Question

1) The net effect of the direct pathway on the thalamus when there is cortical input is _. (2 points) 2) Describe the basal ganglia pathway that must be utilized to prevent an antagonistic muscle from interfering with an agonist muscle. Describe each step in detail using complete sentences. Do not use a drawing or diagram. (4 points) 3) Describe how the neurons of the SNPC influence the direct pathway and the indirect pathway. (4 points)

Answer 1

1) Net effect of direct pathway on the thalamus:

• Activation of ventral lateral nucleus of thalamus and transmission of activating signals to motor cortex.
• Activation of motor activity in cortex which causes muscle contractions.

2) Indirect Pathway of Basal Ganglia: This pathway prevents unwanted muscle contractions from interfering with voluntary movements.

• Signals reaching the cerebral cortex which causes activation of GABA neurons in the striatum.
• Projections coming from striatum reach the Globus Pallidus external and inhibit the activity of neurons there.
• This prevents Globus Pallidus External from inhibiting neurons which are present in Subthalamic nuclei.
• Subthalamic neurons are activated by the signals coming from cerebral cortex and in turn Subthalamic nucleus stimulates GABA neurons present in Globus Pallidus Internal and Substantia nigra pars reticulata.
• These neurons project to Thalamus and inhibit thalamic neurons which eventually inhibits movement which prevents unwanted movements sfrom interfering.

3) Effect of neurons from Substantia Nigra Pars Compacta (SNPC) on indirect pathway:

• Neurons from SNPC reach Striatum via nigrostriatal pathway and can modulate the activity of this pathway by release of Dopaminea.
• One of its effect is to inhibit activity of Inhibitory pathway which facilitates movement as seen in Parkinson's disease.

Effect of SNPC on Direct pathway:

• It activates direct pathway via D1 receptors which facilitates motor movements.