Synopsis ‘Networks and Neurons’
This synopsis will be short, because much of the material was basic neuroscience and
can be found online by a simple Google search. Nevertheless, I attach the story of
Cajal and Golgi, which can be found in a wonderful book by Richard Rapport: Nerve
Endings.
Basic overview of the material to guide your Google-search:
Neuron
A neuron consists of a soma, dendrites, and an axon.
The soma is the cell body, and many crucial and fantastically exciting things occur
there that unfortunately go beyond the scope of this lecture series.
The neuron as a communicative element has a few important properties:
- it either signals, or it does not, there is no half-way signal.
- It receives signals from other neurons on its cell body and its extensive
network of dendrites.
- It sends signals to other neurons through its axon.
A signal is called an action potential. When a neuron receives a signal, at the point of
contact (synapse), the membrane potential of the neuron is affected. A potential is a
difference in voltage between the inside and the outside of the cell. The membrane is
the wall of the cell.
The potential at rest is at around -70 mV. When the signal is excitatory, or more likely
to make the neuron fire, this is called an EPSP, and moves the potential closer to 0
mV. When the signal is inhibitory, or less likely to make the neuron fire, this is called
an IPSP, and moves the potential further down from -70 mV, away from 0 mV. These
changes in potential are caused by ions (atoms with charge) moving through the cell
membrane and inside/outside the cell. When the membrane potential around the axon
hillock, the start of the axon, reaches a certain threshold (around -50 to -40 mV), the
neuron will fire. Youtube the terms ‘action potential’, ‘neuronal signalling’, ‘action
potential animation’, for the movies shown in the lecture. You’ll find several similar
ones that can refresh your memory of this story.
The action potential is a sort of travelling wave of changing membrane potential, that
by the end of the axon causes neurotransmitters to be released in the synaptic cleft, the
contactpoint with another neuron. Depending on these neurotransmitters, the signal
can be an EPSP, or an IPSP, exciting or inhibiting the receiving neuron.
Neurons can integrate incoming signals from thousands of neurons, in a process of
spatial summation. They can also integrate incoming signals from the same neuron
that occur repetitively, in a process of temporal summation. All this integration
determines the pattern of firing of a single neuron.
Youtube ‘cocaine + neurons’ to find movies explaining how cocaine and basically
many other drugs affects neuronal communication.
Neurotransmitters bind to receptors on the receiving neuron, causing EPSPs or IPSPs.
Enzymes in the synaptic cleft then ‘clean out’ these chemicals, in a procedure called
‘reuptake’. This procedure is for example disrupted by certain drugs like cocaine,
which blocks the reuptake of dopamine in centers of the brain related to ‘reward’.
Columns
The brain is organized neatly in a way that makes sense. Neurons are organized in
columns, which process the same features throughout the cortical depth. For instance,
a single orientation, in columns in the visual cortex. These columns again are
organized in a way that makes sense: the columns coding for a particular region of
space, are placed next to the columns coding for the neighboring region of space. The
fact that there are such things as ‘visual regions’ shows that again one level up,
regions of the brain are organized and placed together that process the same (kinds of)
information.
Networks:
When something is perceived that consists of different features, they must somehow
be connected: bound together by a particular kind of brain process. This is called the
binding problem: why do you perceive a red Ferrari rather than something red and a
Ferrari? It’s bound into one. One proposed solution, and at least a good way to think
about it, is that all the regions/neurons that must be connected fire in oscillations that
are linked. Oscillations are just waves of firing, MORE firing LESS firing MORE
firing LESS firing. If the MORE phase of the wave is at the same time for two
separate regions, they are firing in synchrony, which could be a way to connect them.
There are regions specialized for processing things like faces, and motion. But most
real-life input is processed throughout the brain, in various regions, that are bound by
mechanisms of anatomical and functional connectivity (such as this synchrony thing).
Thus, the brain works in networks, that are active. Which regions belong to the
network depends on the inputs, the thoughts, the behaviour, etcetera.
Networks in real life often have hubs. Thus, not all nodes are connected to all other
nodes, but some nodes fulfil an important function in connecting other regions. Such
as big airports that often function as the link between two smaller airports. I
recommend the book ‘Linked’ by Barabasi to those interested in networks in real life.
It really is a fascinating book, even if it is about networks.
Karl Lashley was an animal friend who set out to find the ‘engram’: the memory
trace, in the brain. He had to sacrifice many rats, only to find out that memories are
not in any one place in the brain. Thus, memories are also networks.
In creating memories, patient HM was instrumental in demonstrating that a region
called the hippocampus, in the temporal lobe, is a hub in the network of memory
formation. If this region is damaged or lesioned, no new memories are stored. At
least, most memories that you can actively recall: HM was still able to learn
‘procedural memories’ – for instance he continued to improve in tasks such as linetracing with a pen, even though he never remembered having already done this task
many times.
Last thing about learning: Hebb said that neurons that fire together wire together,
thus, the actual memories in your brain are nothing more than connections between
neurons strengthened by repeatedly co-occuring stimulation. This is how you learn!
Please see other document for the story of Cajal and Golgi.
Tip from Brain Storm attendee - Tuesday night on Belgian TV there is apparently an
interesting show on mind/brain! Check it out… (this may be posted too late for you to
notice – because I forgot to write the synopsis, my apologies in that case!)