SENSORY

Question 1

General characteristics of the sensory organs

*

Answer 1

• Sense
  
  • is the ability to detect certain type of stimuli from internal and external enviorment
  • Stimuli can be of physical nature (light, sound), or chemical nature
  • the information from stimulus is transfered by nerves and recieved by receptors
• The sensory receptors can be classified according to the stimulus origin:
  
  • Exteroceptors
    
    • From an external source, environment.
    • Includes taste, smell, light, hearing and touch.
  • Interoceptors
    
    • From the internal organs.
    • Sensation of nausea, pain, stretch and pressure.
    • Proprioceptors
      
      • Sense the body’s position (without vision) by mechanoreceptors in the joints (fibrous capsule), muscles (muscle spindles) and tendons (Golgi tendon organs).

Sensory organs, general characteristics

• Sensory receptors
• A sensory receptor is a structure specialized to detect a stimulus.
• Functional calssification
• The receptors can be classified into 5 basic functional types:
  
  • Mechanoreceptors
  • -respond to physical deformation of tissues, like touch, pressure, stretch, and vibration. Include the hearing receptors, skin, joints and viscera. (hair follicles, Merkle cells, free nerve endings, Pacinian corpuscles, meissnerscorpuscles, ruffinicorpuscles)
• 2. nociceptors-are pain receptors, respond to tissue damage from trauma (blow, cut), ischemia (low blood flow), burns from chemicals or heat.
• 3. thermoreceptors-respond to surrounding temperature.
• 4.chemoreceptors-respond to chemicals, including odor, taste and fluid compositions.
• 5. photoreceptors-respond to light (electromagnetic radiation).
• The sensory receptors can also be classified according to the stimulus origin:
• 1. exteroceptors-from an external source, environment. Includes taste, smell, light, hearing and touch.
• 2. interoceptors-from the internal organs. Sensation of nausea, pain, stretch and pressure.
• 3. proprioceptors-sense the body’s position (without vision) by mechanoreceptors in the joints (fibrous capsule), muscles (muscle spindles) and tendons (Golgi tendon organs).

SENSORY EPITHELIUM

Sensory epithelium consists of:

• sensory cells
• supporting cells

Sensory cells

• are receptors,
• convert external stimuli into electric impulses
• Classification according stimuli:
  
  • Photoreceptors: rods and cones of the retina
  • Chemoreceptors: olfactory cells (smell, olfaction) and taste cells
  • Mechanoreceptors: hair cells of the inner ear (sense of hearing, and sense of equilibrium and balance)
• Classification according structure:
  
  • Primary sensory cells,
    
    • rods,
    • cones,
    • olfactory cells,
    • have an own axon and generate action potentials.
  • Secondary sensory cells
    
    • hair and taste cells,
    • have no axon, just depolarize according to the stimulus. T
    • he appropriate action potentials are generated by another neuron that is in a synaptic contact with the secondary sensory cell.

The retina is the innermost layer of the eye: contain distinct layers

• Outer layer: pigmented epithelial cells
• Internal layer: the retina (nervous tunic)
  
  • Pars ceaca retinae: blind part
    
    • Ciliary part
    • Iris part
  • Ora serrata: (“tooth like”): border
  • Optic part
    
    • From optic nerve papilla to ora serrata
• Inner retinal plexsus

General:

• Development
  
  • Develops from the optic cup;
  • Its outer layer differentiates into the retinal pigment epithelium (RPE)
  • Inner layer thickens and begins a long process of differentiation into neurons, photoreceptors, and neuro- glial cells of the neuronal retina.
  • Two differentiation gradients of retina:
    • a) from the inner to the outer layers,
    • b) from the central to the peripheral parts

Internal layer: the retina (nervous tunic)

• 10-11 layers
  
  • brukes membrane (can be considered apart of choroid or apart of retina)

1- Pigment epithelium

• Simple cuboid
• Apical processes of cells surround the outer segments of rods and cones
• absorbs light passing through the neural retina
• isolates the retinal cells from blood-borne substances:
• blood-retina barrier
• participates in restoring photo- sensitivity of visual pigments
• phagocytoses and disposes of membranous discs from the rods and cones

2- Layer of rods and cones

• outer and inner segments of rods and cones

3- External limiting membrane:

• apical boundary of Müller ́s cells, glial cells

4- Outer layer core (cell bodies+nuclei of rods and cones)

• cell bodies (nuclei) of rods and cones

5- Outer layer plexiform

• contains the processes of rod and cones and horizontal, and bipolar cells that connect to them; synapses of rod and cone axons and bipolar cells

6- Inner layer core/ nucelar layer

• contains the cell bodies (nuclei) of bipolar, amacrine, and Müller ́s cells

7- Inner layer plexiform

• contain the processes of horizontal, amacrine, bipolar, and ganglion cells that connect each other; synapses of bipolar and ganglion c.

