L15 Motor Cortex, Upper and Lower Motor Neuron Lesions Flashcards
Cortex contains two somatotopic maps on either side of the central gyrus. Where are each located?
Cortex contains two somatotopic maps on either side of the central gyrus:
- Motor Map in the PRECENTRAL GYRUS
- Sensory Map in the POSTCENTRAL GYRUS
__________________ are known as UPPER Motor Neurons
__________________ are known as LOWER Motor Neurons
Betz Cells are known as UPPER Motor Neurons
- Largest Neurons in the brain!! (Only Purkinje cells in cerebellum are comparable)
- Axons project to the spinal cord and cranial nerve nuclei and control peripheral motor neurons
Spinal Motor Neurons are known as LOWER Motor Neurons
Corticospinal Tracts?
- Axons project from CORTEX through the white matter ____________
- Axons gather and pass between ____________ and __________________NUCLEI via the _____________________
- Pass through the __________________________, past the PONS to the ________________________________________
- Majority of fibers decussate at the _________________
- ALL Fibers descend the cord as ________ and __________ CORTICOSPINAL TRACTS
Corticospinal Tracts?
- Axons project from CORTEX through the white matter CORONA RADIATA
- Axons gather and pass between THALAMUS and BASAL GANGLIA NUCLEI via the INTERNAL CAPSULE
- Pass through the CEREBRAL PEDUNCLE, past the PONS to the PYRAMIDS of the MEDULLA
- Majority of fibers decussate at the PYRAMIDS
- ALL Fibers descend the cord as ANTERIOR and LATERAL CORTICOSPINAL TRACTS
Lateral (_____________) Corticospinal Tract
- ____________ % of neurons
- Decussation?
- Muscle Control?
- Arrangement?
Lateral (PYRAMIDAL) Corticospinal Tract
- 80-90% of neurons
- Decussates at the pyramids
- Controls DISTAL limb muscles
- Somatotopically Arranged (Cerivlca on INSIDE, Sacral on OUTSIDE)
Anterior (_____________) Corticospinal Tract
- ____________ % of neurons
- Decussation?
- Muscle Control?
Anterior (VENTRAL) Corticospinal Tract
- 10-20% of neurons
- Does NOT Decussate
- Controls PROXIMAL, AXIAL Muscles
Damage to either upper or lower motor neurons will result in disrupted motor control, but the effect is very different for upper and lower motor neuron lesions. Describe the differences in the way each functions normally.
Lower Motor Neurons: Direct control of contraction of muscle fibers in a one-to-one manner (ie. Motor neuron action potential leads to muscle action potential and contraction in the muscle unit it innervates)
Upper Motor Neurons: Recruitment of LOWER Motor Neurons and control of their activity to produce fine, coordinated behavior control.
- Can be DIRECT or INDIRECT (via spinal cord interneurons)
- Upper motor neurons can exert INHIBITION of spinal reflexes (ie. Spindle Reflexes)
What do Lower Motor Lesions result from and what can they lead to?
Lower Motor Lesions result from:
- Damage to motor neurons
- Damage to myelin on motor neurons (ie. Guillain-Barré Syndrome)
Lower Motor Lesions results in:
- Muscle Weakness: partial (PARESIS) or complete (PARALYSIS)
- Fasciculations (Spontaneous muscle twitching): visible or detected ad fibrillations on EMG. Sign of muscle fiber death
- REDUCTION in muscle tone
- DECREASED deep tendon reflexes (Spindle Reflexes)
What do Upper Motor Lesions result from and what can they lead to?
Upper Motor Lesions result from:
- Damage to ANY part of the upper motor neuron path from the cortex (Stroke) to the spinal cord
- Abnormal development of upper motor neurons (Cerebral Palsy)
Upper Motor Lesions results in:
- Fine Control of LIMBS is most affected (Trunk muscles have multiple pathways controlling them)
- Hemiparesis or Hemiplegia
- NO Fasciculations
- General INCREASE in Muscle Tone (LOST INHIBITION. Still have input from spindles => driving muscle tone up)
- INCREASED Deep Tendon Reflexes (In absence of cortical input, spinal afferents have dominant control over lower motor neurons => INCREASED Spindle Reflexes)
Sign of Upper Motor Lesion?
Facial nerve motor nucleus receives upper motor neuron input from the cortex via the ___________________ (as opposed to the pyramidal tract). Lesions to this tract are therefore UPPER MOTOR LESION:
- LOWER Motor neurons of ________________receive inputs from BOTH sides of cortex
- LOWER Motor neurons in ________________ ONLY receive inputs from contralateral side of cortex
Implications of this for a Unilateral Upper Motor Lesion?
Facial nerve motor nucleus receives upper motor neuron input from the cortex via the Corticobulbar Tract (as opposed to the pyramidal tract). Lesions to the corticobulbar tract are therefore UPPER MOTOR LESION.
- LOWER Motor neurons of DORSAL facial motor nucleus (UPPER face) receive inputs from BOTH sides of cortex
- LOWER Motor neurons in VENTRAL facial motor nucleus (LOWER face) ONLY receive inputs from contralateral side of cortex
=> UPPER MOTOR LESION on one side will result in Paralysis in the LOWER face of the OPPOSITE side, but NOT the UPPER.
