Long Thoracic Nerve
The long thoracic nerve is also known as the external respiratory nerve of bell or posterior thoracic nerve, It arises from the superior trunk of the brachial plexus after which it descends posterior to the brachial plexus and anterior to the posterior scalene muscle, passes over the 1st rib, descends along the lateral aspect of the thoracic wall at the mid-axillary line for a distance of approximately 22-24 cm. This long and superficial course of the nerve makes is susceptible to various injuries. 
Anterior Ramii of C5, C6, C7
In a small percentage of population (approximately 10%), the long thoracic nerve receives supply from C8 nerve root as well.
The long thoracic nerve is a motor nerve that supplies the Serratus anterior muscle which is responsible for shoulder protraction (during punching) and upward rotation of scapula during lifting.
Injury to the Long thoracic nerve
Injury to long thoracic nerve can cause paralysis of the serratus anterior muscle leading to a phenomenon called Winging of scapula.
There are various causes for injury to long thoracic nerve but they can be mainly divided into 3 categories i.e. Non-traumatic, traumatic and Iatrogenic.
The causes of non-traumatic injury to long thoracic nerve include Viral illness (influenza), Tonsillitis-bronchitis, Poliomyelitis, allergic drug reactions and muscular dystrophy (facio-scapulohumeral dystrophy).
The causes of traumatic injury include sudden scapular depression, repetitive arm movements as seen in athletes and house-hold activities such as digging, car washing or prolonged lying in bed with arm abducted and propping up head to read.
The causes of Iatrogenic injury include use of a single axillary crutch, mastectomies with axillary node dissection and spontaneous pneumothorax.
 Lung K, Lui F. Anatomy, Thorax, Long Thoracic Nerve. [Updated 2020 Jul 31]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK535396/
 Martin RM, Fish DE. Scapular winging: anatomical review, diagnosis, and treatments. Curr Rev Musculoskelet Med. 2008;1(1):1-11. doi:10.1007/s12178-007-9000-5
 Murphy MT, Journeaux SF. Case reports: long thoracic nerve palsy after using a single axillary crutch. Clin Orthop Relat Res. 2006;447:267-269. doi:10.1097/01.blo.0000205880.27964.a3