The Autonomic Nervous System.
ANS mediates many of our classical homeostatic process
such as temperature regulation,
glucose specially in the blood, as well as blood levels of lipids.
The plasma levels of oxygen, carbon dioxide and even pH.
The heart and blood vessels are also innervated
by the autonomic nervous system
and will help us to control and regulate blood pressure.
These doesn’t not only in the level of the heart,
where you can control heart rate in contractility
but also blood vessels where controls vascular resistance.
The kidneys where also under
ANS mediated homeostatic control
things like pH, sodium levels and other electrolytes,
osmolality even total body water.
All of these items are being constantly
regulated throughout the body
and miniscule, and large changes are occurring
because of the autonomic nervous system.
So what are the structural divisions of the ANS?
These are important, and also, it’s important
to contrast this to the somatic motor system
so do you have a good feel for what the differences are.
In the somatic motor system, we go right from
the spinal cord to the muscle of action.
So this is one nerve that goes from
the spinal cord to the muscle.
In the ANS works different.
The sympathetic nervous system synapses in
sympathetic chain ganglia or autonomic ganglia.
And that travels up and down in the spinal cord,
innervating a multitude of various organs and
of blood vessels all at the same time.
So either somatic motor system really only
innervates a muscle in more specifically
a few fibers in that muscle.
The ANS can innervate large slots
of organs in our organ systems.
The parasympathetic nervous system, usually
has a synapse both in the spinal cord as well as
in an autonomic ganglia that’s located in
close proximity to the organ or other item
that its going to be innervating.
So that’s a big difference between
the sympathetic and parasympathetic.
And that is where the autonomic ganglia are located.
Autonomic ganglia are located in a close
proximity for the sympathetic nervous system.
And then, for the parasympathetic nervous system
they are located close to the organ of action.
That big difference is important because it means
that the presynaptic versus postsynaptic nerves
are gonna be different lengths for the different
divisions of the autonomic nervous system.
Sympathetic has a short pre, long post.
While the para sympathetic has a long pre, short post.
So once you have the nervous system output to the receptor,
what different neurotransmitters are being released?
And what receptors are gonna be engaged?
If we go back to our example, in where we gonna
compare things to the somatic motor system,
lets look at that first.
So normally, here again, we have the
soma or cell body in the spinal cord.
The nerve will go the length until it innervates
a few muscle fibers it releases acetylcholine.
It is a cholinergic nerve.
That acetylcholine leaves the acts on terminal,
goes across the synaptic cleft in this case
the neuromuscular junction, and binds to a nicotinic
type 1 acetylcholine receptor that is ionotropic.
Meaning, that allows ions to travel once
the acetylcholine binds to it.
This is a very classic way of signaling.
Now the automatic nervous system utilizes a
little bit more complex of a process.
So let’s compare and contrast.
If we take the sympathetic nervous system first,
you have your first spinal nerve soma and then
it will synapse in an autonomic ganglia.
This autonomic synapse also use acetylcholine.
So this is a presynaptic nerve.
It releases that acetylcholine goes cross the synaptic cleft
and bind to a nicotinic type 2 receptor.
These particular receptors also allow
for ions to travel through and
will stimulate another nerve to be released.
That other nerve to be released can be one of two types.
Either another cholinergic nerve or a noradrenergic nerve.
So lets go through the cholinergic nerves first.
Usually in these case, that cholinergic nerve
will synapse and release its acetylcholine
go across the synaptic cleft and
bind to muscarinic type 3 receptors.
Now these muscarinic receptors are
G-protein coupled receptors (GPCRs)
rather than, letting the ions through, you’re gonna
stimulate or cause a transduction of their signal
via G-protein and then stimulate something else,
such as, allowing for calcium influx,
changing the amount of phospholipase C, so forth.
Norepinephrine or noradrenergic nerve can also be part
of this sympathetic nervous system response.
This organ, postganglionic nerves that
released norepinephrine into a synaptic cleft.
They have a plethora of receptors to bind too.
They can bind to alphas or betas.
And there is a number of different
types of each of this receptors.
There is alpha 1, there’s alpha 2, there’s alpha-2C.
There’s beta 1, beta 2s, beat 3s,
and they will be a different low-cals in the body
depending upon the density of which they are.
Then they will get a metabolic response, vascular response
or whatever change that particular organ will undertake.
So that’s the noradrenergic pathway through
adrenergic receptors, whether they’re alpha or beta.
But remember, there are few examples
of cholinergic sympathetic nerves.
Finally, we use our last comparing contrast by
looking at the parasympathetic nervous system.
Here the preganglionic nerve, the nerve is longer
in nature, but its still uses acetylcholine.
He releases acetylcholine and that goes across the
synaptic cleft and binds to a nicotinic type 2 receptor.
From here, that stimulates another nerve and
that second nerve is also cholinergic.
So then, this postganglionic nerve is cholinergic
releasing acetylcholine into the synaptic cleft.
This can bind to a number of different types
of muscarinic receptors M1s, M2s, M3s.
You know, in there are it seems to keep increasing
the amount of the M's they want to denote.
But these was engaged things like smooth muscle,
glands various types of processes
in which you would want to change from
the parasympathetic nervous system.
This are G-protein coupled receptors as well.
Therefore, when they get stimulated,
they will stimulated G-protein,
that will then do a particular action like
activating enzymes or open a channel.
If we compare and contrast all of these,
we’ll notice some similarities.
In first similarity, acetylcholine is also
always the first nerve in this system.
The first synapse is always nicotinic.
In the somatic, its nicotinic type 1.
In the sympathetic comparison sympathetic,
its nicotinic type 2.
This is then where a things start
to change after the first synapse,
cause now whether you could have a cholinergic
or adrenergic nerve, and then of course,
the downstream receptors are quite a few of those.
Another difference with nerves are the
ANS axon terminals are a little bit different.
Instead of being a single axon terminal, we are
kind of bugs out and has a nice synapse
with one particular low-cal.
Axon terminals in the ANS are
often times in bead like strings.
These allows for more wide spread engagement of the
peripheral tissue rather than just a single response.
These particular axon terminals, also turns
varicosities, will allow for more neurotransmitter
to be released across that range of tissue.