The Cortical Outflow As An Inhibitory Brake

The cerebral cortex represents a uniquely mammalian innovation and it's evolutionary emergence coincides with the following:

• The transition to endothermy via the four-chambered heart.


• The up-regulating of the sympathetic nervous system.


• Remodeling of the pharyngeal arch system via myelinated parasympathetic neurons.


• The emergence of the nucleus ambiguous and the poly-vagal regulation of the heart.


• Reintegration of the head senses with respect to the neck and overall body posture.


• The onset of real-time environmental foraging.

The mammalian neocortex sends inhibitory output to all levels of the nervous system, including the pharyngeal-hindbrain complex - systems collectively refered to as the subcortical nervous system.

This subcortical nervous system is actually the conserved core of the archaic vertebrate nervous system, with the neocortex representing a functional and architectural overlay.

The Corticothalamic Loop and Tracts

The Thalamocortical system of mammals is functionally and anatomically preceeded by a more primitive system that integrated the thalamus with the archicortex, septum, and striatum - this system is and integral part of the limbic system and contains components of Herrick's Loop.

The corticothalamic tract represents a new and uniquely mammalian modulatory input into the archaic vertebrate system at the level of the thalamus allowing further refinement and differentiation of the neural signals between the thalamus and the more ancient parts of the telencephalon.

The implications of this for the differentiation of the primary emotions - governed by the limbic system - into more rarefied cognitive evaluations should not be underestimated (Greenspan and Shanker 2004).

The corticothalamic tract projects from layer VI of the cerebral cortex to the reticular nuclei, relay cells, and interneurons of the thalamus.

The reticular nuclei of the thalamus, in turn, projects to the relay cells of the thalamus which then project back to layer IV of the cerebral cortex forming a corticothalamic loop.

The Corticostriatal Loop and Tracts

The allocortex, or entorhinal cortex, projects to the paleostriatum (i.e. the globus pallidus) of early vertebrates.

In mammals layers II and III of the entorhinal cortex send input to the dentate gyrus and hippocampus - while the deep zone of layer V projects to the patch zones of the dorsal striatum, i.e. the globus pallidus of the basal ganglia.

The mammalian neocortex, via the corticostriatal tract, projects from upper zone of layer V in the sensory and motor association areas to the ventral striatum, more specifically - to the matrix zones of the caudate and puatamen where the neurons synapse at striatal neurons possessing excitory D₁ receptors that project to the globus pallidus.

The mammalian corticostriatal tract is part of a loop connecting the neocortex back to the thalamus via the straitum and globus pallidus.

The Corticorubral Tract

The corticorubral tract projects from motor areas 4 and 6 to the red nucleus in the mesencephalon.

The corticorubral tract is primarily responsible for allowing cortical modulation of the rubrospinal tract that innervates the limbs.

According to Humphrey (1984):

• The Magnocellular Nuclei of the Red Nucleus receives projections mainly from Layer V_b of Area 4 and 6.


• Parvocellular Nuclei of the Red Nucleus receives projections mainly from Layer V_a of Areas 4 and 6, as well as from Layer V_b of the Parietal Area.

The Corticotectal and Corticoreticular Tracts

The corticotectal tract projects from the cerebral cortex to the tectum of the midbrain.

The corticoretucular tract projects from the cerebral cortex to the reticular formation of the midbrain and brainstem.

The Corticopontine Tract

Cortical input to the cerebellum is via the pontine nuclei.

The corticopontine tract carries projections from the cerebral cortex to the pontine nuclei.

The Corticobulbar Tract

The Corticobulbar tract sends myelinated inhibitory outputs to the pharyngeal hindbrain complex connecting to both general and special cranial nerve source nuclei.

The corticobulbar tract may be divided into two divisions: Somatic and Visceral.

The Special Visceral and General Visceral Corticobulbar Tracts

The Special Visceral Corticobulbar (SVE) tract projects from the facial motor areas of the cerebral cortex to the motor nuclei of the pharyngeal-hindbrain complex which innervate the muscles of the face, head and neck of mammals.

The SVE Corticobulbar tract is the equivalent of a special visceral motor (SVE) pathway which terminates on cranial nerves V, VII, IX, X, and XII and facilitates the operation of the mammalian social engagement and polyvagal systems.

This system emerges by integrating the pharyngeal arch components into the head, neck, and facial structures of mammals.

The General Visceral Corticobulbar (GVE) tract is equivalent to a general visceral motor pathway (GVE) terminating on the cranial nerves V, VII, IX, X, and XII.

The General Somatic Corticobulbar Tract

The General Somatic Corticobulbar (GSE) tract projects from the frontal eye fields of the cerebral motor cortex to the reticular formation of brainstem.

This GSE Corticobulbar tract is the equivalent of a general somatic motor pathway terminating on cranial nerves III,IV,VI,XI.

The GSE Corticobulbar pathway projects from the frontal eye fields in the motor areas of the cerebral cortex to neurons in the reticular formation which then innervate the motor neurons of cranial nerves governing visual gaze and tracking.

The Corticospinal Tract

The corticospinal tract projects from Betz cells in layer V_b of the precentral gyrus somatomotor cortex to the primary motor neurons of the spine.

The corticospinal tract may be divided into two divisions:

• Lateral Corticospinal Tract - ~ 85% of the corticospinal neurons decussate at the pyrimidal layer and thence project to the level of the spine from which they will exit.


• Medial Corticospinal Tract - ~ 15% of the corticospinal neurons decussate the pons and then proceed to the level of the spine at which they exit.