Olfactory and Gustatory Pathways

Afferent Division of Nervous System

• The afferent division of neurons convey action potentials from receptors in the somatic or special senses to the central nervous system (CNS).
• This section will emphasize conscious and subconscious sensory information that reaches the sensory afferent division of the nervous system.

Functional Areas of the Cerebral Cortex and Special Senses

• Key functional areas include the primary motor cortex, premotor cortex, prefrontal cortex, Broca's area, primary somatosensory cortex, somatosensory association area, and common integrative area.
• Other key regions are the visual association area, orbitofrontal cortex, primary auditory cortex, auditory association area, gustatory cortex, and olfactory cortex.

Olfactory Epithelium

• Superior nasal concha houses olfactory epithelium.
• Contains 3 types of cells, supporting cells, olfactory receptor cells, and Basal cells.
• Bipolar neurons are olfactory receptor cells with a single knob-shaped terminal.
• Dendrites with olfactory cilia contain olfactory receptor proteins.
• Olfactory proteins are G Protein-linked receptors embedded in mucus.
• Proximal end contains an axon (CN I) that carries information to the olfactory bulb.
• Glomeruli and mitral cells synapse with secondary sensory neurons.
• Columnar epithelial cells in the membrane that lines the nose are supporting cells.
• Supporting cells help detoxify chemicals.
• Basal cells are stem cells located in the olfactory epithelium, between the bases of supporting and receptor cells.
• Basal cells divide to produce receptor cells and turnover about every two months.

Olfactory Pathway: Anatomy

• Paired masses of gray mater located on underside of frontal lobe form the olfactory bulb, containing secondary sensory neurons.
• Axons of olfactory bulbs project to olfactory cortex to form the olfactory tract.
• Temporal lobe's inferior and medial surface is where the olfactory cortex is located.
• Olfactory cortex connects to limbic and cerebral cortexes.
• Olfactory neurons show rapid adaptation, exhibiting a 50% adaptation rate within the first second of exposure.

Olfaction Pathway

• Odorant chemical molecules dissolve in and penetrate mucus before binding to olfactory receptor.
• Golf G protein is stimulated, which activates adenyl cyclase to produce cAMP.
• cAMP ultimately opens cation channels to allow Na+ and Ca+ ions to enter the cytosol.
• The depolarization triggers the formation of a depolarizing receptor potentiail in the membrane of the olfactory receptor cell.

Olfaction: Steps 7-10

• Step 7: Olfactory neurons synapse in the olfactory bulb, containing glomeruli.
• Glomeruli receive input from one type of olfactory receptor, and axons converge onto mitral cells where integration and sorting of smells takes place.
• Step 8: Sensory neurons in the olfactory bulb conduct action potentials along mitral cell axons, which bundle together to form the olfactory tract.
• Step 9: Axons of the olfactory tract carry information to the olfactory cortex on the temporal lobe.
• Awareness of smell originates in the olfactory cortex.
• Step 10: Impulses continue into the limbic system and cerebral cortex.
• In both the limbic system and cerebral cortex, there is a link between smell, memory, and emotions.
• Olfaction does NOT go first to the thalamus.

Gustation

• Taste is closely linked to olfaction.
• Non-neuronal epithelial cells with non-neuronal epithelial cells with microvilli are the taste receptors.
• Microvilli contain protein (chemical) receptor molecules in their membrane.
• Five distinguishable types of tastes include sour, sweet, bitter, salty and umami (MSG).
• Each taste cell is sensitive to only one taste.
• Most taste receptor cells are in the taste buds, clustered on the surface of the tongue.
• Each taste bud is composed of 50–150 taste receptor cells (TRC's), support cells, and regenerative basal cells.
• CN's VII (facial nerve) innervates anterior 2/3 of tongue, IX (glossopharyngeal nerve) innervates posterior 1/3 of tongue, & X (vagus nerve) innervates surface of throat and epiglottis.
• The gustatory epithelial cells live only 10-12 days before being replaced.

