Organ of hearing and equilibrium. Internal ear. Cochlear part of membranous labyrinth. Structure of spiral organ of Corti. Vestibular part of membranous labyrinth

General characteristicOrgan of hearing and of equilibrium includes external, middle and internal ear, which realize perception of sound, gravitation, vibration stimulus, linear and angular accelerations. The organ of hearing occupies the second place according to volume of entering information.

DevelopmentThe organ of hearing and of equilibrium is founded in embryogenesis in ectoderm from special thickenings – placode.

StructureThe organ of hearing consists of three parts: external ear, middle ear and internal ear. External earincludes: 1. auricle, 2. external acoustic meatus and 3. tympanic membrane (eardrum).

Internal ear consists of bony and membranous labyrinths. In the membranous labyrinth receptor cells – hair-shaped sensor epitheliocytes of ear and of balance are located.

The auditory receptor cells are located in cochlea. The receptor cells of organ of equilibrium are located in the utricle and in saccule and in ampular crista) of semicircular ducts (canal).

CochleaPerception of sounds is realized in spiral organ, which arranges on cochlear duct of membranous labyrinth.

Cochlear ductThe cochlear duct is triangular canal lying between basilar membrane, vascular stria and vestibular membrane. Cochlear duct is filled by endolymph. Scala vestibuli contains perilymph. External wall of cochlear duct is formed by vascular stria (stripe) arranging on spiral ligament.

Its epithelium consists of flat basal light cells and high prismatic dark cells, which are rich in mitochondria. Between cells hemocapillaries pass. Vascular stria realizes secretory function. It produces endolymph and plays role in trophic of spiral organ. Upper medial wall of cochlear duct is made by vestibular membrane. It is thin fibrillar connective tissue lamina, covered by simple squamous epithelium. Lower wall is formed by basilar lamina. Basilar lamina represents connective tissue lamina.

Lower wallOn the side related to spiral organ, it is covered by basal membrane of epithelium of this organ. Thin collagen fibers underlie the basilar lamina. These fibers consist of fine fibrils with diameter 30 nanometers. From the side of scala tympani layer of flat cells covers basilar lamina. They are of mesenchyme origin. Spiral limbus The surface of spiral limbus is covered by squamous epithelium. Its cells capable to secretion. Lining of spiral sulcus represents few lines of large flat polygonal cells, which continue to supporting epitheliocytes. They fit closely to internal hair cells of spiral organ.

Tectorial membrane is connected with epithelium of vestibular lip.It is lamina of jelly-like consistence. It consists of radial thin collagen fibers. Between fibers there is transparent adhesion substance containing glycosaminoglycans.

Spiral organ of CortiIt consists of 2 group of cells – sensor and supporting. Each of these groups of cells is divisible into the internal and external. Their border is tunnel.

Internal sensor hair epitheliocytes (A) are jug-shaped. They have widen basal part and lie in one line. Apical surface is covered by cuticle. At the apical surface they contain 30-60 stereocilia. In human 3500 internal hair cells are present.

StereociliaApical part of stereocilia is covered by cuticle, through which stereocilia pass. In their cytoplasm mitochondria, elements of smooth and granular endoplasmatic reticulum, actin and myosin microfilaments are present.

External sensor hair epitheliocytes are cylindrical, lie in 3-5 lines.

In human 12 000-20000 external hair cells are present. Stereocilia of external hair cells attach to the internal surface of tectorial membrane. Stereocilia contain fibrils with contractile protein actomyosin. External hair sensory cells are more sensible to sounds of high intensity, then internal. During sound influence onto tympanic membrane, its vibrations pass to hammer, anvil and stirrup, further through oval window to perilymph, basilar and tectorial membranes. Stereocilia move, receptor cells are excited. Receptor potential appears (microphone effect). Afferent information along acoustic nerve passes to central parts of acoustic analyzer.

Receptors of organ of hearingInternal sensor hair epitheliocytes and external sensor hair epitheliocytes are receptor cells of the organ of hearing.

