A plexus is a part of nervous system

Nervous system

The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

. Plexus has a slightly different definition in vertebrates and in invertebrates.

In vertebrates

In vertebrates, a plexus is an area where nerve

Nerve

A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...

s branch and rejoin. The electrical signals do not mix; rather, the fibres travel together with their electrical signals separate. The brachial plexus

Brachial plexus

The brachial plexus is a network of nerve fibers, running from the spine, formed by the ventral rami of the lower four cervical and first thoracic nerve roots...

 is an example. It is made up of the spinal nerves which enter the upper limb.

Almost a hundred such plexi have been described in the human body, but the four primary nerve plexi

Nerve plexus

A nerve plexus is a network of intersecting nerves. Except for the ventral rami of Th2-Th11 nerves, they combine sets of ventral rami of spinal nerves that serve the same area of the body into one large grouped nerve...

 are the cervical plexus

Cervical plexus

The cervical plexus is a plexus of the ventral rami of the first four cervical spinal nerves which are located from C1 to C4 cervical segment in the neck. They are located laterally to the transverse processes between prevertebral muscles from the medial side and vertebral from lateral side...

, brachial plexus

Brachial plexus

The brachial plexus is a network of nerve fibers, running from the spine, formed by the ventral rami of the lower four cervical and first thoracic nerve roots...

, lumbar plexus, and the sacral plexus

Sacral plexus

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.

The choroid plexus

Choroid plexus

The choroid plexus is a structure in the ventricles of the brain where cerebrospinal fluid is produced...

 is a part of central nervous system

Central nervous system

The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

 in the brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 and it consist of capillaries, ventricles and ependymal cells.

In invertebrates

The plexus is the characteristic form of nervous system in the coelenterates and persists with modifications in the flatworm

Flatworm

The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...

s. The nerves of the radially symmetric echinoderm

Echinoderm

Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

s also take this form, where a plexus underlies the ectoderm

Germ layer

A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...

of these animals and deeper in the body other nerve cells form plexi of limited extent.