The nervous system, including the brain, consists of nets of interconnected nerve cells to form neural circuits.
Donald Hebb proposed the idea of neuronal assemblies to describe functionally coherent groups that bind individual feature-coding neurons together1. Neurons constituting an assembly interact selectively with one another ensuring cohesion of the assembly and identify themselves as members of that assembly2.
Neural circuits and neuronal assemblies are both useful concepts, and in this essay, I will use them interchangeably.
Understanding the way in which any portion of the nervous system works requires identifying the neural circuits or assemblies underlying a particular function.
The anatomical arrangement of such networks of neurons forming a particular circuit is often referred as the ‘wiring’ of the circuit. The strength of the wiring of a circuit depends on the strength of the coupling between neurons, ie the strength of the synapses. If synaptic transmission operates mostly with simple arithmetic calculations as described above (eg, spatial and temporal summation), the circuits can be called ‘hard wired’. If modulation of synaptic transmission occurs so that the coupling is weaker, then the circuit is regarded more plastic. Modulation of neurotransmission contributes to as neural plasticity or neuroplasticity which underlies memory, learning and other changes of neural functions over time.
In addition to the fast action potentials, some neurons exhibit slower oscillations (0.1-10 Hz) of their membrane potentials as a result of oscillating depolarising and repolarising currents (often generated or modulated by calcium channels). Such slower oscillations may be intrinsic to the neurons in which case they are called pacemaker neurons3. Alternatively, they can be triggered by the action of neurotransmitters so that they act as conditional pacemakers4. At a given threshold value of the membrane potential, the slow depolarisations trigger a barrage of action potentials. Thus, neural circuits, especially those with pacemaker neurons, behave as active media.
The overall behaviour of the neural circuits depends on its architecture, ie the wiring (connectivity), the intrinsic properties of its neurons, and the nature of the synaptic connections. As a consequence, different neural circuits may exhibit dramatically different functional properties5.
- “a diffuse structure comprising cells … capable of acting briefly as a closed system, delivering facilitation to other such systems.” DO Hebb (1949): The Organization of Behavior – A Neuropsychological Theory. Republished in 2012, Psychology Press. ↩︎
- For example, see Wolf Singer (2013): Cortical dynamics revisited. Trends in Cognitive Sciences 17(12), P616-P626.
CH Papadimitriou et al (2020): Brain computation by assemblies of neurons. Proceedings of the National Academy of Sciences USA 117,14464-14472;
G Umbach et al (2022): Flexibility of functional neuronal assemblies supports human memory. Nature Communications 13, 6162. ↩︎ - Neural circuits containing pacemaker neurons are known as central pattern generators. For examples, see: https://en.wikipedia.org/wiki/Central_pattern_generator ↩︎
- J-M Ramirez et al (2004): Pacemaker neurons and neuronal networks: an integrative view. Current Opinion in Neurobiology 14, 665-674. ↩︎
- For example, see TH Bullock et al (2005): The Neuron Doctrine, redux. Science 310: 791-793;
George Dragoi (2023): The generative grammar of the brain: a critique of internally generated representations. Nature Reviews Neuroscience 25, 60-75.
Rafael Yuste et al (2024): Neuronal ensembles: Building blocks of neural circuits. Neuron 112, P875-P892. ↩︎