Muscle Fiber

Cardiac muscle fibers are striated and composed of similar sarcomere structures as skeletal muscles.

From: Biofluid Mechanics , 2012

Contractile Mechanisms in Skeletal Muscle

Joseph Feher , in Quantitative Homo Physiology, 2012

Publisher Summary

Musculus fibers are big multinucleated cells whose most obvious histological feature is cantankerous-striations. The cytoplasm contains many myofibrils that include tiny cylinders consisting of bundles of myofilaments. The myofilaments include thick filaments mainly composed of myosin and thin filaments mainly composed of actin. The thick and thin filaments cause the cross-striations because of their regular overlap that is kept in register all beyond the bore of the muscle cobweb. The A-band corresponds to the thick filament. The I-band forms where the thin filament does not overlap the thick filament. Z-disks are centered in the I-band. The functional unit of wrinkle is the sarcomere, which extends from one Z-disk to the side by side. The whole signal of muscles is to evangelize force to the exterior of the muscle. This is accomplished by transferring the force to cytoskeletal elements that connect the internal myofilaments to special proteins that penetrate the membrane. These proteins transfer the forcefulness to the extracellular matrix at special regions called costameres. It is found that dystrophin, the protein lacking in some kinds of muscular dystrophy, is concentrated at the costameres.

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Physiological Foundations

David A. Rosenbaum , in Human Motor Control (Second Edition), 2010

Motor Units and Recruitment

The muscles fibers within a muscle group are innervated past motor neurons. Any given muscle cobweb is innervated by just 1 motor neuron. A motor neuron and the muscle fibers it innervates are chosen a motor unit.

The muscle fibers within a motor unit usually have similar mechanical backdrop. Such mechanical homogeneity may simplify the recruitment of motor units. For a given task, the motor units to be recruited can be the ones that are mechanically best suited to the task. Similarly, tasks with dissimilar mechanical demands may telephone call for the recruitment of different motor units. Swinging the leg or maintaining stance, for case, are carried out with unlike motor units (Loeb, 1985).

The number of muscle fibers in a motor unit varies from effector to effector. In the paw and eye, fewer than 100 muscle fibers occupy a motor unit; in the lower leg, a single motor unit may contain as many every bit one,000 muscle fibers (Buchthal & Schmalbruch, 1980). Generally, the larger the number of muscle fibers in a motor unit of measurement, the less precise the associated movements.

Activation of a motor unit is all or none. Information technology is impossible to voluntarily activate some only not all of the muscle fibers within a motor unit. In this sense, the motor unit is the nearly basic unit of motor control. On the other manus, it is possible to voluntarily recruit some motor units but not others. With feedback, such as visual or auditory signals concerning the action of single motor units, people tin learn to activate simply ane motor unit at a time (Figure three.iv).

FIGURE 3.four. Arrangement for learning to activate single motor units. The setup shown illustrates the original engineering science used.

 From Basmajian, J. V. (1974). Muscles alive: Their functions revealed by electromyography (3rd Edition). Baltimore: Williams & Wilkins. With permission.

When movements are produced without overt feedback about the activity of single motor units, motor units tend to exist recruited in an orderly fashion (see Figure three.v). The first activated motor units are normally the ones whose muscle fibers are smallest and least forceful. As recruitment continues, the motor units that turn on have larger and more forceful muscle fibers. This orderly relation is called the size principle (Henneman, Somjen, & Carpenter, 1965).

FIGURE 3.5. The size principle of Henneman, Somjen, & Carpenter (1965). Open and filled circles stand for to data obtained from the same subject at different times.

 From Brooks, V. B. (1986). The neural basis of motor command. New York: Oxford Academy Press. With permission.

What is the physiological ground of the size principle? Pocket-size motor neurons have depression thresholds for generating action potentials (neural firings), whereas large motor neurons accept high thresholds for generating action potentials. Thus weak inputs to the motor neuron pool tin can produce activity potentials in minor motor neurons. As the forcefulness of input grows, larger motor neurons begin to burn owing to their increasing thresholds for activation.

