Special senses

Taste buds are replaced every

10 days

olfactory cells are replaced every

50 days

cyclic guanosine monophosphate (cGMP)

A second messenger within photoreceptors that is responsible for maintaining the dark current by opening sodium channels

Accommodation

Accommodation is the ability of the eye to alter its focusing ability on different objects. This is achieved by altering the shape and focal length of the lens.

Amacrine cells

Amacrine cells are laterally orientated neurons that form synapses with bipolar and ganglion cells. They modulate photoreceptor inputs, including color and brightness.

Aqueous humor

Aqueous humor is a clear, watery fluid that is filtered from blood capillaries in the ciliary body and is constantly secreted. It plays an important role in maintaining intraocular pressure and drains via an opening, the scleral venous sinus, located at the junction of the cornea and sclera. It also provides nutrients to the avascular cornea and sclera.

external ear parts

Auricles External acoustic meatus (auditory canal)

Basal cell

Basal cells are another type of supporting cell present in the olfactory epithelium. They sit at the base, sandwiched between the olfactory receptor and supporting cells, and do not make contact with the free surface. They act as stem cells, constantly replenishing the supply of olfactory receptor cells.

Bipolar cells

Bipolar cells are radially positioned cells with two processes that extend from either side of their cell bodies. Their axons synapse with a ganglion cell, while their dendrites synapse with the photoreceptor cells. Bipolar cells relay and integrate information from the photoreceptor cells to the ganglion cells, as well as any inputs from surrounding horizontal and amacrine cells.

Parts of the cochlea

Bony labyrinth Membranous labyrinth

Olfactory receptor cells consist of three parts

Cell body, dendrite and axon

__________on the microvilli detect molecules of food (tastants) dissolved in saliva.

Chemoreceptors

Vascular tunica: the pigmented, vascular coat parts

Choroid Ciliary body Iris

parts of the membranous labyrinth

Cochlear duct (scala media)

Cone cells

Cone cells are sensitive to high light levels and provide us with color vision. Cone cells are sensitive to high light levels and provide us with color vision. There are three types of cone cells: blue, green, and red and the combination of signals from all three types enables us to see different colors. Their inputs are perceived in color, as the photopigments they contain absorb various wavelengths of light. However, unlike rod cells, they have fewer connections with bipolar cells and as a consequence, the information passed onto ganglion cells is much more detailed and results in higher resolution vision.

Convergence

Convergence is the term given to the ability of the eyes to maintain binocular vision and track a common object moving towards the eyes. In order to do this, both eyes rotate medially to maintain the same focal point on the back of both retinas.

How vision happens

Cornea Light hits the cornea and passes through the pupil. Pupil Light passes through the aperture of the pupil to reach the lens. Lens The lens refracts the light so that it is focused onto the retina at the back of the eye. The ciliary muscles adjust the curvature of the lens so that the light is focused correctly on the back of the retina, in a process known as accommodation. Retina Light is detected by the rod and cone cells of the retina and is converted into neural signals that travel along the axons of the ganglion cells. These axons converge at the optic disc to form the optic nerve. During the passage of light through the retina, the visual field is reversed (the image is flipped), so that the image appears upside down and the left side appears on the right side of the retina and vice versa. Optic nerve (II) Axons from the ganglion cells in the retina pass into the optic nerve, which runs through the optic canal (foramen) to the middle cranial fossa, before reaching the optic chiasma. Optic chiasma Here, half of the visual information crosses to the opposite side: fibers from the medial side of the retina enter the contralateral (opposite) optic tract whereas those from the lateral side of the retina remain uncrossed, traveling in the ipsilateral (same side) optic tract. Optic tracts These fibers continue in the optic tracts to the brain. Thalamus With some exceptions, both contralateral and ipsilateral tract fibers synapse at the thalamus (some fibers terminate in the superior colliculus and suprachiasmatic nucleus). From here, the fibers form the optic radiation and project to the visual cortex of the occipital lobe. Association tracts connect the visual cortex to the visual association area for interpretation.

direct gustation transduction

Direct passage is the passing of ions into the cell through ion channels in the plasma membrane. The direct passage mechanism is associated with salty and sour tastants, which use sodium and hydrogen ions, respectively. Sodium ions enter the receptor cell through the sodium ion channels in the cell's plasma membrane. The entry of positively charged ions into the plasma membrane causes the inside of the cell to become more positive and this change in membrane potential triggers depolarization of the cell. Na+ Salty H+ Sour

ampullae

Each ampulla contains mechanoreceptors, known as the crista ampullaris.

cupula

Each crista ampullaris consists of a group of hair cells and supporting cells together with a gelatinous mass that sits on top of these cells known as the cupula. The cupula is a gelatinous mass that sits atop the hair cells and supporting cells of the crista ampullaris. It functions to increase inertia and movement of the stereocilia and kinocilia.

Lacrimal canal

Each lacrimal punctum leads to a lacrimal canal, which drains the tears into the lacrimal sac.

