Basal Ganglia (Week 4--Chesselet) Flashcards
Basal ganglia
Ensemble of interconnected brain regions
Striatum: caudate nucleus and putamen
Globus pallidus (external and internal segments)
Substantia nigra (pars compacta and pars reticulata)
Subthalamic nucleus (only recently included in definition of basal ganglia)
Lentiform nucleus
Putamen and globus pallidus
(just because they are close together)
Are the caudate and putamen separate structures?
Not really, the caudate and putamen are two parts of the same structure (the striatum)
However, the internal capsule (white myelinated fibers) go through this structure to create the caudate medially (near ventricles) and the putamen laterally
Caudate and putamen have same connections, except for somatotopic arrangements
External globus pallidus (GPe) vs. Internal globus pallidus (GPi)
Have VERY different connections and functions
GPe: projects within the basal ganglia; uses GABA; gets input from medium spiny neurons from striatum that use enkephalin; indirect pathway, uses D2 receptors, inhibitory
GPi: projects outside the basal ganglia; uses GABA; gets input from medium spiny neurons from striatum that use substance P; major output pathway of striatum (projects to/inhibits VA,VL of thalamus); direct pathway, uses D1 receptors, excitatory
Substantia nigra pars compacta (SNc) vs. Substantia nigra pars reticulata (SNr)
Very different
SNc: contains dopaminergic neurons projecting to caudate-putamen (striatum)
SNr: contains GABAergic neurons projecting outside the basal ganglia
Ansa lenticularis
Output from the internal pallidum (GPi) to the thalamus
Lenticular fasciculus, or “H2 field of Forel” is same?
Inputs to the basal ganglia
The main “entry” into the basal ganglia is input to striatum
These inputs come mainly from the cerebral cortex
What regions of the cortex project to what regions of the striatum?
Medial cerebral cortex projects to caudate nucleus (more medial!)
Lateral cerebral cortex projects to putamen (more lateral!)
Two things about the cortex to striatum connections that are important to remember
1) Convergence: a huge region of cortex (almost the whole thing) projects to a small striatum
2) Topographical organization: medial cortex projects to caudate and lateral cortex projects to putamen
Inputs to the striatum
Cerebral cortex (not directly though!): uses glutamate
Nigrostriatal pathway (SNc to both putamen and caudate): uses dopamine; modulates input, not direct input (?)
Thalamus (centrum medianum/parafascicular nucleus): uses glutamate
Dorsal raphe: uses serotonin; modulates input, not direct input
Basolateral nucleus of amygdala: uses glutamate
Basal ganglia “loops”
Inputs, outputs and internal connections are topographically organized which results in “parallel circuits”
Motor loop
Oculomotor loop
Prefrontal loop
Limbic loop
So patient with basal ganglia disorder will have symptoms in multiple regions (not just motor!)
Motor loop
1) Motor, premotor, somatosensory cortex
2) Putamen
3) Lateral globus pallidus internal (GPi)
4) Ventral lateral (VL) and ventral anterior (VA) nuclei of the thalamus
5) Primary motor, premotor, supplementary motor cortex
Oculomotor loop
1) Posterior parietal, prefrontal cortex
2) Caudate
3) Globus pallidus internal (GPi), substantia nigra pars reticulata (SNr)
4) Mediodosal and ventral anterior (VA) nuclei of the thalamus
5) Frontal eye field, supplementary eye field
Prefrontal loop
1) Dorsolateral prefrontal cortex
2) Anterior caudate
3) Globus pallidus internal (GPi), substantia nigra pars reticulata (SNr)
4) Mediodorsal (MD) and ventral anterior (VA) nuclei of the thalamus
5) Dorsolateral prefrontal cortex
Limbic loop
1) Amygdala, hippocampus, orbitofrontal, anterior cingulate, temporal cortex
2) Ventral striatum
3) Ventral pallidum
4) Mediodorsal nucleus of the thalamus
5) Anterior cingulate, orbital frontal cortex
Neurons in the cortex vs. neurons in the striatum
Cortex: pyramidal neuron
Striatum: medium spiny neuron
What are the neurons of the striatum?
