Anatomy Notes

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Department
Anatomy and Cell Biology
Course
Anatomy and Cell Biology 3319
Professor
Peter Merrifield
Semester
Fall

Description
Anatomy NotesOct 1811 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 bylower motor neuron that exits through the ventral horn Thelower 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 thelower motor neuron Corticospinal tracts are also called pyramidal tracts and this tract controls the precision and speed of skilled movements hands Pyramidal cells are large neurons found in the primary motor area and the axons of these cells form the pyramidal tracts The axons descend from the primary motor cortex and premotor cortex down the corona down the internal capsule down the cerebral peduncles and down the pyramids The lateral corticospinal tract decussates in the pyramids of the medulla and then continues on to the spinal cord lumbar spinal cord where it synapses in the ventral horn directly or indirectly onandlower motor neurons This results in motor impulses going from the cerebrum to spinal cord motor neurons which activate skeletal muscles on the contralateral side upper and lower limbs A lesion that occurs above where the tract decussates will cause contralateral paralysis A lesion after the crossing will affect the same side that the lesion is on ipsilateral The ventral or anterior corticospinal tract doesn
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