Squamous epithelium

Each taste bud is formed from the epithelial and connective tissue of the tongue. The apical surface of each taste bud is comprised of non-keratinized stratified squamous epithelium. This epithelium consists of a gradient of flat cells stacked on top of one another.

Sensory neurons of the gustatory cells

Each taste bud is innervated by gustatory nerve fibers that are in contact with the base of gustatory receptor cells. These weave in and around the cells in the taste bud, innervating each gustatory cell and carrying nerve impulses to the brain. Activation of different groups of taste neurons enables the brain to evaluate the different combinations of impulses, giving us the ability to distinguish between different tastes.

Eyebrows

Eyebrows are thought to direct perspiration and rays of the sun away from the eyes.

Eyelashes

Eyelashes help to protect our eyes from airborne particles, while the eyelids react rapidly to protect the eyes from injury.

indirect gustation transduction

G protein coupled receptor Tastants do not enter the cell, but instead bind to G protein coupled receptors on the surface of the plasma membrane. The G protein coupled receptor mechanism is associated with bitter, sweet, and umami tastants. Mechanism Ions bind to plasma membrane receptors linked to a G protein called 'gustducin'. Upon binding to the receptor, the G protein separates and activates enzymes also present on the plasma membrane, such as phospholipase-C or adenylate cyclase. These enzymes initiate complex cascades that lead to the release of secondary chemical messengers, provoking depolarization of the cell by either increasing calcium ion inflow or reducing potassium ion outflow.

Ganglion cells of retina

Ganglion cells are multipolar neuron cells. Their dendrites synapse with bipolar, horizontal, and amacrine cells. They also have long axons that form the nerve fiber layer. They are the output neurons of the retina as they relay sensory information about visual stimuli to the brain, via the optic nerve.

glutamate

Glutamate hyperpolarizes the bipolar cells and prevents them from sending signals to the ganglion cells.

Gustatory receptor cells

Gustatory receptor cells are the main sensory cells of the taste bud. Due to their exposure, they are replaced every 10 days. These cells are capable of transduction Gustatory receptor cells have a large, round nucleus. Their apical surface is covered in numerous microvilli (or gustatory hairs), which serve to increase the surface area of the cell that is exposed in the mouth. The basal surfaces of these cells are also in contact with the nerve fiber endings of the various cranial nerves innervating the tongue.

maculae parts

Hair cells Supporting cells Striola Otoliths Otolithic membrane

hair cells in ear

Hair cells are the sensory transducers of the organ of Corti. They rest on the basilar membrane with their stereocilia (hairs) embedded in the underside of the tectorial membrane. There are two rows of hair cells, outer and inner, which are separated by rows of supporting cells. There are three or four rows of outer hair cells, but only a single row of inner hair cells.

auricles (pinna) of the ear parts

Helix Tragus Lobule

Horizontal cells

Horizontal cells are laterally extending cells that synapse with photoreceptors. They act as inhibitory interneurons.

Hyperopia

Hyperopia, or far-sightedness (the person can see far away objects), is a refractive abnormality of the eye caused either by a thinner than normal lens or when the eyeball is shorter than usual. Both result in the image converging behind the retina, meaning that distant objects can be seen clearly but close objects cannot.

strabismus

If the contraction of the extrinsic eye muscles is not co-ordinated properly, either because a muscle is paralyzed or because it has failed to develop all its nerve connections, then a strabismus (cross-eye) arises. This will affect the quality of vision, and in particular, the ability to see in 3D.

gustatory pathway

Impulse sent from gustatory receptor cells to: Glossopharyngeal nerve (IX) Facial nerve (VII) Vagus nerve (X) to: Medulla oblongata: gustatory nucleus to: Thalamus, hypothalamus, and limbic system to: Primary gustatory area

Limbic system function for taste

Impulses that reach the limbic system from the gustatory nucleus help us to appreciate what we are tasting. The limbic system is important in determining emotional states and memory formation, so when taste messages arrive here, we experience pleasant or aversive feelings towards certain foods.

cis-retinal, trans-retinal

In darkness, retinal (the light-absorbing part) has a bent shape, called cis-retinal, which fits tightly into the opsin part. Cis-retinal When cis-retinal absorbs light, it straightens out to a shape called trans-retinal. This cis-to-trans conversion is called isomerization.

Inner hair cells

Inner hair cells are pear-shaped cells that curve slightly towards the surface of the organ of Corti. Owing to their position on the basilar membrane, they do not move much in response to vibrations, as this part of the membrane is immobile. Each inner hair cell has 3 or 4 wavy rows of stereocilia and it is these structures that make contact with the tectorial membrane. Function Both the inner and outer hair cells of the organ of Corti act as sensory transducers, converting mechanical energy into neural energy. Together, they detect and send signals to the brain, conveying information about the amplitude and frequency of sound waves that enter the ear.

