The Peripheral Nervous system is divided into two sub-systems: The Autonomic Nervous System and the Somatic Nervous System. The automatic nervous system is responsible for routine adjustments in one’s body outside of their conscious awareness. The ANS is responsible for regulating body temperature and coordinating cardiovascular, respiratory, digestive, excretory, and reproductive functions. It is also responsible for adjusting and maintaining the blood, water, electrolyte, and dissolved-gas concentrations in the bodily fluids.
The main difference between the Somatic Nervous System and the Autonomic Nervous System is that the somatic nervous system is connected to the CNS via the afferent and efferent pathways whereas; the Autonomic Nervous System is connected to the visceral organs and their associated receptors.
The ANS contains further two major subdivisions: the sympathetic subdivision and the parasympathetic subdivision (SEE THE FIGURE).
These two subdivisions usually have opposing effects. The sympathetic subdivision is often also termed as the “fight or flight” system because an increase in sympathetic activity generally stimulates tissue metabolism, increases alertness, and prepares the body to deal with emergencies. As opposed to this, the parasympathetic subdivision is termed as the “rest and repose” system because it conserves energy and promotes sedentary activities, such as digestion.
The parasympathetic pathways are controlled by the upper and lower ends of the spinal cord and the sympathetic pathways are controlled by the nerves origination from the central region of the spinal cord.
Functions of the Sympathetic Pathway
The sympathetic pathway deals with target cell responses in times of emergencies or during times of sudden, intensive physical activity. Its major functions include dilation of respiratory pathways, dilation of pupils for focusing on distant objects, stimulation of secretion by the sweat glands, accelerating the heart rate, blood vessel constriction, stimulation of energy production for the use of skeletal muscles, and release of stored lipids from adipose tissue. The sympathetic pathway is also responsible for stimulating orgasms.
The two main hormones, or neurotransmitters, released and controlled by the sympathetic division are norepinephrine (or, noradrenaline) and epinephrine (or, adrenaline). These are mainly responsible for the fight-or-flight response generated by the human body.
In an emergency situation or crisis, the sympathetic pathway responds by an event called the sympathetic activation. The effects of this activation include heightened alertness, feeling of increased energy or euphoria, increased heart and respiratory rates, and an overall increase in the mobilization of the energy reserves. This activation has two results: secretion of norepinephrine at the effector junctions’ i.e. where the response to the stimulus is going to take place, or the secretion of both epinephrine and norepinephrine into the bloodstream.
‘Caffeine’ which is generally found in beverages such as coffee, tea, or sodas like energy drinks stimulates a sympathetic neural reaction. It induces short-term physiological effects like an increase in blood pressure and an increase in nerve activity. The consumption of caffeine on a daily basis reduces these effects and makes them next to negligible.
Functions of Parasympathetic Pathway
The parasympathetic pathway deals with sending signals from the CNS to the target cells/organs. The major effects produced by parasympathetic pathway include contraction of urinary bladder during urination, increasing the smooth muscle activity in the digestive tract, controlling and stimulation of secretions by the digestive glands, secretion of hormones that aid in nutrient absorption, reduction in heart rate, coordination and stimulation of the rectum, contraction of respiratory passageways, pupil constriction, and stimulating salivary glands. The parasympathetic pathway also plays a major role in sexual arousal.
In the case of parasympathetic activation, the nerve fibres associated with the division release acetylcholine (Ach) at the synapses and neuroeffector junction. But its effects are generally short-lived as the actions of enzymes in the neighbouring tissues overpower the activation.
Relationship between sympathetic and parasympathetic divisions
The sympathetic division is widespread and reaches all visceral organs and tissues throughout the body. The parasympathetic division, on the other hand, controls the activity of organs and tissues managed by the cranial and pelvic nerves. Most vital organs in the body receive dual innervation-they receive signals from both sympathetic division and the parasympathetic division. This is most prominently observed in the heart, digestive tracts, and the lungs.
Comparison between sympathetic and parasympathetic divisions
Apart from their responses stated above, the sympathetic and parasympathetic divisions have some other key differences. The sympathetic nerve fibres usually release norepinephrine in response to a stimulus, whereas the nerve fibres of the parasympathetic division always release acetylcholine. The sympathetic division has a quick and immediate response and it creates a feeling of urgency as opposed to the parasympathetic division which is responsible for the generation of a more relaxed feeling.
Diabetes and the autonomic nervous system
A person suffering from diabetes usually also has a dysfunction autonomic nervous system. As the blood glucose levels in the body are elevated and the body is unable to absorb this glucose and use it as an energy source, a variety of physiological problems arise which results in diabetic neuropathy. Its symptoms often include a reduced sympathetic response and control of the cardiovascular system which results in low blood pressure and a slower heart rate. These individuals also have difficulty in urination, delayed gastric emptying and impotence.