8-Layer of ganglion cells

• contains perikarya of ganglion cells (typical neurons)

9-Layer of nerve fibers

• contains axons of ganglion cells
• (optic nerve fibers that lead from the retina to the brain)
• Papilla of optic nerve (also where central artery enters eye)

10-Internal limit membrane

• composed of basal lamina of Müller ́s cells

NEURAL RETINA

• Photoreceptor cells: rods and cones
• Conducting neurons: bipolar neurons and ganglion cells
• Association neurons: horizontal and amacrine neurons 7
• Supporting (glial) cells: Müller’s cells, microglial cells, astrocytes

Photoreceptor cells:

• Primary sensory cells: Sensory process and short axon, but have simillarities to neurons

Visual pigments

• RODS
  
  • Rods are more sensitive to light visual pigment rhodopsin
  • 120 million rods
• CONES:
  
  • Each cone is specialized to respond to one of three colors – red, green or blue- three types of visual pigments - iodopsins
  • 6 - 7 millon cones
  • mostly found in fovea centralis (retina is thinner, allows light rays to get closer to cones so there is minumun light scattering)
• Outer segment
  
  • RODS:
    
    • has cylindrical shape,
    • contains horizontally flattened membrane discs
  • CONES:
    
    • Contrary to rod the interior of cone discs is continuous with the extra-cellular space
  • acctual receptors, sensory processes
• Connecting stalk with cilium
  
  • (BB = basal body, R = striated rootlet)
• Inner segment (metabolic region)
  
  • contains protein synthesis organelles (Golgi complex, ER, free ribosomes, mitochondria)
• “outer limating membrane”
• is formed by a row of zonulae adherentes (ZA) between the apical parts of Müller ́s cells and photoreceptor cells
• Cell body
  
  • contains nuceus
• Axon
• Presynaptic part (synapse with bipolar neuron)

Types of neural cells (photoreceptor cells, bipolar cells, and ganglion cells) that:

Form nine layers connected by synapses

Perceive and transform light into neuronal signals that travel to the brain via the optic nerve.

Two notable structures: optic disc and macula

has 10 layers (sorted from outermost to innermost)1

*Choroid

• General
  
  • Vscular sheet that lies between the sclera and the photosensitive retina;
  • Loose CT
  • Blood vessels and melanin pigment give the choroid an intense dark brown color.
  • The pigment absorbs scattered and reflected light to minimize glare within the eye.
• Micro: 4 layers
  
  • Lamina suprachoroidea
    
    • Loose pigmemnted CT
      
      • Melanocytes
      • Pigmented epithelial cells in retina>?
  • Zona vasculosa
    
    • Arteries and viens, melanocytes
  • Lamina Choriocapillaris
    
    • Capillary network
    • Fenestrated capillaries provide nutrients to the cells of retina
  • Bruch’s membrane
    
    • Amorphous refractile layer
    • Basal lamina of capillary endothelium
    • Elastic and reticular fibrils
    • Basal lamina of outer most layer of pigmemted endithelium

Question 2

Histological structure of the anterior part of the eye

Answer 2

GENERAL STRUCTURE OF THE EYE

• 25 mm in diameter.
• It is suspended in the bony orbital socket by six extrinsic muscles that control its movement.
  
  • The extraocular muscles are coordinated so that the eyes move symmetrically around their own central axes.
• A thick layer of adipose tissue partially surrounds and cushions the eye as it moves within the orbit.
• Contains 3 chambers
  
  • The anterior chamber
    
    • is a space located between the cornea, iris, and lens.
  • The posterior chamber
    
    • is a small space situated between the iris, ciliary process, zonular fibers, and lens.
  • The vitreous chamber
    
    • is a larger, posterior space that is situated behind the lens and zonular fibers, and surrounded by the retina.
• The aqueous humor
  
  • Watery fluid that fills the anterior and posterior chambers
  • Continually produced by the ciliary process located behind the iris.
  • Circulates from the posterior chamber to the anterior chamber, where it is drained by veins.
• Vitreous body.
  
  • The vitreous chamber is filled with the gelatinous substance
  • Contains liquid similar to ground substace, high proportion of hylauric acid
• Layers of the eyebal
  
  • Fibrous layer (dense CT): cornea, sclera
  • Uvea (loose CT): choroid, cillary body, iris
  • Retina: optic part (photosensitive part), caeca part

Layers of the anterior part of the eye

1- Corneoscleral coat: Outer/fibrous layer

• Encloses the inner two layers except where it is penetrated by the optic nerve.
  
  • Includes:
    
    • Sclera: White portion
    • Cornea: Transparent portion

2- Vascular coat, the middle layer, or uvea

• Includes:
  
  • Choroid and the stroma of the ciliary body
  • Iris.

3- Retina, the inner layer

• Includes
  
  • Outer pigment epithelium
  • Inner neural retina,
  • Epithelium of the ciliary body and iris.
• The neural retina is continuous with the central nervous system through the optic nerve.

*Cornea

• General:
  
  • Covers the anterior one-sixth of the eye
  • 50 um in average thickness
  • The cornea is continuous with the sclera
  • No blood vessels!
    