Supporting epitheliocytesSupporting epitheliocytes of spiral organ are arranged on basement membrane. In their cytoplasm are tonofibrils. Internal phalangial epitheliocytes lie under internal hair sensory epitheliocytes. Onto basilar lamina external phalangial cells are located. These prismatic shaped cells are located in 3-4 lines. They attach to external hair cell. In spiral organ also internal and external pilar epitheliocytes are located. They form triangular channel – internal tunnel, filled by endolymph. Bases of pilar cells fit closely to each other and are located on basilar membrane. Through the tunnel non-myelinated nerve fibers pass from neurons of spinal ganglion to sensory cells.

Vestibular part of membranous labyrinth is place where receptors of organ of equilibrium are arranged. It consists of two saccules: elliptic - utricle and spherical – saccule. They are connected with the help of narrow channel and are joined with 3 semicircular channels. They are localized in bony channels and are located in 3 perpendicular directions. Vestibular part of membranous labyrinthThese channels in the place of junction with elliptic saccule (utricule) have widen ampules. In the wall of membranous labyrinth in the region of elliptic and spherical saccules and ampules parcels, containing sensor cells are present. In saccules these parcels are called spots or macula. In the ampules they are called crista.

Macule is lined by epithelium arranging on basement membrane and consisting of sensor (1) and supporting cells (2).

The surface of receptor macula is covered by otholith membrane (3). It consists of crystals of calcium carbonate – otholits or statoconia. Macule of utricle is receptor of linear accelerations and gravitation. Macule of saccule also is receptor of gravitation and perceives vibrations. Hair sensor cells of macule are divided into two types: I type cells (pear-shaped), II type cells (prismatic).

On the external surface of these cells 60-80 immobile stereocilia (2) and 1 mobile kinocilia (1) are present. Spherical macule of human being contains about 18 000 receptor cells, oval contains about 33 000. Supporting cells (sustenocytes) are located between sensory cells. They have dark oval nuclei. They contain numerous mitochondria. On their apexes number of cytoplasmic microvilli are located.

Ampular crista are lined by sensor hair and supporting epitheliocytes. Apical part of these cells is surrounded by gelatinous transparent cupola. Gelatinous cupula is receptor of angular accelerations. Moving of cupola under the action of moving of endolymph in semicircular channels stimulates hair cells. Their excitation causes reflex in those part of skeletal muscles, which is responsible for position of body and moving of eye muscles. Hair cells of ampular crista are receptors of organ of equilibrium of moving body.

General characteristicOrgan of hearing and of equilibrium includes external, middle and internal ear, which realize perception of sound, gravitation, vibration stimulus, linear and angular accelerations. The organ of hearing occupies the second place according to volume of entering information.

DevelopmentThe organ of hearing and of equilibrium is founded in embryogenesis in ectoderm from special thickenings – placode.

StructureThe organ of hearing consists of three parts: external ear, middle ear and internal ear. External earincludes: 1. auricle, 2. external acoustic meatus and 3. tympanic membrane (eardrum).

The auricle consists of a thin lamina of elastic cartilage covered by skin. This skin has hair follicles, sebaceous glands and sweat glands. The external acoustic meatus is made up of elastic cartilage. The surface of external acoustic meatus is covered by thin skin. This skin also has hair follicles and sebaceous glands. There are ceruminous glands under these sebaceous glands. They secrete the earwax. They are modified sweat glands lined by columnar, cuboidal or squamous epithelia.

Tympanic membraneThe tympanic membrane has oval shape. One of the auditory ossicles is called the hammer. The hammer knits with the tympanic membrane. The tympanic membrane has three layers: internal, middle, external. Internal fibrous layer contains collagen fibers and some elastic fibers. The fibers are arranged in two layers. In the external layer they are placed radially, while in the inner layer they run circularly. Middle layer is made up of fibrous tissue, which is covered from the outside by skin, and inside is lined by mucous membrane of the tympanic cavity. External layer of tympanic membrane is covered by stratified squamous keratinizing epithelium. Internal surface is lined by simple squamous epithelium.

