Brachial Plexus

Question 1

What are the roots of the brachial plexus?

What are the spinal cord segments?

Answer 1

• Ventral rami = roots of brachial plexus
• C5-T1
• Nerves containing C5 and C6 fibers tend to innervate muscles that act at the shoulder
• Nerves containing C8 and T1 fibers tend to innervate muscles that act within the hand
  
  • in fact, a nerve must contain T1 fibers to be “allowed” to enter the hand and innervate intrinsic hand muscles

Question 2

What makes the M of the plexus?

Answer 2

• Branches of terminal nerves, the 3 that are anterior (flexors)
• Observe the “M” created by the reorganization of the cords containing anterior division fibers
  
  • The musculocutaneous nerve (C5, C6; most superolateral); the median nerve (C6, C7, C8, T1), and the ulnar nerve (C8, T1; most inferomedial).

Question 3

How is the plexus biaxial?

Answer 3

• anterior/posterior; proximal/distal
• Anterior and posterior divisions: Anterior division fibers tend to innervate flexor muscles, whereas posterior division fibers tend to innervate extensor muscles.
  
  • AF
• Proximal-to-distal gradient: The lower the spinal cord segment, the more distal the muscular innervation. That is, nerves containing C5 and C6 fibers tend to innervate muscles that act at the shoulder; nerves containing C8 and T1 fibers tend to innervate muscles that act within the hand (in fact, a nerve must contain T1 fibers to be “allowed” to enter the hand and innervate intrinsic hand muscles).
  
  • Lower = more distal

Question 4

How are flexion and extension different in upper and lower limbs?

Why?

Answer 4

Upper limb:

• Flexor side faces anteriorly. Flexion occurs when move body part anteriorly in sagittal plane.
• Extensor faces posteriorly. Extension occurs when increase the angle between two body parts in the sagittal plane.
• Ex: hip flexor faces anterior (bend forwarD), extensor posterior (stand back up)
• Why: rotates laterally 90* during development

Lower limbs:

• For any joint distal to the hip, flexion is a posterior force instead
  
  • Ex: flexion at the knee when knee is bent back
• Why: rotates medially 90* during development

Question 5

axilla

Answer 5

• axilla = armpit
• Transmits the axillary vessels and the brachial plexus, which constitute the primary blood supply and innervation of the upper limb.
• It’s walls are skeletal muscles

Question 6

Which muscles are on the anterior wall of the axilla?

Answer 6

Pectoralis major, pectoralis minor

Question 7

Which muscle lies over the roots of the brachial plexus?

Answer 7

• More superiorly, we note the anterior scalene muscle, behind which emerge the roots of the brachial plexus, C5 through T1.
• The anterior scalene will come to be an important landmark in the neck during the head and neck block for a number of reasons, not the least of which is this important relationship to the brachial plexus
• The phrenic nerve, can be seen running down the anterior surface of the anterior scalene, as well

Question 8

What artery goes through the brachial plexus?

Name change?

Answer 8

• subclavian artery –> axillary artery
  
  • the axillary artery is a continuation of the subclavian artery that simply changes its name at the lateral edge of the first rib
• Branches of axillary artery: Some Times Life Seems A Pain
  
  • Superior thoracic, Thoracoacromiol trunk, Lateral thoracic, Subscapular, Anterior circumflex humeral, Posterior circumflex humeral

Question 9

What are the cords of the brachial plexus?

Which cords are anterior, which posterior?

Answer 9

• Medial, posterior, lateral
• The cords are named for their position relative to the axillary artery.
• The medial and lateral cords come from anterior divisions, the posterior from posterior division
  
  • Medial and lateral cords = anterior = flexors
  • Posterior cord = posterior = extensors

Question 10

What are the anatomic subdivisions of the brachial plexus?

Answer 10

• roots, trunks, divisions, cords, and branches (aka terminal nerves)
  
  • Roudy Truckers Drink Cold Beer. Or think tree (roots–>trunks–>DC->branches)
• The 5 roots are the ventral rami of spinal cord segments C5 through T1.
• The 3 trunks are formed from the five roots: C5 and C6 come together to form a superior trunk, C7 continues as the middle trunk, and C8 and T1 come together to form an inferior trunk.
• Each trunk then subdivides into 6 anterior and posterior divisions
• These divisions reorganize to form 3 cords
  
  • The anterior divisions of the superior and middle trunks come together to form a lateral cord, the three posterior divisions come together to form a posterior cord, and the anterior division of the inferior trunk continues as the medial cord​
• Finally, the cords reorganize to form 5 branches/terminal nerves.