Parts of the retina, 10 layers

Internal limiting membrane Nerve fiber layer Ganglion cell layer Inner synaptic layer Bipolar layer (Syn. Inner nuclear layer) Outer synaptic layer Outer nuclear layer External limiting membrane Photoreceptor layer Pigmented epithelium

Olfactory dentrites have

It terminates in a bulb-like process, from which 10-20 olfactory cilia project. It is here that the trapping of odorants and sensory transduction occurs: odor molecules are inhaled and get trapped in the overlying layer of mucus where they bind to receptors in the plasma membrane.

lacrimal apparatus parts

Lacrimal gland Lacrimal punctum Lacrimal canal Lacrimal sac Nasolacrimal duct

Foliate papillae

Located along the lateral surface of the tongue, close to the termination of the terminal sulcus. Foliate papillae are similar in structure to fungiform papillae, with a different orientation due to their position on the sides of the tongue. They bear numerous taste buds in their walls and on their apical surfaces. Innerved by: Glossopharagyl Nerve

Anterior chamber

Located between the cornea anteriorly and the iris posteriorly, the anterior chamber is filled with aqueous humor which allows light to pass through it.

Fungiform papillae

Located on the anterior part of the dorsal surface of the tongue. Fungiform papillae Fungiform papillae look like mushroom-shaped elevations. They have taste buds present both in their walls and on their apical surfaces. Innervation Facial nerve (VII).

Filiform papillae

Located over most of the presulcal, dorsal area of the tongue. Filiform papillae are small, conical, cylindrical projections. They are tall and pointed and can be found arranged in diagonal rows extending anterolaterally across the tongue. Their apical ends are often split into numerous processes. This type of papilla has no taste buds. Innervation Glossopharyngeal (IX) and facial nerves (VII).

Motion sickness

Motion sickness is an equilibrium disorder caused by the brain receiving conflicting signals about the immediate environment, both from the eyes and the vestibular organs. For example, if you are in a cabin on a boat, visual information sent to the brain from the eyes would indicate that the surrounding environment is stationary and fixed. However, the vestibular organs would still detect the motion of the ship at sea. As the brain usually receives consistent information about the surroundings from these two sense organs, it becomes confused and responds by inducing the symptoms of nausea, dizziness, and sickness.

inferior rectus of eye

Moves the eye so that the cornea is directed downwards (depression). down and toward the opposite ear Oculomotor nerve (III)

lateral rectus of eye

Moves the eye so that the cornea is directed laterally (abduction). Innervation toward the ear Abducens nerve (VI)

Medial rectus of eye

Moves the eye so that the cornea is directed medially (adduction). Innervation middle toward nose Oculomotor nerve (III).

superior rectus of eye

Moves the eye so that the cornea is directed upwards (elevation) and medially (adduction). up toward the opposite ear Oculomotor nerve (III)

Mucus

Mucus is a viscous fluid produced by the olfactory glands in the olfactory epithelium. It is secreted onto the free surface of the olfactory epithelium, keeping it moist so that odorants can dissolve and bind to receptors on the surface of the olfactory receptor cells.

Myopia

Myopia, or near-sightedness (the person can see near objects), is a refractive abnormality of the eye caused either by a thicker than normal lens or when the eyeball is longer than usual. Both result in the image converging in front of the retina, meaning that close objects can be seen clearly but distant objects cannot.

Olfactory gland

Olfactory (or Bowman's) glands are short, single-cell lined mucous glands that penetrate the olfactory epithelium. The duct of the gland sits in the connective tissue below the cribriform plate. They produce mucus, which functions to keep the surface of the olfactory epithelium moist, allowing odorants to dissolve within it.

Olfactory supporting cell

Olfactory supporting cells are tall, columnar epithelial cells that sit adjacent to olfactory receptor cells in the olfactory epithelium. These cells provide nourishment, support, and insulation to the olfactory receptor cells.

Opsins

Opsins are the glycoprotein part of photopigments. They combine with the light absorbing part, known as retinal. The type of opsin to which a molecule of retinal is bound determines the wavelength of light that a photopigment can absorb. Rods only have one type of opsin, whereas cones have three.

Otoliths

Otoliths are small, calcium carbonate crystals that sit on top of the otolithic membrane. They function to increase the inertia of the membrane by increasing its weight.

Outer hair cells

Outer hair cells are long, cylindrical cells usually about 20-50 µm in length; almost twice as tall as inner hair cells. They are arranged by size in a gradient, with the outermost hair cell and those closest to the apex of the cochlea being the tallest. Each outer hair cell has a flat apical surface, through which 3 rows of stereocilia protrude. Function The outer hair cells in particular are the key to the high sensitivity and sharp tuning ability of the organ of Corti. When the stereocilia of the outer hair cells are displaced, the entire cell shortens and elongates causing displacement of their stereocilia. The outer hair cells then push up against the tectorial membrane, amplifying the vibrations felt by the basilar membrane at specific points, thus enabling us to hear quieter sounds. Outer hair cells therefore play a special role in sound transduction as mechanical amplifiers

Parts of a cone cell

Outer segment Inner segment Outer fiber Cell body Inner fiber Stalk Synaptic ending

Parts of a rod cell

Outer segment Inner segment Outer fiber Cell body Inner fiber Stalk Synaptic ending

Pigment epithelial cells

Pigment epithelial cells are cuboidal-shaped, melanin-containing epithelial cells that house the outer segments of the photoreceptor cells. The pigment epithelium breaks down the discarded membranous discs of the outer segments of the photoreceptor cells.

superior oblique of eye

Posterosuperior surface of the eyeball. Uses the trochlear as a pulley. Depresses the posterior aspect of the eye, depressing the cornea. Innervation Down and toward outer cheek Trochlear nerve (IV).

