The five roots of the brachial plexus are formed by the spinal nerves.
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A 25-year-old man complains that he has noticed increased difficulty moving his right arm that started when he took up volleyball. He also sometimes feels pain in his right shoulder (posterior aspect). Upon visual inspection of his back, you notice that the muscles above and below the spine of the scapula are atrophied. When you perform a motor exam, he has weak abduction and external rotation of the right arm. The nerve that is most likely damaged arises from which of the following segments of the brachial plexus?
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Content Reviewers:Rishi Desai, MD, MPH, Ameed Raoof, Justin Ling, MD, MS, Evan Debevec-McKenney, Will Wei, Antonella Melani, MD
Now that the nerves are all drawn, let’s begin labeling the brachial plexus, starting with the cervical and thoracic nerves - C5, C6, C7, C8, & T1.
The brachial plexus is also divided into roots, trunks, divisions, cords, and terminal branches.
The order can be remembered using the acronym “Remember To Drink Cold Beer.”
Then to remember the terminal branches, you can use the word “MARMU.” Which stands for the musculocutaneous, AXILLARY, radial, median, and ulnar nerves respectively.
Next, we can start labeling the collateral nerves starting with those that branch off the roots.
The dorsal scapular nerve branches off of the C5 root, and the long thoracic nerve arises from the C5, C6, and C7 roots.
And the cords have a total of 7 collateral branches.
The lateral cord gives rise to the lateral pectoral nerve.
The POSTERIOR cord gives rise to the upper, middle, and lower subscapular nerves.
And the medial cord gives rise to the medial pectoral, medial cutaneous nerve of the arm, and the medial cutaneous nerve of the forearm.
The nervous system is divided into the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system, which is further divided into the somatic and the autonomic nervous systems.
Broadly speaking, the nervous system is split into an afferent and an EFFERENT division.
The afferent division brings sensory information from the outside into the central nervous system, and includes visual receptors, auditory receptors, and touch receptors.
On the other hand, the EFFERENT division brings motor information from the central nervous system to the periphery, ultimately resulting in contraction of skeletal muscles to trigger movement through the somatic nervous system, as well as contraction of the smooth muscles to trigger activity of the glands and organs through the autonomic nervous system.
Now, the peripheral nervous system consists of nerves, which are enclosed bundles of fibers that connect the central nervous system to every other part of the body.
Cranial nerves arise from the brain and exit from the skull and innervate the head and neck, and spinal nerves exit various regions of the spinal cord and innervate the rest of the body.
There are 31 pairs of spinal nerves, which are grouped into eight pairs of cervical nerves, twelve pairs of thoracic nerves, five pairs of lumbar nerves, five pairs of sacral nerves, and one pair of coccygeal nerves.
The brachial plexus is a network of nerves that innervates the shoulder, arm, and hand, by supplying afferent or sensory nerve fibers from the skin, as well as EFFERENT or motor nerve fibers to the muscles, and there’s a brachial plexus on each side of the body.
The plexus is really important because it can get injured during sports injuries, industrial accidents, surgical procedures and other traumatic injuries to the upper limbs. And the resulting functional deficit can be significant.
In terms of anatomy, the brachial plexus is divided into five roots, three trunks, six divisions, three anterior and three POSTERIOR, three cords, and five terminal branches.
Additionally, there are pre-terminal or collateral branches that leave the plexus at various points along its length.
The five roots come from the last four cervical nerves - C5, C6, C7, and C8 - as well as the first thoracic nerve or T1.
These five roots give off some collateral branches: the long thoracic nerve - which arises from C5, C6, and C7, and innervates the serratus anterior muscle that draws the scapula laterally - the dorsal scapular nerve from C5 - which innervates the rhomboid muscles that retract the scapula - and a contribution to the phrenic nerve from C5, which gives motor and sensory innervation to the diaphragm to help breathing.
Then, the five roots combine to form three trunks: C5 and C6 merge to form the superior or upper trunk, C7 remains as the middle trunk, and C8 and T1 merge to form the inferior or lower trunk.
The superior trunk gives off another collateral branch - the suprascapular nerve, which gives motor innervation the supraspinatus and infraspinatus muscles, and also gives sensory innervation to the acromioclavicular and glenohumeral joints.
Then, each trunk splits into an anterior and a POSTERIOR division - giving rise to a total of six divisions.
The six divisions then regroup with each other to form three cords.
The cords are named for their relationship to the AXILLARY artery, so there’s the lateral cord - formed by the anterior divisions of the superior and middle trunks - the POSTERIOR cord - formed by the POSTERIOR divisions of the superior, middle, and inferior trunks - and the medial cord - formed by the anterior division of the inferior trunk.
The three cords give rise to various collateral branches.
The lateral cord gives rise to the lateral pectoral nerve.
The POSTERIOR cord gives rise to the upper, middle, and lower subscapular nerves, which help innervate the muscles that move the scapula.
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