MSK Session 1 - Development of the Limbs

Question 1

What is the pectoral region?

Answer 1

The pectoral region is located on the anterior chest wall. It contains four muscles that exert a force on the upper limb; the pectoralis major, pectoralis minor, serratus anterior and subclavius.

Question 2

Outline the pectoralis major in terms of its:

• Structure
• Function
• Innervation
• Attachments

Answer 2

• Structure: The pectoralis major is the most superficial muscle in the pectoral region. It is large and fan shaped, and is composed of a sternal head and a clavicular head

- Function: Adducts and medially rotates the upper limb, and draws the scapular anteroinferiorly. The clavicular head also acts individually to flex the upper limb.

• Innervation: Lateral and medial pectoral nerves.

- Attachments: The distal attachment of both heads is into the intertubercular sulcus of the humerus.

I. Clavicular head – originates from the anterior surface of the medial clavicle.

I. Sternocostal head – originates from the anterior surface of the sternum, the superior six costal cartilages and the aponeurosis of the external oblique muscle.

Question 3

Outline the pectoralis minor in terms of its:

• Structure
• Function
• Innervation
• Attachments

Answer 3

• Structure: The pectoralis minor lies underneath its larger counterpart muscle, pectoralis major. Both of these muscles form part of the anterior wall of the axilla region.

- Function: Stabilises the scapula by drawing it anteroinferiorly against the thoracic wall.

- Innervation: Medial pectoral nerve.

- Attachments: Originates from the 3rd-5th ribs, and inserts into the coracoid process of the scapula.

Question 4

Outline the serratus anterior in terms of its:

• Structure
• Function
• Innervation
• Attachments

Answer 4

• Structure: The serratus anterior is located more laterally in the chest wall, and forms the medial border of the axilla region.

- Function: Rotates the scapula, allowing the arm to be raised over 90 degrees. It also holds the scapula against the ribcage.

- Innervation: Long thoracic nerve.

• Attachments: The muscle consists of several strips, which originate from the lateral aspects of ribs 1-8. They attach to the costal (rib facing) surface of the medial border of the scapula.

Question 5

Outline the subclavius in terms of its:

• Structure
• Function
• Innervation
• Attachments

Answer 5

• Structure: The subclavius is small muscle, which is located directly underneath the clavicle, running horizontally. It affords some minor protection to the underlying neurovascular structures E.g. in cases of clavicular fracture or other trauma.

- Function: Anchors and depresses the clavicle.

- Innervation: Nerve to subclavius.

- Attachments: Originates from the junction of the 1st rib and its costal cartilage, inserting into the inferior surface of the middle third of the clavicle.

Question 6

What is the axilla region?

Answer 6

The axilla is the name given to an area that lies underneath the glenohumeral joint, at the junction of the upper limb and the thorax. It is a passageway by which neurovascular and muscular structures can enter and leave the upper limb.

Question 7

Outline the borders of the axilla region in terms of its:

• Apex
• Lateral wall
• Medial wall
• Anterior wall
• Posterior wall
• Size and shape

Answer 7

• Apex – Also known as the axillary inlet, this is formed by lateral border of the first rib, superior border of scapula, and the posterior border of the clavicle.

- Lateral wall – Formed by intertubercular groove of the humerus.

- Medial wall – Consists of the serratus anterior and the thoracic wall (ribs and intercostal muscles).

- Anterior wall – Contains the pectoralis major and the underlying pectoralis minor and the subclavius muscles.

- Posterior wall – Formed by the subscapularis, teres major and latissimus dorsi.

• Size and shape: The overall 3D shape of the axilla looks slightly like a pyramid. The size and shape of the axilla region varies with arm abduction. It decreases in size most markedly when the arm is fully abducted – at this point, the contents of the axilla are at most risk of injury

Question 8

Outline and identify the three main routes by which structures leave the axilla.

Answer 8

• The main route of exit is immediately inferiorly and laterally, into the upper limb. Most contents of the axilla region leave by this method.
• Another pathway is via the quadrangular space. This is a gap in the posterior wall of the axilla, allowing access to the posterior arm and shoulder area. Structures passing through include the axillary nerve and posterior circumflex humeral artery (a branch of the axillary artery.
• The last passageway is the clavipectoral triangle, which is an opening in the anterior wall of the axilla. It is bounded by the pectoralis major, deltoid, and clavicle. The cephalic vein enters the axilla via this triangle, while the medial and lateral pectoral nerves leave.

