Skeletal Muscle Structure
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General Overview
Introduction to Muscle
Skeletal muscle is a classic example of a biological structure-function relationship. At both macro- and micro-scopic levels, skeletal muscle is exquisitely tailored for force generation and movement. This page is an attempt to at least mention some of the important aspects of the basic science of the neuromuscular system, a kind of table of contents in prose.
Development
The development of the neuromuscular system is typically divided into at least three phases. Myogenesis (perhaps synchronous with axonal outgrowth) refers to the fusion of the precursor myoblasts into true muscle fibers. Nerves attach to, or innervate, fibers as (or perhaps just before) they develop during the phase of synaptogenesis. This process results in most fibers being multiply innervated. The several axons in contact with each fiber compete for control during synapse elimination until each fiber is synapsed with only one axon. Single innervation is believed to be very important, as the axon is thought to have a strong influence on the properties of the fiber.
Muscle Fiber Structure
Muscle cells are roughly cylindrical, with diameters between 10 and 100 µm but up to a few centimeters long. Each cell is embedded in a basal lamina of collagen and large glycoproteins. Between the fiber and the basal lamina are large numbers of satellite cells, that are important in the growth and repair of the fiber. The fiber itself contains specialized structures for excitation-contraction coupling to ensure that a contractile stimulus (received at the synapse) is rapidly and evenly communicated to the whole fiber. Contractile and performance characteristics vary, but are closely linked to the myosin heavy chain isoform expressed by the fiber. Force production occurs in the myofibrils, which are chains of sarcomeres running from one end of the fiber to the other. Energy for contraction comes from metabolism of fats and sugars.
Muscle Architecture
The properties of a whole muscle depend not only on the properties of the fibers, but also on the organization of those fibers: the muscle architecture. Fibers rarely run the whole length of the muscle, tending to be somewhat oblique to the muscle's line of action. Peak force production is related to the physiological cross sectional area (PCSA), which estimates the sum of the cross sectional area of all the fibers. Contraction velocity and excursion range are related to fiber length.
Control of Contraction
Although each fiber is innervated by a single axon, a motorneuron may have a hundred or more axons. A single motorneuron, with all the fibers it controls, is called a motor unit. As the brain's signal for contraction increases, it both recruits more motor units and increases the "firing frequency" of those units already recruited. Even during a "maximal voluntary contraction", it is unlikely that all the motor units (and hence muscle fibers) are activated.
Biomechanics of Strength
The above discussion focused on the muscle itself. All joints, however, are set up as lever systems: the fulcrum where two bones meet, one force produced by the muscle, and the other by a load. Strength is not just muscle force, but muscle force as modified by the mechanical advantage of the joint. To complicate matters further, this mechanical advantage usually varies with joint rotation (as does the muscle force). The net result is strength that varies with joint angle and may be somewhat decoupled from muscle force. Joint strength can (obviously) be increased with exercise.
Skeletal muscle comprises the largest single organ of the body. It is highly compartmentalized, and we often think of each compartment as a separate entity (such as the biceps muscle).
Each of these individual muscles is composed of single cells or fibers embedded in a matrix of collagen. At either end of the muscle belly, this matrix becomes the tendon that connects the muscle to bone.
Muscle cells contain most of the structures common to all cells. Each cell is enclosed by a cell membrane or plasmalemma; they contain mitochondria for the oxidative metabolism of nutrients; and all the machinery necessary for protein synthesis. Skeletal muscle fibers are multinucleated and can be as much as two centimeters long.
The principal force generating components are actin and myosin molecules. These myofilaments are arranged in interdigitating matrices capable of sliding across each other. To produce force, crossbridges from the myosin filaments associate with the actin filament, then rotate slightly to pull the filaments across each other (much like the oars of a rowboat pull across the water).
Muscle fibers, though, are just the building blocks for whole muscles. The precise way in which fibers are arranged into muscle is referred to as architecture.
Fiber Types
The three types of muscle (skeletal, cardiac and smooth) have significant differences. However, all three use the movement of actin against myosin to create contraction. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by the nerves, the motor nerves and motoneurons in particular. Cardiac and smooth muscle contractions are stimulated by internal pacemaker cells which regularly contract, and propagate contractions to other muscle cells they are in contact with. All skeletal muscle and many smooth muscle contractions are facilitated by the neurotransmitter acetylcholine.
