Ch. 18 Modulation of movement by the basal ganglia Flashcards
Why are the putamen and caudate known as the ‘input zone’ of the basal ganglia?
Medium spiny neurons eminating from these are synapsed upon by cortical pyramidal neurons, dopaminergic neurons and local circuit neurons (of corpus striatum), and confer their input to globus pallidus or substantia nigra pars reticulata neurons. This effectively allows cortex, thalamus and brainstem structure influence upon the basal ganglia.
What does the corpus striatum concist of and whe the name?
It is made up of two principle nuclei, the caudate and the putamen. The name is derived from the striped appearance of a parasaggitall cut of the capsula interna that communicates the connection between the two, made up of “slender bridges of gray matter”.
What is the corticostriatal pathway?
The route of cortical neurons, mainly from associative areas of the frontal and parietal lobes, to the corpus striatum and ultimately caudate and putamen.
How do the inputs to the putamen and the caudate differ?
The caudate mainly receives input from the multimodal association ateas (cortex) and motor areas from the frontal lobe controlling the eyes, whilst the putamen receives input from secondary somatic sensory cortices, premotor and motor cortices, extrastriate visual cortices and auditory association areas.
What signifies the corticostriate synapses?
They are all excitatory and glutamergic.
Where does dopaminergic input onto the striate corticostriate pathway come from?
Substantia nigra pars compacta.
What is the function of the basal ganglia and how is this portrayed in their firing.
They generally control decision to move-actions. E.g. the neurons of the caudate can fire seconds before eye movement, and similarly the medium spiny neurons in the putamen before limb movement.
What types of synapses does theninput zone make with target areas!
GABAergic.
What signifies the convergence of the corpus striatum neurons onto the globus pallidus.
A high degree, 100 medium spiny neurons for every cell in the globus pallidus. However each medium spiny neuron makes a lot of synapses with different pallidal neurons, making for a weak but broad connection and not singularly effective transmission.
Outline the organization of the intrinsic circuitry and outputs of the basal ganglia and the nature of rach synapse.
- Caudate and putamen - (-) -substantia nigra pars reticulata - (-) - superior colliculus
- Caudate and putamen - (-) - globus pallidus internal - (-) - VA/VL complex of thalamus - (+) - frontal cortex
How do the efferent connections from the globus pallidus and substantia nigra differ?
Globus pallidus: emanates mainly from the internal segment, contacts cortex upper motor neurons via relay synapses in the VL and VA nuclei of the thalamus
SN pars reticulata: mainly (but not only) projects to upper motor neurons of superior colliculus
Explain the general sequence of events of the action of basal ganglions on upper motor neurons a) during no excitation of the striatum b) during excitation of the striatum.
a)
no input from cortex - striatum - globus pallidus (tonically active) - (-) - VA/VL complex of thalamus - (-) - motor cortex (upper motor neuron)
b)
input from cortex - (+) - striatum transiently excited - (-) - globus pallidus transiently inhibited - (+) - VA/VL complex of thalamus disinhibited (receptive to other input) - (+) - motor cortex excited
What is the relationship between the direct and indirect pathway through the basal ganglia?
They are antagonists, where the firect pathway serves to remove inhibition of the thalamic nuclei and the indirect pathway serves to inhibit the thalamic nuclei.
What is the value of having both the direct and indirect pathways of basal ganglia, and how is their interplay governed?
Competing muscle programmes are filtered away, leaving only the disired movement performed. The interplay of the two follows the center-surround functional organization where the transient inhibition of the globus pallidus internal segment by the caudate/putamen releases VA/VL complex of thalamus from tonic inhibition for the desired movement, whilst surrounding competing programmes as inhibited through opposite affecting.
Through what mechanism and how does dopamine released from substantia nigra pars compacta affect the basal ganglia pathway?
Dopamine released on spiny neurons recruit either D1 or D2 receptors that serve to further excite (former, direct pathway) or inhibit (latter, indirect pathway) the neuron. In this way dopamine serves to strengthen the responses of the corpus striatum and basal ganglia.
