Nerves

Peripheral (outside of the central nervous system) nerves are tubes that are special in their ability to transmit electric impulses along their length and into or away from the central nervous system. Nerves have specialized receptors for different inputs like hot, cold, sharp and vibration. Smaller nerves are grouped into larger rope-like groups that travel up and down the body.

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Peripheral Nerves

Ulnar nerve

The ulnar nerve is formed from the C8 and T1 nerve roots. The C8 and T1 roots are part of the brachial plexus that travels from the cervical spine, under the clavicle, through the armpit (axilla), and down the inside of the arm to the inner elbow.

When you hit your “funny bone” and get numbness and tingling in your fingers as well as a sharp electrical pain, you are actually hitting your ulnar nerve as it crosses the inner (medial) part of the elbow joint. The ulnar nerve is very close to the skin at the elbow, so it is easy to bump. 

The ulnar nerve can also be compressed when it crosses the elbow and cause numbness and tingling in the small and ring fingers and weakness of the hand.  This is called cubital tunnel syndrome.  Cubital tunnel syndrome is a very common problem cared for by hand surgeons. 

The ulnar nerve then travels on the inside of the forearm.  It powers the forearm muscles that bend the tips of the small and ring fingers (the flexor digitorum profundus) and also one of the muscles that bends the wrist (the flexor carpi ulnaris). The main action of the ulnar nerve is in the hand.  The nerve powers almost all of the small muscles in the hand including the hypothenar muscles, the lumbricals to the ring and small finger, the palmar and dorsal interossei muscles, the adductor pollicus, and the deep head of the flexor pollicus brevis.  The ulnar nerve provides sensation to the small finger side of the palm, the small finger, and the side of the ring finger next to the small finger.  It also provides sensation to the back of the small finger side of the hand. 

The ulnar nerve can be injured by a cut anywhere along its course.  A cut to the ulnar nerve in the forearm can lead the clawing of the small and ring finger of the hand and weakness of most of the hand muscles. Additionally, such an injury can result in loss of sensation to the small and ring fingers. Learn more about nerve injuries.

Radial nerve

The radial nerve is formed from the C5, C6, C7, C8 and T1 nerve roots of the brachial plexus.  The brachial plexus is a group of nerves that branches from the cervical spine (neck).  The brachial plexus travels under the clavicle and through the armpit (axilla).  

In the upper arm the radial nerve wraps around the back side of the humerus bone.  The nerve gives function to the triceps muscles on the back of the arm to straighten the elbow.  Because the radial nerve wraps around the humerus bone, it can be stretched or torn when the humerus bone is broken. 

The nerve then travels on the outside of the elbow (the lateral side) and into the outside of the forearm.  In the forearm the nerve powers all the muscles that straighten the wrist and fingers.  Radial nerve injury can lead to wrist drop – the inability to straighten the wrist.

The radial nerve also provides sensation to the thumb side of the back of the hand. 

Median nerve

The median nerve is formed from the C5, C6, C7, C8 and T1 nerve roots of the brachial plexus.  The brachial plexus is a group of nerves that branches from the cervical spine (neck).  The brachial plexus travels under the clavicle and through the armpit (axilla).  

The median nerve travels along the inside of the arm near the brachial artery.  The median nerve does not provide any function until after it crosses the elbow.  

In the forearm, the median nerve supplies almost all the flexor muscles and all the pronator muscles of the forearm with nerves. Its function allows the wrist and fingers to bend. It also pronates the forearm (rotating the hand to face palm down).  The exception is that the ulnar nerve, not the median nerve, bends the flexor carpi ulnaris and the flexor digitorum profundus muscles to the small and ring fingers. 

The median nerve is likely the most well known nerve of the brachial plexus because it can be compressed as it crosses the wrist and cause carpal tunnel syndrome.  

In the hand, the median nerve supplies the thenar eminence -- the muscles at the base of the thumb --  with nerves.  It also supplies nerves to the lumbrical muscles to the index and middle fingers. The median nerve provides sensation to the thumb side of the palm, the thumb, index, middle and half of the ring finger. 

Musculocutaneous nerve

The musculocutaneous nerve is formed from the C5, C6, and C7 nerve roots of the brachial plexus. The brachial plexus is a group of nerves that branches from the neck (cervical spine). The brachial plexus travels under the clavicle and through the armpit (axilla). 

The musculocutaneous nerve travels along the front of the humerus and provides function to the Coracobrachialis, biceps, and brachialis muscles. The coracobrachialis bends and adducts (moves the arm toward the body) the shoulder.  The biceps bends the elbow and supinates the forearm (rotating the palm to face up), and the brachialis muscle bends the elbow.

