Upload
brandi
View
122
Download
1
Tags:
Embed Size (px)
DESCRIPTION
Functional Anatomy of the Basal Ganglia. Sharif Taha, Ph.D. [email protected] Department of Neurobiology and Anatomy. Outline. Anatomy a. BG components b. Anatomical connectivity Function: Modulation through disinhibition Action Selection Neuromodulators: dopamine. - PowerPoint PPT Presentation
Citation preview
Functional Anatomy of the Basal Ganglia
Sharif Taha, [email protected]
Department of Neurobiology and Anatomy
Outline
1. Anatomya. BG componentsb. Anatomical connectivity
2. Function: Modulation through disinhibition
3. Action Selection4. Neuromodulators: dopamine
What do the basal ganglia do?
1. Modulate the initiation, termination, amplitude, and selection of movement
- Initiation and selection
2. Learning-Response-outcome associations- Stimulus-response associations
Basal ganglia: a modulatory cortical loop
1. Basal Ganglia receives robust input from the cortex
- Almost all parts of cortex; excludes primary sensory cortices
2. Principal projection of the BG - back to cortical targets
- Motor associated areas- Via ventral thalamic relay
(Other targets: superior colliculus)
Overview of BG organization• Input:
– Caudate and putamen (together, the striatum)
• Intrinsic: – Subthalamic nucleus (STN)– External segment of globus pallidus
(GPe)
•Output: •Substantia nigra pars reticulata (SNr)•Internal segment of globus pallidus (GPi)
•Neuromodulator: •Substantia nigra pars compacta (SNc)
SNc
Striatum: Medium spiny neurons
• Caudate and putamen• Medium spiny neurons
– ~90% of neurons; primary projection neurons
– GABAergic; inhibitory
– Very little spontaneous activity; dependent on excitatory input for discharge
Up and down states• Inwardly rectifying potassium
channels keep striatal neurons (very) hyperpolarized
• Membrane potential shifts from hyperpolarized potentials (−80 mV) to more depolarized potentials (−50mV)
• Transitions to the up state are correlated among nearby striatal neurons
• Selection mechanism – requires concerted cortical activation to move to upstate
Wilson 1998 Science
Striatum: Intrinsic interneurons
2 principle types
– 3 GABAergic interneurons
– Tonically active neurons (TANs)• Cholinergic • Large cell bodies
Globus pallidus
Two segments→ Internal: Principle output nucleus→ External: intrinsic circuitry
Neurons in both areas -high tonic firing ratesGABAergic, inhibitory
Subthalamic nucleus
Alone among the BG circuit elements –glutamatergic
Target for deep brain stimulation (DBS)
Nigral Complex• Midbrain
• Substantia nigra pars reticulata (SNpr)– GABAergic– Output of BG– Developmentally,
related to Gpi
• Substantia Nigra pars Compacta (SNpc)– Neuromelanin-
containing cells– Dopaminergic (A9)
SNc
Basal ganglia connectivityCortical input
Subthalamic nucleus
Thalamus Cortex
Three organizing principles of basal ganglia connectivity
Cortical input
Subthalamic nucleus
Thalamus Cortex
• Anatomically parallel loops with distinct function
• Finer-grain topographic organization within loops
• Patch/matrix
Functional topography: Parallel loops w/in the BG subserve distinct functions
•J.H. Martin, Neuroanatomy: Text and Atlas 2nd Ed., 1996
Functional topography: Parallel loops w/in the BG subserve distinct functions
• 4 pathways:– Skeletomotor– Oculomotor channel– Association
• Behavior, learning, cognition
– Limbic• Addiction, emotional
behavior
•J.H. Martin, Neuroanatomy: Text and Atlas 2nd Ed., 1996
Topography is also maintained within loops: Somatotopy
•J.H. Martin, Neuroanatomy: Text and Atlas 2nd Ed., 1996
Oculomotor topography
Patch/matrix compartments: neurochemical organization
• Neurochemically distinct areas (patch, mu opioid receptor; matrix, calbindin)
• Dendrites observe boundaries
• Afferents/efferents are distinct
• Functional roles –– Patch: limbic– Matrix: sensorimotor
Outline
1. Anatomya. BG componentsb. Anatomical connectivity
2. Modulating action through disinhibition
3. Direct and Indirect Pathways4. Action Selection5. Neuromodulators6. Pathology
Movement modulation through disinhibition
Movement modulation through disinhibition
Output nuclei of the basal ganglia are inhibitory
Output nuclei maintain a high tonic level of discharge, suppressing activity in target regions
Firing under quiescent conditions (in the absence of movement)
Movement modulation occurs through disinhibition of thalamocortical target regions
What advantages does modulation through inhibition confer?
• Strong tonic inhibition allows basal ganglia to serve as a master regulator – arbitrating between multiple excitatory inputs
• Initiating and• Discriminating
Saccadegenerator
Cortical regions
Basal ganglia: movement modulation through disinhibition
1. Output nuclei of the basal ganglia are inhibitory
2. Output nuclei maintain a high tonic level of discharge, suppressing activity in target regions
3. Phasic decrease in firing rate transiently releases target regions from inhibition.
4. Disinhibited thalamocortical circuit discharges, promoting movement.
Outline
1. Anatomya. BG componentsb. Anatomical connectivity
2. Modulating action through disinhibition
3. Direct and Indirect Pathways4. Action Selection5. Neuromodulators6. Pathology
Direct and Indirect Pathways
Direct Pathway
Basal firing rates in the striatum are very low,and dependent upon strong cortical excitation.
Under these conditions, striatal firing has little impact on GPi/SNr discharge
Phasic cortical excitation drives excitatory discharge in the striatum.
Activation of the direct pathway promotes action.
This causes a transient inhibition of GPi/SNr firing.
