Anatomical terminologyis used to uniquely describe aspects ofskeletal muscle,cardiac muscle,andsmooth musclesuch as their actions, structure, size, and location.
Types
editThere are three types ofmuscle tissuein the body: skeletal, smooth, and cardiac.
Skeletal muscle
editSkeletal muscle,or "voluntary muscle", is astriated muscle tissuethat primarily joins tobonewithtendons.Skeletal muscle enables movement of bones, andmaintains posture.[1]The widest part of a muscle that pulls on the tendons is known as thebelly.
Muscle slip
editAmuscle slipis a slip of muscle that can either be ananatomical variant,[2]or a branching of a muscle as inribconnections of theserratus anterior muscle.
Smooth muscle
editSmooth muscleis involuntary and found in parts of the body where it conveys action without conscious intent. The majority of this type of muscle tissue is found in thedigestiveandurinary systemswhere it acts by propelling forward food,chyme,andfecesin the former andurinein the latter. Other places smooth muscle can be found are within theuterus,where it helps facilitatebirth,and theeye,where thepupillary sphinctercontrolspupilsize.[3]
Cardiac muscle
editCardiac muscleis specific to theheart.It is also involuntary in its movement, and is additionally self-excitatory, contracting without outside stimuli.[4]
Actions of skeletal muscle
editAs well asanatomical terms of motion,which describe the motion made by a muscle, unique terminology is used to describe the action of a set of muscles.
Agonists and antagonists
editAgonist muscles and antagonist muscles are muscles that cause or inhibit a movement.[5]
Agonist musclesare also calledprime moverssince they produce most of the force, and control of an action.[6]Agonists cause a movement to occur through their own activation.[7]For example, thetriceps brachiicontracts, producing ashortening (concentric) contraction,during the up phase of a push-up (elbow extension). During the down phase of a push-up, the same triceps brachii actively controls elbow flexion while producing alengthening (eccentric) contraction.It is still the agonist, because while resisting gravity during relaxing, the triceps brachii continues to be the prime mover, or controller, of the joint action.
Another example is the dumb-bell curl at the elbow. Theelbow flexor groupis the agonist, shortening during the lifting phase (elbow flexion). During the lowering phase the elbow flexor muscles lengthen, remaining the agonists because they are controlling the load and the movement (elbow extension). For both the lifting and lowering phase, the "elbow extensor" muscles are the antagonists (see below). They lengthen during the dumbbell lifting phase and shorten during the dumbbell lowering phase. Here it is important to understand that it is common practice to give a name to a muscle group (e.g. elbow flexors) based on the joint action they produce during a shortening contraction. However, this naming convention does not mean they are only agonists during shortening. This term typically describes the function ofskeletal muscles.[8]
Antagonist musclesare simply the muscles that produce an opposing joint torque to the agonist muscles.[9]This torque can aid in controlling a motion. The opposing torque can slow movement down - especially in the case of aballistic movement.For example, during a very rapid (ballistic) discrete movement of the elbow, such as throwing a dart, the triceps muscles will be activated very briefly and strongly (in a "burst" ) to rapidly accelerate the extension movement at the elbow, followed almost immediately by a "burst" of activation to the elbow flexor muscles that decelerates the elbow movement to arrive at a quick stop. To use an automotive analogy, this would be similar to pressing the accelerator pedal rapidly and then immediately pressing the brake. Antagonism is not an intrinsic property of a particular muscle or muscle group; it is a role that a muscle plays depending on which muscle is currently the agonist. During slower joint actions that involve gravity, just as with the agonist muscle, the antagonist muscle can shorten and lengthen. Using the example of the triceps brachii during a push-up, the elbow flexor muscles are the antagonists at the elbow during both the up phase and down phase of the movement. During the dumbbell curl, the elbow extensors are the antagonists for both the lifting and lowering phases.[10]
Antagonistic pairs
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Antagonist and agonist muscles often occur in pairs, calledantagonistic pairs.As one muscle contracts, the otherrelaxes.An example of an antagonistic pair is thebicepsandtriceps;to contract, the triceps relaxes while the biceps contracts to lift the arm. "Reverse motions" need antagonistic pairs located in opposite sides of a joint or bone, includingabductor-adductorpairs and flexor-extensor pairs. These consist of anextensor muscle,which "opens" the joint (by increasing the angle between the two bones) and aflexor muscle,which does the opposite by decreasing the angle between two bones.
