Classify skeletal muscle according to shape and action. Describe its nerve supply and add a note on the functional groups of muscles.

Types of Muscle

Classification of Skeletal Muscle BY Shape and Fibre Direction

Parts and Attachments

• Origin — the more fixed and proximal attachment; insertion — the more mobile and distal
• The two may reverse in function — in a pull-up, the insertion becomes fixed and the origin moves
• Belly — the fleshy contractile part
• Tendon — cord-like; aponeurosis — flattened sheet; raphe — where two flat muscles interdigitate

Functional Classification

• Reciprocal innervation — when the agonist contracts, the antagonist is reflexly inhibited
• Shunt and spurt muscles — a spurt muscle inserts far from the joint and produces movement (biceps); a shunt muscle inserts close and stabilises (brachioradialis)

Nerve Supply

• Motor fibres — large myelinated alpha motor neurones to extrafusal fibres; gamma efferents to intrafusal fibres of the muscle spindle
• Sensory fibres (about 40% of the nerve) — from muscle spindles (length) and Golgi tendon organs (tension); the basis of proprioception
• Sympathetic fibres — vasomotor
• Motor unit = one motor neurone and all the fibres it supplies; small in precise muscles (extraocular, 1:5) and large in powerful ones (gluteus maximus, 1:2000)
• The nerve usually enters at the "motor point", near the junction of the upper and middle thirds

Blood Supply

• Muscles are richly vascular, with many anastomosing branches
• Vessels usually enter with the nerve at the neurovascular hilum
• Blood flow rises many-fold during exercise; capillary density reflects the muscle's oxidative type

Muscle Fibre Types

• Most human muscles are mixed; the proportion is genetically determined and modified by training
• Endurance training increases type I capacity; sprint training hypertrophies type II fibres

Applied Aspects

• Denervation causes rapid wasting with fibrillation; reinnervation within about 18 months may restore function, after which fibrosis is irreversible
• Volkmann ischaemic contracture — muscle necrosis from compartment ischaemia, followed by fibrosis and permanent deformity
• Intramuscular injection into the gluteal region must be in the upper outer quadrant to avoid the sciatic nerve; the deltoid and vastus lateralis are alternatives
• Muscle biopsy distinguishes myopathy from neuropathy — grouped atrophy indicates denervation
• Tendon transfer exploits the fact that a muscle retains its own nerve supply when its insertion is moved, restoring lost function after nerve injury