Function and Dysfunction of the Cerebellum and Basal Ganglia

Question 1

Understand the difference between a feedback and feedforward controller, and the value of prediction in motor control

Answer 1

The feedback one purely REACTS to a change, it has no though process of predicting what might happen (like a fish that reacts to a shark, it didn’t predict it, it just knows to swim as fast and far away as possible).

The feedforward controller actually makes contingency plans and plans ahead! Kind of like a college savings plan for parents, if the kid is awesome they will have all the money right there for college, but if the kid really sucks in the future, that money is available for a great lawyer to keep the kid out of jail!

Question 2

Understand the role of the parietal cortex in matching joint position sense with visual coordinates for a reaching motion

Answer 2

• Stream of info for “where” advances dorsally from occipital cortex into parietal lobe
• Parietal association cortex: between the visual “where pathway and joint position, it senses info from somatosensory cortex.
• Then, it integrates info about the target (limb position) and somatosensory info about limb position.
• Integrated info is fed to motor corticies in frontal lobe to guide movement of limb to target.

Question 3

Describe how the cerebellum can adapt cortical networks to changes in the relationship between joint position sense and visual coordinates

Answer 3

Basically this is how to fix mismatches between visual coordinates of a target and the joint angles required to get to the effector that happened differently than you expected

Cerebellar pathways connecting parietal and premotor corticies effect a re-calibration so that if the visual coordinations (like adding a prism) and a given set of joint angles are different, the brain can adapt for this. ONLY PERIOD OF ADAPTATION DEPENDS ON CEREBELLUM!!!

•Cerebro-cerebellum’s mossy fibers arise from contralateral pontine grey. These are next innervated by cortico-pontine fibers descending through internal capsule. Parietal cortex contributes these cortico-pontine fibers that synapse on ponto-cerebellar neurons.
Mossy fiber input = reflection of present state of parietal cortical mapping between visual + proprioceptive signals. Form synapses on granule cells. Granule cells form synapses on purkinje cells. Purkinje cells output to dentate nucleus to thalamus to motor and premotor corticies

Question 4

Describe how the inferior olivary nucleus might contribute to changes in the relationship between joint position sense and visual coordinates

Answer 4

Sends climbing fibers to purkinje cells, and simultaneous discharge of olivary climbing fibers and parallel fibers leads to changes in synpatic strengths of parallel-purkinje synapses, which can reconfigure the network.

GABAergic neurons are the major input to ION from deep cerebellar nuclei. Here ION receives proprioceptive and visual feedback. Acts as a COMPARATOR and compares the expected state with the observed state. If feedback is different than the expected state, a signal is generated by ION. The error signal conveyed by climbing fibers to cerebellar cortex, where it initiates complex spikes in purkinje cells. The parallel fiber synpses on surkinje cells

Question 5

Understand the role that dopaminergic neurons in the VTA/SNc play in conveying reward prediction errors to the striatum

Answer 5

Aka: How to associated a symbol with a reward and get a spike in dopamine!

• The mu-opioid receptor is throughout the striatum (of basal ganglia) except for in certain patches. These patches are the striosome with medium spiny GABAergic neurons who project through direct pathways to the SNc/VTA dopaminergic neurons.
• Convey current predictions about rewards – reflect present state of cortico-striatal network to these neurons
• Additional inputs to the SNc/VTA convey info about actual reward experience.
• ACTS AS A COMPARATOR – activated by mismatch between observed and expected rewards

Question 6

Describe similarities between the role the ION plays in modifying cerebellar circuitry and the role that VTA/SNc neurons play in modifying corticostriatal circuitry

Answer 6

Cerebellum:
•error signal = ION neurons
•Return projection: comes to ION neurons directly from deep cerebellar nuclei or indirectly through red nucleus. Carries a prediction signal so the ION neurons can compute error between prediction and inputs that actually occur.
•Comparator
•What conveys the prediction: GABAergic inhibitory neurons through deep cerebellar nuclei to IO.
•Critic of the parallel-purkinje network

Basal Ganglia
•Error signal = dopaminergic neurons
•Return Projection: comes to DA neurons from small groups of striatal medium spiny neurons in striosomes. Carries a prediction signal so the DA neurons can compute the error between the prediction and inputs that actually occur.
•Comparator
•What conveys the prediction: GABAergic inhibitory neurons through the striosome to SNc.
•Critic of cortico-striatal network

Question 7

What is a feedback controller?

Answer 7

Feedback controller: simple system in which the environment must first change and then the controller can elicit a change to compensate. Slow and often will overcompensate when you attempt to speed it up.

Ex:
•A thermostat that can turn on a heater or a cooler to keep the room at a steady temperature.
•If there is a decrease in temperature, it turns on the heater. However, the time and sense delay cause it to overshoot and need to turn on the cooler. This cycle continues…
•Problem: this can not consider alternative variables or predict probable outcomes

Question 8

What is a feedforward controller?

Answer 8

Feedforward controller: using sensory data to calculate a state that is coming at some future time, the predicted state. Senses variables that allow it to make contingency plans.

Ex:
•A thermostat that can turn on a heater or a cooler to keep the room at a steady temperature.
•This smart controller can sense variables like the window opening, how much it opens, the outside temp, and PREDICT the change in room temp.
•The system activates the heater pre-emptively to counteract the temp drop and maintain a stable target value with minimal energy waste.
•The controller has a model of the system to calculate how changes in one input variable (window opening) can affect the output (room temperature).