    • Nutrition comes mostly from posterior surface, diffusion
• Micro: Three cellular layers and two noncellular layers.
• Corneal epithelium:
  
  • Stratified squamous (5 layers), non-keratinized 70 um
    
    • The basal cells:
      
      • low columnar with round, ovoid nuclei
    • Surface cells acquire a squamous or discoid shape
    • Tear film covers cornea, so it is not directly exposed to air
  • Dentritic langerhans cells in margins
  • Free nerve ending within epithlium? branchs from V1?, afferent link of pupillary reflex
  • Regenerative capacity: turnover time of approximately 7 days
    
    • The actual stem cells for the corneal epithelium reside at the corneoscleral limbus, the junction of the cornea and sclera.
  • DNA in corneal epithelial cells is protected from UV light damage by nuclear ferritin
    
    • (iron-containing protein).
• Bowman ́s membrane (anterior basement membrane)
  
  • 3-9 um thickness
  • Homogenous layer consisting of meshwork collagen fibrils
  • Acts as a barrier to the spread of infections.
• Corneal stroma: substantia propria
  
  • 90% of the corneal thickness.
  • Collagenous Lamellar regular dense CT:
    
    • Parallel bundles collagen fibrils
    • Arranged at right angles to those in adjacent lamellae
  • Inbetween:
    
    • Keratocytes: CT cells of cornea, specilized fibrocytes
    • Proteoglycans (sulfated glycosaminoglycans)
      
      • Keratan sulfate (lumican)
      • Chondroitin sulfate
    • Covalently bound to protein (decorin).
      
      • Lumican regulates normal collagen fibril assembly in the cornea and is critical in the development of a highly organized collagenous matrix.
• Descement ́s membrane (posterior basement membrane)
  
  • Thick basal lamina of corneal endothelium
    
    • Intensely positive to periodic acid–Schiff (PAS) and can be as thick as 10 um.
  • Extends peripherally beneath the sclera as a trabecular meshwork forming the pectinate ligament.
    
    • Strands from the pectinate ligament penetrate the ciliary muscle and sclera and may help to maintain the normal curvature of the cornea by exerting tension on Descemet’s membrane.
• Corneal endothelium
  
  • Low cuboidal ion-transporting cells,
  • Regulate the water content in stroma
  • Zonulae adherentes, relatively leaky zonulae occludentes, and desmosomes
    • Anterior chamber was filled with mesenchyme, the cavity formed inside this tisse. the mesenchymal cells flattened and became endothelium. similar to endothelium of blood vessels.
• Anerior chamber filled with aqueous humor

2- Sclera

• Proper dense connective tissue, fibrous capsule
• Provides attachment for the extrinsic muscles of the eye.
• The sclera constitutes the “white” of the eye.
  
  • In children, it has a slightly blue tint because of its thinness;
  • elderly people, it is yellowish because of the accumulation of lipofuscin in its stromal cells.
• Micro
  • 0.3-1 mm
  • flat budnels of collagen fibers, moderate amount of ground substace and fibroblasts
  • fibers are mainly parallel with surface
  • The sclera is pierced by blood vessels, nerves, and the optic nerve (see Fig. 24.2).
  • It is 1 mm thick posteriorly
• The sclera is divided into three rather ill-defined layers:
  
  • The episcleral layer (episclera),
    
    • External layer, loose connective tissue adjacent to the periorbital fat.
  • Substantia propria (sclera proper, also called Tenon’s capsule),
    
    • The investing fascia of the eye and is composed of a dense network of thick collagen fibers.
  • Suprachoroid lamina (lamina fusca),
    
    • the inner aspect of the sclera, is located adjacent to the choroid and contains thinner collagen fibers and elastic fibers as well as fibroblasts, melanocytes, macrophages, and other connective tissue cells.

*Ciliary body (CB)

• General:
  
  • Thickened portion between the iris and choroid
  • Production of aqueous humor
    
    • ​Drained through trabecular meshwork in the iridocorneal angle into schlemm’s cannal (brings liquid into venous system)
  • Anchors the lens, attacment place for zonular fibers
  • Accommodation: adjusts the lens for seeing at different distances
• Micro:
  
  • Composed of the ciliary ring, epithelium with ciliary processes, and ciliary muscle
• Ciliary muscle, CM
  
  • Smooth muscle composed of:
    
    • Medridional fibers: streching of the choroid, opening of the iridocorneal angle and facilitate drainage of the aqueous humor
    • Radial fibers: flatten the lens – focusing for distant vision)
    • Circular fibers: accomodation for near vision
• CM is covered by the vascular coat which consists of:
  
  • Vascularized loose CT
  • Ciliary epithelium
    
    • Double layers of low columnar cells:
      
      • Outer cells are heavily pigmented,
      • Inner nonpigmented cells
        
        • Fluid transporting epithelium, production of the aqueous humor
    • Each layer sits on its own basal lamina, apical surfaces attached to each other by junctions
  • Ciliary processes: fibers that encircle the border of the lens
    
    • fibers of the zonule arise from the grooves between the ciliary processes
• Posterior part of CB is flat, anterior part is projecting into the ciliary processes (anchoring of the zonular fibers of lens, suspensory ligament of lens).

*Iris

• General:
  
  • Extension of cillary body
  • contractile disc attached to the sclera.
  • Circular membraneous process, surrounding pupil
  • Regulates the amount of light that impinges on the retina
  • Determines the color of the eye
• Micro:
• Anterior surface:
  
  • fibroblats and melanocytes
  • No epithlium!, only
• Stroma
  
  • Blood vessels
  • Loose CT with melanocytes
    
    • Less: blue/green
    • More: dark/brown
• Posterior epithelium
  
  • 2 layers
  • Both are pigmented
  • Functions as a “mirror”
    
    • Felects light that hits this layer
    • So the only light that enters the eye is through the pupil
• Central aperture- Pupil
  
  • Size is controlled by contraction of:
    
    • Sphincter pupillae muscle
      
      • circular band of the smooth muscle
      • decrease diameter
    • Dilator pupillae muscle
      