95% efferent neurons (medium spiny neurons): use GABA; 50% to GPi/SNr use GABA/substance P; 50% to GPe use GABA and enkephalin
5% interneurons: use GABA or ACh; few but important
Note: see that most of the efferent neurons out of the striatum are inhibitory
Main outputs of basal ganglia
Main “exits” are outputs from internal pallidum (GPi) and substantia nigra pars reticulata (SNr)
Types of inputs to caudate vs. putamen
Caudate = cognitive = inputs from limbic prefrontal areas
Putamen = motor = inputs from motor and sensory regions
Medium spiny neuron and output of striatum
Neurons of the striatum
Very spiny dendrites
Spines receive inputs from the cortex (use glutamate)
Shafts receive dopaminergic inputs from SNc (use dopamine)
GABAergic and project outside the striatum
Two main classes based on their output projections (enkephalin output to GPe and substance P output to GPi/SNr)
Interneurons of the striatum
Few interneurons (5%) but they are important functionally
Drugs that block muscarinic ACh receptors help in Parkinson’s disease (“dopamine/ACh balance” important therapeutically)
Important in processing cortical info within the striatum
Input to the subthalamic nucleus (of the basal ganglia)
Cortex directly projects to subthalamic nucleus and is excitatory
Called “hyperdirect pathway” and is functionally important
Outputs from the basal ganglia originate from which two regions?
Internal palladium (GPi)
Substantia nigra pars reticulata (SNr)
Outputs from basal ganglia (originating from GPi and SNr) use which NT?
GABA
Where do outputs from GPi project to?
Thalamic motor nuclei (VA, VL)
Reticular formation
Where do outputs from SNr project to?
Thalamic nuclei of the thalamus (VA, VL and dorsomedial nucleus)
Reticular formation
Superior colliculus
How does the basal ganglia send information back to the cortex
Via the thalamic motor nuclei (VA, VL)
Dorsal vs. ventral part of internal palladium
Neurons in dorsal part of internal pallidum more involved in movements based on internal cues (remembered sequences)
Neurons in ventral part of internal pallidum more involved in movements guided by external cues
Conections within the basal ganglia
Striatum sends GABAergic projections to GPe, GPi, SNc (just a little), SNr
Two distinct pathways:
1) Striatum –> GPi and SNr (contains GABA and substance P)
2) Striatum –> GPe (contains GABA and enkephalin)
Subthalamic nucleus
Only excitatory pathway WITHIN the basal ganglia (uses glutamate)
Direct and indirect pathways of output neurons of the striatum
Direct: SNc onto D1 receptors in striatum –> input of GABA and substance P from medium spiny neurons of striatum –> GPi/SNr –> VA/VL of thalamus –> frontal cortex to increase movement
Indirect: SNc onto D2 receptors in striatum –>input of GABA and enkephalin from medium spiny neurons of striatum –> GPe –> subthalamic nucleus –> GPi –> VA/VL of thalamus –> frontal cortex to decrease movement
Is there tonic activity in the indirect and direct pathways?
Yes!
Direct pathway has tonic inhibition of GPi to thalamus but when cortex fires to cause striatum to inhibit GPi, you get movement
Indirect pathway has tonic inhibition of GPe to subthalamic nucleus but when cortex fires to inhibit this inhibition, the subthalamic nucleus is free to stimulate the GPi which inhibits the VA/VL of the thalamus which decreases movement
How does the indirect pathway act as a “brake” to suppress competing movements?
It’s a brake because it inhibits movement
1) Because of additional synapses, the effect of inhibiting movement will be delayed compared to the direct pathway
2) Anatomical organization creates a “center/surround” effect with direct pathway affecting a small group of neurons in output regions and indirect pathway a large number of “surrounding” neurons; this focuses facilitation of specific motor units (via direct pathway) while inhibiting competing, unwanted movement (via indirect pathway)
How does dopamine act on direct and indirect pathways?
Dopamine exerts opposite effects on the direct and indirect pathways in the striatum, but same consequence of actually increasing movement
Direct pathway: dopamine stimulates GPi/SNr by acting on D1 receptors (facilitates movement)
Indirect pathway: dopamine inhibits GPe by acting on D2 receptors (inhibits inhibition to cause movement)
Role of basal ganglia disinhibition in the generation of saccadic eye movements
Stimulate caudate nucleus –> inhibition of SNr –> now superior colliculus free to fire –> stimulation of horizontal and vertical gaze centers (eye movements!)
