basal nuclei network Flashcards
what regions form the inhibitory output circuits for the basal nuclei?
SNr and GPi
what region forrm the regulatory input of the basal nuclei?
dopaminergic input from the SNc
what is tonic activity?
cells are instrincally active- producting action potentions without excitatory input
what regions are tonically active?
GPe, STN, inputs from striatum to SNc and outputs from the SNr/GPi to the thalamus
what is a convergent network?
takes in a lot of information to a small number of neurons
is the basal nuclei network convergent or divergent?
convergent
what neurons does the striatum contain?
90-95% of neurons are medium spiny neurons
where do the MSN connect to?
100 MSN connect to 1 cell in the globus pallidus
what are MSN innervated by?
5000 pyramidal cells (layer v) in the cortex connect to one medium spiny neuron in the striatum
loss of somatotopy in the globus pallidus
high pattern of intergration of motor information from cortex to GP
lots of information so do not know where movement information comes from just by anaylsis GP activity (e.g was it right arm or left finger?)
what are the two pathways of the basal nuclei?
direct pathway from striatum to thalamus
indirect pathway from striatum to thalamus
direct basal nuclei pathway
- motor cortex sends excitatory glutamatergic input to the striatum
- striatum sends inhibitory GABAergic signals to the GPi and SNr
- GPi at rest tonically inhibits the thalamus but since the striatum inhibits the GPi, the GPi reduces its inhibition of the thalamus — this is called disinhibition.
- thalamus can send excitatory signals back to the motor cortex
- voluntary movement occurs
what is disinhibition?
inhibiting something that is already inhibitory (equivalent to already being excitatory)
inhibitione of the motor cortex
when striatum is at rest then GP is tonically active thereby inhibiting the VA/VL complex so there is no excitation of the upper motor neurons in the cortex
excitation of the motor cortex
when striatum is transiently excitated then the GP is transiently inhibited so the VA/VL are disinhibited so other inputs can excite it leading to excitation of upper motor neurons in the cortex
indirect pathway of the basal nuclei
- motor cortex sends excitatory glutamatergic input to the striatum
- activated striatum sends inhibitory GABAergic signals to the GPe
- when GPe is inhibited, it stops inhibiting the STN → the subthalamic nucleus becomes more active
- active STN sends excitatory glutamatergic input to the GPi and SNr
- sends stronger inhibitory GABAergic signals to the thalamus
- inhibited thalamus sends less excitatory input to the motor cortex
direct pathway overview
activation leads to removal of inhibitory
input to thalamus – get excitation of motor
cortex
this facilitates/initiated wanted movements
indirect pathway output
activation enhances inhibitory input to
thalamus – no excitation of cortex
inhibits/suppresses unwanted movements
STN pacemaker
projects back to GPe when STN is activated
this reintroduces inhibition of STN
pulsatile output from STN means the output from the GPi/SNR is not constant
combined basal nuclei pathways
in the absence of cortical input, the indirect pathway dominates the direct pathway- default position of motor system is nothing (not excited)
why is the indirect pathway more dominant than the active pathway?
striatal-pallidal MSNs can generate a tonic inhibitory current (indirect)
striatal-nigral
MSNs and are
quiescent at rest
and have a low
firing rate when
active (direct)
what is the role of the nigral-striatal projection?
to balance the direct and indirect pathways and favour movement
what are the two subtypes of MSN?
those that express the D1 dopamine receptor and those that express the D2 dopamine receptor
these control the nigral-strital projection (tonically active)
D1 receptor role
positively modulatory receptor (excitatory)
enhances activity of the direct pathway striatal neurons
promotes movement
D2 receptor role
negative modulatory receptor (inhibits)
supress activity of direct pathway
effect of nigral-striatal projection balance
strength of input to direct/indirect will determine how far initation of movement will go
this allows smooth initation of movement
other parallel basal nuclei loops
- oculomotor loop control of gaze
- prefrontal and orbitofrontal loops concerned with cognition
- limbic loop concerned with emotional and visceral functions
impact of limbic loops
- may play a central role in reward learning
- involved in drug addiction mechanism
- may also be involved in diseases such as schizophrenia