Middle earconsists of tympanic cavity, auditory ossicles and auditory (Eustachian) tube.

The tympanic cavity is flattened space containing air. It is covered by simple epithelium. The lining epithelium varies from region to region. Typically it is cuboidal or squamous. On the medial wall of the tympanic cavity there are two holes or windows. The first one is oval. It separates the tympanic cavity from vestibule stairs of cochlea.

Sound-passing apparatus of organ of hearing includes: tympanic membrane, auditory ossicles, membrane of oval window, perilymph of scala vestibuli of cochlea.

Auditory ossicles are hammer, anvil and stirrup.They are formed by lamellar bone tissue. Auditory ossicles pass vibrations of tympanic membrane of external ear to oval window. Tympanic stair contains perilymph.

Auditory (Eustachian) tubeAuditory tube joins tympanic cavity with the nasal part of pharynx. Diameter of this tube is 1-2 mm. The wall of the auditory tube is partly bony (lateral part) and partly cartilaginous (medial part, nearer the nasopharynx). Auditory tube is lined by pseudostratified prismatic ciliated epithelium. There are goblet cells in this epithelium. On the surface of this epithelium ducts (canals) of mucous glands are opened. Through the auditory tube pressure of air in the tympanic cavity of the middle ear is regulated.

Internal ear consists of bony and membranous labyrinths. In the membranous labyrinth receptor cells – hair-shaped sensor epitheliocytes of ear and of balance are located.

The auditory receptor cells are located in cochlea. The receptor cells of organ of equilibrium are located in the utricle and in saccule and in ampular crista) of semicircular ducts (canal).

CochleaPerception of sounds is realized in spiral organ, which arranges on cochlear duct of membranous labyrinth.

Cochlear ductThe cochlear duct is triangular canal lying between basilar membrane, vascular stria and vestibular membrane. Cochlear duct is filled by endolymph. Scala vestibuli contains perilymph. External wall of cochlear duct is formed by vascular stria (stripe) arranging on spiral ligament.

Its epithelium consists of flat basal light cells and high prismatic dark cells, which are rich in mitochondria. Between cells hemocapillaries pass. Vascular stria realizes secretory function. It produces endolymph and plays role in trophic of spiral organ. Upper medial wall of cochlear duct is made by vestibular membrane. It is thin fibrillar connective tissue lamina, covered by simple squamous epithelium. Lower wall is formed by basilar lamina. Basilar lamina represents connective tissue lamina.

Lower wallOn the side related to spiral organ, it is covered by basal membrane of epithelium of this organ. Thin collagen fibers underlie the basilar lamina. These fibers consist of fine fibrils with diameter 30 nanometers. From the side of scala tympani layer of flat cells covers basilar lamina. They are of mesenchyme origin. Spiral limbus The surface of spiral limbus is covered by squamous epithelium. Its cells capable to secretion. Lining of spiral sulcus represents few lines of large flat polygonal cells, which continue to supporting epitheliocytes. They fit closely to internal hair cells of spiral organ.

Tectorial membrane is connected with epithelium of vestibular lip.It is lamina of jelly-like consistence. It consists of radial thin collagen fibers. Between fibers there is transparent adhesion substance containing glycosaminoglycans.

Spiral organ of CortiIt consists of 2 group of cells – sensor and supporting. Each of these groups of cells is divisible into the internal and external. Their border is tunnel.

Internal sensor hair epitheliocytes (A) are jug-shaped. They have widen basal part and lie in one line. Apical surface is covered by cuticle. At the apical surface they contain 30-60 stereocilia. In human 3500 internal hair cells are present.

StereociliaApical part of stereocilia is covered by cuticle, through which stereocilia pass. In their cytoplasm mitochondria, elements of smooth and granular endoplasmatic reticulum, actin and myosin microfilaments are present.

External sensor hair epitheliocytes are cylindrical, lie in 3-5 lines.

