Download the autonomic nervous system (ANS)

neurophysiology /lec 7 / dr.muhannad
the autonomic nervous system (ANS):
_definition
it is part of the peripheral nervous system concerned with the control
and regulation of bodily visceral functions like :(blood pressure, heart rate,
GI motility, GI secretion, regulation of body temperature, sweating ect...)
*the most striking characteristic of the ANS is it's rapidity & intensity
(means: the ANS can change the visceral functions so rapid and so intense
)
ex1: A normal resting heart rate(pulse rate) for adults ranges from 60 to
100 beats per minute but through the function of ANS this rate can be
increased twice normal within 3-5 seconds especially during intense
exercises.
ex2:normal resting blood pressure in young adults range from (120/80 in
men) (110/70 in women) also through the effect of ANS the blood pressure
can be doubled within 5-10 seconds in case of performing unusual
exercise.
_ general organization of ANS
the control centers of this system are located at the level of spinal cord ,
brainstem ,hypothalamus(especially);which is a big control center of the
ANS ,as well as a certain area in the cerebral cortex called the limbic cortex
which is part of wide spread system called limbic system concerned with
emotions ,motivations and behaviors.
_physiologic anatomy of ANS
the ANS is divided into 2 main subdivision: 1/sympathetic division
2/parasympathetic division .
means: the nerves carrying the responses to the effector organs after
exciting the ANS centers will be brought in 2 types (sympathetic nerves &
parasympathetic nerves).
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1_sympathetic subdivision:
the
sympathetic division arises from the spinal cord in between the segments
(T1- L2).
the nerve fibers originating from the intermediolateral horns of the the
spinal cord will emerge and pass through the ventral route of the spinal
cord and terminate in one of the ganglia in the ganglion chain on both sides
of the vertebral column (paravertebral chains).A second neuron will start
from the ganglia to the effector organ.
note :the anterior horns of
spinal cord are for motor
neurons
-posterior horns are for
sensory neurons
-intermediolateral
horns
are sites of origin of
sympathetic nerves
Q/ the nerve fibers of the sympathetic NS are: a/similar , b/different from
the spinal motor nerves???
A/ in the spinal motor nerves: there is a single neuron passing from the
motor neurons in the ventral horn of the S.C. passing through the ventral
route to the skeletal muscles directly (one neuron); while in the sympathetic
division of ANS there are 2 neurons so as the parasympathetic:
These two neurons of a single system (the symp. or parasymp.) can be
divided into two neurons.
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1/preganglionic neuron: arising from the intermediolateral horn to the
sympathetic chain.
2/postganglionic neuron: start from the ganglia to the effector organ.
basic characteristics of ANS:
in both subdivisions of ANS one of two neurotransmitters is released from
the nerve endings, this neurotransmitter is either acetylcholine or nor
epinephrine (nor adrenaline)
-at the preganglionic neurons: acetylcholine is the neurotransmitter
released in both subdivisions of ANS(sympathetic and parasympathetic)
-at the postganglionic neuron: in case of sympathetic subdivision the
neurotransmitter released at the terminal nerve ending is nor epinephrine.
2_parasympathetic subdivision :
can be subdivided into 2 main parts:
1/cranial (arising from the cranium)
2/sacral (arising from sacral segment of spinal cord)
the parasympathetic subdivision also
consist of 2 main neurons(similar to
the sympathetic division): the one that
start from the intermediolateral horns
sending its nerve fiber directly to the
effector organ is called preganglionic
neuron, and a postganglionic neuron
which is very short segment (few mm)
present in the wall of the effector
organ. (this is point of similarity
between
sympathetic
&
parasympathetic subdivisions)
*in
sympathetic
division:
the
preganglionic neuron arising from the
spinal cord to the ganglion chain is
too short & the postganglionic have a
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long fiber to pass to the effector organ.
*while in parasympathetic division (exactly the opposite): preganglionic
neuron originating from the spinal cord (sacral division) will pass all the way
to the effector organ (too long) whereas the postganglionic will start in the
wall of the effector organ and passing very short distance (few millimeters
up to few centimeters).This is a point of difference between the 2
subdivisions.