Major diseases of the basal ganglia
Parkinson’s disease
Huntington’s disease
Dystonia
Restless Leg Syndrome
Psychiatric diseases (OCD)
Parkinson’s disease
1-2% of population, more men, onset before 50 years old
Loss of nigrostriatal dopaminergic neurons (loss of melanin pigment of SNc)
Symptoms: akinesia (difficulty initiating movements)/bradykinesia (slowness of movement), muscle rigidity (cogwheel sign), resting tremor, postural imbalance (can have stooped posture and gait disorder)
Triad of symptoms in Parkinson’s Disease
Akinesia/bradykinesia
Muscle rigidity
Resting tremor
Which neurons are lost in PD?
Dopaminergic neurons of the substantia nigra pars compacta (SNc)
Other dopaminergic neurons in the brain are fine!
What part of the brain is first affected in PD and what does this mean?
Putamen is the first brain region to be affected in PD
Since putamen = motor (putamen is part of “motor loop”), the first symptoms you have are motor symptoms
How does PD affect the direct vs. indirect pathways to decrease movement?
Direct: decreased activity of caudate/putamen to GPi/SNr because less stimulation by dopamine –> decreased inhibition of GPi/SNr –> increased activity of GPi/SNr –> increased inhibition of VA/VL of thalamus –> less movement
Indirect: decreased inhibition of caudate/putamen –> increased activity of caudate/putamen –> increased inhibition of GPe –> decreased activity of GPe –> decreased inhibition of subthalamic nucleus –> increased activity of subthalamic nucleus –> increased activity of GPi –> increased inhibition of VA/VL –> less movement
General consequences of loss of dopamine in the brain of someone with PD
1) Striatum has loss of “balanced” control
2) GPi/SNr has disorganized firing
3) Subthalamic nucleus has hyperactivity/bursts
Note: easy to think about increases/decreases but in reality it’s the disorganization of firing that causes the basal ganglia not to be able to regulate thalamocortical neurons that causes PD symptoms
Treatments for PD
L-dopa therapy
Surgery: lesions of GPi, high frequency stimulation to inhibit GPi or subthalamic nucleus
How might deep brain stimulation (DBS), or high frequency STIMULATION actually reduce the activity of structures??
1) Induce “depolarization block” because you’ve stimulated so much that the cells just stop firing
2) Stimulates the release of GABA from nearby axons which inhibits the structure itself from firing
What causes the tremor in PD
We don’t really know yet!
Animal models of PD don’t show tremor, only akinesia so no good way to study it
Lewy body
Main neuropathological “signature” of PD
Cytoplasmic inclusion made of various proteins
Mainly made of alpha-synuclein, which is toxic when it accumulates and misfolds within the neuron (sometimes called a “synucleopathy”)
Present in dopaminergic neurons of substantia nigra
Possible etiology of PD
Mutations in genes linked with the proteasome or toxins (pesticides) altering function of proteasome
Proteins can’t be degraded as well and get accumulation of proteins and misfolding of proteins
Proteasome dysfunction may affect mitochondria function also –> free radicals and oxidative stress
Treatments for Parkinson’s disease
Currently only symptomatic and for motor symptoms
1) Give drug that becomes dopamine in the brain
2) Give agonists at dopamine receptors
3) Change brain activity with implanted electrode
The many systems that PD can affect
Akinesia
Rigidity
Tremor
Postural imbalance
Olfactory loss
Cognitive disorders (implicit memory)
Affective disorders (depression/anxiety/apathy)
Sleep disturbances
Autonomic disorders (hypotension)
Digestive symptoms (constipation)
Environmental factors that may cause PD
MPTP (in heroine addicts, see Melega lecture)
Rotenone in experimental studies
Pesticides, heavy metals
Environment vs. genes in PD
Environmental pesticides increase the risk of PD but this effet is modulated by a person’s genes
Genes for VMAT2 and DAT (which control amount of dopamine in cell/synapse) altered in those who have been exposed to pesticides
