The way that signals work depends a lot on the
distance. If there is a person right next to you
that you're trying to talk to in this busy room,
then you can just whisper to them right here.
However if you want to get a hold of someone all the
way across the room, you might have to travel
across the room first, or you may even have to
yell across the room in order to signal to them.
Cells deal with this in much the same way.
So we have direct contact, paracrine signalling,
endocrine signalling and synaptic signalling. And
even autocrine signalling which we'll investigate here.
Direct contact, you'll recall we looked at gap
junctions in our previous lecture. Gap junctions allow
passage of material directly between cells. So, gap
junctions allow for direct contact.
The signal molecule doesn't have to go very far.
It can go straight from the cytoplasm of one cell
to the cytoplasm of the next cell. So the next
mechanism that we can look at is paracrine signalling.
And paracrine means sort of signalling your neighbour.
So if we were to use sort of an analogy here,
direct contact might be like holding someone's hand
and saying "Hey" you know, "I mean this.
Come over this way". Paracrine signalling would be
sort of whispering to them or you might shout
a little bit louder and reach them at a little bit
further distance. But still we're dealing with
neighbouring cells. So in paracrine mode of signalling,
one cell is releasing signal molecules in order to
signal a neighbouring cell. How far that signal can
reach really depends on how quickly the signal
is broken down or metabolized. So here, signalling
neighbour cells local or a little bit further away
but not too far away. Autocrine signalling is an
interesting thing. Auto means self.
So some cells actually signal themselves and this is
used in development fairly often where one cell
is going to secrete a product based on what's
happening developmentally to almost give itself
a checking balance and tell itself where it is
developmentally and whether it needs to continue
on this path or we're done with that stage. We also
see autocrine signalling a lot in the immune system.
The next mechanism we can look at is endocrine
signalling. Endocrine signalling is really pretty cool.
We have hormones that are released at a origin site.
For example we might release them from the
pituitary gland. And the hormones are released from
the pituitary gland. They'll flow through the blood
until it reach a target tissue, say the adrenal gland.
So the pituitary gland is dangling right of the bottom
of the brain and signal travels all the way through
the bloodstream and ends up eliciting a response at
any cell that has a receptor for it. So in the case of
the adrenal glands, it would have to have a cell
that had receptors for the particular hormone that's
dropped into the system. The final mode of signalling
that I'll introduce is synaptic signalling. This is
what neurons use to communicate with their cells.
The cell body will be located say at the spinal cord
and the axon will extend to a distant target.
And that axon will synapse on a target cell. So the
end of the axon will drop off some kind of signal molecule.
Perhaps it's acetylcholine. We're looking at muscle
contraction maybe. And then it will bind to the target cell
receptors and elicit an effect in its cell. So these
are cell surface receptors. When we were considering
the endocrine system, sometimes those are going to be,
almost often those are going to be
intracellular receptors for steroid hormones. So, tying
those two concepts together. So not that we've considered
how signals arrive at a cell, let's start thinking
about what happens after they arrive.
How do these signal transduction pathways work to
elicit some sort of cellular response?
So once a signal arrives at a cell membrane and binds
to the receptor, we could have a number of different
options. We could have either a single response.
For example if we have the glucagon receptor,
glucagon is all about releasing blood sugar. Either
way, whatever cell glucagon signals it should be
releasing blood sugar. So a single response of releasing
blood sugar. Or you could have a variety of different
responses depending on the cell type. For example,
in epinephrine, epinephrine causes a number of
different responses. Our eyes may dilate, our heart
may start to beat more strongly, we may start to sweat,
So each of those cells has a different response
to the same signal molecule whereas with glucagon,
well its all about releasing blood sugar
no matter which pathway you take to get there.
So many of the systems that we'll look at are second
messenger systems. We'll take a deeper look at those
in the next lesson although we're going
to introduce them briefly in this lesson.