Professor Dave here, let’s look at the autonomic nervous system.
After looking at the central nervous system and certain divisions of the peripheral nervous
system, we have a reasonable understanding of how sensory information makes it to the
brain, which then tells the body what to do, like when you touch something hot, and then
quickly pull your hand away.
We also understand how we might decide to execute a particular action, like walking
across the room.
But there is so much happening in our nervous system that we are not consciously aware of.
We don’t decide to beat our hearts, it just happens.
We don’t decide to digest our food, it’s automatic.
All of this is due to the autonomic nervous system.
As we already learned, the peripheral nervous system is divided into the sensory division
and motor division.
The latter is what produces motion, and this is divided into the somatic nervous system
and autonomic nervous system.
The autonomic nervous system is in turn divided into a sympathetic division and a parasympathetic
division, so let’s talk a bit more about what this means.
First let’s understand that while the somatic nervous system innervates skeletal muscle
to generate deliberate motion, the autonomic nervous system innervates smooth muscle and
cardiac muscle, to generate the involuntary motions we talked about.
There are differences in neuronal structure as well.
For the somatic, axons are heavily myelinated and singular, releasing acetylcholine.
For the autonomic, neurons are lightly myelinated or nonmyelinated, and there will be a two-neuron
chain from the central nervous system to effector organs.
The first of these is the preganglionic neuron, with its preganglionic axon, and the latter
is the postganglionic neuron, with its postganglionic axon, and these release either acetylcholine
or norepinephrine, resulting in either a stimulatory or inhibitory response, in contrast with somatic
which is always stimulatory.
As we said, the autonomic nervous system has two divisions.
These divisions actually serve the same parts of the body, but they have opposing roles.
The parasympathetic division is for “rest and digest actions”.
It directs the digestion of food and the expulsion of waste, along with other maintenance-related
actions that occur best in a state of relaxation.
The sympathetic division, on the other hand, springs into action in emergency situations,
enacting the fight-or-flight instincts, raising the heart rate, constricting blood vessels,
and releasing more glucose from the liver, all actions that help an organism evade danger.
These two systems work together most of the time to keep the body running smoothly, a
process called dual innervation, but in specific moments one may dominate depending on the
immediate needs of the organism.
To get a bit more specific about the anatomy of these divisions, parasympathetic fibers
originate in the brain and sacral region of the spinal cord, which are the opposite ends
of the central nervous system, whereas sympathetic fibers originate in between, in the thoracic
and lumbar regions of the spinal cord.
Parasympathetic has long preganglionic and short postganglionic fibers, with the ganglia
residing in the effector organs, while the opposite is true for sympathetic, so the ganglia
sit very close to the spinal cord.
Here we can see the various nerves of the two divisions and the organs they innervate.
The sympathetic division also has some interesting roles.
It is responsible for the regulation of sweating, meant to control body temperature.
It has an influence on metabolism, as well as kidney activity.
And all the activity in the autonomic nervous system is regulated by components of the central
nervous system.
So with that we come full circle.
Let’s quickly review the divisions of the nervous system before moving forward with
other systems.
The nervous system is divided into the central nervous system, which is the brain and spinal
cord, and the peripheral nervous system, which is everything else.
This is divided into the sensory, or afferent division, which is the half that goes from
sensory input to the brain, and the motor, or efferent division, which goes from the
brain to whatever the brain needs to control.
This division is comprised of the somatic nervous system, when the conscious mind is
involved, and the autonomic nervous system, when activity is automatic.
And finally, the autonomic nervous system is split into the sympathetic division, which
takes over in times of emergency, and the parasympathetic division, which is in charge
of regular maintenance.
We have just barely scratched the surface on the complexity of the nervous system, and
we will continue to refer back to it in order to build upon our knowledge as necessary,
while we continue to learn about human anatomy.
So now let’s move forward and learn about some other systems in the body.