Question 11

What forms each cord?

Answer 11

• The anterior divisions of the superior and middle trunks come together to form a lateral cord
• The three posterior divisions come together to form a posterior cord
• The anterior division of the inferior trunk continues as the medial cord

Question 12

Anterior = ___.

Posterior = ___.

Answer 12

• Think AF
• Anterior division fibers tend to innervate flexor muscles
• Posterior division fibers tend to innervate extensor muscles.
  
  • Remember that the flexor side of all joints in the upper limb faces anteriorly, and the extensor side faces posteriorly.

Question 13

What do nerves containing C5 and C6 fibers tend to innervate?

C8 and C1

Answer 13

Think proximal to distal gradient. Lower spinal cord origin is more distal.

Brachial Plexus goes C5-T1.

• Nerves containing C5 and C6 fibers tend to innervate muscles that act at the shoulder
• Nerves containing C8 and T1 fibers tend to innervate muscles that act within the hand
  
  • A nerve must contain T1 fibers to be “allowed” to enter the hand and innervate intrinsic hand muscles

Question 14

What are the terminal branches of the brachial plexus?

Answer 14

• 5 important terminal branches.
• “M” created by the reorganization of the cords containing anterior division fibers: musculocutaneous nerve (C5, C6; most superolateral); the median nerve (C6, C7, C8, T1), and the ulnar nerve (C8, T1; most inferomedial).
• The posterior cord divides asymmetrically to form the axillary nerve (C5, C6) and the radial nerve (C5, C6, C7, C8).

Question 15

musculocutaneous nerve:

Spinal cord segments? What muscles does it innervate?

What dermatome does it innervate?

Answer 15

• Step back: three major terminal branches composed of anterior division fibers: the musculocutaneous, the median, and the ulnar. All three predominantly innervate flexors.
• The musculocutaneous nerve contains anterior division fibers of C5 and C6.
• Higher, so proximal, so innervates flexors that act mostly at the shoulder and/or elbow.
• Innervates the 3 muscles in the anterior arm: the biceps brachii, the brachialis, and the coracobrachialis.
  
  • coracobrachialis flexes the arm at the shoulder
  • brachialis flexes the forearm at the elbow
  • biceps brachii flexes at both of these joints, as well as supinating the forearm.
• The nerve can be found piercing the substance of the coracobrachialis.
• After giving off small muscular branches to the other two anterior arm muscles, the remainder of the nerve continues as the lateral antebrachial cutaneous nerve, which innervates the skin of the lateral forearm.

Question 16

median nerve

Spinal cord segments? What muscles does it innervate?

What dermatome does it innervate?

Answer 16

• The median nerve contains anterior division fibers from C6, C7, C8, T1, and thereby innervates mostly flexors within the forearm and hand.
• The median nerve innervates all muscles of the anterior forearm except one and a half (the flexor carpi ulnaris and the medial half of the flexor digitorum profundum).
• It also innervates five muscles in the hand: the three muscles of the thenar eminence at the base of the thumb, and two lumbricals for the second and third digits. On the cutaneous side, the median nerve innervates the palmar surface of the lateral 3½ digits, as well as their nail beds.
• Clinical correlate: compression of median nerve –> carpal tunnel

Question 17

ulnar nerve

Spinal cord segments? What muscles does it innervate?

What dermatome does it innervate?

Answer 17

• The ulnar nerve contains anterior division fibers from C8 and T1
• Mainly innervates flexors in the hand, with a few anterior forearm muscles as well.
  
  • Innervates the flexor carpi ulnaris and the medial half of the flexor digitorum profundus in the anterior forearm
  • Innervates all of the intrinsic hand muscles not innervated by the median nerve.
• Its dermatome is the medial one and a half digits, including both the anterior (palmar) and posterior (dorsal) sides.
  
