You know – sometimes we forget how
different the cells in the body can be – we kind of imagine them as all as
little circle blobs when in reality, there is so much body cell diversity.
Parietal cells in the stomach as part of the digestive system – they can make stomach
acid! Thankfully, cells in other systems do not. Mast cells as part of the immune system
– they contain substances like histamine that they can release which is critical
for the inflammatory response. Skeletal muscle cells -which are also called muscle
fibers – as part of the muscular system, they’re shaped like cylinders with
multiple nuclei – and their structure includes thin and thick filaments which
are essential for muscle contraction. We could on with all the specialized
cells in all the body systems and the cells themselves structurally sure
are different – specialized for their function. And if I had to pick my favorite
specialized body cell – it’d be a neuron –a cell that is part of the nervous system.
The system that is the topic of this video. But before we talk about neurons
or other cells in the nervous system because it’s not just neurons – let’s give
a little general tour of the nervous system. Then we’ll get to cells of the nervous system
and briefly mention the action potential! First, structure wise, you can divide the nervous
system into 2 very general regions: the central nervous system (CNS) – which consists of the brain
and spinal cord - and peripheral nervous system (PNS) –which consists of all other components of
the nervous system -such as nerves throughout the body. The PNS can provide sensory information for
the CNS while the CNS can process that information and act as a command center – the CNS can execute
motor responses or regulate body mechanisms. So, we said the CNS consisted of the spinal
cord and brain. Let’s talk a bit about the amazing human brain – although realize we
are being very general here – as we are going to divide it into 3 general regions:
the hindbrain, midbrain, and forebrain. Let’s look at the hindbrain first.
It includes the medulla, pons, and cerebellum. The medulla has many regulation
functions such as the regulation of breathing, blood pressure, and heart rate. The pons is
involved with some of these type of functions as well and also coordinating signals with
this area to the rest of the brain. And the cerebellum? Balance and movement coordination
are some functions of the cerebellum. The midbrain: deep in the brain, this area is
involved in alertness and the sleep/wake cycle, motor activity, and more. If you’ve heard
the term “brainstem,” this includes some of the structures we just mentioned: the
medulla, pons, and midbrain specifically. Finally, the forebrain. Most notably, this
includes the cerebrum, which itself is divided into two hemispheres: right and left.
So many functions are done by our amazing cerebrum depending on specific location whether
it’s our speech, our thinking and reasoning, our sensing, our emotions – check out
the further reading to explore this! The forebrain also technically includes some
structures in it like the thalamus – which is involved with sensory and motor information-
and hypothalamus – which if you remember from our endocrine system video, has
major control of the endocrine system. There are a lot of myths about the brain. One
quick myth I heard all the time as a kid that I’d like to put to rest. It’s the myth that
“humans only use 10% of their brain” – it’s not correct. We have a great reading suggestion on
that as well as some others that circulate around. Now, that was all the central nervous system
(CNS). What about the peripheral nervous system or PNS? Functionally speaking, we can
further divide the PNS based on what it does. The somatic nervous system (SNS) and autonomic
nervous system (ANS). The SNS is involved with motor functions of skeletal muscle. This will
include voluntary actions under conscious control but also somatic reflexes that involve
skeletal muscle. The ANS is all about what’s going on in the internal environment in
regard to gastrointestinal or excretory or endocrine or smooth and cardiac muscle
and it also includes autonomic reflexes. And the ANS itself can be further divided –
I know, I know, there’s a lot of dividing but stay with me – the ANS can be divided into the
sympathetic and parasympathetic systems. The sympathetic system – the shorter word of the two–
helps me remember it’s part of the quick fight or flight response. I know the whole running
from a bear is a very popular example. For me, it’d be more realistic if I was face to face with
my personal nemesis: the copy machine which I may or may not have had some very bad experiences
with before – and the warning bell just rang so you now know you have 60 seconds to get your
copies – but it’s making crazy machine noises and giving you vague warnings– this also could
activate your fight or flight response. A response that can cause your heart to race and breathing
rate to increase and some things to not be active: like the digestive system. Because if you’re
desperately trying to run from a bear or take on the copy machine, you don’t really need to be
digesting your food at that very moment…right? The parasympathetic system – longer word
– this is often called rest and digest. Heart rate will decrease, digestion will
occur – again, rest and digest. Many times, these two systems can therefore have
opposite effects on the same organ. So let’s talk about two major types
of cells in the nervous system that makes up nervous tissue. That means
these are cells that you’ll find in the central nervous system and
the peripheral nervous system. Most of the time, neurons are what come to mind.
