Neuro: Chapter 17

The Special Senses

• Sensory organs feature receptors enhancing environmental sensitivity.
• Special senses are smell, taste, vision, hearing, and equilibrium.
• Special sensory pathways share resemblances with general pathways.
• A key difference lies in the organization of special sense receptors within complex sensory organs like the nose, tongue, and eyes.

Chemical Senses

• Olfaction (smell) and gustation (taste) are chemical senses that rely on molecule interactions with receptor cells.
• Smell and taste are closely linked to the limbic system, triggering emotional responses and memories.

Olfactory Epithelium

• A one-square-inch membrane holds 10-100 million receptors.
• Spanning 5 cm² (less than 1 in.²), it lines the superior nasal cavity and cribriform plate.
• Three cell types are present.
• Turbinates or nasal conchae in the nasal cavity direct air towards it.

Olfactory Receptor Neurons

• They are first-order neurons of the olfactory pathway.
• Bipolar neurons possess an exposed knob-shaped dendrite and an axon which projects through the cribriform plate, ending in the olfactory bulb.
• Olfactory hairs (cilia) projecting from the dendrite respond to inhaled chemicals as the site of transduction.

Odorants

• Odorants are chemicals with an odor that stimulate olfactory hairs.
• Olfactory receptors create a generator potential upon odorant stimulation, initiating the olfactory response.

Olfactory Epithelium Cell Types

• Supporting cells are columnar epithelial cells lining the nose, which support receptor cells physically, metabolically, and electrically, along with detoxifying the epithelium.
• Basal cells undergo continuous cell division to produce new olfactory receptors and supporting cells, and are stem cells located between the supporting cells.
• Bowman’s glands produce mucus; it moistens the surface, dissolves odorants for transduction, and is innervated by CN VII. This stimulation prompts tears and a runny nose.

Olfactory Receptors

• Bipolar neurons are present with cilia, and the olfactory hairs enable the process of detecting smells.
• Columnar epithelium forms supporting cells.
• Basal cells are stem cells replacing epithelium monthly.
• Olfactory glands produce mucus.
• Cranial Nerve VII innervates both epithelium and glands.

The Sense of Smell

• Hundreds of primary scents exist.
• Recognizing 10,000 odors is based on activity patterns arising from combined olfactory receptor activation.
• A generator potential develops and triggers nerve impulses in olfactory reception.

Olfaction Physiology

• Odorant molecules bind to transmembrane receptor proteins (GPCR) in the olfactory hair plasma membrane. Stimulation of a G protein activates Adenylate cyclase, generating cAMP.
• cAMP opens sodium ion channels, causing an influx of sodium and depolarizing the receptor, which initiates a nerve impulse along the olfactory receptor axon.

Olfactory Transduction

• Odorant molecule binding activates G protein and adenylate cyclase, making cAMP.
• Cyclic AMP opens sodium ion (Na+) channels; Na+ ions enter, causing depolarization, which may create an action potential along the olfactory receptor axon.
• The olfactory receptors that are located on the olfactory receptor cells, which occupy a small area in the upper part of the nasal epithelium, detect inhaled odorant molecules.
• Upon binding of an odorant molecule to receptors, the olfactory receptor cells are activated, sending electric signals to the brain, and the signals are relayed in the glomeruli, well-defined micro regions in the olfactory bulb.
• In the glomerulus, the receptor nerve endings excite mitral cells, and the mitral cells forward the signal to higher regions of the brain with maintained specificity.

Olfactory Pathway Adaptation

• It adapts rapidly, exhibiting a 50% reduction in the first second; it adapts more slowly thereafter. Complete insensitivity to strong odors occurs about a minute after exposure.
• Impulses are conveyed to olfactory bulbs, olfactory tracts, cerebral cortex, and limbic system through olfactory nerves. Approximately 20 olfactory foramina exist in the cribriform plate of the ethmoid bone for unmyelinated olfactory receptor axons.
• Forty axon bundles in the right and left olfactory nerves terminate in the olfactory bulb's gray matter (cerebrum's frontal lobes, crista galli of the ethmoid bone). The axon terminals create synapses with cell bodies of olfactory bulb neurons. These extend to project to the olfactory cortical area though an olfactory tract.
• The cerebral cortex is on the medial temporal lobe, the base of the frontal lobe is sometimes included.
• The cerebral cortex receives these sensations without initial thalamus synapsing.

Influence and Memory

• Emotional memories are accounted for in Collateral axons along this tract which project to the limbic system and the hypothalamus. Examples include attraction from perfumes, and aversion from foods that once made someone ill.
• Odor identification and discrimination in the Orbitofrontal is influenced by the primary olfactory area extending to the frontal lobe.