*the neurotransmitter released in the parasympathetic subdivision at the
site of the preganglionic neuron is acetylcholine (in a similar way to the
sympathetic). Whereas the neurotransmitter released at the site of
postganglionic neuron in parasympathetic subdivision is again
acetylcholine. (there is only 1 neurotransmitter in the parasympathetic
division)
the cranial part of parasympathetic subdivision:
it is represented by 4 cranial nerves :
- oculomotor (3rd cranial nerve)
- facial (7th cranial nerve)
- glossopharyngeal (9th cranial nerve)
- vagus nerve (10th)
mechanism of release of neurotransmitter:
1-arrival of action potential to the nerve terminal
2-depolorization of the nerve terminal.
3-change in the permeability of the cell membrane of the nerve terminal to
Ca++ ions. When Ca++ channels are activated the permeability of the
nerve terminal to Ca ions will increase.
4-Ca++ ions influx ensues.
5-Ca++ ions activate the vesicles in the nerve terminal that contain the
neurotransmitter to attach to the cell membrane of the nerve terminal.
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at the site of attachment the membranes burst and the contents of the
vesicles (the neurotransmitters) will be released to the synaptic cleft
Q/the neurotransmitter molecules remain in the synaptic cleft for how
long??
A/ They remain for few seconds, and it will be removed afterward (removal
of neurotransmitter from the synaptic cleft depends on the type of
neurotransmitter).
1-removal of acetylcholine is by destruction of acetylcholine molecule by a
specific enzyme, called acetylcholine esterase within few seconds.
2-removal of nor epinephrine from the synaptic cleft occurs by 3
mechanisms:
a/ It is mainly removed by re-uptake of norepinephrine into the nerve
terminal itself (about 80% of total amount of the released nor epinephrine is
re-up-taken by the nerve terminal).
b/ most of the remaining norepinephrine will diffuse into the surrounding
tissue .
c/ only a little amount will be removed by destruction mechanism by an
enzyme called mono amino oxides (which is present in the nerve terminal)
or by another enzyme Catechol-O-Methyl Transferase (C-O-MT) which is
present in all tissues especially in the liver for the destruction of
norepinephrine released from the adrenal medlla.
effect of neurotransmitter on the organ:
in order to exert its effect , the neurotransmitter must bind to a specific
receptor on the cell membrane of the effector organ.
The receptors are integral proteins passing through the cell membrane and
consisting of 2 main parts: (external part which represents the binding site
to bind to the neurotransmitter, and internal part which can be an enzyme
that changes the function of the effector organ).
*the neurotransmitter changes the function of an organ through the
following mechanisms:
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1- The outer portion of the receptor represents the binding site of the
transmitter molecule. The reaction between the neurotransmitter and the
receptor induces a certain change in the configuration of the receptor itself.
in case of the receptor is an ion channel changing the configuration will
activate this channel and open it up and the permeability of the membrane
will increase eventually.
Q/what will happen to the cell in case of the receptor is?
1- Na+ or Ca2+ channel.
2- K+ or Cl- cannel.
note: Na+ & Ca2+ & Cl- are extracellular ions
K+ is an intracellular ion
A/-1-when the receptor is an ion channel either for Na+ or Ca2+. the reaction
of the neurotransmitter will activate and open up the receptor , as a result
Na+ or Ca2+ ions will enter into the cell & depolarization will occur to the
polarized cell at rest (the cell will be activated).
2- a/ since K is an intracellular ion, activation of the receptor and increasing
the permeability of the membrane will force K ions to move out of the cell
down it's concentration gradient (carrying with it positive charges) causing
an increased negativity inside the cell & the cell will be more difficult to be
excited (inhibited cell).
b/ when the cell membrane's permeability is increased to chloride , Cl ions
will move to the interior of the cell carrying with it negative charges ,in both
cases (K+ & Cl-) there will be an increased negativity inside the cell & the
cell will be more difficult to be excited (inhibited)
2-(second mechanism)
The internal portion of the receptor is either an enzyme itself or an enzyme
might be attached to it
so the reaction between the neurotransmitter and the outer portion of the
receptor can create a change in the activity of the inner portion (that is
either an enzyme or to the enzyme attached to the inner portion), this
activated enzyme will create a change in the chemical machinery inside the
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cell (activating a series of chemical reactions inside the cell) so the cell will
undergo a change in its function either in the form of inhibition or excitation.
types of receptors:
receptors can be subdivided into 2 main types.
1- alpha receptors: (subdivided into alpha1 & alpha2).
2- β receptors : (subdivided into β1 & β2 ).
