The researchers at Howard Hughes medical Institute, led by Bruce Lahn have found evidence that the pressure of natural selection has lead to dramatic changes in two genes known to control brain size in humans. Brain size or intelligence is naturally selected for in evolution for obvious survival reasons, and larger brains require more oxygen. Although the brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization. The larger the brain, the greater the demand of oxygen and hence the more sophisticated the nervous system needed to provide that oxygen...the evolutionary payoff for larger brain size of course being survival. As a natural extension of mammalian evolution we can see that the human neocortex was an inevitable consequence of evolutionary pressure.
According to the Poly-Vagal Theory during evolution the mammalian nervous system developed two vagal systems. Built onto the relic of amphibians and reptiles is an evolutionary modification unique to mammals. Looking at the history of evolution Poly-Vagal Theory notes the importance of the need for oxygen in evolving the mammalian nervous system. During evolution as the mammalian nervous system got more complex than its amphibian and reptilian brothers, there was a greater demand for oxygen. Porges says that it was this need for extra oxygen that may have provided the evolutionary pressure leading to the development of the highly adaptive and sophisticated autonomic nervous system found in mammals; and that behaviors such as orienting, attention, emotion and stress are by-products of the evolutionary pressure to optimize oxygen resources. The Polyvagal Theory addresses the relative roles of the vagus nerve in energy conservation and survival.
In Stephen Porges's Polyvagal Theory he uses the term Polyvagal to distinguish between the two main branches of the vagus nerve:
1: The Vegetative Vagus--originates in the dorsal motor nucleus (DMNX), descends visceral efferent fibers regulating smooth and cardiac muscle and is associated with passive reflexive regulation of visceral functions: peristalsis of the GI tract, sweating, lungs, diaphragm, stomach. At the heart it is connected to stretch receptors of the aortic arch and chemoreceptors of the aortic bodies and is responsible for heart rate, dilation of blood vessels and blood pressure. The output from the dorsal motor nucleus does not convey a respiratory rhythm. The most primitive function of the vagal complex is the freeze response, which is dependent on the unmyelinated vagus which is part of the reptilian system.
2: The Smart Vagus--which originates in the medullary source of the nucleus ambiguus (NA), serving efferent fibers regulating the somatic muscles of speech and eating: the larynx, pharynx, and esophagus. The ventral vagal complex (including NA) related to processes associated with attention, motion, emotion and communication. The functional output of the NA-vagus on the heart is part of a common neuronal network producing a cardiorespiratory rhythm. The most evolutionary recent component--the communication system functions through the new-mammalian or myelinated vagus that regulates the heart and the bronchi to promote calm and self-soothing states.
In mammals the two vagal systems are neuroanatomically distinct, have different origins, and are programmed with different response strategies and may respond in a contradictory manner. Thus Porges attributes various medical disorders to competition between DMNX and NA originating fibers. The different vagi may have oppositional outputs to the same target organ. The vagus is a complex of neural pathways originating in several areas of the brainstem. The vagus nerve consists of afferent and efferent parasympathetic (acetylcholine) fibers that run from the brainstem (medulla oblongata) down to the traverse colon and urinary organs; providing both motor and sensory parasympathetic activation for everything from the neck to the G spot. Efferent fibers originate primarily in two medullary nuclei (NA, DMNX). The vagus is not solely an efferent or motor pathway, at least 80% of the vagal fibers are afferent; that is they conduct impulses from the periphery of the body to the brainstem.
According to the Polyvagal Theory the growth of the autonomic nervous system evolves through three stages:
Freeze--First a primitive
unmyelinated visceral vagus that fosters digestion and responds to threat by depressing metabolic activity eg: freeze response.
Flight/Fight--The mobilization or flight/flight is dependent on the functioning of the sympathetic nervous system; increasing metabolic output and inhibiting the visceral vagus to foster mobilization behaviors necessary for fight or flight.
