4.1 Skeletal Muscle Structure: Myofibrils, Sarcomeres, and Bands

4.1 Skeletal Muscle Structure: Myofibrils, Sarcomeres, and Bands

Quick Answer: A sarcomere is the repeating contractile unit of a myofibril, bounded by Z bands. The A band spans the full length of thick myosin filaments, the I band contains thin actin filaments without myosin, and the H band is the myosin-only center of the A band. A motor nerve conduction study (NCS) records a compound muscle action potential (CMAP) from this tissue—not a sensory nerve action potential (SNAP) from nerve.

Why NCS candidates must know muscle structure

The Registered Nerve Conduction Study Technologist (R.NCS.T.) candidate spends the workday stimulating nerve and recording either a SNAP or a CMAP. It is easy to treat both waveforms as nerve responses. They are not. A SNAP is the summed action potentials of sensory axons under a recording electrode on the nerve or in its cutaneous field. A CMAP is the summed muscle-fiber membrane depolarizations of the muscle innervated by the stimulated motor axons. If you cannot name what a skeletal muscle fiber contains, you cannot explain why a technically perfect, supramaximal nerve stimulus can still yield a tiny CMAP when the muscle is diseased, inexcitable, or replaced by fat and connective tissue.

OpenExamPrep teaches this chapter so you keep the recording target straight: motor NCS interrogates nerve plus neuromuscular junction (NMJ) plus muscle. Outline topic I.C.1 names the myofibril, the sarcomere, and the H, A, I, and Z bands because those landmarks are the structural vocabulary of skeletal muscle. This study guide is independent OpenExamPrep teaching for candidates using that outline. It is not a statement of official American Association of Electrodiagnostic Technologists (AAET) approval or partnership.

The skeletal muscle fiber and the myofibril

A skeletal muscle fiber (skeletal myocyte) is a long, multinucleated cell. Its plasma membrane is the sarcolemma. Inside the fiber, the cytoplasm (sarcoplasm) is packed with cylindrical organelles called myofibrils, arranged parallel to the long axis of the fiber. Each myofibril is a stack of repeating sarcomeres in series, like boxcars in a train. The striated appearance of skeletal muscle under the light microscope is the optical consequence of those sarcomeres lining up in register across adjacent myofibrils.

A single muscle fiber contains hundreds to thousands of myofibrils. Surrounding them is the sarcoplasmic reticulum (SR), a specialized calcium store, and a network of transverse tubules (T-tubules) that carry the muscle action potential from the surface membrane into the fiber interior. Those membranes execute excitation-contraction coupling, which is section 4.2. This section stays at the structural level of the contractile lattice: thick filaments, thin filaments, and the bands you must be able to name on a sarcomere diagram.

Nuclei sit just under the sarcolemma, which is a histologic clue that you are looking at a skeletal fiber rather than a cardiac myocyte. Mitochondria and glycogen sit between myofibrils and supply the adenosine triphosphate (ATP) that myosin will use in section 4.2. For NCS purposes, the important packing fact is simple: almost the entire cross-section of a healthy extrafusal fiber is contractile lattice. When myopathy, denervation atrophy, or fatty replacement removes that lattice, fewer membranes remain to generate extracellular current, and the CMAP recorded at the skin shrinks.

The sarcomere as the contractile unit

The sarcomere is the segment of a myofibril between two successive Z bands (also called Z lines or Z discs; Z from the German Zwischenscheibe, between disk). Thin actin filaments are anchored at the Z band and project toward the center of the sarcomere. Thick myosin filaments occupy the center and are linked at the M line (Mittelscheibe, middle disk). At typical resting length, the thin filaments overlap the ends of the thick filaments but do not meet in the middle. That overlap geometry is exactly what the classical bands describe.

Typical landmark distances you should be able to picture—not as laboratory NCS cutoff values, which this guide does not invent—are on this order: a resting sarcomere is about 2.0 to 2.2 micrometers from Z band to Z band. The thick filament is about 1.6 micrometers long; that length is the A band and does not change during ordinary shortening. The thin filament is about 1.0 micrometer long. When the fiber is stretched, overlap decreases and the I band and H zone widen. When the fiber shortens, Z bands approach each other, overlap increases, and the I band and H zone narrow.

Titin, a giant elastic protein, runs from the Z band to the M line and helps keep thick filaments centered when the sarcomere is stretched. Nebulin runs along thin filaments and contributes to thin-filament length control. You do not need the full Z-disc proteome for this exam, but you do need to know that the Z band is an anchor, not an empty gap, and that the sarcomere is a bipolar machine: myosin heads on each half of the thick filament pull actin toward the midline.

