CIDPUSA.ORG Autoimmune

Neurology Anatomy Physiology

December 14, 2021

Contents

neurons and Nerves
neurotransmitter
The Brain & Spinal Cord
Cranial Nerves
Peripheral Nervous System
Autonomic Nervous System
Senses: Eye diagrams, Hearing, Smell, Taste, Taste & Tongue Sensation, Balance
Memory , Memory types, Creation of Memory
Higher Functions
Altered States

Lens - It is a fat disk that completes the focusing of light rays into a clear, sharp image on the retina. Actually, the cornea provides about 80% of the focusing power. The lens is stretched by the ciliary muscles to fine-tune the focus by changing the shape, so it becomes fat for near objects, thin for far ones.

Vitreous humor - The posterior cavity behind the lens contains a viscous, gelatinous material. It forms the bulk inside the eyeball and, with the outer sclera, gives it firmness. It also helps to refract light rays toward the retina.

Retina - Inside each light-sensitive cell (rod or cone) in the retina are up to 100 million molecules of photopigment, each of which contains a smaller molecule known as retinal (Figure 12a). When retinal receives light energy, it changes shape by twisting around its backbone. The altered retinal sets off a chain of chemical reactions inside the cell, which triggers an electro-chemical change in the cell membrane creating a nerve impulse. The retinal returns to its original configuration when the signal jumps across a synapse to a bipolar cell. Curiously, in order to

Figure 12a Retinal
[view large image]

Figure 12b Retina
[view large image]

reach the photoreceptors, incoming light must first pass through all the other layers of cells in the retina. There are five layers altogether (see Figure 12b). Starting from the outermost layer:

1. Pigment epithelium - Epithelial cells are the guards and protectors of the organ. They cover the surface and determine which substances are allowed to enter.
2. Rods and cones - These are the photoreceptors. The rods are responsible for night vision, and the cones for color vision. The retina has as many as 150 million rods but only 1 million ganglionic cells and optic nerve fibers. This means that there is considerable mixing of messages and a certain amount of integration before nerve impulses are sent to its final destination. The photoreceptors achieve the remarkable sensitivity (of a millionfold difference in luminance) with adjustment relative to the average background. But if the overall background level of illumination were to change drastically, as it does when we enter a dark room, we are effectively blind for a few minutes until the rod photoreceptors have adapted to this level of reduced intensity. Vision is most acute in the fovea centralis (see Figure 09), where there are only cone cells. Clear vision is possible only when the fovea inspects a scene. This provides a very restricted window of clarity. Thus in order to obtain a clear picture, the eyes have to dart about frenetically and automatically under the direction of the brain.
3. Bipolar cells - Nerve signals from the rods and cones pass inward to this layer. The bipolar cells together with the horizontal and amacrine cells form the network of pre-processing nerve cells. The network helps to simplify, and code information before it reaches the optic nerve.
4. Ganglionic cell layer - It receives input from the pre-processing network, and send output to the brain via the axons of the ganglion cells.
5. Optic nerve - It is made up of nerve fibers, which come from across the whole retina. They pass in front of the retina, forming the optic nerve, which turns to pierce the layers of the eye. The signals eventually end up in the occipital lobe to form an image.

Optic Pathway - As shown in Figure 13, information about the left visual world is transmitted to the right side of the brain and vice versa. As the visual fields of the eyes overlap in the front, this division is achieved by sorting retinal ganglion cell axons according to whether they look at the left (dotted line) or the right (solid line) visual field. So some axons from the right eye go to the right side of the brain and others to the left. The sorting occurs in the optic chiasm. The retinal axons proceed to the lateral geniculate nuclei where the first synapses are fromed with the visual neurons in the brains.

Figure 13 Optic Pathway
[view large image]

continued to hearing page

Powered by cidpusa.org