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.