The differences in the two divisions of the autonomic nervous system are not limited to structure. The neurochemistry of the two pathways is also quite distinct. Lets have a look at these differences by comparing the neurotransmitters released by the pre- and postganglionic neurones of both divisions as well as the types of postsynaptic receptor that mediate their effects.
A.Sympathetic Division
Sympathetic preganglionic neurones utilise acetylcholine (ACh) as their neurotransmitter and the postsynaptic effects of this neurotransmitter are mediated by the nicotinic acetylcholine receptor. The nicotinic acetylcholine receptor is a ionotropic receptor which results in depolarisation of the postsynaptic membrane by opening of a non-selective cation channel. The resultant EPSP is often large enough to cause an action potential in the postganglionic neurone.
Sympathetic postganglionic neurones use noradrenaline (called norepinephrine by citizens of the US of A) as their principal neurotransmitter. The receptors for noradrenaline (known as adrenergic receptors) are located on the plasma membrane of the cells innervated by the postganglionic neurones. These features are summarised in the diagram below.
There are five different types of adrenergic receptor and whilst the regional variation in expression of these can be confusing for the student of physiology, they have turned out to be very useful clinically. Adrenergic receptors are all metabotropic receptors whose intracellular effects are produced by G-protein-mediated upregulation of a number of second messengers. The distribution of some adrenergic receptors as well as their effects are summarised in the table below.
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Adrenergic
Receptor Types in the Autonomic Nervous System.
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Receptor
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Location
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Effect of
Activation
|
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α1
|
Smooth muscle of blood vessels in skin |
Contraction (vasoconstriction). |
| Smooth muscle in sphincters of urogenital tract. | Contraction (urine retention). | |
| Smooth muscle in sphincters of digestive tract. | Contraction (inhibition of defecation). | |
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α2
|
Smooth muscle of digestive tract. | Decrease motility. |
| Secretory glands of digestive system. | Decrease in secretion. | |
| Pancreas. | Decrease in insulin secretion. | |
|
β1
|
Cardiac muscle. | Increase in rate & force of contraction. |
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β2
|
Smooth muscle of blood vessels in heart. | Relaxation (vasodilation). |
| Smooth muscle of blood vessels in skeletal muscle. | Relaxation (vasodilation). | |
| Smooth muscle of airways | Dilation (increased flow). | |
| Smooth muscle of digestive tract. | Decrease motility. | |
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β3
|
Adipose tissue. | Lipolysis (fatty acid release). |
B. Parasympathetic Division
The preganglionic neurones of the parasympathetic division have exactly the same neurochemistry as those of the sympathetic division. They utilise ACh as their neurotransmitter and it exerts its effects on the postganglionic neurone by binding to nicotinic acetylcholine receptors which open ion channels. The postganglionic neurones of the parasympathetic also utilise ACh as their neurotransmitter but the effectors cells express muscarinic acetylcholine receptors which are functionally distinct from the nicotinic receptors. Like adrenergic receptors, muscarinic ACh receptors are metabotropic and thus mediate their effects by an intracellular second messenger system. These features are summarised in the diagram below.
There are three major types of muscarinic Ach receptors and their distribution and effects are summarised in the table below.
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Muscarinic
Acetylcholine Receptor Types in the Autonomic Nervous System.
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Receptor
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Location
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Effect of
Activation
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M1
|
Gastric glands of stomach. | Increased secretion. |
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M2
|
Cardiac muscle. | Decrease in rate of contraction. |
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M3
|
Smooth muscle of digestive tract. | Contraction. |
| Salivary glands. | Increased secretion. | |
Note that the names for the two different types of acetylcholine receptor is due to the fact that they were originally distinguished by two separate drugs. Nicotine (a toxin which can be isolated from tobacco) selectively activates nicotinic receptors whilst muscarine (a chemical present in poisonous mushroom, Amanita muscaria) selectively activates muscarinic receptors. It is important to realise that whilst these exogenous drugs can be used to distinguish between the two types of receptor, acetylcholine activates them both under physiological circumstances.
It is also interesting to note that the preganglionic neurones of the both the sympathetic and parasympathetic divisions utilise an ionotropic receptor (characterised by fast synaptic transmission and short duration of action) the postganglionic neurones employ metabotropic receptors (typically slow in onset but with a much longer duration of action).