The musculocutaneous nerve also provides sensation to the lateral (outside) forearm.

Axillary nerve

The axillary nerve is formed from the C5 and C6 nerve roots of the brachial plexus.  The brachial plexus is a group of nerves that branches from the neck (cervical spine).  The brachial plexus travels under the clavicle.  At this point the axillary nerve travels behind the humerus bone to supply muscles around the shoulder. 

The axillary nerve provides function to three muscles: the deltoid, the teres minor, and the long head of the triceps muscle.  The deltoid is the big fan-shaped muscle over the shoulder that allows you to lift your arm.  The teres minor is one of the rotator cuff muscles that allows you to turn your arm out (externally rotate).  The triceps straightens the elbow.  Most of the triceps is supplied nerves by the radial nerve; however, one part is supplied by the axillary nerve. 

The axillary nerve also provides sensation to the side of the shoulder, sometimes called the "regimental badge" area due to the area that badges were placed on uniforms. The axillary nerve can be injured during shoulder dislocations, from improper use of crutches, or with fractures of the neck of the humerus. 


Brachial plexus 

The brachial plexus is a group of nerves that control the muscles of the shoulder, arm, forearm, and hand. These same nerves also provide sensations (feeling) of the whole upper limb. There are five components of the brachial plexus: roots, trunks, divisions, cords, and branches. 

Roots

The brachial plexus is a group of nerves that control the muscles of the shoulder, arm, forearm and hand.  These same nerves also provide sensation (feeling) of the whole upper limb.

The brachial plexus nerves begin as “roots” off of the spinal cord.  The brachial plexus roots come out of the spinal column between the vertebrae.  The roots are labeled C5, C6, C7, C8 for the cervical vertebrae and T1 for the first thoracic vertebra.

Brachial plexus injuries range from mild stretch injuries that resolve naturally to complete root tears that result in an arm that does not feel and does not function.  Brachial plexus injuries can be caused by motorcycle or car accidents when the neck and the shoulder are pushed in different directions.  Infants can also sustain brachial plexus injuries during childbirth.

Some information about each of the roots: 

  • C5 is the nerve "root" that exits the spinal cord above the fifth vertebra in the neck.  It travels into the brachial plexus and eventually becomes the nerves that feed muscles around the shoulder and chest.  It also provides sensation to parts of the upper arm.
  • C6 is the nerve "root" that exits the spinal cord above the sixth vertebra in the neck.  It travels into the brachial plexus and eventually becomes the nerves that feed muscles that bend the elbow and straighten the wrist.  It also provides sensation to parts of the thumb side of the forearm and hand.
  • C7 is the nerve "root" that exits the spinal cord above the seventh vertebra in the neck.  It travels into the brachial plexus and eventually becomes the nerves that feed muscles that straighten the elbow, bend the wrist, and straighten the fingers.  It also provides sensation around the middle finger in the hand.
  • C8 is the nerve "root" that exits the spinal cord below the seventh vertebra in the neck.  It travels into the brachial plexus and eventually becomes the nerves that feed muscles in the hand and muscles that bend the fingers.  It also provides sensation on the small finger side of the hand and forearm.
  • T1 is the lowest nerve "root" that becomes part of the brachial plexus.  It exits the spinal cord below the first vertebra in the thoracic spine.  It eventually becomes the nerves that feed muscles in the hand.  It also provides sensation around the inside of the elbow and upper arm. 

Trunks

The second level of the brachial plexus consists of three "trunks."  The superior, middle, and inferior trunks are formed from the five main "roots" coming from the spinal cord.  As the three trunks continue toward the shoulder, they each divide into two nerves called an "anterior division" and a "posterior division."

Divisions

The third level of the brachial plexus is called "divisions."  There are six divisions that come from the three "trunks" – three anterior divisions and three posterior divisions.  The divisions then reorganize to create three new nerves, called "cords."

Cords

The nerves in the fourth level of the brachial plexus are called "cords" and come from the "divisions" in level three.  The three cords are named lateral, posterior, and medial based on their position in relationship to the brachial artery.  Many important nerves come from the cords.  Some of these nerves produce motion around the shoulder.  Others travel on to become the five main "branches" of the brachial plexus.

Branches

The fifth and final level of the brachial plexus are the five nerves that feed the shoulder and arm called "branches."  These five branches are named the musculocutaneous, axillary, radial, median, and ulnar nerves.