Download TSM73 - Innervation of the Upper Limbs

TSM73: INNERVATION OF THE UPPER LIMBS
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13/11/08
Note: anatomy in this document focuses on the arm and forearm – nerve branches and muscles
relating to the wrist and hand are considered in subsequent lectures
The arm and the forearm share some important common structural elements:
o Sub-cutaneous deep fascia surrounds all deep structures within the arm and forearm
o Intermuscular septum defines anterior (flexor) and posterior (extensor) compartments
o Muscles in the arm act mainly at the elbow whilst those in the forearm act mainly at the wrist
LEARNING OUTCOMES
Outline the structure of the brachial plexus
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The brachial plexus is a network of cervical and thoracic spinal nerves supplying the upper limbs
o Roots and trunks are supraclavicular and are found in the posterior triangle of the neck
o Cords and branches are infraclavicular and are found in the axilla
There are five roots (ventral rami) from C5 to T1 inclusive
o These form three trunks:
 Upper – C5-6
 Middle – C7
 Lower – C8-T1
o Each trunk bifurcates to give anterior and posterior divisions
 Branches of the anterior division go on to supply flexors of the upper limb
 Branches of the posterior division go on to supply extensors of the upper limb
o The divisions regroup to form three cords relative to the axillary artery:
 Lateral – anterior divisions of upper and middle trunks – C5-7
 Posterior – all posterior divisions – C5-T1
 Medial – anterior division of lower trunk – C8-T1
o Five terminal branches are then formed from the cords:
 Musculocutaneous nerve – lateral cord – all of arm anterior compartment
 Axillary nerve – posterior cord – deltoid and teres minor
 Radial nerve – posterior cord – all of both posterior compartments
 Median nerve – lateral and medial cords – most of forearm anterior compartment
 Ulnar nerve – medial cord – most of intrinsic hand muscles
The brachial plexus can be damaged through stretching of the nerves resulting in two types of injury:
o Upper brachial plexus – from sudden lateral head movement e.g. falling onto the neck
 Affects nerve roots C5 and C6
 Loss of sensation in lateral aspect of arm (see below)
 Loss of flexion, abduction and lateral rotation of the shoulder
o Lower brachial plexus – from sudden over-extension of the shoulder joint
 Affects nerve roots C8 and T1
 Loss of sensation in medial aspect of arm (see below)
 Generalised loss of wrist and finger movements
Describe the relationship between spinal nerves and dermatomes
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A dermatome is an area of skin with sensory innervation from the dorsal root of a single spinal nerve
o Adjacent skin areas supplied by spinal nerves from the same segment (e.g. lumbar) overlap
o Axial lines distinctly divide skin areas supplied by spinal nerves from different segments
The skin of the arm and forearm is divided into the following dermatomes:
o Medially – T1 and C8 – medial cutaneous nerves (medial cord)
o Laterally – C5, C6 and C7
 Axillary nerve on the shoulder – ‘sergeant’s patch’ (posterior cord)
 Radial nerve on the arm (posterior cord)
 Musculocutaneous nerve on the forearm (lateral cord)
Describe the boundaries and contents of the axilla
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The axilla is bounded:
o Superiorly by the clavicle and first rib; inferiorly by the skin of the underarm
o Medially by serratus anterior; laterally by the intertubercular groove of the humerus
o Anteriorly by pectoralis major and minor; posteriorly by latissimus dorsi
The axilla contains various important structures:
o Cords and branches of the brachial plexus
o The axillary artery as a continuation of the subclavian artery passing the first rib
o The axillary vein which becomes the subclavian vein passing the first rib
o A number of axillary lymph nodes
Describe the elbow joint, its normal range of movements and the muscle groups responsible
THE ELBOW JOINT
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The elbow joint is a complex hinge-type synovial joint involving three bones:
o Humerus (distal) – medial and lateral epicondyles, olecranon fossa, capitulum, trochlea
o Radius – rounded head articulates with capitulum of the humerus and radial notch of the ulna
o Ulna – curved trochlear notch articulates with trochlea of the humerus, olecranon posteriorly
In the anatomical position the radius is lateral and the ulna is medial
o There are proximal and distal radio-ulnar joints between the ends of the two bones
o The interosseous membrane holds the radius and ulna together along their whole length
o The radius rotates about the capitulum allowing pronation and supination of the arm
o The ulna does not rotate but allows flexion and extension at the elbow
A collateral ligament arises from each of the epicondyles and attaches to each of the forearm bones
o These ligaments prevent lateral dislocation of the radius and ulnar and stabilise the elbow
o On the lateral side the annular ligament also surrounds the radial head facilitating its rotation
MUSCLES ACTING AT THE ELBOW
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The arm contains two large muscles and one much smaller muscle that act on the elbow joint:
o Biceps brachii – from scapula and coracoid onto radial tuberosity – flexor / supinator
o Brachialis (small) – from lower anterior humerus onto ulnar tuberosity – flexor
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o Triceps brachii – from scapula and humerus onto olecranon of ulna – extensor
Muscles in the forearm are arranged in superficial and deep layers within the two compartments
o Those relating to the wrist and hand are omitted here (see TSM77 - Anatomy of the Wrist)
o The superficial layers in each compartment contain common muscle origins:
 Common flexor origin – medial epicondyle of humerus
 Common extensor origin – lateral epicondyle of humerus
The flexor compartment of the forearm contains:
o Superficial layer
 Brachioradialis – from lateral humerus onto styloid process of distal radius – flexor
 Pronator teres – from common flexor origin centrally onto lateral radius – pronator
o Deep layer
 Pronator quadratus – from distal anterior ulna to distal anterior radius – pronator
The extensor compartment of the forearm contains:
o Superficial layer
 Anconeus – from common extensor origin onto lateral ulna – extensor / pronator
o Deep layer
 Supinator – from lateral epicondyle of humerus onto lateral radius – supinator
All of the above muscles are supplied by terminal branches of the brachial plexus:
o The musculocutaneous nerve supplies all of the above flexors except brachioradialis
o The radial nerve supplies both of the above extensors as well as brachioradialis and supinator
o The median nerve supplies all the pronators
Describe the cubital fossa and the structures which pass through it
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The cubital fossa is a triangular region anterior to the elbow joint in the anatomical position
Four key structures pass through it:
o Brachial artery – before dividing into the radial and ulnar arteries
o Biceps brachii tendon – and surrounding aponeurosis
o Median cubital vein – joining the cephalic laterally to the basilic medially
o Median nerve – medial to the brachial artery