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Life Control > Basal Ganglia, Thalamus and Internal Capsule > Flashcards

Basal Ganglia, Thalamus and Internal Capsule Flashcards

1
Q

What is the function of the basal ganglia? When may these be disrupted?

A

20% initiation of voluntary movements, motor control

80% cognitive and emotional processing

Disturbed in neuro disorders such as Parkinson’s disease and Huntington’s disease

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2
Q

Why might movement disorders be referred to as “extra-pyramidal”?

A

caused by defects in the basal ganglia

as in most cases, the pyramidal (corticospinal) motor pathway is not affected

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3
Q

What is the largest anatomical component of the basal ganglia?

A

CORPUS STRIATUM

=> substantial collection of grey matter
=> located in the base of each cerebral hemisphere

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4
Q

What are the main components of the corpus striatum?

A

LENTIFORM NUCLEUS

CAUDATE NUCLEUS

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5
Q

Which other nuclei are located within the basal ganglia?

A

SUBSTANTIA NIGRA (of midbrain)

SUBTHALAMIC NUCLEUS (of diencephalon)

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6
Q

Is the thalamus part of the basal ganglia?

A

No.

However, projections do pass from the basal ganglia to the thalamus in order to reach the cerebral cortex

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7
Q

What is the structural appearance of Corpus Striatum?

A

striated

  • > bundles of myelinated axons
  • > create striped appearance
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8
Q

What is the CAUDATE NUCLEUS?

A

C-shaped structure with head, body and tail

nestles into curvature of the lateral ventricle

head and body: occupy lateral wall of the frontal horn of lateral ventricle

(v. thin) tail: lies in the roof of the temporal horn, sitting superior to the hippocampus

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9
Q

Where is the hippocampus?

A

inferior to the temporal horn of the lateral ventricles

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10
Q

What is the LENTIFORM NUCLEUS?

A

cone-shaped

base faces laterally beneath the insular cortex

globular apex points towards to the midline

outer portion = putamen
inner portion = globus pallidus

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11
Q

What is the PUTAMEN of the lentiform nucleus?

A

functionally similar to the caudate nucleus

has same dark grey appearance on fixed brain sections

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12
Q

What is the STRIATUM?

A

functional unit = putamen + caudate nucleus

INPUT unit of the basal ganglia

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13
Q

What is the GLOBUS PALLIDUS?

A

is pale or pallid in appearance in fixed brain sections

whitish colour due to bundles of myelinated axons

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14
Q

What is the INTERNAL CAPSULE?

A

thick sheet of white matter

contains projection fibres passing to and from the cerebral cortex

axons include:

  • DESCENDING motor/premotor projections
  • ASCENDING sensory projections from thalamus
  • afferent and efferent connections of the basal ganglia
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15
Q

What is the 3D structure of the basal ganglia?

A

most well appreciated in axial (horizontal) section
-> allows full antero-posterior extent of the internal capsule to be seen at once

=> sheet of white matter
=> wrapped around the apex of lentiform nucleus

3 parts of the internal capsule:

  • ANTERIOR LIMB: between caudate nucleus and lentiform nucleus
  • GENU (knee-bend): at the apex of the lentiform nucleus
  • POSTERIOR LIMB: between thalamus and lentiform nucleus
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16
Q

What is the association between the thalamus and internal capsule in brain sections?

A

IN GENERAL

if in a section, you CAN SEE the thalamus, then the POSTERIOR LIMB of the internal capsule is visible

However, if the thalamus is not visible, then the section is anterior to the thalamus and therefore the ANTERIOR LIMB of the internal capsule will be visible

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17
Q

At which level (of brain section) is the GENU of the internal capsule visible/present?

A

only seen in a single coronal slice

occurs at level of the INTRAVENTRICULAR FORAMEN OF MONRO

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18
Q

What is the clinical significance of the POSTERIOR LIMB of the internal capsule?

A

contains the corticospinal tract

and immediately posterior the ascending sensory pathways (spinothalamic and dorsal column pathways)

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19
Q

What is the CORPUS STRIATUM?

A

used in gross anatomy

= caudate nucleus + lentiform nucleus

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20
Q

Where does the striatum receive projections from?

A

receives AFFERENT projections from the (overlying) cerebral cortex

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21
Q

What is the PALLIDUM of the basal ganglia?

A

aka global pallidus

EFFERENT portion of basal ganglia

comprised of internal and external segments

INTERNAL PALLIDUM = OUTPUT of the basal ganglia

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22
Q

Where does the output of the basal ganglia project?

A

output = internal pallidum (of lentiform nucleus)

projects to the thalamus and then back to the cortex in a loop

23
Q

What other minor nuclei are part of the basal ganglia?

A
  • Substantia nigra (midbrain)

- subthalamic nucleus

24
Q

How are connections of the basal ganglia arranged?

A

As LOOPS - arise and terminate in the TEMPORAL LOBE

Frontal lobe -> striatum -> pallidum -> thalamus -> frontal lobe

25
Q

What are the 4 type of basal ganglia loops?

A
  • movement
  • cognition
  • emotion
  • behaviour

considered as both separate and working in parallel BUT actually function in OVERLAPPING FASHION

26
Q

Where are the COGNITION LOOPS of the basal ganglia located?

A

arise and terminate in the PREFRONTAL CORTEX

pass through the caudate nucleus

i.e. Caudate = Cognition

27
Q

Where are the MOTOR LOOPS of the basal ganglia located?

A

voluntary movement

arise and terminate in the MOTOR AND PREMOTOR AREAS

pass through the putamen

i.e. Putamen = Pure movement

28
Q

Where are the EMOTION/BEHAVIOUR LOOPS of the basal ganglia located?

A

arise from the LIMBIC LOBE, HIPPOCAMPUS and AMYGDALA

pass through the ventral striatum

involved in emotion, behaviour and particularly reward-based learning

29
Q

What regulates basal ganglia loops?

A

DOPAMINE

SUBSTANTIA NIGRA
(midbrain)
dopamine for caudate nucleus + putamen
via the nigrostriatal tract

VENTRAL TEGMENTAL AREA
(midbrain)
dopamine for the ventral striatum

30
Q

What is the SUBSTANTIA NIGRA?

A

large, pigmented nucleus

brown pigment -> accumulation of neuromelanin

found inside the cerebral peduncle of the midbrain

lies between the crus cerebri (in front) and the tegmentum (behind)

31
Q

What is the PARS COMPACTA?

A

one of the components of the substantia nigra (the other ones pars reticulata)

PARS COMPACTA
projects to basal ganglia
degnerates in Parkinson’s disease
contains dopaminergic neurons

32
Q

What is NEUROMELANIN?

A

pigment which makes substantia nigra appear dark

is a by-product of dopamine synthesis and degradation

33
Q

How does PD affect the appearance of the substantial nigra?

A

PD = Parkinson’s

loss of dopaminergic neurons in PARS COMPACTA

causes PALLOR of the substantia nigra

particularly in IDIOPATHIC PD
(seen at post-mortem)

34
Q

How many dopamine receptors are there?

A

5 types (D1-D5)

most are GPCRs

divided into 2 groups:

  • D1-like
  • D2-like
35
Q

What are D1-like dopamine receptors?

A

include D1 and D5 subtypes

Neurons expressing D1-like receptors are DEPOLARISED by dopamine

36
Q

What are D2-like dopamine receptors?

A

include D2, D3, D4 subtypes

Neurons expression D2-liek receptors are HYPERPOLARISED by dopamine

37
Q

What controls the activity of basal ganglia loops?

A

Projections called the Direct + Indirect pathways

these have opposing actions

38
Q

How does dopamine affect the DIRECT PATHWAY?

A

striatal neurons in direct pathway express D1-like dopamine receptors

they are excited by dopamine when it binds

this leads to increased activity via depolarisation of these neurons
=> facilitates movement
=> by lowering threshold for movement initiation (conscious action)

39
Q

How does dopamine affect the INDIRECT PATHWAY?

A

Neurons belonging to the indirect pathway express D2-like dopamine receptors

dopamine binding causes HYPERPOLARISATION and therefore inhibition of activity

dopamine signalling therefore inhibits activity of the indirect pathway

(movement is indirectly initiated)

40
Q

How does Parkinson’s lead to an ‘inability to initiate movement’?

