cytoplasm all the organel all the things
that you would
expect in a ordinary cell
body now if you think back about what we
said about the definition of axons and
dendrites in the case of the motor
neuron we can now apply these names to
the sensory
neuron remember we said a
dendrite is any fiber which can cares
impulses towards the cell
body and in the case of the motor neuron
the dendrites were
short but if we apply the same
definition to the sensory neuron that of
a dendrite being anything which carries
information towards the cell body then
the whole length of this must be the
dendroid so this is the dendro
carrying
information from the periphery towards
the cell
body that's why it's referred to as the
dendroid remember the definition of an
axon an axon is anything which carries
information from the cell body
away so a dendr is carrying information
to the cell body an axon is carrying
information away from the cell body
therefore this is the
axon in the case of a sensory
neuron now just like
the motor
neuron the sensory neuron is covered
with a my and
sheath that
protects
insulates and
nourishes again the myelin
sheath is made up of Schwan cells just
as in the mot neuron schan cells which
are wrapped round about the
axon to comprise the myelin
sheath so just as in the motor neuron
Schwan cells comprising the m and
sheath just as in the motor neuron gaps
between the mile and
sheath the gaps are referred to as the
nodes of Ramia just as they are in the
motor
neuron so very many of the components
are the
same and the key thing with the neuron
is to remember the
direction that the impulse is
transmitted
in now we said the impulse is generated
in the
periphery it's then carried in this
direction along the
dend towards the central nervous
system actually goes into the cell body
sometimes here then back
out
and on toward Wards the central nervous
system so the dendrite carrying the
impulse towards the cell body the axon
carrying
it away from the cell
body the other major components the same
as in the motor
neuron
now just before we uh pass on from these
diagrams there's another important point
to note and it relates again to the
function of the my and
sheath
now to make the impulses travel more
quickly
along the nervous impulse the nerve
impulse does not travel along the full
length of the neuron but what it does is
it actually
jumps from one node of Ramia to the
next it's a little hard to understand
this process it's a little bit like when
you skim a stone across the top of some
water and it
skips like that the impulse Skips from
one node of Ramia to the
next and this greatly
increases the rate at which the nerve
impulse is
transmitted the process of this jumping
conduction is referred to as saltatory
conduction
saltator Rec conduction or
saltatory
transmission now we can note at this
point that there's two factors that
determine how quickly a nerve impulse is
transmitted in a nerve
fiber the first factor is the diameter
of the nerve nerve
fiber The Wider the diameter of the
fiber the faster the nerve impulses
transmitted the more narrow the diameter
of the fiber the more slowly the nerve
impulse is
transmitted and some uh animals for
example the
squid have very very wide diameter axons
to facilitate their skatee response when
they sort of swish to get away from
things very wide axons so the impulse is
transmitted very
quickly but no matter how wide you make
the axon well you can't make it that
wide for practical reasons but no matter
how wide the axon is the rate of
transmission is not as fast as if the
nerve fiber is
mated because when the nerve fiber is
mated saltatory conduction can take
place and nerve transmission is much
much quicker
so the second factor which affects the
rate of transmission after the diameter
is whether that fiber is mated or not
mated fibers transmit nerve impulses
much more
quickly because they use the process of
saltatory
transmission
now let's uh think about this neuron
now which we haven't mentioned
yet and uh we'll refer to this as a
relay
neuron because we're going to assume
that what we're going to look at now is
is a reflex arc we'll assume this is
part of a reflex
arc now let's think about what we mean
by this term
reflex if you touch something hot or
sharp you pull your hand away
quickly or if you're walking along and
one leg gives way because you twist your
ankle or something the other leg
automatically straightens up so you
don't fall
over these are things that protect the
body from damage that we don't think
about
they are reflexes we just do them we
become aware of them only after the
reflex activity has been carried
out this is because in physiological
terms a structure called a reflex arc
has been
used and what we're going to do now is
look at what we mean by this term reflex
arc
a reflex
arc and this is a useful exercise
because what it does is it combines what
we know about the sensory neuron and the
motor neuron and it puts them together
to form a reflex
arc now let's imagine you touch
something very hot and you burn your
fingers what sort of neuron is going to
carry that information to towards the
central nervous
system well that information will be
picked up in a peripheral sensory
receptor so we can start off by drawing
some peripheral sensory
receptors and they join together to form
a sensory
neuron so the next thing we can draw is
the