Muscles Flashcards
(56 cards)
ANATOMY OF SKELETAL MUSCLE FIBERS
Skeletal muscle is found attached to the skeleton either directly via tendons or indirectly through a connective tissue sheet called an aponeurosis. It is under voluntary control, though not always conscious control, and can tire quickly. Between the origin and insertion of a muscle is the muscle belly. Muscles can have multiple origins, insertions, and bellies. Skeletal muscle is able to contract, respond to stimulation from the nervous system, stretch beyond its normal resting length, and revert to its original resting length.
Each skeletal muscle is made up of constituent parts which are then grouped (in successive levels) to form the entire muscle, as follows:
A skeletal muscle is composed of bundles of muscle fascicles.
Each muscle fascicle is composed of bundles of muscle fibers (muscle cells).
Each muscle fiber is composed of bundles of myofibrils (tubes composed of protein filaments).
Each myofibril is composed of bundles of myofilaments (the proteins responsible for muscle contraction).
Cells are wrapped in bundles of connective tissue, divided into three levels of organization:
The endomysium is a thin layer of connective tissue that surrounds each muscle fiber (cell).
The perimysium is a thick layer of connective tissue that groups the muscle fibers into fascicles. It can be seen in a cross-section of a muscle as the marbling in a steak. It protects the fascicle from damage and contains capillaries and nerve fibers to allow nutrient transfer within the muscle.
The epimysium is a sheet of thick connective tissue that surrounds the entirety of a skeletal muscle. It may continue at the end of the muscle belly as the tendon and thus become continuous with the periosteum of bone. It separates the muscle from surrounding tissues and organs.
MICROANATOMY OF A SKELETAL MUSCLE FIBER
A skeletal muscle fiber is an individual cell that can extend up to 30 cm in length. Each cell has a plasma membrane known as the sarcolemma, contains cytoplasm known as sarcoplasm, and is composed of bundles of protein fibers known as myofibrils:
The sarcolemma is the plasma membrane of a muscle fiber. It is invaginated in places to form transverse tubules and contains numerous nuclei. It receives electrical stimuli and conducts an action potential to the internal structure of the muscle fiber via transverse tubules.
The plasma cell membrane of the muscle fiber, the sarcolemma, is invaginated in places to form membranous tunnels known as transverse tubules or T tubules. The tubules penetrate through the fiber and conduct electrical stimuli from the sarcolemma.
The sarcoplasmic reticulum is a special type of smooth endoplasmic reticulum found only in skeletal muscle cells. It contains large, concentrated stores of calcium.
The sarcoplasmic reticulum becomes enlarged and forms large bands that wrap around the muscle fibers on either side of the T tubules, known as terminal cisternae. As the action potential passes down the T tubule, it stimulates the terminal cisternae to release calcium from the sarcoplasmic reticulum and thus triggers contraction of the myofibrils.
Each muscle fiber cell contains multiple, flattened nuclei which lie beneath the sarcolemma.
The sarcoplasm is the cytoplasm of the cell and contains large amounts of glycogen and myoglobin. Glycogen provides energy during muscle contraction while myoglobin contains stored oxygen.
Myofibrils are thread-like organelles 1-3 µm in diameter that extend the length of the muscle fiber. Each myofibril is composed of bundles known as myofilaments which are arranged into the contractile elements of a muscular cell, i.e., the sarcomere.
SARCOMERE
A sarcomere is the basic functional unit of a myofibril, consisting of a complex arrangement of contractile proteins, known as myofilaments, which are joined end to end.
The sarcomere is supported by structural and elastic proteins which work together to shorten and lengthen muscle fibers. The arrangement of the myofilaments inside themyofibrils are the reason that muscle cells appear striped or striated under magnification.
Myofilaments include thin and thick filaments known as actin and myosin. When triggered by a release of calcium, the actin and myosin filaments slide over each other to shorten the length of the sarcomere.
A sarcomere is described in terms of bands, according to the type of filaments present in each section:
The I-band (I = isotropic, meaning uniform in each direction).
It is a lighter band consisting of only thin actin filaments. The I-band is bisected by a thin, dark Z-line.
The Z-line (Z = in between).
It is a dense protein disc defining the end of each sarcomere and bisecting each I-band. It is composed of the large elastic protein titin and provides anchorage for both thin filaments and coiled elastic titin filaments, which aid elastic recoil of muscle during relaxation.
The M-line (M = middle).
It is the thin line in the middle of the thick myosin filaments.
The H-zone (H = bright)
It is the lighter region in the center of each A-band and is deficient in thin actin filaments.
The A-band (A = anisotropic, meaning directionally dependent). It is a dark band consisting of parallel, thick filaments, with thin filaments partly overlapping them.
