Astrocyte

combra_loihi supports an astrocyte module simulating the functions of tripartite synapses. We can easily convert an Spiking Neural Network into a Spiking Neural-Astrocytic Network by using this module.

import combra_loihi.api as combra
import nxsdk.api.n2a as nx

net = nx.NxNet()

# Define Spiking Neural Network on Loihi
...

# Add Astrocyte module onto the network
astro = combra.Astrocyte(net, other parameters)
astro.connectInputNeurons(presyn_compartments, number, connectionMask, weight)
astro.connectOutputNeurons(postsyn_compartments, number, connectionMask, weight)
astro.probe([combra.ASTRO_SPIKE_RECEIVER_PROBE.COMPARTMENT_VOLTAGE,
             combra.ASTRO_IP3_INTEGRATOR_PROBE.COMPARTMENT_VOLTAGE,
             combra.ASTRO_SIC_GENERATOR_PROBE.COMPARTMENT_VOLTAGE,
             combra.ASTRO_SPIKE_GENERATOR_PROBE.SPIKE,
             combra.ASTRO_IP3_INTEGRATOR_PROBE.SPIKE])

# Run network
net.run(simulation time)
net.disconnect()

More information about Astrocyte module api and parameters can be found here.

Astrocyte and Tripartite Synapse

Computational roles of astrocytes in our brain have been neglected for a long time. As electrically silent non-neuronal cells, astrocytes were once thought only serving supporting roles for neural networks. Recent techniques help to observe the active signaling between astrocytes and neurons and show astrocytes play a key computational role in brain function.

(Figure for tripartite synapse)

Our Computational Astrocyte in Loihi simulates the behavior and function of tripartite synapse, a concept of neuron-astrocyte interaction. Astrocyte in tripartite synapse modulate synaptic activity by receiving neuron transmitters from presynaptic neuron and releasing gliotransmitters to postsynaptic neuron. Biological interactions between neurons and astrocyte is shown below.

1. Presynaptic neuron releases transmitters to astrocyte receptors causes the raise of astrocytic intracellular IP3 concentration.

2. A Ca2+ wave is then generated inside astrocyte when IP3 concentration reaches a threshold.

3. This Ca2+ wave activates the release of gliotransmitters in turn generates a SIC current input to postsynaptic neuron.

(Figure for astrocyte in Loihi)

Astrocyte module in Loihi

Our astrocyte module in Loihi uses 4 compartments. It can be used on a spiking neural network without any additional requirement or modification on the network. The setup and function of each compartment are listed below.

1. Input Spike Receiver (SR) simulates the receptors on astrocyte guarding the integration of IP3.

2. IP3 integrator (IP3) activates itself in a much slower activation rate than regular neurons, in timescales ranging from hundreds of milliseconds to seconds. Whenever IP3 spikes, output generation will be triggered to generate SIC bursting spikes to postsynaptic neurons.

3. Non-spiking SIC generator (SIC) and Burst Spike Generator (SG) form a dendritic tree with SIC feeds voltage directly into SG. SIC produces a voltage signal similar to the observed biological SIC current. SG transforms the voltage signal into bursting spikes and send to postsynaptic neurons as a discrete form of SIC current.