The designation brachial plexus derives from the latin translation of the greek term (βραχιόνιο πλέγμα) describing the network of nerves of the arm. Both latin ang greek terms refer to the elaborate network( namely Plexus or πλεγμα ) of nerves responsible for the innervation of the regions of the neck, shoulder, arms and hands. Furthermore, all the impulses concerning sensibility, pain and self-perception of the arms are conducted through these network of nerves and are brought to the corresponding brain centers. Therefore, the nerves originating from the spinal cord, are called mixed nerves meaning that they contain both afferent and efferent axons, and thus conduct both incoming sensory information and outgoing muscle commands in the same bundle. There are also pure motor nerves that originate as individual branches from the brachial plexus and are responsible for the motor stimulation of specific muscles.
The brachial plexus is composed of nerve roots that exit through hollow spaces (the spinal canal) between the vertebrae of the cervical spine. Specifically, it is formed formed by the lower four cervical ( C for cervical) and first thoracic ( T for thoracic) nerve roots (C5-C8, T1). When the C4 cervical root contributes to the formation of the brachial plexus, then the plexus is characterized as a Prefixed Brachial Plexus whereas when the T2 thoracic root contributes to its formation, the plexus is characterized as a Postfixed Brachial Plexus. As soon as the nerves of the spinal cord exit the intervertebral foramen (hollow spaces between the vertebrae ), they merge and form the so called trunks. There is an superior trunk ( C5-C6) , middle ( C7) and an inferior trunk ( C8-T1) (Truncus superior, Truncus medius and inferior ). Each trunk will then split into two, to form six divisions : three anterior and similarly three posterior divisions of the upper, middle and inferior trunks. The posterior divisions join each other and form the posterior fascicle.