quick revision:- - judoctors · physiology 10jun. 25 1 quick revision:- we`ve started talking about...
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Physiology 10
Jun.
25
1
Quick Revision:-
We`ve started talking about the Autonomic Nervous System:
First: There are two divisions of the Autonomic Nervous System:
Sympathetic ANS:- { Thoracolumber origin}
Parasympathetic ANS: - {Craniosacral origin}
Second: Each sympathetic and parasympathetic pathway from the brain or the
cord to the stimulated tissue is composed of two neurons, a preganglionic neuron
{the first one} and a postganglionic neuron {the second one} , forming the axis
of both systems.
Third: Physiologic Anatomy of Sympathetic NS:
Paravertebral ganglia: Chains of ganglia that is located near the vertebral
column in both sides where we have the synapse of the first and second
neurons.
Prevertebral ganglia : Some preganglionic fibers that enter ganglia pass
without any synapse at the paravertebral ganglia and continue to these
ganglia which are located in the abdomen where they have the synapse with
the second neuron > there are three solitary ( unpaired ) prevertebral ganglia
: celiac , superior mesenteric and inferior mesenteric ganglia .
Some preganglionic fibers pass without synapse in either location, these fibers
continue directly toward the suprarenal gland where they synapse with
endocrine cells that release the Epinephrine hormone.
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The sympathetic NS have short preganglionic axons (fibers) and long
postganglionic axons.
Fourth: Physiologic Anatomy of Parasympathetic NS:
The preganglionic fibers leave the central nervous system and go toward
the stimulated organ where they synapse with the second postganglionic
neuron located within the tissue of the effector organ.
The preganglionic fibers are long compared to the short postganglionic
fibers.
Fifth: The General function of the ANS is: Control and Adaptation of
body systems to internal and external changes.
Finally: In the sympathetic NS we have characteristic organizations
such as Convergence and Divergence. This organization is very important
in the function of the sympathetic NS to generate a Diffused response,
while in the parasympathetic we don`t have this type of organization so its
effect is limited to the specific stimulated organ.
Now we will continue our journey of exploring our ANS
What are the Effects of Sympathetic stimulation??
1- Effects on Blood pressure : the sympathetic NS have an effect
on the : blood vessels supplying skeletal muscle , heart muscles and the
conductive tissue of the heart .So it change the blood pressure via :
Changing the rate of contraction of the heart muscles
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Changing the propulsion of blood by the heart {Contractility or the
force of contraction and propelling of blood }
Changing the diameter of the blood vessels { By controlling the
smooth muscles of the vessels }
Note:-
Because blood vessels are widely distributed tissues these structures have only
sympathetic innervations as it is the widely distributed neural system .In fact all
widely distributed tissues in our body are innervated by the sympathetic NS such as
skin , sweat glands, piloerector muscles ……etc.
2- Control Body temperature : the sympathetic NS have many
mechanisms by which it can regulate body`s temperature :-
If the temperature goes high our body can get rid of excessive
heat by :
Vasodilation of cutanous blood vessels
Stimulating the sweat glands to secret large quantities of
sweat.
Our body can reduce heat loss by vasoconstriction for example.
3- Effects on the Cardiovascular system :-
Effects on blood vessels will result in redistribution of blood by
enhancing blood flow to skeletal muscle and reducing blood flow to
skin and mesentery {cause vasoconstriction in these vessels}.
Effects on heart: - In general sympathetic stimulation increases
the overall activity of the heart. This is accomplished by
increasing the rate of contraction by affecting the conductive
tissue of the heart, and increasing the force of contraction by
affecting the heart muscles.
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Note:- The parasympathetic NS can only affect the conductive tissue of
the heart but not the heart muscles them self.
4- Effects on the respiratory system: sympathetic stimulation causes
relaxation of the bronchial smooth muscles which result in
bronchodilation.
5- Effects on Digestive system: - It is not important in the fight
and flight reaction so the main effect is inhibitory, it cause inhabitation
of motility and secretion of the gastrointestinal tract.
6- Metabolic effects : These include :
Mobilization of glucose for the sake of energy for muscles.
Mobilization of Lipids {Lipolysis} as source of energy.
Metabolic activities are increased {Set of chemical reactions that
maintain life in any organism. Two types of metabolic processes
are anabolism and catabolism }
NOW what are the effects of PARASYPMATHETIC STIMULATION?