Qualities of Olfaction

• Adaptation diminishes sensitivity
• Its is rapid, experiencing 50% in 1 second, and complete in 1 minute
• It has a low threshold, meaning only a few molecules need to be present
• Methyl mercaptan is added to natural gas as a warning.
• Noxious stimuli are detected by the trigeminal nerve in the posterior nasal cavity.

Olfactory Bulb

• It transmits smell from the nose to the brain.
• Direct input from olfactory nerves comes through the glomerular layer, made up of axons of approximately 10 million olfactory receptor neurons in the olfactory mucosa.

Glomeruli

• The axon all cluster here in spherical structures
• Input is received from olfactory receptor neurons that express the same olfactory receptor.
• Mitral cells relay signals to the olfactory cortex, dendrites from neurons are permeated by mitral cells.
• Interneuron types exist in the olfactory bulb, including periglomerular cells with synapse with glomeruli, and granule cells that synapse with mitral cells.
• It functions as sensory input from receptor neurons through mitral cell axons to the olfactory bulb.
• The bulb may be a filter as opposed to an associative circuit.

Mitral and Granule Cells

• Basal dendrites of mitral cells connect to interneurons known as granule cells. Some theories propose the granule cells produce inhibits mitral cells
• It’s not clear if that boosts odeor signal to noise ratio.
• Granule cell/ mitral cell synapse is called dendro-dendritic, and it releases neurotransmitters.
• The transmitter Glutamate is released from Mitral cells, and GABA is released by granules.
• Mitral cells are auto inhibited this way, while lateral inhibition occurs in neighboring mitral cells.

Olfactory Nerve Transduction

• The olfactory nerves (CN I) form from the axons of olfactory receptors, and synapse in the olfactory bulb.
• Axons pass in 40 foramina of the cribriform plate.
• The primary olfactory area for second order synapse if in the temporal lope, where awareness begins
• The limbic system receives collaterals.

Nasal and Neurological Links

• United-States experiences olfactory dysfunction. Causes include head trauma, respiratory infections, and tumors.
• Hyposmia, reduced ability to smell, has an increasing incidence with age, and affects 4 million Americans. It is related to neurological changes, such as head injury, Parkinson’s, or Alzheimer’s, and is caused by drugs.
• Some pharmaceuticals can alter ones sense of smell and taste.

Olfactory Ailments

• Anosmia is a lack of smell:
• Hyposmia includes decreasd sensations
• Dysosmia is distorted smell
• Parosmia is the perception of smell in the ABSENCE of of a stimulus
• Cacosmia is smelling foul smells
• Phantomia is smelling things that aren’t present.
• Complex Epilepsy and temporal lobe aura is associated with altered smells in the temporal lobe near the uncus.
• Early Parkinson and Huntingtons may impair smell
• Diagnosis can help

Olfactory Questions

• Receptors for smell are located in the (superior? inferior?) portion of the nasal cavity.
• Bipolar Neurons- receptor cells can be found here
• Columnar cells are the located in the nasal epithelium or lining
• Olfactory (Bowman’s) glands function in the nose? They produce mucus to moisten the surface and dissolve odorants for transduction.
• Because smell is a chemical sense, protein receptors in olfactory hair membranes respond to different chemical molecules, leading to a generator potential and nerve impulses.
• Rapid adaptation to sell occurs at first.
• Upon stimulation of olfactory hairs, impulses pass to cell bodies and axons . Cranial nerve 1 then terminates inferior to the Frontal Lobe of the Cerebrum.
• Limbic systems has the sensation of projection of what smell is associated with
• Pathways for smell are terminate in the lobe, and does not involve neurons of the thalamus enroute to the Cerebral Cortex. The left side of the hemisphere of the brain is more involved in smell.

Gustation Taste

• Taste, similar to olfaction, functions as a “chemical” sense
• Molecules need to be dissolved.
• The human taste comes from primary groups, salty, sour, sweet, savory or “umami”, and bitter, with umami indicating the presence of amino acide
• Odors of food enter through the nose and are involved in a more sensitive process called retronasal Olfaction
• Many receptors exist
• Primary tastes are linked to different molecules and receptors

Taste is a Sensory Input

• Sweet tastes: alcohol
• Salty: metal
• Sour: free hydrogen acid ions that produce stronger more favorable levels
• The receptors are not accurate, but stronger acids may lead to lower PH and more flavorful tastes
• Taste receptions and if foods are nutritious and helpful may not be accurate
• Amino acide/ protein may have umami indications.

