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Question: With reference to the role of the basal ganglia in the control of motor functions, describe the pathology and symptoms of huntingtins disease.
Question: With reference to the role of the basal ganglia in the control of motor functions,
describe the pathology and symptoms of huntingtins disease.
The basal ganglia is a collection of a group of subcortical nuclei that reside at the base of the
forebrain. These structures include the striatum (caudate and putamen), Globus pallidus
(internal and external), substantia nigra (pars compacta and reticularis) and subthalamic
nucleus. These structures all form a circuit with each and connect with the cerebral cortex.
The basal ganglia is responsible for performing a variety of functions including voluntary
motor control, procedural learning, emotion and habits. Each component of the basal
ganglia is responsible for either projecting excitatory glutamate neurotransmitter or
inhibitory GABA neurotransmitter. However, the substantia Nigra pars compacta is
responsible for the release of the neurotransmitter dopamine.
The primary function of the basal ganglia is to regulate and control motor and premotor
areas of the brain to ensure that voluntary movements can be performed correctly. The
basal ganglia network has two distinct pathways, the direct and indirect circuit. The direct
pathway is responsible for the allowance of voluntary movement. Conversely, the indirect
pathway functions to suppress unwanted movement. The indirect pathway utilises several
additional structures of the basal ganglia and follows a different circuit to carry out its
function.
The direct pathway involves an initial excitatory (glutamate) input to the striatum (caudate
and putamen) from the cerebral cortex. This causes the inhibitory striatum to increase its
functionality and therefore, the striatum releases more GABA to inhibit the globus pallidus
internal and the substantia nigra pars reticularis (SNpr). The globus pallidus internal and
substantia nigra par reticularis are both also inhibitory structures and hence, the
subsequent stimulation of the direct pathway causes the globus pallidus internal and SNpr
to be inhibited more. Thus, less inhibitory GABA is projected to the excitatory thalamus
which in turn, increases the excitatory projection to the motor cortex from the thalamus.
The indirect pathway on the other hand utilises several additional structures of the basal
ganglia and takes a different route to the thalamus and works to inhibit motor control. Like
the direct pathway, the indirect circuit requires an excitatory stimulation to the striatum
from the cerebral cortex. However, the straitum then sends inhibitory projections to the
globus pallidus external. This then sends fewer inhibitory signals to the subthalamic nucleus
which then sends excitatory signals to both inhibitory structures, substantia nigra pars
reticularis and globus pallidus internal. Both these nuclei then inhibit the thalamus thereby
decreacing the excitatory signals to the motor cortex via the thalamus.
The balance of the activity between the direct and indirect pathway is modulated by the
neurotransmitter dopamine. Dopamine is produce by the substatia nigra pars compacta.
The dopamine producing neurones of the substantia nigra pars compacta project to the cells
in the striatum. The cells in the direct pathway express D1 receptors. When dopamine binds
to these receptors, the neurone become excited and further help the direct pathway.
Conversely, the cells of the indirect pathway have D2 receptors. When dopamine bind to
these receptors, it inhibits the neurones and hence, suppresses motor movement.
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