The Nervous System: General and Special Senses

Home , Auricle (anatomy), Basilar membrane, Bony labyrinth, Ear, Endolymph, Fovea centralis

18 The Nervous System: General and Special Senses

PowerPoint® Lecture Presentations prepared by Steven Bassett Southeast Community College Lincoln, Nebraska

• Sensory information arrives at the CNS • Information is “picked up” by sensory receptors • Sensory receptors are the interface between the nervous system and the internal and external environment • General senses • Refers to temperature, pain, touch, pressure, vibration, and proprioception • Special senses • Refers to smell, taste, balance, hearing, and vision

• Receptors and Receptive Fields • Free nerve endings are the simplest receptors • These respond to a variety of stimuli • Receptors of the retina (for example) are very specific and only respond to light • Receptive fields • Large receptive fields have receptors spread far apart, which makes it difficult to localize a stimulus • Small receptive fields have receptors close together, which makes it easy to localize a stimulus.

Receptive Receptive field 1 field 2

Receptive fields

• Interpretation of Sensory Information • Information is relayed from the receptor to a specific neuron in the CNS • The connection between a receptor and a neuron is called a labeled line • Each labeled line transmits its own specific sensation

• Classification of Receptors • Tonic receptors • Always active • Photoreceptors of the eye constantly monitor body position • Phasic receptors • Normally inactive but become active when necessary (for short periods of time) • Touch and pressure receptors of the skin (for example)

• Central Processing and Adaptation • Adaptation • Reduction in sensitivity due to a constant stimulus • Peripheral adaptation • Receptors respond strongly at first and then decline • Central adaptation • Adaptation within the CNS • Consciously aware of a stimulus, which quickly disappears

• Classification of the General Senses • One classification scheme: • Exteroceptors: provide information about the external environment • Proprioceptors: provide information about the position of the body • Interoceptors: provide information about the inside of the body

• Classification of the General Senses • Another classification scheme: • Nociceptors: respond to the sensation of pain • Thermoreceptors: respond to changes in temperature • Mechanoreceptors: activated by physical distortion of cell membranes • Chemoreceptors: monitor the chemical composition of body fluids

• Nociceptors • Known as pain receptors • Associated with free nerve endings and large receptor fields. This makes it difficult to “pinpoint” the location of the origin of the pain • Three types • Receptors sensitive to extreme temperatures • Receptors sensitive to mechanical damage • Receptors sensitive to chemicals

• Nociceptors • Fast pain: • Sensations reach the CNS fast • Associated with pricking pain or cuts • Slow pain: • Sensations reach the CNS slowly • Associated with burns or aching pains • Referred pain: • Sensations reach the spinal cord via the dorsal roots • Some visceral organ pain sensations may reach the spinal cord via the same dorsal root

Heart

Liver and gallbladder

Stomach Small intestine Ureters Appendix Colon

• Thermoreceptors • Found in the dermis, skeletal muscles, liver, and hypothalamus • Cold receptors are more numerous than hot receptors • Exist as free nerve endings • These are phasic receptors • Information is transmitted along the same pathway as pain information

• Mechanoreceptors • Receptors that are sensitive to stretch, compression, twisting, or distortion of the plasmalemmae • There are three types • Tactile receptors • Baroreceptors • Proprioceptors

• Mechanoreceptors • Tactile receptors • Provide sensations of touch, pressure, and vibrations • Unencapsulated tactile receptors: free nerve endings, tactile disc, and root hair plexus • Encapsulated tactile receptors: tactile corpuscle, Ruffini corpuscle, and lamellated corpuscle

• Mechanoreceptors • Unencapsulated tactile receptors • Free nerve endings are common in the dermis • Tactile discs are in the stratum basale layer • Root hair plexus monitors distortions and movements of the body surface

