Muscle fibers contain what kind of receptors

In addition to nuclei, skeletal muscle fibers also contain cellular organelles found in other cells, such as mitochondria and endoplasmic reticulum.

Howver, some of these structures are specialized in muscle fibers. Within a muscle fiber, proteins are organized into structures called myofibrils that run the length of the cell and contain sarcomeres connected in series. Because myofibrils are only approximately 1. The sarcomere is the smallest functional unit of a skeletal muscle fiber and is a highly organized arrangement of contractile, regulatory, and structural proteins.

It is the shortening of these individual sarcomeres that lead to the contraction of individual skeletal muscle fibers and ultimately the whole muscle. A sarcomere is defined as the region of a myofibril contained between two cytoskeletal structures called Z-discs also called Z-lines , and the striated appearance of skeletal muscle fibers is due to the arrangement of the thick and thin myofilaments within each sarcomere Figure The dark striated A band is composed of the thick filaments containing myosin, which span the center of the sarcomere extending toward the Z-dics.

The thick filaments are anchored at the middle of the sarcomere the M-line by a protein called myomesin. The thin filaments extend into the A band toward the M-line and overlap with regions of the thick filament. The A band is dark because of the thicker mysoin filaments as well as overlap with the actin filaments. The H zone in the middle of the A band is a little lighter in color, because the thin filaments do not extend into this region.

Because a sarcomere is defined by Z-discs, a single sarcomere contains one dark A band with half of the lighter I band on each end Figure During contraction the myofilaments themselves do not change length, but actually slide across each other so the distance between the Z-discs shortens.

The length of the A band does not change the thick myosin filament remains a constant length , but the H zone and I band regions shrink. These regions represent areas where the filaments do not overlap, and as filament overlap increases during contraction these regions of no overlap decrease.

The thin filaments are composed of two filamentous actin chains F-actin comprised of individual actin proteins Figure These thin filaments are anchored at the Z-disc and extend toward the center of the sarcomere. Within the filament, each globular actin monomer G-actin contains a mysoin binding site and is also associated with the regulatory proteins, troponin and tropomyosin. The troponin protein complex consists of three polypeptides.

Troponin and tropomyosin run along the actin filaments and control when the actin binding sites will be exposed for binding to myosin. Thick myofilaments are composed of myosin protein complexes, which are composed of six proteins: two myosin heavy chains and four light chain molecules. The heavy chains consist of a tail region, flexible hinge region, and globular head which contains an Actin-binding site and a binding site for the high energy molecule ATP.

The light chains play a regulatory role at the hinge region, but the heavy chain head region interacts with actin and is the most important factor for generating force. Hundreds of myosin proteins are arranged into each thick filament with tails toward the M-line and heads extending toward the Z-discs. Other structural proteins are associated with the sarcomere but do not play a direct role in active force production.

Titin, which is the largest known protein, helps align the thick filament and adds an elastic element to the sarcomere. Titin is anchored at the M-Line, runs the length of myosin, and extends to the Z disc. With increasing rates of descending pathway activity, intermediate-size alpha motor neurons are activated in addition to the small neurons.

Because more motor units are activated, the muscle produces more force. Finally, with the highest rates of descending activity, the largest alpha motor neurons are recruited, producing maximal muscle force. The motor system requires sensory input in order to function properly. In addition to sensory information about the external environment, the motor system also requires sensory information about the current state of the muscles and limbs themselves.

The muscle spindle signals the length of a muscle and changes in the length of a muscle. The Golgi tendon organ signals the amount of force being applied to a muscle.

Muscle spindles are collections of specialized muscle fibers that are located within the muscle mass itself Figure 1. These fibers do not contribute significantly to the force generated by the muscle. Rather, they are specialized receptors that signal a the length and b the rate of change of length velocity of the muscle.

Because of the fusiform shape of the muscle spindle, these fibers are referred to as intrafusal fibers. The large majority of muscle fibers that allow the muscle to do work are termed extrafusal fibers. Each muscle contains many muscle spindles; muscles that are necessary for fine movements contain more spindles than muscles that are used for posture or coarse movements.

