Olfaction and the Sense of Smell

• Special senses include smell, taste, vision, hearing, and equilibrium.
• Special senses are housed in complex sensory organs, unlike general senses.
• Special senses are located in specific areas of the body.

Sense of Smell (Olfaction)

• The sense of smell is also known as olfaction.
• Sensory receptors for smell are chemoreceptors.
• Chemoreceptors are located at the top of the nasal cavity.
• These receptors detect tiny chemicals that bind to the olfactory nerve which is cranial nerve I.
• The binding of chemicals to receptors creates an action potential to olfactory areas of the brain.
• Sensory cells are located in the epithelial lining of the nose's mucous membrane.
• Olfactory neural chemoreceptors have specialized cilia.
• Specialized cilia help detect chemicals in the air.
• Neurons connected to the olfactory bulb send messages to the brain's olfactory center, which interprets smell.

Olfactory Pathway

• Odorants bind to receptors, opening sodium channels.
• Depolarization occurs.
• A nerve impulse is triggered.
• Information travels along the olfactory nerve to the temporal lobe.
• The temporal lobe is what processes the sense of smell.

Adaptation and Odor Thresholds

• Adaptation is decreasing sensitivity over time to a stimulus.
• Olfactory adaptation occurs rapidly at 50% in 1 second, completing in about a minute.
• A low threshold for odors means only a few molecules are needed for detection.
• As an example, natural gas is odorless and needs a chemical added for detection.

Sense of Taste (Gustation)

• Taste is also known as gustation.
• Taste buds are found on the tongue, soft palate, and larynx.
• Taste receptors are chemoreceptors
• The five basic taste sensations are sweet, salty, sour, bitter, and umami.
• Taste buds are located on the tongue, and in other areas of the mouth..
• Papillae are small elevated projections on the tongue.
• The types of papillae on the tongue are:
• Fungiform (sweet)
• Circumvallate (bitter)
• Foliate (sour)
• All taste buds contain taste salty.
• Filiform papillae are sensitive to touch.
• Gustatory cells are specialized receptors found within each taste bud (50-125 per bud).
• Gustatory cells have special hair-like projections.
• Hair like projections are sensitive to various chemicals and act as stimuli for various taste sensations.
• Gustatory nerves are connected by cranial nerves to the medulla oblongata.
• Messages are relayed to the thalamus and then the gustatory center of the brain, where the stimulus is interpreted.
• Taste and smell are closely associated.

Taste Bud Structure

• Structures are oval-shaped.
• Consists of about 50 receptor cells.
• Surrounded by supporting cells.
• Single gustatory hair protrudes through taste pore.
• Basal cells develop into new receptor cells every 10 days.

Physiology of Taste

• Complete taste adaptation takes 1-5 minutes.
• Varying taste thresholds exist.
• It is more sensitive to bitter tastes (potential poisons).
• It is least sensitive to salty and sweet tastes.

Taste Pathway

• Dissolved substances contact gustatory hairs.
• It results in neurotransmitter release.
• A nerve impulse is formed in the first-order neuron.
• Signals travel to the thalamus, limbic system, and hypothalamus.
• Conscious perception occurs in the primary gustatory area of the parietal lobe.

Auditory (Hearing) and Balance

• The ear is a dual organ responsible for hearing and functioning as the organ of balance.
• The sensation of balance and hearing are both associated with specialized receptors with hair-like mechanoreceptors (hair cells).
• The ear is divided into three major regions:
• External: Pinna or auricle and auditory canal
• Middle: Tympanic membrane (eardrum) and auditory ossicles (malleus/hammer, incus/anvil, and stapes/stirrup)
• Inner: Semicircular canals and vestibular nerve (balance); cochlea, round or oval window and the cochlear nerve (sound)

Sense of Hearing

• Three parts of the ear: outer, middle, and inner.
• The middle ear contains ossicles: hammer, anvil, and stirrup (malleus, incus, stapes).
• The inner ear structure known at the cochlea contains hearing receptors.
• The Organ of Corti has mechanoreceptors (hair cells).

Hearing (Outer Ear)

• The pinna is the ear flap that collects and directs sound waves into the ear.
• The auditory canal channels waves to the eardrum and houses ceruminous glands.
• Ceruminous glands secrete ear wax (cerumin) to keep the eardrum moist.
• The tympanic membrane/eardrum catches vibrations and sends them to inner ear bones; separates the outer and middle ear.