Hair Merkel cells and Tactile Free nerve tactile discs corpuscle ending

Ruffini corpuscle Lamellated corpuscle

Root hair plexus

Free nerve endings

Sensory nerves

Hair Merkel cells and Tactile Free nerve tactile discs corpuscle ending

Merkel cells

Ruffini corpuscle Tactile disc Lamellated corpuscle

Root hair plexus

Merkel cells and tactile discs

Sensory nerves

Hair Merkel cells and Tactile Free nerve tactile discs corpuscle ending

Ruffini corpuscle Lamellated corpuscle

Root hair plexus

Free nerve endings of root hair plexus

Sensory nerves

• Mechanoreceptors • Encapsulated tactile receptors • Tactile corpuscle: common on eyelids, lips, fingertips, nipples, and genitalia • Ruffini corpuscle: in the dermis, sensitive to pressure and distortion • Lamellated corpuscle: consists of concentric cellular layers / sensitive to vibrations

Ruffini corpuscle Lamellated corpuscle

Root hair plexus

Dermis

Tactile corpuscle LM  550

Sensory nerves Capsule Accessory cells

Dendrites

Sensory nerve fiber

Hair Merkel cells and Tactile Free nerve tactile discs corpuscle ending

Ruffini corpuscle Lamellated corpuscle Root hair plexus

Collagen Sensory Capsule fibers nerve fiber

Dendrites

Sensory nerves Ruffini corpuscle

Ruffini corpuscle Lamellated corpuscle Root hair plexus

Dermis

Dendritic process Accessory cells (specialized fibrocytes)

Concentric layers (lamellae) of collagen fibers Sensory separated by fluid nerves

Lamellated corpuscle LM  125

Concentric layers (lamellae) of collagen fibers separated by fluid

Dendritic process

• Mechanoreceptors • Baroreceptors • Stretch receptors that monitor changes in the stretch of organs • Found in the stomach, small intestine, urinary bladder, carotid artery, lungs, and large intestine

Baroreceptors of Carotid Sinus and Aortic Sinus Provide information on blood pressure to cardiovascular and respiratory control centers

Baroreceptors of Lung

Baroreceptors of Digestive Provide information on lung Tract stretching to respiratory Provide information on volume of rhythmicity centers for tract segments, trigger reflex control of respiratory rate movement of materials along tract

Baroreceptors of Colon

Baroreceptors of Bladder Provide information on volume Wall of fecal material in colon, Provide information on volume of trigger defecation reflex urinary bladder, trigger urinary reflex

• Mechanoreceptors • Proprioceptors • Monitor the position of joints, tension in the tendons and ligaments, and the length of muscle fibers upon contraction • Muscle spindles are receptors in the muscles • Golgi tendon organs are the receptors in the tendons

• Chemoreceptors • Detect small changes in the concentration of chemicals • Respond to water-soluble or lipid-soluble compounds • Found in respiratory centers of the medulla oblongata, carotid arteries, and aortic arch

Chemoreceptive neurons Blood vessel Chemoreceptors in and Trigger reflexive near Respiratory Centers adjustments in of Medulla Oblongata depth and rate of Sensitive to changes in pH respiration

and PCO2 in cerebrospinal fluid

Chemoreceptors of Carotid Bodies Via cranial nerve IX Sensitive to changes in pH,

PCO2, and PO2 in blood Via cranial Trigger reflexive Chemoreceptors nerve X adjustments in of Aortic Bodies respiratory and cardiovascular Sensitive to changes in activity pH, PCO2, and PO2 in blood

Carotid body LM  1500

• The special senses include: • Olfaction (smell) • Gustation (taste) • Equilibrium • Hearing • Vision

• Olfaction • The olfactory epithelium consists of: • Olfactory receptors • Supporting cells • Basal cells

• Olfactory Pathways • Axons leave the olfactory epithelium • Pass through the cribriform foramina • Synapse on neurons in the olfactory bulbs • Impulses travel to the brain via CN I • Arrive at the cerebral cortex, hypothalamus, and limbic system

Olfactory tract Cribriform plate of ethmoid Olfactory epithelium

The distribution of the olfactory receptors on the left side of the nasal septum is shown by the shading.