The size principle has several functional advantages for motor control. 1 is that large forces are not produced when they are unnecessary; recruitment tin stop when the advisable forces have been generated. The size principle also has a computational benefit. Because hundreds or even thousands of motor units may be involved in the activation of a muscle grouping, the number of possible recruitment orders tin can be very large—larger in fact than the number of neurons in the brain (Enocka & Stuart, 1984). Thus a regular recruitment order based on size helps reduce the degrees of freedom problem at this depression level of control.

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Contractile Mechanisms in Skeletal Muscle

Joseph Feher , in Quantitative Homo Physiology (Second Edition), 2017

Muscle Fibers Have a Highly Organized Structure

Muscle fibers are typically big cells, some xx–100  μm in bore and many centimeters long, with the longest fibers beingness most 12   cm. These cells are multinucleated, because they need many nuclei to govern protein synthesis and degradation. The nuclei are typically located virtually the periphery of the cell and oft are more highly full-bodied nigh the myoneural, or neuromuscular, junction. The most hitting feature of muscle cells viewed nether the lite microscope is their banded appearance. The fibers have stripes, or striations, that result from the highly organized arrangement of proteins in the musculus fiber. These striations consist of alternating A-bands and I-bands, named because the I-bands are isotropic to polarized low-cal (meaning that they announced the same from all directions) whereas the A-bands are anisotropic to polarized light. The cross-striations are perpendicular to the long axis of the muscle fiber. Effigy 3.5.i shows the microscopic appearance of frog skeletal muscle fibers using phase dissimilarity microscopy.

Effigy 3.5.ane. Microscopic appearance of skeletal musculus. A packet of frog sartorius muscle fibers was teased out and viewed under phase contrast microscopy. Cross-striations are readily apparent in these unstained muscle fibers.

Muscle cells are also striated longitudinally past the organization of contractile proteins into tiny threads chosen myofibrils. These are generally cylinders of material about 1   μm in bore that also conspicuously bear witness cross-striations. The myofibrils are kept in register across the entire cell to give rise to the cross-striated appearance. The electron micrograph in Effigy 3.5.2 shows how the striations in the myofibrils line up across the cell.

Figure 3.v.two. Electron micrograph of musculus. The spaces between myofibrils are filled with membranes of the sarcoplasmic reticulum, mitochondria, and glycogen granules. The myofibrils are bundles of filaments arranged longitudinally parallel to the long axis of the muscle fiber. The various bands are named according to their position, appearance, or by how they rotate the plane of polarized light.

But as each muscle fiber contains many myofibrils, each myofibril is in plough composed of many filaments. These filaments come in two principal varieties: the thin filament and the thick filament. The major constituent of the thin filament is actin; the master component of the thick filament is myosin. The microscopic striated appearance of the musculus is due to the fashion in which the filaments overlap each other.

The thick filaments define the commencement and terminate of the A-band. The myosin component of the A-band gives rise to the anisotropic beliefs under polarized light. Because the thick filaments are 1.6   μm long, the A-ring is also one.vi   μm long. Figure iii.5.3 shows a schematic illustration of the structure of the muscle fibers and myofibrils.

Effigy 3.5.3. Construction of the muscle cobweb and myofibrils. The A-ring corresponds to the length of the thick filaments, ane.6   μm. The I-ring corresponds to the thin filaments where they do not overlap with the thick filaments. Its width depends on the activation of the muscle. The Z-line or disk is where the thin filaments from reverse sarcomeres are attached. The Yard-line in the middle of the A-band keeps the thick filaments centered and in annals. The clear zone in the middle of the A-band is the region where sparse filaments do not overlap thick filaments.

