Skeletal muscle tissue

Textus muscularis striatus skeletalis

Also known as: striated muscle, voluntary muscle, skeletal muscle

Striated, voluntary muscle made of long, multinucleated fibres arranged in bundles. It forms the named muscles that move the skeleton.

Sarcomere & Sliding Filaments

Overview

Fibre diameter≈10–100 µm

Fibre lengthmm up to ≈30 cm (sartorius)

Share of body mass≈40%

Myonuclear domainEach nucleus serves a fixed volume of cytoplasm

Skeletal muscle tissue consists of very long cylindrical cells (muscle fibres) formed by the fusion of embryonic myoblasts. Each fibre has many peripheral nuclei and regular cross-striations, produced by sarcomeres in register across neighbouring myofibrils. The tissue is under voluntary control by somatic motor neurons. Each fibre normally has a single neuromuscular junction.

Three connective tissue sheaths organise the tissue and carry vessels and nerves: endomysium around each fibre, perimysium around fascicles, and epimysium around the whole muscle. They pass force to the tendon. The tissue is highly vascular, with a capillary network around every fibre that can open up greatly during exercise. Satellite cells beneath the basal lamina allow limited regeneration, but large injuries heal with fibrous scar.

Location: In the ≈600+ named skeletal muscles of the head, trunk and limbs. Also in the upper third of the oesophagus, the tongue, pharynx and larynx, and the external urethral and anal sphincters.

Parts & subdivisions

Epimysium
Dense irregular connective tissue surrounding the whole muscle. It is continuous with deep fascia and the tendon.
Perimysium
Surrounds each fascicle and carries larger vessels and nerves. Muscle spindles lie within it.
Endomysium
Delicate reticular fibres and basal lamina around each fibre. It contains capillaries and nerve terminals.
Fascicles
Bundles of fibres. Their arrangement (parallel, pennate, convergent, circular) defines the muscle's architecture.
Muscle fibres
The multinucleated contractile cells, 10–100 µm in diameter and up to ≈30 cm long.
Tendon / aponeurosis
Dense regular collagen that transmits force to bone.

Anatomy

Histology: Longitudinal sections show long unbranched fibres with alternating dark A bands and light I bands, and flattened nuclei at the periphery under the sarcolemma. Transverse sections show polygonal fibre profiles, peripheral nuclei and myofibrils that appear as dots (Cohnheim fields). Nuclei in the centre of fibres suggest regeneration or myopathy.
Development: Develops from paraxial mesoderm (somites and somitomeres). Myoblasts fuse into myotubes under the control of the myogenic regulatory factors MyoD, Myf5, myogenin and MRF4.

Function

Generates force and movement under voluntary control
Maintains posture and stabilises joints
Produces heat (thermogenesis, shivering)
Stores glycogen and protein and takes up glucose

Physiology: Contraction follows excitation–contraction coupling triggered by acetylcholine at the neuromuscular junction. Force is graded by motor-unit recruitment and by firing frequency.

Clinical significance

Atrophy and sarcopenia

Disuse, denervation, cachexia and ageing reduce fibre size, and denervation can also reduce fibre number. After ≈50 years of age, adults lose roughly 1–2% of muscle mass per year (sarcopenia), mainly type II fibres. This raises the risk of falls.

Myopathy vs neuropathy

Myopathies cause proximal weakness, a raised CK and short, small polyphasic motor unit potentials on EMG. Neurogenic disorders cause distal weakness, fasciculations, and large, long motor unit potentials with fibre-type grouping on biopsy.

Test yourself

In skeletal muscle, what directly opens the ryanodine receptor (RyR1) of the sarcoplasmic reticulum?

Ca²⁺ entering through L-type channels (Ca²⁺-induced Ca²⁺ release)
Mechanical coupling to voltage-sensing dihydropyridine receptors in the T-tubule
IP₃ generated by muscarinic receptors
cAMP-dependent phosphorylation by protein kinase A

Answer: Mechanical coupling to voltage-sensing dihydropyridine receptors in the T-tubule

In skeletal muscle, the DHPR (Caᵥ1.1) acts as a voltage sensor that is physically linked to RyR1, so depolarisation alone opens it. Ca²⁺-induced Ca²⁺ release through RyR2 is the mechanism in cardiac muscle.