  • Hitting the “funny bone” refers to transient trauma of this nerve as it passes under the medial epicondyle of the humerus; next time this happens to you, notice that it’s only the medial half of the ring finger that gets the “pins and needles” sensation, not the lateral half.

Question 18

axillary nerve

Spinal cord segments? What muscles does it innervate?

What dermatome does it innervate?

What space does it run through?

Answer 18

• Step back: it’s one of 2 terminal nerve branches, with posterior fibers (the other is the radial nerve)
• C5 and C6
• Innervates just two muscles: the deltoid muscle (an aBductor of the arm at the shoulder) and the teres minor (a lateral rotator of the arm at the shoulder).
  
  • The axillary nerve finds its way to these muscles by passing through the quadrangular space. Runs with the circumflex humeral artery
• The dermatome of the axillary nerve is the lateral shoulder.

Question 19

radial nerve

Spinal cord segments? What muscles does it innervate?

What dermatome does it innervate?

What space does it run through?

Answer 19

• The radial nerve is composed of posterior division fibers from C5 - C8.
• It innervates all of the muscles of the posterior arm and forearm, which are predominantly extensors.
• To get into the posterior compartment of the arm, the radial nerve passes through the triangular interval, bounded by the long head of the triceps brachii laterally, the lateral head of the triceps brachii laterally, and the teres major superiorly.
• The deep (profunda) brachial artery travels alongside the radial nerve.
• After innervating the triceps brachii and the other posterior arm muscles, the radial nerve continues into the posterior forearm to innervate all muscles there, as well.
• It also continues down as a cutaneous branch into the dorsum of the hand; the full dermatome of the radial nerve consists of the posterior arm, posterior forearm, and the dorsum of the lateral 3½ digits; excluding the nail beds.

Question 20

quadrangular space -

What artery and nerve pass through it?

Answer 20

• The axillary nerve and posterior circumflex humeral artery pass through the quadrangular space
• Bounded by the teres minor superiorly, the teres major inferiorly, the long head of the triceps brachii medially, and the humerus laterally.

Question 21

triangular interval:

What nerve and artery run through it?

Answer 21

• To get into the posterior compartment of the arm, the radial nerve passes through the triangular interval
• Bounded by the long head of the triceps brachii laterally, the lateral head of the triceps brachii laterally, and the teres major superiorly.
• The deep (profunda) brachial artery travels alongside the radial nerve.

Question 22

collateral nerves -

what’s this?

Answer 22

• The collateral nerves of the brachial plexus refer to the additional ten to eleven nerves that branch from the brachial plexus at some point before the five terminal branches.
• Grouped by where they emerge from the brachial plexus for convenience
  
  • The first three nerves are “pre-divisional” - dorsal scapular nerve, the suprascapular nerve, long thoracic nerve
  • A single nerve arises from the lateral cord: the lateral pectoral nerve.
  • The medial cord gives off three branches, all of which have “medial” in their names: the medial pectoral nerve, the medial brachial cutaneous nerve, and the medial antebrachial cutaneous nerve
  • The posterior cord also gives off three nerves: the upper and lower subscapular nerves, and the thoracodorsal nerve (aka the middle subscapular nerve).

(less important to study)

Question 23

predivisional collateral nerves:

what are they?

Answer 23

• The first three collateral nerves are “pre-divisional” - emerge from the brachial plexus from the roots or trunks, before anterior and posterior divisions are established.
• dorsal scapular nerve, the suprascapular nerve, and the long thoracic nerve

Question 24

dorsal scapular nerve

Answer 24

• predivisional collateral nerve
• C5
• Innervates 3 muscles that retract the scapula: the l_evator scapulae, the rhomboid major, and the rhomboid minor_. The levator scapulae also helps elevate the scapula (hence the name).

Question 25

suprascapular nerve What spinal segments? What muscles does it innervate?

Answer 25

* The suprascapular nerve is a predivisional collateral nerve * **C5 and C6** * **Innervates two (of the four) muscles of the rotator cuff** * **​****supraspinatus** - an abductor of the arm at the shoulder * **infraspinatus** - a lateral rotator of the arm at the shoulder * (The other two muscles of the rotator cuff are the Teres minor and Subscapularis.)

