It’s Professor Dave, let’s talk about the nervous system.
The human body is a finely tuned machine, and perhaps its most impressive feature is
the way that we can use sentience to command this machine at will.
If you decide to get up out of your chair and walk to the fridge, the body obeys.
But how does this work?
Just as with everything else, it’s not magic.
It is the result of a staggering amount of chemistry happening on the molecular level,
and the system that carries signals from the brain to the rest of the body is called the
nervous system.
The cells that comprise the nervous system qualify as the fourth type of tissue that
we will examine in this series.
We looked at epithelial, connective, and muscle tissues, so now let’s learn about nervous
tissue.
The main type of cell that makes up the nervous system is called a neuron.
These look very different from the cells we are familiar with thus far, in that there
is an incredible amount of branching within each neuron, where cytoplasmic extensions
project in many directions, allowing a neuron to respond to stimuli and transmit electrical
impulses over very long distances.
These are the very same signals that stimulate the muscle contraction we learned about earlier
in the series.
Neurons are amitotic, meaning they do not divide, and they have the potential to live
a hundred years or more, so the neurons we are born with last a lifetime.
Beyond neurons there are also supporting cells in the nervous system called neuroglia or
glial cells, which are not neurons, but rather wrap around delicate parts of neurons for
protection.
Let’s get a closer look at a neuron now.
First, the central part is called the cell body.
This contains a nucleus and a nucleolus, surrounded by all the typical organelles we learned about
in the biology series.
Projecting from the cell body we can see many dendrites, which produce a tremendous surface
area for receiving signals.
We also see a single axon.
This initiates from an axon hillock, and extends for some distance.
For many neurons, the axon accounts for the majority of the length of the cell, sometimes
extending macroscopic distances, at which point we can call them nerve fibers.
At the terminus of the axon there will be many terminal branches, sometimes thousands,
which are the axon terminals that interface with other neurons, or with muscles to form
the neuromuscular junctions we discussed earlier in the series.
The axon is the conducting region of the neuron, in that it generates electrical signals for
communicating with other neurons, starting at the axon hillock and traveling through
to the axon terminals, also called the secretory region.
When the impulse gets here, neurotransmitters are released which will then either excite
or inhibit nearby nerve cells.
Many axons are protected by a myelin sheath, which also increases the speed at which impulses
are transmitted, and nerve fibers covered in this manner are called myelinated fibers.
Particularly in the peripheral nervous system, the myelin sheath is often made of individual
units called Schwann cells.
These first envelop the nerve fiber, and then wrap themselves around it many times, which
results in numerous layers of plasma membrane to protect the nerve.
These are membranes which lack most of the channel proteins found in other types of cells,
making them great electrical insulators, and allowing them to increase the speed of electrical
conduction down an axon.
There are tiny gaps in between each Schwann cell where a bit of the fiber is exposed,
and these are called nodes of Ranvier, and sometimes axon collaterals branch out at these gaps.
Now we want to be aware that there are several types of neurons, and just like some other
anatomical features, we can classify them either by structure or by function.
Going by structure first, neurons can be unipolar, bipolar, or multipolar.
These terms refer to the number of processes extending from the cell body.
A unipolar neuron has a single axon that quickly divides into proximal and distal branches,
so this still technically qualifies as a single axon.
One of these is a central process that goes towards the central nervous system, while
the other is a peripheral process that acts as a sensory receptor.
Then there are bipolar neurons, which have two processes, one axon and one dendrite,
extending from opposite sides of the cell body.
These are not very common but they are found in certain places like specific parts of the
eye and nose.
Then, multipolar neurons have three or more processes, precisely one of which is an axon,
and the rest of which are dendrites.
Around ninety-nine percent of our neurons are of this type.
Then we can describe function.
A sensory, or afferent neuron, transmits information from sensory receptors towards the central
nervous system.
These are typically unipolar.
Motor, or efferent neurons, transmit information from the central nervous system to muscles
and glands.
These are always multipolar.
Then there are interneurons, which sit between the other two types and help shuttle signals
around the system.
These are typically multipolar neurons, found entirely within the central nervous system,
and most of our neurons are of this type.
For any neuron, regardless of type, we can describe some basic common features.
Any neuron will have a receptive region, where a stimulus is received, typically at dendrites.