      • a thin sheet of radially oriented contractile processes of pigmented myoepithelial cells

LENS

• General:
  
  • Lens is a transparent avascular biconvex structure
  • Suspended between the edges of the ciliary body by the zonular fibers:
    
    • The pull of the zonular fibers keeps the lens in a flattened condition.
    • Release of tension causes the lens to fatten or accommodate to bend light rays originating close to the eye so that they focus on the retina.
  • 10 mm diameter
  • 3.7-4 mm thickness
• Capsula
  
  • Composed primarily of
    
    • IV collagen
    • Proteoglycans
  • is elastic
  • Thickest at the equator where the fibers of the zonule attach to it.
• Subcapsular epithelium
  
  • Simple cuboidal epithelium is present only on the anterior surface of the lens.
  • Gap junctions
  • Few cytoplasmic organelles and stain faintly
• Zona cilliaris: position of the lens fixed by zonule fibers (oxitilane fibers- primary part of elastic fibers?, fibrilin protien)
• Lens fibers (from elongated epithelial cells)
  
  • Epithlial structure:
    
    • Structures derived from subcapsular epithelial cells.
    • New lens fibers develop from the subcapsular epithelial cells located near the equator.
    • Cells in this region increase in height and then differentiate into lens fibers - highly elongated, thin and flattened structures, which lost their nuclei and other organelles, and are completely filled with proteins called crystallins.
• Nucleus of the lens
  
  • Composed of the compressed and condensed lens fibers.
    
    • Loss of transparency of the lens is a relatively common condition associated with aging. This condition, called cataract, can be corrected surgically by removing the lens and replacing it with a plastic lens.
• Apical region of the cell is directed toward the internal aspect of the lens and the lens fibers, with which they form junctional complexes.
• The lens increases in size during normal growth and then continues to produce new lens fibers at an ever-decreasing rate throughout life.
• The new lens fibers develop from the subcapsular epithelial cells located near the equator
• Cells in this region increase in height and then differentiate into lens fibers.
• As the lens fibers develop, they become highly elongated
• and appear as thin, flattened structures.
• They lose their nuclei and other organelles as they become filled with proteins called crystallins.
• Near the center of the lens, in the nucleus, the fibers are compressed and condensed to such a degree that individual fibers are impossible to recognize.
• The high density of lens fibers makes it difficult to obtain routine histologic sections of the lens that are free from artifacts.

*Vitreous body:

• jelly substance
• Amoufous, transparent
• Rich in hyaluric acid
• Small amount of collagen
• few cells
  
  • hyalocytes: produce hyaloric acid, of monocyte-macrophage origin)

Vitreous body

• General:
  
  • Jelly like structure behind lens
• Function:
  
  • Pigmented epithlium and retina are not attached firmly attached
  • Viterous body: mainatain the possition of the retina on pigminted epithlium
  • maintains the internal eyeball pressure
  • compress retina to the choroidea
• Structure:
  
  • Membrana:
    
    • Collagen envelope
  • Stroma
    
    • sparse web of collagen fibers and halocytes (produce onyl prenatally)
  • Humor vitreus
    
    • 4 ml, 98% water, hyaloric acid
    • hyaluronic acid, collagen fibres
    • cells – hyalocytes – during development only!
    • no regeneration ! – in trauma flows out and replaced with aqueous humor
  • Canalis hyaloideus Cloqueti:
    
    • remnant of fetal hylaloidea a.
  • Fossa hyaloidea: impression of the lens
• It consists of the capsule,
• subcapsular epithelium and lens fibers.

EYELID: slide

• Eyelids protect the eye
• Organ
• Tarsal plate -
  
  • flexible support
  • dense fibrous and elastic tissue
• Meibomian glands
  
  • Embedded in the tarsal plate
  • Branched acinar, sebaceous glands
  • their secretion forms oily layer on the surface of the tear film
• Conjunctiva
  
  • mucous membrane; stratified columnar epithelium with goblet cells, lamina propria - loose connective tissue
• Eyeleshes
  
  • (hair follicles..)
  • ​sebaceous (gland of Zeis)
  • apocrine glands of Moll
• Lacrimal glands:
  
  • Produce tears
  • Moisten cornea and pass to the nasolacrimal duct
• Tears keep the conjunctiva and corneal epithelium moist and wash foreign material from the eye.
  
  • Thin film of tears covering the cornea is a mixture of products secreted by the lacrimal glands, the accessory lacrimal glands, the goblet cells of the conjunctiva, and the tarsal glands of the eyelid.

Identification:

• Skeletal muscle: musculus orbitalis occuli
• Looks similar to lip, but it is much thinner!
  
  • in lip there is also skeletal muscle
  • but there isnt a tarsal plate with elongated sebacous gland

LACRIMAL GLAND

• located beneath the conjunctiva on the upper lateral side of the orbit.
• Consists of several separate lobules of tubuloacinar serous glands.
  
  • The acini (A) are lined with columnar secretory cells.
  • Myoepithelial cells aid in the release of tears.
  • Intralobular CT is rich in lymphocytes (L) and plasmocytes.
• Approximately 12 ducts empty into the superior conjunctival fornix between the upper eyelid and the eye.
• Tears protect the corneal epithelium, contain: fluid with electrolytes, lactoferrin, lysozyme and secretory IgA.

distinguish from lacrating lacimary gland!

