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The somatic nervous system is the organism’s apparatus for responding to the external environment. It sends information to the brain from the body’s various sensory detectors. The somatic nerves enable us to respond to these stimuli by moving through our environment, taking voluntary action, reacting and speaking. One of the principal roles of the somatic NS is maintaining homeostasis in the external environment, as discussed later in this chapter.

The autonomic nervous system (ANS) maintains homeostasis in the internal environment by regulating vital organs, such as those involved in digestion, respiration, blood circulation, excretion, and the secretion of hormones. The ANS is divided into two subsystems, the sympathetic and parasympathetic systems. The two branches of the ANS generally work in opposite directions, enabling a continuous upward or downward control of the internal organs to maintain homeostasis in the internal environment.

Figure 2.6 Brainstem structures responsible for neuromodulation

The sympathetic nervous system (SNS) goes into action to prepare the organism for physical or mental activity of ‘fight or flight’. When the organism faces a major stressor, it is the SNS that orchestrates the fight-or-flight response. It dilates the bronchi and the pupils, accelerates heart rate and respiration, and increases perspiration and arterial blood pressure, but reduces digestive activity. Two neurotransmitters are primarily associated with this system: epinephrine and norepinephrine. The parasympathetic nervous system (PNS), on the other hand, causes a general slowdown in the body’s functions in order to conserve energy. Whatever was dilated, accelerated or increased by the SNS is contracted, decelerated or decreased by the PNS. The only things that the PNS augments are digestive functions and sexual appetite. One neurotransmitter is primarily associated with this system: acetylcholine. The two divisions of the ANS and their functions are illustrated in Figure 2.7.

Figure 2.7 Schematic diagram of the autonomic nervous system

Source: Adapted from Geo-Science-International