Special Senses Overview: The Eye Anatomy

Special Senses Overview

• The special senses include vision, hearing and balance, taste, and smell.
• Specialized sensory organs are dedicated to each sense.
• The eye, ear, tongue, and nose are the sensory organs for vision, hearing, taste, and smell respectively.

The Eye

• The eye has two main parts: the eyeball and accessory organs.
• Accessory organs include eyebrows, eyelids, extra-ocular muscles, fascia, conjunctiva and lacrimal apparatus.
• The eyeball has three layers: outer fibrous, middle vascular, and inner retinal layers.
• The outer fibrous layer is composed of the sclera and cornea.
• The sclera maintains the shape of the eye.
• The cornea refracts light entering the eye.
• The middle vascular layer is called the choroid.
• The inner retinal layer consists of the visual and nonvisual parts.
• The visual part is sensitive to light, with features like the optic disc (blind spot) and macula lutea (high visual sensitivity).
• The nonvisual part doesn’t respond to light.
• The junction between visual and nonvisual parts is the ora serrata.

Internal Components of the Eyeball

• The eyeball has three chambers: anterior chamber, posterior chamber, and posterior (vitreous) chamber.
• Aqueous humor fills the anterior and posterior chambers.
• Vitreous humor fills the posterior (vitreous) chamber.
• Aqueous humor nourishes the cornea and lens and maintains intra-ocular pressure.
• Intra-ocular pressure is important for maintaining eye shape.
• Increased intra-ocular pressure is associated with glaucoma.
• The lens is a transparent structure that focuses light onto the retina.
• Cataract is a condition where the lens becomes opaque.
• Vitreous humor is a transparent gel that fills the posterior cavity and maintains the eye's shape.

Extra-ocular Muscles

• The eye has six extra-ocular muscles.
• These muscles control eye movement.
• They are innervated by the oculomotor nerve (III), trochlear nerve (IV), and abducens nerve (VI).
• The muscles move the eye in different directions.
• Paralysis of these muscles can cause eye drooping (blepharoptosis) or misalignment (strabismus).

The Lacrimal Apparatus

• The lacrimal apparatus produces tears.
• It includes the lacrimal gland and its ducts, lacrimal canaliculi, lacrimal sac, and nasolacrimal duct.
• Lacrimal fluid is secreted by the lacrimal gland and distributed across the eye surface.
• The fluid is collected by the tear ducts and transported to the nasal cavity via the nasolacrimal duct.

Arterial Supply to the Eye

• The ophthalmic artery provides blood to the eye.
• It has branches that supply different parts of the eye.
• Central retinal artery occlusion (CRAO) can lead to blindness.

Venous Drainage of the Eye

• The superior and inferior ophthalmic veins drain blood from the eye.
• These veins drain into the cavernous sinus, a major venous structure of the brain.
• Infections in the face can spread to the cranial cavity.

The Ear

• The ear consists of three parts: external ear, middle ear, and internal ear.
• The external ear includes the auricle (pinna) and external acoustic meatus (canal).
• The middle ear has the tympanic membrane and auditory ossicles (malleus, incus, and stapes) to transmit sound vibrations.
• The internal ear contains the bony and membranous labyrinths, with structures involved in hearing balance.
• Structures include cochlear duct, vestibule (with utricle and saccule), semicircular ducts (with crista).
• Structures include the pharyngotympanic (eustachian tube).
• Auditory muscles, the tensor tympani muscle, and stapedius are innervated by branches of the facial nerve which helps control sound volume.
• The eardrum (tympanic membrane) separates the external ear from the middle ear.

The Internal Ear (Labyrinth)

• Bony and membranous parts make up the internal ear.
• The cochlea is a spiral-shaped structure, which plays a major role in hearing.
• Scala vestibuli and scala tympani run through the cochlear duct.

Sensory Organs in the Internal Ear

• Two sensory organs are in the internal ear; the balance and hearing.
• The crista within semicircular ducts responds to movement.
• The macula in the utricle and saccule respond to acceleration.
• Inflammation in these structures may cause vertigo.
• The hearing organ rests on the basilar membrane of the cochlear duct.
• Inflammation can cause hearing loss (deafness).

Arterial and Venous Supply to the Ear

• Numerous arteries supply the ear, with the external and internal carotid arteries and branches like maxillary and basilar.
• Veins drain blood from the ear, with the external and internal jugular veins and branches like the facial vein.

Innervation of the Ear

• Various nerves innervate the ear, including the facial nerve (VII), mandibular nerve (V3), glossopharyngeal nerve (IX), vagus nerve (X), and the vestibulocochlear nerve (VIII).
• The vestibulocochlear nerve (VIII) has two branches:
  • vestibular nerve (responsible for balance)
  • cochlear nerve (responsible for hearing)