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The PaNS, as stated earlier tends to have opposite effects to the SyNS, It is most active in the body's restorative processes such as digestion. This system is dominant when the individual is non-threatened and relaxed. The activities of the PaNS tend to result in energy being conserved and stored.
Neurones of the PaNS emerge from the brainstem as part of the Cranial nerves III, VII, IX and X (Vagus nerve) and also from the sacral region of the spinal cord via Sacral nerves 2, 3 and 4.
Because of these origins the PaNS is often referred to as the 'craniosacral outflow'.
Unlike the SyNS the PaNS preganglionic neurones synapse with only a relatively few
postganglionic neurones For this reason they are much more precise and localised in their
effects.
In the PaNS both pre- and postganglionic neurones are cholinergic (i.e. utilise the neurotransmitter acetylcholine ) Unlike adrenaline and noradrenaline, which the body takes around 90 minutes to metabolise, acetylcholine is rapidly broken down after release by the enzyme cholinesterase. As a result the effects are relatively brief in comparison to the SyNS.
The neurotransmitters used by the PaNS are targeted at various specific receptors in the effector organs. The receptors are usually proteins with which the neurotransmitters can combine which results in some change within the plasma membrane of the effector organ cells causing the desired effect ( e.g. release of hormones, muscular relaxation or contraction etc ).
An understanding of this mechanism is quite important clinically as certain drugs can have effects upon the receptors which may either mimic, enhance, diminish or prevent the effects of ANS neurotransmitters.
These effects may be desired for the effective treatment of the patient or may provide unwanted side effects.
Acetylcholine activates two types of cholinergic receptors namely muscarinic and nicotinic (which has already been mentioned in the section on the SyNS).
Muscarinic receptors are so called because they are activated by muscarine which is a poisonous drug found in certain toadstools.
Nicotinic receptors are activated by nicotine another poisonous drug found in tobacco. In fact many of the effects of tobacco smoking can be explained by the effects of nicotine.
Muscarinic receptors are found in all effector cells stimulated by PaNS nerves and also in those effector organs stimulated by cholinergic SyNS neurones (e.g. most sweat glands).
Nicotinic receptors are found in the postganglionic
neurones of both .the PaNS and the SyNS. They are also present, in skeletal muscle and in
the hormone producing cells of the adrenal medulla.
Last updated on Wednesday, 07 April 1999 11:04 +0100