1- Effects on the gastrointestinal system: Parasympathetic
stimulation in general increases overall degree of activity of
the gastrointestinal tract by promoting motility and secretary
activity.
Remember!!!!
The parasympathetic effect is limited, that means for example
stimulating the neurons innervating the stomach will cause a
limited response changes the activity of the stomach only!!
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2- Effects on Glands of the body : The nasal,
lacrimal,salivary and many gastrointestinal glands are strongly
stimulated by the parasympathetic nervous system ,usually
resulting in increased secretary activity of these glands { BUT
REMEMBER : sweat glands are under sympathetic control }
3- Effects on heart: - It causes mainly the opposite effects of
the sympathetic NS Decreased heart rate and force of
heart contraction. Its effect is limited to the conductive
tissue of the heart.
How Does this Happen??
Conduction tissue of the heart has the special property of depolarizing without any
external influence. This is known as cardiac muscle automaticity {Automatic
generation of action potential by some structures in this tissue}
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In the Diagram:
Phase 4: is slow depolarization which continues till reaching threshold.
Phase 0: is the upstroke of the action potential.
Phase 3: is repolrization going back to the lowest point then slow depolarization
again.
What` s the effect of the parasympathetic NS???
It slows the rate of {SLOW depolarization} this means less number of heart beats
per minute.
BUT the sympathetic NS increases the rate of depolarization so increasing the
number of heart beats per minute.
4- Effects on pupil :- parasympathetic NS controls pupil
diameter by papillary light reflex (myosis){ This reflex
regulate the amount of light falling on retina }
In fact, our parasympathetic NS is adapting our eyes to
increased light intensity by causing constriction of the pupil
opening to decrease amount of light that strikes the retina.
5- Lens: focusing of the lens is controlled almost entirely by
the parasympathetic NS. It is involved in the accommodation
of the lens for near vision .this is usually achieved by changing
the convexity of the eye lens.
6- Urinary Bladder: cause contraction of the urinary bladder
which results in micturation.
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MOLECULAR BASIS of PHYSIOLOGICAL ACTIONS of the
ANS:
Sympathetic NS:-
All the sympathetic preganglionic autonomic nerve endings
secrete the neurotransmitter Acetylcholine.
Most of the sympathetic postganglionic autonomic nerve
endings release the neurotransmitter Norepinephrine.
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An exception is the postganglionic fibers innervating
sweat glands and piloerector muscles their autonomic
nerve endings secretes Acetylcholine.
Parasympathetic NS:
All the parasympathetic preganglionic autonomic nerve
endings secrete the neurotransmitter Acetylcholine.
All the parasympathetic postganglionic autonomic nerve
endings secrete the neurotransmitter Acetylcholine.
Remember!!!! Sweat glands are under the control of the sympathetic
but stimulated by Acetylcholine.
Receptors and Signal transduction mechanism:
At ganglia:-
Sympathetic and parasympathetic have nicotinic receptors at the post synaptic
membrane for acetylcholine.
{{It`s called nicotinic because they can be stimulated by
nicotine}}
The Ach cause the generation of action potential on the
second neuron`s membrane that`s mean that these
receptors are linked to Na+ channels.
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Note that:
In skeletal muscles somatic fibers release acetylcholine which have receptors
on the skeletal muscle`s cells those receptors are also nicotinic.
On effector cells:-
First (Ach):
Sympathetic and parasympathetic have Muscarinic receptors for acetylcholine on the
effector cell`s membrane {{They are called Muscarinic because they can be
stimulated by muscarine [A natural product found in toxic types of mushrooms ]}}
Clinical Application:
If someone gets intoxicated by these toxic mushrooms the Muscarinic receptors in
his body will be stimulated and this will cause: _
Stimulation of parasympathetic fibers and sympathetic fibers innervating sweat
glands. So the major symptoms of this case are:-
Vomiting and diarrhea as a result of increased gastrointestinal motility.