In human 12 000-20000 external hair cells are present. Stereocilia of external hair cells attach to the internal surface of tectorial membrane. Stereocilia contain fibrils with contractile protein actomyosin. External hair sensory cells are more sensible to sounds of high intensity, then internal. During sound influence onto tympanic membrane, its vibrations pass to hammer, anvil and stirrup, further through oval window to perilymph, basilar and tectorial membranes. Stereocilia move, receptor cells are excited. Receptor potential appears (microphone effect). Afferent information along acoustic nerve passes to central parts of acoustic analyzer.

Receptors of organ of hearingInternal sensor hair epitheliocytes and external sensor hair epitheliocytes are receptor cells of the organ of hearing.

Supporting epitheliocytesSupporting epitheliocytes of spiral organ are arranged on basement membrane. In their cytoplasm are tonofibrils. Internal phalangial epitheliocytes lie under internal hair sensory epitheliocytes. Onto basilar lamina external phalangial cells are located. These prismatic shaped cells are located in 3-4 lines. They attach to external hair cell. In spiral organ also internal and external pilar epitheliocytes are located. They form triangular channel – internal tunnel, filled by endolymph. Bases of pilar cells fit closely to each other and are located on basilar membrane. Through the tunnel non-myelinated nerve fibers pass from neurons of spinal ganglion to sensory cells.

Vestibular part of membranous labyrinth is place where receptors of organ of equilibrium are arranged. It consists of two saccules: elliptic - utricle and spherical – saccule. They are connected with the help of narrow channel and are joined with 3 semicircular channels. They are localized in bony channels and are located in 3 perpendicular directions. Vestibular part of membranous labyrinthThese channels in the place of junction with elliptic saccule (utricule) have widen ampules. In the wall of membranous labyrinth in the region of elliptic and spherical saccules and ampules parcels, containing sensor cells are present. In saccules these parcels are called spots or macula. In the ampules they are called crista.

Macule is lined by epithelium arranging on basement membrane and consisting of sensor (1) and supporting cells (2).

The surface of receptor macula is covered by otholith membrane (3). It consists of crystals of calcium carbonate – otholits or statoconia. Macule of utricle is receptor of linear accelerations and gravitation. Macule of saccule also is receptor of gravitation and perceives vibrations. Hair sensor cells of macule are divided into two types: I type cells (pear-shaped), II type cells (prismatic).

On the external surface of these cells 60-80 immobile stereocilia (2) and 1 mobile kinocilia (1) are present. Spherical macule of human being contains about 18 000 receptor cells, oval contains about 33 000. Supporting cells (sustenocytes) are located between sensory cells. They have dark oval nuclei. They contain numerous mitochondria. On their apexes number of cytoplasmic microvilli are located.

Ampular crista are lined by sensor hair and supporting epitheliocytes. Apical part of these cells is surrounded by gelatinous transparent cupola. Gelatinous cupula is receptor of angular accelerations. Moving of cupola under the action of moving of endolymph in semicircular channels stimulates hair cells. Their excitation causes reflex in those part of skeletal muscles, which is responsible for position of body and moving of eye muscles. Hair cells of ampular crista are receptors of organ of equilibrium of moving body.

Nerve tissue

Characteristic of nerve tissue

Significance of nerve tissue in organism is determined by basic properties of nerve cells (neurons) to percept irritation, to get condition of excitation, to produce impulse and pass it.

Nerve tissue is the basis of structure of organs of nervous system providing regulation of the all tissues and organs, their integration in the organism and connection with surroundings.

Development of nerve tissue

Nerve tissue is derived from dorsal thickening of ectoderm – nerve lamina.

Neurons of spinal ganglia, neurons of vegetative ganglia and pigment cells of skin are derived from nerve comb.

Glioblasts of nerve tube are source of development of ependimocytes, astrocytes and oligodendrocytes.

Neurons of ganglia of cranial nerves (V, VII, IX, X) are developed from neural placoda and nerve comb.

