NS 2: Environment of the brain

Question 1

how does the ICA enter the skull?

Answer 1

through the carotid canal in the petrous part of the temporal bone, allowing entry into the middle cranial fossa

Question 2

what does the ICA branch to give in order to supply the medial surfaces of the frontal and parietal lobes?

Answer 2

the anterior cerebral arteries

Question 3

what does the ICA branch to give in order to supply the lateral surfaces of the cerebral cortex?

Answer 3

the middle cerebral arteries

Question 4

how do the vertebral arteries enter the skull, and where do they arise?

Answer 4

through the foramen magnum

arise from the subclavian arteries

Question 5

what do the vertebral arteries form on entering the skull?

Answer 5

the basilar artery

Question 6

what does the basilar artery supply?

Answer 6

the cerebellum and brainstem: midbrain, pons and medulla

Question 7

what does the basilar artery split to give in order to supply the inferior surface of the brain and the occipital lobes?

Answer 7

paired posterior cerebral arteries

Question 8

how are the cerebral arteries joined together to form the circle of Willis at the base of the brain?

Answer 8

by communicating branches
(anterior cerebral arteries joined by anterior communicating artery, posterior cerebral joined to ICA by posterior communicating arteries which arise from the ICA)

Question 9

clinical importance of circle of Willis?

Answer 9

collateral circulation can be provided if arterial blockage, but this is usually inadequate following sudden occlusion of cerebral vessels e.g. with cerebral thrombosis, haemorrhage or embolism), so vascular stroke commonly occurs.

Question 10

describe how the L and R anterior cerebral arteries arise from the arch of the aorta

Answer 10

R: brachiocephalic trunk, R common carotid, R ICA, R anterior cerebral
L: L common carotid, L ICA, L anterior cerebral

Question 11

describe how the R posterior cerebral artery arises from the aortic arch

Answer 11

brachiocephalic trunk, R subclavian, R vertebral, joins with L vertebral to form basilar, gives rise to posterior cerebral

Question 12

what must cerebral veins cross to enter the dural venous sinuses and why is this problematic?

Answer 12

the subarachnoid space

hamorrhage may occur in space if head trauma

Question 13

why is CSF leakage common with fractures of the base of the skull? and why with base fractures is a CT scan required, rather than an X-ray?

Answer 13

the dura lining this region is strongly adherent to the periosteum, so dural tears are common with fracture, causing rhinorrhoea and otorrohoea, and can allow organisms to enter.
skull base is more dense and its L and R sides are superimposed.

Question 14

how does serious bleeding from the nose occur?

Answer 14

tearing of ICA and fracture of the sphenoid

Question 15

type of blood characterising an exxtradural haemorrhage and visual appearance on scan*?

Answer 15

arterial- tearing of anterior branches of middle meningeal artery
lens shaped (lenticular)- flexible periosteal layer of dura mater is moved away from the cranium.

Question 16

CSF functions?

Answer 16

cushioning, protection of brain and SC
reservoir of metabolites
buoyancy- lightens weight of brain

Question 17

why is a meningeal haematoma more worrying if arterial blood rather than venous?

Answer 17

arterial blood= higher pressure, so more damage may result

subarachnoid haemorrhage= arterial blood- often result of a ruptured aneurysm

Question 18

contrast effects of upper and lower motor neurone lesions on muscle tone?

Answer 18

upper e.g. stroke, causes hypertonia, presented as spastic paralysis- resistance to passive movement of a limb, maximal at beginning of movement.
lower produces hypotonia and flaccid paralysis- body limbs hand loose.

Question 19

with what disease is cog wheel rigidity seen (result of hypertonicity with an element of tremor present)?

Answer 19

Parkinson’s disease

Question 20

what would an absent ankle jerk in a patient with lower back pain suggest?

Answer 20

prolapsed disc at S1-S2 level

Question 21

which vertebral arches fail to fuse in spina bifida occulta?

Answer 21

L5/S1

Question 22

concerns with myelomeningocele?

Answer 22

neural problems

exposed neural tissue and meninges susceptible to life-threatening infections

Question 23

describe the difference between communicating and non-communicating hydrocephalus?**

Answer 23

communicating- flow of CSF through ventricles and into SA space is not impaired, but movement from space into venous system is partly or completely blocked.
non-communicating- flow of CSF is obstructed within ventricles or between the ventricles and the subarachnoid space e.g. aqeductal stenosis due to nearby tumour in midbrain or by cellular debris following intraventricular haemorrhage or bacterial and fungal infections of CNS. obstruction may also occur in an interventricular foramen.

