Properties of Neuronal circuits
•Specific connections between neurons
–Established embryonically
–Activity
•Maintains & strengthens
•Weakens if no activity
•Complexity and variation
–Two levels of organization
•Indy neurons generating dif signal types
•Combine into complex and varied circuitry
•Sensory filter networks
•Central pattern generating networks
–Cyclic (respiration)
–Noncyclic (frog feeding)
•Motor command system
–Sensory input modulates motor output
•Variability of circuits
–Divergence
•Repeated branching
•Widespread postsynaptic influence
–Ex: center of visual field
–Convergence
•Multi inputs into a single neuron
•integration from many neurons
–Ex: peripheral vision
Sensory Neural Networks
•First step in generating behavioral responses
•Sort
•Refine
–Vs individual
–Tuning curve
•Receptors and sensory networks
•Filter
–Magnify, amplify, add, subtract & reconfigure
•Light has driven evolution
Vision
–Organisms “eye” varies greatly
–Visual transduction
•conserved set of protein molecules
–Opsins
–Coupled to photopigment
•Optics
–Convergence
–Fish vs humans
•Eyes
Evolution
–Simple eyespots
–Restriction of acceptance angle leads to
•Pattern recognition
•Controlling locomotion
–Pinhole
–Spherical lens
–Series of lenses
–Vertebrate
•Small aperture
•Refractile lens
•Ommatidium
Compound eye
•Corneal lens
•Photoreceptor
–Retinular cell
•12 surrounding single dendrite of eccentric cell
•Rhabdomere
•Mv surface
•Rhodopsin
–Limulus
•Much of early single unit recording
•Hartline and Barlow
•Lateral eye
–Compound
•Ventral eye
–Eyespot
Lateral inhibition
Lateral inhibition
•Single ommatidium
–Lateral plexus
–Border phenomena
Vertebrate eye
Cornea
Pupil
5x change
1x106 intensity
Adaptation
Visual pigment state
Neural adaptation
•Photoreceptors
–1o
–Absorb photons
–Transduction
•Release NT continuously (dark)
•Hyper to light
•Bipolar cells
–2o
–PR to ganglion cells
–Graded hyper or depolar to light
•Ganglion cells
–3o
–Same polarity as bipolar cell
Retina
•Horizontal cells
–Lateral
•Input from PR
–Graded hypo to light
•Synapse to BP
Amacrine cells
Transient response to onset and offset of light stimulus
–Lateral
–BP and Ganglion cells
•photoreceptors\
–Rods
•Scotopic
–Dim light
–Low resolution
–Cones
•Phototopic
–Bright light
–High resolution
–Color vision
Transduction
•Photon hyperpolarizes membrane
•Decrease gNa
•Dark
–Na+ leaks into OS
–Dark current
–Pump keeps Na+ from accumulating
–Steady secretion of NT due to depolar
•Light absorption by photopigment
–gNa decreases
–Vm hyperpolarizes
–Decrease NT
•Note no axons in PR
•Convergence and divergence
–Fovea
•Minimal
•One to one
•Cones to BP to GC
•High acuity
–Non fovea
•GC many inputs
•Increase sens
•Lower acuity
•Visual streak
–Horizontal regions with high cones
–Flat open environments
•Area centralis
–Fovea
–High visual acuity
•Hartline 1940s
•GC spont in dark
fovea
Receptive field
–On or off response
–Center and surround
•On bipolars
•Off bipolars
•4 neurons
Lateral geniculate nucleus
o
–Synapse to primary visual cortex
–Six layers
•Outer 4
–Small somata
–Parvocellular
•Inner two
–Large
–Magnocellular
•Parallel processing
Visual cortex
•Hubel and Wiesel 1960
•Simple cells
–Receptive field
•Long and bar shaped
•Straight border off/on
•Receives excitatory connections from LGN cells
–on centers arranged linearly on retina
•Complex cells
–6o order
–Innervated by simple cells
–Straight bars or borders at specific angles
–Do not have topographically fixed RF
•Correct stimulus anywhere on retina
–Bars of specific orientation
–On one side/off other
–Border moving in only one direction
•Cortical columns