Olfactory Cribriform nerve fibers plate Lamina propria

Developing olfactory receptor cell Olfactory receptor cell Supporting cell Olfactory epithelium

Mucous layer Knob Olfactory cilia: surfaces contain receptor proteins

Substance being smelled

• Olfactory Discrimination • The epithelial receptors have different sensitivities and we therefore “detect” different smells • Olfactory receptors can be replaced • The replacement activity declines with age

• Gustation • The tongue consists of papillae • Papillae consist of taste buds • Taste buds consist of gustatory cells • Each gustatory cell has a slender microvilli that extends through the taste pore into the surrounding fluid

• Gustation Pathways • Dissolved chemicals contact the taste hairs (microvilli) • Impulses go from the gustatory cell through CN VII, IX, and X • Synapse in the nucleus solitarius of the medulla oblongata • The impulses eventually arrive at the cerebral cortex

Gustatory cortex Thalamic nucleus Medial lemniscus

Nucleus solitarius Facial nerve (N VII)

Glossopharyngeal Vagus nerve nerve (N IX) (N X)

• Gustation Discrimination • We begin life with more than 10,000 taste buds • The number declines rapidly by age 50 • Threshold level is low for gustatory cells responsible for unpleasant stimuli • Threshold level is high for gustatory cells responsible for pleasant stimuli

• Equilibrium and Hearing • Structures of the ear are involved in balance and hearing • The ear is subdivided into three regions • External ear • Middle ear • Inner ear

ANIMATION The Ear: Ear Anatomy

• The External Ear • Consists of: • Auricle • External acoustic meatus • Tympanic membrane • Ceruminous glands

EXTERNAL EAR MIDDLE EAR INNER EAR

Auditory ossicles Semicircular Petrous part Facial nerve canals of temporal (N VII) bone Auricle

Vestibulocochlear nerve (N VIII)

External acoustic Bony labyrinth meatus of inner ear

Tympanic membrane

Tympanic Elastic cavity Vestibule cartilage Oval window To nasopharynx Round window Auditory tube Cochlea

• The Middle Ear • Consists of: • Tympanic cavity • Auditory ossicles • Malleus, incus, and stapes • Auditory tube (pharyngotympanic tube)

EXTERNAL EAR MIDDLE EAR INNER EAR

Auditory ossicles Semicircular Petrous part Facial nerve canals of temporal (N VII) bone Auricle

Vestibulocochlear nerve (N VIII)

External acoustic Bony labyrinth meatus of inner ear

Tympanic membrane

Tympanic Elastic cavity Vestibule cartilage Oval window To nasopharynx Round window Auditory tube Cochlea

Auditory tube

Auditory ossicles

Tympanic membrane

External acoustic meatus Tympanic cavity (middle ear) Inner ear

Temporal bone (petrous part)

Stabilizing ligament Malleus Incus

Chorda tympani Base of stapes nerve (cut), a at oval window branch of N VII Tensor tympani External acoustic muscle meatus Stapes Round window Tympanic cavity (middle ear) Stapedius Tympanic membrane muscle (tympanum) Auditory tube

Incus Malleus

Points of attachment to tympanic membrane

Stapes

Base of stapes

The isolated auditory ossicles

Malleus Incus Tendon of tensor tympani muscle Base of stapes at Malleus attached oval window to tympanic Stapes membrane Inner surface Stapedius of tympanic muscle membrane

The tympanic membrane and auditory ossicles as seen through a fiber-optic tube inserted along the auditory canal and into the middle ear cavity

• The Inner Ear • Consists of: • Receptors • Membranous labyrinth (within the bony labyrinth) • Bony labyrinth • Vestibule • Semicircular canals • Cochlea • Utricle • Saccule

EXTERNAL EAR MIDDLE EAR INNER EAR

Auditory ossicles Semicircular Petrous part Facial nerve canals of temporal (N VII) bone Auricle

Vestibulocochlear nerve (N VIII)

