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30 Cards in this Set
- Front
- Back
Functions of CSF |
1. Mechanical protection 2. ICP buffering 3. Source of glucose/O2 and for removal of metabolites/CO2 4. Acid-base - low protein so small CO2 changes = larger pH changes 5. Transport of hormones/neuropeptides to other parts of brain 6. Interstitial fluid absorption as no lymph in brain 7. Maintains constant ionic environment conducive to neuronal function. |
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Describe CSF production |
- 150mL total with 3-4x turnover/day ie 500mls/day - produced in choroid plexus (67%) and directly via ependymal cells of lateral ventricles (33%) - decreased formation only when CPP<70mmHg. NOT affected by ICP unless ICP reduces CPP. - plasma ultrafiltrate via fenestrated capillary endothelium. - promoted by bulk flow and hydrostatic pressure - active transport of Na, glucose and K out - passive transport in of Cl, HCO3 - H2O down osmotic gradient |
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Differences between plasma and CSF |
- higher pCO2 and lower pH - decreased protein ++ - decreased K, glucose - identical osmolality 295, Na, HCO3 - Increased Cl |
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Path of CSF |
1.Choroid plexus/lateral ventricles down foramen of Monro 2. 3rd ventricle 3. Sylvian aqueduct 4. 4th ventricle 5. Foramen of Luschka/ Magendie 6. Cisterna magna 7. Subarachnoid of brain/SC 8. Arachnoid villi into venous system |
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Describe CSF absorption |
Occurs in arachnoid villi or directly via cerebral venules via a pressure gradient - ICP 15mmHg - venous 9mmHg
Therefore increased CSF pressure favours absorption.
Normally production = absorption |
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What is structure of BBB |
- formed primarily by endothelial layer of capillaries Physical - tight junctions w/o fenestrations + increased mitochondria to allow increased active transport. Chemical - enzymes such as MAO/DA decarboxylase degrade toxic substances crossing BBB
- Astrocytes not part of BBB but apply foot processes W/I perivascular areas to form clefts and channels |
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Permeability of BBB |
- generally permeable to small (<30kDA), non-polar,lipophilic substances including resp gases, H2O - active transport of other substances such as glucose, aas, electrolytes, some drugs |
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What are the circumventricular organs of brain |
- Small areas surrounding 3rd/4th ventricles outside the BBB E.g. - choroid plexus - area postrema - median eminence of hypothalamus |
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Functions of the blood-CSF barrier |
- regulate contents of newly-formed CSF - release of hormones into CSF - sensory analysis of CSF ie with CTZ |
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Monro-Kellie doctrine |
In adults, rigid/closed cranial vault forms a fixed brain volume that contains: - parenchyma 1500g (80%) - CSF 75mLs (10%) - cerebral blood/vessels 75ml (10%)
Therefore any change in any of these components will alter one or both of the other components. |
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Cerebral blood flow |
CBF = MAP - ICPor CVP/CVR |
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Features of REM sleep |
1. CNS - Rapid, low voltage irregular EEG similar to awake state - Active dreaming - remembered - CMRO2 matches awake - Difficult to rouse 2. Resp\\ - Irregular RR - ↓ TV/MV - much more than non-REM - ↓ response to hypercarbia/hypoxia - Profound loss of upper airway tone + ↑ resistance - OSA - ↓ FRC 3. CVS - BP/HR irregular 4. Muscle - Marked ↓ muscle tone 5. Other - Decreased temperature, metabolic rate - Release GCs |
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Neurological features of Non-REM sleep |
EEG - 4 stages that gradually generate more synchronised, lower frequency and higher amplitude waveform.
