Special somatic afferent pathways

Question 1

What nerve and foramen starts the visual pathway?

Answer 1

• Optic nerve
• Optic foramen

Question 2

How does decussation happen in the visual pathway?

Answer 2

• Happens at the chiasm- partial decussation of axons from each retina
• Proportion of axons crossing to the opposite side is inversely correlated with the degree of binocular vision
• Small binocular vision then more fibers decussate

Question 3

What happens after the optic chiasm in the visual pathway?

Answer 3

• Continue on as optic tracts that arch over the lateral surface of the thalamus
• Axons terminate within the LGN of the thalamus
• Axons of second order neurons project via optic radiation within internal capsule to visual cortex (in occipital lobe)

Question 4

What if the optic fibers do not synapse at the nuclei?

Answer 4

• Do not synapse at LGN then project to mes nuc
• Pre-tectal region - between thalamus and midbrain - to oculomotor nerve in midbrain which would do PLR
• Rostral colli - visual integration center for gaze - and axons from here end on LMN in cervical SC and constitute tectospinal tract

Question 5

Where do vestibular axons arise from?

Answer 5

• From the vesetibular sensory apparatus in the inner ear traveling within the common vestibulocochlear nerve that enters brain at the level of the trapezoid body

Question 6

What happens to the neurons of the vestibular apparatus?

Answer 6

• Axons terminate on or detach collateral branches to neurons of vestibular nuclei
• Some axons enter caudal peduncle to synapse within the vestibular portion of the cerebellum

Question 7

What happens after the axons terminate on the vest nuclei in the vest apparatus?

Answer 7

• Second order axons arise from neurons in vest nuclei are divided between those that pass to the cerebellum and rest travel to the spinal cord via vestibulospinal tract and to brainstem via MLF

Question 8

What happens in the SC of the vestibular apparatus?

Answer 8

• Vestibulospinal axons project via a series of interneurons onto LMN of the ipsi ventral gray matter

Question 9

What happens in the BS of the vestibular apparatus?

Answer 9

• Follow MLF and reticular formation to synapse on the nuclei of cranial nerves supplying external ocular muscles

Question 10

What happens to the vest axons that lead to conscious perception?

Answer 10

• Axons travel from vest nuc via lateral lem and thalamic nuc to region of cerebral cortex of temporal lobe

Question 11

What is the auditory pathway?

Answer 11

• Axons of cochlear component of the vestibulocochlear nerve synapse in dorsal and ventral cochlear nuc located on dorsal surface of the BS
• Second order neurons from ventral coch nuc then proceed to synapse in ipsi or contra nuc of trapezoid body, those from the dorsal coch nuc will join ipsi or contra to lat lem and follow cours eof ventral coch nuc
• Axons of third stage neurons carried with in lat lem and a proportion of them with synpase within the nuc of the lat lem and a second group with travel to and synapse with caudal colli
• Third set will travel farther rostrally to synapse at MGN of thalamus and send axons to auditory cortex - this is for conscious perception