NE
Red, theory; black,
fact
Background about the cerebellum
The sensory inputs to
the cerebellum are the mossy fibers, which drive the granule cells of the
cerebellar cortex, whose axons are the parallel fibers. The spatial arrangement
of the parallel fibers suggests a bundle of raw spaghetti or the bristles of a
paint brush. These synapse on Purkinje cells at synapses that are probably
plastic and thus capable of storing information. The Purkinje cells are the
output cells of the cerebellar cortex. Thus, the synaptic inputs to these cells
are a kind of watershed at which stimulus data becomes response data. The
granule-cell axons are T-shaped: one arm of the T goes medial (toward the
midplane of the body) and the other arm goes lateral (the opposite). Both arms
are called parallel fibers. Parallel fibers are noteworthy for not being
myelinated; the progress of nerve impulses through them is steady and not by
jumps. The parallel fibers thus
resemble a tapped delay line, and Desmond and Moore seem to have [paywall] proposed this
in 1988.
The space-time
graph of one granule-cell impulse entering the parallel-fiber array is thus
V-shaped, and the omnibus graph is a lattice, or trellis, of intersecting
Vs.
The cerebellar cortex
is also innervated by climbing fibers, which are the axons of neurons in the
inferior olive of the brainstem. These carry motion error signals and play a
teacher role, teaching the Purkinje cells to avoid the error in future. Many
error signals over time install specifications for physical performances in the
cerebellar cortex. The inferior olivary neurons are all electrically connected
by gap junctions, which allows rhythmic waves of excitation to roll through the
entire structure. The climbing fibers only fire on the crests of these waves.
Thus, the spacetime view of the cortical activity features climbing fiber
impulses that cluster into diagonal bands. I am not sure what this adds up to,
but what would be cute?
A space-time theory
Cute would be to have
the climbing fiber diagonals parallel to half of the parallel-fiber diagonals
and partly coinciding with the half with the same slope. Two distinct motor
programs could therefore be stored in the same cortex depending on the
direction of travel of the olivary waves. This makes sense, because each action
you make has to be undone later, but not necessarily at the same speed or force.
The same region of cortex might therefore store an action and it’s recovery.
The delay-line theory, revisited
As the parallel-fiber
impulses roll along, they pass various Purkinje cells in order. If the response
of a given Purkinje cell to the parallel-fiber action potential is either to
fire or not fire one action potential, then the timing of delivery of
inhibition to the deep cerebellar neurons could be controlled very precisely by
the delay-line effect. (The Purkinje cells are inhibitory.) The output of the
cerebellum comes from relatively small structures called the deep cerebellar
nuclei, and there is a great convergence of Purkinje-cell axons on them, which
are individually connected by powerful multiple synapses. If the inhibition
serves to curtail a burst of action potentials triggered by a mossy-fiber
collateral, then the number of action potentials in the burst could be
accurately controlled. Therefore, the gain of a single-impulse reflex loop
passing through the deep cerebellar nucleus could be accurately controlled.
Accuracy in gains would plausibly be observed as accuracy in the rate, range,
and force of movements, thus explaining how the cerebellum contributes to the
control of movement. (Accuracy in the ranges of ballistic motions may depend on
the accuracy of a ratio of gains in the reflexes ending in agonist vs.
antagonist muscles.)
Control of the learning process
If a Purkinje cell
fires too soon, the burst in the deep cerebellar nucleus neuron will be curtailed
too soon, and the gain of the reflex loop will therefore be too low. The firing
of the Purkinje cell will also disinhibit a spot in the inferior olive due to inhibitory
feedback from the deep nucleus to the olive. I conjecture that if a movement
error is subsequently detected somewhere in the brain, this results in a burst
of synaptic release of some monoamine modulator into the inferior olive, which
potentiates the firing of any recently-disinhibited olivary cell. On the next
repetition of the faulty reflex, that olivary cell reliably fires, causing
long-term depression of concurrently active parallel fiber synapses. Thus, the
erroneous Purkinje cell firing is not repeated. However, if the firing of some
other Purkinje cell hits the sweet spot, this success is detected somewhere in
the brain and relayed via monoamine inputs to the cerebellar cortex where the signal
potentiates the recently-active parallel-fiber synapse responsible, making the
postsynaptic Purkinje cell more likely to fire in the same context in future.
Purkinje cell firings that are too late are of lesser concern, because their
effect on the deep nucleus neuron is censored by prior inhibition. Such post-optimum
firings occurring early in learning will be mistaken for the optimum and thus
consolidated, but these consolidations can be allowed to accumulate randomly
until the optimum is hit.
Photo by Robina Weermeijer on Unsplash
