Brachial plexus palsy. Torticollis (Wryneck)
OK, we’re going to use this question as an excuse to NAIL DOWN THE NEURAL FREAKING ANATOMY OF THE ARM!!!
First, the plexus itself.
Each point (C5-T1) gives off a root, we need to track what happens to these roots and understand how the terminal nerves are derived from them. We shall review each root in turn:
Roots
C5
C6
C7
C8
T1
Dorsalscapular nerve splits off C5 (innervates rhomboids major and minor and levator scapulae - causes scapula alata - winged scapula) Long Thoracic nerve splits off C5-C7 (innervates serratus anterior - susceptible to damage from trauma, often a blow to the ribs. Can lead to winged scapula also).
Trunks
C5 + C6 = Upper Trunk
C7 = Middle Trunk
C8 + T1 = Inferior Trunk
Suprascupluar nerve splits off upper trunk (innervates supraspinatus and infraspinatus muscles and sensory information of the top of the shoulder - can be damaged due to damage of the shoulder, limits rotation and abduction of the arm).
Subclavius nerve splits off upper trunk (innervates subclavius muscle)
Devisions
Each trunk now divides into two, Anterior and Posterior. The Anterior divisions of Upper Trunk and Middle Trunk join to form the Lateral Cord, while all of the Posterior divisions from all three trunks come together to form the Posterior Cord. The Anterior division of the Lower trunk stays separate as the Medial Cord.
Anterior divisions go on to innovate muscles of the Anterior (Flexor) group, and Posterior divisions go on to innovate muscles of the Posterior (extensor) group.
No nerves split off divisions
Cords
Lateral Cord - Anterior divisions of upper and middle trunk
Posterior Cord - Posterior divisions of upper, middle and lower trunk
Medial Cord - Anterior division of lower trunk
NB! The terms lateral, posterior and medial relate to the cords orientation to the Axillary artery.
Lateral Pectoral nerve splits off from Lateral cord (innervates pectoralis major)
Upper, middle and lower subscapular nerve split off from Posterior cord (innervates subscapularis and teres major)
Medial Pectoral nerve splits off from Medial cord (innervates pectoralis minor and major)
Medial brachial cutaneous nerve splits off from Medial cord (sensory innervation of medial brachial side of the arm)
Medial Antebrachial cutaneous nerve splits off from Medial cord (provides sensory innervation of the medial antebrachial region)
Terminal branches
The three cords go on to form five terminal branches, these are: