Task 2 Flashcards
(26 cards)
what is the basal ganglia?
= comprises a set of grey matter structures
components involved in movement
- corpus striatum = input zone
-> caudate
-> putamen
- pallidum = main sources of output
-> globus pallidus
-> substantia nigra pars reticulata
what is the corpus striatum?
- input zone of the basal ganglia
- consists of
-> caudate
-> putamen - strongest projections from association areas
- activity of the cell may encode the “DECISION TO MOVE” towards a goal
= rather than the actual movement
what is the caudate?
- part of the corpus striatum (with putamen)
- receives cortical projections
- does not process sensory information
- its neurons fire prior to eye movement
What is the putamen?
- part of the corpus striatum (with caudate)
- receives input from
-> parietal lobe (= somatic sensory cortices)
-> occipital lobe (= visual information)
-> frontal lobe (= motor cortices)
-> temporal lobe (= auditor association aresas) - its neurons discharge in anticipation of limb/ trunk movements
What is the input zone ?
- the destination of most pathways from other brain regions
- collateral axons of excitatory glutamatergic synapse project on medium spiny neurons
- low spontaneous activity
what are medium spiny neurons?
- class of cells in the corpus striatum
- destinations of incoming axons from the cerebral cortex
- receives input from a lot of brain structures
= big number of spiny neurons contacted by a single axon - dopaminergic = main input
- when active, the firing is associated with the occurrence of movements
- neurons increase the rate of discharge before an impending movement
->part of movement selection process - GABAergic = inhibitory
What is the output?
- medium spiney neuorns (of caudate & putamen)
-> give rise to inhibitory GABAergic projection
-> those end in the globus pallus & substantia nigra pars reticulata
-> this resembles the cortico-striatal pathway - regions the basal ganglia projects to
-> tectum
-> brainstem areas - main output is inhibitory
- high spontaneous activity = quiescent medium spiny neurons
-> prevention of unwanted movement (double inhibition)
-> tonic inhibition of thalamus & superior colliculus
what is the globus pallidus?
- for control of limb movement
- part of the pallidum
- send to ventral anterior & ventral lateral nuclei of the thalamus
- interacts directly with circuits of upper motor neuorns in frontal cortex
what is the substantia nigra pars reticulata ?
- projects to the superior colliculus
- controls orienting movements of eyes and head
- part of the pallidum
what are circuits within the basal ganglia?
- direct pathway
-> fast
-> direct
-> inhibitory connections from the striatum to GP / SN - indirect pathway
-> slower
-> roundabout route to GP / SN - hyper-direct pathway
-> direct pathway from cerebral cortex to subthalamic nuclei = excitatory
-> faster stop
-> supports the indirect pathway
What is the direct pathway?
- transiently inhibitory projections from
-> the caudate and putamen project to tonically active inhibitory neurons (globus pallidus) - they then project to the VA/VL complex of the thalamus
- transiently excitatory inputs to the caudate and putamen
-> from the cortex and substantia nigra - transiently excitatory input from the thalamus back to the cortex
- initiation of movement
- pallidum has tonic inhibition = constant inhibition of the thalamus
- Movement can only happen if pallidum is inhibited
= + cortex
= facilitation of movement
what does tonic mean? what does transient mean ?
tonic
= coinstant activity (+/-)
= highly spontaneous
transient
= not active by themselves
what do the neurotransmitters mean exactly?
glutamate +
GABA -
what happens in the direct pathway when the cortex is active?
- inhibited internal segment of globus pallidus
-> less tonic inhibition for the thalamus - excited thalamus
- GO = movement
what is the indirect pathway?
- transiently active inhibitory neurons from the caudate and putamen
-> project to tonically active inhibitory neurons of the external segment of the globus pallidus - the globus pallidus (external segment) neurons project to the subthalamic nucleus
-> receives strong excitatory input from the cortex - the subthalamic nucleus projects to the internal segment of the globus pallidus
-> its transiently excitatory drive acts to increase the disinhibitory action
-> opposing to the direct pathway = MODULATION
= basal ganglia increases in inhibition
= antagonist to direct pathway
what happens in the indirect pathway when the cortex is active?
- internal segment of globus pallidus = excited
- incrase inhibitory outflow of basal ganglia
-> increase inhibition of thalamus - NO GO
what are non-motor loops ?
- basal ganglia has parallel loops that originate in different regions of the coretx
-> engage in specific subdivisions of basal ganglia + thalamus - prefrontal loop
-> initiation & termination of cognitive processes (planning)
-> WM
-> attention
-> involving DLPFC and the head of caudate - limbic loop
-> regulates emotional & motivational behaviour
-> transition from one mood to another
what might be the cause of the tourette´s syndrome?
might be due to excessive activity in basal ganglia loops
-> regulate the cognitive circuitry of the prefrontal speech areas
what are theories of the basal ganglia?
- centre sourround model
- gate-keeper function model
what is the centre sourround model?
- centre-surround functional organisation of the two pathways
-integration of cortical input by the striatum
-> activates direct and indirect pathways
direct pathway
- centre
- focus on particular functional units
- axons from striatum synapse densly on single pallidal neurons
- activation
-> results in the disinhibition of the VA/VL complex
-> results in the expression of the intended motor program
-> reinforces the strongest motor program
indirect pathway
- surround
- covers broader range of functional units
- neurons from subthalamic nucleus are distributed evenly through internal segment
- activation (simultaneously)
-> neurons are driven by excitatory inputs from the subthalamic nuclei
-> reinforces the suppression of a broad set of competing motor programs
-> illuminates the weaker programs
what is the gatekeeper function model?
- two opposing pathways
- basal ganglia can act as gatekeeper of cortcial activity
- strong inhibitory baseline
-> keeps motor system in check
-> allows possible movements to become activated without triggering movement
direct pathway
- faster = decreased inhibition
- target neurons in the output nuclei = inhibited
-> hindering the connection to the thalamus
- leads to excitation of the thalamus + cortical motor areas
= BREAKS THEM DAM with strongest input
indirect pathway
- striatal activation
-> increased inhibition of the cortex
- slower (catches up with the direct pathway)
-> increases the inhibition
= BUILDS THE DAMN AGAIN - catches up with the direct pathway
what happens when dopamine is taken into consideration in the gatekeeper function model?
Direct pathway
- D1 receptors
- (+)
- EPSP
indirect pathway
- D2 receptors
- (-)
- IPSP
=> opposing effects on both pathways
dopamine release has the effect of
-> promoting selected actions = direct pathway
-> discouraging non-selected actions = indirect pathway
what happens in the direct pathway in a dopaminergic modulation?
- excitatory input to spiny neurons
- D1 receptor cells = EPSP
- increased responsiveness to cortical input
- GO = MORE GO
- ACTIVATION OF INITIATION
what happens in the indirect pathway in a dopaminergic modulation?
- inhibitor input to spiny neurons
- D2 receptor cells = IPSP
- decreased responsiveness to cortical input (!!in the indirect pathway)
- LIFTING THE BREAK
- ACTIVATION OF MOVEMENT
