PHGY 209 Lecture Notes - Baroreceptor, Carotid Sinus, Baroreflex

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6 Apr 2012
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Naveen Sooknanan McGill Fall 2011
1
Autonomic Nervous System:
The autonomic nervous system (ANS) is a sensory and motor system which innervates visceral
tissue and organs of the body
The ANS is responsible for many involuntary responses to stimulus input
The ANS is intimately connected to the idea of homeostasis we discussed earlier
o This system allows the maintenance of a relatively constant milieu interieur which
is isolated from the external environment
Factors like blood pressure, temperature control, and many others are all controlled by
the ANS
The output of the ANS can be subdivided into two main subcategories and is related to another
system called the enteric system
The first subcategory is called the sympathetic system and the second is the
parasympathetic system
o The sympathetic system is in charge of “fight or flight” mechanisms such as panic
or excitement
For example, raising of heart rate when running away in fear
o The parasympathetic system is in charge of “rest and digest” functions
For example, lowering of the heart rate when resting
o These two systems contrast each other and have opposing effects on target tissues
o Both systems are always active; they work together to make fine adjustments to
the system
o These systems innervate cardiac, smooth muscle, and glandular tissue
The enteric system is a separate system which controls the contraction of gut muscles
o This allows food to move through the intestines and through the digestive tract
Unlike the somatic motor pathway (the one we discussed before) then autonomic motor system’s
neurons, called post-ganglionic neurons, are located outside the spinal cord in groups called
autonomic ganglia
A ganglia is defined as a group of cell bodies, dendrites and synapses in the peripheral
nervous system
These autonomic ganglia are activated by
pre-ganglionic neurons which are located
either in the spinal cord of brain stem
o These synapses for white dots on
either side of the spinal cord; these
are the axons of the autonomic ganglia
In contract to the somatic motor unit which directly connects the motor neuron to the
skeletal muscle, the autonomic motor unit consists of 2 neurons, 1 from the brain of
spinal cord to the autonomic ganglia, and the other from there to the tissue
o The synapse between the pre and post ganglionic neurons is excitatory
In more detail, the sympathetic system motor unit differ from the motor units of the
parasympathetic system
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Naveen Sooknanan McGill Fall 2011
2
In the sympathetic motor unit, the preganglionic neuron is very short and makes a
synapse right outside the spinal cord. The postganglionic neuron is very long and travels
all the way to the desired tissue
o The preganglionic neuron released ACh (an
excitatory synapse) to the postganglionic neuron
like a neuromuscular junction
o The post ganglionic axon synapses with either an α or β adrenergic receptor on
the target tissue. This is also an excitatory synapse
There is a release of norepinephrine (NE) at this synapse which activate
metabotropic receptors on the target tissue
o This can activate sympathetic responses, such as an increased heart rate, or
relaxation of bronchial tubes to stop intestinal contraction
You don’t want to waste energy digesting during an emergency
o A shot of epinephrine does in effect the same thing as a sympathetic response
releasing norepinephrine
In the parasympathetic system, preganglionic neurons are very long and postganglionic
neurons are very short (in contrast to above)
o The preganglionic/postganglionic synapse is the same as before, release ACh at
an excitatory synapse
o The postganglionic neuron’s synapse with the
target tissue is different than the one above,
however. This synapse releases ACh, not NE,
to the target tissue
This activates metabotropic receptors, different from the NE receptors,
which cause generally opposing effects to the sympathetic system
These receptors are called muscarinic ACh receptors (mAChR)
o The activation of these receptors cause a biochemical change in the target tissue
Such events can lower the heart rate during relaxation or increase
digestive tract activity
The different effects of the sympathetic and parasympathetic systems on the same target tissue
can be seen on the heart, known as cardiovascular reflexes
Normally, the two systems work in balance to produce a stable
milieu interieur. Sometimes, however, one system can take
dominance in special cases
The dominance of the sympathetic system causes a substantial, sudden increase in the
heart rate and the strength of each pulse
Dominance of the parasympathetic system causes a sudden
decrease in the frequency and strength of the heart beatings
The autonomic nervous system generates responses to stimuli received from various sensory
input sources all over the body
For example, the stimulation of pain sensing neurons in the skin activate sympathetic
neurons which cause local vasoconstriction
The sensory inputs first travel to the brain for processing
o They are received by nuclei within the brain
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