Human Anatomy
Muscle Histology
Muscle histology for MBBS: skeletal, cardiac and smooth muscle, sarcomere structure, intercalated discs and membrane systems, mapped to NMC codes AN67.1 to AN67.3.
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Muscle Histology
Muscle histology for MBBS: skeletal, cardiac and smooth muscle, sarcomere structure, intercalated discs and membrane systems, mapped to NMC codes AN67.1 to AN67.3.
This chapter distinguishes skeletal, cardiac and smooth muscle by their microscopic features and correlates each with its function. It covers the sarcomere, the membrane systems and the connective tissue framework of muscle.
High-yield: Muscle Histology
- The three types of muscle are skeletal, cardiac and smooth, distinguished by structure and control.
- Skeletal muscle is striated, multinucleated and under voluntary control, with peripherally placed nuclei.
- Cardiac muscle is striated, branched and involuntary, with one or two central nuclei and intercalated discs.
- Smooth muscle is non-striated and involuntary, with spindle-shaped cells and a single central nucleus.
- Intercalated discs are the junctions between cardiac muscle cells and contain desmosomes and gap junctions.
- The gap junctions of intercalated discs allow cardiac muscle to contract as a functional syncytium.
- The striations of skeletal muscle come from the ordered arrangement of actin and myosin in sarcomeres.
- The sarcomere runs from one Z line to the next, and the A band stays constant while the I band shortens in contraction.
- The connective tissue coverings are endomysium around fibres, perimysium around fascicles and epimysium around the whole muscle.
- The sarcoplasmic reticulum stores calcium, and its terminal cisternae flank the T tubule to form a triad in skeletal muscle.
- Cardiac muscle forms diads rather than triads and depends partly on extracellular calcium for contraction.
- Smooth muscle contracts slowly, sustains tone and lacks the regular sarcomere pattern of striated muscle.
- Skeletal muscle regenerates through satellite cells that lie beneath the basal lamina.
- Cardiac muscle has very limited regenerative capacity, so damaged myocardium is replaced by scar.
- Smooth muscle can both regenerate and increase in number, for example in the pregnant uterus.
Three muscle types compared
- **Skeletal:** Striated, multinucleate with peripheral nuclei, voluntary. Regenerates via satellite cells.
- **Cardiac:** Striated, branched, one or two central nuclei, involuntary. Intercalated discs; poor regeneration.
- **Smooth:** Non-striated, spindle-shaped, single central nucleus, involuntary. Slow, sustained contraction.
- **Sarcomere:** Z line to Z line. A band constant, I band shortens; the basis of the striated pattern.
NMC competencies in this chapter
- **AN67.1:** Identification of the types of muscle under the microscope
- **AN67.2:** Classification of muscle and its structure-function correlation
- **AN67.3:** Ultrastructure of muscular tissue
Frequently Asked Questions
How can you tell the three muscle types apart under the microscope?
Skeletal muscle is striated with many peripheral nuclei, cardiac muscle is striated and branched with central nuclei and intercalated discs, and smooth muscle is non-striated with spindle-shaped cells and single central nuclei.
What are intercalated discs?
They are the specialised junctions between cardiac muscle cells. They contain desmosomes that hold the cells together and gap junctions that let the heart contract as a coordinated unit.
What is a sarcomere?
It is the contractile unit of striated muscle, extending from one Z line to the next. During contraction the I band and the H zone shorten while the A band stays the same length.
Which muscle type regenerates best?
Smooth muscle regenerates readily and can increase in number. Skeletal muscle regenerates through satellite cells, while cardiac muscle has almost no regenerative capacity and heals by scar formation.
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