CONTRACTILE PROTEINS
The two contractile proteins, actin and myosin, are the main myofilaments that form the sarcomere. They are the force generating proteins of the sarcomere, and they work together during the muscle contraction cycle in order to produce movement.
Myosin
Myosin is the contractile protein that forms the thick filaments.
It lies mainly in the A-band and H-zone of the sarcomere and interacts with actin to create movement.
Myosin filaments are made up of three domains: head, tail, and neck.
Function
Myosin mainly involves coupling hydrolysis of ATP to conformational changes in the head region of the filament that enables it bind and move along actin filaments.
Actin
Actin is the contractile protein that forms the thin filaments.
Each actin microfilament is a polymer known as F actin and is composed of individual monomeric protein subunits known as G actin. The F actin polymers twist together, and being composed of G actin subunits, gives the appearance of two strings of beads twisted together. All acting filaments are of the same length and contain myosin binding sites, to which the myosin heads attach and ‘walk’ along. This results in the contraction.
Function
Actin is bound to by the myosin molecules.
REGULATORY PROTEINS
Regulatory proteins work together with actin and myosin during the muscle contraction cycle in order to produce movement.
Tropomyosin is a regulatory component of actin filaments. Tropomyosin filaments are long molecules comprising of a coil of alpha helices. They twist around each filament of actin and bind to it in seven places.
Function
It is involved in the uncovering of myosin head binding sites on the actin filaments during excitation-contraction coupling.
Troponin is a regulatory component of actin filaments. It is involved in moving tropomyosin away from the myosin binding site on actin filaments.
Troponin molecules have three sub-units:
TnT: binds to tropomyosin near the ends of the tropomyosin sub-units.
TnI: binds to actin filaments.
TnC: binds to TnI and TnT sub-units and also binds calcium ions.
Binding of calcium to troponin causes a conformational shape change to occur that moves troponin away from the myosin head binding sites, present on the actin molecules, freeing them for crossbridge formation.
SLIDING FILAMENT MECHANISM
The sliding filament mechanism explains how skeletal muscle fibers contract and relax. It involves the movement of thick and thin filaments, relative to one another, to cause active shortening of a muscle fiber.
Muscle contraction occurs because thick filaments bind onto the thin filaments by forming chemical bonds called crossbridges. Once bound, the thick filaments ‘walk’ along the thin filaments and pull them towards the center of the sarcomere.
This movement causes sarcomere shortening because the thin filaments are attached to the Z-line, and the thick filaments are able to grip their way along, making the H-zone almost non-existent. The combined shortening of the sarcomeres along a number of myofibrils causes whole muscle contraction.
Relaxed muscle
Relaxed muscle contains sarcomeres which do not have many crossbridges. For this reason, the H-zone is seen as large (as is the I-band). As for contracted muscle, the A-band remains a constant length because the length of the myofilaments does not change during contraction.
Contracted muscle
Contracted muscle contains sarcomeres which have many formed crossbridges. This means that the H-zone is small and almost non-existent in some fully contracted muscles. The I-band is smaller but the A-band remains constant.
CARDIAC AND SMOOTH MUSCLE
Cardiac and smooth muscle differ from skeletal muscle in that they do not fatigue and are not under voluntary control, but can be stimulated by autonomic nerves and are affected by hormones.
Cardiac muscle is found in the walls of the heart and is able to contract continuously, pumping blood around the body. Smooth muscle is found in the walls of internal organs, such as organs of the digestive system, walls of blood vessels, and the intrinsic muscles of the eye.
Cardiac muscle
The muscular walls of the heart, known as cardiac muscle or myocardium, must continually contract and relax for life to be sustained. Cardiac cells, known as cardiomyocytes, cardiac myocytes or cardiac muscle fibers, are adapted to never tire and to contract without any stimulation.
Cardiac muscle fibers are shorter than skeletal muscle fibers, at 50-100 μm long, and are less circular in cross-section, with a diameter of about 14 μm. They are branched and adjoin tightly to one another via step-like junctions known as intercalated discs. This gives histology images of cardiac muscle a striated appearance. A broad intercellular junction, called the fascia adherens forms a patch of adhesion anchoring the actin filaments to the inner side of the membrane of a cardiac muscle cell.
Prominent features of cardiomyocytes include: oval nuclei, abundant mitochondria (for a constant energy supply), sarcolemma (plasma membrane) and transverse tubules for co-ordinated muscle contraction, sarcoplasmic reticulum (a reservoir of calcium ions needed for contraction), and contractile elements arranged into sarcomeres and myofibrils.
cardiomyocytes
Typically, cardiomyocytes have one nucleus located centrally within the cell. It is pale in color, oval in shape, and is the largest organelle, measuring approximately 5 μm in diameter.
