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