Taste vs Sensory

• Localized distribution in the tongue is a classic, but untrue statement. A more accurate statement would distribute receptor sites uniformly

Tongue Facts

• Soft palates exist on the soft palate, epiglottis and pharynx
• The tastebuds have shown links to the lungs
• Aging results in declined tastebuds
• Papillae is a tongue’s elevation , and the buds within these elevated area are three in number
• Circumvallate and fungi-form have around 5 with an inverted row of v papillae, about a few hundred
• Buds are near connective material. Supporting cells have a 20 or so layer. Basal cells near that tissue, and the cells develop in those supporting layers.

Taste Buds Cont.

• Filliform papillae cover the tongue and and contain the food friction to aid it in touch and manipulation and receptors for tactile reasons
• Dendrites that synapse with first receptors start this route

Tasting Physiology

• Chemicals stimulating hair are "tastants"
• These dissolve in saliva first
• Then generate receptor in hair plasma to release near nerves such as neurotransmitters
• thresholds vary with taste and may be natural, or toxic
• Bitter is often sensitive due to poisonous or toxic properties of the substance

Reception of Taste

• Na+ salty ions enter cell membrane and stimulate neurotransmitters
• Bitter , sweet and savory, flavors bind with G proteins
• Taste occurs in groups for unique combination/ neurons that are specialized

Adaptation and Perception

• It is extremely variable and changes easily
• Bitter needs a 0.000004m volume, unlike the 0.002
• It adapts readily after moments to receptors , pathways and to olfactory as well.
• Although technically a bud it involves other aspects lie the texture, and temperature
• Smell greatly influences taste by about eighty percent
• Loss of smell , may come with cold or depression.

Tongue Nerves

• Certain areas of the mouth send “taste” indications
• V-X fibers help transport from the front 2/3 and after to the last end
• From their signals go to thalamus, cortical, cortex areas, or the thalamus.

Cranial Nerves in the Tongue

• Info goes to nuclei, and the spinal reflex
• Links to hypothalamus or system nodes
• Goes to thalamus
• Is involved in the function, tasting and production of brain stimulus
• Sensory data passes back from the facial. The motor neurons passes from facial to the cortex.
• That signals to the thalamus for processing

Hearing and Vision

• Cerebral sensory: half the receptors in the body is in the eyes
• Cerebral receptors and visual recognition is a large factor to processing data and receptors for vision

Eyelid Functions.

• They protect from sleep, debris, and light.
• The plates consist of: epidermis, skin, connective layer, orbiculares oculus, lacrimal glands, and conjunctiva
• Conjunctiva: mucus membrane that cover/ support the body, the front.
• The Tarsal plates has many structures to help in support and shape
• It also secretes meibum fluids to not stick to or connect areas

Overview

• One inch the the diameter of the eyeball.
• Most stays protected within the ocular cavity
• Muscles protect by sunlight.
• The “Sty” comes from congested or enflamed eyelash.
• They keep to eyelids as separate entities

Ocular Muscle Structures

• Exterior: muscle connections to these structures
• 3-4- 6 pairs and functions is linked
• Rectus muscles: medial and lateral
• Oblique: inferior position connected by the CN

Lacrimal Fluids

• It generates tears that aid by blinking spread, about ml or so. There is presence of lysosomes within it.

Eye Layers

• Their are three main categories
• Outer, Fibrous: which consist of other sublayers
• A vascular tunic: is an area that contains sublayers as well
• Neural system of the eye: Inner sublayers

Cornea and Transparency

• The surface transports as such, helps protect and functions in light functions
• Multiple epithelial exist
• Transplant procedures come as reliable functions since they have few risks, and it also is known as a humor

Structural White

• It it strong, the lens must not be damaged
• Fibers protect those ligaments and sections
• Glaucoma is commonly the issue

Vascular Choroid

• It carries pigmented epithelial and is known for nutrient values
• Muscular tissue allows the lense to bend

Iris Attributes

• It may influence the shape and sizes of functions in the pupil
• Smoothly regulating eye activity with contraction and pupil shape

Pupillary Control

• Sympathetic dilates, parasympathetic constrict. Can lead to different levels of light.

Lens and Eye Functions

• Lens: protein fibers and light
• Ciliary muscle focus the light
• If this cannot happen, it calls for a retina check. These also must account for potential damage to diseases that harm them

Lens Functions

• This attach to each other around the tension to it
• Tension aids in seeing
• Ciliary has a large influence on the bodies function

Optic

• Nerve function: posterior portion carries a signal. Has retina attached
• The BV supply from a point spreads, they also release from a section
• Insepection must be done because of pressure and diabetes functions
• The back detaches for multiple reasons: trauma, fluid etc.

• Cone types have differing outer
• Dim is specialized to rods
• Cones helps identify high definition scenarios