Evolutionary Pathways of the Thalamus

Divisions of the Dorsal Thalamus: Collothalamic and Lemnothalamic

Archaically, the dorsal thalamus can be divided into two divisions:

• The collothalamus [caudal division] receiving afferent input from colliculi of the midbrain tectum and projecting efferent output ipsilaterally to the striatum mainly, and more sparsely to the pallium,


• The lemnothalamus [rostral division] directly receiving retinal afferent input via the lemniscal pathways and projecting efferent output bilaterally to the pallium mainly, and more sparsely to the striatum.

In amniotes, the dorsal thalamus is modified such that:

• The collothalamic nuclei now serve as a relay for midbrain visual, auditory, and somatosensory-multisensory efferent paths on their way to the ipsilateral striatum and pallium.
  
  In Amniotes, collothalamic nuclei consist of:
  
  
  • Reptiles - Nucleus rotundus
  • Mammals - The lateral posterior nucleus [LPN]-pulvinar complex



• Lemnothalamic nuclei mainly receive lemniscal inputs; some projecting efferent output bilaterally to the pallium, but not the striatum.
  
  In Amniotes, lemnothalamic nuclei consists of:
  
  
  • Reptiles* - Dorsomedial nucleus [DN], lateral dorsal nucleus [LDN], anterior nucleus [AN], and the dorsal lateral optic nucleus [DLON]
  • Mammals* - Anterior nucleus [AN], medial nucleus [MN], intralaminar nuclei [IN], ventral lateral nucleus [VLN], ventral posterior medial nucleus [VPMN], ventral posterior lateral nucleus [VPLN], lateral geniculate nucleus [LGN].

*These nuclei need to be double checked and confirmed correct...

The Dorsomedial Nucleus is a target of melatonin release.

The Archaic Somatosensory Circuit: The Tectothalamic System

It seems that a Tectothalamic system phylogenetically precedes the Thalamocortical System, which is a mammalian innovation.

Diencephalic (thalamic and hypothalamic) and telencephalic (septal nuclei, archistriatum-amygdala, archicortex-hippocampus, paleocortex-olfactory cortex) structures would be important peripheral components of the archaic brainstem-midbrain system relaying sensory experience to the integrating center of the somatovisceral animal.

Also, prior to tetrapods and the evolution of the limbic system, the primary hedonic evaluation system would occur in the Pharyngeal-Hindbrain Complex where pharyngeal, visceral, and somatic components converge; indicating it's archaic role in somatovisceral integration.