neuro ch. 9

What is light?

electromagnetic energy in the form of waves.

How much of the human cerebral cortex is involved with analyzing the visual world?

More than a third

What is the function of the retina in the eye?

The retina, which contains photoreceptors specialized to convert light energy into neural activity, is responsible for detecting light and initiating the process of vision.

How does the rest of the eye function in relation to the retina?

The rest of the eye acts like a camera, adjusting for clear images.

Is the retina part of the brain or part of the eye?

The retina is actually part of the brain.

How are the retinas in each eye specialized?

Each eye has overlapping retinas: one specialized for low-light levels (dusk-dawn) and one for high-light levels (sunrise-sunset) and detection of color.

What is the retina specialized to detect?

differences in the intensity of light falling on different parts of it.

What distributes visual information to several brain structures?

The axons of retinal neurons are bundled into optic nerves

What are the three characteristics of electromagnetic radiation?

Electromagnetic radiation has a wavelength, frequency, and amplitude.

What is the relationship between energy content and frequency of electromagnetic radiation?

The energy content of electromagnetic radiation is proportional to its frequency.

What is the range of wavelengths for visual electromagnetic radiation?

400-700 nanometers

What is a light ray and how does it travel in a vacuum?

In a vacuum, a wave of electromagnetic radiation travels in a straight line, forming a light ray.

What is the study of light rays and their interactions called?

Optics

What is reflection in terms of light?

bouncing of light rays off a surface

What is absorption in terms of light?

transfer of light energy to a particle or surface

How are images formed in the eye?

refraction, which is the bending of light rays that can occur when they travel from one transparent medium to another.

What is the function of the pupil in the eye?

opening that allows light to enter the eye and reach the retina.

Why does the pupil appear dark?

light-absorbing pigments in the retina.

What is the iris and what is its function?

pigmented part of the eye that surrounds the pupil, and it contains two muscles that can vary the size of the pupil. One muscle contracts to make the pupil smaller, while the other makes it bigger.

What is the cornea and how is it related to the sclera?

glassy transparent external surface of the eye that covers the pupil and iris, and it is continuous with the sclera. The sclera is the tough, white outer layer of the eyeball that forms the wall of the eye.

Where is the eyeball located?

sits in a bony eye socket in the skull, also called the eye's orbit.

What are extraocular muscles and what is their function?

Three pars of muscles that are inserted into the sclera and move the eyeball in the orbit.

Why are the extraocular muscles usually not visible?

they are behind the conjunctiva

What is the conjuctiva?

a membrane that folds back from the inside of the eyelids and attaches to the sclera

What is the optic nerve and where does it exit the eye?

The optic nerve carries axons from the retina and exits the back of the eye. It passes through the orbit and reaches the base of the brain near the pituitary gland.

What is an ophthalmoscope, and what is its function?

* a device that enables one to peer into the eye through the pupil to the retina.


* allows examination of the retina and its associated structures.

What is the most obvious feature visible through an ophthalmoscope, and where do these vessels originate?

blood vessels on the surface of the retina; originate from a pale circular region called the optic disk, which is also where the optic nerve fibers exit the retina.

Why can't the sensation of light occur at the optic disk, and how does the brain compensate for this?

* The optic disk lacks photoreceptors
* Large blood vessels exit from this region, so the sensation of light cannot occur there.
* Brain fills in the missing information

What is the macula, and how is it distinguished from other regions of the retina?

part of the retina responsible for central vision, and relative absence of large blood vessels.

What is the trajectory of the optic nerve fibers from the macula, and why is the relative absence of large blood vessels in this region beneficial?

The optic nerve fibers from the macula travel to the optic disk. The relative absence of large blood vessels in this region of the retina is one of the specializations that improves the quality of central vision.

What is the fovea, and how is it used as an anatomical reference point?

dark spot at the center of the retina, approximately 2 mm in diameter

How is the retina different in the fovea than elsewhere?

It is thinner there and used as an anatomical reference point

The part of the retina closer to the nose is called

nasal

The part of the retina closer to the temple is called

temporal

The part of the retina above the fovea is called

superior

The part of the retina below the fovea is called

inferior

What is the nourishment source for the cornea?

aqueous humor

why does cornea lack blood vessels?

to maintain its transparency and clarity.

What is located behind the iris in the eye, and how is it suspended?

The lens and it is suspended by ligaments called zonule fibers, which are attached to the ciliary muscle.

What compartments are present in the interior of the eye, and what fluid is present in each?

* The aqueous humor is the watery fluid that lies between the cornea and lens,
* The vitreous humor (jelly-like) lies between the lens and retina, serving to keep the eyeball spherical.

What is the site of most of the refractive power of the eye, and why?

The cornea, rather than the lens, because light reaches the eye from air, and the cornea is mostly water. A good deal of refraction occurs because light travels significantly slower in water than air.

What is the focal distance, and how does it relate to the curvature of the cornea?

* distance from the refractive surface to the point where parallel light rays converge
* The focal distance depends on the curvature of the cornea.
* The tighter the curve, the shorter the focal distance.

What is a diopter?

A diopter is the reciprocal of the focal distance in meters and is a unit of measurement for refractive power.

What is the cornea's refractive power in diopters

The cornea has a refractive power of about 42 diopters.

How does the lens contribute to the formation of a sharp image, and what distance range is it most important for?

The lens contributes another 12 diopters to the formation of a sharp image of a distant point less than 9 meters from the eye.

What is accommodation, and why is it necessary for near vision?

* when lens changes shape to provide additional focusing power for near objects.
* light rays originating from near objects are no longer parallel and require greater refraction power to bring to focus.

What happens to the lens during accommodation, and what is the role of the ciliary muscle

* ciliary muscle contracts and lens becomes rounder and thicker
* relaxation of ciliary muscle increases tension of suspensory ligaments and flattens the lens

What is the role of refractive power in accommodation?

ability of the lens to bend light rays and bring them to a focus. During accommodation, the lens changes shape to increase its refractive power, becoming rounder and thicker due to its natural elasticity.

What is the pupillary light reflex, and what is its purpose?

* adjustment of the pupils in response to changes in ambient light levels.
* regulate the amount of light entering the eye

What is the neural pathway involved in the pupillary light reflex?

Retina → neurons in brainstem that constrict pupils

What is consensual pupillary light reflex?

constriction of the pupils of both eyes in response to shining a light into only one eye

* because neural connections are bilateral, transmitted to both sides of brainstem

How does the pupillary light reflex differ from the accommodation reflex?

light level adjustment vs lens shape to focus on far/near objects

What is the visual field, and how is it defined?

Visual field = area of vision (can’t see past certain point on right if moving pencil out to periphery)

How is the visual field related to the retina?

image of objects in the visual field → projected onto the retina

What is the relationship between the left visual field and the right visual field, and how are they represented on the retina?

* left visual field → right side of the retina
* right visual field → left side of the retina

How is the upper visual field represented on the retina and why?

bottom of retina, inverted orientation of retinal image

What is visual acuity?

ability of the eye to distinguish two points near each other

What factors determine visual acuity?

spacing of photoreceptors in retina

the precision of the eye’s refraction

What is visual angle?

Distance across the retina in degrees

How is light energy converted to neural activity in the retina?

direct pathway of photoreceptors → bipolar cells → ganglion cells

How do photoreceptors impact bipolar cells?

respond to light and influence the voltage of bipolar cells connected to them

What do ganglion cells do in response to light?

fire action potentials and transmit the impulses along the optic nerve to the brain

Where do horizontal cells receive input from?

Photoreceptors

What signals do horizontal cells transmit?

lateral to influence surrounding bipolar cells and photoreceptors

Where do amacrine cells receive input from?

Bipolar cells

What signals do amacrine cells transmit?

laterally to influence surrounding ganglion cells, bipolar cells, and other amacrine cells

What are the only light-sensitive cells in the retina?

rod and cone photoreceptors

What is the only source of output from the retina?

ganglion cells

What is unique about ganglion cells in the retina?

They fire APs which is essential to transmit info outside the eye. Al other retinal cells depolarize or hyperpolarize, but no APs

4. Why are rod and cone photoreceptors important for retinal processing?

They respond to light and influence the voltage of bipolar cells connected to them, which in turn influence the firing of ganglion cells.

How do ganglion cells transmit information from the retina to the brain?

along their axons, which form the optic nerve and project to other visual areas of the brain

What is the organization of the retina?

The retina has a layered (laminar) structure.

What is the path that light must take to reach the photoreceptors in the retina?

Light must pass from the vitreous humor through the ganglion cells and bipolar cells before it reaches the photoreceptors.

