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Primary FRCA > Physiology 13 > Flashcards

Physiology 13 Flashcards

1
Q

Approximately many neurones are present in the human body?

A

Approx. 1 trillion

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2
Q

What proportion of energy use in neurones is accounted for by 3Na+/2K+ ATPase?

A

Approx 70%

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3
Q

How does the Gibbs-Donnan effect relate to nerve cell physiology?

A

Trapped intracellular ions affect distribution of other ions at equilibrium.

This is counteracted in part by Na+/K+ ATPase, maintaining a physiological osmolality

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4
Q

What is the resting membrane potential of a neurone?

A

-70 mV

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5
Q

What is orthodromic neuronal conduction?

A

Conduction of an action potential from a synaptic junction down the axon to the terminus (ie. physiological)

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6
Q

What is antidromic neuronal conduction?

A

Retrograde +- anterograde conduction of an action potential from anywhere along an axon. Transmission can still only occur at the terminus

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7
Q

Outline the changes in membrane potential during a neuronal AP

A
  1. Initial depolarisation from -70mV due to stimulus
  2. Threshold reached at 15mV of depolarisation (-55mV)
  3. Rapid depolarisation to +35mV (spike potential)
  4. Rapid repolarisation
  5. Slowing down of repolarisation at 70%
  6. Period of hyperpolarisation prior to resuming resting potential (relative refractory period)
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8
Q

How can neurones be classified?

A

Number of axons:

  1. Monopolar neurones (eg. ganglia of autonomic NS)
  2. Bipolar neurones (eg. special senses)
  3. Multipolar neurones - 3 dendrites or more than one axon. Most CNS neurones are multipolar
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9
Q

How are active molecules transported in the neurone?

A

Following synthesis in the cell body, transmitters are transported to the axonal terminus (axoplasmic flow)

Axoplasmic flow can be anterograde or retrograde (eg. flow of used vesicles back to cell body)

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10
Q

Define the process of action potential conduction along a myelinated axon

A

By ‘saltatory conduction’ between nodes of Ranvier

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11
Q

How can axons be classified?

A

A, B, C fibres according to degree of myelination (A > B > C -unmyelinated)

A fibres further subdivided into α, β, γ, δ on basis of diameter:

α: 12-20 um
β: 5-12 um
γ: 3-6 um
δ: 2-5 um

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12
Q

What functions to Aα nerve fibres serve?

A

Proprioception

Motor neurones

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13
Q

What functions to Aβ nerve fibres serve?

A

Touch

Pressure

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14
Q

What functions to Aγ nerve fibres serve?

A

Motor to muscle spindles

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15
Q

What functions to Aδ nerve fibres serve?

A

Pain
Cold
Touch

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16
Q

What functions to B nerve fibres serve?

A

Preganglionic autonomic

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17
Q

What functions to C nerve fibres serve?

A

Pain
Temperature
Postganglionic sympathetic

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18
Q

What is the diameter and conduction velocity of Aα nerve fibres?

A

12-20 um

70-120 m/s

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19
Q

What is the diameter and conduction velocity of Aβ nerve fibres?

A

5-12 um

30-70 m/s

20
Q

What is the diameter and conduction velocity of Aγ nerve fibres?

A

3-6 um

15-30 m/s

21
Q

What is the diameter and conduction velocity of Aδ nerve fibres?

A

2-5 um

12-30 m/s

22
Q

What is the diameter and conduction velocity of B nerve fibres?

A

<3 um

3-15 m/s

23
Q

What is the diameter and conduction velocity of C nerve fibres?

A
  1. 3-1.3 um

0. 5-2.3 m/s

24
Q

Explain the morphology of electrical readings from stimulation of mixed peripheral nerves

A

Sum of action potentials of all the individual axons in the nerve.

‘Compound action potential’ containing multiple peaks due to arrival of APs from axons of varying conduction velocity

25
Q

What types of neuroglial cells exist?

A

Microglia
Astrocytes
Oligodendrocytes
Schwann cells

26
Q

What is the function of microglia?

A

Immune phagocytes which may be resident or derived from blood monocytes

27
Q

Summarise the function of astrocytes

A

Two types - Fibrous and Protoplasmic

Fibrous: Form white matter of CNS. Form a matrix to support neurones, isolate synapses and absorb released transmitters

Protoplasmic: Maintain electrolyte balance (particularly K+). Have a membrane potential but do not generate APs

28
Q

Outline the structure and function of the blood-brain barrier?

A

Formed by processes of astrocytes (both fibrous and protoplasmic) which surround capillaries, preventing capillary leak of electrolytes/drugs etc. Also produces nerve growth factor.

Not present at birth, hence risk of brain damage from neonatal jaundice

29
Q

What is the function of oligodendrocytes?

