The brain gets a continuous flow of stimulus from the sense, organs, through the nervous impulses to the specific region of the cerebral cortex, as well as the consciousness. Partly, the message is also conveyed to a unique system within the spinal cord and the brain. This is designed to interpret the stimulus as a danger signal or one of pleasure and enjoyment, by exciting the nervous system. The reticular formation develops to gather messages from the sense organs, and coordinates them to get a complete picture of the threat. This takes place on a continuous basis.
The messages are exaggerated or suppressed depending upon the importance of the threat. The interpretation as to when a particular threat is important and needs to be exaggerated or when it is of less importance in comparison to another message and needs to be suppressed, is also the task performed by the brain. The ordering of priorities depends on a number of factors which the brain takes into consideration. The stimulus to the sense organ can also get suppressed at the source, even before the brain is able to receive it. A blinding flash-light of the photographer shuts out all other subsequent stimulus to the eye. A jet plane overhead cuts off all conversation. So momentarily or for a period of time, depending upon the intensity and the duration of the 'suppression', the brain becomes 'blind' or 'deaf'.
Involuntary suppression of the sense stimulus when a particular sense organ is under severe stress, is quite different from the practice of pratyahara in yoga. In pratyahara, the individual is conscious of what is happening around him but at the same time, consciously cuts out or filters the stimulus to the brain.
Arousal in the face of threat or stress triggers five separate areas of the brain which sends message to the endocrine glands to secret hormones, such as adrenaline, 17-OCHS and testosterone.
The psychic centre in the frontal lobe and the limbic cortex are the parts of the cerebral cortex involved in stress adaptation. In the limbic cortex are many nuclei which are scattered around the hypothalamus, and connected to it through a nervous network. Initially, any stress like fear, etc. is experienced in these two areas of the brain- the frontal lobe and the limbic cortex. Increased activity in these regions stimulates the hypothalamus, triggering off the automatic nervous system and the endocrine system, leading on to several physiological changes which are needed to provide protection to the organism.
The central nervous system (CNS) is the 'communications' network in the body. Through the CNS, stimuli from the sense organs are conveyed to the appropriate regions of the brain. In the reverse function, messages from the brain are conveyed, through the CNS, to the various parts of the body and the glands, and other individual and collective body-mind mechanisms. The CNS is a two-way communications network. Part of the CNS controls all the involuntary functions of the body. This is known as the autonomous nervous system (ANS). The ANS is subdivided into the sympathetic nervous system or the pingala nadi; and the parasympathetic nervous system or the ida nadi.
Right inside the brain, covered by large folds of cerebral cortex, is the hypothalamus. Located almost in the centre of the brain, the hypothalamus is the highest centre of the sympathetic nervous system. Nerve fibres from the sympathetic nervous system spread on either side of the spinal cord and connect to the various plexuses or yogic chakras; the laryngeal plexus or vishuddhi chakra; the cardiac plexus or anahata chakra; the solar plexus or manipura chakra; the lumbar plexus or swadhisthana chakra; and the sacral plexus or mooladhara chakra.
In the face of a threat, the hypothalamus is able to send electrical impulses throughout the sympathetic nerves, thus activating the eyes, the salivary glands, the heart, the lungs, the stomach, the kidney and the intestine.
The parasympathetic nerve is a large nerve called the vagus. It descends from the brain stem down the neck, chest, abdomen and is connected from organ to organ. During stress adaptation, the parasympathetic nervous system aids the sympathetic nervous system, though the two work at variance to each other.
The sympathetic nervous system, as its name suggests, is sympathetic to the needs of the body. At the first impulse received from the hypothalamus, the sympathetic nerve initiates action: increases the pulse rate, the blood pressure, sweating, the blood glucose, and the muscle tone; opens the air passages and the eyelids, and dilates the pupils.
The para sympathetic nervous system, at the same time, decreases pulse rate, blood pressure and sweating; it makes the blood glucose and the muscle tone normal; it constricts the air passages; it makes the eyelids relax and droop, and it makes the pupils actually constrict.
The seemingly anti-sympathetic role of the parasympathetic nervous system, has the finesse of nature's intelligence about it. By virtually countering the actions of the sympathetic nervous system, the parasympathetic nervous system acts as a moderator and prevents the sympathetic nervous system from getting itself carried away. Besides, it is the counter action of the parasympathetic nervous system which 'pulls' the body system back to its state of normality.
However, stress problems do not always originate from over-arousal of the functioning of the sympathetic nervous system. The parasympathetic nervous system too, sometimes, responds to 'stress', leaving the individual more and more withdrawn within himself, failing to respond to the environment around him.
