Lecture 02 – Sensory Organs (Part 2 – The Human Eye) Genalin Lagman Taguiam

Course Outcomes On the completion of the week’s activity, the students should be able to Describe the structures and functions of the eye Describe the conduction of light waves from the exterior to the brain

Accessory Structures of the Eye EYEBROWS Help to shade and protect the eyes EYELIDS Protect and lubricate the eyes by reflex blinking Supported by cranial nerve V

Accessory Structures of the Eye CONJUNCTIVA Mucosa that lines the eyelids and covers the anterior eyeball surface Lubricates the eyeball surface with its mucus

Accessory Structures of the Eye LACRIMAL APPARATUS consists of the lacrimal gland Produces a saline solution containing mucus, lysozyme, and antibodies lacrimal canaliculi lacrimal sac nasolacrimal duct

Accessory Structures of the Eye EXTRINSIC EYE MUSCLES

Accessory Structures of the Eye CONTROLLING CRANIAL NERVES

Structure of the Eyeball Outermost fibrous layer consists of SCLERA Protects the eye and gives it shape CORNEA Allows light to enter the eye

Structure of the Eyeball Iris that control the size of pupil

Structure of the Eyeball What causes the pupils to dilate (mydriasis) and to constrict (miosis)?

Structure of the Eyeball What causes the pupils to dilate (mydriasis) and to constrict (miosis)? There are two types of muscle that control the size of the iris: the iris sphincter, composed of circularly arranged muscle fibers, and the iris dilator, composed of radially arranged muscle fibers. The former is innervated by the parasympathetic nervous system; the latter by the sympathetic nervous system. Sympathetic stimulation of the adrenergic receptors causes the contraction of the radial muscle and subsequent dilation of the pupil. Conversely, parasympathetic stimulation causes contraction of the circular muscle and constriction of the pupil.

Structure of the Eyeball Sensory layer (or RETINA) Consists of neural layer that contains photoreceptors (rods and cones), bipolar cells and ganglion cells Ganglion cell axons form the optic nerve which exits via the optic disc (blind spot)

Structure of the Eyeball Sensory layer (or RETINA)

Structure of the Eyeball Sensory layer (or RETINA) outer segment of the photoreceptors that contain the light-absorbing pigment in membrane-bounded discs

Structure of the Eyeball Blind spot

Structure of the Eyeball Fovea centralis

Structure of the Eyeball The anterior segment filled with aqueous humor formed by capillaries in the ciliary processes and drained into the scleral venous sinus

Structure of the Eyeball The posterior segment contains vitreous humor Help support the eyeball and keep the retina in place

Structure of the Eyeball The biconvex lens Suspends within the eye by the ciliary zonule attached to the ciliary body Only adjustable refractory structure of the eye

Physiology of Vision Light Made up of those wavelengths of the electromagnetic spectrum that excite the photoreceptors Refracted (bent) when passing from 1 transparent medium to another of different density

Physiology of Vision

Physiology of Vision Concave lenses disperse light Convex lenses converge light and bring its rays to a focal point The greater the lens curvature, the more light bends

Physiology of Vision Cornea accounts for the most refraction but lens allows active focusing for different distances

Physiology of Vision Focusing for close-up/ near vision requires the following reflex activities of the eye Accommodation (bulging of the lens) Pupillary constriction (mioisis) Convergence of the eyeballs All three reflexes are controlled by cranial nerve III

Physiology of Vision Accommodation

Physiology of Vision Pupillary constriction (mioisis) P

Physiology of Vision Convergence of the eyeballs

Physiology of Vision Rods Cones Respond to low-intensity light Provide night and periphery vision Cones Are bright-light, high discrimination receptors Provide for color vision

Physiology of Vision During light adaptation In dark adaptation Photopigments are bleached and rods are inactivated Then, as cones decrease their light sensitivity, high acuity vision ensues In dark adaptation Cones cease functioning Visual acuity decreases

Physiology of Vision Visual pathway to the brain begins with the optic nerve fibers (ganglion cell axons) from the retina At the optic chiasma, fibers from the medial half of each retina cross over and continue on in the optic tracts to the thalamus

Physiology of Vision Thalamic neurons project to the visual cortex via the optic radiation Fibers also project from the retina to the midbrain pretectal nuclei and superior colliculi and to the suprachiasmatic nucleus of the hypothalamus

References Marieb, E.N. (2010). Human anatomy and physiology (8th Ed.). San Francisco, Pearson. Marieb, E.N. (2006). Human anatomy and physiology (7th Ed.). San Francisco, Pearson. Tortora, G. J. (2006). Principles of anatomy and physiology. (11th. Ed.). New Jersey, John Wiley and Sons, Inc.