The thin filaments are about 1.0   μm long but their length varies with muscle types and species. In human deltoid muscle biopsies, thin filament length averages ane.19   μm whereas in the pectoralis major it is i.37   μm. Reverse thin filaments are continued, back to back, at a construction called the Z-line (from the German "zwischen" meaning "betwixt"). Considering the myofibrils are cylindrical, the Z-line is actually a disk of textile and it is likewise called the Z-disk. The sparse filaments typically overlap the thick filaments. The H-zone is a clearer area in the middle of the thick filaments. Its proper name derives from the German "helles," meaning "articulate." This is the function of the A-band in which the thin filaments do not overlap the thick filaments. The thick filaments are connected in their center by material that forms the M-line (from the German "mittel" meaning "middle"; run into Figures 3.5.ii and 3.v.3).

Electron micrographs show that the thick filaments form a hexagonal-centered lattice. The thin filaments likewise form a hexagonal lattice, but it is rotated 30° from the thick filament lattice. Each thick filament is in the center of a hexagon of thin filaments, whereas each thin filament is located equidistant from a triangle of iii thick filaments. Thus each thin filament is surrounded by three thick filaments and each thick filament is surrounded past six thin filaments. In some electron micrographs, cross-bridges can be seen betwixt the thick and thin filaments. The interaction of the filaments through these cantankerous-bridges produces either shortening or force.

The functional unit of measurement of contraction or force production is the sarcomere, extending from one Z-disk to the next. The myofibrils consist of thousands of these sarcomeres strung end to end. The length of the sarcomere varies with muscle activation. Typically, the residual length is virtually ii.0–2.2   μm, depending on the length of the thin filament. This rest length is less than the thick filament (1.half dozen   μm) plus ii thin filaments (one.0–i.37   μm each) because of the overlap of the filaments at balance.

At regular intervals, the surface membrane of the muscle fiber, the sarcolemma, is invaginated to form a long tubule running perpendicular to the surface and penetrating into the farthest parts of the fiber's interior. These are the transverse tubules or T-tubules. The function of these T-tubules is to bring the activity potential on the sarcolemma into the interior of the cell. The T-tubules allow for the rapid spread of the excitation to all parts of the sarcoplasm.

Adjacent to the T-tubules and surrounding each myofibril is a membranous network called the sarcoplasmic reticulum or SR. This organelle surrounds the myofibril similar a loosely knit sweater surrounds your arm. Information technology forms an internal compartment, the lumen, separate from the cytoplasm of the muscle fiber. The SR is divided into a longitudinal SR and final cisternae. The final cisternae are sacs that make contact with the T-tubules, whereas the longitudinal SR are thin tubes of membrane that connect the concluding cisternae from one side of the sarcomere to the other. The longitudinal SR and terminal cisternae are continued and course a unmarried enclosed space. In skeletal muscle, the junction of the T-tubule and the SR forms a triad, because information technology is the junction of one T-tubule and two terminal cisternae. In mammalian skeletal muscle, the triads occur at the junction of the A-band and I-band, so that there are two triads per sarcomere. This arrangement of the T-tubules and SR junction varies with the species and musculus type. Figure 3.5.4 shows the anatomical relation of the SR to the myofibril.

Figure 3.five.4. Structure of the SR around the myofibril.

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Gastric and Pancreatic Function Tests

F.J. BAKER F.I.M.L.S., F.I.S.T. , R.Eastward. SILVERTON F.I.1000.L.S., Fifty.I.Biol. , in Introduction to Medical Laboratory Technology (Fifth Edition), 1976

Musculus fibres

Muscle fibres, which are derived from meat, are stained yellowish-dark-brown by faecal stercobilin. They are unremarkably excreted fully or partially digested. The presence of undigested fibres indicates digestive impairment, for example pancreatic disease. The various stages of muscle digestion are equally follows, simply it must be realized that there is no sharp demarcation betwixt the types; it is a gradual transition.