Question 3

Histological structure of the posterior part of the eye

Answer 3

Posterior segment of eye:​ from inner to outer

• Retina
  
  • Pigment epithelium (1), photoreceptors (2), bipolar neurons (3), ganglion cells (4)
• Choroid
  
  • Choroidal stroma, choriocapillary layer Bruch ́s membrane
• Sclera
  
  • Fibrocollagenouscoat (irregulardense CT), lamina fusca

The retina is the innermost layer of the eye: contain distinct layers

• Outer layer: pigmented epithelial cells
• Internal layer: the retina (nervous tunic)
  
  • Pars ceaca retinae: blind part
    
    • Ciliary part
    • Iris part
  • Ora serrata: (“tooth like”): border
  • Optic part
    
    • From optic nerve papilla to ora serrata
• Inner retinal plexsus

General:

• Development
  
  • Develops from the optic cup;
  • Its outer layer differentiates into the retinal pigment epithelium (RPE)
  • Inner layer thickens and begins a long process of differentiation into neurons, photoreceptors, and neuro- glial cells of the neuronal retina.
  • Two differentiation gradients of retina:
    • a) from the inner to the outer layers,
    • b) from the central to the peripheral parts

Internal layer: the retina (nervous tunic)

• 10-11 layers
  
  • brukes membrane (can be considered apart of choroid or apart of retina)

1- Pigment epithelium

• Simple cuboid
• Apical processes of cells surround the outer segments of rods and cones
• absorbs light passing through the neural retina
• isolates the retinal cells from blood-borne substances:
• blood-retina barrier
• participates in restoring photo- sensitivity of visual pigments
• phagocytoses and disposes of membranous discs from the rods and cones

2- Layer of rods and cones

• outer and inner segments of rods and cones

3- External limiting membrane:

• apical boundary of Müller ́s cells, glial cells

4- Outer layer core (cell bodies+nuclei of rods and cones)

• cell bodies (nuclei) of rods and cones

5- Outer layer plexiform

• contains the processes of rod and cones and horizontal, and bipolar cells that connect to them; synapses of rod and cone axons and bipolar cells

6- Inner layer core/ nucelar layer

• contains the cell bodies (nuclei) of bipolar, amacrine, and Müller ́s cells

7- Inner layer plexiform

• contain the processes of horizontal, amacrine, bipolar, and ganglion cells that connect each other; synapses of bipolar and ganglion c.

8-Layer of ganglion cells

• contains perikarya of ganglion cells (typical neurons)

9-Layer of nerve fibers

• contains axons of ganglion cells
• (optic nerve fibers that lead from the retina to the brain)
• Papilla of optic nerve (also where central artery enters eye)

10-Internal limit membrane

• composed of basal lamina of Müller ́s cells

NEURAL RETINA

• Photoreceptor cells: rods and cones
• Conducting neurons: bipolar neurons and ganglion cells
• Association neurons: horizontal and amacrine neurons 7
• Supporting (glial) cells: Müller’s cells, microglial cells, astrocytes

Photoreceptor cells:

• Primary sensory cells: Sensory process and short axon, but have simillarities to neurons

Visual pigments

• RODS
  
  • Rods are more sensitive to light visual pigment rhodopsin
  • 120 million rods
• CONES:
  
  • Each cone is specialized to respond to one of three colors – red, green or blue- three types of visual pigments - iodopsins
  • 6 - 7 millon cones
  • mostly found in fovea centralis (retina is thinner, allows light rays to get closer to cones so there is minumun light scattering)
• Outer segment
  
  • RODS:
    
    • has cylindrical shape,
    • contains horizontally flattened membrane discs
  • CONES:
    
    • Contrary to rod the interior of cone discs is continuous with the extra-cellular space
  • acctual receptors, sensory processes
• Connecting stalk with cilium
  
  • (BB = basal body, R = striated rootlet)
• Inner segment (metabolic region)
  
  • contains protein synthesis organelles (Golgi complex, ER, free ribosomes, mitochondria)
• “outer limating membrane”
• is formed by a row of zonulae adherentes (ZA) between the apical parts of Müller ́s cells and photoreceptor cells
• Cell body
  
  • contains nuceus
• Axon
• Presynaptic part (synapse with bipolar neuron)

Types of neural cells (photoreceptor cells, bipolar cells, and ganglion cells) that:

Form nine layers connected by synapses

Perceive and transform light into neuronal signals that travel to the brain via the optic nerve.

Two notable structures: optic disc and macula

has 10 layers (sorted from outermost to innermost)1

*Choroid

• General
  
  • Vscular sheet that lies between the sclera and the photosensitive retina;
  • Loose CT
  • Blood vessels and melanin pigment give the choroid an intense dark brown color.
  • The pigment absorbs scattered and reflected light to minimize glare within the eye.
• Micro: 4 layers
  
  • Lamina suprachoroidea
    
    • Loose pigmemnted CT
      
      • Melanocytes
      • Pigmented epithelial cells in retina>?
  • Zona vasculosa
    
    • Arteries and viens, melanocytes
  • Lamina Choriocapillaris
    
    • Capillary network
    • Fenestrated capillaries provide nutrients to the cells of retina
  • Bruch’s membrane
    
    • Amorphous refractile layer
    • Basal lamina of capillary endothelium
    • Elastic and reticular fibrils
    • Basal lamina of outer most layer of pigmemted endithelium