Question 24

function of an epidural block?

Answer 24

anaesthetic inserted into lumbar cistern to block structures innervated by cauda equina

Question 25

what structure is the uncus the anterior part of?

Answer 25

the parahippocampal gyrus ( an elevation of the cerebal cortex)

Question 26

give 3 reasons why familiarity with the inferior (ventral) aspect of the brain is important?

Answer 26

- almost all cranial nerves of brain emerge from here, and base of skull very vulnerable to impact trauma so cranial nerves and inferior aspect is just as vulnerable to these injuries aswell. - relationship between pituitary, hypothalamus and optic nerves clearly visible here, can therefore better understand effects of hypothalamic tumours on neuronal function. - uncus readily seen, so can readily see relationship between uncus and oculomotor nerve, so can understand that uncus herniation leads to impairment of peripheral targets of oculomotor nerve.

Question 27

why are the bones of the calvaria separated in infants with hydrocephalus?

Answer 27

sutures are yet to fuse posterior fontanelle between sagittal and lambdoid sutures closes within 6-9mnths anterior between coronal and sagittal sutures closes within 2 years.

Question 28

what may cause a communicating hydrocephalus throguh blockage of CSF drainage from SA space into venous sytem?

Answer 28

- congenital absence of arachnoid granulations - blocked granulations by rbc due to subarachnoid haemorrhage - fibrosis due to meningitis

Question 29

Intracranially, through what does the ICA pass anteriorly, alongside the abducens nerve?

Answer 29

the cavernous sinus

Question 30

what are the terminal branches of the ICA and where do they lie?

Answer 30

anterior and middle cerebral arteries subarachnoid space between the arachnoid mater bridging the gaps between adjacent gyri, and the pia mater which travels down into the sulci of the cerebral cortex.

Question 31

what forms the anterior circulation of the brain?

Answer 31

the ICAs and their branches

Question 32

when the posterior communicating arteries of the ICAs join the posterior cerebral arteries, this completes the cerebral arterial circle around which structure?

Answer 32

the interpeduncular fossa: deep depression on inferior surface of midbrain between the cerebral peduncles

Question 33

through what do the cervical parts of the vertebral arteries ascend through?

