Cutaneous Nerve Introduction .
Cutaneous Nerves are the continuation of the spinal nerves and carry sympathetic fibres for supplying the sweat glands, arterioles in the dermis and arrector pilorum muscles in relation to the hair follicle. Thus, the effects of sympathetic on the skin are sudomotor (increase sweat secretion); vasomotor (narrow the arterioles of skin) and pilomotor (contract arrector pilorum muscle to make the hair erect or straight) respectively.
The nerves also carry sensation of pain, touch, temperature and pressure. Superficial veins are seen along with the cutaneous nerves. These are utilized for giving intravenous transfusions, cardiac catheterization and taking blood samples
Principles of distribution of Cutaneous Nerve .
There are certain general principles about the distribution of nerves to the skin which are of clinical importance.
Each nerve which passes to the skin is distributed to a circumscribed area. The area of the upper limb skin supplied by spinal nerves C. 4 to T. 2 .
Limb plexuses are formed by the plaiting together of the ventral rami of several spinal nerves.
(a) each part of the plexus and each branch of the brachial plexus contain nerve fibres from more than one ventral ramus—examples: the upper trunk contains fibres from C. 5 and C. 6; the ulnar nerve contains fibres from C. 8 and T. 1 .
(b) several of these branches contain some nerve fibres from the same ventral ramus—example: the suprascapular nerve and the axillary nerve contain fibres from C. 5 and C. 6 . Thus the area of distribution of a cutaneous nerve and consequently the area of sensory loss, when it is destroyed is different from the loss resulting from the destruction of a ventral ramus.
Cutaneous nerves supplying adjacent areas of skin overlap with each other to a considerable degree. Thus, the destruction of a single cutaneous nerve leads to a total loss of sensation only in a small area within the area of distribution of that nerve. Surrounding this anaesthetized area, there will be an area of altered sensation due to the presence of nerve fibres from adjacent uninjured nerves.
Each thoracic spinal nerve (except for T. 1 which is involved in the formation of a plexus) supplies a strip of skin (dermatome) which overlaps those of adjacent nerves, so that destruction of a single thoracic spinal nerve produces only altered sensation within its dermatome. In the plexus regions, each ventral ramus supplies a circumscribed area of skin in sequence with and overlapped by, the areas of adjacent ventral rami. The overlap of these dermatomes is accounted for by the presence of nerve fibres from multiple ventral rami in every branch of the plexus.
Major branches of the plexus, i.e. the main nerves of a limb, give rise to several cutaneous branches which leave them at different points. Destruction of a nerve before it has given off any branches will produce a different distribution of sensory loss to that which occurs when the nerve is destroyed after giving off one or more branches. It is more important to know the total distribution of these major nerves than that of their individual cutaneous branches.
But the ability to make a detailed diagnosis of the site of an injury depends on knowing the distribution of the individual cutaneous nerves and the approximate site of origin of these nerves from the parent trunk . The diagrams of nerve distribution in this book take no account either of the overlap or of the fact that nerve fibres may sometimes pass to their destinations by unusual routes and hence modify the expected clinical effects of destruction of a particular nerve.
In both upper and lower limbs, the nerves which pass to the anterior surface supply a greater area of the skin than those which pass to the posterior surface. In the upper limb, this means that a greater part of the skin is supplied by nerves arising from the medial and lateral cords of the brachial plexus which are formed from the anterior divisions of the trunks of the plexus.
Cutaneous nerves from the spinal nerves adjacent to the brachial plexus .
The supraclavicular nerves (C. 3, 4) descend from the neck, cross the superficial surface of the clavicle and acromion, and supply the skin over the upper part of the front of the chest and deltoid muscle to the level of the sternal angle .
The intercostobrachial nerve (T. 2)—this lateral cutaneous branch of the second intercostal nerve enters the axilla from the second intercostal space. It descends obliquely across the axilla, communicates with the medial cutaneous nerve of the arm and sends branches to the floor of the axilla. It pierces the deep fascia below the axilla to supply the skin on the upper posteromedial part of the arm .
