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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)[b] - 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. [c]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).[d]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.
↑Subject to major change, revision ,and/or retraction at any moment.
↑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]
In the mid 1990s, Stephen W. Porges developed his Polyvagal Theory. The work of Porges on the phyletic origins of the autonomic nervous system and its CNS nuclei in the brainstem adds an additional layer of nuance to Romer's picture of vertebrate organization with respect to mammals. Porges postulates that the neuroanatomical basis of a social-engagement system was constructed out of a recently remodeled and myelinated-pharyngeal arch system and its brainstem nuclei, a newly emergent neocortex with inhibitory outflow, and the myelinated division of the parasympathetic nervous system.[4][5][6]
Porges envisages a social-engagement system of neural architecture constructed out of the remnants of the pharyngeal arch system as amniotes adapted to land, switched modes of breathing; and, ultimately in mammals, and closed the cardiopulmonary loop reestablishing fully-oxygenated bloodflow required for endothermy and optimum utilization of the metabolic oxidative potential. Every 10 degrees Celsius the temperature rises, the metabolic rate doubles. Once the metabolic resources were available, the sympathetic nervous system was "revved-up" and constantly idling with an inhibitory "brake" on it. This allowed the transition from a stimulus-induced behavior typical of cold blooded organisms with limited metabolic resources - where the autonomic nervous system operates with a threshold response to engage, activation, peak execution, and a decay back to baseline to recharge resources; to a pattern or real-time behavioral foraging and ecological exploration that is made possible by the increase in available metabolic resources that allows the sympathetic to remain in a state of high output, but is tamped down by inhibitory myelinated parasympathetic and cortical outflow. The basic picture is one where the anatomy of the social-engagement system is riding on top of a metabolically revved-up subcortical and sympathetic nervous system - and, acting as an inhibitory and modulatory brake on the lower systems.
The social-engagement component of the system are related to the way that the gill arch cartilages, muscles, and nerves are remodeled into larynx and the vocalization apparatus, the ear and the auditory-orienting system within the newly emergent head-neck system, and the muscles of the head and neck. The auditory system of mammals is tuned to the species-specific frequencies of their larynx and vocalization apparatus. The system evolves the capacity for social-engagement through reciprocal co-engagement between members of the same species as they become cognitively and behaviorally entrained via vocalization and posturing.
Edelman describes the neuroanatomy of the somatic division, the central nervous system (CNS), as organized into a structure that is made up of nerve tracts as well as nuclear, laminar and columnar cell populations - and in contact with the external world via the primary sensory sheets and muscle ensembles. Since Edelman focuses primarily on the operation of the neocortex, his hedonic feedback systems emanate from the world of subcortical structures that have roots deep in the brainstem and connections to the visceral body via the autonomic nervous system.
Communication between the two divisions can occur via the autonomic, endocrine, and immune systems but, the key neural integration point is where the reticular network of the brainstem ties together the autonomic and cranial nuclei of the pharyngeal arch system, the general and special tracts of the peripheral nervous system, and the global neurotransmitter fountains that diffusely project upward to the midbrain, thalamus, and cortex, as well as back down the spinal column. This is the link between somatic and visceral divisions that allows the visceral nervous system to act as a mechanism of selection in the recruitment of neuronal groups whose configurations are adapative such that the hedonic needs of the visceral body are being met.
In a selective system, oftentimes differential amplification occurs via inhibition of competitors. In the nervous system most excititory pathways synapse with inhibitory neurons - and, most often the activation of a specific pathway involves suppressing or inhibiting the expression of other pathways that would require the use of the same structural components or resources.
Reciprocal inhibition allows the system to collapse to a solution, rather than calculate a solution. Under the conditions of canalization, this process can result in a set of reflexes that are adaptive under normal circumstances, but under stress conditions the inhibition can be released to reveal the latent variation that was outside the canalized pathway, offering potentially novel and on occasion adaptive, behavioral responses to novel stresses. There is an enormous amount of latent variation or "buffering" in a canalized system that allows the system to respond to stress rapidly.
The reservoir of latent inhibited networks just outside the conditioned range of behavior, is a reservoir of creativity and resourcefulness for the nervous system. There are pathways and networks of neuronal groups capable of exploring new possibilities and solutions outside the range of prior conditioning that lay latent and below threshold activation expect under extraordinary circumstances.
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