Question 9

Outine the main and clinically important contents of the axilla region (including muscles, nerves, vasculature and lymphatics)

Answer 9

• Axillary artery – It is the main artery supplying the upper limb. It is commonly referred as having three parts, one medial to the pectoralis minor, one posterior to pectoralis minor, and one lateral to pectoralis minor. The medial and posterior parts travel in the axilla.

- Axillary vein – The main vein draining the upper limb, its two largest tributaries are the cephalic and basilic veins.

- Brachial plexus – A collection of spinal nerves that form the peripheral nerves of the upper limb.

- Biceps brachii and coracobrachialis – These muscle tendons move through the axilla, where they attach to the coracoid process of the scapula.

- Axillary Lymph nodes – The axillary lymph nodes filter lymph that has drained from the upper limb and pectoral region. In women, axillary lymph node enlargement is a non-specific indicator of breast cancer.

Question 10

What is the brachial plexus?

Answer 10

• The brachial plexus is a network of nerve fibres that supplies the skin and musculature of the upper limb.
• It begins in the root of the neck, passes through the axilla, and enters the upper arm.
• The plexus is formed by the anterior rami (divisions) of the cervical spinal nerves C5, C6, C7 and C8, and the first thoracic spinal nerve, T1.
• The brachial plexus is divided into five parts; roots, trunks, divisions, cords and branches. There are no functional differences between these divisions.

Question 11

Outline what is meant by the ‘roots’ of the brachial plexus.

Answer 11

• The ‘roots’ refer the beginning of the brachial plexus. They are formed by the spinal nerves C5, C6, C7, C8 and T1.
• At each vertebral level, paired spinal nerves arise. They leave the spinal cord via the intervertebral foramina of the vertebral column.
• Each nerve then divides into anterior and posterior nerve fibres. The roots of the brachial plexus are formed by the anterior divisions of spinal nerves C5-T1 (the posterior divisions go on to innervate the skin and musculature of the trunk).
• After their formation, these nerves pass between the anterior and medial scalene muscles to enter the base of the neck.

Question 12

Outline what is meant by the ‘trunks’ of the brachial plexus.

Answer 12

• At the base of the neck, the roots of the brachial plexus converge, forming three trunks. These structures are named by their anatomical position:

I. Superior trunk: A combination of C5 and C6 roots.

II. Middle trunk: A continuation of C7.

III. Inferior trunk: A combination of C8 and T1 roots.

• The trunks begin to move laterally, crossing the posterior triangle of the neck.

Question 13

Outline what is meant by the ‘divisions’ of the brachial plexus.

Answer 13

• Within the posterior triangle of the neck, each trunk divides into two branches. One division travels anteriorly (toward the front of the body) and the other posteriorly (towards the back of the body). Thus, they are known as the anterior and posterior divisions.
• We now have three anterior and three posterior nerve fibres. These divisions leave the posterior triangle and pass into the axilla region. They recombine in the next part of the brachial plexus.

Question 14

Outline what is meant by the “cord” of the brachial plexus.

Answer 14

Once the anterior and posterior divisions have entered the axilla, they combine together to form three nerves. These nerves are named by their position relative to the axillary artery.

• The lateral cord is formed by:

I. The anterior division of the superior trunk

II. The anterior division of the middle trunk

• The posterior cord is formed by:

I. The posterior division of the superior trunk

II. The posterior division of the middle trunk

III. The posterior division of the inferior trunk

• The medial cord is formed by:

I. The anterior division of the inferior trunk.

II. The cords give rise to the major branches of the brachial plexus.

Question 15

What are the major branches of the brachial plexus?

Answer 15

• Musculocutaneous Nerve
• Axillary Nerve
• Median Nerve
• Radial Nerve
• Ulnar Nerve

Question 16

Outline the roots, motor functions and sensory functions of the musculocutaneous nerve.

Answer 16

• Roots: C5, C6, C7.

- Motor Functions: Innervates the brachialis, biceps brachii and coracobrachialis muscles.

- Sensory Functions: Gives off the lateral cutaneous branch of the forearm, which innervates the lateral half of the anterior forearm, and a small lateral portion of the posterior forearm.

Question 17

Outline the roots, motor functions and sensory functions of the axillary nerve.

Answer 17

• Roots: C5 and C6.

- Motor Functions: Innervates the teres minor and deltoid muscles.