Sliding filament overview at the structural level

The sliding filament description is a statement about geometry, not yet about ATP hydrolysis or calcium. Thick and thin filaments slide past one another; the filaments themselves do not shorten as coils. Because thick-filament length is constant, A band width stays essentially constant. Because the Z bands are pulled toward the thick filaments as overlap increases, I band width decreases. Because the myosin-only central gap shrinks as actin tips approach the M line, H band width decreases. At very short lengths, thin filaments can overlap each other in the center and the H zone disappears from the optical picture. The chemistry of cross-bridge cycling that produces that sliding is the next section.

This structural fact matters for CMAP thinking in a limited but useful way. The electrical CMAP is generated by sarcolemma and T-tubule membranes. The twitch you may see in the thenar eminence is the later mechanical result of sliding filaments. A muscle can fail electrically (inexitable membrane, lost fibers) or fail mechanically after a membrane spike (failed calcium release or failed cross-bridges). Motor NCS as performed by the technologist records the electrical muscle response. Do not equate a large visible twitch with a large CMAP, or a small twitch with a small CMAP, without looking at the waveform: recording geometry, movement artifact, and stimulus spread can dissociate what you see from what you measure.

The bands: what each contains

Name the bands by which filaments occupy them, not by dark versus light alone. Classic stains make the A band dark and the I band light, but exam items are won by content, not color memory.

Z band (Z line, Z disc). The sarcomere boundary. It contains alpha-actinin and related Z-disc proteins that anchor the barbed ends of thin actin filaments. Titin also attaches here. Neighboring sarcomeres share a Z band, so each Z band is the meeting place of two sarcomeres. During shortening, the two Z bands of one sarcomere move closer together.

I band (isotropic band). The region that contains thin filaments only—actin plus the regulatory proteins tropomyosin and troponin, without myosin. A single I band is bisected by a Z band and therefore belongs to two adjacent sarcomeres. Under polarized light it is isotropic, which is why it looks pale on classic stains. The I band shortens during contraction as actin is drawn into the A band.

A band (anisotropic band). The region that contains the entire thick myosin filament, including both the zone of overlap with actin and the central zone without overlap. Anisotropy under polarized light makes it dark on classic stains. Because thick-filament length is fixed, A band length does not shorten with ordinary contraction. Exam trap: the A band is not myosin only. Overlap with actin is part of the A band.

H band (H zone). The central portion of the A band that contains thick filaments without overlapping thin filaments at rest (heller means brighter). The M line sits in the middle of the H band and cross-links myosin tails. The H band narrows as thin filaments slide inward.

How this lattice sits inside a motor unit

A motor unit is one anterior-horn motor neuron and all the muscle fibers it innervates. Each of those fibers is a bag of myofibrils. When a motor axon fires, essentially all of its muscle fibers are asked to produce action potentials and then to shorten sarcomeres. The CMAP at the skin is a summed recording of many fiber depolarizations, not a recording of a single sarcomere. If disease drops the number of electrically active fibers—axonal denervation, myopathic fiber loss, or replacement of muscle by fat—the summed CMAP amplitude falls. If remaining fibers depolarize as a less synchronous population, the CMAP may also disperse in time and lose peak amplitude even when area is relatively better preserved.

Belly-tendon montages put the active electrode near the motor point, where incoming axons reach the muscle and where many fibers can be recorded in phase. That is a muscle landmark, not a nerve landmark. When the active electrode is off the muscle belly, you are no longer sampling this lattice efficiently, and amplitude falls for a technical reason that has nothing to do with sarcomeres—but you still need to know that the generator you missed is muscle.

Practical NCS implications for this section

1. Electrode placement is over muscle, not over the nerve ending. The CMAP generator is the muscle-fiber membrane whose interior is myofibrils and sarcomeres.
2. A low CMAP is not automatically a bad nerve stimulus. After technical causes are excluded (submaximal stimulus, wrong muscle, co-recording from a neighbor, excessive electrode distance, cold limb), think about lost motor axons, failed NMJ transmission, or a muscle that cannot generate a membrane potential or has too few remaining fibers.
3. SNAP versus CMAP is a tissue distinction. Sensory studies do not require sarcomeres. Motor studies do. A pure sensory axonopathy can flatten SNAPs while CMAPs remain. A myopathy can reduce CMAPs while SNAPs remain.
4. Do not confuse mechanical twitch with electrical CMAP. Sliding filaments produce force. Motor NCS measures the electrical prelude to that force.

Keep the picture simple for the exam: Z to Z is one sarcomere; A is myosin's full length; I is actin alone; H is myosin alone in the middle. Section 4.2 adds calcium, T-tubules, and the cross-bridge cycle that makes those bands move.