From neural placoda neurolemmocytes (Schwann cells) are developed.

Structure of nerve tissue

• Nerve tissue consists of neurons and neuroglia.

• Neuroglia provides existence and functioning of neurons and realizes supporting, trophic, limiting, secretory and protective functions.

• Nerve tissue is derived from dorsal thickening of ectoderm – nerve lamina.

• Neurons or neurocytes consist of body (pericaryon) and processes – axon (3) and different number of branched dendrites (1).

• Axon is processus along which impulse passes from body of cell.

• Dendrite is branched processus passing impulse to body of neuron.

Morphological classification of neurons

According to number of processes there are neurons:

1.unipolar

2.bipolar

3.multipolar

4.pseudounipolar

Unipolar neurons have just 1 axon.
Neuroblasts have unipolar shape.

Bipolar neurons have axon and dendrite.
They are localized in sense organs.

Multipolar neurons have axon and many dendrites. Great number of neurons, neurons of CNS and of vegetative ganglia are multipolar.

Pseudounipolar neurons have 1 processus further dividing into axon and dendrite. Pseudounipolar neurons are present in spinal ganglia.

Functional classification of neurons

Depending on function there are neurons:
1.Receptor (or sensitive, sensory, afferent). They percept impulse.

2.Associative (or intercalated). They realize connection between neurons.

3.Effector (or efferent, motor). They pass impulse onto tissues of working organs, impel them to action.

Structure of neuron

• Neurons contain one nucleus located at the center.

• Shape of nucleus is round.

• Chromatin is dispersed.

• In nucleus 1 or 2 big nucleoli are located.

• Cytoplasm is rich is organelles – Golgi complex, ER, mitochondria, lysosomes, components of cytoskeleton.

Chromatophilic substance

• During staining of nerve tissue by anilin dyes in cytoplasm of neurons basophilic blocks and granules appear.

• This is chromatophilic substance.

• Also it is called tigroid substance or Nissl substance.

• Chromatophilic substance is accumulation of granular ER, free ribosomes and polysomes.

• This substance is localized in pericaryons and dendrites of neurons.

Structure of neuron

• Corresponding to high specificity of functional activity neurons have specialized plasmalemma.

• Plasmalemma of neurons is able to pass excitation.

• Golgi complex in neurons is seen as accumulation of different shape rings, twisted threads and granules.

• Cellular center is located between nucleus and dendrites.

• Mitochondria are present in body of neuron and in the all processes.

Cytoskeleton of neurons

• During impregnation of nerve tissue by silver in cytoplasm of neurons neurofibrils are revealed.

• They form compact net in pericaryon of cell and are oriented parallel in dendrites and neurites.

• Neurofilaments and neurotubules correspond to neurofibrils.

Secretory neurons

• All the neurocytes are able to synthesize and secrete biologically active substances, particularly mediators.

• But there are neurocytes specialized predominantly for realizing of this function.

• These are secretory neurocytes.

• They are large neurons with neurosecretion in cytoplasm.

• Neurosecretions realize role of neuroregulators.

• Neurons of hypothalamus possess neurosecretory function.

Neuroglia

• Neuroglia realizes in nerve tissue supporting, limiting, trophic, secretory and protective functions.

• There are glia of central (CNS) and of peripheral (PNS) nervous system.

• Glia of CNS is divided into macroglia and microglia.

Macroglia

Macroglia is derived form glioblasts of nerve tube.

These are ependimocytes, astrocytes and oligodendrocytes.

The most numerous group of cells are oligodendrogliocyts.

Ependimocytes (1) line ventricles of brain and central channel of spinal cord.

• These cells are cylindrical.

• They form layer as epithelium, realize secretory function – produce cerebrospinal fluid.

Astrocytes are cells of stellate shape poor in organelles.

They realize mainly supporting and limiting functions.

There are protoplasmic astrocytes (a) localized in grey matter of CNS and fibrous astrocytes (б) present in white matter of brain.

Oligodendrocytes surround bodies of neurons in central and peripheral nervous system.