External acoustic Bony labyrinth meatus of inner ear

Tympanic membrane

Tympanic Elastic cavity Vestibule cartilage Oval window To nasopharynx Round window Auditory tube Cochlea

KEY Semicircular canal Membranous labyrinth Anterior Bony labyrinth Semicircular Lateral ducts Posterior Vestibule Cristae within ampullae Maculae Endolymphatic sac

Cochlea

Utricle Saccule

Vestibular duct

Cochlear duct Tympanic Organ of Anterior view of the bony duct Corti labyrinth cut away to show the semicircular canals and the enclosed semicircular ducts of the membranous labyrinth

Perilymph

Bony labyrinth

Endolymph

Membranous labyrinth

• The Inner Ear • The vestibular complex and equilibrium • Part of inner ear that provides equilibrium sensations by detecting rotation, gravity, and acceleration • Consists of: • Semicircular canals • Utricle • Saccule

• The Vestibular Complex and Equilibrium • The semicircular canals • Each semicircular canal encases a duct • The beginning of each duct is the ampulla • Within each ampulla is a cristae with hair cells • Each hair cell contains a kinocilium and stereocilia • These are embedded in gelatinous material called the cupula • The movement of the body causes movement of fluid in the canal, which in turn causes movement of the cupula and hair cells, which the brain detects

• The Vestibular Complex and Equilibrium • The utricle and saccule • The utricle and saccule are connected to the ampulla and to each other and to the fluid within the cochlea • Hair cells of the utricle and saccule are in clusters called maculae • Hair cells are embedded in gelatinous material consisting of statoconia (calcium carbonate crystals) • Gelatinous material and statoconia collectively are called an otolith

ANIMATION The Ear: Ear Balance

• Equilibrium Process • When you rotate your head: • The endolymph in the semicircular canals begins to move • This causes the bending of the kinocilium and stereocilia • This bending causes depolarization of the associated sensory nerve • When you rotate your head to the right, the hair cells are bending to the left (due to movement of the endolymph) • When you move in a circle and then stop abruptly, the endolymph moves back and forth causing the hair cells to bend back and forth resulting in confusing signals, thus dizziness

Vestibular branch (N VIII)

Cochlea Anterior Ampulla Semicircular Posterior ducts Lateral Endolymphatic sac

Endolymphatic duct

Utricle

Anterior view of the maculae and semicircular ducts Saccule Maculae of the right side

Displacement in Displacement in this direction this direction stimulates hair cell inhibits hair cell Ampulla filled with endolymph Kinocilium Cupula Stereocilia

Hair cells

Crista Hair cell Supporting cells

Sensory nerve

A section through the ampulla of a semicircular duct

Direction of Direction of relative Direction of duct rotation endolymph movement duct rotation Sensory nerve ending Supporting cell

Semicircular duct Ampulla Structure of a typical hair cell showing details At rest revealed by electron microscopy. Bending the stereocilia toward the kinocilium depolarizes the cell Endolymph movement along the and stimulates the sensory neuron. Displacement in length of the duct moves the cupula the opposite direction inhibits the sensory neuron. and stimulates the hair cells. © 2012 Pearson Education, Inc. Figure 18.14 The Function of the Semicircular Ducts, Part II

Anterior semicircular duct for ―yes‖

Lateral semicircular duct for ―no‖

Posterior semicircular duct for ―tilting head‖

Location and orientation of the membranous A superior view showing the planes of labyrinth within the petrous parts of the sensitivity for the semicircular ducts temporal bones

• Equilibrium Process (cont.) • When you move up or down (elevator movement): • Otoliths rest on top of the maculae • When moving upward, the otoliths press down on the macular surface • When moving downward, the otoliths lift off the macular surface • When you tilt side to side: • When tilting to one side, the otoliths shift to one side of the macular surface

Statoconia

Otolith

Gelatinous material Statoconia Otolith

Hair cells A scanning electron micrograph showing the crystalline structure of Nerve fibers otoliths