Urination as a result of urinary bladder contraction
Slowing of the heart rate {Bradycardia}
Contraction of pupil
Increased secretory activity causing :
Sweating
Salivation
Tearing
Nasal and bronchial discharge
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All these symptoms can be reversed by blocking muscarine receptors by ATROPIN
this will prevent the effect of muscarine on these receptors and reverse all these
effects
BUT Atropin has some side effects such as
Increase heart rate {Trachycardia}
Inhibition of glandular secretion dry mouth ,dry eyes ,dry nasal
passages
Loss of papillary light reflex and cause dilation of pupil { actually the drop we
use for eye therapy is composed of atropine }
Loss of ability to focus the lens for near vision
Subtypes of Muscarinic receptors: _
There is five subtypes of Muscarinic receptors have been identified numbered
{M1 --- M5}
Inhibitory:
M2 is an example of inhibitory receptor for Ach.
It is found in the conductive tissue of the heart.
It slows the rate of depolarization.
It function via G proteins :: The Gi protein binds to the enzyme
Adenylyl Cyclase and inhibits it , now the enzyme will stop converting
ATP to cAMP so the concentration of c AMP will decrease .
The final effect is decreasing the rate of depolarization and this can be
achieved in two methods :-
First: - Inactivating sodium channels
Second: - Activating potassium channels
Excitatory:-
{M1, M3, M5} are examples of excitatory receptors for Ach.
They are found in smooth muscle and glands.
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They are linked to Gq protein which when these receptors are
activated will dissociate and bind to the enzyme Phospholipase C
and activate it to break PIP2 and increase the concentration of
IP3 and this will result in increasing the calcium concentration in
the cytosol and causing the smooth muscles to contract .
SO…. We might have an inhibitory or excitatory effect of Ach depending on the
type of receptor.
Second {Norepinephrine}:
Both the hormone epinephrine and the neurotransmitter Norepinephrine which are
called Catecholamines have the same receptors called adrenergic receptors.
These Adrenergic receptors have many subtypes:
Alpha receptors:
The Alpha 1 adrenergic receptor (α1) :- It is excitatory linked to the Gq
protein which when bind to Phospholipase C enzyme it activate it and increase
the concentration of IP3 causing smooth muscles contraction
Note that:
During fight & flight reaction the cutanous blood vessels undergo
vasoconstriction so this means they have more alpha 1 receptors
The Alpha 2 adrenergic receptor (α2):- It is found in the nervous system on
the membrane of neural cells and it has inhibitory effect on neural activity.
Example:-
If we get any injury during any activity as long as our body is moving and
having stress we don`t feel pain .Pain sensation occurs in the relaxation state.
The reason is during stress Norepinephrine & epinephrine are binding to alpha
2 receptors over sensory neurons inhibiting the transmition of pain sensation.
Alpha 2 Heteroreceptors: - They are inhibitory for Adenylyl Cyclase enzyme
and they are noradrenergic.
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Beta receptors:-
Beta 1 receptor (β1):- It is found in heart and it is excitatory increasing the
rate of heart beats.
Beta 2 receptor (β2):- It is found in tracheal and bronchial smooth muscles,
in the gastrointestinal tract and on smooth muscles of blood vessels supplying
skeletal muscles. It is inhibitory of adenylyl Cyclase causing relaxation of
smooth muscles.
So receptors have excitatory or inhibitory effects over cells what determines
the final effect is the concentration of each type of receptor on these cells.
Note:-
Asthmatic patients usually have bronchoconstriction in order to cause
bronchial relaxation sometimes we give these patients epinephrine but we
have to be careful not to increase the heart rate as a side effect .
Blood vessels supplying muscles respond by vasodilation but vessels
supplying the skin are responding by vasoconstriction this difference
although this response is for the same hormone or neurotransmitter is
due to the difference in the type of receptors they bear
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Doctor`s answers to some questions:
Q: What is the difference between equilibrium potential and resting membrane potential??
A: The resting membrane potential includes the activity of the sodium-potassium
pump without this activity we get the equilibrium potential only
So resting membrane potential = equilibrium potential +activity of Na-K pump
Q: When can we use the Nernst or Goldman equation??
A: To see the activity of the membrane for one ion we use the Nernst equation but
for many ions we use Goldman equation.
Q; what is meant by Enteric Nervous System??
A: It is a subdivision of the ANS that directly controls the gastrointestinal system ,
If we removed the ANS the GI will function normally because it have its own
nervous system but the ANS have some effects on the GI like inhibition of motility
and secretion .
Bye …. Bye DR Khatatba ……….
Forgive Me for Any Mistakes
YOUR Colleague: - Rowa2 Lahaseh <3 <3