Case 7 - direct and indirect pathways

Question 1

what is the basal ganglia responsible for

Answer 1

fine tuning the initiation, control and termination of movements

Question 2

what are the interrelated neurone populations in the basal ganglia

Answer 2

putamen
caudate
globus pallidus internis
globus pallidus externis
substantia nigra
sub thalamic nucleus

Question 3

what does the term striatum refer to

Answer 3

the caudate and the putamen only

Question 4

what happens upon initiation of voluntary movements in the direct pathway

Answer 4

the frontal lobes send an excitatory signal via the neurotransmitter glutamate to the striatum

Question 5

what do the neurones in the striatum do

Answer 5

send an inhibitory signal to the globus pallidus interns and the substantia nigra pars reticulata

Question 6

what is the result of this inhibitory signal

Answer 6

as a result, GPi and SNpr are no longer able to inhibit the thalamus which is usually their normal resting function

Question 7

what now happens to the thalamus

Answer 7

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

Question 8

where is dopamine secreted

Answer 8

the substantia nigra pars compacta

Question 9

where do these neurones secrete dopamine onto

Answer 9

specific cells in the striatum

Question 10

what does dopamine do to D1 receptors

Answer 10

interacts with D1 causing them to become more active. has a stimulatory effect via the D1 receptor

Question 11

what is the overall effect of dopamine

Answer 11

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

Question 12

what happens in the striatum in the indirect pathway

Answer 12

striatum sends an inhibitory signal via the neurotransmitter GABA to the external segment of the globes pallidus.

Question 13

series of events in the indirect pathway

Answer 13

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

Question 14

how is the messaged relayed between the internal and external globus pallidus

Answer 14

relayed through another nucleus known as the sub thalamic nucleus (STN)

Question 15

what is the STN usually inhibited by

Answer 15

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

Question 16

what does the STN do to the internal segment of the globus pallidus

Answer 16

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

Question 17

what dopamine receptors are involved in the indirect pathway

Answer 17

binds to the D2 receptors on neurones in the striatum

Question 18

what does this binding to D2 receptors cause

Answer 18

this causes striata neurones to decrease their inhibitory message to the external segment of the globus pallidus.

Question 19

what happens to the globus pallidus after this

Answer 19

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.

Question 20

what does less excitation going to the internal globus pallidus translate to

Answer 20

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

Question 21

what is the overall relationship between the indirect and direct pathways

Answer 21

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

Question 22

when the primary motor cortex is ready to stimulate muscle movement, where does it send signals to

Answer 22

the substantia nigra compacta

Question 23

what does the substantia nigra compacta do

Answer 23

stimulates the direct pathway to cause muscle movement and at the same time it inhibits the indirect pathway