- Sensory Functions: Gives off the superior lateral cutaneous nerve of arm, which innervates the inferior region of the deltoid (“regimental badge area”).

Question 18

Outline the roots, motor functions and sensory functions of the median nerve.

Answer 18

• Roots: C6 – T1. (Also, contains fibres from C5 in some individuals).

- Motor Functions: Innervates most of the flexor muscles in the forearm, the thenar muscles, and the two lateral lumbricals that move the index and middle fingers.

- Sensory Functions: Gives off the palmar cutaneous branch, which innervates the lateral part of the palm, and the digital cutaneous branch, which innervates the lateral three and a half fingers on the anterior (palmar) surface of the hand.

Question 19

Outline the roots, motor functions and sensory functions of the radial nerve.

Answer 19

• Roots: C5-C8 and T1.

- Motor Functions: Innervates the triceps brachii, and the extensor muscles in the posterior compartment of the forearm.

- Sensory Functions: Innervates the posterior aspect of the arm and forearm, and the posterior, lateral aspect of the hand.

Question 20

Outline the roots, motor functions and sensory functions of the ulnar nerve.

Answer 20

• Roots: C8 and T1.

- Motor Functions: Innervates the muscles of the hand (apart from the thenar muscles and two lateral lumbricals), flexor carpi ulnaris and medial half of flexor digitorum profundus.

- Sensory Functions: Innervates the anterior and posterior surfaces of the medial one and half fingers, and associated palm area.

Question 21

Briefly outline prenatal limb development.

Answer 21

• The limb buds first appear as small projections on the lateral body wall during the fourth week of development.
• The buds consist of a mass of mesenchyme covered by a layer of ectoderm; at the tip of the bud the ectodermal cells divide to form an apical ectodermal ridge.
• The development of the upper limb buds precedes that of the lower limb buds by a few days.
• The limb buds elongate by the proliferation of mesenchyme.
• The apical ectodermal ridge is thought to exert an inductive influence on the limb mesenchyme that promotes growth and development of the limbs.

Question 22

Outline the ossification involved in prenatal limb development/

Answer 22

• The mesenchymal skeleton is formed by cell aggregation in the limb. To begin with the entire limb skeleton is cartilaginous.

- Ossification of the long bones begins by the appearance of primary centres of ossification in the middle of the long bones.

• As bones are forming, myoblasts aggregate and develop a large muscle mass in each limb bud.
• In general, this muscle mass separates into dorsal (extensor) and ventral (flexor) components.
• At first the flexor aspect of the limb is ventral and the extensor aspect dorsal. The pre-axial border is cranial and postaxial border is caudal.

Question 23

Explain rotations in limb development.

Answer 23

• The developing upper and lower limbs rotate in opposite directions and to different degrees.
• The upper limb rotates laterally through 90° on its longitudinal axis; thus the future elbows point backwards (or posteriorly) and the extensor muscles come to lie on the lateral and posterior aspects of the limb.
• The lower limb rotates medially through almost 90°; thus, the future knees face forward (or anteriorly) and the extensor muscles come to lie on the anterior aspect of the lower limb.

Question 24

Identify the three spatial axes in limb development.

Answer 24

• Shoulder to fingertips: Proximal – distal axis

- Palmar surface to dorsal surface : Dorsal – ventral axis

- Side-to-side: Anterior – posterior axis

Question 25

In limb development, identify the controllers of axial specification.

Answer 25

- Anterior – posterior axis: Zone of polarising activity (ZPA)

- Proximal – distal axis: Apical Ectodermal Ridge (AER)

- Dorsal – ventral axis: Ectoderm

Question 26

What is the structure and function of the apical ectodermal ridge (Proximal-Distal axis generation)?

Answer 26

• AER exerts an inductive influence on the immediately underlying mesenchyme. This means it remains undifferentiated. We want the mesenchyme to stay undifferentiated so the limb bud can extend. Hence, only mitosis occurs.
• Proximal mesenchyme – begins to differentiate into constituent tissues. The proximal mesenchyme is now further away from the influence of the AER. Hence, it cannot be suppressed and it can begin to differentiate and some cartilaginous templates can develop
• Finally, AER induces development of the digits within the hand/foot plates

Question 27

Outline the Zone of Polarising Activity.

Answer 27

• Generation of asymmetry in the limbs – anterior – posterior axis determination
• A signalling centre located at the posterior base of the limb bud
• Controls both patterning and maintains the AER

Question 28

Describe the formation of hand and foot plates.

Answer 28

• Digital rays

I. Mesenchyme condensations within plates

II. Cartilaginous models of the digital bones

• Apoptosis – of the tissue between the digits
• AER breaks up and is maintained only over the tips of the digital rays
• Interdigital spaces are progressively sculpted by programmed cell death

Question 29

Describe in simple terms the compartmentalisation of structures (e.g. muscles, neurovascular elements, etc.) and their positional changes during development of the limbs.

Answer 29

• Myogenic precursors migrate into limbs from somites (body segments).
• Coalesce into 2 common muscle masses around the newly formed skeletal elements.
• Ventral = flexor
• Dorsal = extensor
• Individual muscles then split from common masses

Question 30

Describe the rotation of limbs.

Answer 30

• Limbs extend ventrally at first
• But, as they elongate, they rotate

- Upper limb: laterally – thumb lateral

- Lower limb: medially – big toe medial

Question 31

What is syndactyly?

Answer 31

The fusion of digits, may involve just connective tissue or bones may be fused.

Question 32

What is polydactyly?

Answer 32

Extra digits, genetic recessive trait.

Question 33

What is amelia?

Answer 33

Amelia - complete absence of a limb.

Question 34

What is meromelia?

Answer 34

Meromelia - the partial absence of one or more limb structures (phocomelia)

Question 35

What are the underlying problems in morphogenesis.

Answer 35

• Malformation – Intrinsic error in coordination of morphogenesis
• Deformation – Constriction bands
• Disruption – External agent

I. E.g. thalidomide

II. E.g. TORCH (infections)

Question 36

Outline the clinical relevance of Winged Scapula.

Answer 36

• One of the actions of the serratus anterior is to ‘hold’ the scapula against the ribcage.
• If the long thoracic nerve is damaged (and the serratus anterior therefore paralysed), a specific clinical sign is produced.
• In cases such as this, the scapula is no longer held against the ribcage – and protrudes out of the back. It is said to have a‘winged’ appearance.
• Long thoracic nerve palsy is thought to most commonly occur from traction injuries, where the upper limb is stretched violently.

Question 37

Outline the clinical relevance of lymph nodes in the Axilla region.

Answer 37

• Approximately 75% of lymph from the breast drains into the axilla lymph nodes, so can be biopsied if breast cancer is suspected.
• If breast cancer is confirmed, the axillary nodes may need to be removed to prevent the cancer spreading. This is known as axillary clearance.
• During this procedure, the long thoracic nerve may become damaged, resulting in winged scapula.

Question 38

Outline the clinical relevance of Erb’s Palsy.

Answer 38

• Erb’s palsy commonly occurs where there is excessive increase in the angle between the neck and shoulder – this stretches (or can even tear) the nerve roots, causing damage.
• It can occur as a result of result of a difficult birth or shoulder trauma.
• Nerves affected: Nerves derived from solely C5 or C6 roots; musculocutaneous, axillary, suprascapular and nerve to subclavius.
• Muscles paralysed: Supraspinatus, infraspinatus, subclavius, biceps brachii, brachialis, coracobrachialis, deltoid and teres minor.
• Motor functions: The following movements are lost or greatly weakened – abduction at shoulder, lateral rotation of arm, supination of forearm, and flexion at shoulder.
• Sensory functions: Loss of sensation down lateral side of arm, which covers the sensory innervation of the axillary and musculocutaneous nerves.
• The affected limb hangs limply, medially rotated by the unopposed action of pectoralis major. The forearm is pronated due to the loss of biceps brachii. This is position is known as ‘waiter’s tip’, and is characteristic of Erb’s palsy.

Question 39

Outline the clinical relevance of Klempke’s Palsy.

Answer 39

• A lower brachial plexus injury results from excessive abduction of the arm (e.g person catching a branch as they fall from a tree). It has a much lower incidence than Erb’s palsy.
• Nerves affected: Nerves derived from the T1 root – ulna and median nerves.
• Muscles paralysed: All the small muscles of the hand (the flexors muscles in the forearm are supplied by the ulna and median nerves, but are innervated by different roots).
• Sensory functions: Loss of sensation along medial side of arm.
• The metacarpophalangeal joints are hyperextended, and the interphalangeal joints are flexed. This gives the hand a clawed appearance.

Question 40

In terms of the branches of the brachial plexus, what are the motor compartments of the upper arm?

Answer 40