Lacrimal gland

Produces tears (containing oils, water, mucin, and antimicrobial proteins), which travel down tiny ducts in the upper eyelid to the surface of the eye: the conjunctiva.

Refraction of light

Refraction (or bending) of light rays occurs when traveling light comes into contact with a surface of a different density to that in which it was previously traveling. When light hits the junction between these two mediums, it changes speed, and as a consequence, meets the surface at a different angle.

Neural tunica: the innermost, neural layer parts

Retina Macula lutea Fovea centralis Optic disc Optic nerve (cranial nerve II)

visual pathway

Retina ↓ Optic nerve ↓ Optic chiasma ↓ Optic tracts ↓ Lateral geniculate nucleus ↓ Optic radiations ↓ Primary visual cortex

retinal

Retinal is the light-absorbing part of photopigments derived from vitamin A and it exists in two forms: trans-retinal and cis-retinal. Cis-retinal is the kinked form retinal takes when it is bound to an opsin, whereas trans-retinal is the straightened form that retinal twists into, which enables it to detach from an opsin.

rod cells

Rod cells also differ from cones in the number of intermediary connections that they receive from bipolar cells. Many rods synapse with one bipolar cell and the resultant high convergence of information causes indistinct and fuzzy vision.

Rod cells

Rod cells are sensitive to low light levels and provide us with 'black and white' vision.

parts of bony labyrinth

Scala vestibuli Scala tympani

Fibrous tunica: the fibrous, outer coat parts

Sclera Cornea

auditory pathway from hair cells

Spiral ganglia ↓ Vestibulocochlear nerve: cochlear branch ↓ Cochlear nucleus ↓ Lateral lemniscus ↓ Superior olivary nucleus ↓ Inferior colliculus ↓ Medial geniculate nucleus ↓ Primary auditory cortex

Stapedius

Stapedius is a small muscle of the middle ear. It is attached to the temporal bone and neck of the stapes. It dampens the vibrations of the ossicles by contracting in response to high intensity sounds.

Hair cells of the ear

Stereocilia (hairs) Kinocilium

Stereocilia (hairs)

Stereocilia are membrane-bound, cellular projections of the inner and outer hair cells. They are arranged in unique patterns on the apical surfaces of both types of hair cell, playing a vital role in sound transduction. Stereocilia protrude from hair cells through the reticular membrane and point towards the tectorial membrane that lies above. It is this relationship that is integral to their function, however, only the tallest of the outer hair cell stereocilia make contact with the tectorial membrane. Function Stereocilia bend forward and backward in response to a shearing force created by the movements of the tectorial and basilar membranes, which also create a streaming movement in the surrounding fluid. These movements cause the stereocilia to bend and subsequently depolarize, sending information to the rest of the hair cell and turning the mechanical energy of a sound wave into a neural signal.

The maximum number of taste cells is located on the tongue, where they are grouped to form

Taste buds

Taste

Taste is detected by taste cells located on the tongue, soft palate, oropharynx, epiglottis, and inner cheeks.

Lacrimal punctum

Tears begin to collect at the medial commissure, where a tiny hole on each eyelid called the lacrimal punctum is found.

Tensor tympani

Tensor tympani is a small muscle of the middle ear. It originates from the pharyngotympanic tube and greater wing of the sphenoid bone. It inserts into the malleus and works with stapedius to tense the tympanic membrane in response to abrupt noise and in anticipation of loud vocalization.

transduction

The ability to convert chemical stimuli into a neural signal that can be interpreted by the brain.

Ampullary nerve

The ampullary nerve is the superior branch of the vestibular branch of the vestibulocochlear nerve (VIII). It sends sensory information about balance and proprioception from the ampullae of the anterior and lateral semicircular ducts to the brain. An inferior branch of the vestibular nerve supplies the ampulla of the posterior semicircular duct.

Anterior semicircular duct

The anterior semicircular duct is orientated almost vertically and is often named the superior semicircular duct.

auricles

The auricles sit on either side of the head and are composed of, and shaped by, elastic cartilage that is covered with skin. The shape of the auricles causes sound to be directed into the external acoustic meatus.

Basilar membrane

The basilar membrane is the membrane that supports the organ of Corti. It separates the cochlear duct (scala media) from the scala tympani. The basilar membrane is not uniform in width or thickness, as it is wider, thinner, and more pliant at the apex than at the base. It deforms in response to pressure waves and provides structural support to the organ of Corti.

Bipolar layer (Syn. Inner nuclear layer)

The bipolar layer consists of bipolar, amacrine, and horizontal cells located between the inner and outer synaptic layers.

bony labyrinth

The bony labyrinth is a tube of bone that coils around a central pillar called the modiolus. It resembles a snail shell. Between the bony labyrinth and membranous labyrinth sit two fluid-filled spaces: the scala vestibuli and scala tympani.

