Textbook Notes (368,098)
Canada (161,641)
Neuroscience (289)
NROC64H3 (81)
Chapter 10

NROC64 - Chapter #10 Notes.docx

8 Pages
135 Views
Unlock Document

Department
Neuroscience
Course
NROC64H3
Professor
Matthias Neimier
Semester
Fall

Description
NROC64 – Chapter #10: The Central Visual System:  Retina extracts information about different facets of the visual image  More than 100 million photoreceptors in the retina but only 1 million axons leaving the eye carrying info to the rest of the brain  Pathway leaving conscious visual perception includes the LGN of the thalamus and the primary visual cortex (Area 17), V1 or striate cortex The Retinofugal Projection:  Retinofugal projection - neural pathway that leaves the eye beginning with the optic nerve o Centrifugal projection = away from the center o Corticogugal projection goes away from the cortex  Retinofugal = away from the retina The Optic Nerve, Optic Chiasm and Optic Tract:  Ganglion axons pass through three structures before reaching the brain stem  Components are: optic nerve  optic chiasm  optic tract  Optic nerves – exit the right and left eyes at the optic disks and pass through holes in the floor of the skull  Optic chiasm – axons originating in the nasal retinas cross from one side to the other o Decussation - crossing of a fiber bundle from one side of the brain to the other o b/c only axons from the nasal retinas cross = partial decussation at the optic chiasm  Optic tracts – axons form these tracts which run just under the pia along the lateral surfaces of the diencephalon Right and Left Visual Hemifields:  Left visual hemifield – objects appearing to the left of the midline (vice versa) o Objects in the binocular region of the left visual hemfield are imaged on the nasal retina of the left eye and on the temporal retina of the right eye o This is because the temporal regions are encoded by the opposite side of the brain o Nasal are encoded by the same side of the brain (ipsilateral) o Optic nerve fibers cross in the optic chiasm  Binocular visual field – central portion viewed by both retinas Targets of the Optic Tract:  Small # of optic tract axons form synaptic connections with cells in the hypothalamus  10% continue past thalamus to innervate the midbrain  Most innervate the LGN of the dorsal thalamus o Neurons in LGN give rise to axons that project to the primary visual cortex o Projection from LGN  cortex = optic radiation  Lesions in the retinofugal projection cause blindness  Cut in the left optic nerve = blind in left eye  Cut in the left optic tract = blindness in right visual field  Midline cut in the optic chiasm = only affects fibers that cross the midline Nonthalamic Targets of the Optic Tract:  Direct projections to the hypothalamus play an important role in synchronizing a variety of biological rhythms including sleep, wakefulness with the daily dark-light cycle  Direct projection to the midbrain (pretectum) control the size of the pupil and certain types of eye movement  10% (150 000 neurons) of the ganglion cells in the retina project to a part of the midbrain tectum called the superior colliculus (little hill)  Tectum of the midbrain major target of retinofugal projection in all nonmammalian vertebrates o In these the superior colliculus = optic tectum o Projection from retina  superior colliculus is called the retinotectal projection in mammals  Superior colliculus a patch of neurons activated by a point of light commands head, eye movements to bring the image to the fovea Box 10.1 – David and Goliath:  Body size is regulated by secretion of growth hormone form the anterior lobe of the pituitary gland  If anterior lobe swollen (hypertrophied) it produces excess amounts of hormone resulting in body growth of unusually large proportions  This is called pituitary hypertrophy – also disrupts normal vision  Loss of peripheral vision b/c enlarged pituitary, results in a loss of peripheral vision = tunnel vision/bitemporal vision The Lateral Geniculate Nucleus:  Right and left lateral geniculate nuclei in the dorsal thalamus = major targets of the 2 optic tracts  Each LGN appears to be viewed in 6 layers of cells (most ventral layer = 1)  Layers arranged like a pancake bent around the optic tract  LGN = gateway to visual cortex (visual perception) The Segregation of Input by Eye and by Ganglion Cell Type:  LGN neurons receive synaptic input from the retinal ganglion cells  Segregation of LGN neurons in layers suggest that diff types of retinal info are being kept separate o Axons from M-type, P-type, nonM-nonP ganglion cells in the 2 retinas synapse of diff LGN layers o LGN: right eye (ipsilateral) axons synapse on LGN layers 2, 3, 5 o Left eye (contralateral axons synapse on layer 1, 4, 6 o Layers 1 and 2 contain larger neurons and the 4 more dorsal layer 3-6 contain smaller cells o Ventral layers = magnocellular LGN layers and dorsal layers = parvocellular LGN layers o P-type ganglion cells in retina project to parvocellular LGN o M-type ganglion cells  magnoceullular LGN o Koniocellular layers- tiny neurons b/w layers that receive inpit from the nonM-nonP types of retinal ganglion cells and project to visual cortex  Info derived from 3 categories of retinal ganglion cells and from 2 eyes remains segregated  Retina