7- Neurophysiology (Vision)- Pt 1.docx

- **Visual System** - Vision is an integral part of neural function. - The eyes are an extension of the brain. - Animals have a wider peripheral vision than humans because their visual fields of each eye do not completely overlap. - The **visual fiel...

- **Visual System** - Vision is an integral part of neural function. - The eyes are an extension of the brain. - Animals have a wider peripheral vision than humans because their visual fields of each eye do not completely overlap. - The **visual field** is the spatial area seen with one eye. - The extent of overlap of the visual field depends on the anatomical placement of the eyes. - For example, prey species have laterally oriented eyes, giving them a wider view. - Dogs have **binocular vision** only in the area directly in front of them. - Monocular vision does not provide good depth perception. - To maintain binocular vision, the eyes move as a functional unit when animals scan their surrounding environments. - In the normal dog, choroidal vessels are visible throughout the fundus. - When the eyes look directly at an object, an image of the object will fall onto the area of centralis. - The **area of centralis** is the retinal area that provides the greatest acuity. - Pathways involved in visual reflexes include: - Vision and visual processing - Intraneuronal connections - Parasympathetic connections for pupillary light reflex - UMN (upper motor neurons) and LMN (lower motor neurons) for the eye, eyelid, and head movement - The **trochlear nerve (facial nerve 4)** controls the superior oblique muscle of the eye. - The **abducens nerve (facial nerve 6)** controls the lateral rectus muscle of the eye. - The **oculomotor nerve (facial nerve 3)** controls the following eye muscles: - Levator palpebrae superioris muscle - Superior rectus muscle - Medial rectus muscle - Inferior rectus muscle - Inferior oblique muscle - **Color** is the brain's interpretation of differences in the wavelengths of light. - **Eye Structure: General Info** - The **pupil** is the opening (hole) of the eye. - The size of the pupil regulates the amount of light that enters the eye. - The **sclera** is the outermost eye layer, which is a modified anteriorly into a clear region called the cornea. - A chamber filled with vitreous humor is located behind the lens. - **Vitreous humor** is a gelatinous (transparent) fluid that gives the eye it's spherical shape. - Vitreous humor contains phagocytic cells. - In contrast, **aqueous humor** fills both the anterior and posterior chambers. - **Eye Structure: Retina** - The **retina** lies behind the vitreous humor, and is the structure where light is transformed into the electrical activity of neurons. - The retina is a specialized sensory epithelium that contains photoreceptors and other cell types arranged in layers. - The **blind spot (optic disc)** is where the retina is interrupted at a point where axons of the retina ganglionic cell layer leave the eye on their way to the brain. - The optic disc lacks photoreceptors. - The following 5 major **cells make up the retina:** - **Photoreceptor cells** are a synapse with bipolar and horizontal cells - Glutamate is the neurotransmitter. - **Horizontal cells** are inhibitory interneurons that transmit signals horizontally from rods and cones to bipolar cells - GABA is the neurotransmitter - **Bipolar cells** are retinal interneurons that transmit signals vertically from the rods, cones, and horizontal cells to ganglion and amacrine cells. - Glutamate is the neurotransmitter. - **Amacrine cells** are mostly inhibitory interneurons that transmit signals in 2 directions which are either directly from bipolar to ganglionic cells, or are horizontally from bipolar cells o ganglionic cells (or to other amacrine cells). - GABA or glycine are the neurotransmitters. - **Ganglionic cells** are projection neurons that transmit output from the retina through the optic nerve into the brain. - Glutamate is the neurotransmitter. - All cell types that make up the retina cause some sort of distortion of light rays. - The **fovea** (area centralis) minimizes this (image/light) distortion. - **Eye Structure: Fovea** - The fovea is a minute area in the center of the retina, which is especially capable of acute and detailed vision. - In animals, it is called the "area centralis". - The fovea is composed almost entirely of cones, and their special long and slender bodies aids their detection in visual images. - The more cones present, the more colors that can be interpreted. - Other cells are all displaced, allowing light to pass unimpeded to the cones. - **Eye Structure: Lens** - The **lens** is located behind the