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Physiology 3120 Lecture Notes - Wilder Penfield, Precentral Gyrus, Somatotopic Arrangement

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Tom Stavraky

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Human Physiology
Monday, November 16, 2009
“Neuro XI”
Motor Cortex
Techniques used to study function of particular brain region
1. Stimulation
Magnetic: rapidly changing magnetic fields can induce currents in cortex)
Epilepsy (natural)
2. Lesion
Stroke, degeneration (natural)
3. Recording
Single neurons
Evoked potentials & EEG (observe populations of neurons)
PET scan & fMRI (record differences in blood flow to brain regions
Motor cortex
Region from which low intensity stimulation elicits muscle contractions
Precentral gyrus (i.e. just anterior to central sulcus)
Usually evoked flexion movement at a joint; elicit jerk-like movement because we elicit large
numbers of neurons that don’t normally fire together
Movement is contralateral of stimulation
Somatotopic organization
Equivalent to somatotopic organization of somatosensory cortex
Larger representations for the fingers & the face; therefore, there is a bigger area from which we
can produce finger movement
Represents a distorted homunculus with larger representations for fingers & face
Hughlings Jackson found that epileptic motor seizures move from medial to lateral (i.e.
Jacksonian March)
Wilder Penfield confirmed this hypothesis in stimulation studies
Lesions to either (I) paralysis, or (II) weakness
Organization is plastic
Laid down at birth, but modifiable by experience
Experimental injury
Blocked small artery that supplied the hand area
Some monkeys were trained after damage, and some were not
The monkeys were trained or not; the monkeys that were trained to use their hand
regained some hand function, and hand representation had relocated to adjacent
area of cortex previously responsible for shoulders & elbows
When patients had to learn a keyboard sequence, there was an increase in finger
representation in the motor cortex over several weeks
3 output pathways
1. Corticoreticular tract
To reticular formation in pons & medulla
Phylogenetically old
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