Answer 33

``` the transverse foramina of the first 6 cervical vertebrae atlantic parts (related to C1) then pass through foramen magnum after perforating dura and arachnoid mater. ```

Question 34

where does the basilar artery form?

Answer 34

at the caudal border of the pons

Question 35

where are the superior cerebral veins and what do they drain into?

Answer 35

on superolateral surface of brain | superior sagittal sinus

Question 36

what do inferior and superficial middle cerebral veins drain into?

Answer 36

from inferior, PI, and deep aspects of cerebral hemispheres, drain into straight, transverse and superior petrosal sinuses.

Question 37

what is the great cerebral vein?

Answer 37

single midline vein formed inside brain by union of 2 internal cerebral veins, ends by merging with inferior sagittal sinus to form straight sinus.

Question 38

what is the venous drainage of the cerebellum?

Answer 38

superior and inferior cerebellar veins which drain into transverse and sigmoid sinuses.

Question 39

How do astrocytes funtion to provide nutrients to neurones?

Answer 39

glucose-lactate shuttle: glucose uptaken by astrocytes from the blood and converted to glycogen for storage as neurones have few energy reserves- cannot store or produce glycogen. Can then be converted to lactate for neurones, so have a very temporary supply of energy. Remember neurones can only respire aerobically, O2 dependent. Glucose from blood metabolised to lactate and excreted into ECF for uptake by surrounding neurones.

Question 40

how do astrocytes maintain a low EC concentration of neurotransmitter?

Answer 40

able to remove neurotransmitters through uptake via proteins in their membranes, so lots of astrocytes found at the synapse. Important for the excitatory neurotransmitter glutamate as too much=TOXIC. glutamate coverted to glutamine via g;lutamine synthetase after uptake, then converted back to glutamate on pre-synsptic terminal by glutaminase. Glutamine also goes to inhibitory neurones for GABA synthesis.

Question 41

why can [K+] in brain ECF increase quite rapidly, and why are astrocytes important for managing this?

Answer 41

K+ release into ECF with intense neuronal activity. As membrane mainly permeable to K+ at rest as K+ channels open, the resting Em is dependent on the transmembrane K+ gradient, so high K+ would mean the Em would reset to a more +ve value, making neurones more electrically excitable, would depolarise causing inappropriate AP firing, but astrocytes have K+ channels and transporters so can redistribute K+ over non active regions, with adjacent astrocytes tightly coupled via gap junctions, allowing K+ to flow down its conc gradient from active zone to a remote site.

Question 42

what are oligodendrocytes responsible for?

Answer 42

myelination of CNS neurones, simultatenously myelinate axons of multiple neurones. Allow rapid AP transmission by saltatory conduction rather than local current flow.

Question 43

what is the brain's main defence system?

Answer 43

microglia: cells involved in immune response. can present antigens to T cells, but don't want this too much as rigid skull doesn't allow volume expansion with inflammatory responses. Dendrites get thicker to allow phagocytosis of material.

Question 44

how do brain capillaries form an effective BB barrier, preventing limiting diffusion of substances from the blood into the brain ECF?

Answer 44

tight junctions between endothelial cells- formed by occludins and claudins (proteins), and end feet of astrocyte processes send messages to endothelium to form tight junctions. basement membrane end feet of astrocyte processes

Question 45

glucose, K+ and aa can cross BB barrier, but in a controlled manner, why is important that aa entry into ECF is controlled?

Answer 45

aa can often act as neurotransmitters and don't want inappropriate neurone activation, but aa are needed.

Question 46

importance of CNS inhibiting initiation of pro-inflammatory T cell response?

Answer 46

rigid skull will not tolerate volume expansion

Question 47

function of swelling of cell body of a neurone, known as the axon hillock?

Answer 47

AP initiation

Question 48

4 main sections of a neurone?

Answer 48

cell body (soma) axon dendrites nerve terminal

Question 49

how does neurotransmitter release occur at a synapse?

Answer 49

depolarisation of nerve pre-synaptic nerve terminal when AP arrives, causes opening of voltage-gated Ca2+ channels, so influx of Ca2+, which is bound by a Ca2+ binding protein= synaptotagmin, which is a vesicle associated protein and so transports vesicle containing neurotransmitter to the presynaptic membrane, where protein activates a SNARE-protein complex, so that veiscle can fuse with membrane at an active zone, and NT released into synaptic cleft.

Question 50

how do botulinum and tetanus toxins (produced by clostridia bacteria) parlayse individuals?

Answer 50

inhibit NT release at synapses by degrading proteins of the SNARE complex through their intrinsic protease activity

Question 51

what does the postsynaptic response depend on?

Answer 51

nature of neurotransmitter nauture of receptor: ligand-gated (ionotropic- ion channels which mediate ion influxes when active e.g. nicotinic AChR which mediates Na+ influx) or GPCRs (metabotropic) e.g. muscarinic AchR

Question 52

3 chemical classes of NTs in CNS?

Answer 52

aa e.g. glutamate, glycine and GABA biogenic amines e.g. Ach, noradrenaline, 5-HT, dopamine, histamine peptides e.g. NPY and somatostatin

Question 53

major excitatory neurotransmitter in the CNS?

Answer 53

glutamate

Question 54

examples of glutamate receptors?

Answer 54

``` ionotropic: AMPA: Na+/K+ permeability Kainate: Na+/K+ NMDA: Na+/K+/sometimes Ca2+. Blocked by Mg at rest, so must be depolarised before ions can flow through when glutamate binds. Important for learning and memory, activation increases Ca2+. metabotropic: GPCRs ```

Question 55

how are fast excitatory responses generated?

Answer 55

by excitatory neurotransmitters e.g. glutamate, which bind to ligand-gated ion channels and cause depolarisation of post-synaptic cell with cation influx. Depolarisation then causes more APs. Particular threshold reached with excitatory PS potential, which causes AP to fire.

Question 56

which receptors do glutaminergic synapses have?

Answer 56

NMDA and AMPA

Question 57

clinical importance of Ca2+ entry through glutamate NMDA ligand-gated ion channels?

Answer 57

too much glutamate can cause excitotoxicity *STROKE: damaged areas can undergo depolarisation, which can then spread and activate NMDA receptors, causing elevated Ca2+, so if these receptros are blocked, it can limit the damage caused by a stroke. Ca2+ entry important for inducing long term potentiation- this can be caused by strong, high frequency stimulation.

Question 58

how may AMPA receptors acted upon by glutamate be upregulated?

