Question

Which neurotransmitters are used for each division of the autonomic nervous system? Be sure to describe the synapse at the autonomic ganglion and at the effector. What component of the system determines if a given neurotransmitter has an excitatory or inhibitory effect on the target effector?

Answer 1

Answer:

The autonomic nervous system is in charge of controlling visceral effectors. Traditionally, it is described by its peripheral nervous components (ganglia, nerves and plexuses) and two divisions are distinguished: the sympathetic and the parasympathetic. Transmission of the excitatory stimulus through the synaptic cleft occurs by release of neurotransmitters; the neurotransmitters of the sympathetic and parasympathetic nervous system are mainly norepinephrine (NA) and acetylcholine (AC). The NA-secreting fibers are called adrenergic and those that secrete AC, cholinergic. All preganglionic neurons, both those of the sympathetic nervous system and those of the parasympathetic nervous system, are cholinergic. The neuron that releases the neurotransmitter is called a presynaptic neuron. The signal receptor neuron is called a postsynaptic neuron. Depending on the type of neurotransmitter released, postsynaptic neurons are either stimulated (excited) or de-stimulated (inhibited).

Explanation:

The autonomic nervous system is the part of the central and peripheral nervous system that is responsible for the regulation of the involuntary functions of the organism, the maintenance of internal homeostasis and the adaptive responses to variations in the external and internal environment  and two divisions are distinguished: the sympathetic and the parasympathetic. Acetylcholine is the preganglionic neurotransmitter of both divisions of the S.N.A. (sympathetic and parasympathetic) and also of the postganglionic neurons of the parasympathetic. The nerves at whose endings acetylcholine are released are called cholinergic. Norepinephrine is the neurotransmitter of postganglionic sympathetic neurons. The nerves into which norepinephrine is released are called adrenergic. Within the efferent sympathetic impulses, the postganglionic neurons that innervate the eccrine sweat glands and some blood vessels that supply the skeletal muscles are of the cholinergic type. Both acetylcholine and norepinephrine act on the different organs to produce the corresponding parasympathetic or sympathetic effects. The peripheral nerve endings of the sympathetic form a reticulum or plexus from which the terminal fibers come in contact with the effector cells. All the norepinephrine in peripheral tissues is found in the sympathetic endings in which it accumulates in subcellular particles analogous to the chromaffin granulations of the adrenal medulla. The release of norepinephrine at nerve endings occurs in response to action potentials that travel through nerve endings. The receptor, when stimulated by catecholamines, sets in motion a series of membrane changes that are followed by a cascade of intracellular phenomena that culminate in a measurable response. There are two classes of adrenergic receptors known as alpha and beta. These two classes are again subdivided into others that have different functions and that can be stimulated or blocked separately. Norepinephrine primarily excites alpha receptors and beta receptors to a small extent. The neurotransmitter acetylcholine is synthesized at the axonal terminal and deposited in synaptic vesicles. Acetylcholine activates two different types of receptors, called muscarinic and nicotinic receptors. Acetylcholine (AC) synthesis takes place at presynaptic termination by acetylation of choline with acetyl-coenzyme A, a reaction catalyzed by acetylcholinetransferase. The energy required for the release of a neurotransmitter is generated in the mitochondria of the presynaptic terminal. Binding of neurotransmitters to postsynaptic membrane receptors produces changes in membrane permeability. The nature of the neurotransmitter and the receptor molecule determines whether the effect produced will be one of excitation or inhibition of the postsynaptic neuron.