Question 4

1- General structure of the inner ear, structure and function of its vestibular part

2- Cochela and organ of corti

• Parts: Labyrinth and Cochlea

Labyrinth

• It is subdivided into
• 1- Osseous labyrinth (bony labyrinth)
  
  • The bony wall of the inner ear in the temporal bone
  • Contains the vestibule, the three semicircular canals, and the cochlea
  • Lined with periosteum
• 2- Membranous labyrinth
  
  • Lodged within the osseous labyrinth in perilymph
    
    • (low potassium, high sodium; transmits sound vibrations from the stapes-covered oval window to the round window)
  • Filled with endolymph (high potassium, low sodium)
  • Contains sensory organs
• Contains complex series of fluid-filled spaces
  
  • Vestibular duct (scala vestibuli):
    
    • filled with perilymph;
    • begins at oval window;
    • connected with tympanic duct at helicotrema (passage at apical end of cochlea)
  • Cochlear duct (scala media):
    
    • filled with endolymph;
    • contains spiral organ of Corti and auditory hair cells
  • Tympanic duct (scala tympani):
    
    • filled with perilymph;
    • ends at round window

Cochlea

• Fluid-filled, snail-shaped cavity in the labyrinth
• Layers of the cochlea
  
  • Bone
  • Vestibular duct
  • Reissner membrane
  • Cochlear duct
  • Basilar membrane: thin at the basal end and wide at the apical end; supports organ of Corti
  • Tympanic duct
  • Bone
• Function:
  
  • transformation of airborne vibrations into auditory neural signals
• Stria vascularis:
  
  • produces potassium-rich endolymph for the scala media, which is essential for the endocochlear potential
• The cochlear duct (scala media) contains hair cells .
  
  • These are specialized cells situated on a basilar membrane.
  • They have apical modifications called stereocilia.
• Vibrations lead to stimulation of a specific location on the basilar membrane, depending on the sound frequency (tonotopy).
  
  • High-frequency sounds stimulate hair cells at the base of the cochlea; the basilar membrane in this area is thin and rigid.
  • Low-frequency sounds stimulate hair cells at the apex of the cochlea near an area called helicotrema; the basilar membrane in this area is wide and flexible.

Answer 4

Opening: The ear

• Organ of hearing and balance.
• It is divided into three sections:
  
  • The outer, the middle, and the inner ear.
    
    • External Ear
      
      • The auricle or pinna of the external ear gathers sound waves and directs them through the external auditory canal interiorly to the eardrum or tympanic membrane.
    • Middle Ear
      
      • Small, air-filled cavity called the tympanic cavity.
        
        • The tympanic membrane separates the external auditory canal from the middle ear.
      • Contains auditory ossicles
        
        • Stapes, incus, and malleus
        • Transmitt the vibration of tympanic membrane to the membrane of the oval window.
        • Causes mechanical vibration of tissues and fluids of the inner ear
      • Auditory (eustachian) tube.
        
        • Communicates with the nasopharynx region of the head
        • Allows for equalization of air pressure on both sides of the tympanic membrane during swallowing or blowing the nose.

Inner ear: general

• Fluid-filled cavity
  
  • Les deep in the temporal bone of the skull.
• It consists of small, communicating cavities and canals
  
  • These cavities, the semicircular canals, vestibule, and cochlea, are collectively called:
    
    • the osseous or bony labyrinth.
• Contains the organ of Corti and the vestibular system.
  
  • The organ of Corti
    
    • Responsible for sound detection,
    • Transmits auditory information to the brain via the cochlear nerve
  • Vestibular system
    
    • Responsible for the registration of body movement and spatial orientation.
    • It transmits information to the brain via the vestibular nerve.

Structure:

• Parts of the middle ear
  
  • Labyrinth
    
    • Bony part: filled with the perilymph,
    • Membranous part: filled with endolymph

1- Bony labyrinth

• contains the following structures:
  
  • Vestibule
  • Cochlea
  • Semicircular canals
• Cavities that communicate with each other.
• Their main function is to house corresponding parts of the membranous labyrinth.
  
  • Vestibule
    
    • Central bony cavity.
    • It contains two sacs:
      
      • the utricle and saccule of the vestibular labyrinth (part of the membranous labyrinth).
      • The vestibule communicates
        
        • Lateral wall: with the tympanic membrane through the oval window
        • Anteriorly: with the cochlea
        • Postero-superiorly: semicircular canals
        • posterior cranial fossa through the vestibular aqueduct. It is a membranous structure that leaves the vestibule, courses medially, passes through the temporal bone and opens on the posterior surface of the petrous part of the temporal bone.

