networks Flashcards
what is a network- scott emmons
when you move onto behaviors that are
more complex than a quick reflex, you’re
dealing with especially complicated pathways
that are not immediately interpretable
because they are not simple circuits—they
are networks
network defintion
patterns of neural connections determines how info flows through networks of the brain and this determines the computations done by the brain and therefore what functions result from brain activity
can we determine the human ‘connectome’?
humans have around 100 billion neurons and thus potentially 100 trillion synapses
caenorhabditis elegans
959 cells
302 neurones; ~ 7000 connections
project has mapped the neuronal ‘connectome’
networks are not just about spatial distribution of connections but also…
temporal properties
temporal properties of neurons
different neurons have different firing patterns
same neuron may have different patterns of firing for different functions
types of connections
- feedforward excitation
- feedforwad inhibition
- convergence/divergence
- lateral inhibition
- feedback/recurrent inhibition
- feedback/recurrent excitation
basal ganglia connections
cerebral cortex sends excitatory neurons to the striatum, different connections activate different ptahywa s
direct inhibitory pathway goes to the globus pallidus interior and substantia nigra
substantia nigra sends connects to thalamus
indirect inhibitory connections go to globus pallidus exterior and this sends inhibitory signals to the subthalamic nucleus
cerebellum neural network
main cell is purkynje cells connect to deep cerebellar nuclei to premotor areas
purkynje gets main input from inferior olive
feedforward excitation
single neuron feeds forward on to another excitatory neuron
e.g corticospinal neuron excitates alpha motor neuron which excites a muscle
stretch response in reflex arc
feedforward inhibition
inhibitory neuron between two excitatory neurons
excitation is going to drive inhibition which inhibits the excitatory neuron in the chain
e.g reflex arc
what is a collateral?
where axons branch
feedforward inhibition part 2
inhibitory neuron on side loop
collateral comes out of first excitatory
inhibitory neurons comes back onto second excitatory which is being excitated by the first one
e.g inhibitory neuron activated in exact same way as second excitatory so there is summation of EPSP, when 2nd and 3rd AP come in the interneuron will fire
reduces EPSP to be low threshold level, 3rd AP will not generate in secind excitatory
feedback inhibition/recurrent inhibition
excitatory branches onto inhibitory but it feeds back onto itself
excitation is under the control of the inhibitory interneuron which forms an active break
this regulates the first neuron in the chain
common in the motor system e.g renshaw cell
divergence
single neuron has connections with multiple other neurons
e.g reflex
convergence
multiple neurons input onto a single neuron
e.g alpha motor neuron
convergence and divergence throughout the nervous system
cerebellar purkinje cell=huge dendritic tree
input from different cells ~100000 different granule cells synapse onto a single purkinje cell (convergent)
cortical chandelier cell=inhibitory neuron that synaspes on several cortical pyramidal neurons (divergent)
what does the overall activity of a network depend on?
balance of excitatory and inhibitory inputs
synaptic weights
timing of activation
what is synaptic weight?
level of activity of individual synapses
e.g synapse may have more powerful effect in post-syanptic cells than others
what would happen if we lost inhibition?
hyper-excitability
which leads to burst firing (not a single fire of action potentials)
this can be seen in people with epilepsy
maintainence depends on inhibitory systems
diffrential distribution of excitation and inhibition on pyramidal cell
apical dendrite can be spilt into proximal and distal
excitation is found distally
proximal is inhibitory input
lateral inhibition
sensory receptors are all activated
determining point of stimulus is difficult
lateral inhibition=area either side of where stimulus is is inhibited which helps determine point of stimulation
mach bands