Studying the types of receptors is important to know what kind of drug we
should give the patient in each condition because there are drugs that can
affect just a certain type of receptors and not the other.
in the parasympathetic system the receptors are described as cholinergic
receptors that are specific for acetylcholine.
these cholinergic receptors can be divided into 2 types :
1-muscarinic receptors: These receptors are present in the effector organs
and are excited by a poison called muscarine which is present in the frog
stool.
2-nicotinic receptors: This type of receptors is present in the nerve terminal
of the preganglionic neurons of the parasympathetic system they can be
excited only by nicotine & nicotine-like substances.
whereas receptors of the postganglionic neuron (on the effector organ) are
muscarinic receptors & can be excited by muscarine and muscarine like
substances.
The receptors on the effector organs of the sympathetic nervous system
are called adrenergic receptors (the name is derived from adrenaline the
British name of epinephrine). These receptors can be divided into alpha
( α1& α2) & beta (β1 &β2). The importance of acquiring this knowledge is
because there are drugs that can excite each certain type of these
receptors but not the other.
A similar mechanism to the excitation of the sympathetic nervous system,
is through the hormones (1-epinephrine 2-norepinephrine) from the adrenal
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medulla (which is the innermost portion of the adrenal gland present at the
top of the kidney) which have similar effects of exciting the symp.NS .
-Basically the dual control mechanisms of the bodily organs (viscera) are:
1- The sympathetic nervous system.
2- The adrenal medullary hormones.
Q/the adrenal medulla can be classified as part of sympathetic nervous
system. Why?
because its innervation comes through the sympathetic nerves
preganglionic segments that arise from the intermediolateral horns of the
spinal cord to the adrenal medulla, and the cells of the adrenal medulla are
considered as the post ganglionic neurons, but are now modified to
perform secretory function (secrete epi. & norepi.) instead of conductive
function.
So the effect of hormones of the adrenal medulla is similar to the effect of
the sympathetic nervous system because it releases epinephrine & nor
epinephrine (which is the same neurotransmitter released from the post
ganglionic nerves of the sympathetic nerve ending).The difference between
the two is that the adrenal medullary hormones have generalized effects on
the body systems.
The reason behind having dual control systems is that, one can substitute
for the other if one is missing, support each other & to induce sympathetic
change in all cells of the body since the sympathetic system can't reach to
all body cells whereas the blood carrying the medullary hormones can.
-action of epinephrine & nor epinephrine :
*nor epinephrine excites alpha receptors more than β receptors.
while epinephrine excite both receptors approximately equally.
ex: The receptors on the heart are of β type.
in case of a patient suffering from inefficient cardiac contractions we give
him epinephrine injection directly into the heart which excites beta
receptors more than norepinephrine (in order to strengthen cardiac
contractions).
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ex2: The receptors on the blood vessels are of alpha type.
in case of a patient with hypovolemia or low blood pressure, we need to
induce vasoconstriction to improve venous return to the heart by giving
norepinephrine injection which excites alpha receptors more than
epinephrine to induce vasoconstriction.
_the effect of ANS on specific organs:
1-The eye: the ANS control two functions of the eye:
a/ the pupillary size varies depending on the muscles of the iris (innervated
by the symp. & parasymp.) that control the amount of light entering into the
eye.
moving from dark room to sunlight will reflexly excite the circular muscle
fibers (innervated by parasymp. fibers) and pupillary constriction will occur
(to decrease the amount of light striking the retina).
moving from sunlight to a darkroom will cause the pupil to dilate due to the
presence of radially arranged muscle fibers around the pupil that can pull
on the periphery of the pupil & dilate it (these radially arranged muscle
fibers are innervated by the sympathetic system).
b/focusing on near objects require a change in the thickness (increased
thickness) of the eye lens .
The thickness of the eye lens is controlled by the ciliary muscles (arranged
around the lens), contraction of these muscles induced by parasymp.
activity will increase the thickness and the refractive power of the lens &
eventually the eye will focus on near objects.
The lens is surrounded by radial ligament, tension of these ligaments make
the lens suspended flat, this reduces the refractive power & allows the lens
to focus for distant objects.
2-the heart:
-sympathetic excitation increases heart rate & force of contraction(heart
acceleration) this is exactly what happens in case of decreased blood
pressure.
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-while parasympathetic excitation can decrease heart rate & force of
contraction.
3-GI tract :
The GIT has it's own nervous system built up in the wall of the GI tract, it is
called the enteric nervous system for the control of the GIT function.