Undigested muscle fibres every bit seen in the faeces are shown in Effigy viii.6a . They have irregular ends, nuclei and transverse striations; an isolated undigested fibre is seen in Effigy viii.6b . Annotation the irregular ends and transverse striations. When acted upon by the digestive juices, the nuclei disappear first and then the ends go rounded. This partially digested fibre tin can be seen in Figure 8.6c . In Figure 8.6d the muscle fibre has been further digested when information technology tin be seen that the ends are notwithstanding round, but now information technology has longitudinal striations. Figure 8.6e shows fragments of fibres free from striations with rounded ends, the digestion being more complete than in Figure 8.6d .

Figure 8.6. Muscle fibres showing various stages of digestion

Soaps appear as plaques with rolled-over edges or equally masses of needle-shaped crystals ( Figure 8.7 ) and are often seen equally a mass of lather crystals. They are insoluble in ether and ethanol, whereas fats and fat acids are soluble. A simple method of identification consists of making a intermission of the faeces in a few drops of saturated copper nitrate on a slide, covering with a coverslip and examining after a few minutes. Soaps are stained greenish owing to their conversion to copper soaps. Fatty acrid crystals are non stained. In a normal specimen of faeces an occasional soap plaque may be seen. Excessive amounts of soaps indicate defective absorption. Fat globules tend to rise to the surface of the preparation. They are called neutral fats and vary in size, are highly refractable and await oily. Simply before deciding that the oil globules are neutral fat (stearin, palmitin and olein) information technology is of import to make sure the patient is not receiving liquid paraffin or other oily drugs ( Figure 8.8 ).

Figure 8.vii. Various forms of soap plaques

Effigy viii.viii. Neutral fat and fatty acid crystals

Fatty acid crystals announced every bit colourless needle-shaped crystals. They are longer than bacilli and are oft slightly curved. Ordinarily seen in groups of two, three or more. They are unaffected by aqueous copper nitrate but are soluble in ether and ethanol.

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Circulate Control for a Large Assortment of Stochastically Controlled Piezoelectric Actuators

Jun Ueda , H. Harry Asada , in Microbiorobotics, 2012

iv.2.2 Bistable actuator control

Muscle fibers have large hysteresis (east.g., Ref. [24]). About materials for bogus musculus actuators have also prominent hysteresis and state-dependent nonlinearities [1, 36–39]. Functional units lower in the muscle bureaucracy take a binary state, which can be modeled as bistable, or ON-OFF, finite state machines. Bistable ON-OFF control can cope with circuitous non-linearities of actuator materials.

Figure 4.ii shows the concept of bistable control of actuator materials. The displacement of the actuator is given by the aggregate sum of the binary outputs of all the cellular actuators. Instead of driving the whole actuator material as a majority, the actuator material is divided into many small segments, each controlled every bit a bistable ON-OFF finite state automobile. Bistable ON-OFF control does not depend on the hysteresis. Every bit the state of the material is pushed toward either ON or OFF state, intermediate states practise not demand to be realized. Dynamic transition may be influenced by the varying not-linearities. Nonetheless, the command problem becomes much simpler for ON-OFF control, equally demonstrated in Ref. [40] for shape-memory alloys (SMAs) and in Ref. [41] for dielectric elastomers.

Effigy 4.ii. Bistable ON-OFF control of actuator materials. Modified and reprinted with permission from © SAGE Publications 2007.

This cellular architecture has some other feature with respect to speed of response. Common to many actuator materials is the fact that speed of response increases when the actuator materials are segmented into many small units or thin films. For example, thin moving picture SMA actuators [two] have a small amount of thermal capacitance, thus the response time is reduced.

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Nanofiber composites in skeletal muscle tissue engineering

