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The nervous control of the circulatory system and the rapid control of arterial pressure.
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Topic: The NervousControl of the
Circulatory System
and the Rapid control
of the Arterial
pressure
Textbook(s): Guyton
(p. 215-225; 241-242)
& Widmaier (p. 411-
416)
,Questions Notes
1. Define the G. p. 215-217, 777.
following terms Medulla: Part of the brainstem closest to the spinal cord; controls many
vegetative functions e.g. breathing, heart rate.
Medulla, Sympathetic Nervous System: Part of the autonomic nervous system that
sympathetic leaves the CNS at the thoracic and lumbar portions of the spinal cord.
nervous system, Parasympathetic Nervous System: portion of the autonomic nervous system
parasympathetic which leaves the CNS from the brainstem and sacral portion of the spinal
nervous system, cord; most of its fibres release acetylcholine.
spinal nerves, Spinal Nerves: one of the 86 peripheral nerves that join the spinal cord.
vagus nerve, Vagus Nerve: cranial nerve; major parasympathetic nerve
vasomotor Vasomotor Center: Located bilaterally mainly in the reticular substance of
centre, the medulla and of the lower third of the pons. Transmits parasympathetic
vasoconstrictor impulses through the vagus nerve to the heart and the sympathetic impulses
area, vasodilator through spinal cord and peripheral sympathetic nerves to all arteries,
area, sensory arterioles, and veins of the body.
area, vasomotor Vasoconstrictor area: Located bilaterally in the arterolateral portions of the
tone, vasovagal upper medulla. Distributes fibres to all levels of the spinal cord- excite
syncope, preganglionic vasoconstrictor neurons of the SNS.
norepinephrine, Vasodilator area: Located bilaterally in the anterolateral portions of the
acetylcholine, lower half of the medulla. Fiber’s project upwards to the vasoconstrictor
nicotine and area- inhibit vasoconstrictions.
muscarinic Sensory area: located bilaterally in the nucleus tractus solitarius in the
receptors, alpha posterolateral portions of the medulla and the lower pons. Neurons receive
and beta sensory nerve signals from the circulatory system mainly through the vagus
adrenergic and glossopharyngeal nerves, and the output signals from this sensory area
receptors then help control activities of both the vasoconstrictor and vasodilator areas
of the vasomotor center.
Vasomotor tone: The vasoconstrictor area transmits signals continuously to
the sympathetic vasoconstrictor nerve fibers over the entire body, causing
slow firings of these fibers at a rate of about one half of two impulses per
second. These impulses normally maintain a partial state of contraction in
the blood vessels.
Vasovagal syncope: emotional fainting; muscle vasodilator system becomes
activated and at the same time, the vagal cardioinhibitory center transmits
strong signals to the heart to slow the heart rate markedly. The arterial
pressure drops rapidly, reducing blood flow to the brain and causes
unconsciousness.
Norepinephrine: sympathetic vasoconstrictor neurotransmitter. secreted at
the endings of the vasoconstrictor nerves, acts directly on the alpha-
adrenergic receptors of the vascular smooth muscles.
Acetylcholine: neurotransmitter released by the pre- and postganglionic
parasympathetic neurons, preganglionic sympathetic neurons, somatic
neurons and other CNS neurons.
, Nicotine and muscarinic receptors: activated by acetylcholine; Muscarinic
receptors use G proteins as signalling mechanism, found on all effector cells
that are stimulated by the postganglionic cholinergic neurons of either SNS
or PSN. Nicotinic receptors are ligand-gated ion channels found in
autonomic ganglia at the synapses between pre- and postganglionic
neurons of both SNS and PSN. Present at many non-autonomic nerve
endings e.g. neuromuscular junction.
Alpha and beta adrenergic receptors: Two major types of alpha receptors;
alpha1 and alpha2- linked to different G coupled proteins.
Beta receptors are divided into eta1, beta2, and beta3- certain chemicals
only affect certain beta receptors. Also use G coupled receptors for
signalling.