Taste Receptor Anatomy and Function

• Receptor transduction ligands bind with taste receptor cells.
• The taste transduction pathway for salty tastes responds to Na+ influx.
• Supporting receptors are salty and sour and are triggered by an influx of Na+ or H+ions.
• Gustatory receptor cells responds to the presence of bitter, sweet and umani, using G protein.
• Basal cells are precursor cells of taste receptors.

Taste Transduction: Salty and Sour

• Taste chemicals (tastant/ligand) dissolve saliva and mucus of mouth before gustatory transduction occurs.
• The salt (Na+) and sour (H+) tastants enter receptor cells through channels.
• Influx of Na+ and H+ causes a receptor potential.
• Depolarization causes voltage-gated Ca+ channels to open, allowing Ca+ ions to flow into the cell.
• The increase in intracellular Ca+ stimulates the release of neurotransmitters (NT) into the synaptic cleft.
• The neurotransmitter binds to and excites the first order neuron.
• Gustatory receptor cells respond to only one type of tastant, and have either ion channels or receptors for only one of the primary tastes.

Taste Transduction: Sweet, Bitter, Umami

• Taste chemicals (tastant/ligand) dissolves saliva and mucus of mouth before gustatory transduction occurs.

• Ligand binding causes a transduction pathway to occur for umani, bitter, and sweet tastes

• Sweet, bitter, and umami molecule (tastant) binds a specific receptor:

  • Gustducin, a G-protein, activates phospholipase C, creating inositol triphosphate, IP3

• IP3 opens transient potential receptor (TRP) channels TRPM5 that are in the plasma membrane.

• Influx of ions causes a depolarization of the receptor.

• The depolarization causes voltage gated Ca+ channels in the plasma membrane to open and allows passage of Ca+ into the cell.

• IP3 activates the endoplasmic retirulum to eject Ca+ into the cyctosol.

• Increase in cytosol Ca+ stimulates the release of neurotransmitters that bind to the first order neuron, eliciting action potentials.

Taste Transduction: The Last Two Steps

• Step 5/7: Taste neurons pass through CN's VII, IX, and X, ending at the gustatory nucleus of the medulla.
• Step 6/8: Taste neurons pass through the thalamus, ending at the gustatory cortex.

General Pathway of Light Rays to the Cerebral Cortex

• Light passes through the cornea, anterior chamber, aqueous humor, pupil, lens, vitreous chamber, vitreous humor, photoreceptors in retina (rods & cones), and optic nerve fibers.
• Information then moves along the optic chiasm, optic tract, lateral geniculate nucleus (body) of the thalamus and then the visual cortex of the occipital lobe of the cerebral cortex.

Vision Pathway

• Light passes through cornea, pupil, lens, and photoreceptors in retina.
• Visual impulses continue to optic nerve, optic chiasm, optic tract, and lateral geniculate body (thalamus).
• Then action potentials continue towards optic radiations(occipital lobe)

Anatomy of the Eye: Accessory Structures

• Eye is protected in orbits by the bones of the skull.
• Foreign objects, sun rays, & perspiration are protected against by the eyebrows and eyelashes. Shade from excessive light and protection from foreign objects are provided by the upper and lower eyelids.
• The lacrimal apparatus produces and drains lacrimal fluid through tears, containing lysozyme, which washes the surfaces of the eye.
• Eye movements are facilitated by extrinsic muscles, including superior & inferior rectus, oblique, and lateral & medial rectus muscles .
• Pupil size is dictated by the iris to modulate light exposure.
• Iris is the colored ring of pigment containing circular and radial muscles.