Cutaneous nerves from the posterior cord .
These supply the posterior surface of the limb. They arise from either the axillary or the radial nerve.
1. Cutaneous branches of the axillary nerve .
The upper lateral cutaneous nerve of the arm (C. 5, 6) pierces the deep fascia at a variable point posterior to the lower half of the deltoid muscle. It curves round the posterior border of that muscle and supplies the skin over its lower half.
2. Cutaneous branches of the radial nerve .
- The posterior cutaneous nerve of the arm (C. 5) arises in the axilla, pierces the deep fascia below the posterior axillary fold and supplies the skin of the back of the arm from the insertion of the deltoid to the olecranon.
- The lower lateral cutaneous nerve of the arm (C. 5, 6) arises posterior to the humerus and pierces the deep fascia below the insertion of the deltoid. It supplies the skin of the lateral side of the arm below that insertion.
- The posterior cutaneous nerve of the forearm (C. 6, 7, 8) arises with the lower lateral cutaneous nerve of the arm and pierces the deep fascia below it. It gives some branches to the lateral side of the arm, descends posterior to the lateral epicondyle and lies in the middle of the back of the forearm. It supplies the skin on the back of the forearm to the level of the wrist or occasionally on to the dorsum of the hand.
- The superficial branch of the radial nerve (C. 6, 7, 8) is a terminal branch of the radial nerve. It descends in the forearm between the extensor and flexor muscle groups passes posteriorly in the distal half of the forearm, and pierces the deep fascia 5 cm superior to the styloid process of the radius. It supplies the lateral two-thirds of the dorsum of the hand, the dorsal surfaces of the thumb and the lateral two and a half fingers through five dorsal digital nerves. These do not supply the terminal parts of the fingers. The area supplied by the nerve varies reciprocally with the other nerves with which it communicates on the dorsum of the hand (ulnar, posterior cutaneous nerve of the forearm, and median).
Cutaneous nerves from the lateral cord .
They arise from the musculocutaneous nerve .
The lateral cutaneous nerve of the forearm (C. 5, 6) pierces the deep fascia just lateral to the biceps, 2–3 cm proximal to the bend of the elbow. It divides into anterior and posterior branches which supply the anterolateral and posterolateral surfaces of the forearm, the anterior extending on to the ball of the thumb.
Cutaneous nerves from the medial cord .
They arise from either the medial cord or the ulnar nerve.
(a) Cutaneous branches of the medial cord .
- The medial cutaneous nerve of the arm (T. 1, 2) pierces the deep fascia on the medial side of the middle of the arm. It supplies the skin on the medial side of the inferior half of the arm, posterior to the basilic vein.
- The medial cutaneous nerve of the forearm (C. 8, T. 1) pierces the deep fascia with the basilic vein. As it descends, it divides into anterior and posterior branches which supply the anteromedial and posteromedial surfaces of the forearm to the wrist. The anterior branch supplies the skin of the distal part of the front of the arm, and together they supply the skin of the medial half of the forearm.
(b) Cutaneous branches of the ulnar nerve (C. 8, T. 1, with fibres of C. 7 received from the median nerve in the axilla) : this nerve supplies only skin in the hand and fingers.
- The dorsal branch of the ulnar nerve (C. 7, 8) arises from the ulnar nerve in the middle of the forearm and descends with it almost to the pisiform bone. It then passes obliquely backwards across the medial surface of the carpus to divide into two dorsal digital nerves. These supply the skin of the medial third of the back of the hand and the dorsal surfaces of the little finger and the medial half of the ring finger, except for the terminal part of the fingers which is supplied by the palmar digital branches of the ulnar nerve.
- The palmar (cutaneous) branch of the ulnar nerve (C. 7, 8) arises in the distal half of the forearm, pierces the deep fascia anterior to the wrist, and supplies the medial third of the palmar skin.
- Two palmar digital nerves (C. 7, 8) arise from the superficial branch of the ulnar nerve, distal to the pisiform bone. The medial nerve is the proper palmar digital nerve to the medial side of the little finger. The lateral nerve is a common palmar digital nerve which divides near the cleft between the little and ring fingers to give a proper palmar digital nerve to the contiguous sides of each. (Palmar and plantar digital nerves and arteries are called ‘proper’ when each is distributed only to one finger or toe. The term ‘common’ indicates that the nerve or artery is distributed to two adjacent fingers or toes as two proper digital branches.)
Cutaneous nerves from branches of the medial and lateral cords .
Cutaneous branches of the median nerve (C. 5, 6, 7 from the lateral cord; C. 8, T. 1 from the medial cord) which supply the skin in the hand :
- The palmar (cutaneous) branch (C. 6, 7, 8) arises a little superior to the wrist, pierces the deep fascia just above it, and descends to supply the lateral two-thirds of the palmar skin.
- Five palmar digital branches (C. 6, 7, 8) arise in the palm. The medial two are common palmar digital nerves. Each divides near the cleft on the medial and lateral aspects of the middle finger to form a proper palmar digital nerve to each side of that cleft. The common palmar digital nerve to the ring and middle fingers communicates with the ulnar nerve.
- The next palmar digital nerve is the proper palmar digital nerve to the lateral side of the index finger. The lateral two nerves curve round the distal border of the thenar eminence as proper palmar digital nerves to the sides of the thumb. The proper palmar digital nerves not only supply the corresponding parts of the fingers, but also send branches to the dorsal skin on the middle and terminal phalanges.
Dermatomes .
The area of skin supplied by one spinal segment is called a dermatome. A typical dermatome extends from the posterior median line to the anterior median line around the trunk . However, in the limbs, the dermatomes have migrated rather irregularly, so that the original uniform pattern is disturbed.
Embryological Basis of Dermatomes .
The early human embryo shows regular segmentation of the body. Each segment is supplied by the corresponding segmental nerve. In an adult, all structures including the skin, developed from one segment are supplied by their original segmental nerve. The limb may be regarded as an extension of the body wall and the segments from which they are derived can be deduced from the spinal nerves supplying them. The limb buds arise in the area of the body wall supplied by the lateral branches of anterior primary rami. The nerves to the limbs represent these branches .
Features of Dermatomes .
1 The cutaneous innervation of the upper limb is derived:
a. Mainly from segments C5–C8 and T1 of the spinal cord, and
b. Partly from the overlapping segments from above (C3, C4) as well from below (T2, T3). The additional segments are found only at the proximal end of the limb.
2 Since the limb bud appears on the ventrolateral aspect of the body wall, it is invariably supplied by the anterior primary rami of the spinal nerves. Posterior primary rami do not supply the limb. It is possible that the ventral and dorsal divisions of the trunks of the brachial plexus represent the anterior and posterior branches of the lateral cutaneous nerves .
3 There is varying degree of overlapping of adjoining dermatomes, so that the area of sensory loss following damage to the cord or nerve roots is always less than the area of distribution of the dermatomes .
4 Each limb bud has a cephalic and a caudal border known as preaxial and postaxial borders, respectively. In the upper limb, the thumb and radius lie along the preaxial border, and the little finger and ulna along the postaxial border.
5 The dermatomes of the upper limb are distributed in an orderly numerical sequence .
a. Along the preaxial border, by segments C3–C6 with overlapping of the dermatomes. b. The middle three digits (index, middle and ring fingers) and the adjoining area of the palm are supplied by segment C7.
c. The postaxial border is supplied (from below upwards) by segments C8, T1, T2. There is overlapping of the dermatomes.
6 As the limb elongates, it rotates laterally and gets adducted and the central dermatome C7 gets pulled in such a way that these are represented only in the distal part of the limb, and are buried proximally.
On the front of the limb, areas supplied by C5 and C6 segments adjoin the areas supplied by C8, T1 and T2 segments. There is a dividing line between them known as the ventral axial line along which C7 is buried proximally. It reaches the skin just proximal to the wrist .
On the back of the limb, C7 reaches the skin just proximal to the elbow. So the dorsal axial line ends more proximal to the ventral axial line. There is no overlapping across the ventral and dorsal axial lines .
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