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
a brain network is a system of neurons or brain areas that are functionally or anatomically connected and communicate with each other to carry out specific tasks
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 pathways
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 excitatory 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=muscle in thigh contracts so muscle in hamstring relaxes
what is a collateral?
collateral axons refer to the side branches of a main axon
feedforward inhibition part 2
inhibitory neuron on side loop
collateral comes out of first excitatory and inhibitory neurons comes back onto second excitatory neuron 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
neuron excites an inhibitory interneuron, which then sends an inhibitory signal back to the original neuron
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
strength or effectiveness of a synapse in transmitting a signal from one neuron to another
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
excited neuron reduces the activity of its neighboring neurons
it sharpens sensory perception by enhancing contrast between stimulated and less-stimulated regions
mach bands
photoreceptors detect light intensity
they send signals to bipolar and horizontal cells
horizontal cells perform lateral inhibition, which:
1. enhances the difference between adjacent areas
2. makes transitions more noticeable to highlight edges
somatic sensory association area
located posterior to the primary somatosensory cortex
lateral inhibition sharpens sensory inputs that arrive from the primary somatosensory cortex
when adjacent areas of the skin are stimulated, lateral inhibition suppresses weaker inputs around the strongest one
this improves your ability to localize where you’re being touched
feedback excitation
neuron or group of neurons excites themselves or their own network, either directly or indirectly through a loop
it’s essentially the opposite of feedback inhibition — instead of damping activity, it amplifies it
