• Introduction
  • The basics of skeletal muscle anatomy
  • Sarcomere
  • Sliding filament model of muscle contraction
  • Muscle fiber types
  • Motor units
  • Final thoughts
  • Sources
  • Acetylcholine (ACh): neurotransmitter that binds at a motor end-plate to trigger depolarization
  • Actin: thin myofilaments found in the sarcomeres of a muscle fiber.
  • Action potential: a rapid shift in the voltage across a membrane.
  • Endomysium: a connective tissue covering each muscle fiber.
  • Epimysium: the outer layer of connective tissue that surrounds skeletal muscles.
  • Excitation-contraction coupling: sequence of events from motor neuron signaling to the contraction of the fiber’s sarcomeres.
  • Fascicle: bundle of muscle fibers surrounded by the perimysium. 
  • Muscle architecture: the structural design of a skeletal muscle, including the arrangement of the muscle fibers, muscle units, and connective tissue within and around which they are embedded.
  • Myofibril: long, cylindrical organelles that extend in parallel columns along the length of the muscle fiber. Contains the sarcomeres.
  • Myosin: thick myofilaments found within a sarcomeres of a muscle fiber.
  • Neuromuscular junction: the synaptic connection between the terminal end of a motor nerve and a muscle.
  • Neurotransmitter: chemical messengers released by nerve terminals that bind to receptors on target cells.
  • Perimysium: a sheath of connective tissue that bundles skeletal muscle fibers into fascicles within a skeletal muscle.
  • Sarcomere: the smallest contractile unit of a muscle fiber consisting of actin and myosin filaments.
  • Sarcolemma: the plasma membrane surrounding a skeletal muscle fiber.
  • Sarcoplasm: cytoplasm of a muscle fiber.
  • Troponin: a complex of three regulatory proteins that binds to actin, tropomyosin, and calcium. An essential regulatory protein in muscle contraction.
  • Tropomyosin: regulatory protein that prevents actin from binding to myosin.

Introduction

The musculoskeletal system consists of three muscle tissue types; cardiac, smooth, and skeletal muscle tissue. 

Cardiac muscle cells (cardiomyocytes) are only found in the walls of the heart. The main function of cardiac muscle tissue is to pump blood throughout the body and maintain blood pressure. Much like skeletal muscle tissue, cardiac muscle tissue is also striated in appearance. However, unlike skeletal muscles, cardiac muscle tissue is not under voluntary control. Cardiac muscle cells also contain branched fibers connected via intercalated discs (gap junctions between adjacent cardiac muscles), several mitochondria, and a single nucleus.

Smooth muscle cells are found in the walls of hollow organs, such as the intestines, stomach, and liver as well as around passages such as the respiratory tract and blood vessels. Smooth muscle tissue is also under involuntary control and has a non-striated, fusiform (spindle-shaped, tapering at both ends) appearance. Smooth muscle cells also have a single centrally located nucleus.

Skeletal muscle cells or muscle fibers (myocytes), are long, cylindrical in shape and have a striated appearance when observed under a microscope. These striations are a result of a repeated arrangement of contractile proteins. Skeletal muscles are also multinucleated – they have multiple nuclei present in a single cell. The main function of skeletal muscles is to control locomotion, which is why they are often referred to as voluntary muscles. The human body comprises of over 600 skeletal muscles which accounts for approximately 40% of body weight.

This post focuses on the anatomy of skeletal muscle tissue, as well as its role in producing movement.

The basics of skeletal muscle anatomy

Each individual muscle is surrounded by a dense connective tissue known as the epimysium. An individual skeletal muscle consists of hundreds, or even thousands, of muscle fibers bundled together into fascicles. Each fascicle is separated by a thin connective tissue covering called the perimysium, whereas the innermost sheath surrounding individual muscle fiber is known as endomysium. Each muscle fiber comprises of hundreds of myofibrils (long contractile fibers that run parallel to the muscle fiber). These, in turn, are composed of overlapping thick and thin myofilaments organized into distinct, repeating units called sarcomeres.

Sarcomeres are the the smallest contractile units of striated muscles. They are composed of two myofilaments; actin (thin filament) and myosin (thick filament), as well as regulatory proteins tropomyosin and troponin (which has three subunits: troponin T, troponin I and troponin C). Actin and myosin arranged to form various bands (see: sarcomere structure) on the skeletal muscle. The basement membrane and sarcolemma (the cell membrane surrounding a skeletal muscle fiber) of the muscle cell also contain specialized stem cells known as satellite cells. When stimulated by growth factors, these cells have ability to replicate (self-renew) and differentiate (a process where unspecialized cells take on individual characteristics and reach a specific form and function) into other types of cells, including muscle cells. Thus, satellite cells act as a source of new nuclei for the muscle fibers, which helps the muscles meet the increased demands of growing muscle cells. 

The main function of skeletal muscles is to produce movement via the shortening of sarcomeres. This is more commonly known as muscle contraction. Because skeletal muscles are attached to bones by tendons, the contraction of the muscle leads to movement of the bone, allowing for specific voluntary movements.

