Neuro Anatomy & CBF Flashcards
What is the organization of the nervous system?
CNS: Brain and Spinal Cord
PNS: Includes the cranial and spinal nerves and their receptors, and it is divided into the somatic and autonomic nervous systems. The somatic nervous system contains sensory neurons for the skin, muscles, and joints. The autonomic nervous system consists of the sympathetic, parasympathetic, and enteric subdivisions and is responsible for involuntary innervation of various organ systems.
What are Neurons?
The basic functional cell of the CNS.
-Responsible for impulse conduction (motor and sensory)
What are Neuroglial cells (Glial)?
5 types:
-Astrocytes
-Ependymal Cells
-Microglia
-Oligodendrocytes
-Schwann Cells
What are Astrocytes?
-Provide structural neuronal support
-Regulate metabolic activity
-Essential in the development of the blood brain barrier.
What are Ependymal Cells?
Form the Choroid Plexus in the 3rd and 4th ventricles to produce CSF.
What are Microglia?
Go to sites of neuronal injury or degeneration and develop into macrophages that phagocytize neuronal debris.
What are Oligodendrocytes?
Form the myelin sheath of axons in the brain and spinal cord (CNS).
What are Schwann Cells?
Produce myelin for the peripheral nervous system.
-If found in CNS it is pathologic (Vestibular or Acoustic Schwannoma)
What is Glutamate?
The primary excitatory transmitter found within the cerebral cortex, the hippocampus, and the substantia gelatinosa of the spinal cord.
-Glutamate plays a significant role in learning and memory (perhaps interactive in memory formation during awareness that occurs during anesthesia) and the appreciation of pain.
-Involved in neuronal injury in TBI as well as ischemic injury.
What is Cerebrospinal Fluid (CSF)?
-Serves to protect the brain and spinal cord
-Contains Na, K, HCO3, glucose, and minimal protein that leak from perivascular fluid
-Normal pressure is 5-15 mmHg
-About 500 mL/day is produced (20-30 mL/hr), mainly in the Lateral Ventricles by the Choroid Plexus
-At any given time, there is only 150 mL of CSF present in adults and 50 mL in children.
-Absorbed in Arachnoid Villi in the Subarachnoid space and absorbed into the blood.
-pH is 7.32
What is the specific gravity of CSF?
1.002 - 1.009
-Can be changed with severe hyperglycemia, hypoglycemia
-Protein levels change SG, this can indicate tumors, infection, bleeds etc.
What are things that can decrease the production of CSF?
-Carbonic anhydrase inhibitors like acetazolamide,
-Vasoconstrictors
-Lasix, steroids
-Isoflurane
Describe the circulation or flow of CSF.
-CSF flows from the lateral ventricles of the cerebral hemispheres through the foramen of Monro into the third ventricle, and through the aqueduct of Sylvius (very tiny - common to obstruct here) in the midbrain into the fourth ventricle. CSF enters the subarachnoid space through the medial foramen of Magendie and the paired lateral foramina of Luschka, opening posterior to the pons and anterior to the cerebellum.
-The cisterna magna, located between the medulla and the cerebellum, is formed from the separation of the arachnoid mater from the pia mater and is filled with CSF. Two additional cisterns exist, the cisterna pontis and the cisterna basalis. CSF drains into the venous blood via the superior sagittal sinus and is absorbed by arachnoid granulations.
How is the Blood-Brain Barrier (BBB) formed?
Collection of Endothelial Cells from the capillary wall that form a very selective semi-permeable membrane plus a capillary basement membrane, a neuroglial membrane, and astrocyte projections.
-These form continuous tight junctions that make the BBB a tight barrier for water soluble molecules.
-Strictly control the passage of substances into and out of the CNS.
-BBB disruption can be caused by traumatic head injury, subarachnoid or intracerebral hemorrhage, or cerebral ischemia. The BBB can be disrupted in several disease processes such as multiple sclerosis, stroke, and brain tumors.
Give an example of one CNS structure NOT protected by the Blood-Brain Barrier?
The Chemoreceptor Trigger Zone (CTZ).
-Allows detection of toxins, emetic agents
How much CO does the brain receive?
~15% (750 mL/min)
-Originates from 2 arterial supplies to form Circle of Willis (Anterior and Posterior)
-The anterior and posterior arterial systems communicate through arterial anastomoses that form the circle of Willis.
Describe the Anterior circulation to the brain.
Receives blood from the Internal Carotid Arteries.
-Form the anterior and middle cerebral arteries.
