Skeleton, Muscle and Movement

Lessons

Senior 6 · Biology

Skeleton, Muscle and Movement

4 lessons

Lesson 2 of 4

Cardiac, smooth and skeletal muscle, and the ultrastructure of striated muscle

30 min

One protein pair, three tissues

At about 70 beats a minute, your heart contracts roughly 100 000 times a day and never needs a rest. The muscles of your arm cannot hold a full jerrycan out at arm's length for even a few minutes. The walls of your intestine squeeze food along for hours without you ever deciding to do it. All three tissues are muscle, and all three contract by the same molecular trick: the proteins actin and myosin sliding past each other. What differs is how the cells are built, how they are controlled and how quickly they tire. This lesson compares the three types and then looks inside a skeletal muscle fibre with the electron microscope.

Definition

Muscle fibre: a single skeletal muscle cell, long and cylindrical, with many nuclei.

Striated muscle: muscle that looks striped under the microscope because its actin and myosin are arranged in repeating units; skeletal and cardiac muscle are striated, smooth muscle is not.

Myofibril: one of the many long protein cylinders, about 1 to 2 µm across, that fill a muscle fibre and do the contracting.

Sarcomere: the repeating contractile unit of a myofibril, from one Z line to the next.

Myogenic: describes muscle whose contraction starts within the muscle itself, without a nerve impulse; cardiac muscle is myogenic.

Sarcoplasmic reticulum (SR): the specialised endoplasmic reticulum of a muscle fibre, which stores calcium ions and releases them to trigger contraction.

T-tubules (transverse tubules): infoldings of the cell surface membrane (sarcolemma) that carry action potentials deep into the fibre.

The three types of muscle

Feature	Skeletal	Cardiac	Smooth
Where found	Attached to bones by tendons	Wall of the heart only	Walls of the gut, blood vessels, bladder, uterus, airways; iris
Cells	Long cylindrical fibres, many nuclei at the edge	Short branched cells, one (sometimes two) central nuclei	Spindle-shaped cells, one central nucleus
Striations	Yes	Yes	No; actin and myosin form a criss-cross network
Cell junctions	Fibres insulated from one another	Intercalated discs with gap junctions, so impulses spread from cell to cell	Many gap junctions in most smooth muscle
Control	Voluntary: somatic motor neurones	Involuntary and myogenic; rate altered by autonomic nerves and adrenaline	Involuntary: autonomic nerves, hormones, stretch
Contraction	Fast and powerful; tires quickly	Rhythmic; never tires; long refractory period prevents sustained contraction	Slow and sustained; uses little ATP; tires very slowly

The intercalated discs of cardiac muscle hold the cells together with strong anchoring junctions (desmosomes) and let ions pass between them through gap junctions. A wave of excitation therefore spreads through the whole chamber wall so that it contracts as a unit, which you studied in Unit 3. Because skeletal fibres are insulated from one another, each must be stimulated by its own motor neurone, which is what allows fine, graded control of movement.

Inside a skeletal muscle

A skeletal muscle is built like a rope, in bundles within bundles:

The muscle (for example the biceps) is wrapped in connective tissue that continues into the tendons.
It contains bundles of muscle fibres (fascicles).
Each muscle fibre is one cell, formed in the embryo when many cells (myoblasts) fuse; this is why it has many nuclei. Its cell surface membrane is the sarcolemma and its cytoplasm the sarcoplasm.
The fibre is packed with myofibrils, lined up so that their stripes are in register across the fibre.
Each myofibril is a chain of sarcomeres, made of overlapping myofilaments: thick filaments of myosin and thin filaments of actin.

A muscle fibre is also rich in other organelles, each linked to its job:

many mitochondria between the myofibrils, to make ATP by aerobic respiration;
an extensive sarcoplasmic reticulum wrapped around every myofibril, to store and release Ca²⁺ ions;
T-tubules running across the fibre and touching the SR, so that an action potential at the surface reaches every myofibril almost at once;
glycogen granules as a store of glucose, and myoglobin, a red pigment that stores oxygen.

