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Neuroscience and Mental Health > 5. Regulation of blood flow > Flashcards

5. Regulation of blood flow Flashcards

1
Q

What happens when blood flow to the brain is reduced by more than 50%?

A
  • More than 50% - function significantly impaired

* Total interruption - unconscious (irreversible damage >few minutes)

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2
Q

What is syncope and what are the causes?

A

• Fainting

  • low BP
  • postural changes
  • vaso-vagal attack
  • sudden pain
  • emotional shock

(results in temporary interruption of blood flow to the brain)

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3
Q

What can be metabolised if there is a shortage of glucose to the brain?

A

Ketones

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4
Q

What are the symptoms of hypoglycaemia when brain function is affected?

A
  • Disorientation
  • Slurred speech
  • Impaired motor function
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5
Q

What is the normal blood glucose range and what level does it have to fall to, to become dangerous?

A
  • Normal: 4-6mM

* Below 2mM - unconsciousness, coma and death

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6
Q

What range of BP can autoregulation of cerebral blood flow occur in order to maintain constant flow?

A
  • 60-160mmHg

* Arterioles dilate or contract

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7
Q

How is cerebral blood flow autoregulated?

A

Myogenic
• Increase in pressure on vessel wall
• Myogenic response - contraction of smooth muscle
• Decreased cerebral blood flow

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8
Q

What happens if blood flow is above the autoregulatory pressure range?

A
  • Swelling of brain

* Increase in intracranial pressure

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9
Q

What are the 2 types of local regulation of cerebral blood flow?

A

Neural and Chemical

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10
Q

Describe the pattern of vascularisation in the CNS tissues

A

(pia matter contains arteries and veins)
• Arteries enter as branches of the surface pial vessels
• These penetrate into the brain parenchyma
• Branch into capillaries => venules => veins => pial veins
• Neurone always with 100μm from capillary
• Deeper down:
- arterioles surrounded by smooth muscle
- capillaries surrounded by pericytes
- innervation of these contractile cells is how dopaminergic neurones regulate blood flow at a capillary level

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11
Q

Describe local regulation of blood flow by neural control

A
  • Sympathetic innervation of main cerebral arteries: high BP => vasoconstriction => less blood flow
  • Facial nerves are innervated by parasympathetic fibres - vasodilation
  • Central cortical neurones releasing neurotransmitters e.g. catecholamines adrenaline/NA - vasoconstriction
  • Dopaminergic neurones - vasoconstriction
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12
Q

What are pericytes?

A
  • Cells that surround capillaries in the brain
  • Brain macrophage with immune function
  • Transport properties
  • Contractile
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13
Q

Describe how dopaminergic neurones cause vasoconstriction?

A
  • Dopaminergic neurones innervate smooth muscle around arterioles and pericytes around capillaries
  • When active => contraction
  • Local action in certain areas allows for diversion of blood to more active areas of the brain
  • Caused via aminergic and serotoninergic receptors
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14
Q

How is local cerebral blood flow chemically controlled?

A
  • CO2, pH, nitric oxide, K+, adenosine, anoxia, histamines, prostaglandins etc. are all vasodilators
  • Increase blood flow to particular tissues
  • Active cells produce lactic acid => H+ causes drop in pH and vasodilation
  • K+ released at one stage of action potential
  • CO2 has indirect effect
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15
Q

How does H+ get to the brain?

A
  • H+ does not cross the BBB - generated within the brain
  • CO2 can cross BBB
  • CO2 + H2O => bicarbonate + H+ [carbonic anhydrase]
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16
Q

How can local changes to cerebral blood flow be imaged?

A
  • PET scan
  • Functional MRI (fMRI)
  • Increased blood flow => increased neuronal activity
17
Q

How does NO affect blood vessels?

A
  • NO stimulates guanylyl cyclase
  • converts GTP => cGMP
  • Vasodilation
18
Q

What lines the ventricles, aqueducts and canals of the brain?

A

Ependymal cells

19
Q

Where and how is CSF produced?

A

• Choroid plexus
- both lateral ventricles
- 3rd and 4th ventricles
• These comprise capillaries surrounded by ependymal cells - tight junctions/non-fenestrated - impermeable (even though capillaries are leaky)
• Ependymal cells secrete molecules into the ventricles to make the CSF

20
Q

What is the path of CSF circulation?

A
  • Lateral ventricles
  • 3rd ventricle
  • Cerebral aqueduct
  • 4th ventricle
  • Subarachnoid space
21
Q

What is the volume of CSF and how much is formed per day?

A
  • 80-150mL

* 450mL/day

22
Q

What is the function of CSF?

A
  • Chemical and physical protection
  • Nutrition
  • Transport
23
Q

How is CSF different to plasma?

A

• Lower: K+, calcium, amino acids, bicarbonate
• Higher: magnesium, chloride
• More acidic
• Little protein - protein in CSF suggests brain injury or infection
(same osmolarity)

24
Q

What is the function of the BBB?

A
  • Protects brain from certain toxins and circulating transmitters e.g. catecholamines
  • Protects from variations in ion concentrations
25
Q

Describe BBB capillaries

A
  • BBB has more dense pericyte coverage
  • More extensive tight junctions
  • Astroglial processes
26
Q

What substances does BBB control and how?

A
  • Tight junctions means that solutes than exchange across peripheral capillaries can’t cross the BBB
  • Applies mainly to hydrophilic solutes - glucose, amino acids, many antibiotics etc.
  • Specific membrane transporters can be used for control - water via aquaporins, glucose via GLUT1
  • Lipophilic molecules can cross easily e.g. O2, anaesthetics, alcohol
27
Q

How is reduced entry of blood-borne infectious agents into the CNS tissue significant?

A
  • Infections of the meninges more common - vessels not BBB

* Evidence that loss of BBB can help clear some infections - immune cell access

28
Q

What are circumventricular organs?

A

• Areas that lack BBB properties
• Found close to ventricles
• Fenestrated capillaries
• CVOs involved in secreting into circulation or sampling the plasma
• Include:
- median eminence region of hypothalamus and posterior pituitary
- area postrema samples plasma for toxins and induces vomiting
- subfornical organ
- organum vasculosum of the lamina terminalis

29
Q

When can the BBB break down?

A
  • Inflammation
  • Infection
  • Trauma
  • Stroke
30
Q

How do old antihistamines cross the BBB and how do second-generation antihistamines differ?

A

First-generation
• Hydrophobic - diffusion
• Histamine is important in wakefulness and alertness, so H1 antihistamines can induce drowsiness

Second-generation
• Polar
• Hydrophilic attachment
• Do not readily cross BBB - less drowsiness

31
Q

Why is the BBB significant in the treatment of Parkinson’s disease?

A
  • Dopamine can’t cross the BBB so peripheral administration doesn’t work
  • L-DOPA can cross the BBB via an amino acid transporter - then converted into dopamine by DOPA decarboxylase
  • However, most is converted outside the CNS - less available to access the brain
  • Can’t increase dose due to peripheral affects of dopamine
  • Can co-administer with DOPA decarboxylase inhibitor, Carbidopa - doesn’t cross BBB so only prevents peripheral conversion

Neuroscience and Mental Health (17 decks)

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