LECTURE 24 SPECIAL SENSES 3

1-lacrimal gland, 2-ducts, 3-lacrimal punctum (or puncta in plural), 4- lacrimal canaliculus, 5- lacrimal sac, 6- nasolacrimal duct, 7- inferior meatus of nasal cavity, 8- nostril

What is the path through which tears travel from the lacrimal gland to the nostril (lacrimal apparatus) [tip:8 structures]

Cleanse and lubricate the eye surface. Deliver oxygen and nutrients to the conjunctiva. Protect from infection (they contain lysozyme and antibodies).

What are the three functions of tears?

The lacrimal apparatus is connected to the nostril, tears drain into the nose.

Why can an excess of tears, such as during crying or watery eyes, lead to a runny nose?

Fibrous, vascular, and neural tunics.

What are the three layers or tunics of the eyeball, starting from the outermost to the inner layer?

The sclera (white of the eye) and cornea.

Name the two structures of the fibrous tunic and specify which one corresponds to the "white" color of the eye

The sclera covers the majority of the outer surface of the eyeball. The cornea is located at the front of the eye and covers the central portion.

Are the structures of the fibrous tunic located throughout the entire surface of the eyeball?

(a) Iris, ciliary body, and choroid. (b) ciliary body. (c) It contains high density of blood vessels, provides nutrients and waste removal to eye structures.)

(a) What are the three structures composing the vascular tunic?, (b) which one is a muscular ring around the lens that secretes aqueous humor? (c) What is the function of the choroid?

The iris and ciliary body are in the anterior part of the eye. The choroid extends from the ciliary body and lines the back of the eye between the sclera (part of the fibrous tunic) and the retina (part of the neural tunic).

Are the structures of the vascular tunic located throughout the entire surface of the eyeball?

(a) In the neural tunic (inner layer). (b) No, It lines the back of the eye.

(a) In which eyeball tunic is the retina located? (b) Is this retina found throughout the entire surface of the eyeball?

[Check LO2 in the study notes]

Create a diagram illustrating the eyeball tunics and associated structures

(a) It is a transparent membrane that protects the eye. (b) It allows light to enter the eye. (c) Yes, it contains stem cells.

(a) What is the cornea?, (b) what function does it serve? (c) Is it capable of regeneration after sustaining damage?

The bending of light rays; cornea and lens because of they are convexes structures [cause parallel rays of light to converge (come together)]. In the case of the eye, this convergence is essential for focusing light onto the retina.

What does the term "refraction of light" mean, and which eye structures are responsible for light refraction?

When light rays strike the center of the cornea, they pass through with minimal deviation. Off-center rays, however, experience greater bending

How do rays behave when striking off-center versus striking the center of the cornea?

Images are projected onto the retina (in 2 dimensions) in an inverted (upside-down) manner; and reversed.

Are images projected onto the retina in an inverted or non-inverted manner?

(a) Between the cornea and the lens, surrounding the pupil.(b) Adjustable diaphragm that controls pupil diameter (size), thus the amount of light entering. (c) It contains cells with melanin (pigment)

(a) Where is the iris located? (b) what is its function, (c) why does it have color?

(a) Aperture through which light enters the eye. (b) Sphincter pupillae and dilator pupillae. (c) The dilator pupillae.

(a) What is the pupil, (b) Which two muscles of the iris control pupil diameter? (c) which one is closest to the pupil?

Dilator pupillae- sympathetic / Sphincter pupillae-parasympathetic innervation

Which muscle of the iris receives sympathetic input, and which one receives parasympathetic innervation?

Sympathetic stimulation of the dilator pupillae, muscle contracts; in dim light.

What physiological mechanism is responsible for the dilation of the pupil? Under what lighting conditions does this occur?

Parasympathetic stimulation of the sphincter pupillae, muscle contracts; in bright light.

What physiological mechanism is responsible for the constriction of the pupil? Under what lighting conditions does this occur?

Pupil constriction (through stimulation of the sphincter pupillae by the parasympathetic NS). To increase the depth of focus and decrease spherical aberration.

