The points of hedonic communication between the visceral and somatic divisions - Connections of the Parasympathetic Nervous SystemGray696Evolutionary somatovisceral welding of the two divisions together - Connections of the Sympathetic Nervous SystemMidbrain cross-section st the top end of the pharyngeal arch system at the point of fusion. Here we see the reticular mesh (mesencephalic reticular formation) binding together key posture related pathways from the trigeminal system (CN V, mesencephalic nucleus), the visual-orienting nuclei (occulomotor CN III, superior colliculus) and postural gait modulators (medial lemniscus, red nucleus)[a] - with, the dopamine global neurotransmitter fountain (substantia nigra) that innervates the basal ganglia and other critical nuclei associate with posture and orienting. We also see a number of ascending spinothalamic sensory tracts and myelinated descending inhibitory cortical tracts. [b]Global neurotransmitter fountains at the point of fusion at the hindbrain-pharyngeal arch interface where hedonic evaluation is globally communicated to the CNS - The chemical neuroanatomy of the brainstemreticular formation (BRF).[c]Brain stem sagittal
The vertebrate nervous system is organized into a somatic division which is fundamentally sensorimotor in structure and dedicated to the ecology - and, a visceral division, characterized by an enteric nerve net, dedicated to the hedonic needs of the organism. The enteric nervous system (ENS) of the visceral division and its autonomic connections to the somatic division, as well as a number of subcortical structures within the somatic division, will provide the vital adaptive hedonic feedback necessary for the CNS and neocortex to form the vital perceptual categorizations and postural motor routines that are meaningful to it.
The Vertebrate as a Dual Animal: Somatic and Visceral
"In many regards the vertebrate organism, whether fish or mammal, is a well-knit unit structure. But in other respects there seems to be a somewhat imperfect welding, functionally and structurally, of two somewhat distinct beings: (1) an external, "somatic," animal, including most of the flesh and bone of our body, with a well organized nervous system and sense organs, in charge, so to speak, of "external affairs," and (2) an internal, "visceral," animal, basically consisting of the digestive tract and its appendages, which, to a considerable degree, conducts its own affairs, and over which the somatic animal exerts but incomplete control."[3]
Romer expounded a vision of the vertebrate as a dual animal.[3] Romer hypothesizes that at the origin of vertebrates these two bodyplans, which were sequentially expressed, came to be expressed simultaneously - and fused only at the hindbrain-gill slits and the sacral nerve. Originally, the only points of communication between the two "animals" was via the unmyelinated neurons of the parasympathetic nervous system. The rest of vertebrate evolution revolves around adaptions that allow the integration of these two bodyplans. Romer describes the gradual emergence of the myelinated sympathetic nervous system and its increasingly sophisticated development of control over the enteric nervous system and viscera by the somatic division as we move along the evolutionary progression of vertebrate anatomy and physiology.
↑The red nucleus modulates gait in vertebrate motor routines. Mammals, and in particular primates, have evolved a dominant secondary system, the corticospinal tracts which allow a more refined higher-order modulation.
↑The two early whole genome events in vertebrate history established the basic themes of vertebrate evolution. Several key innovations emerged with each duplication event. The first event appears to be associated with the emergence of jawless vertebrates - and, the phylums key evolutionary embryological innovation, the neural crest. The neural crest is the germ-layer derived tissue that welds the visceral and somatic divisions together. The developmental biologist Brian K. Hall has proposed that the neural crest is a fourth germ-layer - making vertebrates the only quadroblastic animals to evolved as of yet.[1] Jawless vertebrates possess limited integration between somatic and visceral divisions, with communication via unmyelinated parasymapathetic neurons limited to the hindbrain-pharyngeal gill slits and the sacral end. The second whole genome event brings about a whole swath of vertebrate innovations: myelin, proprioceptors, jaws, fins, sympathetic neurons. After the second whole genome duplication event, the trigeminal system arose with the emergence of jawed vertebrates. The new vertebrate jaw was derived from the first gill arch cartilages; the nerve component would be remodeled into the trigeminal nerve and cranial nerve V and will be a critical part of the newly emergent symapthetic nervous system. These modifications are made possible by the advent of myelin and fast nerve conduction.
↑Original description: "Fig. 2. The chemical neuroanatomy of the brainstem reticular formation (BRF). This cartoon offers a schematic description of those brainstem areas properly belonging to the reticular formation (RF). The diagram shows the constellation of the RF nuclei following a neurotransmitter chemical classification. The isodendritic morphology of the neurons composing the RF nuclei, configures them as crucial stations of both afferent and efferent projections descending and projecting up to the cortex and spinal cord (SC). This network of overlapping connections is involved in a plenty of either extrapyramidal motor and non-motor functions. The major monoamine containing areas, mainly localized in the lateral RF except from C3, are the noradrenergic (A1–A7) adrenergic (C1–C3) dopaminergic (A8–A10) and cholinergic (Ch5–Ch6) nuclei. These are crucial for respiratory activity and for regulating blood pressure and heart rate, micturition, sweat, sleep-waking cycle as well as descending motor control. Serotonergic nuclei are found in the median RF raphe nuclei, mainly in the B3, B8 and B9 areas. They control vegetative functions such as mood, sleep and sexual behavior, depression and pain. The medial RF, found between the median and the lateral column, is a region lacking monoamine nuclei, but whose giganto-cellular and paramedianpontine nuclei act as a station for fibers connecting with monoamine regions such as A6 (LC) and Ch6. They are involved in voluntary movement regulation, as well as in optical, acoustic and olfactory control due to their connections respectively, to the spinal cord and to the main cranial nerves’ nuclei."[2]
↑Hall 1998. sfn error: no target: CITEREFHall1998 (help)
