The appearance of the nervous system marked a momentous event in the evolution of multicellular organisms. Around 630 million years ago (in the Ediacaran period), before the appearance of bilateralians (ie, animals with bilateral symmetry), some cells became specialised to develop kinetic contractile force (muscle cells) while other elongated cells (nerve cells; neurons) used electrochemical signals, called action potentials or nerve impulses, to enable fast communication between different distant parts of the organisms, in response to external and internal events1.
4.1 Nerve Cells
The nervous system in vertebrates consists fundamentally of three types of nerve cells: sensory neurons, interneurons and motor neurons.
Sensory neurons respond to specific physical events transforming them into nerve signals. They are called ‘afferent‘ neurons (from afferre = carry to) as they convey information from outside to the inside the nervous system. Evolution selected different populations of sensory neurons to be specifically responsive to events in the external environment (exteroceptors), to the body’s own movements and position in space (proprioceptors) and to physico-chemical events of the viscera (enteroceptors).
Motor neurons are the only output of the nervous system and lead to bodily movements by contractions of muscle cells. They are called ‘efferent‘ neurons (from efferre = carry from) as they carry information (motor commands) from the central nervous system to the muscles. Most muscle contractions in vertebrates normally only occur via activation of the motor neurons. This means that muscle cells of the body on their own are quiescent. The nervous system organises muscle contractions in space and time resulting in specific actions and consequent behaviour.
There are also motor neurons that control secretions and movements of viscera. These belong to the autonomic nervous system which works in parallel with motor pathways controlling the muscles of the body. Autonomic motor pathways are not under direct voluntary control2 and will not treated in detail in this essay.
The nerve cells in between sensory and motor neurons are called interneurons and are involved in integrating input and output signals. Most of the nervous system can be considered to consist of interneurons.
Neurons, like all living cells, are open dissipative systems. In addition, they are excitable because at ‘rest’ they are kept far from thermodynamic equilibrium by an active separation of ionic charges (mostly sodium and potassium ions) across the neural cell membrane. This maintains an unstable dynamic equilibrium, called the resting membrane potential, with the inside of the cell being electrically negative ( about -70mv) compared with the outside of the cell.
- Nicholas D Holland (2003): Early central nervous system evolution: an era of skin brains? Nature Reviews Neuroscience 4, 617-627.
Detlev Arendt (2020): The evolutionary assembly of neuronal machinery. Current Biology, 30(10), R603-R616. ↩︎ - See Bill Blessing & Ian Gibbins: http://www.scholarpedia.org/article/Autonomic_nervous_system ↩︎