Head in Neutral Position Head Tilted Posteriorly Gravity Gravity

Otolith moves ―downhill,‖ Receptor distorting hair output cell processes increases

Diagrammatic view of changes in otolith position during tilting of the head

To superior colliculus and relay to cerebral cortex

Red nucleus

N III Vestibular N IV ganglion

Semicircular Vestibular canals branch

N VI Vestibular nucleus

To cerebellum Vestibule

Cochlear N XI branch

Vestibulocochlear nerve (N VIII)

Vestibulospinal tracts

• The Cochlea • Consists of “snail-shaped” spirals • Spirals coil around a central area called the modiolus • Within the modiolus are sensory neurons • The sensory neurons are associated with CN VIII • Organ of Corti

Stapes at oval window

Cochlear duct Vestibular duct Tympanic duct

Apical turn

Vestibular membrane Cochlear Vestibular Tectorial membrane branch branch Vestibulocochlear Spiral ganglion Basilar membrane nerve (VIII) Semicircular Middle turn Structure of the cochlea in partial canals Vestibular duct (scala section Modiolus vestibuli—contains perilymph) KEY From oval window Organ of Corti to tip of spiral From tip of spiral Cochlear duct (scala to round window media—contains endolymph) Tympanic duct (scala tympani—contains perilymph) Basal turn Temporal bone (petrous part)

Cochlear nerve Vestibulocochlear nerve (VIII) From oval To round window window Structure of the cochlea within the temporal bone showing the turns of the vestibular duct, cochlear duct, and tympanic duct

• The Cochlea (cont.) • Each spiral consists of three layers • Scala vestibuli (vestibular duct): consists of perilymph • Scala tympani (tympanic duct): consists of perilymph • Scala media (cochlear duct): consists of endolymph / this layer is between the scala vestibuli and scala tympani • There is a basilar membrane between each layer • The scala vestibuli and scala tympani are connected at the apical end of the cochlea • Sense organs rest on the basilar membrane within the scala media

• The Cochlea • The Organ of Corti • Also known as the spiral organ • Rests on the basilar membrane between the scala media and the scala tympani • Hair cells are in contact with an overlying tectorial membrane • This membrane is attached to the lining of the scala media • Sound waves ultimately cause a distortion of the tectorial membrane, thus stimulating the organ of Corti

• Auditory Pathways • Sound waves enter the external acoustic meatus • The tympanic membrane vibrates • Causes the vibration of the ossicles • The stapes vibrates against the oval window of the scala tympani • Perilymph begins to move

EXTERNAL EAR MIDDLE EAR INNER EAR

Auditory ossicles Semicircular Petrous part Facial nerve canals of temporal (N VII) bone Auricle

Vestibulocochlear nerve (N VIII)

External acoustic Bony labyrinth meatus of inner ear

Tympanic membrane

Tympanic Elastic cavity Vestibule cartilage Oval window To nasopharynx Round window Auditory tube Cochlea

Stapes at oval window

Cochlear duct Vestibular duct Tympanic duct

Apical turn

Vestibular membrane Cochlear Vestibular Tectorial membrane branch branch Vestibulocochlear Spiral ganglion Basilar membrane nerve (VIII) Semicircular canals Middle turn Structure of the cochlea in partial Vestibular duct (scala section Modiolus vestibuli—contains perilymph) KEY From oval window Organ of Corti to tip of spiral From tip of spiral Cochlear duct (scala to round window media—contains endolymph) Tympanic duct (scala tympani—contains perilymph) Basal turn Temporal bone (petrous part)

Cochlear nerve Vestibulocochlear nerve (VIII) From oval To round window window Structure of the cochlea within the temporal bone showing the turns of the vestibular duct, Apical turn cochlear duct, and tympanic duct

Middle turn

Vestibular duct (scala vestibuli) Vestibular duct (from oval window) Cochlear duct (scala media) Vestibular membrane Tympanic duct Organ of Corti (scala tympani) Cochlear branch Basal turn Spiral ganglion Basilar membrane Tympanic duct (to round window)

