Hey guys. Dr. Brown here with another exciting histology video. You didn't get
to hear my dulcet tones in the last module, so I thought I'd include a little
self-made video here to walk you through the neuron. The nervous system, as we know,
has over 10 billion cells and the functional cells in those systems is the
neuron. So when we say the functional unit of the nervous system, what we mean
is the smallest piece of an organ. A functional unit is the smallest piece of
an organ, that's doing the job of the whole organ, and since it's the job of
the nervous system to communicate the neuron is the cell that facilitates that
function that is responsible for that communication. And they're able to do
this because neurons exhibit two special characteristics. Like muscle cells,
neurons are irritable or excitable, meaning they respond to an electrical
stimulus, or in the case of, you know, kind of the natural function of the human
body, they're gonna respond to an electrochemical stimulus, but the other
important characteristic that neurons exhibit is they're able to propagate
that signal from themselves to some other cell. And we'll talk about how they
do that later on in this lesson, but for now keep that in the back of your mind,
neurons are able to carry out their job, because they are both irritable and they
are able to propagate signals from one cell to the next.
A neuron has three main pieces or parts to it, so if we're gonna discuss the
general morphology of a neuron we need to know what those parts are. The trophic
center of the cell, that is the place where we find most of our organelles, is
here, and this part of the cell is called, most commonly, the soma meaning body. So
you'll hear me talk about neuronal cell bodies or soma, but your textbook uses
the term perikaryon, which means around "peri" meaning around. Karyose is
that Greek word meaning kernel, but in biology it's what we use to refer to a
nucleus. So eukaryotes have that true nucleus. So
Perikaryon literally means around the nucleus, and then coming into the
perikaryon are one or more branched processes. So this thing right here, this
thing right here, and these are called dendrites, which means little branches,
and the dendrites and the soma together make up what we call the receptive
portion of the cell, meaning this is the part of the cell where electrochemical
signals are coming in. So the dendrites and the soma are where signals are
coming into the cell. Now if the amount of signaling coming into the cell if the
stimulatory signaling reaches a certain threshold amount then the cell will then
propagate that signal to the next cell down, and it will do that via this long
process extending out from the cell, and this little guy here is called the axon.
A neuron can have many dendrites it can have as few as one or it can have many
hundreds of dendrites, but generally you only see a single axon coming out of the
neuron. The axon is considered the conductive segment, because it is the one
that is conducting this electrochemical signal from the soma of this cell to the
next cell down, but the actual transmission of the electrochemical
signal is going to happen at the tip of the axon, at a special cell-cell junction
known as the synapse. N-A-P-S-E. Spell it right. So the synapse is the actual
transmissive portion; this is the part that...M-I-S-S-I-V-E...pardon the handwriting.
So the synapse that's actually going to transmit the signal from this neuron to
the next cell down, and that next cell down the line maybe another neuron, it
may be a muscle cell, it may be a glandular a secretory cell, but the...the
synapse is how the message gets from this neuron to the
next cell down. So let's look at some different ways that we can classify
neurons. The first way is morphologically. We can classify them based on their
shape. The most common shape you're gonna find, and we'll just sketch it out here,
is a neuron with multiple dendrites, and a single axon. Something like this. So
there's the axon terminus it's branched, and here we have you know dendrites
coming in, and dendrites can be highly, highly, highly branched it's called
arborization. We'll learn more about this as we're learning about the details
about these...these specific structures. So there'll be another video where we're
learning about the details about dendrites, and axons, and soma, etc.
and there's our nucleus. So because this neuron has many dendrites attached to
the cell body, as well as the axon coming out of the cell body the cell body has
many poles. So we refer to this as a multipolar neuron. These are the most
common neurons. Ninety nine percent of the time when I show you a picture of a neuron in one
of these videos or in your lessons in class, it's gonna be a multipolar neuron.
These are by far the most common neuron type found in the nervous system. One of
the least common neurons, but still very very interesting is one where we have a
single dendrite coming into the cell. We have our soma here. There's our nucleus,
and then once again we've got an axon coming out. So the cell body, or soma, or
perikaryon of this cell has two poles, right? It has a dendritic pole here, and
an axonic pole here. So we would refer to this cell as bipolar neuron. These are
really rare, probably the best known example are the bipolar cells in the
retina of your eye, and these are responsible for transmitting some of the
visual information between the rods and the cones, and the cells that actually
make up your optic nerve. And then finally, the really another slightly more
common...Oh...not as common as multipolar cells.
