AKA: Rhinencephalon (contribute to sense of smell): consists of: Olfactory Epithelium, Olfactory Bulbs, Olfactory Striae, Olfactory cortex and additional areas of brain. Functions in special sense of smell or olfaction.
Substances that are smelled (odorants) release small quantities of gas, oil. esters (fragrant compounds) acids, etc. into air which reach nasal mucosa
Olfactory System: Neurosensory Cells
First order, bipolar sensory neurons. In olfactory epithelium projected peripheral processes into nasal mucosa; peripheral processes act as receptors (unlike other special sensory nerves that have separate receptors) that are stimulated by odorants.
Sensations are transmitted over central processes of neurosensory cells to form olfactory nerves (CN1)
Olfactory Nerve Fibers
Olfactory nerve fibers ascend in small bundles (fasciuli) across cribiform plate of ethmoid bone to synapse on mitral and tufted cells (2nd order neurons) in olfactory bulb
Comprised of axons from mitral and tufted cells as well as anterior olfactory nucleus. CNS: we are in tracts now. Project centrally to make up olfactory tracts. Splits into lateral, medial and intermediate striae.
projects to anterior olfactory nucleus and to ipsilateral pyriform cortex in uncus (primary olfactory cortex). Pyriform fibers project to: mediodorsal thalamic nucleus for consious analysis of odor, 3rd order.
Come from lateral striae. Entorhinal area (secondary olfactory cortex) to parahippocampal gyrus and orbitofrontal cortex: associating smell with previous experience
Arises from anterior olfactory nucleus and projects to contralateral olfactory bulb via anterior commissure; mediates emotional/autonomic response to odors
Rods (low intensity vision, monochromatic) and cones (sharp visual acuity, color) of retina compose receptors of visual pathway; light falling on these receptors is transducer into retinal potentials
Visual Information Passes through...
Visual info passes forward in the retina (back of eye through internal layer, which is more forward) to the bipolar cells which are the primary sensory (1st order neuron) neurons in visual pathway.
1st order neuron synapse with?
These cells synapse with the ganglion cells (2nd order neurons) of the retina whose axons converge near the center of the retina to form the optic nerves
Located predominately in the central retina which contains the macula lutea (area where light is striking), a small circular area lateral to the optic disk
Located within the macula lute; inner retinal layer almost absent in fovea forming a pit or foveola. Allows for max. amt of light to reach fovea which is focal point of central vision.
They predominate in rest of retina farther from fovea and participate in peripheral vision (peripheral portion of retina)
Optic nerves travel medially from fovea to converge at optic disk (aka papilla or optic nerve head) where they exit from retina. Blind spot area where nerve exit from retina: because no photoreceptors are present in optic disc area
Optic nerve fibers become myelinated as they emerge from optic disc.
Optic nerves pass from eyeball to leave the orbits through the optic canals; each optic nerve enters the middle cranial fossa and joins together to form the optic chasm (optic cross).
At Optic Chiasm
nerve fibers from medai half of each retina decussate, while the lateral fibers from each retina remain uncrossed or direct.
Optic fibers continue without interruption behind the chasm as two diverging optic tracts that go to left and right lateral geniculate bodies.
Cells in the LGB are 3rd order sensory neurons; their axons give rise to fibers that form the geniculocalcarine tracts or optic radiations to primary cortex of the occipital lobe (region surrounding calcarine fissure)
How does light fall on the retina? How does it reflect back?
Visual fields have central and peripheral regions. Central field is projected onto macula (area of most accurate vision. straight back). Peripheral field is projected onto paramacular (medial) and peripheral portions of retina
Each eye is divided into nasal (medial) and temporal (lateral) halves which contain upper and lower quadrants. Light rays from temporal visual field project to nasal half of retina; rays from nasal visual field project to temporal half of retina.
Visual fields are projected onto each retina in a reversed and inverted pattern, follow point-to-point projection to occipital cortex. When we get to retina, it will follow same pathway back from this point on
Origin of light rays
Light rays from upper quadrant strike lower retina, rays from lower quadrant strike upper retina.
Each LGB receives projections from the homonymous (right or left) halves of the field for both eyes. At this point we have "full" fields. 1 temporal field and 1 nasal field.
Upper and Lower Quadrants
Fibers from upper retinal quadrants project to medial LGB. Lower visual field.
Fibers from lower retinal quadrants project to lateral LGB. Upper visual field.
These divide into dorsal and ventral bundles.
projects via parietal lobe to upper wall of calcimine cortex (cuneus). Takes upper - medial LGB projects over.
AKA: loop of Meyer. Projects via temporal lobe to lower wall of calcimine cortex (lingual gyrus). Take lower. Lateral LGB projects over.
Calcarine Cortex = Striate Cortex
Follow all the way back
Homonymous Visual Field Detects:
1. Similar regions of visual field defects for each eye
2. Results from lesion in visual pathway distal to optic chiasm
Heteronymous Visual Field Detects:
Not as frequent
1. Two different parts of visual field impaired (eg. bitemporal hemianopsia - tunnel vision)
2. Due to lesions at level of optic chiasm
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