Week 2 - Embryology, osteology, anterior and posterior arm

Question 1

Name the structures that form the following boundaries of the cubital fossa:

1. Lateral border
2. Medial border
3. Superior border

Answer 1

1. Medial border of the brachioradialis
2. Lateral border of pronator teres
3. An imaginary line connecting the two epicondyles of the humerus

Question 2

Outline the contents of the cubital fossa (in the direction lateral to medial).

Answer 2

1. Radial nerve
2. Tendon of the biceps brachii
3. Brachial artery
4. Median nerve

Question 3

Outline the clinical relevance of the cubital fossa.

Answer 3

1. Brachial artery: Blood pressure
2. Biceps brachii tendon: Test reflex
3. Median cubital vein: Site used for taking blood

Question 4

Name the origins and insertions of the following anterior arm muscles:

1. Biceps Brachii
2. Brachialis
3. Coracobrachialis

Answer 4

1. Biceps brachii
   Long head - Origin: Supraglenoid tubercle
   Short head - Origin: Coracoid process
   Insertion for both heads: Radial tuberosity via the bicipital aponeurosis
2. Brachialis
   Origin: Shaft of the humerus
   Insertion: Tuberosity of the ulna
3. Coracobrachialis
   Origin: Coracoid process of the scapula
   Insertion: Medial aspect of the shaft of the humerus

Question 5

Which ligament holds the tendon of the biceps brachii in the intertubercular groove? Is this an extra capsular/intra capsular ligament of the joint capsule of the shoulder joint?

Answer 5

Transverse humeral ligament

It is an extra capsular ligament

Question 6

Name the origins and insertion of the three heads of the triceps brachii.

Answer 6

Long head: Origin - Infraglenoid tubercle
Lateral head: Origin - From the humerus, superior to the radial groove
Medial head: Origin - From the humerus, inferior to the radial groove
The three heads converge to a tendon.
Attachment: Olecranon of the ulna

Question 7

Outline how the radial nerve can be damaged in the axilla, and the motor and sensory functions that are compromised as a result of damage to this nerve.

Answer 7

How it usually occurs:

1. Dislocation of the humerus at the glenohumeral joint
2. Fractures of the proximal humerus
3. Can also happen via excessive pressure on the axilla, e.g. due to a badly fitting crutch

Motor functions:

1. Triceps brachii and muscles in the posterior compartment: Paralysed
2. The patient is unable to extend the forearm, wrist and fingers
3. Unopposed flexion of wrist: Wrist drop

Sensory functions:

1. All four cutaneous branches of the radial nerve are affected
2. Loss of sensation over the lateral and posterior upper arm, posterior forearm and the dorsal aspect of the lateral three and a half digits

Question 8

What are the embryonic origins of the musculoskeletal system?

Answer 8

1. Somites

2. Lateral plate mesoderm

Question 9

When do the limb buds first appear?

Answer 9

Towards the end of the fourth week

Question 10

Where does the limb bud come from?

Answer 10

Limb development begins with activation of mesenchyme in lateral mesoderm - derived from somatic layer of lateral plate mesoderm

Question 11

State the embryonic origin of the limb skeleton.

Answer 11

Lateral plate - Somatic mesoderm

Question 12

State the embryonic origin of limb musculature.

Answer 12

Somites

Question 13

Describe the functions of the apical ectodermal ridge in limb development.

Answer 13

1. Critical for limb bud outgrowth
2. Orchestrates limb development proximal to distal
3. Induces the development of digits in hand and foot plates
4. Marks the boundary between dorsal and ventral limb ectoderm

Question 14

Briefly outline the process of proximal-distal axis generation in limb development.

Answer 14

1. AER exerts an inductive influence on the immediately underlying mesenchyme, which remains undifferentiated.
2. Proximal mesenchyme begins differentiating into constituent tissues

Question 15

What are the functions of the zone of polarising activity (ZPA) in limb development?

Answer 15

1. Generation of asymmetry in the limbs (anterior-posterior axis determination)
2. Controls both patterning and maintains the AER

Question 16

What is the location of the zone of polarising activity in the limb bud?

Answer 16

The ZPA is a signalling centre located at the posterior base of the limb bud

Question 17

Briefly describe the formation of digits within hand and foot plates during limb development.

Answer 17

Digital rays: Condensations of mesenchyme within plates, Cartilaginous models of digital bones

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

Question 18

Outline the features of polydactyly.

Answer 18

1. Extra digits

2. Genetic recessive trait

Question 19

Outline the features of syndactyly.

Answer 19

1. Fusion of digits
2. May involve just connective tissue or bones may be fused
3. No sculpting of the digital rays

Question 20

Distinguish between amelia and meromelia.

Answer 20

Amelia: Complete absence of a limb
Meromelia: Partial absence of one or limb structures, phocomelia

Question 21

Briefly outline the development of the musculature of the limbs.

Answer 21

1. Myogenic precursors migrate into limbs from somites
2. Coalesce into 2 common muscle masses around the newly formed skeletal elements
   Ventral = Flexor
   Dorsal = Extensor
3. Individual muscles split from common masses

Question 22

Briefly outline rotation of the limbs.

Answer 22

1. Limbs extend ventrally at first
2. But as they elongate, they rotate:
   Upper limb: Laterally - Thumb lateral
   Lower limb - Medially - Big toe medial

Question 23

Which spinal segments does the upper limb appear opposite to?

Answer 23

Caudal cervical spinal segments

Question 24

Define the terms “dermatome” and “myotome”.

Answer 24

Dermatome: Strip of skin supplied by a single spinal nerve
Myotome: Muscle/group of muscles supplied by a single spinal nerve

Question 25

State the preaxial and postaxial structures during limb development.

Answer 25

Preaxial: Thumb and big toe
Postaxial: Little finger and toe