Yoga attaches great importance to the balancing of the ida and pingala nadis or the parasympathetic and sympathetic nervous systems. The two nervous systems are linked to the left and the right brain hemispheres respectively, and to the breath. Yogis understood that by controlling or modifying the function of the breath, the nervous system, and in turn, the brain hemispheres can be controlled - the breath in the left nostril influencing the activities attributed to the right hemisphere, and the breath in the right nostril influencing the activities of the left hemisphere.
The yogis of yore also understood the strong bond between the eyes and the brain, and realised that if the eyes are 'stilled', then the thought waves of the brain could also be stilled. Thus, most meditative and some yogic techniques are practised with the eyes closed. At the same time, they also developed trataka, a technique to still the brain waves even while the eyes are open. Trataka involves the concentration on an object or a symbol, and the one-pointed concentration achieved through trataka has a similar effect on the brain.
There is a small, pea-sized gland above the top of the spine called the pineal gland. It is located between the two cerebral hemispheres and above the cerebellum. Modern medical science does not recognize any specific physiological function of this small body. Yoga on the other hand, says that this gland is the link, the antenna for higher vibrations. When it is stimulated or awakened, then one can start to tune in with less stressful and more subtle patterns of existence.
The pineal gland is stimulated through trataka, as the eyes are connected directly with the pineal gland, via the sympathetic nervous system. The centre of the sympathetic (as well as the parasympathetic) nervous system is the hypothalamus. The hypothalamus is also the centre of wakefulness and pleasure, together with sleep and pain. Thus, the eyes, the pineal gland, the sympathetic nervous system, the hypothalamus and the wakefulness of an individual are all intimately connected with each other, directly or indirectly. Mental concentration of any type will tend to bring these systems into a high level of sensitivity. In trataka, the whole system is stimulated and awakened by concentration through the eyes. This induces higher sensitivity of the pineal gland, and directly via the sympathetic nervous system, greater wakefulness in the individual. All other disturbing or stressful thoughts are blocked out, at least for a temporary period.
The endocrinal glands supplement the nervous system, in particular the sympathetic and parasympathetic nerves. Together, they make up a single neuro-endocrine system that integrates and coordinates the metabolic activities of the body, and controls the ability of the body to face changes in the internal and external conditions. The two systems work hand in hand, but with one major difference. The nervous reactions are quick and short lived, whereas the endocrinal responses are slow and protracted to give permanent changes to the body. The nervous system produces speedy changes in the body, while the endocrinal system gives continuity to these changes.
The glands are distributed throughout the body and the hormones secreted by them into the bloodstream are thus distributed to all parts of the body. The glands trigger definite organs that react to specific hormones, and in this manner, changes are induced within the body according to the dictates of the brain, which is also the master controller of the endocrinal system.
The glands do not work independently - each gland reacts with and modifies the influence of the other glands. Thus, any disorder of the endocrinal system in general, or in one of the glands alone, can have negative repercussions on the health of the whole body.
Some of the important glands which secrete hormones at times of stress in the endocrine system are the two adrenal glands and the thyroid gland. The central portion of the adrenal glands, the adrenal medulla, secretes adrenalin and noradrenalin, while the outer layer or adrenal cortex secretes corticosteroid hormones. These hormones bring about changes similar to the sympathetic stimulation. Besides, the thyroid gland secretes thyroxin and the pituitary gland in the skull secretes growth hormones. A rise in the level of these hormones in the blood assists in stress adaptation, and therefore, these hormones are known as stress hormones.
The pituitary gland, which controls the secretion by the sex glands - ovary and testis - also controls the secretion of corticosteroids by the adrenal cortex by releasing adreno-corticotrophic hormone. The pituitary gland also controls the secretion of thyroxin by the thyroid gland, through the release of thyroid stimulating hormone. Thus, without the impulse from the pituitary gland, neither the adrenal nor the thyroid gland would secrete the stress hormones. However, the secretion of both the adreno-corticotrophic hormone and the thyroid stimulating hormone by the pituitary gland, is in turn, controlled by the secretion of a hormone by the hypothalamus.
In this bureaucratic setup, the hypothalamus is right at the apex, and is in control of several functions of the body associated during adaptation to stress. One of the major aims of yoga in stress management is to harmonize the working of the entire endocrine system. While some of the yogic practices render the individual glands efficient, the practice of surya namaskara influences the entire endocrine system, as it combines asanas, pranayama and awareness of the chakras or the psychic centres in the body.