Chapter 2: Smooth, Skeletal & Cardiac Muscle

This chapter takes the three muscle types one at a time: where each is found, how it is controlled, and above all what it looks like on the slide. For every type, learn the three or four features that identify it and the one that most often confuses it with the others.

Smooth muscle

Smooth muscle (otot polos, visceral muscle) consists of fusiform cells without striations, and it contracts slowly and involuntarily. It is the muscle of the internal organs:

• walls of blood vessels (tunica media of arteries and veins),
• the gastrointestinal tract (the muscularis externa, in inner circular and outer longitudinal layers),
• the respiratory tract (around bronchi and bronchioles),
• the uterus (myometrium) and the urinary bladder (detrusor),
• smaller sites such as the arrector pili muscles of hair (Chapter 5) and the iris.

It is innervated by the autonomic nervous system. Autonomic axons do not form a discrete motor end plate on each cell; they release transmitter from swellings (varicosities) near the cells, and the practicum simply calls this myoneural junction a synapse. Neighboring cells are coupled by gap junctions, so contraction spreads through a sheet of cells.

How it looks

• Fusiform (spindle-shaped) cells, thick in the middle and tapering at both ends, packed so that the thick middle of one cell lies next to the thin ends of its neighbors.
• No visible cross-striations. The cytoplasm looks evenly pink.
• One elongated nucleus, centrally located. In a contracted cell the nucleus may be twisted like a corkscrew.
• Each cell is wrapped in endomysium (reticular fibers), which binds the cells into a sheet.

In transverse section the cells appear as round profiles of very different sizes, because the knife cuts some cells through their thick middles and others through their tapered ends. Only the larger profiles contain a nucleus, and it sits in the center. That mix of large profiles with central nuclei and small empty ones is characteristic.

Easily confused with

Dense regular connective tissue (tendon) and nerve also look like wavy pink strands with elongated nuclei. Clues that favor smooth muscle: the nuclei are inside the cells and centered in them, the cytoplasm is more strongly eosinophilic, and the cells taper at both ends. In tendon the nuclei are flattened and squeezed between collagen bundles.

Smooth muscle regenerates well: its cells can still divide by mitosis, and new cells can also arise from pericytes around blood vessels.

Skeletal muscle

Skeletal muscle is also known as striated muscle or cross-striated muscle (otot lurik, otot rangka). It contracts voluntarily, and its contraction is triggered by acetylcholine (ACh) released at the motor end plate (MEP), also called the neuromuscular junction (NMJ). It forms the muscles of the limbs (extremities) and trunk, and also the tongue, the external eye muscles and the upper esophagus.

How it looks

• Cylindrical fibers with multiple nuclei. Each fiber formed by the fusion of many myoblasts in the embryo, so one cell has dozens to hundreds of nuclei.
• The sarcolemma is clearly visible as the sharp outline of each fiber.
• Oval nuclei located peripherally, just beneath the sarcolemma. This is the single most useful feature: no other muscle puts its nuclei at the edge.
• Cross-striations: regular alternating light and dark bands across the fiber.

Connective tissue sheaths

From the outside in:

1. Epimysium: the outermost layer of dense irregular connective tissue, surrounding the entire muscle.
2. Perimysium: a layer of dense irregular connective tissue around each fascicle (bundle of fibers). Larger vessels and nerves travel here.
3. Endomysium: a very thin layer of fine reticular fibers around each muscle fiber, carrying capillaries right up to the cell.

A classic question: "Which connective tissue wraps a single fascicle?" The answer is the perimysium; "a single fiber" is the endomysium. The peri- layer goes around a group, the endo- layer around one cell.

Transverse section: Cohnheim's areas

In transverse section you can see the whole hierarchy: epimysium, perimysium, fascicles, and muscle fibers surrounded by endomysium. The fibers are large, polygonal profiles, and their nuclei sit at the periphery.

Look closely inside a single fiber and the cytoplasm is not uniform: it is broken into tiny dots or polygons separated by pale lines. Those are the bundles of myofibrils cut across, and the island-like pattern they make is called Cohnheim's areas (Cohnheim's fields). They are the answer to "What are the island-like structures inside a cross-cut skeletal fiber?"

Longitudinal section: the bands

In longitudinal section, the fiber shows alternating light and dark bands:

• A band: the dark band, called anisotropic because it changes polarized light. It contains the thick myosin filaments (overlapping with actin at its ends).
• I band: the light band, called isotropic. It contains only thin actin filaments.
• Z line (Z disc): a dark transverse line in the center of the I band, where the thin filaments are anchored.
• M line: a dark transverse line in the center of the A band, where the thick filaments are linked.
• H zone: a lighter region in the center of the A band, where only myosin is present (no overlapping actin).

The segment from one Z line to the next is a sarcomere, the smallest contractile unit of striated muscle. A myofibril is simply a long chain of sarcomeres.

In the practicum microscope

Skeletal muscle has limited regeneration. The fibers themselves cannot divide, but satellite cells (small stem cells tucked under the basal lamina) can fuse to repair small injuries. Larger losses heal with scar.

Cardiac muscle

Cardiac muscle (otot jantung) is found only in the heart (myocardium) and the roots of the great veins. It is striated but involuntary: it contracts rhythmically on its own, with its rate adjusted by the autonomic nervous system.

How it looks

• Parallel, striated fibers: the cross-bands are the same A and I bands as in skeletal muscle, although usually less crisp.
• Branching and anastomosing: fibers split and rejoin, forming a three-dimensional network. The practicum calls this intercellular anastomosis (anastomose serabut otot).
• Intercalated discs: irregular dark transverse lines, often step-like, crossing the fiber. Each disc is the junction between two cells, containing desmosomes and fasciae adherentes (which hold the cells together) and gap junctions (which let the impulse pass from cell to cell).
• One large, oval, central nucleus, occasionally two. A pale perinuclear zone (glycogen and, with age, lipofuscin pigment) often surrounds it.

Easily confused with

• Skeletal muscle: both are striated. Cardiac muscle has central nuclei, branches and intercalated discs; skeletal muscle has peripheral nuclei and unbranched fibers.
• Smooth muscle: both have central nuclei. Look for striations and intercalated discs; smooth muscle has neither.

Cardiac muscle shows essentially no regeneration. After a myocardial infarction, the dead muscle is replaced by fibrous scar tissue, which does not contract.

Putting it together

For the underlying molecular events (how calcium and ATP drive the crossbridge cycle), see the Block 1.2 physiology ebook. For the general features of the four basic tissues, see Muscle & Nervous Tissue in the Block 1.1 histology ebook.