Plexus
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A plexus is a network. In biology the term plexus has multiple meanings:
Nervous system
In many animals the processes of neurons join together to form a plexus or nerve net.
In vertebrates
In vertebrates, a plexus is an area where nerves branch and rejoin. The electrical signals do not mix - rather, the fibres travel together with their electrical signals separate. The brachial plexus is an example. It is made up of the spinal nerves which enter the upper limb.
Almost a hundred such plexuses have been described in the human body, but the four primary nerve plexuses are the cervical plexus, brachial plexus, lumbar plexus, and the sacral plexus.
In invertebrates
Plexuses is the characteristic form of nervous system in the coelenterates and persists with modifications in the flatworms. The nerves of the radially symmetric echinoderms also take this form, where a plexus underlies the ectoderm of these animals and deeper in the body other nerve cells form plexuses of limited extent.
Circulatory system
A plexus is also a network of blood vessels. The choroid plexus of the brain is an example.
In a vascular plexus, the contents of the vessels mix. A plexus allows blood to flow via multiple routes. If one branch of the plexus is obstructed, the blood may flow via the open branches.
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