Sectional view of cochlear spiral LM  60

Bony cochlear wall

Vestibular duct Spiral ganglion Vestibular membrane Cochlear duct Tectorial membrane Basilar membrane Tympanic duct Organ of Corti

Cochlear branch Three-dimensional section of N VIII showing the detail of the cochlear chambers, tectorial membrane, and organ of Corti

Tectorial membrane Cochlear duct (scala media) Vestibular membrane Tectorial membrane Outer hair cell

Tympanic duct Basilar Hair cells Spiral ganglion Basilar membrane Inner hair cell Nerve fibers (scala tympani) membrane of organ cells of of Corti Diagrammatic and histological sections through the cochlear nerve receptor hair cell complex of the organ of Corti Organ of Corti LM  125

Stereocilia of inner hair cells

Stereocilia of outer hair cells A color-enhanced SEM showing a portion of the receptor surface of the organ of Corti Surface of the organ of Corti SEM  1320 © 2012 Pearson Education, Inc. Equilibrium and Hearing

• Auditory Pathways (continued) • As the perilymph moves: • Pressure is put on the scala media • This pressure distorts the hair cells of the organ of Corti • This distortion depolarizes the neurons • Nerve signals are sent to the brain via CN VIII

ANIMATION The Ear: Receptor Complexes

Bony cochlear wall

Vestibular duct Spiral ganglion Vestibular membrane Cochlear duct Tectorial membrane Basilar membrane Tympanic duct Organ of Corti

Cochlear branch Three-dimensional section of N VIII showing the detail of the cochlear chambers, tectorial membrane, and organ of Corti

Tectorial membrane Cochlear duct (scala media)

Vestibular membrane Tectorial membrane Outer hair cell

Tympanic duct Basilar Hair cells Spiral ganglion Basilar membrane Inner hair cell Nerve fibers (scala tympani) membrane of organ cells of Diagrammatic and histological sections through the of Corti cochlear nerve receptor hair cell complex of the organ of Corti Organ of Corti LM  125

Auditory cortex (temporal lobe) High- Low-frequency frequency sounds sounds Cochlea Thalamus To ipsilateral auditory cortex

Low-frequency sounds Medial geniculate nucleus (thalamus) High-frequency sounds Vestibular Inferior colliculus branch (mesencephalon) Cochlear branch Motor output to cranial nerve nuclei Vestibulocochlear KEY nerve (N VIII) First-order neuron Second-order neuron Superior olivary nucleus Third-order neuron Fourth-order neuron Cochlear nuclei

• Vision • Accessory structures of the eye • Palpebrae (eyelids) • Medial and lateral canthus (connect the eyelids at the corners of the eye) • Palpebral fissure (area between the eyelids) • Eyelashes (contain root hair plexus, which triggers the blinking reflex) • Conjunctiva (epithelial lining of the eyelids) • Glands: glands of Zeis, tarsal glands, lacrimal gland, lacrimal caruncle

Eyelashes

Palpebra Lateral canthus Palpebral fissure Sclera Medial Corneal canthus limbus Lacrimal Pupil caruncle

Superficial anatomy of the right eye and its accessory structures

Superior rectus muscle Tendon of superior Lacrimal oblique muscle gland ducts Lacrimal gland Lacrimal punctum Superior lacrimal Lateral canthus canaliculus Lower eyelid Medial canthus Inferior lacrimal canaliculus Inferior Lacrimal sac rectus muscle Nasolacrimal duct Inferior Inferior nasal oblique muscle concha Opening of nasolacrimal duct Diagrammatic representation of a deeper dissection of the right eye showing its position within the orbit and its relationship to accessory structures, especially the lacrimal apparatus

• Accessory Structures of the Eye • Conjunctiva • Covers the inside lining of the eyelids and the outside lining of the eye • Fluid production helps prevent these layers from becoming dry • Palpebral conjunctiva • Inner lining of the eyelids • Ocular conjunctiva • Outer lining of the eye