Muscular activity accounts for much of the body's energy consumption. All muscle cells produce adenosine triphosphate (ATP) molecules which are used to power the movement of the myosin heads. Muscles conserve energy in the form of creatine phosphate which is generated from ATP and can regenerate ATP when needed with creatine kinase. Muscles also keep a storage form of glucose in the form of glycogen. Glycogen can be rapidly converted to glucose when energy is required for sustained, powerful contractions. Within the voluntary skeletal muscles, the glucose molecule can be metabolized anaerobically in a process called glycolysis which produces two ATP and two lactic acid molecules in the process (note that in aerobic conditions, lactate is not formed; instead pyruvate is formed and transmitted through the citric acid cycle). Muscle cells also contain globules of fat, which are used for energy during aerobic exercise. The aerobic energy systems take longer to produce the ATP and reach peak efficiency, and requires many more biochemical steps, but produces significantly more ATP than anaerobic glycolysis. Cardiac muscle on the other hand, can readily consume any of the three macronutrients (protein, glucose and fat) aerobically without a 'warm up' period and always extracts the maximum ATP yield from any molecule involved. The heart, liver and red blood cells will also consume lactic acid produced and excreted by skeletal muscles during exercise.
Fiber functional properties, peak force, contraction velocity, resistance to fatigue, oxidative and glycolytic capacities, and actino-myosin ATPase activities, fall across a broad spectrum. Nonetheless, it is possible to divide this continuum into a few clusters.
Based on observations of the contractile properties of motor units (force, velocity and fatiguability), Burke and coworkers created four motor unit types. Histochemical assays of the motor unit fibers striking similarites within a unit.
Slow
The most distinct type had long twitch times, low peak forces and high resistance to fatigue. Biochemically, these fibers were found to be high in oxidative enzymes, but low in glycolytic markers and ATPase activity. These have been termed "slow" fibers.
Fast, Fatigue Resistant
Of the fibers with faster contraction times, some were found to maintain their force production even after a large number of contractions. They tend to be high in oxidative and glycolytic enzymes and ATPase activity. These have been termed Fast Resistant (FR) or (histochemically) Fast Oxidative-Glycolytic (FOG).
Fast Fatiguable
The last clearly definable group displayed high contraction rates and extremely large forces, but was unable to maintain these tensions for more than a few contractions without rest. These properties correlated with high ATPase and glycolytic activities and low oxidative capacity. These have been termed Fast Fatiguable (FF) or Fast Glycolytic (FG) fibers.
Fast Intermediate
Basically a catch-all group for a small number of fibers that didn't clearly belong to the other fast groups. These fibers have fast contraction times and maintain some, though not a great amount of their force production with repeated activity.
With the development of techniques capable of identifying specific proteins (or even isoforms of the same family), specifically, antibody techniques and gel electrophoresis, it has been found that these functional properties are closely related to the myosin heavy chain (MHC) isoform. In fact, most of the proteins of the contractile machinery exist in several isoforms, with one or two being associated with each MHC. It should be noted that there is not a one-for-one correspondence between the functional classification and the MHC based classification schemes: you can find fibers that contract quickly but express the slow myosin isoform (and vice versa). These are relatively uncommon, though.
There are at least nine different mammalian MHC isoforms. Two are developmental, termed embryonic and neonatal, based on the time of their expression. Two are "slow" forms, expressed in the heart and termed cardiac alpha and beta. The cardiac beta is also found in slow skeletal muscle fibers (in which case it is called type 1). The remaining forms are found in fast skeletal muscle. Type 2a is found in most FOG fibers, and type 2b and 2x in FG fibers. The last two are relatively rare and appears to be expressed primarily in the extraocular, laryngial and jaw muscles.
Different Fiber Lengths
Let us consider the effects of architecture using an example of two muscles with identical PCSAs and pennation angles but different fiber lengths. Before reading ahead, try to draw the appropriate length-tension and force-velocity curves.
As shown below, the effect is to increase the muscle velocity (or, stated identically, to increase the muscle excursion). The peak absolute force of the length-tension curves is identical, but the absolute muscle active range is different. That sounds a lot like active range of motion (ROM), a measurement that is extremely important in clinical evaluation. In fact, it is directly related to ROM--ROM is a direct result of muscle architecture and the joint properties on which the muscle acts.
