How Synapse works
By: S.Raza Ali |
First steps towards
understanding the electrical properties of individual neurons. We learned how
electrical forces and diffusion give rise
to membrane potentials,
and we learned how cells can generate and propagate signals called action
potentials, or 'spikes', along the membrane. Understanding the properties of
the neuronal membrane is essential, but understanding just these properties
isn't sufficient to give us insight into the collective behavior of the
billions of connected neurons in our brains. Luckily for us, we can approach
neuroscience at many different scales and levels of analysis, and we don't have
to confront the full complexity of everything all at once. That's what we'll be
exploring throughout the rest of this course as we slowly go from our
understanding of single molecules such as ion
channels'..to the
electrical behavior of neurons...to their collective behavior in small circuits...and
finally onto how they become organized in large functional regions of the
brain. Let's start simple, though. Since we've examined one neuron, a logical
next question is how do two neurons connect? We'll first examine some basic
cellular anatomy of neurons. So far, we haven't made too much of the fact that
the majority of neurons are 'polarized' cells. That is, they have one portion
of the cell for receiving inputs and another portion for sending outputs. The
parts of the cells that are specialized for receiving inputs from other cells
are called 'dendrites.' The word 'dendrite' comes from the Greek word
'dendron', meaning tree, and as you can see the dendrites have a branching,
tree-like shape. A signal received by a dendrite is passed to the cell body. If
there is a sufficient depolarization of the cell body membrane to initiate an
action potential, then an action potential is sent down the axon. The axon then
carries the propagating action potential to another neuron. So what happens at
the boundary between two neurons, between the axon of one neuron and the
dendrite of another? This interface is called a 'synapse',
and it'll be the focus
of this lesson. There are two general types of synapses that we'll cover
in-depth later: electrical synapses, and chemical synapses. Electrical synapses
are less common in our nervous systems, but they're simpler to think about, so
let's start with them.
Electrical synapses are
pores between two cells that allow ions to pass through. They allow the passage
of that electrical signal
through to a neighboring
cell without much fuss. It's not so different than just combining two cells
into one larger
Cell. There are lots of
reasons that nature might need synapses like this from time to time -- they're
fast, and they allow cells to couple together with a high degree of
synchronicity. But most neurons are connected by a much more complicated
structure called a chemical synapse. In a chemical synapse, rather than simply
passing along an electrical signal from one cell to another, the action
potential travels to the end of the axon and causes a chemical to be released
into a very small space between the two neurons called the 'synaptic cleft'.
This chemical is taken up by the downstream neuron, on the other side of the
cleft. This chemical signal can cause the downstream neuron to depolarize its
membrane, converting the chemical signal back into an electrical one, or it can
have other effects on the cell.
This chemical step is
slower than transmission across an electrical synapse, but it opens up an
enormously diverse repertoire of different and more complex kinds of signaling,
and synaptic function plays a critical role in computations performed by
neurons. We'll spend the rest of this unit exploring the inner workings of chemical
synapses, on our way to beginning to look at how networks of interconnected
neurons give rise to behavior. We'll also look at the role of defective
synaptic physiology in neurological and psychiatric disorders,
and we'll see how
synapses can be targeted by various psychoactive drugs and poisons. Finally,
we'll wrap up by looking at how synapses can change with time in response to
external stimuli, playing a foundational role in how we learn and remember.
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