Are cells in which we have a single dendrite going into a single axon like so,
and the cell body soma...perikaryon...whatever you want to call it, sits off to
the side. So there's our axon terminus there. Here's the dendritic fibers coming
in, and then here's our soma off to the side,
there's the nucleus. So this neuron we only have a single pole attached to the
soma. So you would think we would call this a unipolar neuron, but that would
imply that there's either only an axon or only a dendrite, and in fact there's
both, a dendrite and an axon. The cell contains the same components as the
bipolar cell, it's just that the perikaryon is stuck off to the side. So it's
not truly unipolar, so we're gonna call this a pseudo unipolar neuron. These are
really common sensory neurons, especially like in the in the ganglia just off the
side of the spinal cord, what we call the dorsal root ganglion. We find the cell
bodies of a lot of pseudo unipolar neurons, and these are bringing in
general sensory information, often like touch information from the periphery. So
whenever you touch something and feel it, you can know that it's a pseudo unipolar
neuron that's bringing that touch information from the touch receptors in
your skin to your brain. So those are morphological classifications when
classifying the neurons based on their shape. We can also classify neurons based
on their function or physiology. So physiologically we can have sensory
neurons. This image is a confocal micrograph of the nasal mucosa, and up in here we
can see those are the cell bodies here, these are the dendrites coming into the
cell, and you can't hardly make out that there are axons back behind all of this
fluorescent signal that are coming out here. So these are sensory neurons,
meaning that they are bringing information
about the external environment into the central nervous system. You can also have
sensory neurons that tell the central nervous system about the internal
environment. For example what is your blood pressure, but sensory neurons are
any neuron that is coming into the CNS. So we refer to them also as afferent
neurons, because they are traveling toward the CNS the counterpart to these
are motor neurons. Now their name implies that they're going to some kind of
muscle, and in fact many of your motor neurons are in fact going to muscle just
like the one we're looking at here. This is a lower motor neuron from the spinal
cord. So this is a neuron whose axon would eventually become part of the
nerve that goes out and tells a skeletal muscle to move. So these neurons are
operating under voluntary control. But a motor neuron is any neuron that is going
outward, that's leaving the CNS, and going to some sort of peripheral effector. So
motor neuron is synonymous with efferent neuron, or I tend to pronounce these
afferent and efferent not because that's the correct pronunciation, but because of
my accent. It's often difficult to sell which one I'm talking about unless I
really put a lot of emphasis on that first letter, so the reason that I
pronounce these so hard is due solely to my redneckery and has nothing to do
with proper scientific pronunciation. So we have sensory neurons coming into the
CNS, we have motor neurons going out of the CNS, and we also have interneurons
that connect other neurons within the central nervous system. So interneurons
are contained almost entirely in the CNS, and in the simplest neuronal circuits,
that is, you know, a neuronal circuit is where we have some sort of input a
decision is made and then some sort of output results from that, as those
simplest neuronal circuits contain a single sensory neuron, a single inter-
neuron, and a single motor neuron. So interneurons are inside the CNS, and
their conduct their connecting other stuff and things. And
finally we have neurosecretory cells. So these cells we're seeing here inside the
adrenal medulla are neurons that are not so much for conducting electrochemical
signals as they are for, as their name implies, secreting something. So the
adrenal medulla contains neurosecretory cells that secrete epinephrine and
norepinephrine as hormones. There are neurosecretory cells in the
hypothalamus of the blo...of the brain that make hypothalamic hormones as well
as posterior pituitary hormones, but any neuronal cell that has specialized for
secretion is now called a neurosecretory cell. So in review a single
neuron is going to have a soma, it's gonna have some dendrites coming in,
it's gonna have an axon going out. The soma, or perikaryon, or cell body, and
the dendrites make up the receptive portion. The axon makes up the conductive
segment, and then the synapse at the end of the axon would be the transmissive
segment, and then we can classify neurons either morphologically as multipolar,
bipolar, or pseudo unipolar. There is a fourth kind of neuron called an anaxonic
neurons, but we're not going to get into them, because as far as I know nobody's
figured out exactly what they do just yet.
And then finally we can classify neurons physiologically as being sensory or
afferrant...afferent...pardon the accent. They can be motor or efferent...efferent,
they can be interneurons, connecting sensory and motor neurons in the CNS, or
they can be neurosecretory where they're usually secreting some sort of hormone.
So that's it for now that's kind of our introduction to the neuron. In the next
video we're going to dive right in, and get into some of the more specifics of
the functional neuronal anatomy. So we're gonna look at how the dendrites, the soma,
and the axon gain their functionality. So we're gonna kind of get into some ultra
structure stuff and things. So I hope that was informative, and you had a good
time. Study hard, and we'll see you later.