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MSU NEU 302 - Exam 1 Study Guide

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Neuroscience Exam # 1 Study Guide Lectures: 1 - 4Lecture 1 (January 15th)Central Visual Pathways: Chapter 12 1. After the signal of light is transduced in the eye, where does the signal go or project to?a. Projections from the retinai. The signal goes from the optic disk of the retina to the optic nerve and finally to the optic chiasm1. 60% of ganglion cell axons are contralaterala. Cross midline at optic chiasmb. They are the ganglion cells from the nasal retina2. 40% of ganglion cell axons stay ipsilateral (on same side)ii. After the signal passes the optic chiasm the axons are referred to as the optic tractb. Retinogeniculostriate Pathwayi. Optic Tract  Lateral Geniculate Nucleus (LGN)  Striate Cortex (V1)ii. Lateral Geniculate Nucleus (LGN)1. Located in the ventral lateral thalamus2. Segregated into 6 separate layersa. Each layer processes specific aspects of the visual informationb. Layers 2, 3, and 5 process visual info from ipsilateral (same side) eye or the Temporal Retinac. Layers 1, 4, and 6 process visual info from contralateral eyeor the Nasal retina3. There are different cells types in different layers of the LGNa. Parvocellular Layers (P-Cells)i. More dorsal layers1. Layers 3, 4, 5 and 6ii. Small neurons (Petite)iii. Sustained responseiv. Convey Color information1. Driven by different classes of cones2. Sensitive to different wavelengthsb. Magnocellular Layers (M-Cells)i. Ventral Layers 1. Layers 1 and 2ii. Large neurons (Mega)1. More extensive dendritic fields2. Larger diameter axonsiii. Larger receptive fieldsiv. Respond to the presentation of visual stimuli in a very quick, short mannerv. Insensitive to different wavelengths4. Segregation of input into the LGN (i.e. what layer the signal goes to) depends on the Ganglion Cell Type as well as the eye that signal is coming from.5. There are also receptive fields in the LGN similar to those found in the retinaa. On-centre fieldsi. When light is on the center of the field, the cell increases firingii. When the cell is on the surrounding receptive field, the cell inhibits firingb. Off-centre fieldsi. Same as the on-centre fields except the opposite.ii. Center = inhibitioniii. Surround = exciteiii. Striate Cortex (myelin stria)1. V1 (physiology) aka Primary Visual Cortex aka Brodman area 17 (cell density)2. 1st visual area3. Characteristics:a. Striated (lines of gennari) or striped tissueb. Mostly buried in a sulcus (Calcarine Sulcus) in the occipital lobeiv. Function: provides your overall visual fields1. Each eye produces a hemifield2. There is overlap between these fields resulting in the binocular visual fielda. This field allows depth perception3.v. Consequences of lesions in pathway:1. Damage to retina or one of the optic nerves (before chiasm)a. Loss of vision that is limited to the eye of originb. Still have bits of each visual field i. Loss of peripheral vision in same eyeii. Loss of binocular vision in contralateral eye (but contralateral visual field is still intact)2. Damage to one of the Optic Tracts, LGN, or Optic radiationa. After Optic chiasmb. Results in Loss of vision of entire contralateral visual fieldc. Complete loss of vision in affected area of binocular visual fieldd. Referred to as Homonymous Hemianopsia3. Damage to Optic chiasma. Loss of vision in temporal visual field onlyi. Parts of visual field loss from both eyesii. Most peripheral part lostiii. Nasal retina info lostb. Referred to as:i. Bitemporal Hemianopsiaii. Heteronomous Hemianopsia4. Smaller visual field deficit referred to as Scotomas instead of anopsias.c. Other targets of Retinal Ganglion Cells:i. Pupillary Light Reflex1. Controls pupil constriction in response to light2. Pathway:a. Optic Nerve  Pretectum  Edinger-Westphal Nucleus (EWN)  Preganglionic parasymp. fiber in cranial nerve 3  Ciliary Ganglion  Pupillary Constrictor Muscle3. This pathway has bilateral projections from the retinaa. Both EWN are activated by only one Pretectumb. Allows both eyes to be influenced simultaneously4. Consequences of lesions:a. Normali. Both stimulated eye (direct response) and unstimulated eye (consensual response) constrict inthe lightb. Damage to Oculomotor Nerve or EWN in Brainstemi. Direct response but no consensual responseii. Problem with visceral motor outflow to problem eyec. Damage to sensory input to problem eyei. Retina or optic nerve damageii. Light in one eye has direct and consensual responseiii. Light in the problem eye has no direct or consensual responseii. Superior Colliculus1. Dorsal surface of the midbrain2. Coordinates head and eye movements to visual (as well as other) targets 2. Organizationa. V1i. Retinotopic Organization1. Left visual field of each eye projects to the right hemisphere and visa versa2. Adjacent neurons have receptive fields that include slightly different, but overlapping parts of the visual field3. Cells in each structure in each structure can be seen as contributing to a map of the visual fieldii. Laminar Organization1. Divided into 6 principle layers2. Lateral Geniculate Neurons terminate most heavily in layers 4C and 4A of the cortexa. 4C is dominated by spiny stellate neurons whose dendrites are confined to this layer.b. Axons from here convey the activity supplied by the LGN toother cortical layers3. Pyramidal neurons located in the superficial layers of visual cortex project to the extrastriate cortical areas4. Pyramidal neurons located in the deeper cortical areas project to subcortical targets like the LGN or superior colliculus5. The organization serves to segregate populations of neurons that have distinct patterns of connectivity.iii. Columnar Organization1. Combines the input from two eyes2. Since the LGN processes the inputs from the ipsilateral eye and the contralateral eye in different layersa. Individual LGN nuclei are strictly monocular3. The information from the left and right eye is conveyed by the geniculate axons to terminate in alternating, eye-specific Ocular Dominance Columns within cortical layer 4.4. The signals now converge as the layer 4 neurons synapse on individual neurons in other cortical layersa. Though the neurons are binocular, the relative strength of the input from the two eyes varies from neuron to neuron.b. Receptive Fieldsi. The single simple cells that capture light in retina go to the LGN and act ason- and off-center activation with overlapping receptive fields.ii. The light activation of the population of receptive fields then synapse


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