The Autonomic Nervous System

The autonomic nervous system regulates individual organ function and homeostasis. It is on the most part not subject to voluntary control and is also known as the visceral or the automatic system.

Functions of the Autonomic Nervous System

The autonomic nervous system (ANS) transmits impulses from the central nervous system to the peripheral organ system. Some of its effects include:

• Control of the heart rate,
• Constriction and dilation of blood vessels,
• Contraction and relaxation of smooth muscles in organs,
• Visual accommodation,
• Secretion from exocrine and endocrine glands, and
• Control of pupil size

For example if a fiber was to get in contact with a lung, it may cause contraction in the smooth muscles such as blood vessels. This may cause a reflex reaction in the heart and even induce simple reflexes like coughing and vomiting.

Anatomy of the Autonomic Nervous System

The ANS is divided into two parts called the parasympathetic and sympathetic systems. This is on the basis of anatomical and functional differences.

Most organs have fibers from both parts of the ANS, and their influence is usually opposing. For example the vagus slows the heart whilst the sympathetic nerves increase the rate. They may also be parallel, for example the functioning of salivary glands.

The Parasympathetic System

The parasympathetic system is concerned with conservation and restoration of energy, as it causes a reduction in heart rate and blood pressure, and facilitates digestion and absorption of nutrients, and consequently the excretion of waste products.

The preganglionic outflow of the parasympathetic nervous system arises from the cell bodies of the motor nuclei of the cranial nerves III, VII, IX and X in the brain stem and from the second, third and fourth sacral segments of the spinal cord. It is therefore also known as the cranio-sacral outflow.

The cranial nerves III, VII and IX affect the pupil and salivary gland secretion, whilst the vagus nerve (X) carries fibers to the heart, lungs, stomach, upper intestine and urethra. The sacral fibers form pelvic plexuses which innervate the distal colon, rectum, bladder and reproductive organs.

Sympathetic Nervous System

The sympathetic system enables the body to be prepared for fear, flight or fight. Sympathetic responses include an increase in heart rate, blood pressure and cardiac output, a diversion of blood flow from the skin and splanchnic vessels to those supplying skeletal muscle, increased pupil size, bronchiolar dilation, contraction of sphincters and metabolic changes such as the mobilization of fat and glycogen.

It may involve the production, storage and recognition of adrenaline, noradrenalin, nicotine and dopamine. For example, the adrenal medulla responds to nervous impulses in the sympathetic cholinergic preganglionic fibers by transforming the neural impulses into hormonal secretion. In situations involving physical or psychological stress, much larger quantities are released.

The parasympathetic and sympathetic system work in order to monitor and regulate our reactions to stimuli. They usually work in opposition to each other to give the body proper balance. For example the sympathetic stimulation with increase heart rate while parasympathetic stimulation will decrease heart rate.

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