Sparse Connectivity

• One thing that we are quite confident in is that neurons are sparsely connected.
• That is each neuron is only connected to a very small proportion of the rest of the neurons in the brain.
• Different types of neurons have a different number of connections leaving them.
• Pyramidal neurons (which are the most common in the cortex) have on the order of 1000s of synapses leaving them.
• Some neurons have perhaps 100,000, and some have has few as 5.
• Neurons tend to be connected to nearby neurons.
• Remember, the brain is a 3D structure.
• Again, chemical synapses are uni-directional, with one neuron effecting another neuron. Topologically these arcs are unidirectional.
• The weight of the synapse specifies how strongly the presynaptic neuron effects the post-synaptic neuron.
• Gap junctions are different. Many (most?) are bidirectional topologically. (I'm not used to seeing models of these, so I'm not sure about weights.)