-Anterior cerebral artery supplies the medial surface of the cerebral hemispheres
-Middle cerebral artery supplies the lateral surface of the cerebral hemispheres.
Describe the Posterior circulation to the brain.
Receives blood from the Vertebral Arteries.
-Form the Basilar artery in the pons.
-Supplies a large portion of the brain and spinal cord.
Nagelhout:
-Branches of the vertebral and basilar arteries supply a wide area, including the cervical region of the spinal cord, the brainstem, the cerebellum, the vestibular apparatus and cochlea of the inner ear, parts of the diencephalon, and the occipital and temporal lobes of the cerebral hemispheres.
The paired anterior, middle, posterior cerebral arteries originate from where?
The paired anterior, middle, and posterior cerebral arteries originate from the circle of Willis.
Thrombosis of the Anterior Cerebral Artery causes what effects?
-Paralysis and loss of sensation of leg and foot
-Personality changes
Thrombosis of the Middle Cerebral Artery causes what effects?
-Hemiplegia and loss of the sensation mainly face and arm
-Aphasia (Broca’s and Wernickes’s area)
Thrombosis of the Posterior Cerebral Artery causes what effects?
-Visual Disturbances
Treatment with thrombolytic agents (TPA) should be started in less than _____ hours of stroke!
Treatment with thrombolytic agents (TPA) should be started in less than 4.5 hours of stroke!
-Must do CT scan to differentiate hemorrhagic and thrombotic stroke before initiating TPA.
How does Cerebral Venous blood exit?
Via 2 systems:
-Cerebellar & cerebral cortex surface veins → dural venous sinuses
-Basal veins → vein of Galen → straight sinus
Nagelhout:
-The blood from the cerebral and cerebellar cortex flows through veins on the surface and empties into overlying dural venous sinuses. Venous blood from the basal portions of the brain empties into the great vein of Galen and the straight sinus.
Where does the blood from the Dural and Straight Sinus drain into?
The IJ Veins.
Nagelhout:
-These sinuses empty into the internal jugular veins. The superficial veins of the scalp are linked to the dural sinuses by the emissary veins.
What can occur with disruption of the venous sinuses?
-High flow area, disruption of sinus can lead to massive EBL.
What can occur with problems with Cerebral Venous flow?
↑Back pressure → microvascular pressure changes → vasogenic edema → BBB compromise
1 – compromised microvascular flow – cytotoxic edema
2- BBB is compromised, increased hydrostatic pressure – vasogenic edema
3- excessive pressure damages vasculature – ICH
What are causes of Cerebral Venous Congestion/Decreased drainage?
-Extreme head flexion (sitting position)
-Jugular compression- trach ties, collars
-Intrathoracic pressure- coughing, peep,
-Vena cava thrombosis
-Venous thrombosis-interventional Radiology (Papaverine, heparin, stenting, suctioning clot out)
What is a Cerebral Venous Thrombus?
Uncommon but devastating.
-75% of patients are female
-Can lead to edema, ICH, increased ICP
-Go to IR: thrombolysis, papaverine, heparin, stent, dilate or suck clot out
How do you calculate Cerebral Blood Flow (CBF)?
Cerebral blood flow = Cerebral Perfusion Pressure/Cerebral Vascular Resistance
What are the major determinants of Cerebral Blood Flow?
-Cerebral Perfusion Pressure
-Cerebral metabolic rate (CMRO2)
-Arterial Carbon dioxide tension
-Venous Pressure – increase venous pressure, decreases venous drainage = increased back pressure= edema, BBB changes
-PaO2 (to a lesser extent)
What is Cerebral Perfusion Pressure (CPP)?
The net pressure gradient that drives O2 delivery into cells.
CPP = MAP - ICP (or MAP - CVP)
Normal CPP = 60-80 mmHg
Normal MAP = 70-100 mmHg
Normal ICP = 5-15 mmHg
What is Autoregulation?
The ability to keep a constant flow/pressure in the brain.
-MAP 50-150mmHg (multiple studies showing slightly different numbers)
-Chronic HTN shifts curve to Right (Patient requires higher perfusion pressures)
-Below limit-cerebral vasculature will dilate to compensate (BP will rise)
-Above limit = cerebral constriction, BBB disruption, edema, hemorrhage
How does MAP decrease with elevation?
For every 1 inch elevation, MAP will decrease by 2 mmHg.
-Ex: 12 inch elevation from where you are measuring = ↓MAP by 24mmhg
-Need to increase MAP when patient is in sitting position.
-Can move art line up to the tragus of the ear to measure cerebral pressure
What is the normal Cerebral Blood Flow (CBF)?
