Case 7 - direct and indirect pathways Flashcards
what is the basal ganglia responsible for
fine tuning the initiation, control and termination of movements
what are the interrelated neurone populations in the basal ganglia
putamen
caudate
globus pallidus internis
globus pallidus externis
substantia nigra
sub thalamic nucleus
what does the term striatum refer to
the caudate and the putamen only
what happens upon initiation of voluntary movements in the direct pathway
the frontal lobes send an excitatory signal via the neurotransmitter glutamate to the striatum
what do the neurones in the striatum do
send an inhibitory signal to the globus pallidus interns and the substantia nigra pars reticulata
what is the result of this inhibitory signal
as a result, GPi and SNpr are no longer able to inhibit the thalamus which is usually their normal resting function
what now happens to the thalamus
now finds itself dis inhibited and is able to send a message back to the cortex saying it is ok to allow the desired movement to occur. the motor cortex then sends a message down the spinal cord causing the desired movement
where is dopamine secreted
the substantia nigra pars compacta
where do these neurones secrete dopamine onto
specific cells in the striatum
what does dopamine do to D1 receptors
interacts with D1 causing them to become more active. has a stimulatory effect via the D1 receptor
what is the overall effect of dopamine
it activates the striatum, which inhibits the internal segment of the globus pallidus. the GPi is then unable to inhibit the thalamus, which in turn allows the thalamus to stimulate the cortex. simply states, dopamine causes an increases propensity towards movement
what happens in the striatum in the indirect pathway
striatum sends an inhibitory signal via the neurotransmitter GABA to the external segment of the globes pallidus.
series of events in the indirect pathway
the external segment of the globus pallidus normally indirectly inhibits its internal counterpart
thus when the striatum inhibits the external segment, it is in effect, releasing the internal segment from inhibition.
now the internal segment of the globus pallidus is able to send its inhibitory signals to the thalamus. which causes thalamic neurones to stop sending excitatory signals to the motor cortex
the cortex is then unable to send an impulse down the spinal cord and the net result is a decrease in movement
how is the messaged relayed between the internal and external globus pallidus
relayed through another nucleus known as the sub thalamic nucleus (STN)
what is the STN usually inhibited by
the external segment of the globus pallidus
therefore when the striatum inhibits the external globus pallidus, it causes the cell in the STN to become more active
what does the STN do to the internal segment of the globus pallidus
in turn, is able to send an excitatory signal to the neurones in the internal segment of the globus pallidus. the cells in GPi then become more active, which means that they suppress the activity of the thalamus more robustly. the thalamus is then unable to send its normal excitatory messages to the motor cortex. the end result is decreased movement
what dopamine receptors are involved in the indirect pathway
binds to the D2 receptors on neurones in the striatum
what does this binding to D2 receptors cause
this causes striata neurones to decrease their inhibitory message to the external segment of the globus pallidus.
what happens to the globus pallidus after this
the external segment of the globus pallidus is then free to carry out its normal job and suppress the excitatory actions of the sub thalamic nucleus on its internal counterpart.
what does less excitation going to the internal globus pallidus translate to
less inhibition of the thalamus, and ultimately more excitation of the cortex. therefore, dopamine causes increased movement by activating the direct pathway and inhibiting the indirect pathway
what is the overall relationship between the indirect and direct pathways
the indirect basal ganglia pathway has the exact opposite effect of the direct pathway. the indirect pathway serves as a negative modulator of movement and the direct pathway serves as a positive modulator of movement
when the primary motor cortex is ready to stimulate muscle movement, where does it send signals to
the substantia nigra compacta
what does the substantia nigra compacta do
stimulates the direct pathway to cause muscle movement and at the same time it inhibits the indirect pathway
