Astrocyte

combra_loihi supports an astrocytic module that connects with neurons by creating tripartite synapses with pre- and post-synaptic neurons. The introduction of an astrocytic module converts an Spiking Neural Network into a Spiking Neural-Astrocytic Network (SNAN.)

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)
probe_list = 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

The tripartite synapse is the key element for information processing in the brain, enabiling astrocytic neuromodulation. Astrocytes modulate the synaptic activity by sensing the neurotransmitters' spillover from the presynaptic neuron and translating it to internal (Calcium) signals that, when reaching a threshold from below, cause a gliotransmitter release into the post-synaptic neuron. Specifically:

1. Neurotransmitters released by pre-synaptic neurons are bounded into astrocytic receptors and cause the production of an intracellular astrocytic messenger molecule, Inositol Triphosphate (IP₃).

2. When IP₃ concentration reaches a threshold, it generates an intracellular Ca²⁺ wave.

3. Ca²⁺ wave triggers the release of gliotransmitters, which in turn induce a Slow Inward Current (I_SIC) that is injected into the postsynaptic neuron.

By controlling multiple tripartite synapses and integrating the neuronal information in both larger spatial and temporal domains (compared to neurons), astrocytes provide a long-neglected neuromodulatory system that has never been introduced into neuromorphic systems.

Astrocyte module in Loihi

The astrocyte module in Loihi simulates the main information pathways taking place in a tripartite synapse. The module has two major components (see above Figure) – (1) input integration, which emulates astrocytic receptors and IP3 production, and (2) output generation, which emulates the SIC inject into postsynaptic neurons. These components are built in Loihi as four compartments:

1. The Input Spike Receiver (SR) compartment: It integrates synaptic activity into the IP3 integrator (IP3).

2. The IP3 integrator compartment: It spikes when its voltage exceeds a pre-defined threshold (configurable by the user).

3. The SIC generator (SIC) compartment: It produces a voltage signal similar to the observed biological I_SIC, when IP3 reaches a pre-defined threshold (configurable by the user).

4. The Burst Spike Generator (SG) compartment: It sends bursting spikes to postsynaptic neurons.