The Brachial Plexus

The Brachial plexus is a group of somatic nerves that innervate and receive sensory input from the majority of the upper limb.

Gross anatomy

Roots

There are 5 roots involved in the brachial plexus:C5, C6, C7, C8 and T1.  There are several points of interest relating to the nerve roots.

  • Firstly, C5-C7 all leave the spinal cord above the level of corresponding vertebrae.  Spinal nerve C8 exits below vertebra C7 (there is no C8 vertebra), and from then on (including T1 for the purpose of the brachial plexus) spinal nerves exit at below the corresponding vertebral level.
  • Secondly, T1 commonly splits into two.  The larger branch enters the brachial plexus (C5-8 do not encounter ribs) while the small becomes the first intercostal nerve (to innervate the muscles of inspiration)
    • C5-8 pass between the gap between the anterior and middle scalene muscles, with the subclavian artery inferiorly.  The subclavian commonly lies in front of (and covers) T1 before it joins the plexus.

Trunks

The brachial trunks are formed shortly after the roots exit the intervertebral foramina in the inferior part of the neck.

  • Superior (upper)trunk- the union of C5 and C6
  • Middle trunk– a continuation of C7
  • Inferior (lower) trunk– the union of C8 and T1

NB Injury to these can occur.

    • Erb’s Palsy is a paralysis of the arm- specifically due to damage to the superior trunk, most commonly seen in babies who have had a difficult delivery (usually very large babies) for which forceps or other assistive measures have been required.  The baby doesn’t move the affected arm, reflexes (including the Moro response) are absent or abnormal and the baby may have an associated Horner’s syndrome or respiratory problems.  This is usually self-limiting and will resolve in several weeks.
    • Klumpke’s palsyis caused in similar circumstances but the damage is to the lower trunk.  This causes a claw hand, loss of grip reflex and weakness of the hand muscles.  There also can be associated Horner’s/respiratory problems.

Divisions

Each trunk then divides into an anterior and posterior division as the plexus passes through the cervicoaxillary canal posterior to the clavicle.

  • Anterior divisions will go on to supply the anterior compartments of the arm (flexor group)
  • Posterior divisions will go on to supply the posterior compartments of the arm (extensor groups)

Cords

The cords pass over the 1st rib close to the dome of the lung and continue under the clavicle, and surround the axillary artery (they are named in relation to this artery.

  • Lateral cord– made up of the anterior divisions of the superior and middle trunks
  • Medial cord– continuation of the anterio division of the inferior trunk
  • Posterior cord- made up of the posterior divisions of all three trunks.

Named nerves

NB NOT ALL nerves arise from the cords of the brachial plexus- some originate at the root, trunk and division level.

Broadly speaking, the nerves arising from the brachial plexus can be split into supraclavicular branches and infraclavicular branches depending on whether they arise above or below the clavicle (respectively).

  • Supraclavicular branches
    • Dorsal Scapular
    • Long Thoracic
    • Suprascapular
    • Subclavian nerve (nerve to subclavius)
  • Infrascapular branches
    • Lateral pectoral
    • Musculocutaneous
    • Median
    • Middle Pectoral
    • Medial cutaneous nerve of arm
    • Median cutaneous nerve of forearm
    • Ulnar
    • Upper and lower subscapulars
    • Thoracodorsal
    • Axillary
    • Radial

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