We begin our study of the body’s response to stress from the very top—the brain. Intuitively, we all understand that our brain controls the functions of our body. We know that we can think of moving an arm or a leg and it happens. In this article we will take a look at how the brain actually exerts control and which systems are activated or suppressed as we undergo stress. In subsequent articles, we will see what the effects of these stress-induced actions are and how they can make us sick.
Autonomic Nervous System
The principal way in which the brain communicates with the rest of the body is through neural telephone lines (like the land-line telephones). The body also uses the mobile phone service—the use of hormones’ in the blood to send messages to distant parts of the body. The ‘land telephone’ system consists of nerves that start from the brain, go down the spinal cord and branch out to the periphery of the body. One part of this system is simple and we all understand it— the voluntary system.
As the name implies this is the system we use to move our arms or legs when we want to. The other half of the system is called the autonomic system. Again, as the name suggests, things here happen automatically and we do not have much control over them. This system controls things like our digestion, heartbeats, sweat glands and so on. It is the autonomic system that is of interest to us as it is involved directly in the body’s response to stress. The autonomic system also consists of two parts—the sympathetic and the parasympathetic systems.
Originating in the brain, sympathetic nerve projections exit the spine and branch out to nearly every organ and blood vessel. These nerves are so pervasive that they are even attached to the tiny muscles attached to the hair on our body. This system is activated in response to any stress. It helps mediate vigilance, arousal, activation and mobilization. A feeble joke can be used to describe the action as mediating behaviour such as flight, fright and fight. This system is turned on when things get too exciting or too alarming, as is the case during stress.
How does the nervous system go about controlling the different organs? The nerve endings of the sympathetic system release epinephrine and closely related substance norepinephrine (the other names in use are adrenaline and noradrenaline). The actual secretion of epinephrine is from the sympathetic nerve endings in the adrenal glands. Norepinephrine is secreted by all the other sympathetic nerve endings throughout the body. Within seconds of the body being stressed, these chemical messengers are secreted and they in turn kick the various organs into action.
The parasympathetic nervous system plays the opposite role. It mediates calm and vegetative activities. It promotes growth, energy storage and other optimistic activities. Everything but the four Fs is controlled by the sympathetic nervous system. Thus, the two parts of the autonomic nervous system work in opposition. The projections from both these parts into an organ when activated bring out opposite results. Therefore, it would be a disaster if both systems were to be activated at the same time. Naturally, our body has a number of mechanisms to ensure that the two systems are not activated simultaneously. In particular, the part of the brain that activates the sympathetic component during a stressful period also inhibits the parasympathetic system.