Question 26

long thoracic nerve

Answer 26

* The long thoracic nerve contains fibers from **C5, C6, and C7.** * Innervates just one muscle: the **serratus anterior,** a **protractor of the scapula**. * Unusual in that it innervates its target muscle from the superficial aspect of the muscle. (Most muscles are innervated from their deep surface, as this provides protection for the nerve.) Given its superficial position, the long thoracic nerve may be lesioned during surgeries taking place in the axilla (classically, a radical mastectomy), which results in paralysis of the serratus anterior muscle. * The patient may complain of difficulty combing the hair, placing an object on a shelf, and other activities that require protraction. When a posteriorly directed force is placed on the scapula (such as leaning on a wall using an outstretched hand), the serratus anterior cannot contract to hold the scapula on the posterior chest wall, so the scapula begins to protrude through the skin of the posterior chest wall (a “winged scapula”).

Question 27

lateral pectoral nerve

Answer 27

* A collateral nerve, the one and only that comes off the lateral cord * **Innervates pectoralis major**

Question 28

What's the collateral nerve from the lateral cord?

Answer 28

**lateral pectoral nerve.** Innervates pectoralis major.

Question 29

What are the collateral nerves from the medial cord?

Answer 29

* The medial cord gives off three branches, all of which have “medial” in their names: * medial pectoral nerve * medial brachial cutaneous nerve * medial antebrachial cutaneous nerve

Question 30

medial pectoral nerve

Answer 30

* The medial pectoral nerve contains anterior division fibers of **C8 and T1** * Innervates two muscles: the **pectoralis major and pectoralis minor** muscles. * mnemonic: **medial does more, lateral does less**. * Medial pectoral nerve – innervates both pec major and minor * Lateral only does the pec major.

Question 31

medial brachial cutaneous nerve

Answer 31

* one of three collateral nerves off the medial cord (in addition to medial pectoral and medial antebrachial) * The medial brachial cutaneous nerve contains anterior division fibers of **C8 and T1** * **Provides cutaneous (skin) innervation for the medial arm**

Question 32

medial antebrachial cutaneous nerve

Answer 32

* The third collateral nerve off the medial cord * Contains anterior division fibers from C8 and T1 * **Provides cutaneous (skin) innervation for the medial forearm.**

Question 33

collateral nerves from posterior cord

Answer 33

* The posterior cord also gives off three nerves: **the upper and lower subscapular nerves**, and the **thoracodorsal nerve** (also called the middle subscapular nerve) * Upper subscapular nerve - contains posterior division fibers from C5 and C6, and **innervates the subscapularis**, a medial rotator of the arm at the shoulder. * Lower subscapular nerve - also contains posterior division fibers from C5 and C6, and **innervates not only the subscapularis, but the teres major as well** * thoracodorsal nerve - contains posterior division fibers from C6 through C8 and **innervates the latissimus dorsi**

Question 34

what are the name changes for the subclavian --\> axillary --\> brachial artery?

Answer 34

* **Subclavian artery** named **axillary artery** after passing first rib * Axillary artery = major artery of the axilla and upper limb * Axillary artery changes name after crossing teres major muscle, named **brachial artery**

Question 35

What are the branches of the axillary artery?

Answer 35

* axillary artery has 6 branches * Most important = **posterior circumflex humeral artery (runs with axillary nerve), deep brachial artery (runs with radial nerve), lateral thoracic artery (runs with long thoracic nerve)** * Axillary artery is divided into sections proximal, deep, distal to pec minor * First part (proximal to pectoralis minor): 1 branch – **superior thoracic artery** * ​Provides blood supply to upper intercostal spaces * Second part (deep to pectoralis minor): 2 branches – **thoracoacromial trunk and lateral thoracic artery** * **​**Provides blood supply to scapula, pec minor * Lateral thoracic artery supplies serratus anterior (runs with long thoracic nerve) * Third part (distal to pectoralis minor): 3 branches – **subscapular artery, anterior humeral circumflex artery, and posterior circumflex humeral artery**. * Subscapular gives blood to subscapularis * The two humeral circumflex arteries create an anastomic ring around neck of humerus, to provide blood supply to muscles that attach to the upper humerus * Mnemonic for branches of axillary artery: Some Times Life Seems A Pain * Superior thoracic, Thoracoacromiol trunk, Lateral thoracic, Subscapular, Anterior circumflex humeral, Posterior circumflex humeral

Question 36

lateral thoracic artery: What nerve does it run with? What happens if damaged?