A
  • > lack of striatal dopamine
  • > excessive activity in the INDIRECT (inhibitory) pathway
  • > difficult to initiate voluntary movements

Rx: replacement of striatal dopamine
e.g. oral levodopa (precursor)
OR
pharmacological agents to modulate D1, D2 or both types of dopamine receptor

BOTH Rx: aim to increase direct pathway and reduce indirect pathway activity

41
Q

What is the function of the ‘BASAL GANGLIA OUTFLOW’?

A

[OUTFLOW]
Internal pallidum -> ventral thalamus

this is entirely INHIBITORY

default action is to suppress unwanted movements or thoughts
-> a braking mechanism

42
Q

How do the direct/indirect pathways affect basal ganglia outflow function?

A

DIRECT PATHWAY ACTIVITY => switches braking mechanisms in basal ganglia OFF

INDIRECT PATHWAY ACTIVITY
=> mimics basal ganglia endogenous function
=> no unnecessary movement/thoughts

43
Q

What are the connections of the direct pathway?

A

arises in the striatum

passes directly to the internal segment of the globus pallidus (internal, output region)

activity in this pathway INHIBITS the normal braking action of internal pallidum
(-> DISINHIBITION)

44
Q

What are the connections of the indirect pathway?

A

arises in the striatum

converges on the internal pallidum but takes an indirect route there via the external pallidum and subthalamic nucleus

these then project back to the internal pallidum

45
Q

What is the net effect of the indirect pathway on its immediate structures?

A

DISINHIBIT (excites) the subthalamic nucleus

this then excites the internal pallidum

this reinforces the normal inhibitory function of the basal ganglia

46
Q

What is the VOLUNTARY MOTOR LOOP (briefly)?

A
  • direct pathway DISINHIBITS the thalamocortical projection
    => movement facilitated
  • indirect pathway INHIBITS same basal ganglia loop by exciting the subthalamic nucleus
    => reinforces the normal braking action of basal ganglia
  • Dopamine shifts balance of activity to FAVOUR DIRECT PATHWAY
    => DISINHIBITION of connections that promote voluntary movement
    (no movement)
47
Q

How do ALTERED DOPAMINE levels affect voluntary motor control?

A

LACK OF DOPAMINE (insufficiency)
difficulty initiating voluntary actions

EXCESSIVE DOPAMINE
unwanted involuntary movements
(dyskinesias)

48
Q

Where are the caudate and lentiform nuclei FUSED?

A

at their most ANTERIOR and VENTRAL portions

they are fused INFERIORLY under the anterior limb of the internal capsule

=> VENTRAL STRIATUM

49
Q

What is the VENTRAL STRIATUM?

A

lowermost or most ventral part of the basal ganglia

bulk is NUCLEUS ACCUMBENS alongside the ventral aspects of the caudate/putamen

50
Q

What is the function of the VENTRAL STRIATUM?

A

aka the limbic striatum

involved in reward-based learning

Some cortical regions project to the striatum. These include:

  • orbitomedial prefrontal cortex
  • anterior cingulate cortex

^^ both involved in emotions, behaviour and decision-making

51
Q

What is the HIPPOCAMPUS?

A

part of cerebral cortex

also projects to the ventral striatum

provides spatial and temporal contextual info about behaviours that are reinforced

52
Q

How is the AMYGDALA related to the ventral striatum?

A

also projects to the ventral striatum

takes part in basal ganglia loops

N.B. Amygdala is more of a nucleus rather than cortical region
hence it is described as “CORTICOID” or cortex-like

53
Q

What kind of receptors are in the ventral striatum?

A

Rich in OPIOID and CANNABINOID receptors

area has been implicated in addiction

such as drugs/EtOH and behaviours e.g. compulsive gambling

54
Q

Are there any addictive behaviours present in Parkinson’s disease?

A

Yes - pts can become ‘addicted’ to levodopa

and therefore take much higher doses than they require

can lead to:

  • hypersexuality
  • pathological gambling
  • obsessive behaviours

=> DOPAMINE DYSREGULATION SYNDROME

Life Control (25 decks)

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