Autonomic Nervous System

Autonomic Nervous System Definition

The autonomic nervous system (ANS) is a complex set of neurons that mediate internal homeostasis without conscious intervention or voluntary control. Cells of the ANS innervate all viscera and influence their activity locally as well as mediate global changes to the metabolic state of the organism. The ANS maintains blood pressure, regulates the rate of breathing, influences digestion, urination, and modulates sexual arousal.

Anatomy of the Autonomic Nervous System

The autonomic nervous system contains two types of neurons that interact with each other at ganglia near the spinal cord. The initial preganglionic neurons begin at the central nervous system in different parts of the spinal cord. These preganglionic neurons form synapses with postganglionic neurons at ganglia that decorate either side of the spinal cord. The postganglionic neuron forms a synapse with effector cells.

There are two main branches to the ANS – the sympathetic nervous system and the parasympathetic nervous system. The neurons of the sympathetic nervous system emerge from the thoracic and lumbar regions of the spinal cord, while the parasympathetic neurons are associated with the cranial and sacral regions. The sympathetic nervous system is usually activated in response to emergencies, especially those that threaten survival. On the other hand, the parasympathetic response is related to enhancing growth and reproduction.

Autonomic Nervous System

Autonomic Nervous System

Functions of the Autonomic Nervous System

The autonomic nervous system controls the cardiovascular system. It can alter the force and rate of heart contractility, as well as the constriction and dilation of blood vessels. Therefore, it also influences blood pressure. The rate of breathing can also be changed by the ANS. It affects both skeletal and smooth muscle fibers across the body, whether it is changing the metabolism of glucose in skeletal muscles or causing pupil dilation in the eye. The ANS can influence digestive efficiency, altering the secretion of enzymes from glands and the rate of peristaltic movement. For instance, activation of the sympathetic nervous system slows down digestion and diverts blood flow towards skeletal muscle. It can impair sexual arousal and shut down most non-essential functions of the body. On the other hand, the parasympathetic nervous system enhances digestive secretions, peristaltic movements, encourages normal cycles of circadian activity, deep sleep and activates the repair mechanisms of the body. In most cases, a physiological response by the parasympathetic nervous system is in direct opposition to the results mediated by the sympathetic nervous system. Colloquially, the sympathetic nervous system is said to influence the fight-or-flight response, and the parasympathetic nervous system is related to the feed-and-breed, or rest-and-digest responses.

Involuntary actions like sneezing, swallowing or vomiting are also controlled by the ANS. There is evidence that the autonomic nervous system not only influences sexual arousal, but also plays a crucial role in maintaining pregnancy and inducing labor. Finally, the autonomic nervous system also changes urinary output and frequency of micturition.

Examples of the Autonomic Nervous System Response

The autonomic nervous system is often described using the response to imminent physical danger and the recovery of the body after the threat has receded. For instance, when faced with a predator, the body increases heart rate and breathing, reduces digestive secretions and activity, and preferentially diverts blood towards skeletal muscles to enable the body to physically combat the challenge. This is usually accompanied by piloerection to conserve body heat. This is why the sympathetic nervous system is said to mediate the fight-or-flight response. Once the situation has become calmer, the parasympathetic nervous system restores the body towards normal functioning, resuming digestion and excretion, reducing blood pressure and restoring normal circadian rhythms.

However, even in the absence of an external threat, the two branches of the autonomic nervous system undergo changes, and interact closely with the endocrine system to minutely monitor the internal and external environment. For instance, sympathetic activation can lead to an increase in circulating plasma levels of epinephrine and norepinephrine secreted from the adrenal gland. On the other hand, hormones can alter the ANS response as well. In fertile, reproducing mammalian females, this interaction between the ANS and the endocrine systems is particularly interesting. Estrogen is involved in increasing the activity of a crucial part of the parasympathetic nervous system – the vagus nerve. Estrogen simultaneously damps sympathetic nervous system activity while progesterone appears to have the opposite effect. This can be seen in the basal level of heart rate variability (HRV). Usually, heart rate increases during inspiration and decreases during expiration. This variation is normal and is influenced by the vagus nerve. When heart rate variability decreases, it indicates reduced parasympathetic activity. In the follicular phase, under the influence of increased plasma estrogen concentrations, there seems to be an increase in parasympathetic nervous activity affecting HRV. On the other hand, during the luteal phase, HRV points towards a decrease in vagal activity, and a shift in the sympathovagal balance. The importance of these changes to the cardiovascular microenvironment is not fully understood, but it is hypothesized that this could explain the differences in the risk faced by men and women for heart disease. However, it is important to note that gross cardiovascular parameters such as blood pressure or heart rate remain mostly unaffected by the phase of the menstrual cycle due to other compensatory mechanisms.

