Somatic motor pathways

Question 1

What are LMN?

Answer 1

• Neurons that have cell bodies in CNS and axons that leave nervous system to synapse on muscle fibers in the periphery

Question 2

What are the axons and cell bodies of the LMN?

Answer 2

• Axons: form the motor component of the peripheral nerves
• Cell bodies: innverate the skeletal muscle of the neck, trunk and limbs, located in the ventral portion of the gray matter of the SC

Question 3

Where are the cell bodies of LMN that innervate the head?

Answer 3

• Within somatic motor nuclei in the brainstem and axons travel in cranial nerves

Question 4

Where are the cell bodies and axons of the UMN located?

Answer 4

• Cell bodies located in the brain (motor area of neopallium)
• Axons travel through the BS and SC to synapse on LMN
• *Axons do not leave the CNS and thus fo no project directly to muscle fibers and exert control by excitation or inhibition of LMN

Question 5

What is the basic structure of the pyramidal system?

Answer 5

• Axons converge as they exit the tel and form an important portion fo the internal capsule
• Continue over lateral aspect of thalamus to enter the crus cerebri on the ventral surface of the brain
• Traverse the ventral portion of the pons and reappear on surface of pyramids in medulla

Question 6

What are three fibers of the pyramidal system?

Answer 6

1. Corticospinal: continue through medulla into SC
2. Corticobulbar: peel off at appropriate levels of the BS to reach LMN in motor nuclei of contra cranial nerves
3. Corticopontine: pass to various nuclei in pons

Question 7

What are the components of the corticospinal tract:

Answer 7

• Certain fibers decussate in the medulla
• And some continue directly into the cord and decussate closer to their terminations
• Fibers in a medullary decussation form a lateral tract in the lat fun.
• Those that continue uncrossed constitute a ventral tract in ven fun.
• Fibers of both tracts project to ventral gray matter of the side contralat to fiber’s origin cortex

Question 8

What is the route of the corticopontine fibers?

Answer 8

• Synapse on neurons in the neuclei of the ventral pons
• Axons of second order neurons decussate and pass within the transverse fibers of the pons to enter the cerebellum through the middle peduncle
• Synapses with deep cerebellar nuc
• Axons then travel back to the cerbral cortex via ventral thalamic nuclei
• This is a feedback system

Question 9

What structures are involved in the extrapyramidal system?

Answer 9

• Basal nuclei- caudate nuc, subthalamic nuc, and substantia nigra
• some output reaches the res nuc of the mesencep, reticular formation and olive in medulla
• Only red nuc and reticular formation contain neurons that project directly on the LMN

Question 10

What is the rubrospinal tract?

Answer 10

• Axons from neurons in the rec nuc in the midbrain decussate immediately before descending through the ventral lat par of the medulla to constitute this tract
• Reaches the most caudal part of the cord, projecting en route on the LMN in the ventral gray

Question 11

What is the reticulospinal system?

Answer 11

• Divided between well defined dorsal and ventral tract location within the lat fun
• And a third (pontine reticulospinal) tract in the ventral fun.