(a) Describe the formation, course and branches of ulnar nerve. Which muscles do each of the branches supply ? Why is ulnar nerve…

(a) Ulnar nerve

The ulnar nerve arises from the medial cord of the brachial plexus (C8–T1). In the axilla it lies posterior to the axillary artery; in the arm it descends medial to the brachial artery in the medial bicipital groove and, at mid-arm, pierces the medial intermuscular septum to enter the posterior compartment, descending in front of the medial head of triceps. It gives no branches in the arm. At the elbow it passes posterior to the medial epicondyle through the cubital tunnel, between the medial epicondyle and olecranon. In the forearm it lies between the heads of flexor carpi ulnaris and on flexor digitorum profundus, medial to the ulnar artery. It gives muscular branches to flexor carpi ulnaris and the medial half of flexor digitorum profundus. The palmar cutaneous branch arises proximal to the wrist, passes superficial to the flexor retinaculum and supplies the skin of the medial palm. The dorsal cutaneous branch arises 5–8 cm proximal to the wrist, passes posterior to the wrist, superficial to the extensor retinaculum, and supplies the dorsum of the hand and medial 1½ fingers. The nerve enters Guyon’s canal at the wrist and divides into superficial and deep branches. The superficial branch supplies palmaris brevis and palmar skin of the medial 1½ fingers. The deep branch supplies the hypothenar muscles (abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi), the third and fourth lumbricals, all interossei, adductor pollicis and the deep head of flexor pollicis brevis. It is called the “musician’s nerve” because these intrinsic hand muscles control fine finger movements, abduction, adduction and opposition required for playing piano, guitar and violin; compression or injury impairs precision grip and dexterity.

(b) Plantar innervation and Morton’s metatarsalgia

The sole is supplied by branches of the tibial nerve and, for the lateral heel, by the sural nerve. Behind the medial malleolus the tibial nerve gives medial calcaneal branches to the skin of the medial heel, then divides into medial and lateral plantar nerves. The medial plantar nerve supplies the skin of the medial sole and medial 3½ toes, and the muscles abductor hallucis, flexor digitorum brevis, flexor hallucis brevis and the first lumbrical. The lateral plantar nerve supplies the skin of the lateral sole and lateral 1½ toes, and the muscles quadratus plantae, abductor digiti minimi, flexor digiti minimi brevis, adductor hallucis, interossei and the second to fourth lumbricals. The sural nerve supplies the lateral heel. Morton’s metatarsalgia is an interdigital neuroma, usually between the third and fourth metatarsal heads, where the common digital nerve is compressed by the intermetatarsal ligament; it causes burning pain, paraesthesia and a clicking sensation in the forefoot, worsened by tight shoes.

(c) Jaundice

Jaundice is classified aetiologically as pre-hepatic, hepatic and post-hepatic. Pre-hepatic jaundice results from increased bilirubin production, e.g. haemolytic anaemia, malaria, G6PD deficiency or transfusion reaction. Hepatic jaundice results from hepatocellular injury, e.g. viral hepatitis A or E, alcoholic liver disease, cirrhosis, drug-induced injury, Wilson’s disease or alpha-1 antitrypsin deficiency. Post-hepatic jaundice results from obstruction to bile flow, e.g. choledocholithiasis, cholangiocarcinoma, carcinoma of head of pancreas, biliary stricture or Ascaris in the common bile duct.

• Pre-hepatic: Blood — Unconjugated bilirubin increased; conjugated normal; Urine — Bilirubin absent; urobilinogen increased; Stool — Normal or dark
• Hepatic: Blood — Unconjugated and conjugated bilirubin increased; Urine — Bilirubin present; urobilinogen increased or variable; Stool — Normal to pale
• Post-hepatic: Blood — Conjugated bilirubin increased; unconjugated normal; Urine — Bilirubin present; urobilinogen decreased or absent; Stool — Acholic/clay-coloured

(d) Nerve impulse

The resting axonal membrane is about –70 mV, maintained by the Na+/K+ ATPase and selective K+ permeability. A stimulus that reaches threshold opens voltage-gated Na+ channels; Na+ influx depolarises the membrane to about +30 mV. Na+ channels then inactivate and voltage-gated K+ channels open, so K+ efflux repolarises the membrane. A brief hyperpolarisation follows because K+ channels close slowly. The absolute refractory period prevents a second impulse, and the relative refractory period requires a stronger stimulus. Conduction occurs because local currents depolarise the adjacent membrane, regenerating the action potential along the axon; it is all-or-none and non-decremental. In saltatory conduction, myelin sheaths formed by Schwann cells in the peripheral nervous system or oligodendrocytes in the central nervous system insulate the axon, leaving nodes of Ranvier rich in voltage-gated Na+ channels. The action potential regenerates only at the nodes and appears to jump from node to node, making conduction faster and more energy-efficient because fewer ions cross the membrane.

(e) Muscle spindle

The muscle spindle is a fusiform proprioceptor within skeletal muscle. It contains intrafusal fibres—nuclear bag and nuclear chain fibres—surrounded by a capsule, with extrafusal fibres outside. Sensory endings are Ia annulospiral endings on the bag and chain fibres and II flower-spray endings mainly on chain fibres. Efferent supply is by gamma motor neurons to the contractile polar ends of the intrafusal fibres (dynamic γ to nuclear bag₁ fibres, static γ to bag₂ and chain fibres) and alpha motor neurons to the extrafusal fibres. Ia endings respond to both the length and the rate of stretch (dynamic response), while II endings signal static length.

Diagram (drawn below):

The Ia afferents enter the spinal cord and form monosynaptic excitatory connections with alpha motor neurons of the same muscle, producing the stretch reflex; they also inhibit antagonist motoneurons. The II afferents contribute to proprioception and slower stretch responses. The gamma motor neurons adjust intrafusal fibre tension, maintaining spindle sensitivity during muscle contraction; this alpha-gamma co-activation allows the spindle to monitor muscle length throughout movement. Thus the spindle functions as a length detector, initiates the stretch reflex, provides proprioceptive information, and helps coordinate posture and fine motor control.