Labyrinth

• It is subdivided into
• 1- Osseous labyrinth (bony labyrinth)
  
  • The bony wall of the inner ear in the temporal bone
  • Contains the vestibule, the three semicircular canals, and the cochlea
  • Lined with periosteum
• 2- Membranous labyrinth
  
  • Lodged within the osseous labyrinth in perilymph
    
    • (low potassium, high sodium; transmits sound vibrations from the stapes-covered oval window to the round window)
  • Filled with endolymph (high potassium, low sodium)
  • Contains sensory organs
• Contains complex series of fluid-filled spaces
  
  • Vestibular duct (scala vestibuli):
    
    • filled with perilymph;
    • begins at oval window;
    • connected with tympanic duct at helicotrema (passage at apical end of cochlea)
  • Cochlear duct (scala media):
    
    • filled with endolymph;
    • contains spiral organ of Corti and auditory hair cells
  • Tympanic duct (scala tympani):
    
    • filled with perilymph;
    • ends at round window

Cochlea

• Fluid-filled, snail-shaped cavity in the labyrinth
• Layers of the cochlea
  
  • Bone
  • Vestibular duct
  • Reissner membrane
  • Cochlear duct
  • Basilar membrane: thin at the basal end and wide at the apical end; supports organ of Corti
  • Tympanic duct
  • Bone
• Function:
  
  • transformation of airborne vibrations into auditory neural signals
• Stria vascularis:
  
  • produces potassium-rich endolymph for the scala media, which is essential for the endocochlear potential
• The cochlear duct (scala media) contains hair cells .
  
  • These are specialized cells situated on a basilar membrane.
  • They have apical modifications called stereocilia.
• Vibrations lead to stimulation of a specific location on the basilar membrane, depending on the sound frequency (tonotopy).
  
  • High-frequency sounds stimulate hair cells at the base of the cochlea; the basilar membrane in this area is thin and rigid.
  • Low-frequency sounds stimulate hair cells at the apex of the cochlea near an area called helicotrema; the basilar membrane in this area is wide and flexible.

Cochlea

• Hearing organ
• Found in the inner ear in the structure called the cochlea.
• Shape and structure:
  
  • It is a spiral bony canal that resembles a snail’s shell.
  • The cochlea makes three turns on itself around a central bony pillar called the modiolus.
  • Interiorly, the cochlea is partitioned into three channels:
    
    • Vestibular duct (scala vestibuli)
    • Tympanic duct (scala tympani)
    • Cochlear duct (scala media)
• Hearing organ of Corti
  
  • Located within the cochlear duct on the basilar membrane
  • Consists of:
    
    • Numerous auditory receptor cells or hair cells
    • Several supporting cells that respond to different sound frequencies.
  • The auditory stimuli (sounds) are carried away from the receptor cells via afferent axons of the cochlear nerve to the brain for interpretation.

Vestibular Functions

The organ of vestibular functions that is responsible for balance and equilibrium is found in the

utricle, saccule, and three semicircular canals.

Question 5

Cochlea and the organ of Corti

Question 6

Types of nerve fibers, their sheaths and endings

• EXTRA
• A-alpha-fibers
• Mylinated
• Afferent: muscle spindles
• Efferent: alpha motoneurons
• A-beta fibers
• Mylinated
• Afferent: cutaneous mechanoreceptors
• A-gamma fibers
• Mylinated
• Efferent: muscle spindles (gamma motoneurons)
• A-delta fibers
• Mylinated
• Afferent: pain (e.g., thermal, mechanical )
• Free nerve endings
• Responsible for the withdrawal response to pain (e.g., rapidly moving the hand when burned)
• B fibers
• Moderately mylinated
• Efferent: preganglionic sympathetic fibers
• C fibers
• Not mylinated (lack of myelin creates the slow conduction as the saltatory conduction is not present)
• Afferent: pain (e.g., chemical, thermal, mechanical)

Myelin is a membranous sheath that insulates the axon. There are regions of the axon between bundles of myelin that remain bare, which are known as nodes of Ranvier. They promote rapid impulse transmission along the axon in a manner known as saltatory conduction. The rate of conduction increases with the diameter of the axon. Larger axons are typically more heavily myelinated than smaller axons, and consequently neurons with larger axons also transmit impulses faster than those with smaller axons:

Neurons have been grouped into two broad categories: those found in the central nervous system (brain and spinal cord) and those in the peripheral nervous system. In the central nervous system, they are found in clusters referred to as nuclei, or in layers also known as laminae. However, in the peripheral nervous system, they are found in ganglia.

Answer 6

Sensory receptors

A sensory receptor is a structure specialized to detect a stimulus.

The sensory receptors can be classified according to the stimulus origin:

• Exteroceptors:
  
  • Help recieve stimuli for outside of body
  • Includes taste, smell, light, hearing and touch.
• Proprioceptors:
  
  • In locomotor system (joints and tendons)
  • Information about mechanical changes
  • Regulation and coordination of movment
  • Sense the body’s position (without vision)
    
    • Mechanoreceptors:
      
      • In joints (fibrous capsule),
      • Muscles (muscle spindles)
      • Tendons (Golgi tendon organs).
• Interoceptiopns
  
  • Stimulation from internal enviorment
  • Sensation of nausea, pain, stretch and pressure.

The receptors can also be classified into 5 basic functional types:

• Mechanoreceptors
  
  • Respond to physical deformation of tissues, like touch, pressure, stretch, and vibration.
  • Include the hearing receptors, skin, joints and viscera.
    
    • (hair follicles, Merkle cells, free nerve endings, Pacinian corpuscles, meissnerscorpuscles, ruffinicorpuscles)
    • Occur as
      
      • Free nerve endings in the skin
      • Proprioceptors (ends in muscles, tendons and inner ear) they perceive muscle tension, joint flexion and body position
• Nociceptors
  
  • Pain receptors
  • Respond to
    
    • Tissue damage from trauma (blow, cut),
    • Ischemia (low blood flow)
    • Burns from chemicals or heat.
• Thermoreceptors
  
  • Respond to surrounding temperature.
• Chemoreceptors
  
  • Respond to chemicals, including odor, taste and fluid compositions.
    
