Motor Systems II:
Basal Ganglia
Competencies:
- Diagram the components of the basal ganglia.
- Distinguish the substantia nigra compact part from the substantia nigra reticular part.
- Illustrate the projections into the basal ganglia and the main output from the basal ganglia.
- Appraise the functions of the direct and indirect loops in the basal ganglia in the brain.
To master the material presented in this
lecture:
Read ...
Purves text, Chapters 18
Haines pp 234 - 241
Look at the Review Questions below ...
Listen to the lecture and focus on
the following points ...
Basal Ganglia: Anatomical Definitions.
-
Basal ganglia in our definition
includes, corpus striatum; i.e. caudate-putamen (in telencephalon) + subthalamic nucleus
(in diencephalon) + substantia nigra (in mesencephalon).
-
The ventral striatum includes
the nucleus accumbens, substantia inominata, ventral portions
of the caudate and putamen, and deep layers of the olfactory tubercle.
Basic Circuits - Functional Considerations.
-
The basal ganglia, via their thalamic
relays, read sensorimotor and cognitive information from broad
areas of the cortex, and process this information through an intricate
array of parallel circuits in order to refine the ultimate cortical
output associated with specific behaviors.
-
Input from sensorimotor, association
and limbic areas of the cortex are segregated.
-
The putamen processes motor
information and projects to the globus pallidus and thalamus
(VA/VL) to the supplementary motor area. The putamen is mainly
involved in the motor functions of the basal ganglia.
-
The caudate nucleus receives
its input mostly from association areas of the cortex and
projects through the globus pallidus and thalamus (VA, dorsomedial
nucleus) largely to prefrontal areas. The caudate is mainly
involved with the cognitive aspects of behavior.
-
The ventral striatum processes
limbic system information via the dorsomedial thalamus to
the anterior cingulate and medial orbitofrontal cortex. The
ventral striatum is primarily associated with the motivational
aspects of behavior.
-
There are two parallel but functionally
distinct circuits within the basal ganglia involved in the modulation
of cortical function. These so-called direct and indirect loops
have opposite effects.
-
The projections from the GPi
and SNr to the thalamus are inhibitory in nature. These neurons utilize the inhibitory neurotransmitter GABA and have
a high rate of spontaneous activity which fresults in tonic inhibition of the flow
of information from the thalamus to the cortex.
-
The direct loop - medium spiny neurons in the striatum that utilize GABA are excited by coticostriate projections; these neurons project to GPi and SNr and inhibit the tonically
inhibitory neurons (disinhibition) in the GPi and SNr, resulting
in the enhancement of cortical activity by excitatory thalamocortical
projections.
-
The indirect loop - increases
the activity of inhibitory neurons, resulting in the suppression
of cortical activity. Medium spiny neurons, excited by corticostriate projections, project to GPe thus inhibiting these tonically atrive GABAergic neurons which normally act to inhibit neurons in the subthalamic nucleus. When active, subthalamic neurons that project back to the GPi act to enhance the activity of the inhibitory neurons which results in the inhibition of the flow of information through the tlalamus.
-
The direct and indirect loop are independent pathways that serve to enhance desired motor and cognitive function while inhibiting extraneous behavior. Medxium spiny striatal neurons associated with either pathway can be distinguished on neurochemical ground (e.g. colocalized neuropeptides, intracellular signaling molecules) whixh has aided our understanding of these two pathways.
Substantia Nigra and Ventral Tegmental
Area.
-
Overall, the nigrostriatal system
reinforces cortically initiated motor activity through a facilitation
of the direct loop and suppression of the indirect loop.
-
Striatal cholinergic interneurons
excite the GABA/enkephalin neurons of the indirect loop, thereby
inhibiting the thalamocortical projection activity. This results
in counterbalancing the cortical reinforcing effect of the dopaminergic
projections. (Given the neurochemical compartmentation of these
systems, these relationships indicate that this network is comprised
of two parallel "cross-talking" systems - fine control
of motor output depends upon these complex yet subtle anatomical
and neurochemical relationships!)
-
The SNr is rich in GABAergic neurons
and receives inputs from the striatum and the subthalamic nucleus
and projects to the VA/VL and the dorsomedial nucleus of the thalamus.
-
The ventral tegmental area (between
the substantia nigra and red nucleus) contains dopaminergic neurons
that project rostrally to the hypothalamus, nucleus accumbens/ventral
striatum, and broad areas of the neo- and paleo-cortices, forming
the mesolimbic dopaminergic system.
Consider the Following Questions ...
- The subthalamic nucleus and the substantia nigra provide feedback to which of the other basal ganglia?
- What pathways and nuclei are involved in transmitting information from the neostriatum to the cerebral cortex?
- How does dopamine modulate transmission through the direct and indirect loop? Given this information, why would stimulation of the internal globus pallidus effectively relieve some of the symptoms seen with Parkinson's disease?
- Describe how information processes are segregated among the nuclei of the basal ganglia. How does this relate to differences in the origins of cortical afferents to the caudate, putamen and ventral striatum?
|