A. Cai , ... J.P. Beier , in Nanofiber Composites for Biomedical Applications, 2017

fifteen.2.ii The extracellular matrix

The muscle fibers in native skeletal muscle are closely packed together in an extracellular matrix (ECM) to form an organized tissue with loftier cell density and three-dimensional (3D) cellular orientation. The ECM plays an essential part in the growth, attachment, alignment, and differentiation of myoblasts and is part of the signaling mechanism involved in myogenesis. Those cellular processes become activated when cells bind to the ECM via cell surface receptors [12–14]. Regardless of the tissue type, in that location are fundamental characteristics of the ECM that are universal: information technology is equanimous of a heterogeneous limerick of macromolecules including proteins and polysaccharides which are typically in fiber grade and include topography at the nanoscale, measuring less than 1   µm of diameter. Those macromolecules provide biochemical and biophysical cues for cell function. Proteins include collagens, laminins, fibronectins, and elastins. Collagen is the major structural protein in skeletal muscle ECM. In that location are different kinds of collagens. I of the near of import collagens is type IV collagen, being an essential part of the basement membrane. Laminin is besides ubiquitous in the basal lamina and tin can self-assemble into networks in clan with other ECM components. Polysaccharides are known as glycoasminoglycans, which are polysaccharide chains, being composed of hyaluronan, keratin sulfate, chondroitin sulfate, and heparin sulfate. They function as a linker between blazon IV collagen and the sarcolemma of the skeletal musculus [8,15,xvi].

The ECM serves important functions in controlling cell behavior such as adhesion signals, growth factor binding sites, and degradation sites. Various ECM molecules contain specific peptide motifs that permit them to directly demark to cell surface receptors. For instance, integrins, the first identified ECM receptor, directly induce biochemical signals into the cells when ligand bounden occurs. The signaling is transduced via the cytoskeleton and induces cell shape changes leading to growth and differentiation [17].

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Development of Muscular System

In Reference Module in Biomedical Sciences, 2014

Conclusion and Differentiation of Skeletal Musculus

The mature skeletal muscle fiber is a complex multinucleated cell that is specialized for contraction. Precursors of most muscle lineages (myogenic cells) have been traced to the myotome of the somite. Committed myogenic cells laissez passer through several additional mitotic divisions before completing a terminal mitotic division and becoming postmitotic myoblasts.

Proliferating myogenic cells are kept in the jail cell cycle through the activeness of growth factors, such equally FGF and transforming growth gene-β. With the accumulation of myogenic regulatory factors (see next section), myogenic cells upregulate the synthesis of the prison cell cycle protein p21, which irreversibly removes them from the cell bike. Under the influence of other growth factors, such as insulinlike growth cistron, the postmitotic myoblasts begin to transcribe the mRNAs for the major contractile proteins actin and myosin, merely the major event in the life cycle of a postmitotic myoblast is its fusion with other similar cells into a multinucleated myotube ( Effigy 2 ). The fusion of myoblasts is a precise process involving their lining up and adhering by calcium (Ca++)–mediated recognition mechanisms, involving molecules such as M-cadherin, and the ultimate union of their plasma membranes.

Figure 2. Stages in the morphological differentiation of a skeletal muscle fiber. Important subcellular elements in a musculus fiber are as well shown.

Myotubes are intensively involved in mRNA and protein synthesis. In addition to forming actin and myosin, myotubes synthesize a broad variety of other proteins, including the regulatory proteins of muscle wrinkle–troponin and tropomyosin. These proteins assemble into myofibrils, which are precisely arranged aggregates of functional contractile units called sarcomeres. As the myotubes fill with myofibrils, their nuclei, which had been arranged in regular fundamental bondage, migrate to the periphery of the myotube. At this stage, the myotube is considered to accept differentiated into a muscle fiber, the final stage in the differentiation of the skeletal muscle cell.

The development of a muscle fiber is not complete, however, with the peripheral migration of the nuclei of the myotube. The nuclei (myonuclei) of a multinucleated musculus cobweb are no longer able to proliferate, simply the muscle fiber must continue to grow in proportion to the rapid growth of the fetus and so the infant. Muscle fiber growth is achieved by means of a population of myogenic cells, called satellite cells, which have up positions betwixt the musculus cobweb and the basal lamina in which each musculus cobweb encases itself (see Figure two ). Operating under a poorly understood control mechanism, possibly involving the Delta/Notch signaling arrangement, satellite cells dissever slowly during the growth of an private. Some of the daughter cells fuse with the muscle cobweb then that it contains an adequate number of nuclei to direct the continuing synthesis of contractile proteins required by the muscle fiber.

