The Autonomic Nervous System (ANS) is part of the Peripheral nervous system. It is an efferent nervous system, used in the control of major organs and homeostasis.
There are 2 branches of the ANS:
Structure of the ANS
The ANS is a "2-neurone" system, with 2 neurones in series, separated by the autonomic ganglia.
- Preganglionic Neurone has its cell body in the Central Nervous System, and synapses at the autonomic ganglia.
- Postganglionic Neurone has its cell body in the autonomic ganglion, and synapses on the target organ.
In the parasympathetic nervous system the autonomic ganglia are close to the target organ (i.e. the preganglionic neurone is much longer than the postganglionic).
In the sympathetic nervous system the autonomic ganglia are located in the "sympathetic chain", a group of cell bodies close to the spinal cord. The postganglionic neurone is much longer than the preganglionic.
These have their cell bodies in the central nervous system, more specifically within the spinal cord. The sympathetic and parasympathetic nervous systems have their origins in different areas of the spinal cord.
- Sympathetic - all preganglionic neurones begin in the thoraco-lumber region of the spinal cord.
- Parasympathetic - preganglionic neurones begin either in the cranial or sacral region. Generally the neurones innervating the upper body and head begin in the cranial region, with those innervating the lower body beginning in the sacral region.
Function of the ANS
The classical view is that the two branches of the ANS (sympathetic and parasympathetic) oppose each other. This is true to an extent, but is not always the case.
- In many organs the different branches have opposing effects, e.g. on the heart and GI tract.
- In some organs the different branches have the same effect - e.g. both parasympathetic and sympathetic stimulation causes an increase in salivary gland secretion.
- Some organs are only innervated by one branch of the ANS (e.g. blood vessels are only innervated by sympathetic)
There are 2 main neurotransmitters in the Autonomic Nervous System:
- Acetylcholine (ACh)
- Noradrenaline (NA) (Norepinephrine in america)
ACh is released by ALL preganglionic neurones at the autonomic ganglia in BOTH the sympathetic and parasympathetic branches. ACh is also released by the postganglionic neurones at the target organ in most parasympathetic neurones. NA is released by MOST postganglionic neurones in the sympathetic nervous system.
Synthesis and Storage
Neurotransmitters are stored in the transmitter cell (the presynaptic cell), and are stored in small membrane-bound organelles called Vesicles.
The precursor molecules are usually actively transported into the neuronal cell for the synthesis of the transmitter molecule.
When stimulated by electrical signals in the axon of the neurone, the vesicles fuse with the plasma membrane.
The transmitter is then released into the synaptic cleft, and passes via diffusion to the target cell, which it activates, giving the response.
Release can be regulated in 2 ways:
- Homotrophic - transmitter inhibits its own release (negative feedback)
- Heterotrophic - a transmitter released from a different type of neurone inhibits release (e.g. ACh inhibits release of NA)
It is important that the transmitter is rapidly removed so that the signal does not continue to be made. The transmitter can either be reabsorbed into the presynaptic cell to be reused, or broken down by the postsynaptic cell into its precursors.
Some neurotransmitter will "overflow" from the synaptic cleft and be lost into the tissue fluid.
There are other transmitters involved in the ANS.
- Co-transmitters are released along with ACh or NA, and can modulate the strength or duration of the signal
- Non-Adrenergic Non-Cholinergic nerves release other transmitters, including Nitric Oxide, VIP and NPY.