Answer 36

* Lateral thoracic artery runs with **long thoracic nerve** * Disruption of the nerve can also **disrupt blood supply to the serratus anterior**

Question 37

posterior circumflex humeral artery: What nerve does it run with? What happens if damaged?

Answer 37

* Runs with **axillary nerve, through _quadrangular space_** * Artery supplies the teres major, teres minor, deltoid, and triceps (long head) muscles

Question 38

deep (profunda) brachial artery: What nerve does it run with? What if damaged?

Answer 38

* Spirals around the humerus in the spiral groove * Runs with the **radial nerve** * A **mid-shaft humeral fracture** can lesion the radial nerve and deep brachial artery * Provides blood supply to triceps brachii (just as radial nerve innervates triceps brachii)

Question 39

Name the 5 terminal nerves of brachial plexus. Anterior or posterior division? (A-Flexion, P-extension) Spinal cord segments?

Answer 39

* Axillary nerve: posterior division fibers of C5 & C6. * Median nerve: anterior division fibers of C6--T1 * Musculocutaneous nerve: anterior division fibers of C5 & C6. * Radial nerve: posterior division fibers of C5--C8 * Ulnar nerve: anterior division fibers of C8--T1

Question 40

Answer 40

**E** The spiral groove of the humerus transmits the radial nerve and its accompanying profunda (deep) brachial artery.

Question 41

Answer 41

E. ## Footnote The **cords get their names from their positions relative to the axillary artery**. This may seem like silly semantics, but the nomenclature of the brachial plexus is precise! (A) Anterior fibers generally cause flexion, and proximal spinal cord segments (such as C5) generally act proximally within the limb, such as at the shoulder. (B) Remember back to the first month of anatomy: all branches of all spinal nerves contain mixed motor and sensory neurons. In this case, where we’re looking at a cutaneous-only nerve, the motor fibers must be autonomic fibers; specifically, sympathetic fibers going to sweat glands, arrector pili muscles, and smooth muscle in vessel walls in the dermatome of the medial brachial cutaneous nerve. (C) Each of the spinal cord segments between C5 and T1 contribute to at least two different terminal nerves; thus, lesioning any level may impact more than one nerve. (D) Each of the terminal nerves contains at least two different spinal cord segments; thus, lesioning any terminal nerve may impact sensation at more than one spinal cord level.

Question 42

Answer 42

## Footnote **C: The myotome of the musculocutaneous nerve is the anterior arm (coracobrachialis, biceps brachii, brachialis). Its dermatome is the lateral forearm (through the lateral antebrachial cutaneous nerve).** In this case, the dermatome (forearm) does not overlie the myotome (arm). (A)The axillary nerve innervates the deltoid and teres minor, and the skin covering these muscles (lateral shoulder). (B)The median nerve innervates muscles in the forearm and a few muscles in the lateral hand; it also innervates some skin of the lateral hand. (C)(correct answer) (D)The radial nerve innervates muscles of the posterior arm and forearm, and its dermatome includes these same areas (as well as much of the dorsum of the hand). (E)The ulnar nerve innervates 1½ muscles of the forearm and many of the intrinsic hand muscles; its dermatome (medial hand) overlies some of these intrinsic hand muscles.

Question 43

Answer 43

C: * The **profunda (deep) brachial artery arises from the brachial artery**. The brachial artery itself is the renaming of the axillary artery once it passes the inferolateral edge of the teres major. * The branches off the axillary artery, in order, are: **superior thoracic artery (not discussed much in this lecture), thoracoacromial trunk, lateral thoracic artery, subscapular artery, anterior and posterior circumflex humeral arteries.**

Question 44

triangular space * What runs through?

Answer 44

* The scapular circumflex artery runs through the triangular SPACE

Question 45

mnemonic for remembering root contribution to marmu (musculocutaneous, axillary, radial, median, and ulnar nerves) **Three musketeers assassinated 5 rats, 5 mice, and 2 unicorns.**

Answer 45