Norepinephrine and epinephrine both have different effects on it.
Norepinephrine mainly excites alpha receptors but excites beta receptors to
a lesser extent.
Epinephrine excites both types of receptors equally.
Table 61-1 (p778)
2. Describe the The Sympathetic Nervous System:
nerve supply to
the heart and Sympathetic vasomotor nerve fibers leave the spinal cord through
blood vessels. the thoracic spinal nerves and through first one or two lumbar spinal
nerves.
pass immediately then into a sympathetic chain- lies on each side of
the vertebral column.
Pass through two routes:
o through specific sympathetic nerves that innervate mainly the
vasculature of the internal viscera and the heart.
o almost immediately into the peripheral portions of the spinal
nerves distributed to the vasculature of the peripheral areas.
Sympathetic innervation to the blood vessels:
Sympathetic nerve fibres are distributed to all the blood vessels
except the capillaries. Precapillary sphincters and metarterioles are
innervated in some tissues.
Innervation of small arteries and arterioles allows the increase of
resistance to blood flow and thus decrease rate of blood flow to the
tissues.
Innervation of large vessels-veins, causes the decrease of the volume
of the vessels. This decrease can push the blood into the heart and
thus plays a major role in the pumping of the heart. Also, increasing
the heart rate and enhancing its strength and contractility.
, Parasympathetic Stimulation:
Most important circulatory function is the control of the heart rate by
way of the parasympathetic nerve fibers to the heart and the vagus
nerves.
It causes the decrease in the heart rate and the slight decrease in the
contractility.
3. Explain the
importance of
cardiac turnover
and total Cardiac output and the total peripheral resistance determine the mean
peripheral systemic arterial pressure because they determine the average volume of
resistance in blood in the systemic arteries over time; it is this blood volume that causes
determining the pressure.
average arterial
pressure. W. p.
407-410
W. p. 409.
4. Give a
summary of the
factors that
determine the
systemic arterial
blood pressure.
(Flow chart)
5. Make a The approximate and long-term control responses can be divide into 3
summary of the groups:
mechanisms
involved in blood 1. those that react rapidly, within seconds or minutes
pressure 2. respond over an intermediate time period, minutes, or hours.
regulation and 3. provide long-term arterial pressure regulation for days, months and
indicate how years.
they are
, 1. Those acting rapidly:
Mostly entirely acute nervous reflexes or responses. These mechanisms are:
baroreceptor feedback mechanism
central nervous system ischemic mechanism
chemoreceptor mechanism
These act in second and are very powerful. After any acute fall in arterial
pressure, the nervous mechanism combine to cause:
the constriction of the veins and transfer of blood into the heart
increased heart rate and contractility of the heart to provide greater
pumping capacity by the heart
constriction of most peripheral arterioles impede flow of blood out of
the arteries.
All these occur immediately to raise arterial pressure.
integrated.
2. Those acting at an intermediate phase:
1. renin-angiotensin vasoconstrictor mechanism
2. stress-relaxation of the vasculature
3. shift of fluid through tissue capillary walls in and out of the
circulation to readjust blood volume as needed.
3. Those acting over long-term:
The kidneys play a role in regulation of the arterial pressure. By the renal-
blood volume pressure control mechanism.
Factors affecting this mechanism include:
aldosterone- decrease in arterial pressure results in an increase in
aldosterone secretion.
Without a functional renin-angiotensin-aldosterone system, blood pressure
becomes very sensitive to changes in the salt intake.
6. Discuss the
The carotid sinus regulates the arterial pressure. The wall of the artery is
role of the
thinner than usual and contains a large number of branching, sensory
carotid sinus and
neuronal processes. And serve as pressure receptors/ baroreceptors.
the aortic arch
The aortic arch baroreceptor is found in the arch of the aorta. These two
baroreceptor in
constitute the arterial baroreceptors.
the baroreceptor
Sensitive to the stretching of blood vessel wall
reflex. W. p. 410
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