For the same reason that fiber length increases the active muscle range of the length-tension relationship, it causes an increase in the muscle's absolute maximum contraction velocity (Vmax). Again, while the fiber length increase causes an increase in these extrinsic properties, it has no effect on the intrinsic properties of the muscle. A similar exercise can be performed comparing muscles with different PCSAs and fiber lengths. Try predicting force-velocity and length-tension curves for the case where both architectural parameters are changed.
Different Physiologic Cross-sectional Areas
Suppose that two muscles had identical fiber lengths and pennation angles, but one muscle had twice the mass (equivalent to saying that one muscle had twice the number of fibers and thus twice the PCSA). What would be the difference in their mechanical properties? How would the length-tension and force-velocity curves be affected?
This schematic demonstrates that the only effect is to increase maximum tetanic tension so that the length-tension curve has the same basic shape but is simply amplified upward in the case of the stronger muscle. Similarly, the force-velocity curve simply changes the location of Po, but the curve retains the same basic shape. Note that if both curves are plotted on relative scales (i.e., percent maximum tension instead of absolute tension), the two muscles of different architecture appear to have identical properties. This demonstrates that while architectural properties profoundly affect the extrinsic muscle properties (i.e., the properties that vary with absolute muscle size, such as PCSA or mass), they have no affect on its intrinsic properties (i.e., the properties that are independent of absolute muscle size, such as fiber length/muscle length ratio).
Skeletal Muscle Architecture
Skeletal muscle is not only highly organized to function at the microscopic level, the arrangement of the muscle fibers at the macroscopic level also demonstrates a striking degree of organization. Skeletal muscle architecture is defined as "the arrangement of muscle fibers relative to the axis of force generation." The functional properties of a whole muscle depend strongly on its architecture. The various types of arrangement are as numerous as the muscles themselves, but for convenience we often refer to three types of fiber architecture.
Examples of Muscle Architecture:
Muscles with fibers that extend parallel to the muscle force-generating axis are termed parallel or longitudinally arranged muscles (Left). While the fibers extend parallel to the force-generating axis, they never extend the entire muscle length. Muscles with fibers that are oriented at a single angle relative to the force generating axis are termed unipennate muscles (Middle). The angle between the fiber and the force-generating axis generally varies from 0° to 30°. Most muscles fall into the final and most general category, multipennate muscles--muscles composed of fibers that are oriented at several angles relative to the axis of force generation (Right). As we will discuss, an understanding of muscle architecture is critical to understanding the functional properties of different sized muscles.
Effect of Muscle Architecture on Muscle Function:
The functional effect of muscle architecture can be simply stated as: muscle force is proportional to physiologic cross-sectional area (PCSA), and muscle velocity is proportional to muscle fiber length. PCSA is the sum of the areas of each fiber in the muscle. It may be apparent, based on the brief discussion of architecture presented above, that neither fiber length nor PCSA can easily be deduced based on gross muscle inspection. Detailed dissections of cadaveric muscles are required for architectural determination (see Sacks and Roy, 1982, for a description of the methodology). However, after determining architectural properties, it is possible to understand how much force the muscle generates and how fast it contracts (or how far it contracts). Let's look at two specific architectural examples and their impact on the length-tension and force-velocity relationships.
Fundamental Functional Properties of Skeletal Muscle
The isometric length-tension curve represents the force a muscle
is capable of generating while held at a series of discrete lengths. When
tension at each length is plotted against length, a relationship such as that
shown below is obtained.
While a general description of this relationship was established early in the history of biologic science, the precise structural basis for the length-tension relationship in skeletal muscle was not elucidated until the sophisticated mechanical experiments of the early 1960s were performed (Gordon et al. 1966). In its most basic form, the length-tension relationship states that isometric tension generation in skeletal muscle is a function of the magnitude of overlap between actin and myosin filaments.
The force generated by a muscle is a function of its velocity. Historically, the force-velocity relationship has been used to define the dynamic properties of the cross-bridges which cycle during muscle contraction.
The force-velocity relationship, like the length-tension
relationship, is a curve that actually represents the results of many
experiments plotted on the same graph. Experimentally, a muscle is allowed to
shorten against a constant load. The muscle velocity during shortening is
measured and then plotted against the resistive force. The general form of this
relationship is shown in the graph below. On the horizontal axis is plotted
muscle velocity relative to maximum velocity (Vmax) while on the
vertical axis is plotted muscle force relative to maximum isometric force (Po).
