Modulation of Movements by the Basal Ganglia - VOR is hard Flashcards
What is the Basal Ganglia?
The motor nuclei of the BG are divided into several functionally distinct groups of subcortical nuclei
The Striatum:
*Caudate & Putamen
*Globus Pallidus (internal and external part)
*Substantia Nigra (reticulata and compacta part where dopaminergic cell bodies of the nigro-striatal dopamine system reside)
*Subthalamic Nucleus
Collection of subthalamic nuclei sitting in the heart of the brain and controlling some fundamental functions
What is the biggest area of the basal ganglia and what is the primary cell type that it consists of?
The striatum (caudate and putamen)
Consists primarily of medium spiny neurons (85%) -> these are neurons, of which their denditres are densly covered in spines that receive synaptic inputs
They are all projection neurons, so they all project out of the neurostriatum
The axons arising from the medium spiny neurons converge on neurons in the pallidum, projecting to both the globus pallidus and substantia nigra pars reticulata which are the two main outputs from the basal ganglia
What are the two main types of receptors in the BG?
*Ones that express D1 type dopamine receptors
*Ones that express D2 type dopamine receptors
D1 type project mainly to the Globus Pallidus internal part and the Substantia Nigra Pars Reticulata
D2 type mainly project to the Globus Pallidus external part
So the biggest area of the BG, the striatum, has basically the same morphological cell type but they express two different kinds of receptors on their cell surface, splitting them into two clear populations of cells
What type of receptor are D1 type and D2 type dopamine receptors in the BG?
GABAergic
They are inhibitory, inhibiting the structures that they project to
What substance does D1 like release alongside GABA?
Substance P which is a peptide neurotransmitter
What substance does D2 like release alongside GABA?
Enkephalin which is a peptide neurotransmitter
What two areas of the BG receive the majority of the inputs to the BG?
The Striatum
Subthalamic Nucleus
Describe the ‘Go Circuit/Pathway’ in the BG
Cortex sends excitatory projection to the striatum and subthalamic nucleus cells, driving up activitiy in the D1 type cells in the striatum, which are GABAergic
D1 type then projects to the globus pallidus internal part and the substantia nigra pars reticulata (inihbitory GABAergic output)
-> these then inhibit cells that are tonically active, releasing the thalamus which then projects to, and activates, the cortex, facilitating action
Describe the ‘Stop Circuit/Pathway’ in the BG
Cortex sends excitatory projections to the subthalamic nucelus and the D2 type striatal neurons, which have inhibitory outputs
D2 neurons have an inhibitory input to the external part of the globus pallidus which releases the subthalamic nucelus and activates the excitatory output from the subthalamuc nucleus (glutaminergic output) to the globus pallidus internal and substantia nigra pars reticulata
This increases the inhibitory tonic stimulation of the thalamus, clamping the thalamus in an off position and inhibiting the cortex
How has the idea of two pathways been confirmed?
By optogenetics (getting cells in the brain to express proteins in their cell membranes that make them sensitive to light)
Been confirmed in mouse models which allow us to turn on D1 or D2 cells in the neo-striatum
Freeze et al. (2013)
Direct pathway= on pathway
Indirect pathway = off pathway
Engineered mice to express channel rhodopsin, an oxin which depolarises the membrane when exposed to an appropriate wavelength of light relating to D1 or D2
Exposed mice to a 1s or 100ms pulse of light
When mice exposed to D1 type dopamine activating wavelength (the go pathway), the velocity of the the animal is increased
When the mice are exposed to D2 type dopamine acitivatinf wavelength (the stop pathway, the velocity of the animal is decreased
This is direct evidence of those two pathways which have opposite effects on the animals behaviour
What is the hypothesis for BG function?
Redgrave et al. (1999)
Propose that the BG is the central ‘switch’ in the vertebrate brain that enables ‘action’ selection
Selecting some actions via the on pathway and inhibiting others via the off pathway
What is action selection about?
Picking an appropriate action within a given context depending on the predisposing conditions, e.g. sensory input (e.g. is there food around), homeostasis (e.g. am i hungry or thirsty), cogntive state (e.g. am i in danger)
Based on these things, one action amy become more salient based on the behaviour that is most adaptive in that particualr context
How does the BG mediate competition between actions?
A computational imperative
Central switch has a ‘wiring’ advantage over the distributed alternative
Much nore efficient by way of connectivity - central switch which meadiates actions by which you may want to perform in a particular context
What is the action selection hypothesis in the BG?
Basal ganglia circuits are organized to selected desired actions and to inhibit potentially competing unwanted actions. This is accomplished through a complex circuitry that is modified through development and learning.
What is the evidence for the action selection hypothesis?
The function of BG and is role in action selection seen in disorders of the BG
Damage to the BG is the cause of:
*Parkinson’s disease
Bradykinesia (slowness of movement), rigidity, tremor
reduce the ability of something to select actions in an appropriate context
*Huntington’s disease
Involuntary movements (chorea)
This leads to disinhibition in terms of movements, the person is making movements they cannot suppress
BG - the way movements are being selected is being affected