A

Provide myelination to axons in the CNS, analogous to Schwann Cells in PNS

30
Q

List the components of the limbic system

What functions are associated with the limbic system

A 
Cingulate gyrus (above corpus callosum)
Olfactory bulb
Amygdala (temporal lobe)
Hippocampus (temporal lobe)
Thalamus
Hypothalamus

Limbic system involved in emotions, learning, behaviour and many hypothalamic endocrine responses

31
Q

What functions does the hypothalamus help to regulate?

A 
Hunger and thirst
Body temperature
Sleep and wakefulness
Fear / anger / emotions
Pain / pleasure / arousal
32
Q

What is the O2 consumption of the brain under normal conditions?

How does this compare to other organs?

A

46ml/min

Relatively high (3rd highest after liver - 51ml/min; Skeletal muscle 50ml/min)

33
Q

What is the brain’s main source of energy?

How much reserve energy does it have?

A

Glucose metabolism (90%)

Can adapt to using ketone bodies during starvation (>24h)

Minimal reserves of energy substrate and O2 are present. Occlusion of perfusion produces unconsciousness in around 10 seconds.

34
Q

How does blood flow vary within brain tissue?

A

Cortical (grey matter): 75-80 ml/100g/min
Subcortical (white matter): 20 ml/100g/min

Avg. 45-55 ml/100g/min

35
Q

What are normal cerebral venous sats?

A

55-70%

36
Q

What is a normal ICP?

A

8-12 mmHg

37
Q

What factors control autoregulation of cerebral blood flow (CBF)?

A

MAP, PaCO2 and PaO2

MAP:
-CBF maintained at ~50ml/100g/min between a MAP of 50 and 150 mmHg

PaCO2:
-Sigmoid shaped curve with increasing CBF with increasing PaCO2. Roughly linear over ranges encountered in clinical anaesthesia

PaO2:

  • No change >8kPa
  • Below 8kPa, steep hyperbolic increase in CBF
38
Q

Outline the theories of cerebral blood flow autoregulation

A

Myogenic / Metabolic

Myogenic:
-Increased MAP causes increased stretch of vessels, increasing diameter and thus CBF. This is counteracted by contraction of vascular smooth muscle, limiting increasing CBF.

Metabolic:
-Increased metabolic activity of the brain increases extravascular concentrations of vasodilatory molecules, increasing CBF. Increased flow leads to efflux of molecules, autoregulating dilatation and thus CBF.

39
Q

What is the Monroe-Kellie doctrine?

How is cerebral perfusion pressure calculated?

A

Brain (1400g), cerebral vessels (75ml blood) and CSF (75ml) are within rigid skull.

This means that volume in the compartment is constant and there is a direct relationship between pressures in the different areas:

Cerebral perfusion pressure:
CPP = MAP - CVP - ICP

40
Q

What happens to intracranial pressures with increasing size of SOL within the skull?

A

Initially pressures are constant due to mainly reduction in CSF volume. When this buffer becomes exhausted, ICP increases sharply

41
Q

Summarise the production of CSF

A
  • Majority produced by choroid plexuses in lateral, 3rd and 4th ventricles
  • Small amounts generated around vessels and ventricular walls
  • Approx 550ml/day produced
  • Total volume around 150ml
42
Q

Describe the normal flow of CSF

A

Lateral ventricles -> Interventricular foramina -> 3rd ventricle -> Mesencephalic aqueduct -> 4th ventricle -> median and lateral apertures (to subarachnoid space) and central canal of spinal cord

43
Q

What are normal CSF biochemical values?

How do these compare to plasma?

A 
pH 7.3 (lower)
Na 150 (higher)
K 2.9 (lower)
Ca 2.3 (lower)
Mg 2.3 (higher)
Cl 130 (higher)
HCO3 21 (lower)
PO4 0.5 (lower)
Protein <0.4 (lower)
Glucose 3.5 (lower)

Osmol 285 (isotonic)

44
Q

How is CSF reabsorbed?

A

Via arachnoid villi into cerebral venous sinuses

45
Q

What are the main functions of CSF?

A
  1. Protection of the brain
    - Net weight in CSF only 50g
  2. Chemical buffer
    - Isolated from chemical changes in blood, protecting neuronal tissue
  3. Transport medium
    - For nutrients, neurotransmitters and metabolites
46
Q

Which brain areas lie outside the BBB?

Why are these areas relevant?

A

Known as circumventricular organs:

  1. Posterior pituitary and median eminence of hypothalamus
  2. Area postrema
  3. Organum vasculosum of lamina terminalis (OVLT)
  4. Subfornical organ

Allow communication between the brain and the systemic circulation

Primary FRCA (34 decks)

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