Choroid

The choroid is the dark, brown layer that contains a rich network of blood vessels and lymphatic vessels to drain excess fluid from the posterior cavity. Arterial supply to the choroid is via the ciliary arteries.

Ciliary body

The ciliary body is the anterior, thickened portion of the choroid that is composed of smooth muscle. It sits just behind the iris and forms a ring around the lens. Together, the ciliary body and lens divide the anterior cavity into anterior and posterior chambers. Suspensory ligaments project from the ciliary body and attach it to the lens. The ciliary body supports the lens and iris, adjusts the curvature of the lens, and secretes aqueous humor into the posterior chamber.

Cochlear duct (scala media)

The cochlear duct is the middle, triangular canal that houses the organ of Corti. Forming its roof is the vestibular membrane, and its floor is the basilar membrane. It is a blind tube with the only outlet being the small ductus reuniens, which connects it to the saccule. The cochlear duct, unlike its neighboring canals, contains endolymph.

cochlear nerve

The cochlear nerve is a branch of the vestibulocochlear nerve (VIII). Its fibers pass from the hair cells of the organ of Corti and through the modiolus as the spiral ganglia, before continuing onto the internal auditory meatus.

Cornea

The cornea is the avascular transparent dome over the anterior part of the eye. Its functions are to refract (bend) light towards the pupil and to limit the amount of light that enters the eye.

crista ampullaris

The crista ampullaris are a saddle-shaped ridge of sensory cells involved in the maintenance of equilibrium. They detect the rotational acceleration and deceleration involved in dynamic equilibrium. When the head is moved, the endolymph moves slightly afterwards and drags over the crista ampullaris. This causes the cupula and the embedded stereocilia of the hair cells to bend, producing a receptor potential, which is transmitted to the brain.

External acoustic meatus (auditory canal)

The external acoustic meatus is an S-shaped passage that begins as an opening in the auricle. It travels about 4 cm through the tympanic portion of the temporal bone to terminate at the tympanic membrane. The external acoustic meatus is lined with mucous membrane containing sebaceous glands and modified sweat glands known as ceruminous glands. Their secretory ducts can combine or open independently into the external auditory canal. The secretions of the ceruminous and sebaceous glands combine to form cerumen (earwax). This waxy secretion prevents the entrance of foreign particles into the ear and reduces the risk of bacterial and fungal infection. The external acoustic meatus directs sound waves to the tympanic membrane.

External limiting membrane

The external limiting membrane consists of synapses between photoreceptors and the surrounding supporting cells.

Eyelids

The eyelids protect the eyeball anteriorly. They are separated by the palpebral fissure and join at the medial and lateral commissures. They are thin layers of skin, surrounding the thin tarsal muscles which are smooth muscles that assist in widening the palpebral fissure in fight or flight situations. The upper lid is elevated by levator palpebrae superioris, which is controlled by CN III. The palpebral margin is closed using orbicularis oculi, a facial muscle controlled by CN VII.

Fovea centralis

The fovea centralis is a small area at the center of the macula lutea. It is the area of the retina where vision is most accurate.

Ganglion cell layer

The ganglion cell layer consists of millions of ganglion cells located between the inner limiting membrane and the inner synaptic layer.

Gustatory nucleus

The gustatory nucleus is a collection of neurons in the medulla oblongata associated with taste. It receives nerve impulses from the glossopharyngeal (IX), vagus (X), and facial (VII) nerves relaying information from the taste buds. From the gustatory nucleus, axons project to the thalamus, hypothalamus, and limbic system. As information is fed through the gustatory nucleus, it sends signals to the stomach and mouth to increase saliva secretion, thus preparing the digestive system for food.

Helix

The helix forms the outer, curved margin of the auricle. It originates at the crus of the helix, superior to the opening of the external acoustic meatus. The helix continues from here to form the free edge of the auricle, terminating superior to the lobule.

hypothalamus function in taste

The hypothalamus is a series of nuclei located anteriorly in the diencephalon, inferior to the thalamus, and in the lateral wall of the third ventricle. The hypothalamus has roles in mood, memory, and emotion, via its limbic system component. The hypothalamus is also involved in determining whether or not we experience aversive or pleasant feelings towards specific foods.

incus

The incus is the middle ossicle and resembles an anvil or a bicuspid tooth. It is located in the epitympanic recess of the tympanic cavity (middle ear) and it articulates with the stapes.

inner ear

The inner ear contains the sensory structures for both hearing and balance. It consists of a network of bony canals contained deep within the temporal bone. The parts of the bony labyrinth are the cochlea and the semicircular canals, which are connected together by a centrally-placed vestibule. Only the cochlea is involved with hearing. Within the bony labyrinth lies the membranous labyrinth. It consists of the cochlear duct and three semicircular canals, which are connected together by the centrally placed saccule and utricle. The membranous labyrinth is filled with a fluid called endolymph. Between the membranous labyrinth and the bony labyrinth is a fluid called perilymph, which acts to cushion the membranous labyrinth.

