new olfactory receptors and supporting cells.
• Mucus secretion is the function of olfactory (Bowman's) glands in the nose
• Smell is a chemical sense where protein receptors in olfactory hair membranes respond to different chemical molecules, so a generator potential is produced and then one or more nerve impulses.
• Smell adaptation occurs rapidly at first and then happens at a much slower rate.
• For stimulation of olfactory hairs, impulses pass to cell bodies and axons and these olfactory cells pass from the nasal cavity to the cranium to terminate in the olfactory bulb.
• The olfactory bulb is located just inferior to the frontal lobes of the cerebrum.
• Cell bodies in the olfactory bulb then send axons through the olfactory tracts, where one route extends to the limbic system.
• The effect of smell impacting the limbic system accounts for emotional and memory-evoked responses to odors, and another olfactory pathway terminates in the temporal lobe.
• Pathways for smell are unique in that they do not involve the thalamus en route to the cerebral cortex, where the right hemisphere is more actively involved in olfaction.

Gustation (Taste)

• To be detected, molecules of flavors must be dissolved in a medium.
• Primary tastes include sour, sweet, bitter, salty, and umami (meaty or savory).
• All other flavors are a combination of 2 or more of the 5 primary tastes, somatic sensations, and retronasal olfaction involving odors.
• Up to 80% of flavors relies on plugging nose to eat, which diminishes perception of taste.

Types of Taste Receptors

• There are taste receptors for sweet, salt, sour, bitter, and umami.
• Sweet receptors are activated by sugars, alcohol, ketones, aldehydes, etc.
• Salt receptors are activated by metal ions, most commonly Na+.
• Sour receptors are activated by free hydrogen ions.
• Bitter receptors are activated by alkaloids such as caffeine and nicotine.
• Umami receptors are activated by L-glutamate.
• Sweet tastes indicate energy-rich foods, salty foods indicate electrolyte-rich foods, and tasting some bitter and sour "toxic" components is considered aversive
• Umami indicates food high in amino acids for building proteins.
• The classic distribution in humans for taste is inaccurate where it is instead more uniform.
• Around 10,000 taste buds are found on the tongue, soft palate, epiglottis & pharynx in elevations called papillae
• A taste bud can be inside the lungs!
• As body ages, the number of taste buds declines. Three types of elevations on the tongue, or papillae, contain taste buds:
  • Circumvallate Papillae have 12 large structures in an inverted V- shaped row at the tongue's back, housing 100-300 taste buds each.
  • Fungiform Papillae are mushroom shaped elevations on the surface of tongue with 5 taste buds each.
  • Foliate Papillae - located in small trenches at tongues sides and diminish in early childhood

Cells in taste buds

Consist of three epithelial cells:

• Gustatory Receptor Cells - sensory receptors with hairs that project into the taste pore
• Supporting Cells - approximately 20 gustatory receptor cells are surrounded by these
• Basal Cells - stem cells at connective tissue layer near periphery of the taste bud - they differentiate and develop into new receptor cells about every 10 days
• The surface of the tongue is also covered in Filiform Papillae, used for friction and general sensations
• Pointed papillae act as tactile receptors, they increase friction, and they do not have taste buds
• Dendrites from first-order neurons branch and contact receptor cells in several taste buds, where the cells synapse

Stimulation (Taste)

• The compounds that stimulate the gustatory receptor cells are called tastants and must dissolve in saliva.
• Once dissolved, these enter papillae, interact with the hair plasma membrane, and produce receptor potentials.
• The potentials encourage neurotransmitter release-
• Thresholds for tastes vary in that the most sensitive is to bitter (harmful toxins are almost always bitter) and least sensitive to salty & sweet

Receptor Potentials- Salts, Acids, Bases

• Receptor potentials arise differently for different gustatory stimulations:
• Salts have Na+ that enters gustatory receptor cells, and the ion buildup causes depolarization.
• Acids have H+ that may flow into taste bud, and the ion influences ion flow (K+). BINDING stimulation occurs for sweeter, savory, umami substances; the ligand binds cell protein- linked (g proteins) to activates messengers and ultimately trigger potentials and sensations
• Activation of different groups of taste nerves result in the interpretation of a certain tastes.
• A given receptor can respond to more than one primary taste but differs in what stimulation it responds to best.
• Variations among population can include people who have less taste buds or the amount and population of taste buds is different
• This accounts for variability in the human interpretation of taste

Thresholds

• Measure of concentration (units) is moles and threshold is variable Bitter substances have different concentrations of stimulating compounds to trigger interpretation of the signal by the brain and these tend to be poisonous which accounts for higher brain stimuli (0.0000004 M). • The higher the concentration, the lower the threshold (easier to stimulate) The rate of interpretation of taste diminishes over time on a matter of minutes after introduction of a taste signal in both taste bud receptors and brain- linked sensations (due to fatigue)