There are different types of neurons but to focus on general neuron structure: you have the cell
body – the nucleus and most other organelles are here. There are dendrites, generally
these branched structures are where signals are received. And you have an axon – I like
to think away axon! – because axons are the fiber where normally a signal will be carried
away to some other cell. The junction area where the neuron will be communicating
with another cell is called a synapse. And the other major cell type? Glial cells. Or
you can call them glia. When I was a student and read that they were supporting cells – I
don’t think the word “supporting” emphasized to me at the time how essential they really
are. Structurally, there was a lot of emphasis on how they actually help the neurons connect
in place – the word “glia” comes from a Greek word that means glue. But glia have huge roles
and they are SO much more than that. Some glial cells keep a balance of certain chemicals
in the space between cells – essential for signaling – and maintain the blood-brain barrier
which keeps a lot of substances in the body from getting into the nervous system. Some glial
cells make myelin – which goes around the axons of neurons as something called a myelin
sheath - insulates the axon and transferring of the signal. Some glial cells produce
cerebrospinal fluid which is protective to the brain and essential for homeostasis -
as well as many other critical functions. Some glial cells have important immune function in the
nervous system. These are all just a few examples. As amazing as glial cells are, it’s time to
move on to the action potential. Generally, action potentials are recognized as something
neurons do – but we did link some interesting reads about certain glial cell types and
action potentials. We’re just going to touch on what an action potential is but we may have
a future video to go into more detailed steps. The main idea is that neurons need to be able
to communicate with each other. And to do that they’ve got to be able to receive a signal in the
dendrite and carry it down the axon. And they need to do that fast – like less than 2 milliseconds
fast. The action potential makes that possible. We can’t talk about an action potential without
talking about when the neuron is at rest – meaning when there is no signal being carried – at
rest, a neuron has something called a resting potential. The resting potential of a neuron is
more negative than its surroundings – in fact it can be measured – it generally is around
-70 mv. Yes, mv, which is millivolts- it has an electrical charge. That’s because there are
ions involved inside and outside of the cell: ions like chloride (Cl-), sodium (Na+), potassium
(K+), certain anions (A-). Specifically, sodium (Na+) and potassium (K+) play huge roles
in keeping the resting potential – they should sound familiar because we talk about the sodium
potassium pump in another video and that is a pump that helps maintain a neuron at resting potential.
At rest, generally the sodium (Na+) concentration is higher outside of the cell and the potassium
(K+) concentration is higher inside the cell. How can we remember that? How about it’s Kool
to be K+ resting in the cell. But overall, at rest, the neuron is more negative
inside compared to its surroundings. So let’s say the dendrite of the neuron receives
a signal. This can generate an action potential along the axon. An action potential is going to
rapidly change the charge in the neuron along the axon - the signal carries from one area
of the axon to the next. Ion channels open allowing Na+ to flood inside the first region
of the axon. Recall Na+ is a positive ion. This event is called depolarization – as the electric
charge is becoming more positive in the axon as Na+ floods in and most K+ channels at that moment
stay closed. This spreads to the next region of the axon and carries along. But as the action
potential spreads to a new region of the axon, the old region where the action potential
already occurred will start to be restored back - to learn more about the different channels
that open and close to achieve this amazing feat – or specific events like the undershoot or
refractory period- check out the further reading links in the video description. Eventually we
hope to have an entire video on this process. Two things to point out about this
action potential. 1. If neurons are myelinated – meaning they have myelin
sheaths that insulate the axon and assist with the transfer of the signal
– the action potential can actually jump from node to node – the nodes being
areas of where it’s not myelinated. 2. Important to realize, the action
potential is considered an “all or none” thing. What we mean by that is that it
either happens or it doesn’t – like a light switch it’s either on or off – there isn’t a
dimmer switch, there aren’t different levels, it’s either off or it reaches a threshold
of when it’s on and if it’s on, it’s going. So that’s all good but what happens next? Let’s
say you have an action potential and it’s going to signal another neuron – how? Well that’s one
way to introduce neurotransmitters. So the action potential goes down the axon and gets to the axon
terminals – the ends of the axon. We had mentioned there is this space called a synapse which
consists of the area between the two neurons. The action potential can signal synaptic vesicles
in that neuron to release something called neurotransmitters. There are different types
of neurotransmitters and they can be derived from different substances: for example, amino
acids or amino acid precursors. Or even a gas such as nitric oxide although the release
is different than other neurotransmitters. Generally, when neurotransmitters are
released from the synaptic vesicles, the neurotransmitters only need to travel
a small space between the neurons specified as the synaptic cleft. Then they can
bind specific receptors of the next neuron – specific receptors to the type of
neurotransmitter that binds it. The dendrite area of the other neuron receives the signal and
can start an action potential across its axon. When we cover a lot of things, we
think it’s important to recap: so, we’ve talked about the peripheral
nervous system (PNS) and the central nervous system (CNS). Since the CNS
includes the spinal cord and brain, we also talked some about major areas of the
brain. Then we focused on the PNS- how it can be divided into the somatic nervous system (SNS)
and autonomic nervous system (ANS) and then how the autonomic nervous system (ANS) can be divided
into the sympathetic and parasympathetic system. We then explored major cell types in the
nervous system: glial cells and neurons. And since neurons can communicate with
each other using an action potential, we gave a brief overview of the action
potential. We then mentioned that once the action potential occurs, this can signal
the release of neurotransmitters in the synapse between neurons. Those neurotransmitters bind
specific receptors of a neighboring neuron. Phew! So, with such a complex system that could be
so many videos long –there continues to be a lot of research done to help diseases and
conditions of the nervous system. If you have an interest in this field– there
are many careers involved in neurology to explore. Well that’s it for the Amoeba
Sisters, and we remind you to stay curious.