give rise to streams of info that are processed in parallel Receptive Fields:  Inserting a microelectrode into LGN can study potential discharges of geniculate neurons in response to visual stimuli  Visual receptive fields of LGN neurons are almost identical to those of the ganglion cells that feed them  Magnocellular LGN neurons: relatively large centre-surround receptor fields, respond to stimulation with a transient burst of APsand are insensitive to diff in wavelength (like M-type ganglion cells)  Parvocellular LGN cells are like P-type retinal ganglion cells – small centre-surround receptive fields, respond to stimulation of receptive field centers with a sustained increase in the freq of Aps, exhibit colour opponency  Receptive field of cells in koniocellular layers – center-surround and have either light/dark opponency  Within all layers of LGN the neurons are activated by only one eye Non-Retinal Inputs of the LGN:  Retina is not the main source of synaptic input to the LGN  Major input (about 80% excitatory synapses) comes from the primary visual cortex  LGN also receives synaptic inputs from neurons in the brain stem (activity: related to alertness and attentiveness)  LGN is the 1 site in the ascending visual pathway where what we see is influenced by how we feel Anatomy of the Striate Cortex:  LGN has a single major synaptic target: primary visual cortex (aka area 17, V1, striate cortex) located in the occipital lobe, surround the calcarine fissure  Close correlation b/w structure and function Retinotopy:  Projection from retina  LGN and V1  Retinotropy – organization whereby neighbouring cells in the retina feed info to neighboring places in their target structures – LGN and striate cortex o 2-D structure of retina is mapped onto 2-D surface of the subsequent structures o 3 important points:  Mapping of the visual field onto a tetinotopically organized structure is often distorted (central few degrees of the visual field are overrepresented or magnified in the retinotopic map)  Discrete point of light can activate many cells in the retina due to the overlap of receptive fields  Don’t be misled by the word “map” – perception is based on the brain’s interpretation if distributed patterns of activity Lamination of the Striate Cortex:  Neocortex and striate cortex have neuronal cell bodes arranged into about half a dozen layers  Just under the pia mater is largely devoid of neurons and consists entirely of axons and dendrites  Full thickness of the striate cortex from white matter to pia = 2 mm  At least 9 distinct layers in reality of striate cortex, however 3 layers are combined The Cells of Different Layers:  Spiny stellate cells – small neurons with spine-covered dendrites that radiate out from the cell body, primary in 2 tiers of layer IVC  Pyramidal cells – outside layer of IVC, covered with spines, a single thick apical dendrite that branches as it ascends toward the pia mater and by multiple basal dendrites that extend horizontally  Only pyramidal cells send axons out of the striate cortex to form connection with other parts of the brain  Axons of stellate cells make local connections only within the cortex  Inhibitory neurons which lack spines are in the cortical layers as well, these neurons only form local connections Inputs and Outputs of the Striate Cortex:  In LGN every layer receives retinal afferents and send efferent to the visual cortex  Visual cortex only a subset of layers receives input from the LGN or sends output to a diff cortical/subcortical area  Axons form the LGN terminate in several diff cortical layers with the largest # going to IVC layer  Output of LGN divided into streams of info  These streams anatomically segregated in layer IVC  Magnocellular LGN  IVC alpha, Parvocellular LGN  IVC beta Ocular Dominance Columns:  Wanted to know how the left and right eye LGN inputs segregated when they reach layer IVC of the striate cortex  Ocular dominance columns – distribution of axon terminals relaying info was not continuous in layer IVC but rather was split up into a series of equally spaced patches each about 0.5 mm wide Innervation of Other Cortical Layers form Layer IVC:  Radial connections – maintain the retinotopic organization established in layer IV  Therefore a cell in layer IV receives the same input as the cell above it in layer IV  Axons of some layer III pyramidal cells extend collateral branches that make horizontal connections within layer III  Radial and horizontal connections play diff roles in analysis of the visual world  Layer IVC stellate cells project axons radially up mainly to layers IVB and III where for the first time info from right and left eye mix  All layer IVC neurons receive only monocular input most neurons in layers II and III receive binocular input coming from both eyes  IVC alpha (magnocellular)  IVB, Layer IVC beta (parvocellular)  layer III  III and IVB an axon may form synapses with the dendrites of pyramidal cells of all layers Striate Cortex Outputs:  Pyramidal cells send axons out of striate cortex into the white matter  Layer II, III and IVB  axons to other cortical areas  Layer V  axons to superior colliculus and pons  Layer VI 
More Less

Related notes for NROC64H3

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit