Brachial Plexus Flashcards
What are the roots of the brachial plexus?
What are the spinal cord segments?
- 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
What makes the M of the plexus?
- 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).
How is the plexus biaxial?
- anterior/posterior; proximal/distal
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Anterior and posterior divisions: Anterior division fibers tend to innervate flexor muscles, whereas posterior division fibers tend to innervate extensor muscles.
- AF
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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
How are flexion and extension different in upper and lower limbs?
Why?
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
axilla
- 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
Which muscles are on the anterior wall of the axilla?
Pectoralis major, pectoralis minor
Which muscle lies over the roots of the brachial plexus?
- 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
What artery goes through the brachial plexus?
Name change?
- 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
What are the cords of the brachial plexus?
Which cords are anterior, which posterior?
- 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
What are the anatomic subdivisions of the brachial plexus?
- 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.
What forms each cord?
- 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
Anterior = ___.
Posterior = ___.
- 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.
What do nerves containing C5 and C6 fibers tend to innervate?
C8 and C1
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
What are the terminal branches of the brachial plexus?
- 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).
musculocutaneous nerve:
Spinal cord segments? What muscles does it innervate?
What dermatome does it innervate?
- 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.
median nerve
Spinal cord segments? What muscles does it innervate?
What dermatome does it innervate?
- 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
ulnar nerve
Spinal cord segments? What muscles does it innervate?
What dermatome does it innervate?
- 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.
axillary nerve
Spinal cord segments? What muscles does it innervate?
What dermatome does it innervate?
What space does it run through?
- 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.