More on Flavors

• Flavors have three components: temperature texture pain
• The majority of components are determined by smell with up to 80% of “taste” determined by the response to what is being smelled • If one can’t smell this diminishes the perception of taste greatly

The Gustatory Pathway

Cranial Nerves + Taste

• Innervation occurs from certain cranial nerves. 3 components are used: nerve 5, nerve 7 (transmits signals to 2/3 of the tongue, nerve 9 (transmits signals to the posterior third of the tongue) and Cranial nerve X that innervates parts of pallet epiglottus and esophagus

From Tongue to Brain

• The gustatory areas have pathways routed to parts of the brain • the brain components stimulated are the thalamus (VPN-part), limbic system including the hypothalamus or cortex for ??? • Reflexes are routed to cranial nuclei

Gustatory Pathway in Depth

• As information goes from tastebuds in the cranial nerves carrying the signal it is directed to the taste bud nucleus (which is where?) where it goes to the thalamus (VPN), limbic system with hippocampus and hypothalamus, and the cortex

Vision

• Over half the bodies sensory receptors are located in the eyes and therefore a large part of the cerebral cortex is devoted to it's understanding.

Eyelids, Eyelashes, Eyebrows and Conjunctiva

• Eyelids (palpebral) shade the eyes during sleep, protect the eyes from sunlight and debris and they are composed of skin, muscle fibers and conjunctiva
• Tarsal plates are part of the eyelids and assist in supporting the shape Tarsal (meibomian) glands produce fluid to retain bulbar conjunctiva hydration
• Connectiva is the delicate membrane that lines eyes and the inner part of the eye lids. Eyelashes and eyebrows prevent foreign body implantation

Eyeballs+Extrinsic Eye Muscles

• Eyeballs function inside the skulls bony orbit and 5\6 in this region.
• Eye lash glands have sebaceous secretions such that with clogging can produce inflamed sty.

Lacrimal Apparatus

Approximately 1 ml of tears are produced, released via blinking and lysozyme and contain bactericidal enzymes

Layers of Oue Tissue

Consisting of three layers

• Tunic Fibrous is the connective and epithelial layer
• Tunic Vascular contains retina
• Tunic Neural innermost- Contains nerves tissue

Fibrous (Outer) Tunic - Clear Cornea and White Sclera

• The transparency aids cornea wih light refracting ability and is transplantable due to a lack of presence antibodies that fight against tissue rejection
• The tears are made of aqueous fluids that supply this tunic, which aid in tissue nourishment
• The eye (Sclera) is densely connective, supportive, white colored and pierces to the back of the optic nerve (CN2)

Vascular (Middle or Uvea) Tunic - Includes Iris, Choroid, and Ciliary Body

• Iris -The hole in the center is the pupil that contains blood, muscle, fiber which regulates light The outer, pigmented epithelial cells of the retina called melanocytes nourish the black-pigmented retina.
• The Ciliary body contains ciliary muscles, smooth fibers alter shape and the the ciliary processes release aqueous liquids

Lenses (Not Part of the Middle Tunic)

• The lense consists of crystallin arrangements and the lens are fixed and supported and suspended by ligaments. With age these lose crystalline proteins arranged like layers in onion
• The clear capsules transparent and allows for focusing of data as with increasing elasticity with increase in age
• When ciliary processes are attached to ligaments will attach and therefore change shape and controls tension with muscles-

Internal Retina+View

• View via an Ophthalmoscope reveals the outer optic disc where nerves reach to give image, a spot on the temporal portion of this layer and visible and able to transmit data from an entire organ Detatched Retina can be fixed with fluids and is seen in the case of trauma , diabetics or AIDS

Photoreceptors

• Outer segments have shapes to receive stimulus Rods are for light and Cones are more distinct by the fovea and enable distinct color acuity.
• the Macula Luta is in alignment to visual center Fovea is found in the central macula lotta and is the sharpest image area due to cones Rod cells are peripheral only
• Photopigments are integral proteins that react to color using A vitamin derivatives to trigger the receptor potential on the disc

Neural Layer Composition from back to Front

• The pigment has light aborbing, light and images. • Three Layers of Nerves: Photoreceptors, Bipolar neuron + Ganglion neuron • The other two types of cells modify nerve signals: horizontal and macular cells modify.