The sarcomere

Seen with the electron microscope, each sarcomere shows a pattern of light and dark bands. The pattern exists because thick and thin filaments overlap only partly:

Z line: a disc of protein that anchors the thin filaments; it marks each end of a sarcomere.
I band (light): thin filaments only. It is centred on a Z line, so half of each I band belongs to each of two neighbouring sarcomeres.
A band (dark): the whole length of the thick filaments, including the regions where thin filaments overlap them.
H zone: the lighter middle of the A band, where there are thick filaments only.
M line: proteins in the centre that hold the thick filaments in place.

Each thick filament is a bundle of several hundred myosin molecules. Each molecule has a long tail and a head that sticks out sideways; the head can bind to actin and can hydrolyse ATP (it is a myosin ATPase). The heads point away from the M line in both directions, so they pull the thin filaments on each side towards the centre. Each thin filament is two twisted chains of actin, with a long thread of tropomyosin lying in the groove and small troponin complexes attached at intervals. At rest, tropomyosin covers the binding sites on actin where the myosin heads would attach. In cross-section through the overlap region, each thick filament is surrounded by six thin filaments in a hexagon.

Worked example 1: measuring sarcomeres on an electron micrograph

On an electron micrograph printed at a magnification of ×20 000, a row of 5 sarcomeres, from the first Z line to the sixth, measures 220 mm. Calculate the mean length of one sarcomere in µm.

Actual length $= \frac{\text{image length}}{\text{magnification}} = \frac{220 \text{ mm}}{20\,000} = 0.011 \text{ mm}$ .
Convert: $0.011 \text{ mm} \times 1\,000 = 11$ µm for 5 sarcomeres.
Mean length: $\frac{11}{5} = 2.2$ µm.

Why measure several? A single sarcomere is only about 2 µm long, so the error in placing a ruler on one is large compared with its length. Measuring across many and dividing reduces that percentage error.

Slow and fast fibres

Not all skeletal fibres are alike. Most muscles contain a mixture of two main kinds:

Feature	Slow-twitch (type I)	Fast-twitch (type II)
Speed of contraction	Slow, with myosin heads that hydrolyse ATP slowly	Fast, with myosin heads that hydrolyse ATP quickly
Main source of ATP	Aerobic respiration	Anaerobic respiration (glycolysis) and creatine phosphate
Mitochondria and capillaries	Many	Fewer
Myoglobin; colour	Much; dark red	Little; pale
Glycogen and SR	Less	More, for rapid Ca²⁺ release
Fatigue	Resist fatigue; used for posture and endurance	Fatigue quickly; used for short bursts of power

The dark meat of a chicken's leg, which the bird uses all day for standing and walking, is rich in slow fibres; the pale breast muscle, which a farmyard chicken uses only for brief bursts of flapping, is mostly fast fibres.

Worked example 2: interpreting a fibre count

Illustrative data. A thin biopsy of the thigh muscle of two athletes was stained so that slow fibres appear dark. Athlete P had 180 dark fibres out of 300 counted; athlete Q had 66 dark fibres out of 220. Calculate the percentage of slow fibres for each and suggest which athlete is the long-distance runner.

P: $\frac{180}{300} \times 100 = 60\%$ slow fibres.
Q: $\frac{66}{220} \times 100 = 30\%$ slow fibres.
Long-distance running needs sustained aerobic ATP production and resistance to fatigue, so P is more likely to be the distance runner, and Q the sprinter.

Limitation. A biopsy samples a few hundred fibres from one spot in a muscle that has hundreds of thousands, and fibre types are not spread evenly, so one sample may not represent the whole muscle.

Warning

Common mistakes

"The A band is where actin is." A and I come from the way the bands affect polarised light (anisotropic and isotropic), not from actin. The A band is the length of the myosin filaments.
"A muscle fibre is made of many cells." A fibre is one cell, with many nuclei; a muscle is made of many fibres.
"Smooth muscle has no actin and myosin." It has both; they are just not arranged in sarcomeres, so there are no stripes.
"Cardiac muscle needs a nerve impulse to contract." It is myogenic; nerves only change the rate.