What is the advantage of the "inside out" arrangement of the retina?

pigmented epithelium can absorb any light that passes entirely through the retina, minimizing the scattering of light within the eye that would blur the image.

What is the tapetum lucidum and what is its function?

reflective layer beneath the photoreceptors that bounces light back at the photoreceptors if it passes through the retina. This makes many nocturnal animals more sensitive to low light levels, but it reduces their activity.

What are the different cell layers of the retina (going outwards), and what do they contain?

* ganglion cell layer
* cell bodies of the ganglion cells
* inner plexiform layer
* synaptic contacts between bipolar cells, amacrine cells, and ganglion cells
* inner nuclear layer
* cell bodies of bipolar cells, horizontal cells, and amacrine cells
* outer plexiform layer
* photoreceptors make synaptic contact with bipolar and horizontal cells
* outer nuclear layer
* cell bodies of photoreceptors

What do plexiform layers contain?

Synaptic contacts with bipolar + horizontal (outer) / bipolar + amacrine + ganglion (inner)

What is contained in the layer of photoreceptor outer segments, and where is it located?

* light-sensitive elements of the retina
* embedded in the pigmented epithelium.

Where does the conversion of electromagnetic radiation into neural signals occur in the retina?

on the photoreceptors at the back of the retina

What are the four regions of every photoreceptor?

outer segment, an inner segment, a cell body, and a synaptic terminal.

What is contained in the outer segment of a photoreceptor, and what is its function?

stack of membranous discs, and light-sensitive photopigments in the disk membrane absorb light, triggering changes in photoreceptor membrane potential.

What are the two types of photoreceptors, and how do they differ in structure and sensitivity?

* Rods have a long cylindrical outer segment with many disks, while cones have a shorter, tapering outer segment with fewer disks.
* Rods have a greater number of disks and a higher photopigment concentration, making them over 1000x more sensitive to light than cones.

What is the difference between scotopic, photopic, and mesopic vision?

* Scotopic vision refers to vision in low light conditions, where only rods contribute to vision.
* Photopic vision refers to vision in high light conditions, where cones do most of the work.
* Mesopic vision refers to vision in intermediate light levels, where both rods and cones are active.

What is the difference between rods and cones in terms of photopigment types?

All rods contain the same photopigment, but there are three types of cones, each with a different pigment that makes different cones sensitive to different wavelengths of light.

Where are the majority of the 5 million cones located in the retina?

fovea, with a smaller proportion in the retinal periphery.

Are there any rods in the central fovea?

No

Are there more rods or cones in the peripheral retina?

rods

During daylight, where do we have greater spatial sensitivity on the retina?

fovea, which has high spatial acuity

Why are we able to discriminate colors better on our peripheral retina?

because there are fewer cones in this region

Why are we more sensitive to low levels of light on our peripheral retina?

because rods respond more slowly to low light levels than cones, there are more rods in the peripheral retina, and more rods project to single bipolar and ganglion cells in the peripheral retina.

Are we able to perceive color differences at night?

No, since there are no cones that function in low light levels.

What is phototransduction?

process by which photoreceptors convert light energy into changes in membrane potential

What is the Vm of the rod outer segment in complete darkness?

30 mV

What is the dark current, and how is it regulated?

* Steady influx of Na+ through special channels in the outer segment membrane.
* Sodium channels are stimulated to open and are gated by cGMP, a second messenger.

What happens when light is absorbed by the photopigment in the rod outer segment?

causes a change in the conformation of retinal, which activates the opsin in the photopigment. This process is known as bleaching.

What is the consequence of using a biochemical cascade for phototransduction?

amplification, which gives the visual system increased sensitivity to light.

What is rhodopsin, and what is its structure?

* Rhodopsin is the photopigment found in rods.
* It is a receptor protein with a prebound chemical agonist called retinal.
* The receptor protein, opsin, has 7 alpha helices and is typical of G protein-coupled receptors.

What is the role of transducin and PDE in phototransduction?

The bleaching of rhodopsin → G-protein called transducin in the disk membrane → enzyme PDE breaks down → cGMP that is present in the cytoplasm of the rod → Na+ channels close → membrane hyperpolarizes

Sodium channels are gated by what

cGMP (2nd messenger)

Do PRs depolarize or hyperpolarize to light?

hyperpolarize