Eye Components

• The cornea bends incoming light towards the retina to initiate vision.
• The sclera is the "white" part of eye, gives its shape, and covers the eyeball.
• Inner surface of sclera is lined with choroid, containing pigment melanin (absorbs light).
• Extension of the choroid forms the ciliary body, containing smooth muscle, that extends the choroid and secretes/produces aqueous humor.
• Suspensory ligaments, or zonular fibers, are attached to lens, alters its shape when viewing objects.
• Iris - colored portion of eye
• Pupil - hole in the center of the iris
• Transparent tissue with 2 convex surfaces causes light to refract in order to be focused on the retina.
• Macula lutea is an oval area within the posterior retina that is responsible for central vision.
• Fovea, within the macula lutea, delivers highest visual acuity or resolution.
• The optic disc, or blind spot, is where the optic nerve and blood vessels don't contain photoreceptors.
• Anterior Cavity - in front of the lens filled with aqueous humor covered by cornea. - replaced
• Posterior Cavity - behind the lens, larger vitreous cambers filled with vitreous humor.
• Canal of Schlemm allows drainage of aqueous humor.

Pupil Responses to Light

• Pupil is the opening in iris that modulates light, enabling light entry to the eye through constricted or dilated muscular movements.
• Iris smooth pupillary muscles control pupillary constriction (circular muscle) & dilation (radial muscle): parasympathetic & sympathetic.
• Pupillary and Consensual reflexes are often measured in parts of neurological exams.
• Pupillary reflex is a consensual reflex in which light shown in the right eye causes the left pupil to constrict.
• Consensual reflex is the inverse effect, in which light shown in the left eye causes the right pupil to constrict.
• Oculomotor nerve CNIII controls pupillary reflexes.

Retinal Organization

• The retina converts light received into action potentials.
• The retina contains A) Pigmented layers containing melanin, responsible for stray light rays reabsorption
• Action potentials originate in the Neural layer: 1) Outer photoreceptors (rods and cones) convert light into receptor potentails 2) Neurons in the bipolar cell layer conduct action potentials from the outer region of the retina to the ganglion cells 3) the inner layer of neurons generate nerve impulse
• Action potentials travel along Optic nerve (cranial nerve II).

Lens Actions

• The eye forms clear images of objects via refraction of light, accomodation of objects on different planes, and constriction of the pupil.
• Light enters the eye and is refracted/bent at the cornea and lens
• Refraction is influenced by incident light.
• Convex lenses cause parralel light rays to focus.
• Concave lenses cause parallel light rays do diverge.

Accommodation

• The process by which adjustments to lens shapes allow for focused vision.
• The nearest distance where the lens can focus on an object sets the near point of accomodation.
• The lens flattens when we are focusing on distant objects.
• Lens becomes rounder/spherical when on nearby objects on retina.

Refraction Abnormalities

• Emmetropic eyes deliver clear images because of accurate focus of light onto retina
• Near-Sighted eyes, or myopic eyes, refract light too strongly, or the the eyeball is too long.
• Visual focus is too powerful with near sightedness, and focal point falls in front of the retina.
• Can't focus on far objects in myopia, but ability to focus on near objects remains.
• Far-sighted eyes, or hyperopic eyes, refract light too weakly, or the eyeball is too short.
• Visual focus is too weak, and so points in front of the retina.
• Limited ability to focus on close objects, distance vision is fine.
• Astigmatism- vision is distorted, with visual blurring.
• Astigmatism usually results from curvature irregularities in lens or cornea.

Photoreceptors: Rods & Cones

• Rods function well in low light and are used in night vision.
• Only grayscale and black/white perception is possible via Rods.
• Cones enable high-acuity and color vision during the daytime
• Outer segment: light transduction region.
• Inner segments houses cellular components.
• Synaptic terminal: vessels (with NT) that stimulate bipolar cell neurons.

Photoreceptors & Light

• Light absorption elicits a change in membrane potential.
• Rods contain rhodopsin, which absorbs most wavelengths of light.
• Cones contain three opsin pigments: red, green, and blue light- sensitive opsins enabling color vision.
• Colorblindness results one or more defective cone types, leading to difficulty distinguishing a range of colors.

