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Anatomy of the basal ganglia
Dana Cohen
Gonda Brain Research Center, room 410
[email protected]
A small number of neurons…
• The basal ganglia receive projections from most cortical
areas
• The basal ganglia project out to cortical areas involved in
the generation of behavior
• Act in parallel with other output systems of the cortex
and thus may not play a primary role in generating
behavior
• Essential for several types of learning
The cortico-basal ganglia
circuits consist of:
• Neocortex
• The striatum (caudate - putamen and the core of
nucleus accumbens)
• The globus pallidus (GP) (lateral and medial)
• The subthalamic nucleus (STN)
• Substantia nigra (SN) (pars compacta and pars
reticulata) and the ventral tegmental area (VTA)
Anterior to posterior coronal view
Anterior to posterior coronal view
Anterior to posterior coronal view
Input and output of the basal ganglia
• Cortex to striatum: glutamate
• MGP and SNr: GABA
The striatum
• The major input nucleus of the BG.
• Made up of the putamen, the caudate nucleus and
nucleus accumbens which have similar histological and
anatomical characteristics.
• Receive input from most of the cortex (and the thalamus)
• Complex bidirectional interaction with the substantia
nigra pars compacta (SNc)
• Output to both segments of the globus pallidus (GP) &
the substantia nigra pars reticulata (SNr)
Striatum – Medium Spiny Neurons I
• MSNs = Medium Spiny Neurons
• > 90% of all cells in striatum
• >10,000 cortical inputs to one MSN
• GABAergic projection neurons
• Dense collateral network
• Two states:
– ‘down’ state with low resting
potential and no firing
– ‘up’ state characterized by short
firing episodes.
Striatum – Medium Spiny Neurons II
• MSNs are typically quiet with no baseline firing.
• Sensory and movement related response comprises of a
short high frequency burst.
• Highly specific to portion of the task and parts of the
movement but can respond to several events.
• Affected by sequence context or reward contingency.
The interneurons of the striatum make
up about 5-10% of the neurons
• TANs: large aspiny neurons – Ach (Was
previously thought to be the projection neuron)
The GABAergic interneurons of the striatum
• FSNs: medium aspiny neurons that contain parvalbumin
• Medium aspiny neurons that contain somatostatin
The medial globus pallidus and the SNr
• Primarily made up of
GABAergic projection neurons.
• Firing rate at rest is 60-100
spikes/s and is highly irregular
(The ultimate Poissonian
neuron).
• Sensory and motor response is
broad and includes increases
and decreases of firing rate.
The lateral globus pallidus (GPe)
• Same morphology as the MGP
• High frequency pausers (HFP) & lowfrequency bursters (LFB)
• Internal to the basal ganglia with no
external connections for input or output
The subthalamic nucleus
• Made up mainly of projection
neurons. Firing rate at rest is 2030 spikes/s with short burst
following movement.
• The projection neurons are
glutamatergic and send their
output to the GPi & SNr.
• In addition to its role in the indirect
pathway, has direct cortical inputs
forming the hyperdirect pathway.
Direct and indirect pathways
• The direct pathway causes disinhibition
• The indirect pathway is more complex but likely to
counterbalance the direct pathway
Feedback pathways
Cortical input to the striatum
originates from most cortical areas
• Primary and higher order sensory areas, motor,
premotor, and prefrontal regions, and limbic cortical
areas.
• The input is organized topographically
–
–
–
–
Frontal areas project to rostral striatum
Sensorimotor cortex projects to dorsolateral striatum
Parietal cortex projects to caudal striatum
Highly interconnected cortices may overlap in the striatum
• Numbers:
– There are 17 million cortico-striatal cells
– There are almost 2.8 million striatal projection neurons
– No 2 striatal neurons share their cortical input
Synaptic input to distal dendrites of
the MSNs
Inputs to the proximal and distal
dendrites of the MSNs
Direct and indirect striatal projection neurons
selectively express the D1 and D2 receptors
Components of the indirect pathway
Synaptic inputs to the SNr
• The SNc and SNr
cannot be distinguished
based on the
morphological
properties of the
neurons
– Dopaminergic neurons
– GABAergic neurons