Lecture 16: The Special Senses Flashcards
Describe the structure and function of accessory eye structures, eye layers, the lens, and humours of the eye
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Accessory structures of the eye to protect the eye and aid eye function. Name them
- eyebrow
- eyelids (palpebrae)
- conjunctiva
- lacrimal apparatus
- extrinsic eye muscles
Be able to label the eye.
Do it!
Give the structure and function of the following:
Eyebrows
Eyelids (upper and lower)
Eyelashes (nerve endings of follicles initiate blinking
Lubricating glands associated with the eyelids
Eyebrow: shading the eye and preventing perspiration from reaching the eye
Eyelids: protect the eye. Levator palpebrae-give the upper eyelid mobility
Eyelashes: nerve endings if follicles initiate reflex blinking
Lubricating glands associated with the eyelids:
-tarsal glands
-sebaceous gland associated with follicles
Conjunctiva
Transparent membrane
-palpebral conjunctiva- lines the eyelids
-bulbar conjunctiva-covers the white of the eyes
-produces a lubricating mucous secretion
Refer to slide 8 for picture
Lacrimal apparatus
Lacrimal gland and ducts that connect to nasal cavity
Lacrimal secretion (tears)
-dilute saline solution containing mucus, antibodies, and lysozyme
-blinking spreads the tears towards the medial commissure
-tears entered paired lacrimal canaliculi via the lacrimal puncta
-drain into the nasolacrimal duct
Slide 10 for picture
Extrinsic eye muscles
6 strap like extrinsic eye muscles
- originate from the bony orbit
- enable the eye to follow moving objects
Four rectus muscles: names indicate the movement they promote
Two oblique: muscles move the eye in the vertical plane and rotate the eyeball
Label the diagram of the eye muscles on side 15 and 16
Do it
Summary of muscle actions on eye and innervating cranial nerves
Lateral rectus muscle-moves eye laterally (abducens)
Medial rectus muscle- moves eye medially (3- oculomotor)
Superior rectus muscle- elevates eye (3)
Inferior rectus muscle - depresses eye (3)
Inferior oblique- elevates eye and turns it laterally (3)
Superior oblique- depresses eye and turns it laterally (trochlear) 4
Structure of the eyeball
Wall of eyeball contains 3 layers, what are they?
Fibrous (outer)
Vascular (middle)
Sensory (inner)
Internal cavity is filled with fluids called humors
The lens separates the internal cavity into anterior and posterior segments
Label diagram on slide 16
Do it
Describe the fibrous layer and what it consists of
Outermost dense avascular connective tissue
Two regions: sclera and cornea
Sclera: opaque (white)
-protects and shapes the eyeball
-anchors extrinsic eye muscles
Cornea: transparent
-numerous pain receptors here contribute the blinking and tearing reflexes
Describe the vascular layer and what it consists of
Middle pigmented layer (uvea)
3 regions: choroid, ciliary body and iris
1. Choroid: posterior portion of the uvea
-supplied blood to all layers of the eyeballs
-brown pigment absorbs light to prevent visual confusion
2. Ciliary body
-ring of tissue surrounding the lens
-smooth muscle bundles (ciliary muscles) controls lense shape
-ciliary zonule (Suspensory ligament) holds lens in position
3. Iris
-the coloured part of the eye
Pupil- central opening that regulates the amount of light entering the eye
-bright light-sphincter papillae (circular muscle) contracts; pupils constrict
Sensory layer: retina
Delicate 2 layered membrane
- Pigmented layer
- outer layer
- absorbs light
- stores vitamin A - Neural layer
- photoreceptors
Ganglion cell axons
-run along the inner surface of the retina
Optic disc (blind spot)
- site where the optic nerve leaves the eye
- lacks photoreceptors
Two sources of blood supply
- choroid supplies the outer third (photoreceptors)
- central artery and vein of the retina supply the inner 2 thirds
Photoreceptors
What are the 2 kinds and their properties
Rods- more of them at peripheral region of retina, away from the macula lutes
-operate in dim light
-provide indistinct , fuzzy, non coloured peripheral vision
Cones:
-found in the macula lutea: concentrated in the fovea centralis
-operate in bright light
-provide high acuity coloured vision
What are the internal chambers and fluids of the eye?
The lens and ciliary zonule separate the anterior and posterior segments
Posterior segments contains vitreous humor that:
-transmits light
-supports posterior surface of the lens
-holds the neural retina firmly against the pigmented layer
-contributes to the intra ocular pressure
Anterior segments is composed of 2 chambers
- anterior chamber- between the cornea and the iris
- posterior chamber- between the iris and the lens
Internal chambers and fluids
Anterior segments contain aqueous humor
- plasma like fluid
- drains via the scleral venous sinus (canal of schlemm) at the sclera- cornea junction
- supplies nutrient and oxygen mainly to the lens and cornea but also the retina, and removes wastes
Lens
Biconvex, transparent, flexible, elastic and avascular
- allows precise focusing of light on the retina
- lens becomes denser, more convex, and less elastic with age
Focusing light on the retina
Pathway of light entering the eye: cornea- aqueous humor- lens- vitreous humor- neural layer of retina- photoreceptors
Light is refracted
-at the cornea
-entering the lens
-leaving the lens
Change in lens curvature allows for fine focusing of an image
Focusing for close vision
Close vision requires:
Accommodation- changing the lens shape by ciliary muscles to increase refractory power
-near point of vision is determined by the maximum bulge the lens can achieve
Constriction- the accommodation of the pupillary reflex constricts the pupils to prevents the most divergent light rays from entering eye
Controlled by oculomotor, parasympathetic fibres
Visual pathway to the brain
- Axons of retinal ganglion cells form the optic nerve
- medial fibres of the optic nerve decussate at the optic chiasm
- most fibres of the optic tracts continue to the superior colliculi
- the optic radiation fibres connect to the primary visual cortex in the occipital lobes
Sense of smell
Taste and smell (olfaction)
- their chemoreceptors respond to chemicals in aqueous solution
- olfactory epithelium in the roof of the nasal cavity
- bundles of axons of olfactory receptor cells form the filaments of the olfactory nerve (CN 1)
Sense of taste
Receptor organs are taste buds
-found on the tongue: CN 7 and 9
-found on the epiglottis: CN 10
Taste is 80% smell
-thermoreceptors, mechanoreceptors and nociceptors in the mouth also influence taste
-Cranial nerves 7, 9 and 10 carry impulses from the taste buds to the gustory cortex in the insula
What are the 5 taste sensations
- Sweet
- Bitter
- Umami
- Sour
- Salty
