For Your Eyes Only

The Senses General senses of touch: Temperature, Pressure, Pain Special senses: Smell, Taste, Sight, Hearing, Equilibrium

The Eye and Vision 70 percent of all sensory receptors are in the eyes Each eye has over a million nerve fibers Adult eye is a sphere that measures about 1 inch in diameter Only see the anterior 1/6 of the eye Protection for the eye Most of the eye is enclosed in a bony orbit A cushion of fat surrounds most of the eye

Accessory Structures of the Eye Eyelids and Eyelashes - both provide protection Eyelids (fig. 8.1b p. 273) meet at the corners and are called Medial canthus and lateral canthus Figure 8.1b

Accessory Structures of the Eye Tarsal Glands – (fig. 8.1 a p. 273) modified sebacious (sweat) glands that produce an oily secretion to lubricate the eye Figure 8.1a

Accessory Structures of the Eye Conjunctiva – delicate membrane that lines the eyelids and covers part of the outer surface of the eyeball (fig. 8.1 a p. 273) Connects to the surface of the eye and secretes mucus to lubricate the eye Homeostatic Imbalance: Conjunctivitis - inflammation of the conjunctiva and causes reddnes and irriatation. Pinkeye – is conjunctivitis caused by a bacteria or virus and is contagious.

Accessory Structures of the Eye Lacrimal Apparatus Lacrimal gland – produces lacrimal fluid (tears) and is located in the superior part of the lateral end of eye Lacrimal canals – drains lacrimal fluid from eyes; Know superior and inferior Figure 8.1a Fig. 8.1 b p. 273

Accessory Structures of the Eye Lacrimal sac – (fig. 8.1b p. 273) provides passage of lacrimal fluid towards nasal cavity Figure 8.1b p. 273

Accessory Structures of the Eye Nasolacrimal duct – (fig. 8.1b p. 273) empties lacrimal fluid into the nasal cavity Figure 8.1b

Function of the Lacrimal Apparatus Properties of lacrimal fluid: 1) Dilute salt solution (tears) 2) Contains antibodies and lysozyme (an enzyme that destroys bacteria) Protects, moistens, and lubricates the eye Empties into the nasal cavity

Extrinsic Eye Muscles Muscles attach to the outer surface of the eye to produce eye movements Figure 8.2

Extrinsic Eye Muscles Lateral Rectus - moves eye laterally Medial Rectus - moves eye medially Superior Rectus - elevates eye Inferior Rectus - Depresses eye  Figure 8.2

Extrinsic Eye Muscles Inferior Oblique - elevates eye and turns it laterally (rolling your eyes at “MOM” Superior Oblique - depresses eye and turns it laterally So the next time you accidently do that, you can say, “ I am sorry, but I had an action potential in my autonomic nervous system and my inferior oblique muscle just contracted Duck! ‘Cause your mom’s autonomic nervous system will have an action potential as her flexor muscles move to slap your face! Figure 8.2

Structure of the Eye The wall is composed of three tunics Fibrous tunic – outside layer Choroid – middle layer Sensory tunic – inside layer Figure 8.3a

The Fibrous Tunic Sclera is the white connective tissue layer. Seen anteriorly as the “white of the eye” Cornea - Transparent, central anterior portion 1) Allows for light to pass through 2) Repairs itself easily 3) The only human tissue that can be transplanted without fear of rejection

Choroid Layer Blood-rich nutritive tunic; VASCULAR TUNIC Dark Pigment prevents light from scattering Modified interiorly into two structures Ciliary body – smooth muscle that attaches to the lens by ligaments called Ciliary Zonule

Choroid Layer 2) Iris - Pigmented layer that gives eye color. Circularly and radially arranged smooth muscles act as a diaphragm of a camera to allow certain amounts of light. Dim light: opens to allow light in. Bright light: closes to keep light out. Think about the morning when the light hits your eyes? Pupil – rounded opening in the iris that allows light into the eye

Sensory Tunic (Retina – common name) The RETINA contains receptor cells (photoreceptors) 1) Rods 2) Cones Signals pass from photoreceptors via a two-neuron chain 1) Bipolar neurons 2) Ganglion cells Signals leave the retina toward the brain through the optic nerve

Neurons of the Retina Figure 8.4

Neurons of the Retina and Vision Rods - Most are found towards the edges of the retina Allow dim light vision and peripheral vision Perception is all in gray tones

Neurons of the Retina and Vision Cones - Allow for detailed color vision Densest in the center of the retina Need bright light (do you see color at night?) Fovea centralis – area of the retina with only cones No photoreceptor cells are at the optic disk, or blind spot

Cone Sensitivity There are three types of cones 1) Blue 2) Green 3) Red Different cones are sensitive to different wavelengths Color blindness is the result of lack of one cone type Blue man https://www.youtube.com/watch?v=W-P9xCkeWZk Figure 8.6

Lens Lens - Biconvex crystal-like structure Held in place by a suspensory ligament attached to the ciliary body Figure 8.3a

Internal Eye Chamber Fluids Aqueous humor - Watery fluid found in chamber between the lens and cornea Similar to blood plasma Helps maintain intraocular pressure Provides nutrients for the lens and cornea Reabsorbed into venous blood through the canal of Schlemm

Internal Eye Chamber Fluids Vitreous humor - Gel-like substance behind the lens Keeps the eye from collapsing Lasts a lifetime and is not replaced

Animal eyes Tapetum - A reflecting pigment layer lying behind the retina of certain mammals (e.g. cats, dogs) gives a shining appearance to the eyes when illuminated in the dark. The role of the tapetum is to reflect light back aiding vision in dim illumination.

Animal eyes

Animal eyes: Rossi

Animal eyes

Lens Accommodation Light must be focused to a point on the retina for optimal vision The eye is set for distance vision (over 20 ft away) The lens must change shape to focus for closer objects Figure 8.9

Images Formed on the Retina Real Image – after accommodation , the real image on the retina is reversed from left to right, upside down, and smaller than the normal object. Figure 8.10

Visual Pathway 1) Photoreceptors of the retina 2) Optic nerve 3)Optic nerve crosses at the optic chiasma Figure 8.11

Visual Pathway 4) Optic tracts 5)Thalamus (axons form optic radiation) 6) Visual cortex of the occipital lobe Overlapping gives “Binocular” vision with depth perception or 3-D vision Figure 8.11

Eye Reflexes Internal muscles are controlled by the autonomic nervous system Bright light causes pupils to constrict through action of radial and ciliary muscles Viewing close objects causes accommodation External muscles control eye movement to follow objects Viewing close objects causes convergence (eyes moving medially) https://www.youtube.com/watch?v=p_xLO7yxgOk