Computational Neuroscience: Neurobiology 101
Neurons, Synapses, and Brain Regions
- You may wish to skip this and the next two lectures if you have a background in
Enter...the Neuron (Brain Cell)
- If the brain is a stage, the undeniable star is a neuron
- Neurons are tiny (25 millionths of a meter)
- The things that look like branches are called dendrites, and the one long, slender branch in the
centre is called the axon.
The Neuronal Zoo - The cortical neuron is not the only kind. There are many different kinds of neurons
o Parametal neurons are the ones with triangular bodies at the bottom of the
visual cortex picture. They form the parametal tract in the motor system
o Purkinje cells in the cerebellum are in the middle, and have some very
interesting branching structure in their dendrites
o The cells in the optic tectum have different branching structures depending on
- Two competing hypotheses in 1900 when Ramon y Cajal made these drawings
o Reticular hypothesis: the brain is a continuous structure
o the brain is composed of discrete cells (this was what Ramon supported, and it
led to the neuron doctrine)
- Neuron Doctrine:
o The neuron is the fundamental structural & functional unit of the brain
o Neurons are discrete cells and not continuous with other cells (there are some
exceptions to this rule, but it holds true most of the time
o Information flows from the dendrites to the axon via the cell body (there are also
some exceptions here)
Generally speaking, which of the following are true under this idealized model of the brain
(check all that apply)?
A) The brain is broken into individual, discrete parts called 'neurons.'
B) The shape of neurons varies in some general way from one area of the brain to another.
C) Dendrites are the input ends of the neuron, whereas axons are the output ends.
D) Axons are the input ends of the neuron, whereas dendrites are the output ends.
The Idealized Neuron
- Dendrites surrounding a cell body, and then an axon
- The inputs to the dendrites look like the following: - When you get the EPSPs arriving, they are added together. Once they reach a certain
threshold, then there is an output spike.
Spikes (output) from a neuron occur when:
A) An input from neighboring neurons occurs.
B) The sum of inputs from neighboring neurons reaches a certain threshold.
C) The sum of output expected from this neuron by neighboring neurons reaches a certain
D) None of these
What is a Neuron?
- A “leaky bag of charged liquid”
- Contents of the neuron enclosed within a cell
- Cell membrane is a lipid (fat) bilayer
o Bilayer is impermeable to charged ion
species such as Na , Cl, and K +
o Ionic channels embedded in membrane
allow ions to flow in or out (without this,
nothing could really happen)
The Electrical Personality of a Neuron
- Each neuron maintains a potential difference across its
o Inside is about –70 mV relative to outside (-70 mV
is called the “resting potential.”
o [Na ] and [Cl ] higher outside; [K ] and organic
anions [A ] higher inside (this could be reflective of the fact that
we began as single celled organisms in a salty ocean)
o Ionic pump maintains -70 mV difference by