Photopigments

• Photopigments contain the protein opsin and the light absorbing portion of the pigment known as retinal.
• Photopigments cycle between light and non-light absorbing compounds: 1) Isomerization converts retinal to its alternative form known as trans-retinal within a dark 2) Bleaching converts trans-retinal completely into nonlight reactive states 3) Retinal isomerase convers to cisretinal 4) Regeneration - resynthesis of photopigment via binding of cis retnal to opsin.

Photoreceptor Distribution in Retina

• Incoming light passes through three cells layers -ganglion cells, bipolar cells, & photoreceptors- before encountering the pigmented epithelium.
• Exception to this structural ordering exists at the fovea, which facilitates high acuity vision.
• Fovea are immediately surrounded by macula, enabling high acuity vision (cones).

Phototransduction

• Phototransduction is described for a rod, but similar in a cone.
• In darkness and with no exposure to incoming light cis-retinal binds to opsin to express rhodopsin
• Expression to rhodopsin in darkness triggers production of cyclic GMP (cGMP)
• Ca2+ influx is triggered, and depolarizes the transmembrane voltage of bipolar vessels (releasing neurotransmitters and inhibiting neural activity)
• Exposure to light halts production of cytoplasmic cGMP, hyperpolarizing transmembrane potential, inhibiting voltage-gated production of neurotransmitters to allow sensory neurons to proceed

Phototransduction Cont.

• Influx of light signals retina and retinal to transform from cis to trans, releasing opsin and bleaching photopigment.
• Upon exposure light to opsin, cellular g-proteins known as transduction are activated.
• Activation of transducing G protein degrades cyclic guanine monophosphate (cGMP)
• cGMP degradation closes Na+ channels and causing cell hyperpolarization.
• Inhibitting entry of CA2+ causes less neurotransmitters in cyctosol, stimulating bipolar cells to release glutamate.

Signal Processing

• Ganglion cell axons form the optic nerves.
• Within the optic chiasm, nerve impulses are sorted into visual fields.
• Right visual field neurons project to the left cerebral hemisphere, vice versa.
• At Thalamus axons bind with lateral geniculate body and then onto occipital cortex.
• Ocipital cortex is topgraphically orgnanized.

Visual Field Defects

• Lesions can cause monocular blindness
• Bitemporal hemianopsia results loss of the nerves in the center of the optic chiasm results in loss of vision of the left vision of the left eye and conversely on the right eye.
• Homonymous henianopia is when action potetials form one of the visual tracts, it cause loss of vision from the visual field to the corresponding damaged tract.

Hearing: Vibrations Transduced Into Sound

• Light enters the eye and is refracted/bent at the cornea and lens
• Light signals are sent to the optic tract
• Light enters through the pupil and is refracted via the lens
• Light must be just right - not too much or too little
• The cornea and lens send parallel light rays down the path to converge at a single point known as the focal point
• If parallel light rays strike a concave lens, light rays diverge
• If parallel light rays strike a convex lens it causes rays to converge at a single point

Human Ear Chambers

• Innermost canal contains malleus
• Outermost canal contains bones with connective ligaments and tendons
• Human ear has 3 regions: external ear, middle ear, & the innermost canal of the inner ear.
• The canal of the external ear funnels soundwaves into the ear canal
• The cannal of the external ear secretes a waxy substance or serumin
• The innermost chamber contains vibrations

Ear Middle Chamber

• Connects to the eustachaion tubes, or auditory tubes
• the air filled cavity found in our eers

Inner Ear

• Contains receptors for hearing.
• Includes cavities of different sizes and shapes
• The bone has various shapes associated with it

The Cochlea

• If uncoiled, the cannal has three parralel fluid filled chambers
• The scala vestibuli's membrane has no known function.

Cochlea cont. pt 2

• Outer partition is referred to the scala tympani.
• Fluid that separates the two endolymphs is the organ corgi.
• Vibration of the basilar of the membrane is what stimulates our hearing.
• The cochlear cannal has the stereocilia.