What is the physiologic basis of the force-velocity relationship? The force generated by a muscle depends on the total number of cross-bridges attached. Because it takes a finite amount of time for cross-bridges to attach, as filaments slide past one another faster and faster (i.e., as the muscle shortens with increasing velocity), force decreases due to the lower number of cross-bridges attached. Conversely, as the relative filament velocity decreases (i.e., as muscle velocity decreases), more cross-bridges have time to attach and to generate force, and thus force increases. This discussion is not meant to provide a detailed description of the basis for the force-velocity relationship, only to provide insight as to how cross-bridge rate constants can affect muscle force generation as a function of velocity.
Muscles are strengthened based on the force placed across the muscle. Higher forces produce greater strengthening. Therefore, exercises performed with muscle activated in a way that allows them to contract at high velocities, necessarily imply that they are also contracting with relatively low force. This is intuitively obvious as you lift a light load compared to a heavy load—the light load can be moved much more quickly. However, these rapid movements would have very small strengthening effects since the muscle forces are so low.
Types of contractions
When we think of a muscle contracting normally, we tend to think of the muscle shortening as it generates force. While it's true that this is a way of muscle contracting, there are many different ways that a muscle can generate force, as seen in Figure 1 below.
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Figure 1: A demonstration of the difference in force responses for between lengthening and non-lengthening active contractions (isometric vs. eccentric), and between active lengthening (eccentric) vs. non-active lengthening (passive stretch). |
Concentric Contractions—Muscle Actively Shortening
When a muscle is activated and required to lift a load which is less than the maximum tetanic tension it can generate, the muscle begins to shorten. Contractions that permit the muscle to shorten are referred to as concentric contractions. An example of a concentric contraction in the raising of a weight during a bicep curl.
In concentric contractions, the force generated by the muscle is always less than the muscle's maximum (Po). As the load the muscle is required to lift decreases, contraction velocity increases. This occurs until the muscle finally reaches its maximum contraction velocity, Vmax. By performing a series of constant velocity shortening contractions, a force-velocity relationship can be determined.
Eccentric Contractions—Muscle Actively Lengthening
During normal activity, muscles are often active while they are lengthening. Classic examples of this are walking, when the quadriceps (knee extensors) are active just after heel strike while the knee flexes, or setting an object down gently (the arm flexors must be active to control the fall of the object).
As the load on the muscle increases, it finally reaches a point where the external force on the muscle is greater than the force that the muscle can generate. Thus even though the muscle may be fully activated, it is forced to lengthen due to the high external load. This is referred to as an eccentric contraction (please remember that contraction in this context does not necessarily imply shortening). There are two main features to note regarding eccentric contractions. First, the absolute tensions achieved are very high relative to the muscle's maximum tetanic tension generating capacity (you can set down a much heavier object than you can lift). Second, the absolute tension is relatively independent of lengthening velocity. This suggests that skeletal muscles are very resistant to lengthening. The basic mechanics of eccentric contractions are still a source of debate since the cross-bridge theory that so nicely describes concentric contractions is not as successful in describing eccentric contractions.
Eccentric contractions are currently a very popular area of study for three main reasons: First, much of a muscle's normal activity occurs while it is actively lengthening, so that eccentric contractions are physiologically common (Goslow et al. 1973; Hoffer et al. 1989) Second, muscle injury and soreness are selectively associated with eccentric contraction (Figure 2, Fridén et al. 1984; Evans et al. 1985; Fridén and Lieber, 1992). Finally, muscle strengthening may be greatest using exercises that involve eccentric contractions. Therefore, there are some very fundamental structure-function questions that can be addressed using the eccentric contraction model and eccentric contractions have very important applications therapeutically to strengthen muscle.
| Figure 2: Plot demonstrating maximal tetanic force prior to and immediately following an exercise bout. While passive stretch causes negligible force decrement, isometric causes a moderate loss and eccentric causes a significant loss of force. |  |
The Virtual Hospital has a more clinical look at this and other forms of muscle injury.
Isometric Contraction—Muscle Actively Held at a Fixed Length
A third type of muscle contraction, isometric contraction, is one in which the muscle is activated, but instead of being allowed to lengthen or shorten, it is held at a constant length. An example of an isometric contraction would be carrying an object in front of you. The weight of the object would be pulling downward, but your hands and arms would be opposing the motion with equal force going upwards. Since your arms are neither raising or lowering, your biceps will be isometrically contracting.
