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.
