Blood Supply and Brain Metabolism PDF

SESSION 8: BLOOD SUPPLY OF CNS AND BRAIN METABOLISM ❖ Blood supply: CNS ❖ Venous drainage: CNS ❖ Cerebral blood flow ❖ BBB ❖ Brain metabolism 1 A. BLOOD SUPPLY Structure of thi...

SESSION 8: BLOOD SUPPLY OF CNS AND BRAIN METABOLISM ❖ Blood supply: CNS ❖ Venous drainage: CNS ❖ Cerebral blood flow ❖ BBB ❖ Brain metabolism 1 A. BLOOD SUPPLY Structure of this part:  Blood supply to brain  Venous drainage: cerebrum  Blood supply: spinal cord  Venous drainage: spinal cord 2  BLOOD SUPPLY TO THE BRAIN  two pairs of vessels: internal carotid a. and vertebral a.  Internal carotid a. (anterior circulation) :  Cerebral part: ascends to base of skull – enters carotid canal in petrous part of temporal bone  immediately gives off opthalmic a.  then branch into :anterior and middle cerebral a. (cavernous part enters subarachnoid space) Anatomic Aspects of Cerebral Circulation Chaves, Claudia J., Netter's Neurology, 14, 164-168 Arteries to Brain and Meninges. Copyright © 2020 Copyright © 2020 by Elsevier, Inc. All rights reserved.  BLOOD SUPPLY …  Vertebral a.(posterior circulation)  from first part of subclavian a.  ascends through transverse foramina of C6 – C1  pierces the dura mater and arachnoid and enters subarachnoid space at level of foramen magnum  runs ant on medulla oblongata  unites with other vertebral a. of opposite side  form basilar a. that runs over pons  ends in 2 posterior cerebral a. Anatomic Aspects of Cerebral Circulation Chaves, Claudia J., Netter's Neurology, 14, 164-168 Arteries to Brain and Meninges. Copyright © 2020 Copyright © 2020 by Elsevier, Inc. All rights reserved.  CEREBRAL ARTERIES: CIRCLE OF WILLIS NB anastomosis between the arteries that supply the cerebrum  encircles the optic chiasma and the infundibulum (of pituitary gland) Circle of Willis Supply brainstem 5  RULE OF THUMB: THE 3 CEREBRAL ARTERIES SUPPLY A SURFACE AND A POLE  Ant cerebral:  Med (and Sup) surface and frontal pole  Middle cerebral:  lat surface and temporal pole  Post cerebral:  inferior surface and occipital pole 6 TOPOGRAPHIC MAPS 7 NEUROLOGICAL DEFICITS -1 Patient with a blockage of the ACA. -Med (and Sup) surface and frontal pole  paresis of the contralateral leg with sensory loss,  deficits in gait and micturition  mental impairment  dyspraxia. 8  NEUROLOGICAL DEFICITS -2 Patient with a blockage of the MCA.-lat surface and temporal pole affected  contralateral paralysis of the arm and the lower part of the face (upper motor neuron lesion)  with contra lateral sensory loss  loss of language if the dominant hemisphere is involved. 9  NEUROLOGICAL DEFICITS -3 Patient with a blockage of the PCA- inferior surface and occipital pole  visual field defect (usually a contralateral homonymous hemianopia with macular sparing, as the foveal representation at the occipital cortical area receives some overlap supply from the MCA)  amnesic syndrome,  disorders of language and  occasionally complex visual perceptual abnormalities. 10 CONTRALATERAL HOMONYMOUS HEMIANOPIA refers to:  the corresponding visual fields of the two eyes (temporal and nasal fields)  the corresponding parts of the visual pathways (e. g., temporal and nasal hemiretinas) 11 NEUROLOGICAL DEFICITS – PATIENTS WITH CARDIOVASCULAR DISEASE [HYPERTENSION] Occlusion/blockage of the penetrating arteries from a large cerebral artery, most commonly from the Circle of Willis = lacunar infarcts (small subcortical lesions) affecting the Internal Capsule  Hemiparesis of contralateral face ,arm and leg - syndromes cross the barriers of specific cerebral artery domains 12 CRANIAL NERVES CLOSELY RELATED TO A’S 13 BRAIN: VENOUS DRAINAGE  Venous sinuses = channels formed between the two layers of the dura mater  Cerebral veins drain into the sinuses Internal jugular veins 14  BRAIN: VENOUS DRAINAGE (Superolateral surface) Superficial veins Inferior Saggital sinus (medial surface) Sigmoid Sinuses FIG 6.7 Arrangement and interconnections of superficial and deep veins. Vanderah, Todd W., PhD, Nolte's Essentials of the Human Brain, 3, 36 Internal Jugular veins Copyright © 2019 Copyright © 2019 by Elsevier, Inc. All rights reserved. 15  SPINAL CORD: ARTERIAL BLOOD SUPPLY  1 Ant (single) and 2 post v (paired) spinal arteries descends inferiorly as branches of vertebral artery  Radicular medullary arteries running along the spinal nerve roots, that arise from spinal branches of lumbar or intercostal arteries 16  SPINAL CORD: VENOUS DRAINAGE  1 ant. & 1 post. spinal veins; single, in midline  2 anterolateral & 2 posterolateral spinal veins; irregular, bilateral, paired (sometimes incomplete)  = total of 6 veins drain via ant. & post. radicular veins  into the internal vertebral venous plexus between the dura mater and the vertebrae (in epidural space);  via ascending lumbar azygos and hemiazygos veins (inf spinal cord), or via vertebral veins (sup spinal cord)  into the vena cava 17 B. CEREBRAL BLOOD FLOW Structure of this part:  Monro-Kellie Doctrine  Control of brain circulation  Effect of O2 and CO2 on brain blood flow  Autoregulation - arterial pressure changes 18 MONROE-KELLIE DOCTRINE  Cranial cavity is closed rigid box and a change in the quantity of intracranial blood can occur only through the displacement /replacement of CSF.  = relationship between the contents of the cranium and intracranial pressure.  three components exist in equilibrium to maintain normal intracranial pressure, the brain tissue, the blood, and the cerebrospinal fluid.  Any change in volume of intra-cranial contents - ↑ ICP  ↑ ICP herniation of cranial content and ultimately death  Warning symptoms and signs 19 ↑ ICP Neck stiffness, severe headache, opisthotonus in kids, papilloedema … ↑ ICP  Dura forms a sheath around optic nerve  Increased ICP – shift to posterior eyeball - swelling of optic disc – seen with ophthalmoscope – papilledema  Thus papilledema = sign of ↑ ICP LP – MEASURE ICP (N= 10-20CMH20) CONTROL OF BLOOD CIRCULATION: BRAIN  brain itself - primary control of cerebral blood flow  2,5 % of the total body weight  15% of the N resting cardiac output (750 to 900ml/min)  Normal blood flow rate (adult) = 50 to 65 ml / 100 g / min.  Total cerebral blood flow and cerebral oxygen consumption - amazingly constant  Increase in regional flow - associated with the release of local vasodilator metabolites - occurs in that region alone.  Metabolites implicated in the control of cerebral blood flow include H+, K+, CO2, O2 and adenosine. 23 EFFECT OF O2 AND CO2 ON CEREBRAL BLOOD FLOW  Increasein CO2 (hypercapnea) and /or H+ ions increases cerebral blood flow – vasodilation  Whereas hypocapnea = hyperventilation – decrease in CO2 – vasoconstriction - syncope  Decrease in O2 (hypoxia) increases cerebral blood flow – vasodilation  Responses to O2 are very small compared to those elicited by changes in CO2. 24 AUTOREGULATION OF CEREBRAL BLOOD FLOW  Constant between arterial pressures of 60 to about 140mmHg  Normocapnic (CO2 normal) - auto regulation very good  Arterial pressure < 50mmHg - cerebral ischemia due to under perfusion  fainting  Pressures > 180mmHg  cerebral oedema or cerebral haemorrhage 25 C. BBB  3 barriers:-  BBB: brain capillaries  Arachnoid barrier  Choroid epithelium  BBB missing at circumventricular organs  (eg. Hypothalamus – in walls of 3rd and 4th ventricles): tight junctions to prevent diffusion into rest of brain  Infection (viruses – polio/rabies); drugs – link to lipid soluble for treatment Astrocytes wrap their podocytes around the capillaries 26 Zona occludens 27 BBB… Permeability  high for gases  O2, CO2 and anaesthetic gases. Alcohol and lipids.  low for antibiotics such as penicillin  difficult to treat infections of the brain.  low for protein and cholesterol - large molecular size  useful in assessing vascular integrity in the brain (LP) 28 BBB… Radioactive isotopes bound to protein - injected into the blood - brain scans: determine if the label is escaping from the vessel.  Ruptures, aneurysms, and increased permeability at tumour sites.  Newtechnology: CAT (computerised axial tomography) scans and NMR (nuclear magnetic resonance) replaced the above 29 FUNCTIONS OF THE BBB 1. It protects the brain from endogenous or exogenous toxins. 2. It prevents escape of neurotransmitters from their functional sites in the CNS into the general circulation. 3. (It maintains a constant environment for neurones in the CNS) 30 BRAIN OEDEMA  Definition: transudation of fluid from the capillaries into the brain  3 protecting systems against brain oedema:  "Glial feet“  thickening of brain capillaries  arterioles remain significantly constrict all the time to prevent transmission of high blood pressure to capillaries 31 BRAIN OEDEMA… Causes:  Increasedcapillary pressure  Damage to capillary walls (Concussion)  Treatment of brain oedema:  Mannitol: how does it work? = intravascular hyperosmotic substance – body wants homeostasis – water diffuse back into capillaries 32 D. BRAIN METABOLISM Resting conditions:  15% of the total metabolism of the body  Although brain mass only 2% of total body mass!  sources of energy:  Glucose (as glycogen): normally  Ketones (DM)  Lactate and pyruvate (DM) 33 BRAIN METABOLISM…  Brain not capable of much anaerobic metabolism:  It doesn’t really store O2 (uses a lot!)  Low glycogen for anaerobic use  Onlya 2-min. supply of glucose (stored as glycogen) in the neurones at any given time  Constant cerebral blood flow NB!  Irreversible neuron damage if blood flow stopped > 5-8 minutes!  Rate of glucose consumption by brain tissue = 110g glucose / day 34 35