The force generated during an isometric contraction is wholly dependant on the length of the muscle while contracting. Maximal isometric tension (Po) is produced at the muscle's optimum length, where the length of the muscle's sarcomeres are on the plateau of the length-tension curve.
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Figure 3: A series of isometric contractions performed at varying muscle lengths (from -40% (slack) to +40% (stretched). The maximum force is produced at optimum length (Lo). Note that as the muscle is stretched, the baseline of the force record is raised due to passive tension (PT) in the muscle and contributes more to overall force than the active tension (AT). |
Passive Stretch—Muscle Passively Lengthening
There is a fourth type of muscle "contraction" known as passive stretch. As the name implies, the muscle is being lengthened while in a passive state (i.e. not being stimulated to contract). An example of this would be the pull one feels in their hamstrings while touching their toes.
The structure(s) responsible for passive tension are outside of the cross-bridge itself since muscle activation is not required. Several recent studies have shed light on what has turned out to be a fascinating and huge protein with skeletal muscle—aptly named, “titin.” A seminal study performed by Magid and Law, demonstrated convincingly that the origin of passive muscle tension is actually within the myofibrils themselves. This is extremely significant because, prior to this study, most had assumed that extracellular connective tissue in striated muscle caused the majority of its passive properties. However, Magid and Law measured passive tension in whole muscle, single fibers and single fibers with membranes removed and showed that each relationship scaled to the size of the specimen. In other words, the source for passive force bearing in muscle was within the normal myofibrillar structure, not extracellular as had previously been supposed.
Joint Moment Arm
Although muscles produce linear forces, motions at joints are all rotary. The rotary torque is the product of the linear force and the moment arm or mechanical advantage of the muscle about the joint's center of rotation. Mechanically, this is the distance from the muscle's line of action to the joint's center of rotation.
Determination of joint moment arm requires an understanding of
the anatomy and movement (kinematics) of the joint of interest. For example,
some joints can be considered to rotate about a fixed point. A good example of
such a joint is the elbow. At the elbow joint, where the humerus and ulna
articulate, the resulting rotation occurs primarily about a fixed point,
referred to as the center of rotation. In the case of the elbow joint, this
center of rotation is relatively constant throughout the joint range of motion.
However, in other joints (for example the knee) the center of rotation moves in
space as the knee joint rotates because the articulating surfaces are not
perfect circles. In the case of the knee, it is not appropriate to discuss a
single center of rotation--rather we must speak of a center of rotation
corresponding to a particular joint angle, or, using the terminology of joint
kinematics, we must speak of the instant center of rotation (ICR), that is, the
center of rotation at any "instant" in time or space. 
Having defined a joint ICR, the moment arm is defined as the perpendicular distance from line of force application to the axis of rotation. This is illustrated for a simulated elbow joint. In A, the elbow joint is almost fully extended. Let the angle, q, between the brachialis muscle and the ulna be relatively small, e.g., q=20°. Let the distance between the brachialis insertion site and the elbow instant center be 5 cm. In this case, the perpendicular distance between the line of force application and the elbow ICR is shown by the dotted line in A and is equivalent to 5 cm x sin(20°) = 1.7 cm. Thus because the joint is nearly fully extended, this presents an unfavorable mechanical advantage to the muscle--the moment arm is relatively small. Much of the force generated by the muscle will simply compress the joint, not rotate it. Contrast this situation with the conditions shown in B, where the joint has now been flexed so that q=50°. Now, the moment arm equals 5 cm x sin(50°) = 4.3 cm. We see that for a simple hinge joint (a joint with a fixed ICR), the maximum moment arm is attained at q=90°. If we plotted moment arm vs. joint angle for this simple hinge joint, we would obtain a simple sine function that has a maximum of 5 cm occurring at q=90°. Such a curve can be generated for any joint. In general, the experimental curves are not quite as simple as the one here.
