Chapter 1: Muscle Tissue Basics

Muscle is the tissue that moves you: it walks your skeleton, pushes food along your gut, narrows your blood vessels and beats your heart. Under the microscope, all of that work comes from one shared feature. Muscle cells are packed with contractile filaments, and the way those filaments are arranged is what lets you tell the three kinds of muscle apart on a slide.

This chapter covers the vocabulary and the ground rules: what muscle tissue is, how it is classified, the special names given to parts of a muscle cell, and how to read the two section planes you will meet in the practicum. Chapter 2 then takes smooth, skeletal and cardiac muscle one at a time. If you want the molecular side of contraction (sliding filaments, calcium, the crossbridge cycle), that is in the Block 1.2 physiology ebook; this ebook stays with what you can see.

What muscle tissue is

Muscle tissue is a tissue composed of a collection of muscle cells characterized by a large number of cytoplasmic filaments that are contractile. The filaments are the proteins actin (thin filaments) and myosin (thick filaments). When they slide past each other the cell shortens, and because muscle cells are anchored to their neighbors and to connective tissue, the whole organ shortens with them.

Three properties follow from that design:

• Muscle cells are elongated, lined up along the direction in which they pull. That is why a muscle looks so different depending on how it was cut (see "Section planes" below).
• Most of the cytoplasm is filled with filaments, so muscle stains strongly with eosin (pink to red in H&E), while nuclei stand out as dark purple ovals.
• Each cell is wrapped in a thin layer of connective tissue, which transmits its pull and carries capillaries and nerves.

Classification

The practicum classifies muscle in two ways at once: by what it looks like (histology) and by how it is controlled (physiology).

• Histologically, muscle is either striated (skeletal and cardiac muscle, whose filaments are organized into repeating units that show up as cross-bands) or non-striated (smooth muscle, whose filaments run in a looser lattice without regular bands).
• Physiologically, muscle is either voluntary (skeletal muscle, under conscious control through somatic motor nerves) or involuntary (cardiac and smooth muscle, controlled by the autonomic nervous system, hormones and, in the heart, its own pacemaker).

Cardiac muscle is the "odd one out" that trips people up: it is striated like skeletal muscle but involuntary like smooth muscle.

The Indonesian names you will hear in the lab are otot rangka or otot lurik (skeletal, "striped" muscle), otot jantung (cardiac) and otot polos ("plain", smooth muscle). Smooth muscle is also called visceral muscle, because it lines the hollow organs (viscera).

The vocabulary of the muscle cell

Muscle cells were described before modern cell biology had settled its terms, so many of their parts carry the prefix sarco- (Greek sarx, flesh) instead of the usual names. Learn these as pairs:

Two traps:

• A myofiber is a whole cell; a myofibril is a bundle of filaments inside that cell. On a slide you can see myofibers easily; myofibrils only show as the fine stippling in cross-sections (Cohnheim's areas, Chapter 2) or as longitudinal striations.
• "Muscle fiber" in histology means a cell, not a connective tissue fiber like collagen. Exam questions sometimes lean on that ambiguity.

Section planes

Because muscle cells are long and parallel, the same muscle looks completely different depending on the plane of section. Before you decide what kind of muscle you are looking at, decide how it was cut.

In the atlas figure used in the practicum, the transverse part is marked OL and the longitudinal part IC. Those letters are just the figure's labels, but the question "What kind of section is the part marked OL?" does come up, so remember: OL = transverse, IC = longitudinal.

A practical routine at the microscope:

1. At low power, find the direction the fibers run. If you see long strips, it is longitudinal; if you see a mosaic of round shapes, it is transverse.
2. In longitudinal section, look for cross-striations (skeletal or cardiac) and for branching (cardiac).
3. In transverse section, look at where the nuclei sit: at the periphery of each profile (skeletal) or in the middle (smooth or cardiac).

A first comparison

Chapter 2 goes through each type in detail. As a map, here is the summary table from the practicum, translated:

Keep this table in mind; almost every identification question in the practicum reduces to two observations from it: striations or not, and where the nuclei are.