The cerebellum and motor control Flashcards
What are some useless facts about the cerebellum?
Contains 1/2 of all neurons but 10% of brain weight
Projects to almost all UMNs
Jerky, erratic and poorly coordinated movements + voluntary movement seems stereotyped with damage;
Intention tremor, dysarthria and ataxia = names for symptoms
You can be born without one and be okay-ish
How does the cerebellum influence motor control?
No direct projections to LMNs - modulates UMN activity
What are the main inputs to the cerebellum?
Cortical - motor cortex (a copy of a motor command), somatosensory + visual areas of parietal cortex
Spinal - proprioceptive information about limb position/movement (muscle spindles/golgi tendon organs)
Vestibular - rotational and accelerational head movement (semicircular canals/otoliths in inner ear)
From the inferior and middle cerebellar peduncles
What is the primary output of the cerebellum?
To the thalamus via purkinje cell axons, inhibitory
Made up of the sum of the parallel fibre inputs multiplied by the weight (strength of parallel-purkinje synaptic connection)
From the superior cerebellar peduncle
What is the function of the cerebellum?
Knows what the motor command is - integrates with feedback about actual body position - projects back to cortex to adjust further movements
Also motor learning (+basal ganglia and cortical circuits)
Also non-motor tasks - sensory prediction (image changes when you move your hands); active sensing (knowing what object is in your pocket without looking); emotioal and cogntiive processing )verbal memory)
Disorders - autism, dyslexia, schizophrenia? (verbal prediction failures = auditory hallucinations)
Describe the cytoarchitecture of the cerebellar cortex
Three layers
Mossy fibre - input (white matter under the cortex) from many cortical and spinal inputs,
synapse onto granule cells
Granular layer - granule cells outnumber mossy fibres 50:1, project axons to top molecular layer where they split into two parallel fibres - these synapse with Purkinje cells
Purkinje layer - middle, sole output, each cell synapses with 150,000 parallel fibres in the top molecular layer; also receive 1 climbing fibre input
Climbing fibres - axons of inferior olive, wraps itself around Purkinje cell dendritic tree and forms 1000+ synapses with same input signal
Describe Purkinje cell output
Fire spontaneously = simple spikes at c.50 spikes/sec but with parallel fibre input can increase to >200/s
Complex spikes = unusual shape, low frequency, produced by climbing fibre input = reliable, when climbing fibre fires as does PC
What did Brindley (1964) suggest about the function of the cerebellum?
Assumptions: cerebellar damage does not usually cause paralysis; other parts of the brain issue motor commands but badly
Possibly to learn motor skills so when a simple or incomplete message from the cortex arrives, its sufficient to produce a proper execution - frees up space in cortex too
What is the core of the Marr-Albus theory of the cerebellum?
Error signals needed in order to process what you’ve done wrong =climbing fibre input
As climbing fibre input/error frequency is low frequency - doesnt really affect your movement = good
What is a synaptic weight and a decorrelation learning rule?
Synaptic weight - between parallel fibre and Purkinje cell - is a change according to the correlation between parallel-fibre signal and the error signal conveyed by he climbing fibre:
+ve correlation = reduce weight (as is contributing to the error); -ve correlation = increase weight
Learning stops when there is not longer a correlation between any parallel-fibre signal and climbing fibre signal = decorrelation learning rule
In this system correlation = causation
Version of the least mean squared rule (used in signal processing technology)
What evidence do we have for the decorrelation learning rule?
Stimulate parallel fibres at the top of the cortex - measure Purkinje cell output - then stimulate climbing fibre input
Purkinje cell activity found to reduce for a substantial time = long term depression (me when i realised this exam was not multiple choice)
What is some evidence that decorrelation learning work?
Vestibulo-ocular reflex (VOR) - inactivate the flocculus in the cerebellum - failures in VOR
Semicircular canal output to brainstem vestibulo- occulomotor nuclei is modulated by floccular output which receives a copy of the eye movement command from the brainstem nuclei via mossy fibre inputs + error signals from retinal slip from climbing fibres
We have made robots that can stabilise camera images in a robot head using the equivalent maths
