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Special senses
Two types of peripheral nerve terminals
• Terminals of axons, which transmit impulses
from the CNS to skeletal or smooth muscles
(motor endings), or to glands (secretory
endings)
• Terminals of dendrites, called sensory nerve
endings or receptors, which perceive various
stimuli and transmit this sensory input to the
CNS
Receptors are classified into 3 types
• Exteroceptors – located near the body surface, are
specialized to perceive stimuli from the external
environment; other exteroceptors include receptors
for vision, hearing, smell and taste
• Interoceptors – are specialized to perceive sensory
information from visceral organs
• Proprioceptors – located in joint capsules, tendons,
muscle spindles, inner ear, are specialized to perceive
information that relates to an awareness of the body
in space and movement
Eye:
organ for visual perception
Anatomy of the eye
Anatomy of the eye
• Tunica externa
– Cornea
– Sclera
• Tunica vasculosa
– Choroid
– Corpus ciliaris
– Iris
• Retina
– Fovea centralis (yellow spot)
– Optic nerve disc (blind spot)
– Pars iridica et pars ciliaris (non-visual retina)
Three layers of the eye
Important components of the eye
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Schlemm’s canal
Anterior chamber
Posterior chamber
Pupil
Ciliary processes
Aqueous humor
Ciliary muscle
Lens
Zonula fibers
Corpus vitreum
Eye development
LM of the cornea
Cornea
The cornea is the transparent bulging anterior
1/6 of the eye. Structure of the cornea:
• Anterior epithelium (stratified squamous
non-keratinized with the naked nerve
endings, responsible for corneal reflex)
• Bowman’s membrane
• Stroma (connective tissue lamina propria)
• Descemet’s membrane
• Posterior epithelium (corneal endothelium)
Sclera
• The white, opaque sclera covers the posterior
five-sixths of the orb
• Sclera is composed of type I collagen fibers
interlaced with elastic fibers
• Sclera is enveloped with the capsule of Tenon
• Thin layer of loose connective tissue between
sclera and capsula of Tenon is called episclera
Anterior segment of the eye
Iris
The iris, the colored anterior extension of the choroid,
is a contractile diaphragm that controls the pupillary
aperture. Its structure:
• Anterior surface covered by incomplete layer of
fibroblasts and melanocytes
• Connective tissue stroma enriched with fibroblasts
and melanocytes
• Dilator and sphincter pupillae muscle
• Posterior surface is smooth, covered by heavily
pigmented melanocytes
Anterior segment of the eye
Ciliary body
• Ciliary muscle contraction has responsibility for the
process called accomodation: contraction of ciliary
muscle makes lens more convex, enabling focusing the
eye on nearby objects
• Ciliary processes (about 70) radiate from the central
connective tissue core towards the lens; ciliary
processes produce aqueous humor
• Zonular fibers, radiating from the ciliary processes to
insert into the lens capsule, regulate convexity of the
lens
LM of iridocorneal angle
Aqueous humor turnover
• Ciliary processes produce aqueous humor
which is responsible for the maintenance of
intraocular pressure
• Corneo-scleral junction (limbus of the eye)
houses canal of Schlemm, which is the site of
outflow of the aqueous humor from the
anterior chamber of the eye into the venous
system
Ciliary body, iris, and pupil viewed from behind
Anterior segment of the eye, LM of the lens
The developing lens and SEM of mature lens fibers
Retina
Retina: 10 layers
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Pigmented epithelium
Rods and cones photosensitive layer
Outer limiting membrane
Outer nuclear layer
Outer plexiform layer
Inner nuclear layer
Inner plexiform layer
Ganglion cell layer
Optic nerve fiber layer
Inner limiting membrane
LM of the retina with its 10 layers
Retina: cellular composition
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Pigmented epithelium
Photosensory (bipolar) neurons
Bipolar (associative) neurons
Horizontal cells (neurons)
Amacrine cells (neurons)
Muller cells (radial gliocytes)
Ganglion cells (neurons)
Cellular composition of the retina
Rods and cones
• Light sensitive (photoreceptor) functions of the retina
are attributed to rods and cones - modified dendrites
of retinal photosensory neurons
• Rods are sensitive to dim light; each eye contains about
100-120 million rods; each rod consists of membranous
discs, into which photosensitive pigment rhodopsin is
incorporated
• There are 3 types of cones, which are sensitive to red,
green, and blue lightwaves; each eye consist about 6
million cones, into membranous discs of which
photopigment iodopsin is incorporated
Morphology of a rod and a cone
LM and EM of retinal rods
EM of rods (A, B) and of cones (C, D) fragments
EM of a retinal rod segments
Blood supply of the eye
Retina and choroid
Ora serrata, optic nerve disc, and fovea centralis
Eyelid
Eyelid and conjunctiva
Lacrimal apparatus
Ophtalmological pathologies
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Eyesore: non-transparent cornea
Glaucoma: excess of intraocular pressure
Cataract: non-transparent lens
Presbyopia: decreased elasticity of the lens
Eye floaters: vitreous opacities
Detachment of the retina
Chalazion: Meibomian cyst, tarsal cyst
Hordeolum (sty): bacterial inflammation of the
sweat gland or gland of Zeiss of an eyelid
• Conjunctivitis: inflammation of conjunctiva