Share this post

Skeletal Muscle Anatomy

Skeletal muscles consist of bundles of fasciclesFascicles consist of bundles of muscle fibers (myocytes)Muscle fibers consist of bundles of myofibrilsMyofibrils consist of sarcomeresSarcomeres consist of actin and myosin filamentsActin and myosin filaments slide past each other to contract the muscle

Sarcomere

Sarcomeres are the basic contractile units of muscle fibers (myocytes). Sarcomeres are composed of two protein filaments; actin (thin filament, approximately 7 nm in diameter) and myosin (thick filament, approximately 15 nm in diameter). Under electron microscopy, these filaments form several distinct lines, as well as light and dark areas, known as bands. These areas give muscles their striated appearance when observed under electron microscopy. Additionally, these dark and grey bands correspond to the arrangement of actin and myosin filaments in specific areas of the sarcomere.

The sarcomere is defined as the area between two Z discs or Z lines. This makes the sarcomere the smallest functional unit in the muscle that can shorten. The light central region of the sarcomere is known as the H zone. It contains no actin, and only a small number of myosin. The I bands are located at both ends of the sarcomere and only contain actin filaments. The dark band in the middle of the sarcomere is referred to as the A band, which consist of both actin and myosin filaments. Finally, the line at the center of the sarcomere is known as the M line.

Sarcomeres are attached to one another at the Z line. This combination of interconnected sarcomeres form a myofibril, whereas a bundle of myofibrils form a muscle fiber.

  • A band: anisotropic bands – dark bands that corresponds to the thick (myosin) filaments.
  • H zone: an area in the middle of the A zone, where only thick filaments are present. Has a somewhat lighter appearance than the overlap region of the A band.
  • I band: isotropic bands – light bands that contain only thin filaments (actin). Located between two thick filaments.
  • M line: the attachment site for the thick filaments located in the center of the A band, and therefore also at the center of the sarcomere.
  • Z disc/Z line: the boundaries of a single sarcomere where thin (actin) filaments attach.

Muscle contraction occurs when actin and myosin filaments slide past each other, resulting in the shortening of the sarcomere. This phenomenon is more commonly known as the sliding filament theory, which forms the basis of how muscles contract and relax.

Sliding filament model of muscle contraction

In order to contract a muscle, the sarcomere must shorten. When a signal from the nervous system (action potential) reaches the neuromuscular junction (location where motor neuron reaches the muscle cell), it triggers the release of a neurotransmitter – acetylcholine (ACh). This starts a chemical reaction that ultimately allows myosin heads to form cross-bridges with actin and pull it inwards towards the sarcomere’s anatomical limit. The entire process that links plasma membrane stimulation with cross-bridge force production is known as excitation–contraction coupling. It forms the foundation of muscle contraction at a cellular level. 

During a muscle contraction, the Z discs/Z lines move closer together while the width of the H zone decreases. The A band remains at a constant length while the I bands shorten with the sarcomere. This means that actin and myosin must slide past one another to shorten the sarcomere and generate muscle tension. This phenomenon is known as the sliding filament theory of muscle contraction.

Muscle fiber types

Muscle fibers (myocytes) consist of a single muscle cell. The number of muscle fibers found in a muscle depends on its size, location, and function. Skeletal muscles consist of two muscle fiber types; slow-twitch muscle fibers (type I) and fast-twitch muscle fibers (type II). Fast-twitch muscle fibers can also be further divided into type IIa and type IIb/IIx muscle fibers. Each muscle fiber type has its own distinct characteristics and responses to physical activity. All skeletal muscles consist of roughly the same amount of slow and fast muscle fibers, although the proportions may differ individually.

Slow-twitch muscle fibers excel in sustained low-intensity activity due to their increased oxygen capacity. This is a result of containing a large amount of mitochondria (the powerhouse of the cell) which use oxygen to create adenosine triphosphate (ATP), a significant amount of capillaries, which helps provide oxygen and nutrients to the muscle while removing unwanted byproducts, as well as elevated levels of myoglobin, which is a protein that binds iron and oxygen. This also gives slow-twitch muscles their red color.

Fast-twitch muscle fibers contract with a significant amount of force but have a low endurance capacity. Fast oxidative (type IIa) muscle fibers primarily use aerobic respiration (due to having a moderate amount of mitochondria, capillaries, myoglobin, etc.) while still contracting relatively fast. This means that type IIa fibers produce more tension than slow fibers but also fatigue quicker. Fast glycolytic (type IIb/IIx) fibers rely primarily on anaerobic glycolysis. They are the largest of all muscle fibers, generate the most force. However, they also have the lowest amount of capillaries, mitochondria and myoglobin of any muscle fiber type, resulting in a low oxidative capacity.

