Source: A Brain-Mind Odyssey, Chapter 2 | MCB C61, UC Berkeley
Tags: nervous system, neurons, glia, glial cells, astrocytes, nerve cells, signal transmission, neural networks, C. elegans, Drosophila, invertebrate nervous systems, nervous system complexity, sponges, hydra, planaria, nematode, connectome
The nervous system is the body's information-processing network, built from neurons (signal carriers) and glial cells (support and signalling). Nervous system complexity varies enormously across species, from animals with no nervous system at all (sponges) to insects with 150,000 neurons, all the way up to the roughly 100 billion neurons in the human brain.
Neuron (nerve cell)
The cellular unit of signal transmission in the nervous system. The human brain contains approximately 100 billion neurons.
Glial cell (glia)
A class of non-neuronal cells in the nervous system that support and protect neurons. Astrocyte glia are also directly involved in signalling, so glia are not purely passive support cells.
Astrocyte
A type of glial cell that participates directly in neural signalling, in addition to providing structural and metabolic support.
Nervous system
A network within the body that functions to manipulate both external and internal information. William James emphasised that without it, sensory organs lose their connection to conscious experience.
Caenorhabditis elegans (C. elegans)
A tiny nematode (roundworm) with only 302 neurons. Its complete wiring diagram (connectome) is known because the organism is semi-transparent and structurally simple.
Connectome
The complete map of neural connections in an organism. C. elegans was the first animal to have its connectome fully mapped.
Drosophila melanogaster (fruit fly)
A model organism in neuroscience. Its brain is less than 0.5 mm wide yet contains around 150,000 neurons.
James, author of Principles of Psychology, argued that if nervous communication between the brain and a body part is severed, the brain loses all experience of that part: the eye goes blind, the ear deaf, the hand insensible.
The reverse is also true: if the brain itself is injured, consciousness is abolished or altered even when every other organ is intact.
This frames the nervous system as the essential mediator between body and mind.
Neurons carry out signal transmission. They are the primary computational units.
Glial cells were historically seen as passive support, but astrocyte glia are now known to participate directly in signalling.
Both cell types are necessary for a functioning nervous system.
The range of nervous system complexity across the animal kingdom is striking:
Sponges
No nervous system at all.
Hydra
Very simple nervous system: a loosely connected network of a small number of cells.
Allows basic signal communication throughout the body, with no centralised brain.
Jellyfish
Relatively simple neural networks, similar in principle to hydra but somewhat more elaborate.
C. elegans (nematode/roundworm)
Only 302 neurons.
Complete wiring diagram is known, thanks to the organism's transparency and simple body plan.
A major model organism for studying how neural circuits produce behaviour.
Planaria (flatworms)
More complex than nematodes.
Nervous system includes an extended network of interconnected neurons plus two clusters of neurons at the head end (a precursor to brain-like centralisation).
Insects (e.g. Drosophila)
Complex brains despite tiny size.
Fruit fly brain: less than 0.5 mm wide, roughly 150,000 neurons.
Capable of sophisticated behaviours including learning and navigation.
⚠️ Know the spectrum of nervous system complexity: sponges (none) → hydra (simple net) → C. elegans (302 neurons, full connectome known) → planaria (head-end clusters) → insects (complex, 150k neurons in Drosophila) → vertebrates (billions).
⚠️ Glial cells are not just structural support. Astrocytes participate in signalling. This is a commonly tested distinction.
⚠️ C. elegans and its 302-neuron connectome come up frequently. Know why the connectome is known (transparency, simple structure) and why the organism matters (complete neural map).
⚠️ The William James quote captures a core principle: the nervous system is what connects sensory organs to conscious experience. Damage to the nerve pathway or the brain itself breaks that link.
Q: How many neurons does the human brain contain, and what are the two main classes of cells in the nervous system?
A: Approximately 100 billion neurons. The two main classes are neurons (signal transmission) and glial cells (support and, in the case of astrocytes, direct involvement in signalling).
Q: Why is C. elegans an important model organism in neuroscience?
A: It has only 302 neurons and its complete wiring diagram (connectome) has been mapped. Its semi-transparent body and simple structure made this mapping possible. It allows researchers to study how a full neural circuit produces behaviour.
Q: Which animal phylum has no nervous system at all?
A: Sponges (phylum Porifera) have no nervous system.
Q: What distinguishes the planarian nervous system from that of C. elegans?
A: Planaria have a more complex system with an extended network of interconnected neurons and two clusters of neurons at the head end, representing early centralisation. C. elegans has a simpler system with only 302 neurons and no comparable head-end clustering.
Q: What role do astrocyte glial cells play beyond structural support?
A: Astrocytes are directly involved in neural signalling, not just passive support. They can modulate synaptic activity and participate in information processing.
neuron, nerve cell, glia, glial cell, astrocyte, nervous system, neural network, signal transmission, C. elegans, Caenorhabditis elegans, connectome, wiring diagram, Drosophila, fruit fly, planaria, flatworm, hydra, sponge, invertebrate neuroscience, nervous system evolution, MCB C61, brain-mind odyssey, William James, Principles of Psychology