by Dr Patrick Quanten MD
The autonomic nervous system comprises a series of reflex arcs which control or modulate involuntary functions, such as blood pressure, heart rate, glandular secretions, intestinal motility and bladder function. The nerves comprise two systems, the sympathetic and the parasympathetic, that receive impulses arising in the central nervous system. Both systems form synapses at autonomic ganglia in the periphery before innervating their target tissues. The sympathetic ganglia form a chain along the spinal column extending from the cervical to the lumbar regions. The postganglionic sympathetic fibres mediate their effects by releasing norepinephrine (adrenaline). There are, however, two exceptions in that the nerves stimulating the sweat glands and those causing a dilatation of the blood vessels in the skeletal muscles are both cholinergic. The final component of the sympathetic nervous system is the adrenal medulla which is derived from the same embryonic tissue as autonomic ganglia and releases adrenaline into the general circulation. In contrast, the parasympathetic ganglia are usually situated near or in the organs they innervate, so that responses tend to be more discrete, subserving local functions. Parasympathetic postganglionic fibres are cholinergic.
The brain regulates the degree of sympathetic and parasympathetic traffic by integrating a number of input signals including those from the baroreceptors (pressure), from the perception of pain and emotional stress, and from mental and physical effort. Although a rather generalised response of the sympathetic nervous system may occur with severe stress, more often the brain controls specific autonomic functions discretely.
The brain plays a key role in the control of arterial pressure by regulating cardiac output, rennin release and sympathetically mediated vasoconstriction. The sympathetic impulses that lead to vasoconstriction arise from the vasomotor centre in the medulla, which is the major site for integrating the excitatory and inhibitory inputs that determine the final level of traffic in peripheral sympathetic neurons. The vasomotor centre is under constant stimulation by fibres arising from higher centres of the brain including the hypothalamus. This tonic level of excitatory impulses is heightened by wakefulness, pain, mental and muscular effort, or emotional stress. A major inhibitory regulator of sympathetic activity is the baroreceptor system. With a rise in arterial pressure and accompanying stretch of the baroreceptors in the carotid sinus and aortic arch, the baroreceptor fibres fire more rapidly. These baroreceptor impulses travel to the medulla where they inhibit the sympathetic outflow from the vasomotor centre. Any rise in arterial pressure results in an inhibition of the resting level of sympathetic output to resistance vessels and to the heart, thereby lowering the blood pressure. This actually means that there is a self-regulating mechanism that controls blood pressure from within by a balanced interplay between pressure and the response of the system to it.
The peripheral effects of the sympathetic nervous system are diverse. The integrated cardiovascular response of arteriolar constriction (constriction of the blood vessels), tachycardia (rapid pulse rate), enhanced cardiac contractility (better pumping function of the heart), and rennin release maintains or raises arterial pressure (your “blood pressure”). Metabolic functions including lipolysis (breaking down of fat), glycogenolysis (production of sugars), and the release of antidiuretic hormone (retaining and preserving water) are stimulated by the sympathetic nervous system, as is mydriasis (widening of the eye pupil), ejaculation, and bronchodilatation (widening of the airway tubes). The sympathetic nervous system also participates in the regulation of body temperature and salivary secretion.
The peripheral effects have been classified as alpha or beta. Alpha receptors mediate vasoconstriction, pupil dilatation and relaxation of the gut. Beta adrenoreceptors are also divided into two types: beta1(mediating increased heart rate and contractibility and probably rennin release) and beta2 (mediating relaxation of bronchial, uterine and vascular smooth muscle). There is also some evidence that both alpha and beta receptors can be regulated at the membrane level, which is difficult to fit in with the general picture presented about the autonomic nervous system. For example, exposure to a stimulant may decrease the number of receptors, while conversely, exposure to antagonists may increase receptor numbers. Another anomaly within the theoretical concept of how the body functions!
The peripheral effects of the parasympathetic nervous system act on the diverse organs which the system innervates. Acetylcholine released from parasympathetic nerves results in a slow sinoatrial rate and atrioventricular conduction (slow heart rate), in a stimulation of salivary (saliva), bronchial (airway tubes), and gastric (stomach) secretions, in a contraction of the iris (eye pupil), in a stimulation of the bronchial smooth muscle contraction and stimulation of the intestinal motility, in a sustaining of the penile erection, and in a promotion of sweating with increased heat loss. Acetylcholine is also the neuromuscular transmitter for skeletal muscle as well as acting on the nicotinic type of receptors in the autonomic neurons.