45-65ml/100g tissue/min or 15% of CO or ~750ml/min.
-The average adult brain is 1300-1400 grams or about 3 pounds.
How is Cerebral Blood Flow regulated?
By flow-metabolism coupling.
-increased activity = increased flow.
-CBF matches CMRO2 in a healthy brain.
EEG slows at what CBF?
CBF <20-25ml/100g/min
EEG becomes isoelectric at what CBF?
CBF <15-20ml/100g/tissue
Irreversible brain damage occurs at what CBF?
CBF 10ml/100g/min.
What is the most important influence on CBF?
PaCO2 (!!!)
-For every 1 increase in Co2 there is a 2ml increase in blood flow.
-This is consistent and linear between PaCO2 25-65
CO2 goes from 30 to 60mmhg – double CBF
CO2 goes from 40 to 20mmhg- halves the CBF
How does Cerebral Autoregulation change with hypertension?
Curve shifts to the right.
-Patient requires higher perfusion pressures
Nagelhout:
-Decreases in MAP should be limited to no more than 20% of preoperative values
What is Cerebral O2 consumed for?
60% used for electrical activity and 40% is for cellular integrity
What is Coupling?
Refers to parallel relationship between Metabolic activity and cerebral blood flow.
-We can choose to give anesthetic agents which will decrease CMRO2 and cause ”uncoupling” (Propofol, Barbiturates, Etomidate)
What is normal CMRO2?
unit of measure/gm/tissue
3 – 3.8 mL/100g/min in adults
5-7 ml/min/100g/min in children.
CMRO2 =~ 50ml/min (average brain is 1400g)
Does the brain have O2 storage?
No. The brain has NO O2 storage.
-ATP stores will deplete in 3-8 min.
-Irreversible cell injury
What does hypoventilation do to CBF?
Hypoventilation -↑Co2 = ↑CBF = vasodilation
What does hyperventilation do to CBF?
Hyperventilation-↓ CO2 = ↓CBF = vasoconstriction
How does CO2 influence CBF?
-CO2 influence on blood flow is thought to be from changes in ph.
-Hypercarbia can increase acidosis, increasing the hydrogen ions in CSF.
-Vasodilating effect by stimulating K+ channels and causing hyperpolarization and therefore, there is less intracellular calcium….more vasodilation.
Hyperventilation effects only lasts about 4-6 hours
At what PaO2 does Cerebral Vasodilation occur?
-PaO2 under 50 mmHg causes cerebral vasodilation
-PaO2 over 350 mmHg has some protection from free radical damage.
-O2 is not as big of a predictor as CO2 is.
What are the neurologic effects of Hypoglycemia?
-Glucose consumption parallels CMRO2
-Glycogen stores in neurons last 2 min
-Acute sustained hypoglycemia is as devastating as hypoxia
-Prolonged hypoglycemia = more neuronal cell death
*Tight glucose control
What are the neurologic effects of Hyperglycemia?
-Marked cerebrovascular changes both during ischemia and reperfusion
-In ischemic tissues, glucose gets depleted and more lactic acid produced, more tissue damage and increased permeability, inflammation and edema
-Decrease integrity of BBB
-Increased morbidity and mortality
*tight glucose control
How does temperature affect CBF?
-For every 1 degree drop in temperature there is a 7% decrease in CBF.
-Moderate decreases in temp (35-36) decrease CMRO2 and may help with inflammatory and immune response
-Prevent hyperthermia
-Use of bair hugger on cool settings, cool fluids, room temperature, anti-pyretics
What is Cerebral Steal/Luxury Perfusion?
Normally, the area of ischemia maximally dilates in an attempt to get more blood and oxygen to the area.
-Ischemia produces metabolites that cause dilation.
-Non areas of ischemia have tone and don’t dilate.
-With Cerebral steal/luxury perfusion, the area that is not injured or ischemic also will vasodilate.
-Let’s say you hypoventilated the patient or gave a vasodilator. This would take blood flow away from the injured area because the uninjured area would dilate also. If you gave vasodilators and hypoventilated you would promote cerebral steal and luxury perfusion.
What is Inverse Steal (Robin Hood Reverse Steal)?
-You are taking from the rich fed area and giving to the poor fed area.
-Locally, the ischemic area is maximally dilated, but in this case the uninjured, well-perfused area constricts and allows more blood flow to go to ischemic areas.
-Hyperventilation and hypocarbia would constrict the unaffected blood vessels and move the blood to the injured ischemic areas.
-Need to keep CO2 30-35
