Nervous System [2]: Reflex arc (A Level Biology)

Uploaded by freeeschool on 02.09.2012

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