ANS
I. Introduction/Overview of the ANS
>A. Comparisons with the SNS
>>1. ANS effectors are smooth muscle, cardiac muscle, and glands (what
does the SNS target?)
>>2. Autonomic motor neurons synapse in ganglia (do SNS neurons?)
>>>the Pre-ganglionic neuron originates in the brain or spinal cord
>>>the Ganglionic neuron has its cell body in a ganglion, and there it
synapses with the axon terminals of the Pre-ganglionic neuron. The
ganglionic neuron sends its Post-ganglionic fiber, or axon, to its effector
(what are the effectors of the ANS?)
>>3. Preganglionic axons are type-B fibers, postganglionic axons are
type-C fibers (what does this mean? Do ANS axons conduct faster or
slower than SNS fibers?)
>>4. SNS motor neurons all release Ach on effectors, and it's always
excitatory. ANS motor neurons, on the other hand, may release Ach
(parasympathetic) orNE (sympathetic) on their effectors. Depending on
the receptor type and effector, Ach and NE can be either excitatory or
inhibitory.
>B. Overview of the divisions
>>1. Parasympathetic- the "resting and digesting" division. When
parasympathetic activity dominates,
>>>heart rate and blood pressure decline
>>>GI tract motility increases, and excretory activity increases
(defecation and urination)
>>>respiratory rate and volume decrease (breaths are shallow and slow)
>>>energy in the form of glycogen and triglycerides is stored
>>2. Sympathetic- the "stress, exercise, and emergency" division. When
sympathetic activity dominates,
>>>heart rate, blood pressure, and muscle tone increase
>>>GI tract motility and excretory activity decrease
>>>respiratory rate and volume increase (breaths are deep and fast)
>>>alertness increases
>>>energy stores are mobilized, so glycogen is broken down to release
glucose to the blood, and triglycerides are released to the blood from
adipose.
II. Parasympathetic division
>A. Preganglionic fibers travel in cranial nerves and sacral spinal nerves
(trivia: 90% of all parasympathetic preganglionic fibers travel through the
vagus nerve & its branches)
>B. The preganglionic neuron synapses with the ganlionic neuron/s in a
ganlion near the effector or in the effector. Preganglionic axons release
Ach onto ganglionic neuron, whose postganglionic axons release Ach
onto its effector.
>C. Preganglionic fibers are long, postganglionic fibers are short.
>D. Functions
-increase GI tract motility
-relax GI tract sphincters
-increase digestive gland activity
-increase excretory activity
-conserve energy (reduce heart rate, respiration rate, store fats, etc)
-constrict pupils
-prepare for sexual activity
III. Sympathetic division
>A. The effectors innervated by the parasympathetic division are also
innervated by the sympathetic division, and the two divisions generally
have opposing effects. However, the sympathetic division serves some
effectors that ARE NOT served by the parasympathetic. Effectors that
only receive sympathetic innervation include: smooth muscle lining blood
vessels, the arrector pili muscles, sweat glands, and the adrenal glands.
So, for instance, the degree of constriction of blood vessels is dependent
on the sympathetic division only.
>B. Synapses and ganglia
>>1. Preganglionic fibers travel through the T1-L2 spinal nerves. They
release Ach onto ganglionic neurons in ganglia (see below). The
postganglionic axons release NE (most) or Ach (some) onto their effectors.
Preganglionic fibers are generally short, postganglionic fibers are
generally long.
>>2. They synapse with their ganglionic neurons in one of 3 places:
>>>a. Sympathetic chain ganglia- paired, near the spinal cord on either
side
>>>b. Collateral ganglia- unpaired, located anterior to the vertebral
column
>>>c. Adrenal medulla- the center of the adrenal glands. The adrenal
glands are located on top of the kidneys. In the adrenal medulla,
specialized neurons produce and release NE and Epinephrine into the
blood. This is unusual because this means that NE can serve both as a
neurotransmitter and as a hormone. Hormones are chemicals that are
released into the blood and can have widespread effects on many
different types of cells throughout the body. NE and E have similar
effects.
>>3. The telodendria, or axon terminal branches, of postganglionic fibers
are specialized to release Nt all along the branches, rather than just at
the synaptic terminal. Remember that the axon is a long fiber that
carries an action potential. The end of the axon contains small branches
called telodendria. Normally, the ends of the telodendria form swollen
knobs that release Nt in response to an action potential.
In the post-ganglionic fibers of the sympathetic division of the ANS, the
swollen areas of the telodendria are not restricted to the very ends.
Instead, several areas along the telodendria swell and synapse with
effectors. These swollen areas release Nt in response to action potentials
arriving from the axon, just like synaptic knobs. They are called
varicosities. Varicosities allow one neuron to wrap its telodendria around
its effector and release Nt all over it at once.
*So, a ganglion is a swollen area of a whole nerve, which contains the
axon terminals and cell bodies of MANY neurons. A telodenrion is a type
of axon terminal which occurs on ONE neuron.
