The Autonomic Nervous System (ANS) Controls the Body's Internal Environment in a Coordinated Manner

• The ANS helps control the heart rate, blood pressure, digestion, respiration, blood pH and other bodily functions through a series of complex reflex actions
• These controls are done automatically, below the conscious level
• To exert this control the activities of many different organs must be coordinated so they work to accomplish the same goal
• In the ANS there are 2 nerves between the central nervous system (CNS) and the organ. The nerve cell bodies for the second nerve are organized into ganglia:
  • CNS -> Preganglionic nerve -> Ganglion -> Postganglionic nerve -> Organ
• At each junction neurotransmitters are released and carry the signal to the next nerve or organ.

The ANS has 2 Divisions, Sympathetic and Parasympathetic, Which Differ in Anatomy and Function

• The sympathetic nerves come from the thoracic and lumbar regions of the spinal cord. The preganglionic nerves are short and synapse in paired ganglia adjacent to the spinal cord
  • The adrenal medulla, technically an endocrine gland, is functionally a part of the sympathetic nervous system
    • Adrenal medulla is like a modified autonomic ganglion + postsynaptic nerve (see diagram below)
• Parasympathetic nerves come from the cranial and sacral regions of the CNS. They have long preganglionic nerves which synapse at ganglia near or on the organ innervated
• The 2 branches use the same transmitter, acetylcholine (ACh), in the ganglia. At the organ quite different transmitters are used:
  • Parasympathetic = ACh
  • Sympathetic = norepinephrine (NE). Note: norepinephrine is the same thing as noradrenaline.
    • Note: there are a few sympathetic nerves where the transmitter at the organ is ACh
    • Sympathetic fibers releasing ACh: sweat glands, piloerector muscles, some blood vessels
• Comparison of the 2 systems:

	Anatomical Location	Preganglionic Fibers	Postganglionic Fibers	Transmitter (Ganglia)	Transmitter (Organs)
Sympathetic	Thoracic/ Lumbar	Short	Long	ACh	NE
Parasympathetic	Cranial/ Sacral	Long	Short	ACh	ACh

• The autonomic nervous system is further complicated by existence of multiple types of neurotransmitter receptors. Click for a more detailed discussion of autonomic receptors.

The Sympathetic is the "Fight or Flight" Branch of the ANS

• Emergency situations, where the body needs a sudden burst of energy, are handled by the sympathetic system
• The sympathetic system increases cardiac output and pulmonary ventilation, routes blood to the muscles, raises blood glucose and slows down digestion, kidney filtration and other functions not needed during emergencies
• Whole sympathetic system tends to "go off" together
• In a controlled environment the sympathetic system is not required for life, but it is essential for any stressful situation

The Parasympathetic is the "Rest and Digest" Branch of the ANS

• The parasympathetic system promotes normal maintenance of the body- acquiring building blocks and energy from food and getting rid of the wastes
• It promotes secretions and mobility of different parts of the digestive tract.
• Also involved in urination, defecation.
• Does not "go off" together; activities initiated when appropriate
• The vagus nerve (cranial number 10) is the chief parasympathetic nerve
• Other cranial parasympathetic nerves are: III (oculomotor), VII (facial) and IX (glossopharyngeal)

The Hypothalamus Has Central Control of the ANS

• The hypothalamus is involved in the coordination of ANS responses,
• One section of the hypothalamus seems to control many of the "fight or flight" responses; another section favors "rest and digest" activities

The Adrenal Medulla is an Extension of the Sympathetic Nervous System

• The adrenal medulla behaves like a combined autonomic ganglion and postsynaptic sympathetic nerve (see diagram above)
• Releases both norepinephrine and epinephrine in emergency situations
  • Releases a mixture of epinephrine (E = 80%) and norepinephrine (NE = 20%)
  • Epinephrine = adrenaline
• This action is under control of the hypothalamus

Comparison of the Actions if the Sympathetic & Parasympathetic Systems (see table)

• Usually (but not always) both sympathetic and parasympathetic nerves go to an organ and have opposite effects
• You can predict about 90% of the sympathetic and parasympathetic responses using the 2 phrases: "Fight or Flight" and "Rest and Digest".
• Special cases:
  • Occasionally the 2 systems work together: in sexual intercourse the parasympathetic promotes erection and the sympathetic produces ejaculation
  • Eye: the sympathetic response is dilation and relaxation of the ciliary muscle for far vision (parasympathetic does the opposite)
  • Urination: the parasympathetic system relaxes the sphincter muscle and promotes contraction of muscles of the bladder wall -> urination (sympathetic blocks urination)
  • Defecation: the parasympathetic system causes relaxation of the anal sphincter and stimulates colon and rectum to contract -> defecation (sympathetic blocks defecation)

Organ	Parasympathetic Response "Rest and Digest"	Sympathetic Response "Fight or Flight"
Heart (baroreceptor reflex)	Decreased heart rate Cardiac output decreases	Increased rate and strength of contraction Cardiac output increases
Lung Bronchioles	Constriction	Dilation
Liver Glycogen	No effect	Glycogen breakdown Blood glucose increases
Fat Tissue	No effect	Breakdown of fat Blood fatty acids increase
Basal Metabolism	No effect	Increases ~ 2X
Stomach	Increased secretion of HCl & digestive enzymes Increased motility	Decreased secretion Decreased motility
Intestine	Increased secretion of HCl & digestive enzymes Increased motility	Decreased secretion Decreased motility
Urinary bladder	Relaxes sphincter Detrusor muscle contracts Urination promoted	Constricts sphincter Relaxes detrusor Urination inhibited
Rectum	Relaxes sphincter Contracts wall muscles Defecation promoted	Constricts sphincter Relaxes wall muscles Defecation inhibited
Eye	Iris constricts Adjusts for near vision	Iris dilates Adjusts for far vision
Male Sex Organs	Promotes erection	Promotes ejaculation

More Information

The autonomic nervous system is described on Eric Chudler's webpage.

Frank Vincenzi of the University of Washington has created a Virtual Lab for studying autonomic drugs such as acetylcholine, epinephrine, atropine. These are among the drugs most often involved in medicine.

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