However, muscles do not always work this way; sometimes agonists and antagonists contract at the same time to produce force, as perLombard's paradox.Also, sometimes during a joint action controlled by an agonist muscle, the antagonist will be slightly activated, naturally. This occurs normally and is not considered to be a problem unless it is excessive or uncontrolled and disturbs the control of the joint action. This is called agonist/antagonist co-activation and serves to mechanically stiffen the joint.
Not all muscles are paired in this way. An example of an exception is thedeltoid.[11]
Synergists
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Synergist musclesalso calledfixators,act around a joint to help the action of anagonist muscle.Synergist muscles can also act to counter or neutralize the force of an agonist and are also known asneutralizerswhen they do this.[12]As neutralizers they help to cancel out or neutralize extra motion produced from the agonists to ensure that the force generated works within the desired plane of motion.
Muscle fibers can only contract up to 40% of their fully stretched length.[citation needed]Thus the short fibers ofpennate musclesare more suitable where power rather than range of contraction is required. This limitation in the range of contraction affects all muscles, and those that act over several joints may be unable to shorten sufficiently to produce the full range of movement at all of them simultaneously (active insufficiency, e.g., the fingers cannot be fully flexed when the wrist is also flexed). Likewise, the opposing muscles may be unable to stretch sufficiently to allow such movement to take place (passive insufficiency). For both these reasons, it is often essential to use other synergists, in this type of action to fix certain of the joints so that others can be moved effectively, e.g., fixation of the wrist during full flexion of the fingers in clenching the fist. Synergists are muscles that facilitate the fixation action.
There is an important difference between ahelping synergistmuscle and atrue synergistmuscle. A true synergist muscle is one that only neutralizes an undesired joint action, whereas a helping synergist is one that neutralizes an undesired action but also assists with the desired action.[citation needed]
Neutralizer action
editA muscle that fixes or holds a bone so that the agonist can carry out the intended movement is said to have a neutralizing action. A good famous example of this are thehamstrings;thesemitendinosusandsemimembranosus musclesperform knee flexion and kneeinternal rotationwhereas thebiceps femoriscarries out knee flexion and kneeexternal rotation.For the knee to flex while not rotating in either direction, all three muscles contract to stabilize the knee while it moves in the desired way.
Composite muscle
editCompositeorhybridmuscles have more than one set of fibers that perform the same function, and are usually supplied by different nerves for different set of fibers. For example, the tongue itself is a composite muscle made up of various components like longitudinal, transverse, horizontal muscles with different parts innervated from a different nerve supply.
Muscle naming
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There are a number of terms used in the naming of muscles including those relating to size, shape, action, location, their orientation, and their number of heads.
- By size
- brevismeans short;longusmeans long;majormeans large;maximusmeans largest;minormeans small, andminimussmallest. These terms are often used after the particular muscle such asgluteus maximus,andgluteus minimus.[13]
- By shape
- deltoidmeans triangular;quadratusmeans having four sides;rhomboideusmeans having arhomboidshape;teresmeans round or cylindrical,trapeziusmeans having atrapezoidshape,rectusmeans straight. Examples are thepronator teres,thepronator quadratusand therectus abdominis.[13]
- By action
- abductormoving away from the midline;adductormoving towards the midline;depressormoving downwards;elevatormoving upwards;flexormoving that decreases an angle;extensormoving that increase an angle or straightens;pronatormovingto face down;supinatormovingto face upwards;[13]Internal rotatorrotatingtowards the body;external rotatorrotating away from the body.
Form
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Insertion and origin
editThe insertion and origin of a muscle are the two places where it is anchored, one at each end. The connective tissue of the attachment is called anenthesis.
Origin
editTheoriginof a muscle is thebone,typically proximal, which has greater mass and is more stable during acontractionthan a muscle's insertion.[14]For example, with thelatissimus dorsi muscle,the origin site is the torso, and the insertion is the arm. When this muscle contracts, normally the arm moves due to having less mass than the torso. This is the case when grabbing objects lighter than the body, as in the typical use of alat pull downmachine. This can be reversed however, such as in achin upwhere the torso moves up to meet the arm.
Theheadof a muscle, also calledcaput musculiis the part at the end of a muscle at its origin, where it attaches to a fixed bone. Some muscles such as thebicepshave more than one head.
Insertion
editTheinsertionof a muscle is the structure that it attaches to and tends to be moved by thecontractionof the muscle.[15]This may be abone,atendonor the subcutaneous dermalconnective tissue.Insertions are usually connections of muscle viatendonto bone.[16]The insertion is a bone that tends to be distal, have less mass, and greater motion than the origin during a contraction.