>C. Functions
-decrease GI activity and all related digestive functions
-shunt blood to skeletal muscle, brain and heart (constrict most
vessels, dilate those going to muscle, brain and heart): increase
blood pressure, alertness, and readiness for exercise
-mobilize energy stores (fat and glycogen breakdown)
-increase heart rate and respiratory rate; dilate airways
-dilate pupils
-complete sexual activity
IV. Neurotransmitters and receptors of the ANS
>A. Fiber types based on the Nt they release- right now we're talking
about what goes on at axon terminals (release of Nt from the presynaptic
neuron)
>>1. Cholinergic- release Ach, which acts directly. Found on:
>>>All preganglionic fibers of the ANS, so the preganglionic fibers of both
divisions release Ach
>>>All parasympathetic postganglionic fibers, and a few sympathetic
postganglionic fibers (ex, those serving sweat glands)
>>2. Adrenergic- release NE, which acts indirectly. Found on:
>>>Most sympathetic postganglionic fibers
>>3. (This will be for extra credit only) Nitroxidergic- release NO (nitric
oxide). NO is a powerful vasodilator (causes blood vessels to dilate).
Found on:
>>>Some sympathetic postganglionic fibers
>B. Receptor types based on what Nt they bind to and how they respondright now we're talking about what goes on at cell bodies and dendrites of
a postsynaptic neuron or effector (binding of Nt released from the
presynaptic neuron) See table 14.3, pg. 513
>>1. Receptors that bind to Ach-cholinergic. There are two types of
cholinergic receptors:
>>>a. Nicotinic- When Ach binds to nicotinic receptors, it causes a direct
EPSP. Nicotinic receptors are found on (in general, neuron cell
bodies/dendrites):
>>>>all ganglionic neuron cell bodies and dendrites
>>>>neurons of the adrenal medulla
>>>>(FYI: also motor end plates of skeletal muscle)
>>>b. Muscarinic- When Ach binds to muscarinic receptors, it can be
either excitatory or inhibitory. Often indirect. Muscarinic receptors are
found on (in general, effectors):
>>>>All parasympathetic effectors- again, excitatory on most, inhibitory
on cardiac muscle.
>>>>Some sympathetic effectors- excitatory or inhibitory. For example,
- Ach binding muscarinic receptors on sweat glands is excitatory;
- Ach binding muscarinic receptors on blood vessels is inhibitory;
blood vessels with muscarinic receptors are found serving
skeletal muscle and the brain.
>>2. Receptors that bind to NE (or E, from the blood)- adrenergic. NE and
E cause indirect effects on target cells. There are two broad categories of
adrenergic receptors, alpha and beta. Within each of these categories are
subcategories.
>>>a. Alpha-receptors- NE binding to alpha receptors can be excitatory
or inhibitory, depending on the cell type. For example, NE binding to
alpha receptors on most smooth muscle lining blood vessels causes
EPSPs, or blood vessel constriction. There are 2 subcategories; here are
some examples of where they are found and how they affect their cells
when NE binds:
-alpha 1 receptors: >on most blood vessels, cause vasoconstriction
(EPSP)
-alpha 2 receptors: >on pancreas and other secretory glands, cause
inhibition
>>>b. Beta-receptors- again, excitatory or inhibitory. For example, NE
binding to beta receptors on smooth muscle lining blood vessels causes
IPSPs, or blood vessel dilation.
NE binding to beta-receptors on target cells also increases general
metabolic activity. For example, adipose cells contain beta-receptors, and
they release triglycerides in response to binding of NE. There are 3
subcategories; here are some examples of where they are found and how
they affect their cells when NE binds:
-beta 1 receptors: >on cardiac muscle, cause increased heart rate
(EPSP)
>on kidneys, cause renin release
-beta 2 receptors: >on blood vessels serving the heart, cause
vasodilation (IPSP)
>on digestive organs, decrease muscular activity
(IPSP)
-beta 3 receptors: >on adipose tissue, cause lipolysis
V. ANS tone
> Autonomic neurons are always active to some extent. The "resting"
condition of ANS effectors is determined by the degree of influence of
each of the divisions.
For instance, heart rate is determined by the amount of NE (excitatory)
from sympathetic fibers versus the amount of Ach (inhibitory) from
parasympathetic fibers. Cardiac muscle is always receiving some of both.
When heart rate increases, the amount of NE increases, and the amount
of Ach decreases.
Another example: remember that blood vessel diameter (degree of
constriction) is determined by the sympathetic division. Smooth muscle
lining vessels is always receiving some NE, so the smooth muscle lining
vessels is always tone. To dilate or constrict a vessel, the sympathetic
division releases more or less NE onto the smooth muscle. Depending on
where the vessel is located in the body, NE can be excitatory (cause
constriction) or inhibitory (cause dilatation). This is, of course, based on
the receptor types.
VI. Visceral Reflexes- like somatic reflexes, these are simple reflex arcs
between sensory and motor neurons that bypass the brain. They can
involve an interneuron between sensory and motor neurons, or they can
be direct connections.
Synapses can occur in the gray matter of the spinal cord, or can occur in
ganglia. When synapses occur in ganglia, the reflex bypasses the CNS;
this is called a short reflex, and it is unique to the ANS.
An example of a long reflex controlled by the ANS is the constriction of
pupils in response to bright light.
Short reflexes occur in the enteric nervous system: an extensive network
of visceral neurons in the GI tract that communicate via reflex arcs, and
function to some extent without input from the CNS.