The interaction between the ANS and cardiovascular system becomes even more important during pregnancy as there are large-scale changes to hemodynamics. Blood volume, basal oxygen consumption, red cell mass, cardiac output, and the heart rate increase in pregnant women. Both systolic and diastolic blood pressure drop and there is extensive remodeling of all blood vessels. While the changing hormonal environment primarily mediates these changes, the ANS is also an important player. Here again, HRV becomes a relatively sensitive and non-invasive measure of ANS activity. Studying the variability in heart rate of pregnant women at different gestational ages points towards an increase in vagal activity in the first trimester, coupled with a decrease in sympathetic nervous system activation. This reverses as gestational age increases, with great spikes in neural activity of the sympathetic nervous system and the release of adrenal hormones – both from the cortex and medulla – as the woman nears term.

After birth, lactation is influenced mostly by the sympathetic nervous system (SNS), since there is very little evidence of parasympathetic innervation of mammary glands. While hormones like oxytocin are important for the stimulation of milk production, activation of the SNS can alter the response of the body to the hormone. Norepinephrine can inhibit blood flow to the mammary glands, inhibit their response to oxytocin, as well as directly influence the release of the hormone from the central nervous system. While these effects have been better studied in cows, there is anecdotal evidence to support this interrelationship in humans as well.

  • Ganglion – In neurobiology, refers to a cluster of neural cell bodies consisting of soma and dendrites. Different cells within ganglia are linked to each other through synapses, different ganglia interact to form a plexus.
  • Piloerection – Involuntary bristling of hair on the body, usually in response to cold, or fear. Commonly called goosebumps.
  • Plexus – Branching network of interconnecting nerves or ganglia, that can innervate a relatively large area within the body.
  • Soma – Bulbous cell body of a neuron, containing the nucleus.

Quiz

1. Which of these statements about the autonomic nervous system is true?
A. Not under voluntary control
B. Influences the cardiovascular system
C. Interacts with the endocrine system
D. All of the above

Answer to Question #1
D is correct. The autonomic nervous system is not under voluntary control, and can activate or inhibit the functioning of nearly every visceral organ within the body. There is extensive interaction between the endocrine system and the ANS. This could be at the level of release of hormones from certain glands, or the effect of some hormones on the autonomic nervous system. Since the ANS is made of both the sympathetic and parasympathetic nerves, it can either increase or decrease every cardiovascular parameter, from blood pressure and heart rate, to stroke volume, contractility, oxygen saturation and heart rate variability.

2. Which of these statements about the vagus nerve is true?
A. Important part of the sympathetic nervous system
B. Increases heart rate variability
C. Increased activity during the third trimester of pregnancy
D. All of the above

Answer to Question #2
B is correct. Heart rate variability is the change in the periodicity of the heart depending on the phase of respiration. Normally the heart beats faster during inspiration and slower during expiration. These minute changes are mediated by the vagus nerve. When there is greater variability, it is indicative of higher vagal activity. The vagus nerve is part of the parasympathetic nervous system and has a number of roles within the body, from the respiratory and cardiovascular systems to the digestive and reproductive systems. Its activity has been found to be higher during the first trimester of pregnancy, with an increase in sympathetic nervous system activity as gestational age increases.

3. A group of surgeons is working hard on a liver transplant. The procedure lasts for over 12 hours during which time they were mostly standing, focusing deeply, and looking intently at the surgical surfaces. If it was possible to measure their physiological parameters, which of these changes would be seen in their bodies?
A. Increased conversion of glucose to glycogen in their skeletal muscles
B. Constriction of pupils
C. Changes to melatonin levels
D. All of the above

Answer to Question #3
C is correct. An extended, high-stress situation like a liver-transplant surgery would have the surgeons in a state of sympathetic nervous system activation. This would mean that their skeletal muscles would be primed for a prolonged period of activity, and therefore, would mobilize stored energy reserves. The metabolism of these cells would prefer the breakdown of glycogen to glucose to fuel their contraction. Sympathetic nervous system activation also causes the dilation of pupils, and the loss of peripheral vision, to enable focused visual stimulation. Melatonin is a hormone released by the pineal gland that affects normal cycles of sleep and wakefulness and is influenced by external light. Sympathetic nervous system activation and operating under a bright light can affect circadian rhythms and can even cause insomnia. Even after an extended and tiring procedure, the surgeons can experience an ‘adrenaline high’, which makes them wakeful and alert irrespective of whether it night or day. Therefore, changes to melatonin levels would be a very likely physiological change seen in these doctors. The sympathetic nervous system would also dampen hunger and digestive secretions, as well as reduce urinary output. These effects also aid the doctors in performing long and complicated procedures.
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