Formation of the Basal and Alar Plates and Their Derivatives

The division of the neural tube into the alar and basal plates sets the stage for the development of the peripheral nervous system (10, 13, 14).

Spinal Cord Division

• Sensory portion: The alar plate gives rise to Rexed’s laminae I-IV.
• Motor portion: The basal plate gives rise to Rexed’s laminae VII-X.

Brainstem Division

The early post-neurulation neural tube develops into dorsal and ventral halves – the alar and basal plates respectively. The two plates are separated by the sulcus limitans.

• Motor neurons: Cells within the basal plate develop into efferent neurons. The basal plate also further develops general motor efferents (GME) innervating somatic muscles, special motor efferents (SME) innervating branchial arch-derived muscles, and visceral motor efferents (VME) innervating autonomic (respiratory, cardiac, and gastrointestinal system) muscles.
• Sensory and interneurons: Cells within the alar plate develop into afferent and interneurons. The alar plate further develops general sensory afferents (GSA) innervating the skin, special sensory afferents (SSA) innervating the branchial arch derivatives, and visceral sensory afferents (VSA) innervating the respiratory, cardiac, and gastrointestinal systems.

 

• Formation of cranial nerve nuclei: In the brainstem, further developments within the alar and basal plates produce a complex of cranial nerve nuclei that are understandable from an embryological standpoint. In particular, the pontine flexure and the resultant thinning of the mesencephalic roof splay the mesencephalon so that the alar plate comes to lie dorsolateral to the basal plate.
• Columnar organization: In general, each type of afferent and efferent is organized along loosely defined columns from dorsomedial to ventrolateral. The sensory system’s organization is in a medial-to-lateral layout of GSA → SSA → VSA. The motor system’s organization is in a medial-to-lateral layout of GME → SME → VME. Each cranial nerve is composed of axons derived from one or more of these sensory and motor groups.

                  Efferent and afferent contributions to cranial nerves

CN	GSE	GVE	SVE	GSA	GVA	SVA
I*
II*
III	X	X
IV	X
V			X	X**
VI	X
VII		X	X	X	X†
VIII						X
IX		X	X		X†	X
X		X	X		X†	X
XI	X		X
XII	X

 

Abbreviations: CN, cranial nerve; GSE, general somatic efferent; GVE, general visceral efferent; SVE, special visceral efferent; GSA, general somatic afferent; GVA, general visceral afferent; SVA, special visceral afferent.

* CN I and II are classified as special sensory nerves. CN II is technically a central tract rather than a cranial nerve.

** Receives contributions from facial, glossopharyngeal, and vagal nuclei but

output is via trigeminal nerve.

† Gustatory input from facial, glossopharyngeal and vagal nuclei;

cardiorespiratory input from glossopharyngeal and vagal nuclei.

 

• Cerebellum: The cerebellum is a greatly expanded outgrowth of the alar plate of the mesencephalon that performs integrative (interneuron) functions.

Supratentorial Brain Division

• Only the alar plate exists cranial to the midbrain: The derivatives of the alar plate give rise to the sensory and integrative functions of the thalamus, hypothalamus, basal ganglia, and telencephalon. Both hypothalamic and pyramidal cells are technically considered interneurons since they innervate other effector cells – the ventral horn motoneurons in the case of the cortical pyramidal cells, and the humeral cells of the pituitary gland in the case of the hypothalamic neurons.