Visual Pathway and Stimulation in Depth

• Light will pentrate retinal cells, causing action and then the code and cones stimulate bipolar and pass through ganglion cell Ganglion signals and axons pass from the nerve disc (also blindspost) and the info passes to the Thalamus later geniculate Nucleus • Third order neurons end at Visual Cortex in the brain's occipotal lobe Intracoular cavities - there are anterior + posterior fluids filled with humorous

Intraocoular Fluid

• The amount that is present controls intraoculour pressure which glaucoma is often linked to that creates eyeball smoothness The aquous humour made with ciliary fluid, with lymph drainage contains shlemm channels and vessels (blood stream drainage

Image Formation

• Light forms images via lens adjustment, with retina in 2 ways: retina + cones Refraction - is a physical property and the most refraction happens wigh light rays that fall to retinga
• Cornea ( 43 diop) Lens ( +20 diop)

Refraction

Is dependent on the difference between light that hits the object, and the refractive surface The eye refracts with greatest degree at the cornea and then then and then refracts more so through lens or with convex qualities Focality is increased with lense adjustment and vision gets adjusted with objects in direction

Retina in Depth

• Images refract onto the retina and the brain mirrors and relearns patterns 75% of is done via light rest 1s by light The degree of how far object rays ( above 20") focus is increased during accomodation and the process involves increasing the lenses Diopter value

Eye Function and Accommodation

• The focus is determined by nerve, lens accommodation and the pupil An overstimulus or stress can lead to a limitation or limitation

Clinical Applications for Vision

4 aspects aid in visual acuity

• Retina position-
• Proper accommodation
• And pupillary constriction An Emmetropia rating indicates normal vision of a distant fixed point with a relaxed cilaruly muscle Myopia indicates near sightedness whereas a hyperotipa is the indication of far sightedness Clinical considerations are done with vision assessment that uses chart for testing a 20 /20 base

Clinical Pathologies

Astigmatism - Irregular curviture on surface and and cannot be offset by accommodation Prebyopia- Lens accommodation looses power over time from thichning and elasciticity los Glacomua- Increase from damage on the retina is caused either as close narrow of the angle or open angle

Visual Physiology &Photoreceptors

• There are four opsins which are connected color vision.The process starts with an absorption of light by pigment and cells and are made of outer segments
• The colors then combine for color vision where green red and blue light are received on outer and the pigment then allows images is a non visual process.

The Optic Nerves, Light Transduction, Visual Information

• Pigment can alter with processes like bleaching or generating to allow vision for dim versus dark places after adaptation
• The signal then can be modulated - horizontal versus amaricrine

What happens during visual acuity for the eyes and receptors? Rods are for movements light and cones are for detailed color stimuli and sharp images

• Signals are transmuted to bipolar with electrical synapsing to axons which generate signals to ganglioside nerve The fibers exit and travel to opoic chlasm in what one pathway

Ganglion and the Visual Pathway

Information then then pass at light / darkness and to other segments from optic at chassam toward thalamus

Vision - Pathology

• Lack of color and abnormalities happens and is genetically linked , night blindness - from defincence , or A vitiatom deficiency
• Retinbulisa pigmento and degeneration
• A chart is used for eyesight test called snelin

Hearing and Equilibrium

The three sections of the process are separated between • Auricle directs sound waves to outer tube Eardrum vibrates in response to pitch range and causes malleus to incus to be to be touched. stapes to be activated • Stapes vibrates and it's vibration are stronger causing cochlear and sound

Middle Ear Components

• Tensor tympani limits movement, increases tension in the chord, is part of facial Nerve vii: the branch innervates in middle part of ear ( stapedius) and hypercusis causes hearing difficulty
• The auditory tube leads to the nasopharynx with an Eustachan or tympani part helps equilibrate ear with opening by whistling of yawning, the pathway of pathogends if there is an middle infection/ Otitis • Bony Labyrinth - consist of a series of three areas names , semi circular canals and a vestibule which the structure aids in maintaining balance

Structure and Equilibrium

• Semi circular Canal function is the maintenance of the orientation in space • The anterior + posterior canals face vertical, while lateral faces horizontal:
• there are 2 primary tubes which all serve two roles which are -The Utricle and saccule contains macula receptors that help provide position for balancing orientation

Nerve Impulses and Signal Transmission

• There many forms and aspects relating to balance. Bony labyrinth contains lymph , and the movement in the direction is with help/
• If that is in balance then its not right/ off

How Does Hearing Actually Work- Cont.

• Inner cells Transform vibrations/action potentials along the membrane and the vibrations work by the outer cells enhancing. • Stereocillia: are hair cells that when open and released there is depolarization+potential

Auditory Signals- and Pitch Volume

• The hearing sounds exist from molecules in the that the and there by air and range by frequency from hertz.

The threshold - frequency 3000 range where to which high low that where and volume relate / pitch

There volume = intensity; volume relates amount of a vibration or is related to how many actions sent Volume: a conversion at DB scale above the norm can be a potential risk of harm to function

Hearing Damage

Bending of stereociiolia generates signals and is transmitted 1  Hair cells bend due to force