Nevertheless, GIT function can be modified by the ANS to increase or
decrease the overall activity of the GIT.
-the parasympathetic division will increase the overall activity of the GI tract
through increasing its motility to push its content at a rapid rate &
increasing the rate of its secretion.
-while sympathetic division decreases the motility & the rate of secretion of
GI tract.
4- emptying the rectum and urinary bladder occur through the activity of
parasympathetic division, this emptying process occur by activation of the
stretch receptors that are present in the wall of the rectum, these receptors
are sensitive to stretch(that's why we call them as stretch receptors) . when
the rectum is full with fecal matter these receptors are stretched, after that
they send the information to the spinal cord (sacral segment) & the
response will be brought back from the spinal cord to the rectum (to the
distal portion of the colon) to induce contraction & evacuation of the
intestinal contents by the action of defecation.
In a very similar way when the urinary bladder is filled with urine through
the stretch receptor in the wall of urinary bladder & sensory information
passes to the spinal cord & a response will be brought back to the bladder
(effector organ) to induce contraction and urination.
_effect of ANS on the glands:
The function of the glands such a lacrimal gland, nasal gland ,salivary
glands and GIT glands is increased by the parasympathetic nervous
system.
- exciting the parasympathetic division will cause an increase in the
secretion of lacrimal, nasal, salivary, GIT glands and even sometimes the
pancreas (pancreatic secretion ).
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These previously mentioned examples indicate that it is not a rule to have
opposite effects of the sympathetic and parasympathetic nervous systems.
Since exciting the glands increases their rate of secretion by
parasympathetic only (the sympathetic activity will do nothing).
_the autonomic reflexes:
1- The Cardiovascular autonomic reflexes:
Because brain is a very sensitive organ in the body so that the blood
pressure in the vessels of the brain mustn't change severely (severe rise in
the BP may cause stroke or intracranial hemorrhage), therefore it require
some control system or mechanism to prevent the elevation of blood
pressure inside the brain. This mechanism is the cardiovascular autonomic
reflex, these reflexes depend another type of receptors that are sensitive to
pressure (baroreceptors), they're present at the very beginning of the
internal carotid artery that goes to the brain. When blood pressure is
increased, these receptors will detect this change in blood pressure, and
send the information to the brain stem (where the cardiovascular control
centers are present) to excite the mechanisms (through inducing
vasoconstriction and decrease heart rate) that will reduce blood pressure
and put it back to normal, so that pressure passing to the brain will be
under the control of this mechanism and prevent hazards as in case of
hemorrhage inside the brain.
2- Gastrointestinal autonomic reflexes:
Of these reflexes are the gastrocolic reflex, the gastoenteric reflex and
others. When the stomach is filled with food this will excite stretch receptors
in the wall of the stomach and the signal will pass to the spinal cord and the
response is brought back to the colon to increase motility (induce
contraction) of the contents of the colon are pushed out, to allow more
space for the newly introduced food. This is especially important for ladies
because when she feed the young kids with breastfeeding or artificial
feeding, the moment that the stomach is filled with milk the gastro colic
reflex will be activated and the kid will pass to stool.
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_The mass response:
Sometimes the activity of autonomic nervous system might be specific or
localized and other times it is generalized we call it mass reflex or mass
response.
- when you expose a certain area of the skin to heat, firstly the area will be
red in color ,secondary it will start sweating to control the increase in
temperature.( it is localized response because you exposed localized area
to increase temperature).
-At other times the reflex might be wide spread and the excitation might be
excessive through the interconnections between the different segments of
the spinal cord at level (T1_L2).
i.e: When the stimulus is severe and arising centrally from the
hypothalamus the response will be massive and wide spread involving
many segments of the spinal cord and eventually many organs of body will
be affected. We call this response as mass response( mass reflex) and
sometimes is called as fight or flight. Because usually in animals in such a
situation when the animal is exposed to something dangerous their blood
pressure increases, The pupils are dilated, The respiratory rate will be
increased, also break down of glycogen stored in the muscles and liver to
release glucose so that the level of glucose in circulation will be increase &
the total metabolic rate of the body is increased. All of these responses
result from the excitation of the nervous system in these abnormal
situations (fear, stress or excitation). So that the animal will be either
prepared to stand and fight or to run away (for this reason it call as fight or
flight reflex).
Thank you so much for the very nice language well written and informative lecture. I highly
appreciate it.
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