Then there is a trigger zone, which initiates a conducting region, where the electrical
signal travels.
This leads to a secretory region with axon terminals that release neurotransmitters.
That covers the basics regarding the structure of a nerve cell.
But how exactly does it generate electrical impulses?
This could be discussed for hours upon hours, but let’s go over a very basic summary here.
First we must understand the concept of membrane potential.
For a more rigorous discussion of electric potential from a physics standpoint, check
out my tutorial on that subject now, otherwise for our purposes here, we just need to know
that opposite charges attract, so if there exist opposite charges near one another, work
must be done to separate them.
If these regions are already separated within a cell, there is an opportunity to use the
potential energy that exists by virtue of their separation.
This charge separation exists within nerve cells due to the concentrations of specific
ions that sit inside and outside of the plasma membrane, so we say there is a potential difference
across that membrane, and the membrane itself resists the current flow, as formally charged
ions have difficulty traversing the nonpolar section of the membrane.
However, there are ion channels in the membrane which can allow specific ions through at specific times.
Some of these are non-gated, meaning they remain open.
Others are gated, meaning they are closed, and only open as the result of a particular signal.
Chemically-gated channels open when a specific neurotransmitter binds.
Voltage-gated channels respond to changes in the membrane potential.
And mechanically gated channels open when the receptor becomes physically deformed.
When these channels open, ions can freely diffuse through, obeying the electrochemical
gradient, which seeks to balance charge, so an electrical current is generated.
A resting neuron has a resting membrane potential, based on the sodium and potassium ion concentrations
in and out of the cell, due to their differing abilities to diffuse across the membrane,
and it is maintained by sodium-potassium pumps that keep the concentration gradient as it is.
But when a signal is received, whether from sensory input or a neurotransmitter from another
neuron, there will be a change in this potential which can produce a signal.
This signal will be either a graded potential, which operates over short distances, or an
action potential, which operates over long distances, like the length of an axon.
In the latter case, depolarization must exceed a particular threshold, meaning sodium channels
must open and a sufficient number of sodium ions must diffuse into the cell, but if achieved,
a nerve impulse will result, and a current will propagate along the axon towards the
axon terminals.
Repolarization will then occur, where voltage-gated potassium channels allow potassium ions to
exit the cell, which leads to a brief hyperpolarization, and then everything resets to original levels
and positions.
As for the impulse, when it reaches the axon terminals and neurotransmitters are released,
these enter the synaptic space.
A synapse is a junction between neurons or between a neuron and an effector cell.
This is where communication happens.
These synapses can be axodendritic, axosomatic, or axoaxonal, depending on where the axon
terminals from the presynaptic neuron connect to the postsynaptic neuron.
So that covers the basics regarding the action potential.
We could get much more detailed and quantitative than this brief description, and perhaps we
will dig a little deeper later in the series.
But as we are currently focused on the big picture, let’s be satisfied with this rudimentary
understanding for the moment.
Now that we know a bit about the structure and function of a neuron, let’s discuss
how these are assembled to form the nervous system.
This system receives sensory input through receptors, integrates this information to
decide what to do, and then generates a motor output, which is a response to stimulus.
This is a lot more complicated than it sounds, so the nervous system has many components.
First, it is divided into two main parts, the central nervous system and the peripheral
nervous system.
The central nervous system consists of the brain and spinal cord, while the peripheral
is all the rest, mainly the nerves that extend all around the body, which are bundles of axons.
That means the central nervous system is the control center, where all the sensory information
is integrated, after which a motor output is determined and implemented.
The peripheral nervous system is what allows for communication throughout the body.
The peripheral nervous system is also divided into two parts.
There is the sensory, or afferent division, which sends signals from receptors to the
central nervous system, and the motor, or efferent division, which sends signals from
the central nervous system to all the muscles and glands to do its bidding.
In turn, the motor division is also divided into two parts.
There is the somatic nervous system, which sends signals to the skeletal muscles we consciously
control, and the autonomic nervous system, which sends signals to smooth muscle and cardiac
muscle that operates without conscious control.
And finally, the autonomic nervous system is also split into two parts.
Those are the sympathetic division and parasympathetic division, which operate in conjunction and
typically have opposing functions.
There is so much to discuss at each tier of structure that we have just mentioned, so
let’s go through these tiers one at a time, starting with the central nervous system.