Answer 58

by activation of NMDAs and mGluRs

Question 59

main inhibitory neurotransmitter in brain?

Answer 59

GABA: gamma amino butyric acid

Question 60

where in the CNS does glycine mainly act as an inhibitory neurotransmitter?

Answer 60

brainstem and SC

Question 61

how do GABA and glycine act as inhibitory neurotransmitters?

Answer 61

receptors have integral Cl- channels so activation causes hyperpolarisation, producing an inhibitory post synaptic potentialand reducing AP firing

Question 62

give examples of drugs which enhance the response to GABA?

Answer 62

benzodiazepines and barbiturates bind to GABA A receptors benzodiazepines used to treat anxiety, sedative effects and epilepsy- suppress too much neuronal activity barbiturates no longer used in anxiety as risk of fatal OD and tolerance, but sometimes used as anti-epileptic also alcohol and steroids

Question 63

describe what neurotransmitters are released in the patellar (knee jerk) reflex (L3, L4)

Answer 63

glycine released from inhibitory interneurones to relax hamstrings (biceps femoris, semimembranosus and semitendinosus) message sent to SC, glutamate released, activating motor neurone- releases ACh to cause quads to contract

Question 64

function of biogenic amines as neurotransmitters in CNS?

Answer 64

neuromodulators | most confined to specific pathways

Question 65

give 4 examples of where ACh acts as an neurotransmitter?

Answer 65

NMJ ganglionic synpases of ANS postganglionic parasympathetic nicotinic and muscarinic receptors in brain receptors often present on presynptic terminals to enhance release of other transmitters

Question 66

which are the 1st neurones to die off in Alzheimer's?

Answer 66

those cholinergic neurones of the nucleus basalis | so can give cholinesterase inhibitors to relieve symptoms in alzheimers.

Question 67

tment for Parkinson's disease?

Answer 67

levodopa: converted to dopamine by DOPA decarboxylase Parkinsons: loss of dopaminergic neurones in substantia nigra

Question 68

disease caused by too much dopamine?

Answer 68

schizophrenia | anti-psychotics: antagonsits at dopamine D2 receptors

Question 69

why don't we want too much dopamine peripherally?

Answer 69

can increase BP so give carbadopa- can't pass BB barrier, so dopamine reduced in blood as carbadopa inhibits DOPA decarboxylase required to convert L-DOPA to dopamine, but doesn't affect enzyme converting L-dopa to dopamine in brain

Question 70

where are the neurones located that are responsible for most NA production in brain?

Answer 70

locus ceruleus activity increases during behavioural arousal, inactove in sleep amphetamines increase NA and dopamine release and increase wakefulness

Question 71

what condition may be associated with a deficiency of NA?

Answer 71

depression

Question 72

where are serotoninergic neurones located in brain?

Answer 72

raphe nuclei

Question 73

serotonin functions?

Answer 73

sleep/wakefulness, mood | serotonin selective reuptake inhibitors increase serotinin, so used in tment of depression and anxiety disorders

Question 74

describe how CSF passes from the lateral ventricles in the superior sagittal sinus?

Answer 74

passes from lateral ventricles into 3rd via intraventricular foramina of Monro then into 4th via cerebral aqeduct then into SA space via foramina of Luschka (2 found laterally) and Magendie ( 1 in midline) CSF then flows rostrally over cerebral hemispheres or down into SC reabsorption then occurs via arachnoid granulations within the superior sagittal and related venous sinuses

Question 75

name of foramina that allows CSF to pass from lateral ventricles into 3rd?

Answer 75

interventricular foramen/foramen of Monro

Question 76

which arteries are given off the vertebral arteries on route to the brain, and what do they supply?

Answer 76

anterior and posterior spinal arteries- anterior spinal= anterior 2/3 of SC, posterior spinal= dorsal column/medial lemniscal tract, arise from vertebral or indirectly from posterior inferior cerebellar. posterior inferior cerebellar artery- lateral part of medulla and cerebellum

Question 77

4 main functions of CSF?

Answer 77

buoyancy- cerebral tissues helped from being compressed by gravity against the skull- this would impede b.flow, causing ischaemia shock absorption and protection volume changes- CSF allows H20 to shift from CSF to cells without causing any gross change in CNS volume homeostasis- K+ levels tightly maintained so protecting neurons from fluctuating plasma concs, constant renewal of fluid to prevent build up of neuronal waste products, transmitters and ions, enabled by high flow rate.