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Summary nervous and hormonal control of the circulation
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Summary of the nervous and hormonal control of the circulation
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Nervous and Hormonal Control of the CirculationThis session will cover:
Key elements of the sympathetic pathway from brain to i) a blood
vessel and ii) the adrenal medulla
How different types of nerves can either constrict or dilate arteries
The Renin-Angiotensin hormonal cascade
The meaning of 'nociceptor axon reflex'
The effects of adrenaline, vasopressin and angiotensin II on vascular
tone
The importance and main mechanisms of venous control
Vascular control can be categorised into local control and extrinsic control. The local
control is things involving the vessel itself such as: the myogenic response (ability of
blood vessels to regulate diameter according to pressure across them), paracrine
agents, nitric oxide, PGs and endothelin released from the endothelial cells.
There is also physical factors such as temperature, pressure and you also have
autoregulation (metabolic reactants/products affect blood vessels).
This lecture mainly focuses on extrinsic control, which include: vasodilator nerves,
sympathetic vasoconstrictor nerves (some produce dilation responses) and
hormones like adrenaline, angiotensin II, vasopressin.
Extrinsic Control
We have extrinsic control in order to regulate TPR to control blood pressure, BP will
influence where blood flows to. Extrinsic control is ultimately controlled by the brain,
which can alter blood flow selectively to organs when required e.g. exercise, sexual
activity – because it has overall control of the vasculature.
Extrinsic controls are “outflows” from the brain, so these are:
, -The release of neurotransmitters – e.g. vasoconstrictors (NA), vasodilators (Ach),
nitric oxide (NO).
-Hormones (adrenaline, angiotensin II, vasopressin)
The sympathetic vasoconstrictor fibres are the dominant extrinsic regulatory control
of vasculature. This mediating of release of NA is most widespread and most
important to the extrinsic control system.
Sympathetic Vasoconstrictor System –
The brain stem controls the outflow to the sympathetic system, the whole process
starts in the rostal ventral lateral medulla (RVLM) which receives info from the
hypothalamus, axons leave the RVLM and project down to the thoracic-lumbar
region to the pre-ganglionic fibres which start at the intermediolateral point. So it is
the main excitatory drive neurones (from RVLM) that stimulate the pre-ganglionic
fibres.
These short pre-ganglionic fibres leave the spinal cord and enter the sympathetic
ganglia, where they stimulate the post-ganglionic neurones, which go off to their
target organs.
At the heart you have release of NA acting at β1 receptors, at the blood vessels there
is release of NA to α1 and at β2 receptors (beta receptors found in specific tissues like
coronary circulation and skeletal muscle circulation).
Sympathetic fibres also go to the adrenal medulla (release Ach on Nic), these are
single fibres with no ganglia. They stimulate the adrenal medulla to release
adrenaline (small amounts of NA can be released from adrenal medulla also) that
enters the circulation.
Noradrenaline is released from sympathetic fibres at strings of varicosities
In the blood vessel wall:
-Tunica media is where the vascular smooth muscle is found and these are mainly
circular in formation, so contraction of them causes constriction of lumen diameter
-Tunica adventitia contains various structures but importantly contains the
sympathetic vasoconstrictor fibres, these fibres come close to the tunica media and
have (special) synapses called varicosities.
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