Hearing (Middle Ear)

• The Eustachian tube connects the ear to the throat and equalizes pressure.
• Middle ear contains ossicles.
• Hammer (malleus) - outermost bones, receives vibrations from the eardrum.
• Anvil (Incus) - middle bone.
• Stirrup (stapes) - innermost bone, transfers vibrations to the cochlea.

Hearing (Inner Ear)

• The vestibule a space between the cochlea and semicircular canals.
• The cochlea is a snail shaped structure filled with fluid from endolymph and perilymph and contains the organ of corti.
• The organ of Corti has tiny hair like cells that pick up vibrations and transfer them to the auditory nerve.
• The semicircular canals contain liquid (perilymph).
• The semicircular canals have tiny hair-like cells that blend with motion to help maintain equilibrium
• The auditory nerve transfers information from the ear to the temporal lobe of the brain.
• The perilymph is a thin liquid in spaces of the inner ear.
• The endolymph is a thick liquid found in the cochlear ducts of the inner ear.
• The vestibule contains organs of equilibrium.
• The cochlea is the organ of hearing.

Vestibular Apparatus

• Deals with equilibrium.
• Equilibrium is coordination, balance, and orientation in three-dimensional space, and it can be static or dynamic
• Vestibular apparatus are receptors equilibrium that consist of:
• Three semicircular ducts that detect only angular acceleration
• Two chambers (saccule and utricle) are responsible for static equilibrium and linear acceleration.

Hearing Pathway

• Sound waves enter the outer ear.
• The eardrum (tympanic membrane) vibrates.
• Ossicles vibrate and amplify sound.
• Fluid in the cochlea moves and hair cells in the organ of Corti bend.
• Action potential is created.
• Signals travel along the vestibulocochlear nerve (Cranial Nerve VIII) to the temporal lobe.
• Signals then travel from pinna, auditory canal, tympanic membrane, malleus, incus, stapes, cochlea, organ of corti, auditory nerve to temporal lobe of brain.

Sense of Balance (Equilibrium)

• Located in the inner ear.
• Two main components: vestibule and semicircular canals.
• Receptors are mechanoreceptors (hair cells).
• Detects head position and movement.

Vestibular System (Static Equilibrium)

• Located in the vestibule.
• Contains otoliths ("ear stones").
• Detects head position relative to gravity.
• Also responds to linear acceleration/deceleration.
• Otoliths move, bending hair cells.
• Action potential is sent via the vestibular branch of CN VIII.

Semicircular Canals (Dynamic Equilibrium)

• Three semicircular ducts exist: anterior, posterior, horizontal.
• Detect rotational movement.
• Contain hair cells covered by cupula.
• Endolymph fluid movement bends cupula.
• Stimulates hair cells to release neurotransmitter.
• Signals indicate direction of head rotation.

Dizziness and the Vestibular System

• Inner ear system can cause rapid spinning of the body which can lead to dizziness
• Endolymph continues moving after body stops, leading to continued stimulation of hair cells.
• Eyes move back and forth (nystagmus).
• Brain receives conflicting information from eyes and the vestibular system.

Sense of Vision

• Located in the eyes.
• Accessory organs: eyebrows, eyelids, eyelashes, and lacrimal apparatus.
• Eyeball has three layers: sclera, choroid, and retina.
• There are two cavities: anterior (aqueous humor) and posterior (vitreous humor).
• Extrinsic eye muscles control eyeball movement.
• Intrinsic eye muscles control pupil size and lens shape.
• Circular muscles constrict the pupil.
• Radial muscles dilate the pupil.

Visual Pathway

• Light enters the eye and stimulates photoreceptors.
• A nerve impulse travels along the optic nerve which is Cranial Nerve II.
• Information crosses at the optic chiasm.
• Signals reach the lateral geniculate nucleus of the thalamus.
• Final processing occurs in the occipital lobe of the cerebrum.

Photoreceptors: Rods and Cones

• Located in the retina.
• Rods are responsible for night vision.
• Cones have three types (red, green, blue) for color vision.
• Light causes hyperpolarization of photoreceptors.
• Neurotransmitter (glutamate) release decreases in the presence of light.

Importance of Special Senses

• Allow interaction with and interpretation of the environment.
• Work together to provide a complete sensory experience.
• Crucial for survival, communication, and quality of life.
• Disorders in special senses can significantly impact daily functioning.