There are 3 types of muscle spindle fibers, characterized by their shape and the type of information they convey Figure 1. Because the muscle spindle is located in parallel with the extrafusal fibers, it will stretch along with the muscle. The muscle spindle signals muscle length and velocity to the CNS through two types of specialized sensory fibers that innervate the intrafusal fibers.

These sensory fibers have stretch receptors that open and close as a function of the length of the intrafusal fiber. Because of their patterns of innervation onto the three types of intrafusal fibers, Group Ia and Group II afferents respond differently to different types of muscle movements. Initially, both Group Ia and Group II fibers fire at a certain rate, encoding the current length of the muscle. During the stretch, the two types differ in their responses.

The Group Ia afferent fires at a very high rate during the stretch, encoding the velocity of the muscle length; at the end of the stretch, its firing decreases, as the muscle is no longer changing length. Note, however, that its firing rate is still higher than it was before the stretch, as it is now encoding the new length of the muscle.

The Group II afferent increases its firing rate steadily as the muscle is stretched. Its firing rate does not depend on the rate of change of the muscle; rather, its firing rate depends only on the immediate length of the muscle. The Group Ia afferent responds at a highest rate when the muscle is actively stretching, but also signals the static length of the muscle because of its innervation of the static nuclear bag fiber and the nuclear chain fiber.

The Group II afferent signals only the static length of the muscle, increasing its firing rate linearly as a function of muscle length. Although intrafusal fibers do not contribute significantly to muscle contraction, they do have contractile elements at their ends that are innervated by motor neurons.

The muscle starts at a certain length, encoded by the firing of a Ia afferent. When the muscle is stretched, the muscle spindle stretches and the Ia afferent fires more strongly. When the muscle is released from the stretch and contracts, the muscle spindle becomes slack, causing the Ia afferent to fall silent. The muscle spindle is rendered insensitive to further stretches of muscle. To restore sensitivity, gamma motor neurons fire and cause the spindle to contract, thereby becoming taut and able to signal the muscle length again.

Motor neurons are divided into two groups. Alpha motor neurons innervate extrafusal fibers , the highly contracting fibers that supply the muscle with its power. Gamma motor neurons innervate intrafusal fibers , which contract only slightly. The function of intrafusal fiber contraction is not to provide force to the muscle; rather, gamma activation of the intrafusal fiber is necessary to keep the muscle spindle taut, and therefore sensitive to stretch, over a wide range of muscle lengths.

This concept is illustrated in Figure 1. If a resting muscle is stretched, the muscle spindle becomes stretched in parallel, sending signals through the primary and secondary afferents. A subsequent contraction of the muscle, however, removes the pull on the spindle, and it becomes slack, causing the spindle afferents to cease firing. If the muscle were to be stretched again, the muscle spindle would not be able to signal this stretch.

Thus, the spindle is rendered temporarily insensitive to stretch after the muscle has contracted. Activation of gamma motor neurons prevents this temporary insensitivity by causing a weak contraction of the intrafusal fibers, in parallel with the contraction of the muscle. This contraction keeps the spindle taut at all times and maintains its sensitivity to changes in the length of the muscle.

Thus, when the CNS instructs a muscle to contract, it not only sends the appropriate signals to the alpha motor neurons, it also instructs gamma motor neurons to contract the intrafusal fibers appropriately; this coordinated process is referred to as alpha-gamma coactivation.

The Golgi tendon organ is a specialized receptor that is located between the muscle and the tendon Figure 1. Unlike the muscle spindle, which is located in parallel with extrafusal fibers, the Golgi tendon organ is located in series with the muscle and signals information about the load or force being applied to the muscle. A Golgi tendon organ is made up of a capsule containing numerous collagen fibers Figure 1.

The organ is innervated by primary afferents called Group Ib fibers , which have specialized endings that weave in between the collagen fibers. It is likely that muscle spindle receptor messages provide the information the central nervous system uses to compute the angle of joints. In addition, all types of receptors in muscle provide information used in systems that control movement and posture.

Muscle spindle afferent fibers The primary and secondary muscle spindle afferent fibers both arise from a specialized structure within the muscle, the muscle spindle , a fusiform structure mm long and m in diameter. The spindles are located deep within the muscle mass, scattered widely through the muscle body, and attached to the tendon, the endomysium or the perimysium, so as to be in parallel with the extrafusal or regular muscle fibers.

