Neurotransmitters in the autonomic nervous system are divided into cholinergic neurotransmitters with the neurotransmitter acetylcholine being the major neurotransmitter in this group of neurotransmitters. The other type of neurotransmitters in the autonomic nervous system are the adrenergic neurotransmitters and include the two neurotransmitters norepinephrine and epinephrine.
Acetylcholine usually functions by stimulating the parasympathetic nervous system such as the stimulation of the gastrointestinal motility of the stomach and intestinal muscles. Thus increasing the propulsion of food in the intestinal lumen. This neurotransmitter has two types of receptors in the cells of organs in the body especially glands and smooth muscles.
These receptors are termed nicotinic receptors and muscarinic receptors. Binding of acetylcholine to nicotinic receptors causes depolarization of the cell with the result of contraction in the case of muscle or transmission of nerve signals in the case of neurons.
Muscarinic receptors can be excitatory or inhibitory depending on the location of the cell in the body. For example, the binding of acetylcholine to muscarinic receptors in the sphincter of the gastrointestine leads to hyperpolarization of the affected cells with an inhibitory effect that relaxes the muscles of the sphincter.
On the other hand, binding of acetylcholine to muscarinic receptors in the circular muscle of the iris of the eye has an excitatory effect which leads to the contraction of the muscle of the iris.
Neurons that secrete acetylcholine are present in the sympathetic and parasympathetic preganglionic neurons, in addition to their presence in sympathetic postganglionic neurons which innervate the sweat glands. also it is secreted by all parasympathetic postganglionic neurons.
Acetylcholine induces contraction of of smooth muscles by binding to specific receptors that are probably excitatory muscarinic receptors. This process opens Na+/Ca++ ion channels in the cell membrane and leads to the inflow of calcium ions to the inside of the cell. This in turn induces the contraction of the muscle.
Receptors for the neurotransmitter acetylcholine are integral membrane proteins which are located within the cellular membrane of the postsynaptic neurons of the affected glands. Acetylcholine does not function by penetrating the phospholipid bilayer due to its polar structure. Instead it binds to receptors on the cellular membrane and it exerts its effect by openning voltage gated ion channels mostly Na+/Ca++ ion channels. This process leads to the enterance of calcium ions into the cell with the result of muscle contraction.
This process is true for processes which occur in excitatory muscarinic receptors. Acetylcholine is released into the synaptic cleft from vesicles inside the neurons by a process that is called exocytosis. Its half life in the synaptic cleft is very short after which it is deactivated by the specific enzyme which is called acetylcholine esterase.
This enzyme is a pharmacologic traget for many drugs that are used to increase the effect of acetylcholine in the synaptic cleft. One type of chemical warfare agent uses this type of effect of inhibiting the action of acetylcholine in order to kill enemies soldiers. This can lead to the death of affected people by this chemical.
The other type of neurotransmitter in the autonomic nervous system is the adrenergic neurotransmitters. The two types of this kind of neurotransmitters are epinephrine and norepinephrine. These two neurotransmitters usually are secreted by the sympathetic division of the autonomic nervous system. This process leads to the shutting down of the parasympathetic nervous system.
Norepinephrine is synthesized in the neurons and is released by exocytosis into the synaptic cleft where it exerts its effect by binding to adrenergic receptors causing either excitatory or inhibitory effects. Norepinephrine can be either secreted as a neurotransmitter from neurons or as a hormone from the adrenal medulla.
In both cases it stimulates the sympathetic nervous system causing symptoms such as those occuring in stressful conditions in which there is increased heart beat rate and hypertension due to the constriction of blood vessels by the effect of norepinephrine on blood arteries. It also causes widening of the pupils of the eyes and increases alertness. The life time of norepinephrine and epinephrine in the synaptic cleft is usually longer than that of acetylcholine. It is deactivated by either being reuptaken by the neurons that secreted them or it can be metabolized by the enzyme catechol-O- methyltransferase and the enzyme mono-amine-oxidase.