Muscle Fiber Type

I

IIa

IIb/IIx

Contraction Speed

Slow
(90-140ms)

Fast
(50-100ms)

Very Fast
(40-90ms)

Fatigue Resistance

High

Medium

Low

Force Production

Low

High

Very High

Mitochondria Content
(powerhouse of the cell)

High

High

Low

Myoglobin Content
(a protein that binds iron & oxygen and gives blood its red colour)

High

High

Low

Capillary Content
(capillaries provide muscles with oxygen and nutrients while removing unwanted byproducts)

High

High

Low

Oxidative Capacity
(ability to use oxygen for energy production)

High

Medium

Low

Glycolytic Capacity
(ability to store and break down glycogen for intense exercises)

Low

High

High

Muscle Fiber Diameter

Small

Medium

Large

Muscle Fiber Color

Dark Red

Dark Red

Pale Red

Motor Neuron Size
(larger neurons provide faster activation)

Small

Large

Very Large

ATPase Level
(enzyme that controls glycogen breakdown and ATP synthesis)

Low

Medium

High

Motor units

A motor unit refers to a single motor neuron and all the muscle fibers innervated by it. This makes it the smallest functional unit of the nervous system and the final output of motor commands. All of the individual motor neurons that innervate a muscle is known as a motor pool. 

The number of muscle fibers innervated by the same motor neuron varies greatly. Generally speaking, smaller motor units tend to produce finer movements that require dexterity, whereas large motor units produce more force in gross movements. The amount of force exerted by a muscle depends on the number of motor units activated as well as the rate at which they discharge action potentials (rate coding). 

Instead of recruiting all muscle fibers simultaneously, the muscles can conserve energy by altering the firing rate of motor units as well as recruiting motor units with different mechanical properties. This allows muscles to gradually increase the amount of force required for specific movements. Motor unit activation follows two main principles; the all-or-none law, and the size principle. The all-or-none law states that when a stimulus exceeds an activation threshold, all the muscle fibers innervated by the same motor neuron will fire. Thus, the nerve fibre will always give a maximum response or none at all. The size principle states that motor units are activated according to size from smallest to largest. Because slow motor units are naturally smaller in size, they have the lowest activation threshold. Fast motor units, which are larger in size, only fire when slow motor units are unable to produce enough force. 

Skeletal muscles are responsible for all voluntary movements of the body.

Final thoughts

Skeletal muscle is one of the three muscle tissues found in humans. They make up 30 to 40% of total body mass. Skeletal muscles connect muscles to bones by tendons, and their main function is to produce voluntary movement. The skeletal muscle fibers have a distinct pattern of red and white lines, giving them a striated appearance. This is also why they are commonly referred to as striated muscles. 

Skeletal muscles contract as the myosin heads pull acting inwards, resulting in the shortening of a sarcomere. This process, also known as cross-bridge cycling, requires adenosine triphosphate (ATP) to function. This process construes until the sarcomere is shortened to its anatomical limit or there is no more ATP available. 

Did you learn anything new about skeletal muscle anatomy? Let us know in the comments. 

Sources

  • Bordoni B, Varacallo M. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Jul 18, 2022. Anatomy, Tendons.
  • Bordoni B, Varacallo M. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Apr 16, 2022. Anatomy, Head and Neck, Scalenus Muscle.
  • Bordoni B, Mahabadi N, Varacallo M. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Jul 18, 2022. Anatomy, Fascia.
  • Chang NC, Rudnicki MA. Satellite cells: the architects of skeletal muscle. Current topics in developmental biology. 2014 Jan 1;107:161-81.
  • Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Actin, Myosin, and Cell Movement.Available from: https://www.ncbi.nlm.nih.gov/books/NBK9961/
  • Guy-Evans, O. (2021, June 24). Motor neuron: function, types, and structure. Simply Psychology.
  • Dave HD, Shook M, Varacallo M. Anatomy, Skeletal Muscle. [Updated 2022 Aug 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537236/
  • Dumont NA, Bentzinger CF, Sincennes MC, Rudnicki MA. Satellite Cells and Skeletal Muscle Regeneration. Compr Physiol. 2015 Jul 1;5(3):1027-59. doi: 10.1002/cphy.c140068. PMID: 26140708.
  • Frontera WR, Ochala J. Skeletal muscle: a brief review of structure and function. Calcif Tissue Int. 2015 Mar;96(3):183-95. 
  • Goodman CA, Hornberger TA, Robling AG. Bone and skeletal muscle: Key players in mechanotransduction and potential overlapping mechanisms. Bone. 2015 Nov;80:24-36.
  • Korthuis RJ. Skeletal Muscle Circulation. San Rafael (CA): Morgan & Claypool Life Sciences; 2011. Chapter 2, Anatomy of Skeletal Muscle and Its Vascular Supply. Available from: https://www.ncbi.nlm.nih.gov/books/NBK57140/
  • Stone WL, Leavitt L, Varacallo M. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): May 8, 2022. Physiology, Growth Factor.
  • Wilke J, Engeroff T, Nürnberger F, Vogt L, Banzer W. Anatomical study of the morphological continuity between iliotibial tract and the fibularis longus fascia. Surg Radiol Anat. 2016 Apr;38(3):349-52.

Join our growing list of subscribers!

Stay informed about the latest in sports science and physical performance. Subscribe to our mailing list for the latest updates, posts, products and much more.

    Motor Unit Muscle Fibers