Structure and Function of the Basal Ganglia Flashcards
What are the components of the extrapyramidal system?
Basal ganglia, thalamus, subthalamic nucleus, substantia nigra, red nucleus (flexors of upper limb)
What are the functions of areas 4 and 6?
Area 4 - primary motor cortex - responsible for execution of movement
Area 6 - premotor / supplementary motor cortex, formulates motor program and lesions will cause akinesia (paucity of movement) / apraxia
What is Bereitschaftspotential (BP)?
Readiness potential - slow negative potential on EEG seen over bilateral areas 4/6 right before a self-initiated voluntary movement occurs
-> early preparation for a motor activity
What is the function of the basal ganglia? When are they active?
Optimization / filtering of motor program
- > integration of sensory and other information
- > no role in motor decisions or basic parameters of movement
- > active even during imaginary motor actions
What is the function of the cerebellum and thalamus for movement?
Cerebellum - dynamic adjustment of motor execution, especially important for automatic / repetitive movements
Thalamus - relays information to and from cortex from BG, cerebellum, and sensory inputs
What is the “pallidum”?
Globus pallidus internus (medial) + substantia nigra pars reticulata
(GPi/SNr)
Give the direct pathway.
Striatum fires GABAnergic to inhibit Pallidum which is GABAnergic to Thalamus
Thalamus becomes disinhibited and can project glutamate to cortex, completing loop
Give the indirect pathway.
Striatum fires GABAnergic to GPe which is GABAnergic to Subthalamic nucleus.
Subthalamic nucleus is disinhibited, can stimulate Pallidium which is GABAnergic to Thalamus, which inhibits cortical input by thalamus.
How does the direct pathway make the movement filtered?
There is a surround inhibition of nearby motor pathways via interneurons (lateral inhibition)
What is the “hyperdirect” connection which increases BG inhibition?
Cortex can direct stimulate subthalamic nucleus to reduce movement
What is the homunculus orientation of cortical projections of sensory information to the putamen?
Anterior / ventral - face
Posterior / dorsal - feet
What is the relative functional difference between the caudate and the putamen? What innervates them?
Putamen - motor nucleus of striatum, receives feedack from motor and sensory system to control limb movement
Caudate - dense innervation from prefrontal cortex, plays a role in planning, memory-based, and psychological aspects of motor function (motivational circuit)
What role does the basal ganglia play in oculomotor function?
Caudate fires during saccadic eye movements
-> loss of caudate particularly affects eyes
How does degeneration of striatum in Parkinson’s and Huntington’s differ and thus relate to symptoms?
Parkinson’s - preferential loss of putamen over caudate -> loss of motor function first
Huntington’s - preferential loss of caudate - cognitive and eye movement abnormalities. Impulsive behavior (loss of Nucleus accumbens) occurs often before choreiform movements
What are the striosomes and what do they more contribute to? What dopamine receptors do they have?
Chemically distinct “islands” which receive cortical input from layers 5 and 6 in the prefrontal, insular, and temporal cortex. Mostly involved in emotional input / motivational influence of movement
Mostly have medium spiney neuronsn with D1 and D3 receptors (mostly excitatory)
What are the matriosomes and what do they more contribute to?
Neuronal areas which makes up both the direct and indirect pathway - Receive cortical inputs from layer V mostly sensorimotor and association cortex. Most high concentrations of D2 receptors, but also have some D1 for the direct pathway.
What are medium spiney neurons and what distinguishes them?
90-95% of striatal neurons
- GABA neurons which are distinguished by their dopamine receptor types
- > D1/D2/D3
What do D1-expressing medium spiney neurons do?
They are primary striosomal, integrating sensory information, but there are also some D1-expressing medium spiney neurons which form the majority of the direct pathway.
D1 receptors are excitatory (increase cAMP)
What do D2-expressing medium spiney neurons do?
They are primarily matriosomal (form the indirect motor pathway
D2 receptors are inhibitory (decrease cAMP)
What are the functions of the large spiney neurons and the medium aspiney neurons?
Large spiney - cholinergic interneurons (interact directly with medium spiney)
Medium aspiney - somatostatin interneurons
Very important in the matrix, interacting with matriosomes, having a large control over motor function
What is the function of dopaminergic input on convergence of the basal ganglia pathways?
Naturally, many neurons converge on a single neuron, but often pathways are kept separate (parallel)
- > controls integration of many neurons and parallel pathways in the basal ganglia
- > Removal of dopamine = convergence of pathways
What is hemiballism?
Lesions in the subthalamic nucleus increase movement tendency (decreased excitation of pallidum from indirect pathway)
What causes disruption in movement in Huntington’s disease?
Preferential loss of caudate = preferential loss of indirect pathway of striatum first -> increased movement
How does Parkinson’s arise?
Loss of SNc
- > loss of dopamine to direct pathway (decreases movement)
- > Loss of dopamine to inhibit indirect pathway (decreases movement)