iris. - The lens is suspended by the **suspensory ligaments**, and these fibers are attached to the ciliary body. - The **ciliary body** is a muscular structure that is located near the base of the iris that helps with accommodation of the lens. - It will increase or decrease the tension of the lens - It makes the lens curvature more or less convex, allowing the lens to focus on near or far objects. - **Eye Structure: Choroid** - The **choroid** consists of loos connective tissue with numerous vasculature and pigmented cells, and it serves a nutritive function. - Some diurnal animals have melanocytes that absorb light that has passed by the photoreceptors without stimulating them. - Nocturnal and most domestic mammals have a patch of reflective material that is called the "**tapetum lucidum**". - The tapetum lucidum has a light-reflective surface that enhances dark-adaptive vision under dim light, causing the eyes to shine. - In the normal cat eye, the tapetal color is typical, and the optic nerve is surrounded by tapetum. - **Eye Structure: Iris** - The **iris** is a diaphragm muscle that can contract/relax to control the rays of light that goes into the eye via modification of the pupil diameter. - In the dark, pupils will dilate (**mydriasis**- pupil enlargement). - In the light, pupils will contract (**miosis**- pupil size is reduced). - The iris has dilator and sphincter muscles. - **Pupillary dilator muscles** - The pupillary dilator muscles are radially arranged, and they oppose the action of the sphincter. - The pupillary dilator muscles are apart of the pigmented anterior epithelial cells, known as myoepithelial cells, which are composed of smooth muscle. - Its contraction results in pupillary dilation (mydriasis) - Pupillary dilation reflects the general state of the sympathetic tone - Examples: pain, fear, anger - **Pupillary sphincter muscles** - Pupillary sphincter muscles are circularly arranged near the pupillary margin that are innervated by parasympathetic fibers. - Their contraction results in decreased pupillary size (miosis). - **Eye Structure: Optic Nerve** - The axons leaving the eye at the optic disk give rise to the optic nerve (cranial nerve 2). - There are more axons in both optic nerves than in all the dorsal roots of the spinal cord. - The surface of the retina has blood vessels, with arteries and veins entering the retina at the optic disk. - Provides the nutrition of the retina together with vessels of the choroid - **Eye Structure: Photoreceptors (rods and cones)** - There are about 130million photoreceptor cells in the retina, which can either be cones or rods. - Rod and cone cells, each have an outer and inner segment. - The **outer segment** is the photosensitive region. - In cone cells, the outer segment is composed mainly of membranous invaginations. - In rod cells, the outer segment contains numerous flattened membranous sacs that are arranged like a stack of coins. - The membrane of these invaginations and sacs contains photopigments, which convert light stimulus to a receptor potential. - The **inner segment** is the metabolic region of the photoreceptor. - **Rods** are photochemical neurotransmitters that are responsible for the perception of shades of grey (black and white vision), making them essential for night vision. - Rod cells have **rhodopsin**, which has a low threshold of excitability. - Rhodopsin is easy to stimulate by low-intensity light. - Vitamin A is important for the formation of rhodopsin. - Severe vitamin A deficiency can cause night blindness. - Rod cells are 300x more sensitive to light than cone cells. - 95% of photoreceptors present in the retina are rods. - **Cones** are photochemical neurotransmitters that are responsible for the perception of color. - Cones have color pigments (or cone pigments), and they are less sensitive to light. - This lack of sensitivity means that cones requires relative high-intensity light than rhodopsin. - Primates have the following 3 cone types: blue sensitive pigment (445 nanometers), green sensitive pigment (535 nanometers), and red sensitive pigment (570 nanometers). - These 3 types make up polychromatic vision. - The visual system mixes and contrasts the effects of each cone cell. - Each one carries a different type of color pigment with a different pick of absorbencies at different wavelengths. - Most mammals are dichromatic, meaning that they have only 2 types of cones, allowing them to detect the blue and yellow portion of the light spectrum, but unable to distinguish reds and oranges. - Some lizards, birds, turtles, and fishes have 4 types (3 types of cones with 1 UV-sensitive cone). - This gives them a richer color perception than humans and primates.