In the last module, we took 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 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 with one another?
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 actually 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 own nervous systems,
but they're simpler to think about, so let's start with them.
Electrical synapses are basically pores between two cell
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 together 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 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 synapse can change with time,
in response to external stimuli, playing a foundational role in how we learn
and remember.