• Accessory Structures • Glands • All of the glands are for protection or lubrication • Glands of Zeis: sebaceous glands / associated with eyelashes • Tarsal glands: secrete a lipid-rich product / keeps the eyelids from sticking together / located along the inner margin of the eyelids • Lacrimal glands: produce tears / located at the superior, lateral portion of the eye • Lacrimal caruncle glands: produce thick secretions / located within the canthus areas

• Accessory Structures • Glands • An infection of the tarsal gland may result in a cyst • An infection of any of the other glands may result in a sty

• Accessory Structures • Lacrimal glands • Part of the lacrimal apparatus • The lacrimal apparatus consists of: • Lacrimal glands (produce tears) • Lacrimal canaliculi • Lacrimal sac • Nasolacrimal duct

• Accessory Structures • Lacrimal glands (continued) • Tears are produced by the lacrimal glands • Flow over the ocular surface • Flow into the nasolacrimal canal (foramen) • This foramen enters into the nasal cavity • Therefore, when you sob heavily, tears flow across your eye and down your face and also through the nasolacrimal canal into your nose and out, resulting in a “runny” nose

ANIMATION The Eye: Accessory Structures

• The Eyes • Consist of: • Sclera • Cornea • Pupil • Iris • Lens • Anterior cavity • Posterior cavity • Three tunics: • (1) fibrous tunic, (2) vascular tunic, and (3) neural tunic • Retina

Ora serrata Fornix

Posterior cavity Palpebral conjunctiva (Vitreous chamber filled with the vitreous body) Ocular conjunctiva

Ciliary body Anterior chamber (filled with aqueous humor) Lens

Pupil Cornea Central retinal artery and vein Iris Optic nerve Posterior chamber (filled with aqueous Optic disc humor) Fovea Corneal limbus Retina Suspensory ligaments Choroid

Sclera

Major anatomical landmarks and features in a diagrammatic view of the left eye

• The Eyes • The Fibrous Tunic (outer layer) • Makes up the sclera and cornea • Provides some degree of protection • Provides attachment sites for extra-ocular muscles • The cornea is modified sclera

• The Eyes • The Vascular Tunic (middle layer) • Consists of blood vessels, lymphatics, and intrinsic eye muscles • Regulates the amount of light entering the eye • Secretes and reabsorbs aqueous fluid (aqueous humor) • Controls the shape of the lens • Includes the iris, ciliary body, and the choroid

ANIMATION The Eye: Uvea Parts

• The Vascular Tunic • The iris • Consists of blood vessels, pigment, and smooth muscles • The pigment creates the color of the eye • The smooth muscles contract to change the diameter of the pupil

• The Vascular Tunic • The ciliary body • The ciliary bodies consist of ciliary muscles connected to suspensory ligaments, which are connected to the lens • The choroid • Highly vascularized • The innermost portion of the choroid attaches to the outermost portion of the retina

ANIMATION The Eye: Ciliary Muscles

• The Eyes • The Neural Tunic (inner layer) • Also called the retina • Made of two layers: (pigmented layer – outer layer) / (neural layer – inner layer) • Retina cells: rods (night vision) and cones (color vision)

Choroid Ciliary body Vascular Iris tunic (uvea) Posterior Anterior cavity cavity

Cornea Fibrous Neural Neural part tunic Sclera tunic (retina) Pigmented part

The lens is suspended between the posterior cavity and the posterior chamber of the anterior cavity.

