The Autonomic Nervous System

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Table of Contents

Comparison between Somatic and Autonomic Nervous Systems
Control and Integration of ANS
The Divisions of the ANS
The Relationship between Structure and Function of the Two Divisions
The ANS Neurotransmitters
Bibliography

Comparison between Somatic and Autonomic Nervous Systems

The autonomic nervous system (ANS) is a division of the peripheral nervous system (PNS) that regulates the functioning of visceral organs and plays a role in homeostatic control[1]. The ANS together with the somatic nervous system (SNS) make up the PNS. Autonomic neurons control hormone and enzyme secretion from glands and the contraction of smooth and heart muscles.

The somatic division differs from the autonomic division with regard to effectors, efferent pathways, and neurotransmitter effects. The skeletal and heart muscles are the major target organs (effectors) of the impulses relayed by the somatic fibers and autonomic neurons respectively[2]. The two systems also differ in relation to their efferent pathways. In the SNS, a motor neuron (efferent pathway) extends from the CNS to the peripheral effectors. In comparison, the pre- and post-ganglionic nerves constitute the efferent pathway of the autonomic system. Thus, the SNS has only one neuron while the ANS has two. The position of the cell bodies is also varies between the two systems. The cell body of the preganglionic neuron of the ANS is located within the CNS while its axon (myelinated) joins the postganglionic motor neuron, which lies outside the CNS, through its synapse. The postganglionic axon (unmyelinated) transmits signals to the peripheral effectors. The cell bodies of somatic neurons lie within the brain or spinal cord. The ANS also contains ganglia, which are absent in the SNS. The two nervous systems also differ with regard to their neurotransmitter effects. For the SNS, the neurotransmitter released at the synaptic cleft is acetylcholine. In comparison, the autonomic impulse transmission involves acetylcholine and norepinephrine[3].The ANS plays an important role in homeostasis and thermoregulation.

Control and Integration of ANS

The functions of ANS are regulated by various brain centers. The medulla and spinal cord centers (higher centers) control the reflex activity function of the ANS. In particular, the reticular formation of the medulla plays a big role in autonomic regulation. The medulla motor centers stimulate various reflex activities, including vessel dilation (vasomotor), heart rate (cardiac), gut activity (peristalsis), and breathing (respiratory). Respiratory centers are also located in the ‘Pons’, i.e., nerve fibers that link the medulla with the midbrain. Oculomotor nuclei located in the midbrain are the centers that control pupil dilation.

Hypothalamic integration centers (lower centers) control autonomic responses through the lower and higher brain centers[4]. Parasympathetic activity (localized effects) is controlled by the anterior and medial brain regions, while sympathetic function (widespread response) is regulated by the posterior part. Sensory impulses from these centers are conveyed via the reticular formation to the ANS motor nerves (preganglionic) located in the brain and spinal cord. Body process such as heart rate, hormone secretion, peristalsis, and blood pressure/sugar levels, among others are regulated by hypothalamic centers. The centers also control emotions and biological feelings of hunger or thirst.

The limbic lobe (cerebrum), in stress situations, sends impulses to the hypothalamus, which stimulates the sympathetic division of the ANS to effect a ‘fight or flight response’[5]. Cortical centers in the ‘higher’ brain link with the limbic system to bring about conscious regulation of the ANS functioning.

The Divisions of the ANS

The ANS consists of two parts, the parasympathetic division, which controls involuntary organ function when the body is in a resting phase and the sympathetic division that is activated in a ‘fight or flight’ situation. The parasympathetic or craniosacral part consists of the cranial and sacral outflows. It controls the digestive system and is normally active when the body is resting. It maintains organ functioning at the lowest level. In contrast, the sympathetic or thoracolumbar ANS plays a role in the activation of the body systems under stressful situations or ‘flight-or-fight’ conditions[6]. It stimulates pupil dilation, sweating, high respiratory and breathing rates, and elevated blood sugar and pressure. It diverts blood to the “brain and heart and skeletal muscles” during heavy exercises or danger[7].

The functioning of the parasympathetic and sympathetic divisions involves two neurotransmitters, which include acetylcholine (cholinergic) and norepinephrine (adrenergic). Cholinergic nerves include all ANS preganglionic pathways and postganglionic fibers of the parasympathetic nerve that terminate in peripheral effectors[8]. In comparison, all adrenergic fibers are sympathetic in function.

