Table of Contents
Proper control of muscle contractility not only requires motor signals from the CNS but also feedback signals originating in the muscles themselves to tell the spinal cord about the functional status of muscles at each instant. These feedback signals arise from the muscle spindles which are the mechanoreceptors in muscles that respond to the stretch stimuli. They detect the changes in muscle length and transmit this information to the central nervous system. They tell the CNS about the degree of stretch and the rate of change in length.
The CNS processes this information and engages the motor neurons to bring about the control of muscle contractility. Muscle spindles are supplied with both afferent and efferent fibers. The afferents are principally responsible for transmitting sensory information to CNS while the efferents carry impulses back to muscle spindles. This feedback control of contractility is crucial to maintain muscle tone, correct body posture, and preventing excessive stretch to avoid tearing of the fibers.
Anatomy of muscle spindles
The muscle spindle is 3 to 10 mm long. It is made up of 3 to 12 small intrafusal fibers which are pointed at their ends and are attached to the glycocalyx of the surrounding large extrafusal fibers. Each intrafusal muscle fiber is a tiny skeletal muscle fiber. The central area of each of these fibers has few or no actin and myosin filaments but the ends do have actin and myosin. Therefore, this central portion does not contract when the ends of fibers contract. Instead, the central portion functions as the sensory receptor. The extrafusal fibers and the ends of intrafusal fibers are excited by the separate efferent motor neurons from the CNS. The end portions of intrafusal fibers are excited by small gamma motor nerve fibers that originate from small type A gamma motor neurons in the anterior horns of the spinal cord. These gamma motor nerve fibers are called gamma efferent fibers. In contrast, the large extrafusal fibers are excited by the large alpha-efferent fibers (type Aα nerve fibers) arising from the spinal cord.
Sensory innervation of muscle spindles
The central portion of muscle spindles acts as sensory receptors and it’s devoid of any contractile proteins. It’s excited by stretching which can be induced by:
- Lengthening of the whole muscle which stretches the muscle spindles and excites the receptor portion.
- Contraction of the ends of intrafusal fibers stretching the central portion of fibers
Whatever the cause is, stretching of the central portion of intrafusal fibers stimulates the sensory afferent fibers which may be primary or secondary fibers. In the central portion of the receptor area, a large sensory nerve ending encircles each intrafusal fiber. It is called primary afferent ending or annulospiral ending. This nerve fiber is a type Ia fiber about 17 micrometers in diameter. It transmits the sensory signals to the spinal cord at a velocity of 70 to 120 m/sec, which is as rapidly as any type of Ia nerve fiber in the entire body. Usually, one but sometimes two smaller sensory nerve fibers belonging to the type II fibers with an average diameter of 8 micrometers innervate the receptor region on one or both sides of the primary ending. They are called secondary afferent endings and may surround the receptor portion in the same way primary afferents surround.
Nuclear bag and nuclear chain-like fibers
Intrafusal fibers are also classified as nuclear bag-like and nuclear chain-like fibers. In nuclear bag-like fibers, nuclei of the intrafusal fibers are aggregated in the center of the fiber to show up a bag-like structure full of nuclei. That’s why they are called nuclear bag-like fibers. In nuclear chain-like fibers, nuclei are arranged in the form of chains throughout the receptor area.
There are one to three nuclear bag-like fibers in each muscle spindle while the remaining are nuclear chain-like fibers (three to nine). Nuclear chain-like fibers are smaller in diameter and length as compared to nuclear bag-like fibers. The primary afferent endings are excited by both nuclear bag-like and nuclear chain-like fibers while the secondary afferent endings are excited only by the nuclear chain-like fibers.
Motor supply of muscle spindles
Muscle spindles are supplied by the gamma motor nerves arising from the anterior horn of the spinal cord. They can be divided into two types: gamma-dynamic fibers (gamma-d fibers) and gamma-static fibers (gamma-s fibers). The first of these gamma motor nerves, gamma dynamic fibers, excite mainly the nuclear bag intrafusal fibers, and the second ones, gamma static fibers, excite mainly the nuclear chain intrafusal fibers.
