Source: Module 1, Chapter Two | Course: A Brain-Mind Odyssey (UC Berkeley)
Tags: nervous system, neuron, glial cell, cerebral cortex, cerebral lobes, meninges, cerebrospinal fluid, vertebrate brain, William James, Golgi stain, Ramón y Cajal, corpus callosum, brain anatomy
This chapter surveys nervous system complexity from sponges (no neurons at all) through to the human brain, then details the gross anatomy of the vertebrate brain: cerebrum, cortex, lobes, meninges, and cerebrospinal fluid. It also introduces the historical figures who shaped early neuroscience, from William James to Golgi and Ramón y Cajal.
Neuron (nerve cell)
One of two general classes of cells that make up the brain. Around a hundred billion neurons in the human brain. Considered the cellular units of signal transmission.
Glial cell (glia)
The other general class of brain cell. At least a hundred billion glia in the human brain. Many glia, especially astrocyte glia, are directly involved in signalling (not just support cells).
Cerebrum
The largest part of the brain, containing many gyri and sulci. Its size increases across evolutionary history from fish and amphibians to birds and mammals.
Cerebral cortex
The outer layer of the cerebrum. A sheet of neural tissue roughly 3 mm thick (varies by region), highly folded to fit inside the skull. Contains four lobes: frontal, parietal, occipital, and temporal.
Sulci
Grooves on the surface of the cerebrum.
Gyri
Bumps (ridges) on the surface of the cerebrum.
Corpus callosum
A bundle of approximately 200 million nerve fibres connecting the right and left cerebral hemispheres.
Meninges
Three protective layers covering the brain: dura, arachnoid, and pia.
Dura (dura mater)
Latin for "hard or tough mother." The outermost, skin-like sheet of tissue covering the brain.
Arachnoid
Latin for "like a spider web." A delicate layer of tissue covering the brain underneath the dura.
Pia (pia mater)
Latin for "soft or tender mother." The innermost, most delicate layer, closest to the brain surface.
Subarachnoid space
The space between the arachnoid and pia layers, containing cerebrospinal fluid.
Cerebrospinal fluid (CSF)
A liquid that cushions the brain inside the skull and transports soluble substances throughout the central nervous system. Found in the subarachnoid space and the ventricles.
Meningitis
An extremely serious condition in which the meninges become inflamed, usually as a result of infection.
Golgi stain (Golgi staining)
A technique developed by Camillo Golgi in 1873, also called la reazione nera (the black reaction). Makes neurons, along with all their dendrites and axons, visible under the microscope. Only about 1% of neurons take up the stain, and the reason is still unknown.
Growth cone
Term first used by Santiago Ramón y Cajal, referring to the growing tip of a developing axon.
Nervous systems range enormously in complexity across the animal kingdom:
Sponges (phylum Porifera, over 500 million years old) have no nerve cells and no nervous system at all
Hydra (phylum Cnidaria) have a very simple nervous system, a loosely connected network of a small number of cells
Jellyfish (also phylum Cnidaria) possess relatively simple neural networks
Roundworm Caenorhabditis elegans (phylum Nematoda) has exactly 302 neurons, and the complete wiring diagram is known
Flatworm/planarian (phylum Platyhelminthes) has an extended network with two clusters of neurons at the head end, sometimes considered a primitive brain
Fruit fly Drosophila has a brain less than 0.5 mm wide containing roughly 150,000 neurons
All vertebrate brains share a basic structural plan. During embryonic development, the neural tube folds in, closes off, and expands at one end. The interior spaces of the tube become ventricles in the mature brain.
The three main divisions:
Forebrain, dominated by the cerebrum
Midbrain, dominated by the optic tectum
Hindbrain, dominated by the medulla and cerebellum
The size of the cerebrum increases as one moves from evolutionarily older fish and amphibians through reptiles to more recent birds and mammals.
The cerebral cortex is a ~3 mm thick sheet of neural tissue, highly folded into sulci (grooves) and gyri (bumps). Small mammals such as mice, rats, and squirrels do not exhibit cerebral folding; larger mammals such as capybaras, dogs, cats, and primates do.
The four cerebral lobes and their landmarks:
Central sulcus separates the frontal lobe from the parietal lobe
Lateral fissure separates the temporal lobe from the frontal and parietal lobes
Longitudinal fissure divides the right and left cerebral hemispheres
The brain is covered by three layers (outer to inner): dura mater, arachnoid, pia mater. Between the arachnoid and pia is the subarachnoid space, filled with cerebrospinal fluid. CSF cushions the brain and transports soluble substances throughout the central nervous system.
William James (1842–1910) – pioneer of the modern scientific study of mind. His key observation: if nervous communication is cut off between the brain and other parts of the body, the experiences of those parts cease to exist for the mind
Andreas Vesalius (1514–1564) – published De Humani Corporis Fabrica, containing some of the most detailed anatomical drawings ever produced
René Descartes (1596–1650) – key figure in Western philosophy and one of the early thinkers on signalling in the nervous system
Luigi Galvani (1737–1798) – studied electrical stimulation of animal muscles; hypothesised that muscles move via internal electrical forces triggered by external stimulation
Camillo Golgi (1843–1926) – developed the Golgi staining technique (1873), enabling visualisation of individual neurons under the microscope
Santiago Ramón y Cajal (1852–1934) – used Golgi's staining method to study brain tissue; first to use the term growth cone
⚠️ Know the two major cell types in the brain (neurons and glia) and their approximate numbers (~100 billion each). Note that glia are not merely support cells; astrocytes are directly involved in signalling.
⚠️ The C. elegans figure (302 neurons, complete wiring diagram known) comes up frequently as a benchmark for nervous system mapping.
⚠️ Be able to name the four cerebral lobes and the landmarks that separate them (central sulcus, lateral fissure, longitudinal fissure).
⚠️ Meninges order from outside in: dura, arachnoid, pia. CSF is found in the subarachnoid space.
⚠️ The Golgi stain is a classic exam topic: know it stains only ~1% of neurons, and that the reason is still unknown.
⚠️ Do not confuse Golgi (the staining pioneer) with Ramón y Cajal (who used the stain and coined "growth cone"). They are distinct figures, though both studied brain slices under microscopes.
Q: What are the two general classes of cells that make up the brain?
A: Neurons (nerve cells) and glial cells (glia). Each type numbers roughly a hundred billion in the human brain.
Q: Name an organism whose complete nervous system wiring diagram is known, and state how many neurons it has.
A: The roundworm Caenorhabditis elegans has 302 neurons, and its complete wiring diagram (connectome) is known.
Q: What are the three layers of the meninges, from outermost to innermost?
A: Dura mater, arachnoid, and pia mater.
Q: Where is cerebrospinal fluid found, and what does it do?
A: CSF is found in the subarachnoid space (between the arachnoid and pia layers) and in the ventricles. It cushions the brain inside the skull and transports soluble substances throughout the central nervous system.
Q: What is the corpus callosum?
A: A bundle of approximately 200 million nerve fibres that connects the right and left cerebral hemispheres.
Q: What was distinctive about the Golgi stain technique?
A: It made individual neurons, along with their dendrites and axons, clearly visible under the microscope. Only about 1% of neurons are stained, and why only some neurons take up the stain remains unknown.
Q: Which landmark separates the frontal lobe from the parietal lobe?
A: The central sulcus.
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