Anatomy and Cell Biology 3319 Lecture : Anatomy Notes
20 Feb 2012
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Elements of descending motor systems: a motor unit comprises a group of extrafusal muscle fibres and the alpha motor neuron that innervates them. A lower motor neuron lesion causes flaccid paralysis, loss of stretch reflex, and atrophy. Major descending pathways are the corticobulbar, corticospinal, reticulospinal, and vestibulospinal (postural). An upper motor neuron lesion causes spastic paralysis, exaggerated stretch reflexes, abnormal babinkski reflex, usually without atrophy. The connections of the cerebellum are such that each cerebellar hemisphere is concerned with ipsilateral muscles. The descending spinal tracts deliver motor (output) instructions from the brain to the spinal cord. Extrafusal muscle fibres are innervated by lower motor neuron that exits through the ventral horn. The lower motor neuron leaves through the ventral horn and goes to the intrafusal fibres of muscle spindle located within the extrafusal muscle fibres. This provides feedback back to the lower motor neuron.