Muscle-Joint Interactions
Maximum Strength
Examination of current physiology texts reveals a good deal of wishy-washiness regarding the definition of the joint angle that corresponds to maximum strength. Typically, it is stated that muscle force is maximum when the joint is in a neutral position, or when the muscle is at resting length. What is the basis for such statements? Unfortunately, there is very little scientific basis for such a statement. Recent studies of torque generation in animals and humans have generally agreed that the joint angle at which the muscle generates maximal force is not the same angle at which moment arm is maximum. Thus, during normal joint rotation, both moment arm and muscle force are constantly changing which results in the "shape" of the strength or torque curve. This concept has recently been addressed in detail by experimental and theoretical modeling (Hoy et al., 90; Lieber and Boakes, 1988; Lieber and Shoemaker, 1992). What an interesting design!
Range of Motion as a Function of Architecture
We can now "combine" the muscle architectural discussion with the joint moment arm concept. We stated that muscles with longer fibers have a longer functional range than muscles with shorter fibers (Muscle Architecture). Does this imply that muscles with longer fibers are associated with joints that have larger ROMs? The answer is No. It is true that a muscle with longer fibers does have a longer working range. However, the amount of muscle length change that occurs as a joint rotates is very strongly dependent on the muscle's moment arm--the perpendicular distance from the muscle insertion to the axis of joint rotation.
This idea is illustrated where we have attached a simulated "muscle" using two different moment arms. In A, the moment arm is much less than in B. This means that in A, the muscle will change length much less for a given change in joint angle compared to the same change in joint angle in B. As a result, the active ROM for the muscle-joint system shown in A will be much greater than that which is shown in B, in spite of the fact that their muscular properties are identical. In fact, in the current example, increasing moment arm decreased range of motion from 40o (A) to only 25o (B)!
This indicates that muscles that appear to be designed for speed because of their very long fibers may not actually produce large velocities if they are placed in position with a very large moment arm. The increased moment arm causes a greater joint moment, and the muscle may actually be better suited for isometric torque production. Similarly, a muscle that appears to be designed for force production due to a large PCSA, if placed in position with a very small moment arm, may actually produce high joint excursions or angular velocities. Thus, muscle design may or may not be a reflection of its actual use in the physiologic torque-generating system. It does seem, in general, that muscle fiber length and muscle moment arm are positively correlated (McClearn, 1985). Thus muscles with long fibers tend to have long moment arms, but this need not necessarily be the case. Muscle architectural features may represent muscle adaptation to kinematic criteria. However, definitive answers to these suggestions await further study.
Nervous control
Efferent leg
The efferent leg of the peripheral nervous system is responsible for conveying commands to the muscles and glands, and is ultimately responsible for voluntary movement. Nerves move muscles in response to voluntary and autonomic (involuntary) signals from the brain. Deep muscles, superficial muscles, muscles of the face and internal muscles all correspond with dedicated regions in the primary motor cortex of the brain, directly anterior to the central sulcus that divides the frontal and parietal lobes.
In addition, muscles react to reflexive nerve stimuli that do not always send signals all the way to the brain. In this case, the signal from the afferent fiber does not reach the brain, but produces the reflexive movement by direct connections with the efferent nerves in the spine. However, the majority of muscle activity is volitional, and the result of complex interactions between various areas of the brain.
Nerves that control skeletal muscles in mammals correspond with neuron groups along the primary motor cortex of the brain's cerebral cortex. Commands are routed though the basal ganglia and are modified by input from the cerebellum before being relayed through the pyramidal tract to the spinal cord and from there to the motor end plate at the muscles. Along the way, feedback, such as that of the extrapyramidal system contribute signals to influence muscle tone and response.
Deeper muscles such as those involved in posture often are controlled from nuclei in the brain stem and basal ganglia.
Afferent leg
The afferent leg of the peripheral nervous system is responsible for conveying sensory information to the brain, primarily from the sense organs like the skin. In the muscles, the muscle spindles convey information about the degree of muscle length and stretch to the central nervous system to assist in maintaining posture and joint position. The sense of where our bodies are in space is called proprioception, the perception of body awareness. More easily demonstrated than explained, proprioception is the "unconscious" awareness of where the various regions of the body are located at any one time. This can be demonstrated by anyone closing their eyes and waving their hand around. Assuming proper proprioceptive function, at no time will the person lose awareness of where the hand actually is, even though it is not being detected by any of the other senses.
Several areas in the brain coordinate movement and position with the feedback information gained from proprioception. The cerebellum and red nucleus in particular continuously sample position against movement and make minor corrections to assure smooth motion.