Communication--The third stage, the mammalian myelinated vagus, can rapidly regulate cardiac output to align with the environment and is associated with cranial nerves that regulate sociability via facial expression and vocalization.
Stephen Porges points out the phylogentic hierarchy of response to challenge: "The hierarchy emphasizes that the newer "circuits" inhibit the older ones. We use the newest circuit to promote calm states, to self-soothe and engage. When this doesn't work, we use the sympathetic-adrenal system to mobilize for flight and flight behaviors. And when that doesn't work, we use a very old vagal system, the freeze or shutdown system."
Stephen Porges suggests that the true freeze response is dangerous to mammals. For example, high tone in the dorsal motor nucleus vagal system may be lethal in mammals through an overdose of the immobility response overdose. Whereas high tone from the NA-vagal system may be beneficial in adaptive significance of mammalian affective processes including courting, sexual arousal, copulation, and the establishment of enduring social bonds. In the development of enduring pair-bounds the mammalian vagus communicates safety and trust, via oxytocin and vasopressin, between the hypothalamus and the medullary source nuclei of the viscera vagus.
Porges suggests that we use our higher cognitive processes to calm the stress response and establish effective connections with others by using our facial muscles, making eye contact, modulating our voice and listening to others. In this way we increase the influence of the myelinated vagus, which calms us and turns off the stress response and makes us more metabolically efficient. He says the social neural circuit supports our health through its calming influences on the heart and lungs and its reduction of HPA axis activation.
The vagus is asymmetrical with the left and right sides performing different tasks, with the right vagus most active in the regulation of the heart. Primary emotions are related to autonomic functioning since they are often survival related, they must be integrated into the regulation of the heart and lungs. Emotions have a right limbic bias, as does the brainstem medullary structures controlling visceral function. Only when the environment is perceived as "safe" is there cortical regulation of the visceral pathways, because while under threat, cortical control of brainstem structures would compromise the individual's ability to mobilize. Therefore when stressed or in danger, cortical control of brainstem is "inhibited" and the brainstem structures are "disinhibited" to allow the sympathetic nervous system to efficiently increase metabolic output.
Stimulation of the ascending fibers of the vagus releases norepinephrine into the amygdala strengthening memory storage in regions of the brain that regulate arousal, memory and feeling responses to emotionally laden stimuli. These ascending fibers is how the peripheral epinephrine from the adrenals released into the blood during the fight-flight response activates the release of norephinephrine in the limbic system sharpening memory of the events. Since the adrenal hormone epinephrine cannot cross the blood brain barrier it activates the vagus nerve, which in turn stimulates neurons in the brainstem known as the "Nucleus of the Solitary Tract (NTS). This third medullary nucleus, located near DMNX, is the terminus of many of the afferent pathways travelling through the vagus from peripheral organs. Vagus afferent sensory fibers carrying information to the brain from the head, neck, thorax, and abdomen relay information to the NTS. These NTS neurons release norephinephrine into the memory processing areas such as the amygdala and hippocampus to activate long term memory storage of emotionally laden events. This explains why vagus nerve stimulation was found to improve memory consolidation of recent events. Researchers found that by microinjecting the NTS with either GABA agonists or glutamate antagonists, they thereby increased GABA or decreased glutamate in the NTS and this blocked seizures.
Stephen W. Porges, Ph.D. found that he could improve autism by stimulating the newer structures and prompting the social engagement system with the use of acoustic sessions using frequencies associated with the human voice. Check out Stephen Porges's fabulous papers on the web:
Love: an emergent property of the mammalian autonomic nervous system, Psychoneuroendocrinology, (1998) Nov;23 (8):837-61.
Orienting in a Defensive World: Mammalian Modifications of our Evolutionary Heritage. A Polyvagal Theory. Psychophysiology, (1995) 32, 301-318.
The Polyvagal Theory: phylogenetic contributions to social behavior, Physiology & Behavior, (2003) 79, 503-513.Neuroception: a subconscious system for detecting threats and safety, Zero to Three, (2004) 32, 19-24.