- They are in structure of layers of nerve fibeкs and nerve endings.

- Functions of oligodendrocytes are trophic, taking part in metabolism of neurons, synthesis of myelin, formation of layers around processes of cells (they are called neurolemmocytes or Shwann cells).

Microglia represents phagocyting cells related to the system of mono-nuclear phagocytes and derived from stem hemopoietic cell.

So, mesenchyme is source of development of microglia.

Function of microglia is protection from infection and from damage and removing of products of destruction of nerve tissue.

Cells of microglia (2) are mobile, have little dimensions, their bodies have oblong shape.

Glia of PNS

Glia of PNS is derived from nerve comb.

These are:

1.neurolemmocytes or Schwann cells – they form layers of processes of neurons in nerve fibres of PNS;

2.gliocytes of ganglia (mantle gliocytes) – they surround bodies of neurons in ganglia and take part in metabolism of neurons.

Processes of nerve cells covered by layers are called nerve fibers.

According to structure of layers there are:

1.myelinated (a)

2.non-myelinated (б).

Processus of nerve cell in nerve fibre is called

axial cylinder (1).

Non-myelinated nerve fibers

They are present in structure of vegetative nervous system.

Non-myelinated nerve fibre consists of a few axial cylinders and layers formed by neurolemmocytes.

With the help of electron microscope in non-myelinated fiber mesaxon is seen.

Mesaxon is dublicator of plasmalemma of neurolemmocyte, on which axial cylinder is suspended.

Myelinated nerve fibres are present both in CNS and PNS.

They are thicker, than non-myelinated.

Myelinated fiber consists of one axial cylinder (1), myelin sheath and neurilemma.

Myelin sheath contains great number of lipids.

In it periodically narrow light lines – incisures (4) of myelin are present.

Through definite intervals (1-2milimeters) parcels of fiber without myelin sheath are seen.

These are nodes of Ranvier (4).

Parcel of fiber between two nodes is called internode segment.

Neurilemma or Schwann sheath is external peripheral layer of myelinated fibre.

It contains cytoplasm and nuclei of neurilemmocytes.

Nerve endings

Nerve fibres end by terminal apparatus - nerve endings.

There are 3 groups of nerve endings:

1.terminal apparatus forming interneuron synapses and realizing connection of neurons with each other;

2.effector endings (effectors) passing impulse onto tissues of working organ;

3.receptor (sensitive).

Effector nerve endings are of 2 types:

1.motor – these are terminal apparatus of axons of motor cells of somatic or vegetative nervous system. With their help nerve impulse is passed onto tissues of working organs (nerve-muscular endings in transversal-stripped muscles, motor nerve endings in smooth muscles);

2.secretory – they represent terminal thickenings of terminals or thickenings along nerve fiber containing synaptic vesicles (cholinergic).

Receptors

• Receptor nerve endings are receptors spread in whole organism.

• They percept different irritations from external environment and from internal organs.

• There are 2 types of receptors:

• 1.exteroreceptors – external these are acoustic, optic, olfactory, taste and tactful;

• 2.interreceptors – internal – these are visceroreceptors (they signalize about condition of internal organs) and vestibular proprioreceptors (receptors of supporting-motor apparatus).

Synapse

Synapse is specialized contact between neurons, where passing of impulse from one cell to another occurs.

According to localization synapses are:

1.axo-dendritic;

2.axo-somatic;

3.axo-axonic.

Interneuronic synapses

• Polarization of passing of nerve impulse along chain of neurons is determined by their specialized contacts – synapses.

• Depending on method of passing of impulse synapses can be chemical and electrical.

Chemical synapse

• Chemical synapses pass impulses onto other cell with the help of special biologically active substances – neuromediators placed in synaptic vesicles (acetylcholin, noradrenalin, dophamin, serotonin, glicin, histamine).

Electrical synapse

• Electrical synapses in nervous system of mammals are present rarely.

• Their structure corresponds to fissure-shaped contact.

