Most Things Biology

Pl. II · Striatus ramosus — cardiac muscle

Time To Read

3–4 minutes

Date Last Modified

Pl. II · FAM. STRIATIDAE

Striatus ramosus var. populinus

the grove  ·  cardiac muscle

The plate

The specimen, drawn

Pl. I

[ plate drawing to come · the aspen grove ]

Habitat & range

Where it lives

Body map

[ body map to come — needs the BioRender export ]

Range

  • The myocardium — the muscular wall of all four chambers, thickest by far in the left ventricle.
  • Thin sleeves of muscle running a short way up the great veins where they enter the atria.
  • The conducting cells — SA node, AV node, bundle branches, Purkinje fibers. These are modified cardiac muscle, not nerve, which surprises everyone.
  • And nowhere else at all. It is the only tissue in the body that lives in a single organ.

Collection record

  • Add your slide numbers here.

Niche

What it does for a living

Tireless and automatic. Cardiac muscle contracts somewhere around three billion times in an average life without ever once being told to start, and it is not allowed to fatigue or to fire out of step. The cell is built for exactly that: branched, so force and signal spread sideways as well as end to end; joined to its neighbours by intercalated discs that are glue and electrical doorway in the same structure; and so packed with mitochondria that it runs aerobically or it does not run. There is no anaerobic option and no rest day.

field collected specimen

How to know it on sight

  • Striations, as in skeletal muscle — but fainter, and the fibers are far shorter.
  • Fibers branch and rejoin, so the tissue reads as a net rather than a bundle of cables.
  • One nucleus to a cell, round or oval, sitting in the middle of the fiber rather than at its edge.
  • Intercalated discs — dark lines crossing a fiber at intervals, marking where one cell stops and the next starts.
Microscopic view of cardiac muscle tissue stained with Masson's trichrome, highlighting elongated, branching cardiac muscle fibers in deep red, separated by blue connective tissue strands.

Specimen 1

A magnified view of cardiac muscle tissue as seen on a histological slide. The image exhibits striated muscle fibers running horizontally across the frame, purple-stained nuclei are squished between sarcolemma.

Right heart wall. The fibers fork and rejoin, and the nuclei sit in the middle of the fiber, not along its rim.

Specimen 2

A magnified view of cardiac muscle tissue as seen on a histological slide. The image exhibits striated muscle fibers running horizontally across the frame, purple-stained nuclei are squished between sarcolemma.

Ventricle. Look for dark lines running straight across a fiber — those are intercalated discs, the join between one cell and the next.

Specimen 3

A magnified view of cardiac muscle tissue as seen on a histological slide. The image exhibits striated muscle fibers running horizontally across the frame, stained a vibrant red. White spaces are visible between the fibers.

Masson’s trichrome. The muscle takes the red; the blue is the collagen scaffold the fibers are slung in.

Illustration showing three types of muscle tissue: smooth muscle (spindle-shaped), cardiac muscle (branched and striated), and skeletal muscle (cylindrical and striated). Each type is shown as a stylized cell with a visible nucleus.

People keep calling her by the wrong name

Look-alikes

Two plates. Read the line underneath each one before you decide.

Skeletal muscle

Microscopic view of skeletal muscle tissue arranged in a longitudinal section. The muscle fibers appear as elongated, pink-stained bands running horizontally across the image. Darkly stained oval nuclei are visible within the muscle fibers. The striated, or striped, appearance of the muscle fibers is also clearly visible.
Dense irregular connective tissue. Pink-stained collagen fibers and cells in thin skin.

Tell them apart by the branch. Skeletal fibers run straight and parallel with many nuclei at the rim; cardiac fibers fork, and carry one nucleus in the centre.

Smooth muscle

Histological cross section of smooth muscle
Micrograph of kidney tissue displaying tubular structures lined by simple cuboidal epithelium. The cells manifest as circular or elliptical forms with centrally positioned, round, blue-stained nuclei.

Tell them apart by the stripes. Both keep one central nucleus, but only cardiac is striated — and smooth muscle never branches.

Examine it yourself

The virtual slide

Find a fiber that forks. Then follow it along until you meet a dark line crossing it at right angles: that is an intercalated disc.

Virtual slide courtesy of Histology Guide.

YOur missions

Field Notes for Future Missions

Mission 1: find the discs

Mission 2: quick check

Field notes

Why this organism

Pando is a single quaking aspen in Utah — some forty thousand trunks that look like a forest and are one organism, every stem grafted to its neighbours underground and sharing whatever it has. Cardiac muscle is a grove. The fibers branch and rejoin, and the intercalated discs between them are root grafts: mechanical anchor and open electrical door in one structure, so a signal that starts in one cell does not stay in that cell. That is why a heart beats as a single thing instead of as a bag of cells all twitching on their own schedule.

Next · Pl. III

Laevis fusiformis var. strangulans

the slow squeeze · smooth muscle.

Continue to Pl. III →

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← Epithelial Tissues: A Guided Journey

All plates — The Tissue Field Guide: Epithelium

Frontispiece Ordo Epithelia (main page)
Pl. I · Simple squamous
Pl. II · Simple cuboidal
Pl. III · Simple columnar
Pl. IV · Stratified squamous
Pl. V · Stratified cuboidal
Pl. VI · Stratified columnar
Pl. VII · Transitional
Pl. VIII · Pseudostratified columnar
Epithelial Identification Game

All Modules
  1. Anatomical Language, Membranes & Homeostasis
  2. Just Enough Chemistry
  3. The Cell & Its Transport
  4. Making Cells & Proteins
  5. The Integumentary System
  6. The Skeletal System
  7. The Muscular System
  8. Nervous Tissue & the Senses
  9. The Spinal Cord
  10. The Brain & the Blood–Brain Barrier
  11. The Autonomic Nervous System
  12. The Special Senses
  13. The Endocrine System
  14. Blood
  15. The Heart
  16. Blood Vessels
  17. The Digestive System
  18. The Respiratory System
  19. The Urinary System
  20. Fluids, Electrolytes & Acid–Base Balance
  21. The Reproductive System
  22. The Immune System

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