A typical musculus is not composed of homogeneous muscle fibers. Instead, usually several types of musculus fibers are distinguished by their contractile properties and morphology and by their possession of dissimilar isoforms of the contractile proteins.

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CENTRAL NERVOUS SYSTEM — LOCOMOTOR SYSTEM

B.R. Mackenna MB ChB PhD FRCP(Glasg) , R. Callander FFPh FMAA AIMBI , in Illustrated Physiology (Sixth Edition), 1990

THE MUSCLE SPINDLE

The intrafusal muscle fibres that make upward a muscle spindle (p. 289) consist of (a) two nuclear bag fibres (purse1 and pocketbookii) with many nuclei in their distended heart 3rd (equatorial region) and (b) iv or more nuclear chain fibres with a unmarried row of nuclei in the equatorial region. The ends of the purse fibres are attached to the connective tissue of the surrounding extrafusal fibres. Nuclear chain fibres, beingness shorter, are attached to the connective tissue of the nuclear bag fibres. Both fibre types have a motor and sensory innervation and their outer thirds (the polar regions) are striated and contractile.

Activation of α-motor neurons by descending fibres from the motor surface area of the cerebral cortex is accompanied by simultaneous activation of γ-motor neurons. This is α-γ coactivation and is very important for the accurate control of all muscle movements. Encounter p. 308.

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Spinal Reflexes

Joseph Feher , in Quantitative Human being Physiology, 2012

The Muscle Spindle Is a Specialized Muscle Fiber

The regular musculus fibers are chosen extrafusal fibers, and the activation and contraction of these fibers are discussed in Chapters 3.4–iii.7 Chapter 3.4 Chapter three.7 . Interspersed amid these fibers are small encapsulated sensory receptors that have a fusiform or spindle shape. The unabridged apparatus is called the muscle spindle, and its job is to inform the primal nervous arrangement (CNS) of the contractile land of the muscle by sending afferent impulses to the spinal cord when the muscle spindle is stretched. The musculus spindle consists of a group of fine muscle fibers, called intrafusal muscle fibers, 4–10   mm long, whose cardinal portions are not contractile.

Typically the connective tissue capsule encloses two unlike types of intrafusal fibers. The nuclear chain intrafusal fibers accept a gear up of aligned nuclei in the middle. The nuclear bag fibers have a clump of nuclei randomly arrayed in a purse-similar structure in the center of the intrafusal fiber. Typically a spindle has two to iii nuclear pocketbook fibers and about 5 nuclear chain fibers. At that place are also 2 distinct types of afferent sensory fiber endings. Large myelinated nerves termed Ia or main afferents surrounds the central portion of all of the intrafusal musculus fibers. This nervus ending forms a coiled construction called the annulospiral ring. Stretching this nerve ending activates stretch-activated channels in its surface that depolarize the neuron and therefore increase its firing rate. Lengthening the muscles stretches these receptors and increases the firing rate. Shortening of the muscle alleviates stretch of these receptors and their firing rate decreases. Thus, the firing charge per unit of the intrafusal sensory nervus is related to the stretch of the muscle relative to the musculus spindle. The nuclear bags actually come up in ii varieties: "static numberless" and "dynamic numberless". The static nuclear pocketbook and nuclear concatenation fibers receive a second kind of innervation, classified every bit 2 afferents. These innervate the juxtaequatorial regions of the intrafusal fibers. The Ii afferents consist of medium myelinated fibers that adapt slowly. Their tonic activity carries information most the static muscle length. In add-on to these sensory receptors, each intrafusal fiber is innervated with a motor neuron, the gamma motor efferent, that controls the length of the intrafusal fiber by activating its contractile mechanism just as an ordinary motor neuron would. This has the upshot of maintaining the sensitivity of the muscle spindle when muscles contract. The arrangement is shown schematically in Figure 4.four.3.