    • Located in the oral cavity (taste)
      
      • The taste buds are arranged in taste buds , which are most often found on the sides of the covered papillae of the tongue
    • Nasal mucosa (smell)
      
      • Olfactory cells can be found in the upper part of the nasal cavity, pars olfactoria
• Photoreceptors
  
  • Respond to light (electromagnetic radiation)
  • Rods and cones in the eye (sight), with which we are able to analyze the shapes and intensity of light and colors

Primary VS seconday sensory organs

• The receptor cells of the sensory organs do not have a common origin
  
  • Therefore we can divide them into primary and secondary cells
  • The difference between them is not only in their appearance, but also in the way they transmit information to the CNS.
• Primary sensory cells
  
  • Neuroepithelial origin
    
    • They originate from the epithelium of the neural tube)
      
      • Like normal nerve cells, have a venous protrusion through which they receive sensations and a conductive protrusion (axon) leading excitation efferently to the CNS.
      • The primary sensory cells are the rods and cones of the visual system and the cells of the olfactory epithelium .
• Secondary sensory cells have an ectoderm origin and, unlike primary cells, have only a clear protrusion. The excitation leads to the CNS fibers of the sensory nerves, formed by the axons of the second neuron, whose dendrites connect synaptically to the axons of the sensory cells. These include hair cells (ear) and taste bud cells .

Primary and secondary sensory cells are not found alone in the sense organs. In addition to them, there are also supportive cells, basal cells and more. They have their own functions (support, replacement of dead sensory cells), but they do not include receiving stimuli from the external environment.

​​

Primary sensory organs-have both sensory and conductive projections. These cells are neurons. Example: olfactory cells, rods and cones.

•Secondary sensory organs-have only sensory projections. These cells are epithelial cells. Example: taste cells and hair cells.

NERVE ENDINGS

General characteristics:

Stimuli: ​Mechanoreceptos, thermoreceptor, nociceptors (pain)

1. Non elncapsuled:

• Free nerve endings
  
  • Lack myleyin or schwann cells- “naked nerve fibers”
  • Afferent fibers
  • Pain and temperature
  • Location:
    
    • Epidermis (all the way to dtratum granulosum), dermis
    • Corneal epithelium
• Merkel disc
  
  • neural crest derived
  • basal layer of epidermis
  • high resolution tactile
  • fingertips and lips
• Palisade: ?
  
  • nerve fibers wrapped arounf of the base of the hair follice, stimulated by hair follicles

2. Encapsuled endings

• Pacinian corpusles
  
  • Lamellae of modified schwann and perineural cells
  • Lymph like fluid bewtween the lamellae
  • Large structures, up to 4 mm in length
  • Deep pressure, pressure change and vibrations
  • Location:
    
    • Hypodermis and deep fascia (mesentary of viseceral organs)
• Meissners corpucles
  
  • Iregular lamellae of schwann cells, spiral pancake
  • Suspended by fibrils which connect to the basal layer of epithelium
  • Mechanoreceptor:
    
    • finner mechanical stimuli
    • shape and texture
    • lower frequency stimuli
  • Location:
    
    • dermal papilla
• Ruffuni corpusles
  
  • Responds to stretching and warmth: redspond to mechanical dispacemt of adjacent collagen fibers
  • Axon ending: mylwn sheet is lost after entrance to corpusle and are dispersed and intertwined in side capsule)
  • Location
    
    • Skin (dermis) and joint capsule (can be considered as proprioceptors)
    • Stratum reticulare
• Karuese end bulb
  
  • Encapsuled thermoreceptor: detects cold
    
    • can also transmit mechanical
  • round sturcture, capsule from perineural cells that encapsule schwann cells with branched nerve endings
  • Location:
    
    • lips and tounge
    • penis and clitoris: genital corpusle

Motor end plate:

• peripheral nrver ending of effernt nerve fibers
• ?????

Muscle spindle and

• afferent and efferent (can be stimulated by gamma motor fiber) endings
• intrafusal (smaller and diffrent morphology that extrafusal)
• contains more neuclei and are thinner
• surroundend by lamellar CT that surounds muscle tissue
• proprioception:
  
  • sensitive to stretch of muscle
  • adapatable senitivity
    
    • more contract- more sensitive to strech

golgi tendon organ

• stimulated by contraction of muscle whuch is transmited to tendon
• located between collagen fibers of tendon

dorsal root ganglion:

• At dorsal root of spinal nrerve
• Pseuodounipolar neurons surround by satellite cells
  
  • T shape

Sympathetic ganglia:

• mutlipolar neurons with many dendrites
• surrounded by saltelite cells
• can be sympathetic: paravertebral ganglia
• parasympathetic: ?

Paraganglia:

• Variable, small up to 3 mm in diameter
• largest: carotid bodies
• aortic sympatheritc ganglia
• small corpuslces consiting of newuroendocrine cells (neural crest origin),
• sympathetic: paraganglion aortic abdomial (childern, chromaffin cells- adrenaline)
• parasympathetic: glomus caroticum, chemorepeption (content of O2 and Cos in bloos)

Structure: PARAGANGLIA

• Cheif cells froming round clupms/nests
• 40% sustenracular cells
• fernistaed capillaries
• *