What response does the pupil have for close-up vision, and why?

It is an optical phenomenon that occurs when light rays passing through the periphery of a lens or optical system focus at a different point than those passing through the center. This can result in a blurring or distortion of the image.

What is spherical aberration?

The focal point refers to the specific point on the retina where light rays converge after being refracted by the cornea and lens.

In the context of vision, what does the term "focal point" refer to?

It is a clear biconvex and flexible structure suspended in the eye by the suspensory ligaments (or zonules). (b) Refraction of light, adjustment of focal length (accomodation)

(a) What is the definition of the eye lens, (b) what functions do eye lenses serve?

The ciliary muscles

Which muscle controls lens shape?

Ciliary ligaments or zonules, ciliary process, and ciliary muscles.

Which structures form part of the ciliary body?

It is produced by the ciliary body and flows out through the trabecular meshwork in the drainage angle. (b) Supplies nutrients to the avascular structures of the eye, cornea, and lens, and maintains intraocular pressure (IOP).

(a) Where is the aqueous humor produced and drained? (b) What function does it serve?

The aqueous humor is found in the anterior chamber of the eye, situated between the cornea and the lens.

In which part of the eye is the aqueous humor located?

The lens is flexible and can change shape through a process called accommodation. This adjustment of the lens's curvature allows the eye to focus on objects at different distances, ensuring a clear image on the retina.

What does "lens accomodation" refers to?

By contraction of the ciliary muscles (caused by parasympathetic stimulation) which produces relaxation of the zonular fibers and bulging of the lens (becomes rounder)

How does the lens accomodate to close-up vision?

Flat, because of relaxation of the ciliary muscles and the tightening of the zonular fibers.

How is the shape of the lens at rest (non-accomodated)? why?

Emmetropia means normal eye. An emmetropic eye sees distant objects clearly, without using accommodation.

What is emmetropia?

By contraction of both medial rectus muscles which orients the visual axis of each eye toward the object for close-range vision. (b) Diplopia (doble vision).

(a) How is eye convergence produced? Which external eye muscles need to contract for eye convergence?(b) What happens if the eyes cannot converge accurately?

Pupillary constriction, lens thickening (accommodation), and inward rotation of the eyes (eye convergence)

What are the 3 processes involved in the adjustment to close-range vision?

Vitreous humor. It maintains the round shape of the eye and help with vision clarity (it is clear so light can pass through)

What liquid fills the globe of the eyeball, and what is its function?

It is a gel-like substance located in the posterior part of the eye, filling the space between the lens and the retina.

What is the vitreous body?

It is the region in the retina where the optic nerve exits the eye. It is devoid of photoreceptor cells, creating a blind spot in the visual field.

What is the optic disc? why is it considered the "blind spot"?

It receives the image that the cornea focuses through the lens and transforms this image into electrical impulses that are carried by the optic nerve to the brain.

What is the overall function of the retina?

It is a small area of about 1mm in diameter from which we get our sharpest vision. In the macula there is a concentration of cone cells. These cells are responsible for color vision, sharp visual acuity and central vision. The highest concentration of cones is in the fovea, a tiny area in the center of the macula.

What is macula (yellow spot) and the fovea?

Photoreceptors (rods and cones), bipolar cells, ganglion cells, amacrine cells, and horizontal cells.

What are the six types of cells found in the neural layer of the retina involved in vision?

Bipolar cells

Which cells do photoreceptors contact and transmit the electrical signals?

(a) Both are located in the neural layer of the retina. They are interneurons that play a role in visual signal modulation by modifying the rate of electrical firing in bipolar cells. (b) NO

(a) Where can amacrine and horizontal cells be found, and what functions do these cells serve? (b) Do they form part of the pathway of electrical signal output?

Ganglion cells

Which cells in the retina give rise to the axons that form cranial nerve II (optic nerve)?

Photoreceptors - bipolar cells- ganglion cells

Sequence the light path from photoreceptor cells to the optic nerve (CN II).

