Introduction to the Nervous System Part 1 – 2.13

Hi. My name is Dr. Matt Ivory.

I'm one of the lecturers in the school pharmacy and

pharmaceutical sciences at Card of University.

In this series of talks,

we're going to look at the nervous system in the body and

we'll look at the different cells that make up the nervous system.

We'll look at how they transmit signals to one another as well.

We'll have a look at the different structures in the

nervous system, particularly the central nervous system,

the brain and the spinal cord,

and we'll look at some of the issues in identifying problems

and resolving those problems in these tissues.

So in the first talk,

we are going to have a look at the different cells involved in

the nervous system,

and we'll look at how they perform their role of signaling within the body.

So all organisms need to be able to detect and react to

changes in their environment.

And these are called stimuli, which is the plural of stimulus.

And they need to do so in order to survive.

So you think about If an organism needs to find food, it

needs to see the food, detect the food and move towards it,

to access it, or it needs to be able to detect danger,

whether that's chemical danger or a predator,

and escape from that if it's going to survive.

So,

single celled organisms will have receptors on their cell

that allow them to detect these things and trigger a response,

so it might be that it moves the bacterial cell away from

danger or towards food.

But in multicellular organisms,

we can't have these receptors on all of our cells because we

need all of our cells to work together.

And it might be that only some of ourselves detect certain stimuli.

So if you think about our eyes detect light and so if we see something,

the cells of our eyes need to be able to transmit that signal

to the rest of the body so that it can respond.

And it's also important that the cells of the body all do

the same thing, so the body acts as one organism.

So you think if you were being chased by a predator and one of

your legs ran away, but the other one stayed still.

You're just going to run-in circles and you're going to be

caught by that predator very quickly.

So there needs to be some way that the body can coordinate

all of its different tissues, all of the cells,

all of the organs that all push in the same direction and

coordinate movement and different responses.

So the nervous system is one of the ways that body does this,

and the other is the hormonal system.

So we'll talk more about the hormonal system in a different set of talks.

But we'll cover the nervous system in this set of talks.

So, nerve cells are also called neurons,

and these are the wiring of the nervous system.

And they transmit signals in the form of electrical impulses.

And if you combine many neurons, you get nerves. So

nerves are made out of many nerve cells.

So nerve cell is made up of a cell body,

which contains a cell nucleus

and long extensions that extend from that cell body.

And so these extensions are what allow the signals or impulses

to be carried along neurons.

And the extensions have different names depending on

where they are on the cell and their purpose.

So there are dendrons and dendrites and there are also axons.

So, signals are received from either sense receptors or other

neurons, and they are transmitted towards the cell

body. By either a dendron or dendrite.

And the nerve signal is then carried away from the cell body down an axon.

So that's one way to differentiate between the two

different types is the direction of the nerve impulses travel.

The length of the neuron,

the position of the cell body within that length,

and the type and the number of dendrites present,

will vary between the different types of neurons found in the

body and the role that they have within the nervous system.

So, sensory neurons have a long dendron,

and they have dendrites branching from it that connect

it to receptor cells, and a central cell body within the

neuron and then a long axon that travels away from that,

which forms synapses with relay neurons.

Relaying your own are much shorter and instead of

possessing a single long dendron,

have multiple dendrites that extend from the cell body.

They do have an axon and this does form synapses with other

relay neurons or with motor neurons.

And motor neurons have multiple pull dendrites that extend from

their cell body and along axon that forms synapse with an effector.

And we'll talk a bit more about all these different terms,

so sensory relay and motor neurons and effectors and

sensory cells, a little bit later in this talk.

So sometimes the axons and or the dendrons of a neuron are

encased in a fatty layer. And this is known as a myelin

sheath. So for this reason,

neurons that have a myelin sheath are called myelinated neurons.

And the myelin sheath is really important because it acts to

insulate the neuron, so it prevents signal from leaving

the cell. And if you think about

a coated wire,

so you often have copper wire that's coated in plastic,

and that insulates the wire.

And it means that if you touch the wire on the plastic bit,

you don't get an electric shock.

The signal keeps traveling down the wire in the direction that

it's supposed to. In that way,

the myelin sheath helps to insulate the nerve cell and

ensures that the impulse isn't lost to the environment that

the cell sits in.

There's also just like when you touch an uncoated wire and you

get an electric shock,

it means that if the length of the neuron comes into contact

with another neuron,

it's not going to accidentally transmit that signal and

trigger a response in another neuron

that isn't needed.

So, nice insulation of the signal,

make sure it gets to the place it needs to be.

Without a loss of its power.

Each part of the sheet, so it's not one continuous covering.

There's little sections of the sheath,

and each section is produced by a separate cell. And so,

where there's two cells next to each other,

there'll be a small gap in between.

And so these little gaps in the myelin sheath allow impulses

to jump down the neuron.

So instead of traveling the entire length,

of the neuron in one go. It jumps in between the gaps in

the myelin sheath. So, by doing this, it increases the speed.

At which the signal can move down the neuron and therefore

increases the speed with which a signal is transmitted from

one end of a neuron to another.

So I touched upon earlier about how some of the different types

of neurons need to transmit signals between each other. So

from a sensory neuron to a relay neuron for example.

And the way that they do this is at the point where they meet.

So instead of them being directly connected and signal

traveling straight from one cell into another.

There's a small gap between the two neurons.

And this small gap is called the synapse.

Substances are released from the first neuron at the synapse

and these substances are called neurotransmitters.

So, at the gap in a synapse, neurotransmitters

are released from the first neuron,

so the neuron where the signal has traveled down,

so it's traveled down its axon. To reach the end of the axon,

which is called the axon terminal. And from the axon

terminal, neurotransmitters

are released into the synapse.

And into the gap that's there.

So the neurotransmitters

diffuse across the synaptic gap. And they are picked up by

the neuron on the other side.

And, this signals that next neuron to initiate a nerve signal.

And it'll keep the nerve signal moving on from one neuron to the other.

So, it does come at a cost. Obviously,

it's slower to have an impulse stop.

And have these neurotransmitters released, and

then absorbed, and then start a new signal.

But it ensures that nerve signals only travel in one

direction, so they can only move one way across a synapse

because axon terminals

aren't sensitive to the neurotransmitters they

released. Otherwise,

they just send loads of signals back the way they came.

So, they release neurotransmitters,

which are picked up on the other side,

but not on their own side.

It also ensures that signal strength doesn't diminish if a

signal is transmitted,

an impulse is transmitted over a much longer distance.

Or over multiple neurons because if you think,

if you have a very long length of wire and you're transmitting

a charge down it, the longer the wire,

the more diminishment of the charge would occur.

So by having these multiple synapses

and by creating a fresh impulse at each of the synapses,

you ensure that the nerve signal doesn't diminish and you

don't end up with a really weak signal that's not going to do

the job that's required of it.

So, for this reason,

if you are transmitting along a single long neuron, it's much

quicker. There's a risk of signal diminishment, of course,

but It's much quicker than if you have multiple small neurons

over the same distance.

And that becomes important in things like reflex arcs which

we'll cover in more detail later.