Figure 4.4.3. The musculus spindle. The musculus spindle is a group of smaller and specialized muscle fibers inside a muscle. Typical spindles comprise two bag fibers and well-nigh five chain fibers. The nuclear bag fibers are classified as static or dynamic. The intrafusal fibers are innervated by motor nerves (the gamma motor neurons) and two different types of sensory fibers: the type Ia stretch receptors and the type II afferent sensory receptors. The annulospiral receptors sense stretch and are rapidly adapting. Thus they sense the charge per unit at which the muscle is stretched. The blazon 2 afferents are slowly adapting and inform the CNS most the static stretch of the musculus.

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PHYSIOLOGICAL FOUNDATIONS

David A. Rosenbaum , in Human Motor Control, 1991

Motor Units and Recruitment

Muscles are equanimous of muscle fibers. The muscle fibers within a muscle grouping are innervated by several motor neurons. Whatever given muscle fiber is innervated by just one motor neuron. A motor neuron and the musculus fibers it innervates class what is called a motor unit.

The muscle fibers inside a motor unit of measurement normally have similar mechanical properties. Such mechanical homogeneity simplifies the task of recruiting motor units. For a given task, the motor units to be recruited can be the ones that are mechanically all-time suited to the job. For tasks with different mechanical demands, different motor units can be recruited. Swinging the leg or maintaining opinion, for example, are carried out with dissimilar motor units (Loeb, 1985).

The number of muscle fibers in a motor unit varies from effector to effector. In the hand and eye fewer than 100 muscle fibers occupy a motor unit, simply in the lower leg a single motor unit may incorporate as many as 1000 muscle fibers (Buchthal & Schmalbruch, 1980). More often than not, the larger the number of muscle fibers in a motor unit, the less precise the movements it allows.

Information technology is incommunicable to actuate voluntarily some but not all of the muscle fibers inside a motor unit. In this sense, the motor unit of measurement is the most bones unit of motor control. However, it is possible voluntarily to recruit some motor units but not others. With feedback, such as visual or auditory signals virtually the activeness of single motor units, people tin learn to activate one motor unit at a fourth dimension (Figure 2.iv). In more natural situations, when movements are produced without overt feedback nearly the activeness of unmarried motor units, motor units however tend to be recruited in an orderly mode (run into Effigy two.5). The get-go activated motor units are usually the ones whose muscle fibers are smallest and to the lowest degree forceful. As recruitment continues, the motor units that plough on have larger and more forceful muscle fibers. This orderly relation is called the size principle (Henneman, Somjen, & Carpenter, 1965).

Effigy 2.4. System for learning to activate unmarried motor units.

 From J. V. Basmajian, Muscles alive: Their functions revealed by electromyography (3d ed.). Copyright © past Williams and Wilkins, 1974. Copyright © 1974

Figure ii.five. The size principle of Henneman, Somjen, & Carpenter (1965). Open and filled circles correspond to data obtained from the same field of study at dissimilar times.

 Adapted from Brooks, 1986.

What is the physiological basis of the size principle? The reply is related to thresholds of activity potentials (neural firing). Motor units with small musculus fibers have small motor neurons, whereas motor units with large musculus fibers have large motor neurons. Modest motor neurons accept low thresholds for generating action potentials (neural "firings"), only large motor neurons have high thresholds for generating action potentials. Thus weak inputs to the motor neuron pool can produce action potentials in small motor neurons. As the strength of input grows, larger motor neurons begin to fire.

The size principle has several functional advantages for motor control. I is that large forces are not produced when they are unnecessary; recruitment can stop when the advisable force has been generated. The size principle also confers a computational benefit. Because hundreds or even thousands of motor units may be involved in the activation of a musculus group, the number of possible recruitment orders can become very large–larger in fact than the number of neurons in the brain (Enocka & Stuart, 1984). Thus a regular recruitment guild based on size helps reduce the degrees-of-freedom problem at this low level of control. [For discussion of some exceptions to the size principle, see Desmedt (1981).]

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