CrashCourse: "Muscles, Part 1: Muscle Cells" (Crash Course Anatomy and Physiology #21). Watch for the three muscle tissues and how a skeletal muscle is built from fibres, myofibrils and myofilaments.

✦

Practice: muscle types and the sarcomere

Name the type of muscle found in (a) the wall of the small intestine, (b) the biceps, (c) the wall of the right ventricle.
Give two structural features that cardiac and skeletal muscle share, and two that differ.
Explain why a skeletal muscle fibre has many nuclei.
State what each of the following contains: (a) the I band, (b) the H zone, (c) the A band.
Describe the role of (a) T-tubules, (b) the sarcoplasmic reticulum, (c) tropomyosin in a resting muscle.
On a micrograph at ×15 000, the distance between two neighbouring Z lines is 33 mm. Calculate the sarcomere length in µm.
Explain how the intercalated discs allow the heart to contract as a single unit.
A cross-section through the overlap region of a sarcomere shows each thick filament surrounded by six thin filaments. Predict what a cross-section through (a) the H zone, (b) the I band would show, and explain your answers.

Show answersHide answers

(a) Smooth. (b) Skeletal. (c) Cardiac.
Shared: striations (sarcomeres); many mitochondria. Different: skeletal fibres are unbranched with many peripheral nuclei, while cardiac cells are branched with one central nucleus; cardiac cells are joined by intercalated discs.
Each fibre forms by the fusion of many embryonic cells (myoblasts), each bringing its nucleus; many nuclei also help supply mRNA for proteins along a very long cell.
(a) Thin (actin) filaments only. (b) Thick (myosin) filaments only. (c) Thick filaments along their whole length, plus thin filaments where they overlap.
(a) Carry action potentials from the sarcolemma deep into the fibre. (b) Stores Ca²⁺ ions and releases them when stimulated. (c) Covers the myosin-binding sites on actin, so the heads cannot attach.
$\frac{33 \text{ mm}}{15\,000} = 0.0022 \text{ mm} = 2.2$ µm.
Gap junctions in the discs let ions flow directly from cell to cell, so a wave of depolarisation spreads quickly through the whole wall; desmosomes hold the cells together as they pull.
(a) Thick filaments only, in a hexagonal pattern, because the H zone has no thin filaments. (b) Thin filaments only, because the I band contains no myosin.

Key points

Skeletal: long, multinucleate, striated fibres; voluntary; fast; fatigues. Cardiac: branched, striated cells with intercalated discs; myogenic; never fatigues. Smooth: spindle cells, unstriated; involuntary; slow and sustained.
Muscle → fascicles → fibres (cells) → myofibrils → sarcomeres → actin and myosin filaments.
Sarcomere: Z line to Z line. I band = thin only; A band = length of thick filaments; H zone = thick only; M line holds the thick filaments.
Fibres contain many mitochondria, an extensive SR storing Ca²⁺, T-tubules, glycogen and myoglobin.
Slow-twitch fibres are aerobic and fatigue-resistant; fast-twitch fibres are anaerobic, powerful and tire quickly.

Sources and credits

10.2 Skeletal Muscle, from Anatomy and Physiology 2e by J. Gordon Betts and others, OpenStax (Rice University), licensed under CC BY 4.0. Used for the structure of skeletal muscle fibres and sarcomeres. Adapted by Ganzaa.
38.4 Muscle Contraction and Locomotion, from Biology 2e by Mary Ann Clark, Matthew Douglas and Jung Choi, OpenStax (Rice University), licensed under CC BY 4.0. Used for the comparison of the three muscle types. Adapted by Ganzaa.
Video: "Muscles, Part 1: Muscle Cells", Crash Course Anatomy and Physiology #21, CrashCourse, embedded from YouTube.

Figures 1 and 2, the tables, the worked examples (with illustrative data) and the practice questions were written by Ganzaa.

1One protein pair, three tissues

2The three types of muscle

3Inside a skeletal muscle

4The sarcomere

5Slow and fast fibres

One protein pair, three tissues

The three types of muscle

Inside a skeletal muscle

The sarcomere

Slow and fast fibres