Function:
A nucleus regulates gene expression and therefore controls the activities of a cell.
Mitochondria are particularly large and abundant in cardiomyocytes and make up about 25-40 % of a cardiac muscle fiber.
Function:
Often referred to as the ‘power plants’ of a cell, mitochondria are highly specialized for the sole purpose of providing cardiac muscle with a constant supply of energy in the form of ATP. Mitochondria have the ability to self replicate when the demand for ATP increases.
The sarcolemma is the plasma membrane of a cardiomyocyte. As in skeletal tissue, it invaginates into the cytoplasm, creating transverse tubules.
Function:
Transverse tubules ensure the spread of excitation deep into the muscle fibers for co-ordinated muscle contraction.
Transverse tubules tunnel through a muscle fiber: they start at the surface, pass deep into the fiber, and emerge on the opposite side. They are filled with interstitial fluid.
Function:
They provide a direct route for supplemental extracellular calcium to pass into the core of a muscle fiber.
Myofibrils are the cylindrical bundles of thick and thin filaments that run from one end of a cardiac myocyte to the other. Unlike the more uniform myofibrils of skeletal muscle fibers, they vary in diameter, branch extensively, and are interspersed with numerous mitochondria.
Function:
Myofibrils contain the contractile elements of the cardiomyocytes and are the organelles responsible for contraction.
As in skeletal muscle, the sarcoplasmic reticulum is the smooth endoplasmic reticulum of a muscle cell, made up of a network of fluid-filled, membrane bound, tubular sacs that surround each myofibril.
Function:
It functions as a reservoir for calcium ions. Distributed throughout its membrane are gated ion channels, which permit the sudden influx of calcium into the cytosol, thus triggering muscle contraction.
INTERCALATED DISCS
Intercalated discs are structures found between cardiomyocytes that provide a site for cell-to-cell adhesion and communication. They are composed of complexes of trans-membrane proteins, which form both mechanical and electrical junctions.
The mechanical junctions, fascia adherens and desmosomes, link one cell to the next and resist mechanical stress. The electrical junctions, such as gap junctions, permit the cell to cell flow of ions, thus providing essential electrical stimulation for co-ordinated contraction.
Desmosome: A type of mechanical junction present on intercalated discs, tightly adjoining one cardiomyocyte to another.
Function:
Desmosomes function to resist mechanical stress, enabling cardiomyocytes to pull on each other during contraction without separating.
Gap junctions, also known as communicating junctions, are a type of electrical junction present on intercalated discs.
Function:
They are fluid-filled channels that permit the diffusion of ions, glucose, amino acids, and other small solutes from the cytoplasm of one cardiac myocyte directly into the cytoplasm of its neighbor.
A sarcomere of cardiomyocytes has the same arrangement of thick and thin filaments, supported by structural and elastic proteins, as those found in skeletal muscle.
SMOOTH MUSCLE TISSUE
Smooth muscle is unstriated, involuntary muscle found in the walls of the internal organs, blood vessels, and the intrinsic (internal) muscles of the eye. It enables involuntary visceral contraction, such as vasoconstriction that restricts blood flow, and peristalsis that aids the digestion and elimination of waste from the body.
Smooth muscle contracts in unison in a slow and synchronized manner. The intensity and rate of smooth muscle contraction, like cardiac and skeletal muscle, is influenced by neuronal and hormonal stimulation, and the mechanism of contraction is similar to that of skeletal muscle in many ways, as it uses thick and thin filaments and excitation contraction coupling. However, the mechanism through which excitation contraction coupling triggers muscle contraction differs slightly in smooth muscle, and it uses the regulatory molecule, calmodulin instead of troponin. Calmodulin binds to Ca2+ and activates an enzyme known as myosin light chain kinase. Myosin light chain kinase adds a phosphate group from a molecule of ATP to the myosin head, allowing it to bind to actin. This process occurs more slowly than in skeletal muscle, which accounts for the difference in speed of contraction.
There are two types of smooth muscle: single-unit (visceral) and multi-unit.
Smooth muscle tissue is made up of small, non-striated, spindle-shaped (thickest in the middle with tapering ends) smooth muscle fibers, containing one centrally located nucleus.
The contractile proteins of smooth muscle are not arranged in a sarcomere, as in cardiac and skeletal muscle, which is why smooth muscle is described as being unstriated. Instead, the thick and thin filaments are arranged in contractile bundles, with each end attached to a structure known as a dense body, and the surface of the smooth muscle fiber is crossed by several intermediate filaments, which interconnect the dense bodies. In addition, smooth muscle fibers have no sheath but are joined by connective tissue and connected via gap junctions, promoting powerful, synchronized contraction.