Indirect pathway
Striatal neurons have low tonic firing rates;again, dependent upon strong cortical inputs
GPe neurons are similar to those in GPi;they have high tonic firing rates
Firing under quiescent conditions (in the absence of movement)
What happens with strong, phasic cortical excitation?
Transient inhibition of GPe firing…
Followed by phasic excitation of the STN (through disinhibition)…
Activation of the indirect pathway suppresses action.
And finally, a increased rate of discharge in the output nuclei -
Rate model & basal ganglia pathology
http://www.youtube.com/watch?feature=player_detailpage&v=fCL7RWaC3RAhttp://www.youtube.com/watch?feature=player_detailpage&v=AvBrP4yRTRA
Indirect pathway suppresses action. Direct pathway facilitates action.
How do they cooperatively regulate motor output?
Outline
1. Anatomya. BG componentsb. Anatomical connectivity
2. Modulating action through disinhibition
3. Direct and Indirect Pathways4. Action Selection5. Neuromodulators6. Pathology
Action selection
Action encoding in output nuclei of the BG
Action encoding in the output nuclei of the BG
Direct pathway inputs are focused and robust
Direct pathway inputs are focused and robust
Indirect pathway inputs are widespread and diffuse
Together, these inputs create a center-surround mechanism for action selection
Movement modulation occurs through disinhibition of thalamocortical target regions
Competing alternatives are actively inhibited
Why do we need to ‘sharpen’ selection mechanisms?
• Multiple/ambiguous stimuli in our environment often demand our attention/action (e.g., visual stimuli)
• However, we’re often confined to making a single action to address these stimuli (e.g., a saccade).
• Particularly where conflicting needs are present, action may require active inhibition
Action selection (in action)• Multiple/ambiguous stimuli in our
environment often demand our attention/action.
• However, we’re often confined to making a single action to address these stimuli (e.g., a saccade).
• Selection through surround inhibition likely occurs on large and small scales – i.e., not only saccade left or right, but how far to saccade?
Direct and indirect pathways together facilitate action selection
• Activation of direct pathway facilitates movement
• Activation of indirect pathway suppresses movement
• Direct output makes focal inhibitory contact on GPi/SNr
• Indirect output makes diffuse, widespread excitatory contact on GPi/SNr
• Co-activation of these pathways facilitates action selection through center-surround mechanism
Outline
1. Anatomya. BG componentsb. Anatomical connectivity
2. Modulating action through disinhibition
3. Direct and Indirect Pathways4. Action Selection5. Neuromodulators6. Pathology
Dopamine input arises from the SNc
Direct and Indirect pathways express distinct dopamine receptors
D2 signaling suppresses firing in indirect pathway neurons
D2 signaling suppresses firing in indirect pathway neurons
Thus, D2 effects on indirect pathway act to facilitate movement
Strong cortical inputs are facilitated by D1 signaling
Strong cortical inputs are facilitated by D1 signaling
Thus, D1 facilitates movement in the presence of strong cortical drive
Up and down states/DA action
• D1 receptor signaling - In down state, increases voltage-dependent
K+ current - In up state, increases voltage-dependent Ca+
+ current• D2
– Generally inhibit firing by decreasing Ca++ currents.
Dopamine effects on direct and indirect pathways
• Dopamine signaling through D2 receptors in the indirect pathway suppresses striatal activity
• Dopamine signaling through D1 receptors in the direct pathway:– Facilitates strong, phasic inputs– Suppresses weak inputs
Acetylcholine effects
Cholinergic signaling promotes firing in the indirect pathway suppresses movement
Cholinergic signaling in the direct pathway inhibits firing suppresses movement
Net effect of cholinergic signaling (through both direct and indirect pathways) is an inhibition of movement
Under what conditions do DA, ACh neurons fire?
• Both neurons are sensitive to reward-related stimuli, particularly reward-predictive cues (i.e., Pavlov’s bell).
• However their response differs:– DA neurons increase firing– ACh neurons decrease firing
• Net effect: facilitation of movement in response to reward predictive cues
Examples of DA firing/release
Tomorrow’s paper discussion!
Outline
1. Anatomya. BG componentsb. Anatomical connectivity
2. Modulating action through disinhibition
3. Direct and Indirect Pathways4. Action Selection5. Neuromodulators6. Pathology
Parkinson’s Disease: What happens when DA input is lost?
Parkinson’s Disease: What happens when DA input is lost?
http://www.youtube.com/watch?feature=player_detailpage&v=3VrnOtmZBtc
Direct pathway become less active; indirect pathway becomes more active
Action selection (direct pathway) is suppressed: action inhibition (indirect pathway) is facilitated
Summary
1. Modulating action through disinhibition
2. Direct and Indirect PathwaysDirect pathway facilitates actionIndirect pathway suppresses action
3. NeuromodulatorsDopamine
Facilitates action through both pathwaysIncreases firing in response to reward directed cues
AcetylcholineSuppresses action through both pathwaysDecrease firing in response to reward directed cues
4. BG Role in Action SelectionSelection through direct pathway; surround suppression through indirect pathway
5. Parkinson’s Disease: DA loss suppresses action selection
Limitations
1. ‘Rate model’ does little to explain other BG-related phenomena, such as tremor…though this model been very useful
2. Dopamine function is not confined to facilitating action – very likely plays an important role in learning.
3. BG function is not confined to regulation of movement!
References• Kandel is fine for the basics• Excellent review of BG function and role of
BG in guiding reward-directed (eye) movements: – Hikosaka 2001, Physiological Reviews - Role of
the Basal Ganglia in the Control of Purposive Saccadic Eye Movements
• General review of striatal function: – Kreitzer Annu. Rev. Neurosci. 2009. 32:127–47,
Physiology and Pharmacology of Striatal Neurons