Fibrous Vascular Neural Ora serrata tunic tunic tunic Fornix (sclera) (choroid) (retina) Posterior cavity Palpebral conjunctiva (Vitreous chamber filled with the vitreous body)

Ocular conjunctiva

Ciliary body Anterior chamber (filled with aqueous humor) Lens

The three layers, or Pupil tunics, of the eye Cornea Central retinal artery and vein Iris Optic nerve Posterior chamber (filled with aqueous Optic disc humor)

Fovea Corneal limbus

Retina Suspensory ligaments Choroid

Sclera

Major anatomical landmarks and features in a diagrammatic view of the left eye

Horizontal cell Cone Rod Choroid Pigmented part of retina

Rods and cones

Bipolar cells Amacrine cell

Ganglion cells

Nuclei of Nuclei of rods Nuclei of ganglion cells and cones bipolar cells The retina LM  70

Histological organization of the retina. Note that the photoreceptors are located closest to the choroid rather than near the vitreous chamber. LIGHT

• Cavities and Chambers of the Eye • Anterior cavity • Anterior chamber • Posterior chamber • Filled with fluid called aqueous fluid • Posterior cavity • Vitreous chamber • Filled with fluid called vitreous fluid

ANIMATION The Eye: Posterior Cavity

• Cavities and Chambers of the Eye • Aqueous fluid • Sometimes called aqueous humor • Secreted by cells at the ciliary body area • Enters the posterior chamber (posterior of the iris) • Flows through the pupil area • Enters the anterior chamber • Flows through the canal of Schlemm • Enters into venous circulation

Pupil Posterior Cornea cavity Lens (vitreous chamber) Pigmented Anterior chamber epithelium Anterior Posterior cavity Suspensory chamber ligaments

Ciliary Canal of process Schlemm

Choroid Body of iris Ciliary body Retina Conjunctiva Sclera

• Cavities and Chambers of the Eye • Vitreous fluid • Gelatinous material in the posterior chamber • Sometimes called vitreous humor • Supports the shape of the eye • Supports the position of the lens • Supports the position of the retina • Aqueous humor can flow across the vitreous fluid and over the retina

Ora serrata

Conjunctiva

Cornea Posterior cavity Lens (vitreous Anterior chamber chamber) Iris Posterior chamber

Suspensory ligaments Ciliary body

• Aqueous fluid • If this fluid cannot drain through the canal of Schlemm, pressure builds up • This is glaucoma • Vitreous fluid • If this fluid is not of the right consistency, the pressure is reduced against the retina • The retina may detach from the posterior wall (detached retina)

• Visual Pathways • Light waves pass through the cornea • Pass through the anterior chamber • Pass through the pupil • Pass through the posterior chamber • Pass through the lens • The lens focuses the image on some part of the retina • This creates a depolarization of the neural cells • Signal is transmitted to the brain via CN II

ANIMATION The Eye: Interior Parts of the Eye

Cornea Anterior cavity Iris Posterior Anterior Edge of chamber chamber pupil Suspensory ligament of lens

Nose Corneal limbus Lacrimal punctum Conjunctiva Lacrimal caruncle Lower eyelid Medial canthus Lateral canthus Ciliary Lens processes

Ciliary body

Ora serrata Sclera

Choroid

Fovea Retina Posterior cavity Ethmoidal labyrinth Lateral rectus muscle

Medial rectus muscle Optic disc

Left eye Binocular vision Right eye only only

Optic nerve (N II)

Optic chiasm

Other hypothalamic Optic tract nuclei, pineal gland, and reticular formation Suprachiasmatic nucleus

Lateral Lateral geniculate geniculate nucleus Superior nucleus colliculus Projection fibers (optic radiation)

• Visual Pathways • The retina • There are rods and cones all over the retina • 100% cones in the fovea centralis area • The best color vision is when an object is focused on the fovea centralis • 0% rods or cones in the optic disc area • If an object is focused on this area, vision does not occur • Also known as the “blind spot”

ANIMATION The Eye: Blind Spot

• Visual Pathways • The retina (cont.) • The cones require light to be stimulated (that’s why we see color) • At night (still has to be at least a small amount of light), the cones deactivate and the rods begin to be activated (that’s why we can see at night but we can’t determine color at night)

ANIMATION The Eye: Lens and Retina

ANIMATION The Eye: Light Path

ANIMATION The Eye: The Retina