Brachial plexus

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Brachial plexus


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USMLE® Step 1 questions

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High Yield Notes

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Brachial plexus

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USMLE® Step 1 style questions USMLE

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A human cadaveric model is being studied to understand better the anatomic distribution and innervation in the upper extremity, chest, and back. A branch of the brachial plexus is stimulated with electrical impulses, which captures this impulse over the lateral aspect of the forearm. Which of the following nerves is being stimulated in the previous experiment?  

External References

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Brachial plexus

Pancoast tumor p. 710

Brachial plexus lesions p. 456


The brachial plexus may look difficult to draw, but here’s a shorthand way to draw the brachial plexus really fast.

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.

The superior trunk has two collateral branches, the suprascapular and subclavius nerves.

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.


The brachial plexus is a network of nerves that originate in the spinal cord in the neck and pass down through the shoulder area to sensory and motor nerve supply to the shoulder area, and the arm, the forearm, and the hand. It originates from spinal roots C5, C6, C7, C8, and T1, which combine to form three trunks. Trunks divide into 6 divisions, because each trunk splits into an anterior and a posterior division. Divisions regroup into three cords, which finally, give rise to five terminal branches of the brachial plexus that are the musculocutaneous nerve, the axillary nerve, the radial nerve, the median nerve, and the ulnar nerve. Injury to any of these nerves can cause paralysis or other problems with movement and sensation. The brachial plexus can be injured during birth when the baby's head or shoulders get stuck in the birth canal. This can stretch or tear the nerve fibers. Other causes of injury include heavy lifting, car accidents, and falls.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Multiple unilateral variations in medial and lateral cords of brachial plexus and their branches" Anatomy & Cell Biology (2014)
  6. "Epidemiology of Brachial Plexus Injuries in a Multitrauma Population" Neurosurgery (1997)

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