Skeletal Muscle

Skeletal muscles consist of tightly packaged muscular bundles (fascicles) surrounded by connective tissue

• Each bundle contains multiple muscle fibres, which are formed when individual muscle cells fuse together

  • Muscle fibres are multinucleated and have many mitochondria (for ATP production)

  • The muscle fibres in a bundle are collectively surrounded by a membrane called the sarcolemma

• Muscle fibres contain long structures called myofibrils that run the length of the fibre and are responsible for muscular contraction

  • The myofibrils are surrounded by a membranous network called the sarcoplasmic reticulum

• The myofibrils are composed of protein myofilaments and can be divided into repeating sections called sarcomeres

  • Each sarcomere represents a single contractile unit of a muscle fibre

Skeletal Muscle Organisation

Motor Units

Skeletal muscles are innervated by motor neurons, with each neuron connecting to multiple muscle fibres via branching axon terminals

• Neurons cause skeletal muscles to contract by releasing neurotransmitters (acetylcholine) into the neuromuscular junction

A motor unit represents a single motor neuron along with all the muscle fibres it stimulates

• The number of motor units activated will determine the intensity of the muscle contraction

• Low intensity muscle contractions activate a low number of muscle units, while high intensity contractions activate more motor units