Spinal cord architecture and descending motor pathways

Question 1

pyramidal tract

Answer 1

• upper motorneurons
• originate in M1, SMA, PMA, parietal cortex and ‘some’ other cortical area
• contains over 1 million axons of which most are myelinated (fast)
• primarily innervate facial, axial (abs and back muscles) and intrinsic musculature
• divided into corticobulbar and corticospinal tracts
• neuron cell body at brain travel all the way down to the spinal cord (long)

Question 2

corticobulbar tract

Answer 2

• upper motorneuron residing in cerebral cortex, projects into the upperbrain stem and innervates lower motorneuron (alpha motorneuron) recieves info from upper corticostructures
• extend from M1 (and other motor regions of cortex) to the ‘bulb’ (i.e. brainstem)
• control facial muscles and muscles of mastication
• rostral component innervates upper facial musculature (innervation is bilateral)
• caudal component innervates lower facial musculature (innervation is contralateral)

Question 3

lower motorneuron injury: Bell’s Palsy

Answer 3

deficit (i.e. inflammation) on the lower motorneurons

facial muscle weakness or paralysis

Question 4

upper motorneuron injury stroke

Answer 4

• selective lower facial muscle paralysis
• damage to upper motorneuron
• the upper facial muscles are innervated by both the right and left upper motorneurons
• the lower facial muscles are only innervated by the opposite sides of the upper motorneurons
  ex) left upper motorneuron controls the right lower facial muscles

Question 5

the ventral corticospinal tract

Answer 5

• extend from M1 (and other motor regions of cortex) to the spinal cord
• descend as “uncrossed” fibers in ventral columns of the spinal cord
• innervate axial (back, trunk) musculature
• few monosynaptic connections
• propriospinal neuron mediated, interneuron mediated

Question 6

ploy- vs monosynaptic connections

Answer 6

polysynaptic - proper spinal neuron synapses on 2 motor neurons on the layers of the spine, coordinated muscle control (power grip)

monosynaptic - fine motor control (fine movement)

Question 7

lateral corticospinal tract

Answer 7

• originate largely from M1
• 70-80% of fibers decussate at the pyramids (cross over to the other side)
• innervate intrinsic (e.g. hand) musculature
• monosynaptic connections (directly connect to alpha motorneuron)

Question 8

lateral corticospinal tract (different animals)

Answer 8

• amplitude of cortico-motorneural EPSPs (more potential for action potentials)
• # of fibers per tract, amount of individual neurons that connect directly to alpha motorneurons (monosynaptic)
• rats have no monosynaptic connections (propriospinal mediated)
• humans have most monosynaptic connections and most excitatory postsynaptic potential

Question 9

spinal cord anatomy and organization

Answer 9

white matter (axons, outer)

grey matter (cell bodies, inner)

ventral median fissure

dorsal median sulcus

central canal (cerebrospinal fluid runs through the canal)

dorsal column - gracile and cuneate (fine touch and proprioception)

ventral corticospinal tract (axial motorneurons)

lateral corticospinal tract (limb motorneurons)

lateral spinalthalamic tract (pain and temp)

anterior spinalthalamic tract (crude touch, not able to locate)

anterior white commissure (where axons cross)

Question 10

corticospinal tract

Answer 10

axial & limb motor

• begins in pre-central gyrus (primary motor cortex, M1)
• 2 neurons, 1 innervates axial muscles, 1 innervates limb muscles
• path to the brainstem
• 75-90% deccusate at medulla (cross, control limbs), the rest decussate at anterior white commissure in spinal cord
• after brainstem, the neurons run through the lateral corticospinal tract and anterior corticospinal tract
• neurons then synapse with anterior horn cells (lower motorneurons) in the grey matter
• lower motorneurons innervate limb and axial muscles