Question: Why do lesions of the primary motor cortex result in flaccid paralysis? I thought upper motor neurone (UMN) lesions resulted in spastic paralysis, is it an exception?
The observation that lesions of the primary motor cortex cause flaccid paralysis is based on 'pure lesional' studies in animals (dogs and primates) and date back to Penfield in the 1950s. It is very rare to see pure motor cortex lesion in clinical medicine; motor lesions usually extend beyond the motor cortex to involve adjacent areas, connecting fibre tracts and deep grey matter structures such as the basal ganglia and thalamus. It is these other systems that contribute to increased tone.
The observation that lesions of the primary motor cortex cause flaccid paralysis is based on 'pure lesional' studies in animals (dogs and primates) and date back to Penfield in the 1950s. It is very rare to see pure motor cortex lesion in clinical medicine; motor lesions usually extend beyond the motor cortex to involve adjacent areas, connecting fibre tracts and deep grey matter structures such as the basal ganglia and thalamus. It is these other systems that contribute to increased tone.
Please note that the increased tone compatible with an upper motor neurone lesion typically evolves over days to weeks after a new lesion. In the acute stage, i.e. within the first few hours to a few days, after a UMN lesion tone may be reduced or flaccid. This is particularly prominent with acute spinal cord lesions and in this context is referred to as spinal shock.
R. Chris Miall. Cortical Motor Control. Neuroscience in the 21st Century pp 1187-1208.
The localization of motor functions within the cerebral cortex has a long history, dating back to the demonstration in 1870 by Fritsch and Hitzig that weak electrical stimulation of the cortex of the dog could evoke movements of the contralateral limbs. At about the same time, Hughlings Jackson made careful clinical observations of the convolutions of epileptic patients, including his own wife. Epileptics often have a spasm of the muscles that may spread sequentially from, for example, the fingers up the arm to the shoulder. Hughlings Jackson realized that the March of the seizures along the limb (Jacksonian March) might reflect some physiological event sweeping across a topographical map of the body within the brain. These ideas were later confirmed by experiments, initially on dogs and monkeys, in which the cortex was stimulated with brief electric shocks. In 1906, Sherrington showed that movements could be evoked most easily from an area now known as the primary motor cortex, and this finding was extended by Penfield in the 1950s, who demonstrated during brain surgery on epileptic patients that the body was topographically mapped on the surface of the human motor cortex (Fig. 37.1). More recently, the unknown events that Hughlings Jackson predicted were shown to be waves of neuronal activation which spread across this topographical map.
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R. Chris Miall. Cortical Motor Control. Neuroscience in the 21st Century pp 1187-1208.
The localization of motor functions within the cerebral cortex has a long history, dating back to the demonstration in 1870 by Fritsch and Hitzig that weak electrical stimulation of the cortex of the dog could evoke movements of the contralateral limbs. At about the same time, Hughlings Jackson made careful clinical observations of the convolutions of epileptic patients, including his own wife. Epileptics often have a spasm of the muscles that may spread sequentially from, for example, the fingers up the arm to the shoulder. Hughlings Jackson realized that the March of the seizures along the limb (Jacksonian March) might reflect some physiological event sweeping across a topographical map of the body within the brain. These ideas were later confirmed by experiments, initially on dogs and monkeys, in which the cortex was stimulated with brief electric shocks. In 1906, Sherrington showed that movements could be evoked most easily from an area now known as the primary motor cortex, and this finding was extended by Penfield in the 1950s, who demonstrated during brain surgery on epileptic patients that the body was topographically mapped on the surface of the human motor cortex (Fig. 37.1). More recently, the unknown events that Hughlings Jackson predicted were shown to be waves of neuronal activation which spread across this topographical map.
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