Psychology › Brain & biology
Neurons and synapses
Neuron doctrine, 1888· Concept
Well replicatedStrong, repeated evidence.
The brain is built from about 86 billion separate nerve cells, neurons, which signal each other across tiny gaps called synapses.
See it on the Psychology map →What it says
In 1888 the Spanish anatomist Santiago Ramón y Cajal, using Camillo Golgi's silver stain, showed that nerve cells are separate units, not one continuous web as Golgi believed. A neuron gathers inputs on branching dendrites; if they add up past a threshold, it fires an electrical pulse down its axon. At the tip it releases chemicals across a gap of about 20 to 40 nanometers, the synapse, a term Charles Sherrington coined in 1897. A typical neuron has thousands of synapses.
Why it matters
It turned the brain into a network of countable, connected parts. Learning, memory, drug effects and mental illness are now all studied as changes in neurons and synapses.
Think of it like
Like a deal network: each broker hears from many contacts, and only makes the call when enough of them say yes.
The evidence
Status: well replicated. Cajal's drawings, then 1950s electron microscopes, which finally showed the synaptic gap. Cajal and Golgi shared the 1906 Nobel Prize; the 86 billion count comes from a 2009 Brazilian study.
Open questions
How roughly 100 trillion synapses (estimates vary) produce thought. A full wiring map exists for the fruit fly (2024, about 140,000 neurons); a human map is far off.
Inside this (2)
Connections (3)
▸Neurotransmitters, the brain's chemical messengers carry signals between Neurons and synapsesWell replicated
The connection
Inside a neuron the signal is electrical; between neurons it becomes chemical. When an impulse reaches the axon's tip, tiny packets spill neurotransmitter into the synapse. The molecules cross the gap in under a millisecond, lock onto receptors on the next cell, and push it toward firing or quiet it. Pumps then recycle them, which is where many drugs act.
Who showed it and when
Otto Loewi's frog-heart experiment, Graz, Austria, 1921; Bernard Katz's studies of transmitter 'packets' in London in the 1950s; Julius Axelrod's work on recycling (reuptake) in the US.
The evidence
Nobel Prizes in 1936 (Loewi, Dale) and 1970 (Katz, Axelrod, von Euler); electron microscopes later showed the packets and synaptic gaps directly.
Open questions
Some neurons also connect through direct electrical synapses, and one neuron can release several chemicals at once; how these mixtures carry information is still being mapped.
▸Neuroplasticity: the brain rewires with use rewires connections between Neurons and synapsesWell replicated
The connection
Plasticity happens mainly at synapses. When neurons fire together repeatedly, the receiving side adds receptors and the link strengthens (long-term potentiation); unused links weaken and get pruned. Neurons also grow new branches and spines. Learning a skill or a street map is literally a change in the wiring between neurons.
Who showed it and when
Donald Hebb's 1949 book at McGill University in Montreal proposed the rule; Tim Bliss and Terje Lømo confirmed it in Oslo in 1973; Eric Kandel traced it in sea slugs.
The evidence
Microscopes now film individual synapses forming and vanishing in living mice; Kandel shared a 2000 Nobel Prize for showing that memory storage changes synapses.
Open questions
How memories stay stable while synapses constantly turn over; researchers debate whether a memory lives in specific synapses or in broader patterns of activity.
▸Neurons and synapses cluster into Brain regions and localization of functionWell replicated
The connection
Neurons with similar jobs bunch together and wire into circuits: visual cortex at the back, movement strips across the top, the hippocampus deep in the temporal lobe. The wrinkled cerebral cortex, a sheet a few millimeters thick, holds about 16 billion neurons; the small cerebellum holds about 69 billion. Regions work in networks, not alone.
Who showed it and when
Korbinian Brodmann mapped 52 numbered cortical areas by cell structure in Berlin in 1909; Wilder Penfield's surgeries in Montreal, 1930s–1950s, mapped body areas by electrical stimulation.
The evidence
Microscopic cell maps, stimulation during surgery and injury cases; a 2016 map by Matthew Glasser's team identified 180 areas per hemisphere from MRI data.
Open questions
How best to divide the brain is still debated: maps range from dozens to hundreds of areas, and borders differ between individuals.