Intrinsic and extrinsic muscles
editIntrinsic muscleshave their origin in the part of the body that they act on, and are contained within that part.[17]Extrinsic muscles have their origin outside of the part of the body that they act on.[18]Examples are theintrinsic and extrinsic muscles of the tongue,andthose of the hand.
Muscle fibers
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A: fusiform. B: unipennate. C: bipennate.
(PCS: physiological cross-section)
Muscles may also be described by the direction that themuscle fibersrun, in theirmuscle architecture.
- Fusiform muscleshave fibers that run parallel to the length of the muscle, and arespindle-shaped.[19]For example, thepronator teres muscleof theforearm.
- Unipennate muscleshave fibers that run the entire length of only one side of a muscle, like aquill pen.For example, thefibularis muscles.
- Bipennate musclesconsist of two rows of oblique muscle fibres, facing in opposite diagonal directions, converging on a centraltendon.Bipennate muscle is stronger than both unipennate muscle and fusiform muscle, due to a largerphysiological cross-sectional area.Bipennate muscle shortens less than unipennate muscle but develops greater tension when it does, translated into greater power but less range of motion.Pennate musclesgenerally also tire easily. Examples of bipennate muscles are therectus femoris muscleof thethigh,and thestapedius muscleof themiddle ear.
State
editHypertrophy and atrophy
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Hypertrophyis increase in muscle size from an increase in size of individual muscle cells. This usually occurs as a result of exercise.
See also
editReferences
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This article incorporates text in thepublic domainfrom the 20th edition ofGray's Anatomy(1918)
- ^Skeletal Muscle
- ^Stimec, Bojan V.; Dash, Jérémy; Assal, Mathieu; Stern, Richard; Fasel, Jean H. D. (1 May 2018)."Additional muscular slip of the flexor digitorum longus muscle to the fifth toe".Surgical and Radiologic Anatomy.40(5): 533–535.doi:10.1007/s00276-018-1991-7.PMID29473094.S2CID3456242.Retrieved13 May2021.
- ^Smooth Muscle
- ^Cardiac Muscle
- ^"Interactions of skeletal muscles their fascicle arrangement and their lever-systems".Archived fromthe originalon 23 March 2022.Retrieved10 May2021.
- ^Saladin, Kenneth S. (2011).Human anatomy(3rd ed.). New York: McGraw-Hill. pp. 236–241.ISBN9780071222075.
- ^Taber 2001,pp. "Agonist".
- ^Baechle, Thomas (2008).Essentials of Strength Training and Conditioning.USA: National Strength and Conditioning Association.ISBN978-0-7360-8465-9.
- ^Taber 2001,pp. "Antagonist".
- ^Walker, H. Kenneth (1990), Walker, H. Kenneth; Hall, W. Dallas; Hurst, J. Willis (eds.),"Deep Tendon Reflexes",Clinical Methods: The History, Physical, and Laboratory Examinations(3rd ed.), Boston: Butterworths,ISBN978-0-409-90077-4,PMID21250237,retrieved2024-02-19
- ^Purves, D; Augustine, GJ (2001)."Neural Circuits".NCBI.Sinauer Association.
- ^"9.6C: How Skeletal Muscles Produce Movements".Medicine LibreTexts.19 July 2018.Retrieved8 May2021.
- ^abcSaladin, Kenneth S. (2011).Human anatomy(3rd ed.). New York: McGraw-Hill. p. 265.ISBN9780071222075.
- ^OED 1989,"origin".
- ^Taber 2001,"insertion".
- ^Martini, Frederic; William C. Ober; Claire W. Garrison; Kathleen Welch; Ralph T. Hutchings (2001).Fundamentals of Anatomy and Physiology, 5th Ed.Prentice Hall.ISBN0130172928.
- ^"Definition of INTRINSIC".www.merriam-webster.com.Retrieved7 May2021.
- ^"Definition of EXTRINSIC".www.merriam-webster.com.Retrieved7 May2021.
- ^Taber 2001,"Fusiform".
- Books
- Taber, Clarence Wilbur; Thomas, Clayton L.; Venes, Donald (2001).Taber's cyclopedic medical dictionary(Ed. 19, illustrated in full color ed.). Philadelphia: F.A.Davis Co.ISBN0-8036-0655-9.ISSN1065-1357.
- J. A. Simpson, ed. (1989).The Oxford English dictionary.Oxford: Clarendon Press.ISBN9780198611868.