Although spindles are scattered widely in muscles, they are not found throughout. Figure shows the distribution of spindles in the medial gastrocnemius of the cat, in dorsal A , and midsagittal projections B , and for comparison the location of Golgi tendon organs C. A drawing of a muscle spindle is shown in Figure The nuclear bag fibers are thicker and longer than the nuclear chain fibers, and they receive their name from the accumulation of their nuclei in the expanded bag-like equatorial region-the nuclear bag.

The nuclear chain fibers have no equatorial bulge; rather their nuclei are lined up in the equatorial region-the nuclear chain. This distinction is illustrated in Figure A typical spindle contains two nuclear bag fibers and nuclear chain fibers.

Drawing of a muscle spndle to show the nature of attachment, the arrangement of the intrafusal fibers, and how the afferent and efferent fibers enter the spindle. The sensory innervation of the muscle spindle arises from both group Ia and group II afferent fibers. As shown in Figure , a single, large group Ia fiber coils around the equatorial regions of both nuclear bag and nuclear chain fibers, forming the annulospiral endings or primary muscle spindle receptors.

There appears to be only one group Ia afferent fiber per spindle, but every intrafusal muscle fiber within that spindle receives innervation from that fiber. Current thought is that all group Ia afferent fibers form annulospiral endings, and therefore the terms primary muscle spindle afferent fiber and group Ia afferent fiber are used interchangeably.

The smaller group II fibers terminate at either end of the nuclear region primarily on the nuclear chain fibers there is apparently some innervation of bag fibers by secondary muscle spindle afferent fibers , but there is disagreement as to how much ; they form flower-spray endings or secondary muscle spindle receptors.

There usually are several group II fibers innervating each spindle. Not all group II fibers form such endings, so these terms are not synonymous. A nuclear bag and a nuclear chain fiber showing their innervation by group Ia and Group II afferent fibers and gamma motoneurons. Matthews PBC: Physiol Rev 44 , The intrafusal fibers are striated muscle fibers that receive innervation from the fusimotor neurons or gamma-motoneurons.

Activity in fusimotor neurons produces a contraction of the striated, polar regions of the bag and chain fibers, putting stretch on the equatorial region where the receptor regions are that has few myofibrils and therefore, has little contractility. It is apparently the stretching of this central region, regardless of how it is accomplished, that is the adequate stimulus for both primary and secondary spindle receptors.

The fusimotor neurons or gamma-motoneurons should not be confused with the larger skeletomotor neurons or alpha-motoneurons , whose activity produces contraction of the extrafusal fibers that do the work of the muscle. The difference in diameter of fusimotor gamma-motoneurons and skeletomotor fibers alpha-motoneurons is illustrated in Figure The former average about 5 m in diameter, the latter 13 m. Contraction of all the intrafusal fibers at once does not produce any measurable tension in the muscle.

The intrafusal fibers are much shorter than the extrafusal fibers, millimeters compared with centimeters. The largest intrafusal fiber, shortening by the same percentage, would only change length by 2. The extrafusal fiber is therefore capable of a fold greater change in length.

The average human striated muscle fiber has a diameter of about 60 micrometers; the intrafusal fiber averages about 10 micrometers. Because the force produced by a skeletal muscle is proportional to its cross-sectional area, the extrafusal fiber is capable of producing at least 36 times more force than the intrafusal fiber.

Add these factors to the relative numbers of intrafusal in cat soleus muscle about and extrafusal fibers again in cat soleus muscle about 25, , and it is not hard to see why intrafusal fibers do not generate much force. Fiber spectrum of efferent portion of a muscle nerve. Indicated are A alpha and A gamma fibers, fusimotor and skeletomotor fibers. Edinburgh, Livingstone, It also appears that the mechanism for generating force in intrafusal muscle fibers may be different than in extrafusal fibers.

There are no action potentials in intrafusal fibers as there are in extrafusal fibers see Chapter 14 , with a consequence that one striated end of an intrafusal fiber may contract without the other end doing so.