How Does the Brain Communicate with the Rest of the Body?

*How does the brain communicate with the rest of the body? How is the autonomic branch of the peripheral nervous system involved in controlling emotions?
The nervous system as a whole includes the Central Nervous System, consisting of brain and spinal cord, and the Peripheral Nervous System, whose nerve fibres connect all parts of the body with the central nervous system. The Peripheral Nervous System is further subdivided into two branches, the Somatic Nervous system and the Autonomic Nervous System. All these nerves are outside the Central Nervous System. The Somatic Nervous System controls musculoskeletal movement, and conducts sensory messages from the body to the CNS. (model is increasing decentralisation)
The Autonomic Nervous System has two branches, the Sympathetic and the Parasympathetic, which regulate the involuntary processes of the body, the viscera, and sense organs, glands and blood vessels. In evolutionary terms it is older than the CNS and its anatomical circuitry is broadly dispersed, creating a general response, quite unlike the highly specific pathways and response of the CNS. This generalised, widely distributed structure enables it to mediate overall changes in state; it is part of the limbic system which has also been known as the mammalian or emotional brain.
It was called autonomic because it was believed to function autonomously – we now know that it is dynamically related to many other parts of the brain especially the orbitofrontal cortex. Autonomic also means self-regulating and this is a key principle of all body systems, which depend of constant feedback in order to maintain homeostasis. There are multiple feedback loops in the body which continually send and receive information about what’s going on and the ANS is part of this wider complex.
In standard physiology the two parts of the ANS have been perceived as functioning reciprocally: the sympathetic governing arousal, the fight or flight reaction and the parasympathetic involving relaxation, recuperation and digestion. The sympathetic nervous system is activated by any stimulus over an individual’s threshold (and the threshold can vary enormously), including feelings, and by noise, light, drugs and chemicals (e.g. caffeine).In response to the stimulus an immediate anticipatory state is generated by the release of adrenaline. This causes the heart to beat more quickly and strongly, increases blood supply to the muscles, raises blood pressure, dilates the bronchii and increases the breathing rate, raises the blood sugar level for increased energy, speeds up mental activity, increases tension in the muscles, dilates pupils and increases sweating. Non-emergency functions, such as digestion are lessened or suspended. (priming phase – short-term) Walter Cannon coined the phrase ‘fight or flight’ to describe the function of the rapid mobilisation of resources.
The parasympathetic nervous system comes into operation after the stimulus has been responded to and action taken. It has the opposite effect to sympathetic activity, allowing the body to wind down and re-balance. The activation of the parasympathetic nervous system encourages relaxation of muscles, slowing the heart rate and lowering the blood pressure. It assists the breathing to return to its normal rate, digestive juices flow, bladder and bowels are ready to function, the pupils constrict and immune functions, such as the production of white blood cells are re-commenced. The parasympathetic mode supports rest and sleep. (the investment phase - long term). The standard physiological model of the ANS is of reciprocal tension - with the two parts keeping each mode in check – When the sympathetic goes up, the parasympathetic goes down. A good example of optimal autonomic balance can be seen in cats who respond alertly to certain sounds or movements, but, as soon as the situation is assessed as safe, return immediately to a relaxed state. (‘The New Anatomy: Exploring the Mind in the Body’ run at Chiron February-March 2001.)