In synapses with chemical passing are:

• 1) presynaptic part;

• 2) synaptic fissure;

• 3) postsynaptic part.

Presynaptic part

n Presynaptic part is formed by parcel of plasmalemma of the first neuron.

n It contains accumulations of mitochondria and synaptic vesicles.

n Synaptic vesicles are filled by neuromediator excreted into synaptic fissure and taking part in passing of excitation onto postsynaptic part.

Synaptic fissure

• Synaptic fissure is located between pre- and postsynaptic parts of synapse.

• Its width is 20-30 nanometers.

Postsynaptic part

3. Postsynaptic part is parcel of plasmalemma of the second neuron which percepts neuromediators.

4. It contains receptor zones for perception of corresponding mediator.

Sense organs provide getting of information about condition and changes of surroundings and of activity of systems of organism. They are peripheral parts of analyzers, i.e. sensory systems.

Analyzers are aggregations of organs and structures, providing perception of different irritations, acting onto organism, formation and coding of external energy into nerve impulse, This nerve impulse passes along nerve ways to subcortical and cortical centers, where analysis of information and formation of subjective sensation occur.

Each analyzer has three parts:

n Peripheral or receptor, is represented by organs in which receptor cells are present, they percept signals from external environment and from internal organs.

n Intermediate or conducting – these are intercalated neurons along which nerve impulse passes from receptor cells to cortical centers.

n Central part represents parcels of cortex of large hemispheres, where terminal analysis and synthesis of percept sensation occur.

Classification of sense organs.

Depending on structure and function of receptor part sense organs are divided into 3 types:

n Sense organs, which receptors are specialized neurosensory primary-sensory cells, transforming external energy into nerve impulse. These are organ of vision and olfactory organ. Source of development is nerve lamina.

n Sense organs, which receptors are sensoepithelial, secondary-sensory. From them irritation passes to dendrites of sensory neurons, which percept excitation and born nerve impulse. These are organ of hearing and of equilibrium and gustatory organ. Source of development is ectoderm.

n Not marked anatomically organ form. These are group of receptor capsulated and encapsulated nerve endings.

Organ of vision.

Eye is peripheral part of visual analyzer, in which receptor function is realized by neurons of retina. Eye consists of eyeball containing photoreceptor cells. They connect with brain by optic nerve. Also eye consists of auxiliary apparatus, which includes eyelids, lacrimal apparatus and oculomotorius muscles.

Development.

Eye is developed from different sources. Retina and optic nerve are formed from rudiment of nerve tube, which called eye bubble. Anterior part of eye bubble sticks out and forms bilateral eye-goblet. Ectoderm located opposite to aperture of eye goblet is thicken and is divided, gives the beginning of rudiment of lens. Internal part of eye-goblet transforms to retina, and external part transforms to pigment layer of retina. On the internal wall of an eye-goblet neurons are present. From these neurons rods and cones are formed. Eye-goblet is surrounded by mesenchyme. From this mesenchyme choroid (vascular layer) and sclera are formed. In the anterior part of eye sclera continues to cornea, which is covered by multi-layered squamous epithelium. Vessels and mesenchyme form vitreous body and iris. Muscles of iris are neural according to origin.

Eye – ball is made of three layers:

1.fibrous (sclera and cornea)

2.vascular and internal (sensory) layers and their appendages (iris, ciliary body)

3.lens, fluid of anterior and posterior chambers of eye, vitreous body.

Structure of an eyeball.

Fibrous layer (tunica fibrous bulbi) is outer part of eye and it represents sclera. Sclera is compact connective tissue layer, which thickness is 0.3-0.4 millimeters in the posterior part and 0.6 mm near the cornea. Around of optic nerve thickness of sclera is 1.5 mm. Knowing of sclera thickness is important for ophtalmosurgeon.