Inner synaptic layer

The inner synaptic layer consists of synaptic connections between cells in the ganglion cell layer and cells in the bipolar cell layer.

Internal limiting membrane

The innermost layer of the retina is the internal limiting membrane, which consists of a simple, thin basement membrane that tops the nerve fiber layer.

Iris

The iris is a pigmented ring of smooth muscle that forms the aperture known as tmacuhe 'pupil'. It contains two rings of muscles: the pupillary constrictor muscles and the pupillary dilator muscles. The iris changes the diameter of the aperture of the pupil.

Kinocilium

The kinocilium is the single, longest cilium protruding from the apical surface of the cell. It is also embedded in the gelatinous mass of the otolithic membrane. Function Stereocilia and kinocilia depolarize or hyperpolarize, depending on the direction in which they are bent. This causes an increase/decrease in their neurotransmitter release, which affects the firing rate of the vestibular branch of the vestibulocochlear nerve (VIII), and the combination of impulses sent to the brain regarding the spatial orientation of the head.

Lacrimal sac

The lacrimal sac is the upper dilated end of the nasolacrimal duct. It is oval in form and measures 12 to 15 mm in length. Its upper end is closed and rounded, and its lower end continues as the nasolacrimal duct, which is lodged in a deep groove formed by the lacrimal bone and frontal process of the maxilla.

lamina propria

The lamina propria is the connective tissue deep to the epithelial layer. It attaches to the basement membrane of the epithelium and contains the nerve fibers and blood vessels that supply the tongue.

Lateral semicircular duct

The lateral semicircular duct is orientated almost horizontally and lies most laterally.

Lens

The lens is a transparent, avascular, elliptical structure held behind the aperture of the pupil by the suspensory ligaments of the ciliary body. It refracts light to a focal point on the retina and is able to change shape to accommodate viewing objects at closer or further distances.

Lobule

The lobule is a highly vascularized flap of skin that forms the most inferior part of the auricle. It does not contain any cartilage.

Macula lutea

The macula lutea consists of a small area of cells on the retina, directly in line with the lens. It is the area of the retina that provides the clearest vision and contains only cone photoreceptors.

Saccule

The maculae in the saccule are nearly vertical and respond to vertical movements.

Utricle

The maculae in the utricle are positioned horizontally and detect horizontal movements.

Malleus

The malleus is the largest and most lateral ossicle of the middle ear; its name is derived from its resemblance to a mallet. It is attached to the tympanic membrane and articulates with the incus.

Membranous labyrinth

The membranous labyrinth is a coiled tube containing the cochlear duct (or scala media).

modiolus

The modiolus is the cone-like, central pillar of bone that forms the center of the bony labyrinth of the cochlea, an axis around which the spiral canal of the cochlea runs. It contains the spiral ganglia of the vestibulocochlear nerve (VIII) in small cavities known as Rosenthal's canals.

Nasolacrimal duct

The nasolacrimal duct drains into the inferior meatus of the nasal cavity, beneath the cover of the inferior nasal concha.

Nerve fiber layer

The nerve fiber layer consists of ganglion cell axons, which leave via the optic disc.

Optic disc

The optic disc (also known as the blind spot) is located approximately 3 mm medial to the macula lutea. It is the point from which the ganglion cells from the retina leave the back of the eye to form the optic nerve. There are no photoreceptor cells on the optic disc, therefore, it cannot respond to light, thus creating the blind spot.

Ossicles

The ossicles are three tiny bones: the malleus (hammer), incus (anvil), and stapes (stirrup), which transmit and magnify sound from the tympanic membrane and across the tympanic cavity to the oval window of the inner ear.

Otolithic membrane

The otolithic membrane is a thick, gelatinous mesh in which the stereocilia and kinocilia of the hair cells are embedded. The otoliths sit on top of the otolithic membrane. Function Together with the otoliths, the otolithic membrane increases inertia and movement of the stereocilia and kinocilia. When the head is tilted forwards for example, gravity pulls the otoliths and otolithic membrane downwards, bringing the stereocilia and kinocilia with it.

Outer nuclear layer

The outer nuclear layer is an extension of the photoreceptor layer, containing the cell bodies of the photoreceptor cells.

Outer synaptic layer

The outer synaptic layer consists of synaptic connections between photoreceptor, bipolar, horizontal, and amacrine cells.

Oval window

The oval window is an opening between the middle and inner ear that articulates with the footplate of the stapes. It allows the footplate of the stapes to transmit the sound vibrations from the middle ear to the inner ear.

Pharyngotympanic tube (Eustachian tube)

The pharyngotympanic tube, also known as the Eustachian tube or auditory tube, connects the middle ear to the throat (nasopharynx) and allows the pressure within the cavity to remain equal to local barometric pressure.

Photoreceptor layer

The photoreceptor layer consists of photoreceptor cells (rod and cone cells). Rod and cone cells are responsible for image formation and the 'light-trapping' power of the retina.

Pigmented epithelium

The pigmented epithelium is the outermost layer of the retina that sits next to the choroid of the eye. It comprises a single layer of pigmented epithelial cells.

