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.
