How molecules drifting across the gap strengthen the synapse and, therefore,
create a preferred path
 A
close-up of the synapse between an ingoing and outgoing neuron.
The outer curves (orange for incoming neuron, red for outgoing) represent the
cell membranes. The voltage potential difference across the membrane (from
inside the cell to outside) changes when the synapse receives an incoming
signal. Those changes across both cells eventually may cause the outgoing cell
to fire. Drawing courtesy of Bruno Dubuc and http://thebrain.mcgill.ca/,
modified by the author.
It's a four step process that's unbelievably complicated. I shall
simplify to describe the main ideas.
- The incoming signal perturbs the potential across the incoming cell's
membrane at the synapse. This stimulates little sacs at the synapse to
release the neurotransmitter, for example, glutamate (an amino acid, shown as red balls in the figure).
- The glutamate molecules drift over to the outgoing side, and bond to big
proteins there (blue in the figure), called receptors. The chemical
bonding causes an electrical disturbance that raises the potential difference
(making it more positive) across the outgoing neuron's cell membrane. If
the potential increase is high enough, cell channels (proteins) open, and
allow a flood
of positive sodium ions into the outgoing neuron cell from outside the
membrane, which creates an electrical pulse.
- If the increase in membrane potential is bigger than the neuron's
threshold voltage, the neuron fires an outgoing signal spike. This
achieves typical firing.
- For a preferred path, we need frequent-firings. If the incoming neuron fires frequently so that the outgoing
neuron's membrane receives many bonding jolts in a short period of time, the jolts excite the outgoing neuron's
membrane long enough to elevate the voltage across the cell membrane for a
sustained time, which
activates yet another kind of receptor proteins (aqua in the figure), called NMDA
(N-methyl d-aspartate)
receptors.
- The NMDA receptors clear channels of blocking magnesium ions, allowing a small number of calcium
ions (green balls in the figure) to move into the outgoing neuron.
The calcium ions trigger a frenzy of catalytic activity. Enzymes
change the arrangement of the atoms within their molecules, usually by adding
a phosphate ion to them. This catalytic activity stimulates growth of new proteins, which
create new receptors and even new synapses. The net result of this
action is to raise the resting potential in the outgoing neuron's membrane for a long
period. The elevated resting potential makes it easier for an incoming signal to
exceed the neuron's firing threshold voltage and, therefore, to fire the outgoing
neuron. The synapse is strengthened, can fire more efficiently and a new
preferred path is created.
We've known for some time neurons make new proteins to trigger long-term
storage and strengthen synapses. But we haven't known how the
proteins do it. In 2004, Nobel laureate
Susumu Tonegawa
and his team at
Picower Institute for Learning and Memory at MIT discovered how the neurons make the needed
proteins. "There is a direct activational signal from the synapse to the
protein synthesis machinery," says Tonegawa. An enzyme called mitogen-activated
protein kinase (MAPK) provides a molecular switch that turns on increased
synthesis of a large number of proteins.
So, in summary, the brain stores information chemically by making proteins that
strengthen certain synapses, which establishes new patterns of neural networks,
and thereby a memory.
The brain retrieves a memory by firing those networks, perhaps across
different areas of the brain, to get the information. "Very limited cues
are sufficient to trigger a chain reaction that permits us to become aware of
the rich and detailed content of a memory," Tonegawa says. "This
phenomenon is called pattern completion because it reflects cellular processes
accompanying memory retrieval in which activation of a pattern of cellular
connections harboring memory is completed by very limited input."
(Answered March 26, 2007)
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