The conversion of light energy into electrical signals; by the photoreceptors (rods and cones), in the neural layer of the retina.

What is the definition of phototransduction? Which cells have this function, and where are they located?

(a) Cones and rods. (b) Rods. (c) Cones. (d) Cones.

(a) What are the two types of photoreceptors?, (b) which one is responsible for scotopic or night vision? (c) Which one is for photopic or day vision, and (d) which one is for trichromatic or color vision?

(a) Outer segment. (b) Approx. 1000 membranous discs containing visual pigments.

(a) What segment of the rods is specialized to absorb light: the outer, inner, or cell body? (b) What makes up the outer segment of the rods?

Rhodopsin is a light-sensitive protein found in the disc membranes of the rod cells. Its main components include a protein called opsin and a light-absorbing molecule called retinal.

Define rhodopsin and identify its primary components, along with its location.

Rhodopsin for rods, photopsin of cones. (b) Cis and trans.

(a) What are the visual pigments for rods and cones? (b) What are the two conformations of the retinal moiety that they posses?

The 3 types of cones are the short-wavelength cones (S- or blue cones), the medium-wavelength cones (M- or green cones), and the long-wavelength cones (L- or red cones).They are defined by the wavelength of light to which they are most sensitive..

What are the three types of cones, and how are they defined? W

They detect short-wavelength (blue), medium-wavelength (green), and long-wavelength (red) light, enabling color vision across the visible spectrum.

What do the S, M, and L-cones detect?

Human eyes only perceive three colors of light: red, blue, and green. The wavelengths of these three colors can be combined to create every color on the visible light spectrum.

What does the trichromatic theory of color vision state?

Receptor: rhodopsin; G protein: transducin; Enzyme: phosphodiesterase (PDE); Second messenger: cGMP

In G protein signaling during phototransduction in rods, which molecules act as the receptor, associated G protein, enzyme, and second messenger?

(A) 11-cis retinal. (b) Inactive. (c) Inactive.

In the dark phase, (a) which form of retinal is bound to rhodopsin?, (b) is transducin active or inactive? (c) how about phosphodiesterase (PDE)?

They are opened since there is cGMP available, the molecule that activate the sodium channels.

Are Na+ channels opened or closed at night, and why? Which molecule stimulates their opening, and is it available at night?

Sodium influx and depolarization of the inner segment of the rods that triggers the release of glutamate located in vesicles.

What is the consequence of having Na+ channels in rods opened at night?

Glutamate, to the bipolar cells.

Which neurotransmitter is released by the inner segment of the rods, and to which neuron?

It is converted to all-trans retinal and dissociates from rhodopsin

Upon light absorption, what happens to 11-cis retinal in the rods?

It refers to the temporary loss of sensitivity to light in rods, where trans retinal needs to be regenerated to 11-cis retinal

What is the definition of "bleaching" in the context of vision?

Light absorption

What event activates transducin and PDE in rods?

Closed, cGMP is being degradated by PDE.

During the daytime, how are Na+ channels: closed or opened, and why?

No, because of hyperpolarization (due to the closening of Na+ channels)

Do rods release vesicles with neurotransmitters during the daytime, and why (or why not)?

(a) It starts at the neural layer of the retina and finishes in the optic chiasm. (b) It starts after the optic chiasm and finish in the thalamus.

(a) When does the optic nerve start and finish? (b) and the optic tract?

It is the point where the optic nerves from both eyes partially cross in the brain, allowing each hemisphere to receive visual information from both the left and right visual fields.

What is the optic chiasm?

(a) Lateral geniculate nucleus. (b) Occipital lobe

(a) Which thalamic nucleus receives visual information and relays it to the visual cortex? (b) In which lobe of the cortex is the visual cortex located?

Superior colliculi (related to visual reflexes) and the suprachiasmatic nucleus (the master circadian clock).

Name two other brain areas that receive visual information, as mentioned in MT3 (Hint: one is located in the midbrain and the other in the hypothalamus).