The primordial neuromechanical loops involved in the lamprey’s swimming represent the foundations of more complex nervous systems including the brain. With the evolution of land locomotion by quadrupeds, superimposed neural loops were added to those described for the lamprey and fish more generally. Loops within the spinal cord became superimposed on lower neuromechanical ones, providing extra levels of proximal to distal control of newly-adapted limb muscles.
The internal loops consist of nets of neurons connected by synapses at one level and linked by descending neurons to the adjacent level below while receiving ascending inputs from neurons at that level, completing the internal neural loop. The ascending pathways can be regarded in general as afferent and represent bottom-up influences (often referred as ‘feedback‘ in relation to a higher centre) and the descending pathways as efferent (often referred to as ‘feedforward‘ in relation to a higher centre) and represent top-down influences1. The concept of circular causality naturally applies to all internal superimposed loops with bottom-up and top-down influences between each of the levels.
CPG, central pattern generator; EC, electrochemical, ie neurotransmission to the muscles. From Pearson et al (2006).
5.1 Double Hierarchy of the Central Nervous System
The nervous system, as all structures in the world, is stratified from lower levels starting, for the sake of simplicity, from atoms and molecules, and all the way up to the full organism.
I propose here a novel principle of organisation of the nervous system which simplifies the otherwise enormous degree of complexity that the brain has acquired through evolution.
The vertebrate nervous system, including the human brain, is organised according to a hierarchical structure consisting of internal neural circuits (loops) superimposed on the lowermost neuromechanical loops, with their few sensory inputs and common motor output.
This proposed architecture of the vertebrate nervous system enables us to bring the complexity of brain functions and their underlying neural circuits to a simpler conceptual frame. Each new superimposed internal loop is associated with a new level of neural state and takes it one ‘synaptic distance’ further away from the external world.
With this simplified, yet robust perspective, I think previously unbridgeable conceptual gaps can be eliminated, bringing within one conceptual frame all neural functions from the lower levels of behaviour all the way to the highest mental functions. Naturally, the idea of a hierarchical organisation of the brain is not new and represents the basis of an excellent book by Joseph LeDoux2, one of the foremost neuroscientists who proposes four fundamental superimposed (realms) of existence: the biological, neurobiological, cognitive, and conscious realms, corresponding to different levels of the nervous system. My conceptual frame is consistent with his but makes it part of a more general detailed architecture of the vertebrate nervous system with many more levels.
I will review next some of the well accepted examples of internal loops involved in different sensory-motor functions, and I will extend the analysis to the higher neural loops of the cerebral cortex.
- K Pearson et al (2006): Assessing sensory function in locomotor systems using neuro-mechanical simulations. Trends in Neuroscience 29, P625-631;
T Bem et al (2003): From swimming to walking: a single basic network for two different behaviors. Biological Cybernetics 88, 79-80.
S Rossignol et al (2006): Dynamic Sensorimotor Interactions in Locomotion. Physiological Reviews, 86, 89-154. ↩︎ - Joseph LeDoux (2023): The Four Realms of Existence: A New Theory of Being Human. Belknap. ↩︎