7) Nervous & hormonal control of vascular tone Flashcards
What is a paracrine signal?
- When a cell produces a signal that affects another cell nearby or next to it. (A cell that produces a hormone may affect a cell next to it or a few cells away)
What is an autocrine signal?
- When a cell produces a signal that affects itself
What are the local and extrinsic factors that control blood flow?
- Local: Myogenic (muscular) response, paracrine and autocrine signalling, physical factors (e.g. temp or stress)
- Extrinsic: Parasympathetic, sympathetic and sensory vasodilator nerves, Sensory vasoconstrictor nerves and hormones such as adrenaline, vasopressin, etc.
What part of the brain controls blood flow?
- The medulla oblangata
Describe the route of a sympathetic vasoconstrictor system.
- A main excitatory drive is sent from the medulla oblongata down the spinal chord.
- It emerges as preganglionic neurones between T1-L2 and enter sympathetic ganglia
- From here it travels down final sympathetic post-ganglionic fibres which innervate all vessels, adrenal glands and the heart
- Noradrenaline is mainly released at the site of the heart and vessels.
- Noradrenaline causes constriction of vessels (in alpha-1 adrenoreceptors) and causes heart rate and stroke volume to increase in the heart (beta-1 adrenoreceptors) as the heart beats harder and faster
- There is some parasympathetic innervation to the adrenal medulla which causes adrenaline to be released.
- Adrenaline interacts with beta-2 adrenoreceptors causing relaxation in vessels
Explain how an action potential causes contraction/ relaxation in smooth muscles.
- An action potential moves down the axon and arrives at varicosities (beads on a string) found in the adventitia layer.
- This causes depolarisation of the varicosities which activate voltage gated Ca2+ channels leading to an ingress of calcium ions.
- This ingress makes the cells release neurotransmitters (mainly noradrenaline)
- Noradrenaline diffuses into the smooth muscles where it binds mainly to a1 adrenoreceptors (causing contraction), a2 adrenoreceptors (causing some contraction) and b2 adrenoreceptors (causing relaxation)
- The noradrenaline can then be taken up again and recycled or broken down
How are responses of contraction and relaxation modulated in the smooth muscles?
- Depending on what receptors are present
What can increase the release of noradrenaline?
- Angiotensin II acts on AT1 receptors increasing noradrenaline release
What can inhibit noradrenaline release?
- Metabolites can prevent vasoconstriction to maintain blood flow through feedback and inhibiting noradrenaline release
- Noradrenaline can also negatively feedback itself through a2 adrenoreceptors to limit noradrenaline release
Why does the required vasodilation and vasoconstriction take place?
- Lots of modulation occurs at varicosities
What part of the medulla oblongata controls sympathetic vasoconstrictor nerves?
- The Rostral Ventrolateral Medulla (RVLM) provides central control of blood flow and blood pressure
- It is controlled by the Caudal Ventrolateral Medulla (CVLM) and the hypothalamus
What sets vascular tone?
- The tonic sympathetic activity.
- Sympathetic activity is tonic which means there is an action potential fired every second
What are the main roles of sympathetic vasoconstrictor nerves?
- Different sympathetic pathways that innervate different tissues allow us to vasoconstrict some vessels and vasodilate others
- Precapillary vasoconstriction which causes pressure drop in capillary leading to increased absorption of interstitial fluid into the blood plasma to maintain blood volume
- Control resistance arterioles which allows increased blood flow to occur (to the brain, heart and kidneys)
- Controls venous blood volume. Venoconstriction leads to decreased venous blood volume. This increases venous return which increases stroke volume through Starling’s law
How does vasodilation occur?
- Turning off vasoconstriction
- Through vasodilator nerves
What are vasodilator nerves?
- Parasympathetic nerves that control a vascular bed rather than global functions.
- These nerves release acetylcholine which binds to muscarinic receptors on smooth muscles.
- On binding to M3 receptors, on vascular endothelium, they cause vasodilation through the formation of nitric oxide (NO)
- On binding to M2 and M3 receptors, in smooth muscles, they cause contraction
Where is adrenaline released from?
- Adrenaline is released from the adrenals within the medulla oblongata through the action of acetylcholine on nicotinic receptors.
- Adrenaline is made during exercise, a Fight-or-Fight situation, hypotension and hypoglycaemia
What effect does adrenaline have on the body?
- Increases glucose mobilisation
- Increases heart rate and stroke volume during exercise
- Causes vasodilation of coronary and skeletal muscle arteries
Describe the action of adrenaline/noradrenaline on a1 adrenoreceptors.
- The neurotransmitter binds to a1-adrenoreceptor which activates phospholipase C
- Active phospholipase C metabolises PIP2 to IP3 and DAG which causes the release of Ca2+
- Ca2+ leads to smooth muscle contraction
Describe the action of adrenaline/noradrenaline on a2 adrenoreceptors
- The neurotransmitter binds to the receptor causing the inhibition of adenylate cyclase.
- This means ATP is unable to be processed into cAMP and smooth muscles do not contract (causing muscle relaxation)
- Furthermore it inhibits Ca2+ in a negative feedback system preventing the release of transmitters
Describe the action of adrenaline/noradrenaline on B1 and B2 receptors.
- The neurotransmitter binds to the receptor causing the activation of adenylate cyclase.
- This leads to the conversion of ATP to cAMP.
- In B1 this leads to heart contraction
- In B2 this leads to vasodilation
Which receptor is the main cause of vasoconstriction?
- A1 adrenoreceptors
Describe the distribution of the two adrenoreceptors in skeletal muscles and coronary arteries
-Skeletal muscles contain more B2 than A1 adrenoreceptors
Which receptor has a higher affinity for adrenaline?
- Beta adrenoreceptors
- They mainly bind to B2-adrenoreceptors to dilate vessels
Which receptor has a higher affinity for noradrenaline?
- Alpha adrenoreceptors
- They mainly act at A1-adrenoreceptors to constrict vessels