1. α waves (8-13Hz) replaced by low amplitude/frequency θ (theta) wave (4-6Hz) 2. Bursts of high frequency α-like waves called "sleep spindles" 3. High amplitude but lower frequency δ waves (1-2Hz) with bursts of rapid waves superimposed (K complexes) 4. δ waves become higher amplitude, lower frequency and synchronised. - Decreased CMRO2 |
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Non-neuro features of Non-REM sleep |
1. CVS - Decreased HR and BP 2. Resp - ↓ RR/TV/MV (less than REM) - ↓ FRC secondary to ↓muscle tone - ↓ hypoxic, hypercarbic response (less than REM) - ↓ upper airway tone and ↑AWR causes OSA (less than REM) 3. Muscle - ↓ muscle tone (less than REM) 4. Other - ↓ temp - ↓ metabolic rate |
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Cycling of sleep phases |
- Usual sleep cycle approx 90 minutes 1. Phases 1-4 of Non-REM 2. REM sleep (5-30mins) 3. Phases 4 to 1 of Non-REM sleep REM sleep apart 15 minutes >50yrs In babies 45-65% spent in REM sleep |
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Basis of EEG |
- Measures the spontaneous electrical activity of superficial layer of pyramidal cells perpendicular to electrodes - Processed by low and high frequency filters to remove muscle artefact - Analysed for frequency/amplitude and recognition of wave patterns |
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Clinical use of EEG |
- Can only detect abnormalities in cortical tissue as this is where electrical potentials originate - 3 types of abnormalities - Generalised excess of slow wave activity i.e. infective or metabolic encephalopathy - Focal excess of slow wave activity i.e. focal abnormality - Abnormal high voltage electrical discharge i.e. epileptiform disturbance. |
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Effect of CO2 on cerebral blood flow |
- ↑PaCO2 causes cerebral vasodilation - 4% per mmHg CO2 increase between 20-80mmHg - PaO2 has no effects unless <50mmHg |
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Nernst equation |
Veq = equilibrium potential of the ion R = universal gas constant T = absolute temp z = Elementary charge of ion F = Faraday's constant |
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What is the Nernst potential? |
- The voltage that exists across a membrane for an ion which is fully permeable when the electrical and concentration gradients are equal
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Resting membrane potential? |
Is the potential difference across the cell membrane when electrical and concentration gradients are at equilibrium. Different in tissues due to differing permeabilities to certain ions Ie Excitable cell = -70mV Cardiac myocyte = -90mV Cardiac pacemaker cell = -60mV Skeletal muscle cell = -80mV |
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How to calculate the resting membrane potential |
Goldman-Katz equation P=relative permeability Goldman-Katz equation: |
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Factors that generate RMP |
K+ equilibrium potential = Main factor - 3Na-2K ATPase pumps Na EC and K IC to establish a large concentration gradient. - membrane very permeable to K+ due to non-gated K channels - Therefore more K lost from cell than Na in and creates a -ve membrane potential. Na+ influx - Membrane impermeable to Na at rest however small leak of Na into the cell causes RMP to be more positive than K+ equilibrium potential. Gibbs-Donnan effect - Large non-diffusible anions (proteins) inside cells affects the distribution of permeable ions. |
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Steps for a nerve action potential |
1. RMP = -70mV 2. Impulse increases the RMP to threshold of -55mV upon which VG-gated Na channels open and depolarise the membrane to 35mV. Open only 1msec before closing. 3. Delayed opening of VG-gated K channels repolarise the membrane. Slowed closing causes hyperpolarisation and then back to RMP |
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Steps for a nerve action potential |
1. RMP = -70mV 2. Impulse increases the RMP to threshold of -55mV upon which VG-gated Na channels open and depolarise the membrane to 35mV. Open only 1msec before closing. 3. Delayed opening of VG-gated K channels repolarise the membrane. Slowed closing causes hyperpolarisation and then back to RMP |
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Refractory periods of AP |
Absolute RP - 1msec of AP when all Na channels are open and an AP cannot occur regardless of intensity
Relative RP - 10-15msecs when supramaximal stimulus needed - only when all Na channels reset and membrane is not hyperpolarised by increased K permeability
- RPs allow flow to be unidirectional and limits frequency of nerve conduction. |
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Steps for a nerve action potential |
1. RMP = -70mV 2. Impulse increases the RMP to threshold of -55mV upon which VG-gated Na channels open and depolarise the membrane to 35mV. Open only 1msec before closing. 3. Delayed opening of VG-gated K channels repolarise the membrane. Slowed closing causes hyperpolarisation and then back to RMP |
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Refractory periods of AP |
Absolute RP - 1msec of AP when all Na channels are open and an AP cannot occur regardless of intensity
Relative RP - 10-15msecs when supramaximal stimulus needed - only when all Na channels reset and membrane is not hyperpolarised by increased K permeability
- RPs allow flow to be unidirectional and limits frequency of nerve conduction. |
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Factors affecting speed of nerve propagation |
1. Axon diameter - larger area produces lower resistance to current flow
2. Myelination - myelin sheath around the nerve between nodes of ranvier has high resistance and low capacitance such that flow can pass undisturbed though the myelin to the next node - causes AP to jump from node to node |
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Front (Term)t |
T |