Neurogenic placodes are bilateral patches of thickened neurogenic surface ectoderm that give rise to neurons and other structures in the sensory nervous system. In the cranium, this includes all peripheral ‘special sensory’ neurons (CN I, VIII), some somatosensory neurons in the trigeminal ganglion (CN V) and all viscerosensory neurons (inferior ganglia of CN VII, IX and X). Besides the olfactory receptor neuron cell bodies that remain in the olfactory epithelium and the lens placode (which forms the eye lens), all other placode-derived neurons migrate and coalesce with neural crest-derived glia to form a ganglion.

Special sensory:

CN I: The olfactory placode forms the entire olfactory epithelium, including the olfactory receptor neurons whose axons make up CN I.

CN VIII: The otic placode forms the entire inner ear, including neurons of the spiral (cochlear) and vestibular ganglia. Axons of these neurons make up CN VIII and transmit hearing and balance information from inner ear hair cells to the cochlear and vestibular nuclei, respectively.

Somatosensory

CN V: Thetrigeminal placode forms a minority of the somatosensory neurons in the CN V trigeminal ganglion – the rest (like all other peripheral somatosensory neurons) come from the neural crest. The neuronal axons transmit somatosensory information from the face and jaws to the trigeminal sensory nucleus.

Viscerosensory (+ taste)

CN VII: The geniculate placode forms the viscerosensory neurons in the geniculate ganglion (inferior ganglion of CN VII). Their axons transmit information from taste buds in the anterior two-thirds of the tongue to the nucleus of the solitary tract.

CN IX: The petrosal placode forms the viscerosensory neurons of the petrosal (petrous) ganglion (inferior ganglion of IX). Their axons transmit information to the nucleus of the solitary tract from:

• taste buds in the posterior third of the tongue
• general visceral information from the pharynx
• chemoreceptors and mechanoreceptors in the carotid body and carotid sinus.
  

CN X: The nodose placode forms the viscerosensory neurons of the nodose ganglion (inferior ganglion of CN X). Their axons transmit information to the nucleus of the solitary tract from:

• taste buds in the epiglottic vallecula
• general visceral information from the pharynx, larynx, oesophagus, thoracic and abdominal viscera
• chemoreceptors and mechanoreceptors in the aortic bodies and aortic arch.

*Neurons in the superior ganglions of CN VII, IX and X are neural crest-derived and transmit somatosensory information to the trigeminal sensory nucleus from:

• VII: part of the external ear
• IX: the posterior third of the tongue, the pharynx, auditory tube and middle ear
• X: the pharynx, larynx, tympanic membrane and external auditory meatus