Sclera consists of connective tissue lamina, located parallelly to surface of eye and containing collagen fibers. Among collagen fibers flattened fibroblasts and single elastic fibers are present. Externally, the sclera is covered in its anterior part by the ocular conjunctiva, and posteriorly by a fascial sheath (or episclera). The deep surface of the sclera is separated from the choroid by perichoroidal space. Delicate connective tissue present in this space constitutes the suprachoroid lamina (or lamina fascia).

Anteriorly, the sclera becomes continues with the cornea at the corneoscleral junction (also called sclerocorneal junction or limbus). A circular channel called the sinus venous scleral (or canal of Schlemm) is located in the sclera just behind the corneoscleral junction. A triangular mass of scleral tissue projects towards the cornea just medial to this sinus. This projection is called the scleral spur.

Vascular layer (tunica vasculosa bulbi) is represented by choroidea, ciliary body, and iris. Choroidea carries out nourishment of retina. There are 4 layers in it.

• suprachoroid

• choroid

• choriocapillaries

• basal complex.

Suprachoroid is made by loose fibrous connective tissue, containing a lot of elastic fibres, fibroblasts and pigment cells (melanocytes).

Vascular layer (choroid) consists of veins and arteries, among them there is loose fibrous connective tissue, containing a lot of pigment cells and single smooth myocytes.

Choriocapillaries layer contains hemocapillaries with different caliber. Part of capillaries is sinusoids. Among them fibroblasts are located.

Basal complex is very thin layer (1-4 micrometers). It arranges between vascular layer and pigment layer of retina. There three layers in it:

• outer – elastic,

• inner – fibrous,

• basal membrane.

There are three basic functional apparatus in eyeball:

n Dioptric apparatus or photo - refracting. It includes cornea, fluid of anterior and posterior chambers of eye, lens and vitreous body.

n Accommodation apparatus (or adaptation), which includes of iris, ciliary body with ciliary stria.

n Receptor apparatus is retina.

Dioptric apparatus of eye

It is formed by system of transparent structures and medium, which refract light.

Cornea is cover which thickness is 0.8-0.9 millimeters at the center and 1.1 mm at the periphery. Cornea is transparent. It refracts light waves. The basic part of cornea consists of lamina of collagen fibres. There are 5 layers in cornea:

• anterior epithelium

• anterior limiting membrane

• proper substance of cornea

• posterior limiting membrane

• posterior epithelium.

Anterior epithelium is multi-layered squamous non-keratinized epithelium consisting of 5-6 layers. In epithelium of cornea a lot of free nerve endings are present. They provide high sensibility of cornea. Cornea surface is wet by secretion of lacrimal and conjunctive glands. Anterior epithelium of cornea continues to multi-layered squamous epithelium of conjunctiva. Basal membrane has dark and light layers.

Anterior limiting is compact layer of proper substance of cornea. It is located under the basal membrane and consists of net of collagen fibres. Its thickness is 6-9 micrometers.

Proper substance of cornea makes up 90% of thickness of cornea. It is compact fibrous connective tissue, consists of bundles of collagen fibres and fibrocytes. Transparence of cornea is carried out by amorphous glycozaminoglycans (keratin sulfates).

Posterior limiting membrane has thickness 5-10 micrometers. It represents collagen fibres.

Posterior epithelium (endothelium) is uni-layered epithelium, consisting of flattened polygonal cells.

Nutrition of cornea is realized at the expense of diffusion of nutritive substances from anterior chamber of eye and blood vessels of limb, cornea does not contain vessels.

Lens is transparent convexo-convex corpuscle, which shape is changed during accommodation. Lens surface is covered by transparent capsule, which thickness is 11-18 micrometers. Lens consists of lentis fibres, developing from differentiating epitheliocytes. Each fibre is transparent hexagonal prism. In their cytoplasm transparent protein –crystalline is present. Lens is supported with the help of fibres of ciliary stria, these fibres from the one side are joined with ciliary body, from the other side – with capsule of lens. Thankful to this contraction of muscles of ciliary body is passed to lens.

Vitreous body is transparent mass of jelly-like substance. It is located between lens and retina. Vitreous body contains protein vitrein and hyaluronic acid.