Posterior chamber

The posterior chamber is located between the iris anteriorly, and the lens, suspensory ligaments, and ciliary body posteriorly. It is filled with aqueous humor and allows light to pass through it.

Posterior semicircular duct

The posterior semicircular duct is orientated almost vertically and lies most posteriorly.

Primary gustatory cortex

The primary gustatory cortex is located on the inferior part of the post-central gyrus of the parietal lobe. It receives and processes gustatory signals from the taste buds of the tongue and enables us to become conscious of taste.

Retina

The retina is attached to the optic disc at the back and to the ora serrata at the front. It contains the photoreceptors and associated neurons and fibers. The retina is the inner lining of the posterior eye. It consists of 10 layers of neural, epithelial, and supporting cells.

Scala tympani

The scala tympani is the lower, triangular canal comprising the space beneath the basilar membrane. It passes through the entire length of the cochlea. At its base sits the round window, which connects it to the tympanic cavity, and at its apex the helicotrema joins it to the scala vestibuli. The scala tympani also contains perilymph.

Scala vestibuli

The scala vestibuli is the upper, triangular canal, bound by the roof of the bony cochlea above and the vestibular membrane below. It passes through the entire length of the cochlea. The oval window sits at the base of the canal, where it connects to the tympanic cavity. The helicotrema, an opening that joins the scala vestibuli to the scala tympani, sits at the apex. The scala vestibuli contains perilymph, which is maintained by the secretory epithelial cells lining the canal.

Sclera

The sclera is the white of the eye, and is avascular. Its functions are to prevent light from entering the eyeball and to maintain eye shape.

crista ampullaris

The semicircular canals contain mechanoreceptors called crista ampullaris that detect dynamic equilibrium i.e., the angular movements of the body.

Spiral ganglion

The spiral ganglion lies in the modiolus. It contains the cell bodies of the cochlear branch of the vestibulocochlear nerve (VIII) and also supplies the hair cells of the organ of Corti. Fewer ganglion cells are found in the middle turn compared with the apical and basal turns.

Stapes

The stapes is the smallest and most medial ossicle of the middle ear. Its name is derived from its resemblance to a stirrup and it articulates with the oval window of the inner ear.

Striola

The striola is the central region of the otolithic membrane. In the utricle, kinocilia are orientated towards the striola, whereas the opposite is true in the saccule.

Terminal sulcus of tongue

The surface of the tongue is split into oral and pharyngeal parts

Taste threshold

The taste threshold or 'lowest concentration" of a tastant you can detect is different for each of the five primary tastes. Bitter tastes have the lowest thresholds (meaning we are most sensitive to bitter tastes), sour and umami tastes have a slightly higher threshold, sweet and salty tastes have the highest (we are least sensitive to sweet and salty tastes).

Tectorial membrane

The tectorial membrane is a semi-transparent, fibrogelatinous structure that overlies the hair cells of the organ of Corti. The tectorial membrane is indented by the stereocilia of the outer hair cells in a W- or V-shaped pattern along a ridge known as Hensen's stripe. Function The tectorial membrane has a purely mechanical role. It can withstand the mechanical stresses associated with cochlear fluid vibrations, meaning that as the basilar membrane moves in response to reception of sound waves from the middle ear, shearing forces move the stereocilia back and forth underneath the tectorial membrane, which in turn, bends the stereocilia and depolarizes them, causing neural signals to be sent to the brain.

Connect to the utricle

The three semicircular ducts are housed in corresponding canals within the bony labyrinth. They are the anterior, lateral, and posterior semicircular ducts that are oriented at right angles to each other. They are connected to the utricle via five openings.

Tragus

The tragus is a flat triangular prominence that partially occludes the entrance to the external acoustic meatus.

Tympanic membrane

The tympanic membrane, commonly known as the eardrum, separates the external ear from the middle ear. About 1 cm in diameter, it is a thin, semitransparent concave sheet composed of epithelium and collagen fibers, fixed within a ring in the temporal bone. It vibrates in response to sound and transmits these vibrations to the malleus, one of the tiny ossicles.

Vestibular membrane

The vestibular membrane separates the cochlear duct from the scala vestibuli. It is a thin, transparent membrane that stretches obliquely from the inner projection of the modiolus. When vibrations of the tympanic membrane are felt in response to sound waves, the pressure waves created are transmitted through the vestibular membrane to the basilar membrane.

maculae

The vestibule contains mechanoreceptors called maculae that detect static equilibrium i.e., the movements of the body relative to the forces of gravity.

Vitreous body

The vitreous body consists of transparent jelly that fills the posterior cavity and is formed during the development of the eye. It is not renewed or replaced during life. The fluid is a thick, gelatinous mass composed of collagen and proteoglycans. It helps maintain the shape of the eyeball.

Rod cells give us black and white vision because

Their inputs are perceived in black and white because of the photopigment

Tympanic cavity

This tympanic cavity is the cavity of the middle ear. It is connected to the throat (nasopharynx) by the pharyngotympanic (Eustachian) tube. It contains the three ossicles.