Single-unit smooth muscle
Single-unit smooth muscle is found in the walls of blood vessels and hollow organs, such as the urinary bladder and digestive tract. It usually has a tubular arrangement, whereby each fiber connects to another via gap junctions. This means that single-unit smooth muscle contracts, like cardiac muscle, as a single-unit.
Multi-unit smooth muscle
Multi-unit smooth muscle is found in the arrector pili muscles of hair follicles, muscles of the iris, the male reproductive system, large arteries, and airways of the lungs. It consists of individual fibers, singularly innervated with few gap junctions between cells. Like skeletal muscle, each muscle fiber functions alone (unlike single-unit smooth muscle which contracts as a unit).
Smooth muscle cells
The nucleus of smooth muscle cells is centrally located and contains the genetic information within the cell.
Function:
A nucleus regulates gene expression and therefore controls the activities of a cell.
The sarcoplasm of smooth muscle fibers contains all of the contractile filaments that produce movement of the muscle fiber as a whole. It contains three types of contractile filament: thick, thin, and intermediate.
Thin filaments
The thin filaments present in smooth muscle are structurally similar to those of skeletal muscle, but their arrangement is less orderly. They have a tropomyosin component, but no troponin.
Function:
The role of thin filaments mainly involves binding to myosin molecules.
Thick filaments
The thick filaments present in smooth muscle are extremely similar to those of skeletal muscle as they comprise an intertwining arrangement of myosin proteins, but the thick filaments are much longer and not arranged in as uniform a manner. In addition, the myosin heads of the thick filaments are arranged differently, as the heads are present all along the length of the thick filaments, which is what gives smooth muscle its great strength.
Function:
Thick filaments mainly involve hydrolysis of ATP which enables it to bind and move along actin filaments.
Intermediate filaments
Intermediate filaments are found distributed throughout the sarcoplasm in smooth muscle as a network of the protein desmin located close to the sarcolemma. They are seen as an interconnecting mesh of fibers joined to the dense bodies of the smooth muscle.
Function:
Intermediate filaments act to support and harness actin filaments, via their attachment to the dense bodies, as they contract and move.
In place of transverse tubules, smooth muscle fibers have small invaginations of the plasma membrane called caveolae (singular: caveola).
Function:
Caveolae contain extracellular calcium, used for contraction.
Dense bodies are small collections of the structural protein alpha-actinin.
Function:
They serve as attachment points for the thin filaments of the contractile bundles in smooth muscle fibers.
Important rules for muscle movement
Muscles only pull, never push.
Whatever one muscle can do, another muscle can undo.
CONTRACTION OF SKELETAL MUSCLE
When relaxed, a muscle is soft and pliable, and when contracted, it is hard and elastic. The degree of muscle contraction is referred to as muscle tension and is determined by the following four factors:
The number of muscle fibers innervated per somatic motor neuron (motor unit).
The frequency of stimulation of the muscle, i.e., the number of impulses per second.
The size of the muscle fibers themselves.
The ability of the muscle to form crossbridges.
MOTOR UNIT
A motor unit consists of a somatic motor neuron and all of the muscle fibers it innervates. The size of a motor unit can vary; fine control motor units only consist of one somatic motor neuron and a few muscle fibers, whereas less precise motor units can have hundreds of muscle fibers per somatic motor neuron.
Motor unit recruitment refers to the number of motor units activated during a contraction. The type of overall muscle movement produced will depend on the recruitment of different size units, different strength of units, and the quantity of the units.
The more motor units recruited, the stronger a contraction will be, and similarly, the heavier an object is to move, the more motor units will be required. Recruitment of different motor units of a specific size also enables muscles to gain more control over fine movements, such as typing or playing the piano, as smaller units creating smaller movements gives an entire muscle more precise control over its actions.
Motor units are used cleverly by active muscles to conserve energy, prevent muscle fatigue, and to ensure that tension can be sustained for as long as possible. In addition to resting some motor units while others are contracting, weaker motor units are used first and stronger units are brought in later, something that contributes to the smoothness of movement during muscle action.
TWITCH CONTRACTIONS
A twitch contraction is a fast, brief contraction of a muscle following a single stimulus. This stimulus is usually brief and can be either strong or weak, depending on the number of motor units recruited. Twitch contractions can be studied and recorded using a myogram and occur in three periods.
Skeletal muscle fibers can be classified as either slow twitch or fast twitch fibers, depending on the speed at which they contract. Their speed is restricted by how quickly ATPase in the myosin heads can hydrolyze ATP.