Vibrating Objets

• The vibrations from certain objects are known as soundwaves and are from something moving
• Soundwaves either travel through the air, on the ground, or any medium for that matter
• Most humans in order to even hear vibrations, one must have a sound from 20 hertz through 20000 hertz.
• Louder sounds or amplified sounds have certain measurements for them to be uncomfortable(120db ) and finally painful (140db).

Transmitting waves

 1) waves hit the pinna and proceed to hit the oval window 
 2) waves strike alternating patterns in the pressure 
 3) bones in the ears vibrate to the respective amplitude it should go
 4) tympanic fluid bulge as they travel through the external auditory canal
 5) as the window bulges waves cause the auditory cannal to travel to the auditory structures
  6) high amounts of waves cause the tympanic fluid to reach the scala tympani
  7) vestibular shifts to the basilar membranes and fluid vibrates the endolymph
 8) this produces the auditory receptors of the generation of waves

Inster Hair Cells

• In most of the time, the inner hair cells translate most of the cell's vibrations into electrical signals
• The hair cells travel and are in embedded in the tectorials. -Hair cells has a top lin and certain channels to allow the correct amount of calcium is correct for potential stimuli is met -A few channels are partially met for k+ and calcium, causing the hair cells to move through the cells. -This leads to the cells not meting with each other on the voltage gated potential due to certain blockage by the ions

Hairs Pt. 2

-The vibrations have the ability to depolarize a cell wall -With certain blockage of k+ and calcium, the ions are not able to enter the cell, causing hyperpolariation

• Vobrations also have the ability to repolarize a cell wall

Pitch and loudness Discrimination

• Pitch and loudness is due to the rate of certain actions.
• waves need to reach past the high point, near the helicotrema due to lack of frequency
• frequency waves need to be close to the oval window to vibrate to frequency
• Loudness needs to be high in order to achieve certain bending of the stereocilia.

Auditory Systems

• After sound waves travel to a certain point it transforms electrical signnals
• With electrical signnal action potential has to occure so you can sense the auditory
• If you have an auditory issue the damage may be due to loss of hair cells
• Due to neural pathway for the ears may be an issue

Neuronal pathways in audio

• certain sensory neurons in audio will send and process signals to you auditory coretexes.
• the signals you hear must first be high otherwise the sound you are trying to hear may not come across
• for any auditory that comes across, they must first pass sensory info

Audio Sensation

• Certain audio sounds are processed and certain sensory and recognition are able to be created
• the brain processes audio, which lets you differentiate certain voices from certain songs you recognize

Losing Hearning and Deafness

• Certain issues may or may not occur when coming across external damage
• there are many reasons for this such as, trauma to the ears or any inner or outer damage.
• certain damages that have been placed on the inner ears leads to issues such as sensory hearing loss.
• one may restore these damages with certain devices such as cochlear inplants.

Equillibirum

• There certain orgens for balence.
• there can be a set pathway to get to a certian sensory location to achieve balance to accelerate to a location
• there are canal system set in place as receptors and for you to be albe to have equilibrium to accelerate to where you wish.

Human Otolithics

• When you go forward or tilt your head the sensory receptors stimulate different parts of the brain for sensory input.
• the otholitic system makes it able to sense gravity or acceleration
• When sensing gravity to walk the otoliths signal the nervous system to do what is needed

Balance Pt. 2

• The balance system has the same properties as the normal hair cells found in our bodies
• Certain movements such as kincililin has certain properties found in it
• One you start the moving system you are able to generate what you wish to hear
• The head helps in directing the vestibular function that is on your body

Explore the mechanisms of smell and taste. This quiz covers olfactory pathways from stimulus to perception, olfactory receptor cells, supporting cells, and the olfactory tract. It also includes gustatory pathways, taste receptors, and cranial nerve involvement.