As you watch this video, pay attention to the fact that you’re able to keep your body
in balance, focus on the screen, listen to the words, feel the device with your fingers,
and interpret all of this information, while your heart, lungs, kidneys, liver and other
organs are simultaneously doing their job.
For this, you can thank your nervous system!
For most of us, the nervous system is almost synonymous with the brain.
We think of the brain as the seat of consciousness and the master controller of the body, calibrating
every organ and their particular jobs, right?
Well… not quite.
The nervous system is so much more than the brain.
It is a system, after all, whose main job can be simplified to this: information processing
and communication from one part of the body to another, like telephone wires that help
us communicate across districts, cities, countries and across oceans.
So, what is the nervous system made of and how does it work?
For a living thing to live, it must sense the world in which it lives, interpret it,
and respond to it accordingly.
In most multicellular animals, some form of a nervous system does this job.
It senses the world, communicates this information to a processing facility, which then tells
the rest of the body how to respond to the world.
In the human body, the nervous system has several parts that we’ve broadly divided
as follows.
In the midline of the body lies the central nervous system or CNS, consisting of the brain
and spinal cord.
The CNS is where information is processed and integrated—the master controllers.
Branching out from the CNS is the peripheral nervous system or PNS.
It connects the CNS to the outside world and the body.
The PNS is further divided into the sensory portion of the PNS, with neurons that sense
the external world, and the motor portion, which controls the various muscles and organs
in the body.
The motor portion can be divided into the somatic nervous system or SNS and the autonomic
nervous system or ANS.
The somatic nervous system controls voluntary movements and sensory perception, like sitting
up and walking.
The autonomic nervous system, on the other hand, regulates involuntary actions, such
as your heartbeat, digestion, and breathing.
We’re not done yet.
The autonomic system is further divided in the sympathetic nervous system and the parasympathetic
nervous system.
You’ve experienced the effects of these two if you’ve ever been on a rollercoaster
and then sat down to catch your breath.
The sympathetic nervous system is central to your fight or flight response, increasing
your heart rate, your breathing, and making your muscles contract.
The parasympathetic nervous system serves the opposite function, controlling the rest
and digest response.
It calms you down and returns your body to homeostasis or balance by lowering your heart
rate, relaxing your muscles, and diverting energy back to your gut.
Interestingly, your intestines may also have a mind of their own.
The enteric nervous system is thought to work independently of the CNS, in some instances!
Now, what makes up and connects these parts of the nervous system?
There are two types of cells that make up the nervous system—the neurons and the glia.
The almost 86 billion neurons in a human body are essentially the wires that send information
across the nervous system.
We have a video on how neurons work, which you can check out up here and down in the
description of the video.
But briefly, the neurons look a little like trees.
They have a head region called the cell body, along with several branches.
The smaller branches—called dendrites—receive signals that are combined and the total signal
is then transmitted through the longest branch—the axon.
Similar to how a wire carries electricity from a switch to an electric bulb, the axon
carries the nerve impulse from a cell body to the next cell.
Neurons often transmit signals over extremely long distances.
In fact, the sciatic nerve is the longest nerve in the body, running from the lower
back to the heel of the foot.
A nerve is the term we use for a bundle of neurons located in the PNS.
If you slice and look into the brain, you will see a lighter region and a darker region.
This is white matter and gray matter.
Gray matter on the outside is where all the cell bodies are, whereas the axons of those
cell bodies form the white matter.
The next type of cell found in the nervous system is the glia or glial cells, of which
there are many types.
We have another video dedicated to these fascinating and often underappreciated cells.
We have a video that explores what different glial cells do, but here is a brief overview.
Oligodendrocytes are the most abundant glial cells.
They’re present in the CNS and wrap themselves around the axons of neurons to provide insulation.
This helps the neurons pass electrical signals rapidly.
Schwann cells provide insulation to neurons in the PNS.
This insulation, called myelination, is why neurons appear white.
Astrocytes, the second most abundant glial cells, are present in the brain and do a host
of jobs to ensure that neurons can function at peak performance.
Lastly, microglia are the CNS’s immune bodyguards.
The brain is a VIP organ, and very few things are allowed to enter its domain, including
white blood cells.
The microglia is basically the brain’s immune system.
The nervous system works by transmitting signals from one neuron to another.
This chain is called a neural circuit and we can classify neurons based on what they
do in the circuit.
There are two types of neurons: Afferent and efferent.
Afferent neurons receive information, and pass it towards the spinal cord and brain.
Efferent neurons receive information from neurons in the CNS and pass it away from the
CNS to the periphery.
There are also interneurons that serve as the in-between of afferent and efferent neurons.
One of the easiest neural circuits to understand is the reflex circuit, which only requires
three neurons—the afferent sensory neuron, the interneuron and the efferent neuron.
Consider this classic example: imagine that you touch something hot.
Sensory neurons in your skin will sense the heat and transmit that signal to a neuron
in the spinal cord.
The spinal cord will read the HOT!
HOT!
message and pass this signal to the motor neurons in your hand.
The motor neurons will contract and relax your muscles so that you pull your hand away.
Shockingly, all of this happens in about 80 to 120 milliseconds, faster than the blink
of an eye!
Here, you will notice that information went from the neurons in the PNS to the neurons
in the CNS, then back to the PNS.
For more complex tasks, like remembering this information for an exam, multiple neural circuits
in the brain and body will come into play.
Neuroscientists are interested in understanding these neural circuits for a host of reasons,
from understanding diseases to creating better prosthetics and even using the information
to create smarter robots!
As this video comes to an end, and before you move on to the next video, take a moment
to appreciate the complex network of essential wires that keeps you alive!