Viewing close objects

To view a close object, the lens becomes more convex, or 'short and thick' to increase its focusing power. This causes incoming light rays to converge more as they enter the lens. Here, the ciliary muscles contract, thereby releasing the tension on the lens so that it becomes fatter.

Viewing distant objects

To view a distant object, the lens becomes less convex or 'long and thin' as incoming light rays from distant objects are almost parallel and do not require much refraction. Here, the ciliary muscles relax, and the zonular fibers of the suspensory ligaments apply tension to the lens so that it becomes thinner.

4 types of lingual papillae

Vallate (circumvallate) papillae Foliate papillae Fungiform papillae Filiform papillae

Vallate papillae

Vallate papillae are the largest of the papillae. There are usually 12 found in an inverted V-shape at the back of the tongue. Each papilla consists of a mushroom-like central projection surrounded by a sulcus and mucosal elevation. Taste buds are present in the walls of the papillae and sulcus only. Innerved by: Glossopharagyl Nerve

Vertigo

Vertigo is an equilibrium disorder that can have many different causes. It manifests as an inability to stand for long periods of time without feeling dizzy and sick.

equilibrium pathway

Vestibular ganglia ↓ Vestibulocochlear nerve (VIII) ↓ Vestibular nucleus ↓ ↓ ↓ ↓ Ventral posterior nucleus, cerebellum, vestibulospinal tract, and cranial nerve nuclei ↓ Vestibular area

mechanoreceptors of the ear

accule and utricle contain some of the mechanoreceptors responsible for balance.

names of the three semicircular canals

anterior, lateral, and posterior

inferior oblique of eye

aspect of the eye, elevating the cornea. Innervation up and out toward eyebrow Oculomotor nerve (III).

ora serrate

attaches retina to eye in front

3 types of cells in taste buds

basal cells, gustatory receptor cells, and supporting cells.

Perilymph

between bony and membranous labyrinth

isomeization

change of chape from cis to trans retinal

retinal isomerase

converts trans-retinal back to cis-retinal

The auditory system is composed of the

external, middle and inner ear

accessory vision structures

eyebrows, eyelashes, eyelids, conjunctiva, and lacrimal apparatus

G protein used for gustation

gustducin

Ear can functionally be split into the

hearing and vestibular systems

Dynamic equilibrium

is the balance maintained by the sense organs in response to angular movements of the body (mainly the head), e.g., rotational acceleration/deceleration. Dynamic equilibrium is detected by mechanoreceptors located in the ampullae of the semicircular canals.

Static equilibrium

is the balance maintained by the sense organs in response to movements of the body (mainly the head) relative to the forces of gravity, e.g., linear acceleration/deceleration. Static equilibrium is detected by mechanoreceptors found in the vestibule of the inner ear.

Photopsin

is the photopigment present in cone cells.

Rhodopsin

is the photopigment present in rod cells.

tunicae of the eye

layers of the eye fibrous, vascular, and neural layer

Taste buds on the tongue are associated with specialized areas called

lingual papillae

At the apex of each stereocilium is a

mechanically-gated ion channel

endolymph

membranous labyrinth fluid

sound transduction takes place in the

middle and inner ear

central pillar of the bony labyrinth

modiolus

emmetropic

normal eye can refract light rays from an object 20 ft away.

Commands from the vestibular nucleus regarding the appropriate response to a vestibular stimulus are sent to

oculomotor (III), trochlear (IV), abducens (V), and accessory(XI).

Auditory transmission, or sound transmission

refers to the path that sound waves take as they enter the ear.

Neural structure of the eye

retina, optic nerve

extrinsic eye muscles

superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, inferior oblique

5 tastes are

sweet, salty, sour, bitter, and umami (meaty)

Microvilli project from each taste cell into the

taste pore The taste pore is a hole at the top of the taste bud; the microvilli of gustatory receptor cells project through this hole to reach the epithelial surface of the tongue and thus expose themselves to incoming tastants.

organ of Corti

the organ of hearing located in the cochlea and filled with endolymph The organ of Corti is the name given to the sensory epithelium that sits atop the basilar membrane. It is responsible for the translation of sound waves that travel through the ear into neural signals that can be interpreted by the brain. We are able to hear different sounds primarily due to the mechanical and electrical events that take place within the sensory cells of the organ of Corti.

Location of taste receptors on tongue

the tip of the tongue is the most sensitive to sweet, the anterior sides to salty, the posterior sides to sour, and the back to bitter. The distribution of umami is not known in detail.

sterocilium are connected to each other by a

tip-link protein

parts of middle ear (tympanic cavity)

tympanic membrane Tympanic cavity Tensor tympani Stapedius Ossicles Pharyngotympanic tube (Eustachian tube) Oval window

The way to pull on ear to straighten ear canal

up and back

Parts of the inner ear that are involved in maintaining and sensing equilibrium (or balance).

vestibule and semicircular canals

Taste buds present in the epiglottis and throat are innervated

via the vagus nerve (X).

Visible eye spectrum

visible spectrum (400-760 nm)