Both divisions of the ANS serve the visceral organs in the body. Their interaction involves a mechanism of dynamic antagonism to facilitate the balance of homeostasis in the body. Either the parasympathetic or the sympathetic division exerts its effects under certain conditions only. Antagonistic interactions occur in the organs of the digestive, respiratory (lungs), and circulatory (heart) systems. The sympathetic division usually elevates respiratory activity and heart rate and slows down peristalsis in the gut[9]. In contrast, the parasympathetic activity overrides all these processes.

Sympathetic activity is prevalent in the blood vessels where they maintain vascular contraction at its basal rate. Parasympathetic fibers regulate muscular contraction (heart and smooth muscles) and hormone and enzyme secretion from glands. When frightened or under stress, the sympathetic fibers stimulate vessel constriction to increase blood flow to the organs and muscles. In contrast, the parasympathetic nerve suppresses an elevated heart rate and maintains basal levels of contraction in the urinary and digestive systems. The sympathetic nerve can counteract these effects when the body is under stress. The parasympathetic fiber stimulates all glandular organs except sweat glands and adrenal medulla[10].

Cooperative effects of the two divisions occur in the genital region. The vasodilation of arteries, which increases blood flow to the genitalia leading to penile or clitoral erection during intercourse, is a function of the parasympathetic division[11]. On the other hand, the sympathetic nerve controls ejaculation and vaginal peristalsis. The unique functions of the sympathetic nerve include the effect on the kidneys and the adrenal and sweat glands.

The Relationship between Structure and Function of the Two Divisions

The parasympathetic division consists of the cranial and sacral flows. The cranial outflow encompasses the midbrain’s oculomotor nuclei whose preganglionic neurons terminate in the ciliary muscles of the eye. It is involved in pupil constriction and the adjustment of the eye lens. The cranial outflow also contains the facial nerve whose preganglionic fibers end in the lacrimal glands. It plays a role in tearing and lubrication of the eye lens. The preganglionic fibers of facial nerve also stimulate the sub-mandibular and sublingual regions, which causes the salivary glands to release enzymes and saliva.

The sympathetic division consists of the thoracolumbar outflow that terminates at various visceral organs in the body. Its unique roles encompass metabolic control, stimulation of blood flow, and regulation of blood pressure. Sympathetic nerves occur in glandular areas, such as adrenal and sweat glands as well as in organs like the kidneys[12]. This explains why, during thermoregulation, the sympathetic nerves stimulate widespread effects (vasodilation), which allow blood to flow to the skin and cool the body through direct heat loss. When the temperature in the external environment is low, the sympathetic nerve stimulates vasoconstriction, which delivers warm blood to vital body organs. It also activates the kidneys to secrete ‘renin’ enzyme that elevates blood pressure through angiotensin activity[13].

The sympathetic nerve also has long-lasting metabolic effects. The effects are long-lived because the parasympathetic nerve cannot override them. They include elevated cellular metabolism, enhanced sugar levels in the blood, utilization of fats as metabolic substrates, and improved consciousness through the stimulation of the brain’s reticular activating system (RAS)[14].

The ANS Neurotransmitters

Impulse transmission across the synapses involves two neurotransmitters, that is, acetylcholine and norepinephrine. Acetylcholine transmits impulses between most SNS and ANS fibers. On the other hand, all sympathetic postganglionic fibers release norepinephrine apart from those innervating the genitalia, the sweat glands, and the skeletal muscle arteries and veins[15]. These neurons release acetylcholine.

Adrenaline is secreted during stressful situations. Fright causes the sympathetic nerve to stimulate the adrenal glands, which secrete adrenaline that elevates the blood pressure. This results in enhanced blood flow to the muscles leading to a prolonged ‘fight-or-flight’ response. Adrenaline also elevates cellular metabolism to produce energy for flight or fight. Enhanced mental alertness allows one to overcome stressful situations. Thus, sympathetic stimulation leads to widespread and long-lasting effects on target organs. In contrast, parasympathetic activity is often localized and temporal.

Bibliography

Janig, Wilfrid. Integrative Action of the Autonomic Nervous System: Neurobiology of Homeostasis. Cambridge: Cambridge University Press, 2008.

Marieb, Elain. Essentials of human Anatomy and Physiology. Upper Saddle River, NJ: Pearson, 2011.

Tortora, Gerald and Bryan Derrickson. Principles of Anatomy and Physiology. New York: Wiley, 2011.

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