Lecture 13 - cerebellum

Question 1

Properties of the cerebellum

Answer 1

• Own cortex = massive cortical area but only has 1 output layer
• Heavily folded = fine compared to cerebral cortex = folds called foliations = large SA
• Exerts influence on movement via influence on motor and pre-motor cortex
• Connects with brain and spinal chord
• Adds coordination, fine control, skill to basic movement patters

Question 2

Flattening the cortical sheet

Answer 2

• When you unfold the cerebellum the length reflects body mass
• Width may more reflect some cognitive properties of animal

Question 3

Cell numbers

Answer 3

• Around 102 billion cells
• Most cells are granule cells
• Purkinje cell inputs from parallel fibres and climbing fibres

Question 4

How cerebellum is wired up

Answer 4

• Granular, purkinje and molecular level
• Input mainly comes from mossy fibres (or cortex/spinal chord) = gives state of context
• Inputs connect to granule cells = projects axon up to molecular layer and axon splits and traverses a portion of cerebellar cortex
• Thousands of axons projecting up = all run in parallel = called parallel fibres
• Parallel fibres connect to only output of cerebellum = purkinje cells
• Purkinje cells = large cells with huge dendritic trees which push up into molecular cell layer
• As parallel fibres transverse the cortex, the purkinje cells sit in opposition to them so receive all signals from different cells
• Purkinje cells capture as much info from parallel fibres as they can with flat dendritic sheets
• Allows you to mix signals together to get contexts from lots of different parts of body
• Each purkinje cells controlled by another cell = climbing fibre
• Climbing fibre = strong, powerful, wraps itself round purkinje cell forming strong connection to it
• One climbing fibre for every purkinje cell
• Output goes to set of structures in deep cerebellum = deep cerebellar nuclei = connect off to various places

Question 5

2 pathways in cerebellum

Answer 5

• Direct = drives the output = excitatory
• Indirect = inhibitory (purkinje cells) = restricts the output
• Nuclei are all excitatory except the purkinje cells which is inhibitory
• Balance how strongly signal driven by altering strength of pathway through cerebellar cortex = changing strength of indirect loop

Question 6

Cerebellar damage

Answer 6

Hypermetria (overshoot)

• Neurologist holding pen and patient holding lid
• Asks patient to put lid on the pen
• With someone with damage they have loss of control
• Demonstrate tendency to miss target at end of movement
• Missing as appear to be overshooting = normally would correct movement as you move, overshoot too much in one direction, need to correct but then overshoot too much in the other direction

Intention tremor
-Shaking during an action

Ataxia:
-Loss of coordination and skill

• Nystagmus, balance, gait, speech
• Cerebellar affective disorder: executive, emotional, personality (children)

Question 7

Cerebellum and motor learning

Answer 7

• Inputs = sensory-motor cortical areas, parietal cortex spinal cord
• Outputs = thalamus mainly to motor and premotor cortex, red nucleus to spinal cord
• Inputs and outputs form loops e.g. if coming in from spinal chord go back out to spinal chord
• Cerebellum alters strength of output by tuning indirect pathway to change amount of excitation relative to inhibition = does this through climbing fibre input = when there is an error causes climbing fibre to signal = forces connections to strengthen (marr-albus model of learning)

Question 8

Marr-Albus model of leaning

Answer 8

• Synapse between granule cells (parallel fibres) and purkinje cells is plastic and can undergo long term depression
• The trigger for LTD is simultaneous activity of parallel fibres and climbing fibres (associative learning) velocity via mossy fibres, error signal via climbing fibres
• LTP reduced P-cell inhibition of cerebellar nuclei and di-inhibits the direct pathway

Question 9

4 examples of cerebellar learning

Answer 9

• Vestibular ocular reflex (VOR)
• Eye blink conditioning
• Skill learning
• Visuo-motor recalibration

Question 10

Cerebellar learning: Vestibular ocular reflex (VOR)

Answer 10

• Input: vestibular system signal of head motion
• Output: modulation of direct path to ocular motor neurons
• Have semi-circular canal input to the vestibular nucleus which projects to the motor neuron

Also have pathway going through cerebellum = controls how much excitation going to the ocular motor muscle

• When semi-circular canals activated by head movement we drive the vestibular nucleus, which drives the ocular motor neuron –> causes the eye to move
• Indirect pathway inhibiting vestibular nucleus
• Learns how much to inhibit based on error signals = retinal slips = error triggers climbing fibres causing LTD of fibres going to purkinje cell = purkinje cell becomes less excited, so produces less inhibition = means reflex gets stronger

Question 11

Cerebellar learning: Eye blink conditioning

Answer 11

• Classical conditioning
• Apply puff of air to eye = causes eye to blink
• If have tone or light that occurs with puff of air = produce CR of eyeblink to tone = when remove air still have response
• If you do this with someone with cerebellar damage = they will not show this conditioning response
• This is due to CS provides context (input) comes in on mossy fibres
• Error = air puff as not expected
• When air puff comes in triggers learning through climbing fibres, causes LTD of synapses onto purkinje cell = means those inputs are de-potentiated
• Means next time tone occurs, association between eye blink

Question 12

Cerebellar learning: Skill learning

Answer 12

• Assumption of LTD
• In some cases when have cerebellum damage counter-learn in this way
• Record from cells directly and see what happens during learning task
• Recordings from cerebellum where weight loaded onto a cone and idea is counterbalance the weight = when load occurs, wrist moves and have to adjust back
• Afterr training learning to counterbalance weight efficiently
• If take recordings from cerebellum, normally purkinje cells = normally fire at fairly low rate but when learning signal comes in, causes LTD to occur = produced complex spike = looks like series of spikes of decreasing magnitude
• -> Happens because so many chemical processes in cells to trigger learning that get something beyond simple AP
• -> Can see it in recording
• -> Allows you to correct movements through loop structure which tunes balance of excitatory drive with the inhibition drive which is imposed on it

Question 13

Cerebellar learning: • Visuo-motor recalibration

Answer 13

• Glasses with lense that’s thicker at one end = means when light enters it refracts so see image at an angle
• Brain will cope with new context and adjust behaviour to allow you to interact with environment