A Level Biology_ Nervous System 2 - Reflex Arc
Thanks for joining me again.
This is my second video to do with nerve cells and to do with the nervous system.
Last time, we were looking at how the nervous system was organized.
The first thing we’re going to look at today are the three types of nerve cells or neurones.
The first type is called a sensory neurone.
Now, a sensory neurone, that takes an impulse from a receptor which is something that detects
a stimulus and takes it to our central nervous system.
If you remember from last time, your central nervous system is made up of your spinal cord
and your brain.
The second of which, which is a relay neurone which carries messages from one bit of your
central nervous system to another.
This is usually shown in diagrams on a spinal cord.
The motor neuron is one that carries it from the central nervous system to an effector.
Now, an effector is something we haven’t come across yet.
An effector is an organ which brings about a response.
So, for instance, it might be the muscle that might contract or it might be the muscle in
the iris that causes the people to dilate or something of that nature.
Reflex arc, essentially is we’re going to discuss just usually how the three types of
neurones can all work together.
If we use a situation if you put in your hand on something hot and cause in a reflex where
you basically whip your hand away from the hot object very, very quickly, we can see
how these three types of neurons interact with one another.
If we think about the first thing that would happen if you put your hand on, let’s say,
a hot plate, the first thing that would happen is that’d be picked up by receptor; that
receptor might be the temperature receptors in your hand.
If the temperature receptors in your hand would then generate an impulse in the sensory
neuron, that would send that impulse along with the sensory neurone through the cell
body and along with dendrites, you’ve got this section here which is a gap between the
sensory neurone and the relay neurone and that’s known as a synapse.
The synapse is something that which we’ll come on to discuss later possibly in the later
video.
But that impulse would then be generated in the relay neurone and, again, it would pass
through here, pass through the cell body, down to here along with the dendrites, and
again, to another synapse which, again, is another gap.
That can then generate an impulse in the motor neurone which travels down to here.
If you remember, the motor neurone connects up to the central nervous system to an effector.
So, what would happen would be that your hand would lift up.
The motor neurone, which is connected to the muscles in your arm, would cause the muscle
to contract and you’d lift your hand away.
With the reflex arc, the first thing that happens is the hand comes into contact with
something hot and in this case, something hot is the candle.
The stimulus, which is the change in the environment, which is the heat in this case, that’s picked
up by a receptor in the hand or a pain receptor in the hand and this can then send the impulse
up to sensory neurone towards the central nervous system.
Now, the central nervous system is the brain and the spinal cord.
This bit of the diagram represents the spinal cord.
It’s the impulse come in through the sensory neuron, there’s a gap here and this gap,
again, synapse.
The next step is the impulses then generate it into a relay neurone, which is this green
one here.
This relay neurone forks into two sections: one, that goes up to the brain and that triggers
a center in the brain which relate to pain, and the second part relays it towards the
motor neurone.
So, this is only going through the spinal cord.
It then gets to another synapse that which generates an impulse which goes through the
motor neuron and arrives at the effector.
The effector, in this case, being the muscle.
This causes the muscle to contract and it pulls the hand away from the heat.
This is known as a reflex arc because it doesn’t involve the brain; it only involves the spinal
cord.
To sum up then, your sensory neurone, those connect up by your receptor to your central
nervous system, which then it connects up to a relay neurone which connects parts of
the central nervous system.
Again, these are mostly represented in diagrams within the spinal cord.
You then have motor neurones and those connect up the central nervous system to an effector,
and an effector is something which carries out an organ which carries out a response,
something like a muscle or a gland.
Synapses, which are the gaps between neurones we’ll come on and look at those in the next
video.