Physiology of muscle spindles
The physiology of muscle spindles can be split into sensory and motor parts.
Read more about the Gogli tendon organ
Sensory physiology of muscle spindles
The sensory endings of muscle spindles which detect the change in the length of muscles produce two kinds of sensory responses, static response, and dynamic response. The static response is the one in which there is the continuous discharge of impulses when muscle remains stretched while the dynamic response is produced when the length of muscle fibers is actively changing.
When the central portion of the muscle spindle is stretched slowly, it gets excited and the number of impulses transmitted from both the primary and the secondary endings increases almost directly in proportion to the degree of stretching. These sensory endings continue to transmit these impulses for several minutes if the muscle spindle remains stretched. This response of primary and secondary endings is called the static response of the spindle receptor. However, when the length of the muscle spindle changes suddenly, the primary nerve endings are stimulated powerfully while secondary endings produce no such response. This is called the dynamic response of the receptor portion of muscle spindles. The dynamic response is produced when the muscle length is actively changing. A small increase in the length of muscle spindles triggers a flood of sensory impulses traveling to the CNS through large 17 micrometers of sensory nerve fibers. So, dynamic response principally detects the rate of change of length in muscles while static response detects only change in length not the rate of change. When the length of muscle shortens, the primary endings send opposite signals to the CNS making an apprehension of a decrease in length. So, they can transmit both positive and negative signals about the changes in muscle length and can trigger an appropriate response.
Motor physiology of muscle spindles
Normally some degree of gamma nerve excitation of muscle spindles is always present. Stretching of spindles due to gamma excitation increases the rate of firing while shortening the spindles decreases the rate of firing. When the gamma-d fibers stimulate the nuclear bag fibers, the dynamic response of the muscle spindle is greatly enhanced, whereas the static response is hardly affected by these fibers. Conversely, stimulation of the gamma-s fibers, which excite the nuclear chain fibers, increases the static response while having little influence on the dynamic response.
The gamma-efferent system of skeletal muscles is excited specifically by the signals arising from the bulboreticular facilitatory region of the brain stem and, by impulses transmitted into the bulboreticular area from the cerebellum, basal ganglia, and cerebral cortex. The bulboreticular facilitatory area is specifically concerned with the antigravity contractions of muscles; the antigravity muscles also have an especially high density of muscle spindles which shows the importance of bulboreticular area and muscle spindles in the management of antigravity actions.
Muscle stretch reflex
The simplest manifestation of muscle spindle functioning is the classical muscle stretch reflex. When a muscle is stretched suddenly, excitation of the receptor portion of the muscle spindles causes reflex contraction of the large extrafusal muscle fibers of the stretched muscle and also of closely related synergistic muscles. It’s a protective mechanism against the overstretching of muscles and protects the stretch-induced damage to the fibers.
Neuronal circuits of the stretch reflex
The sensory stimulus of stretching travels through the type Ia proprioceptor nerve fiber originating in a muscle spindle and entering a dorsal root of the spinal cord. A branch of this proprioceptive fiber goes directly to the anterior horn of the cord gray matter and synapses with anterior motor neurons controlling the skeletal muscles. Anterior motor neurons send motor nerve fibers back to the same muscle from which the muscle spindle fiber originated. This monosynaptic pathway allows the motor signal to return with the shortest possible time delay back to the stretched muscle after excitation of the spindle and induces reflex contraction. This action is rapid and occurs instantaneously when the muscle is stretched. The type II fibers from the muscle spindle synapse with the multiple interneurons in the cord gray matter transmit delayed signals to the anterior motor neurons to serve other functions of muscle control.
The dynamic and static stretch reflex
Stretch reflexes of the muscles can be classified into dynamic and static reflexes. The dynamic stretch reflex is exhibited by a strong dynamic signal transmitted from the primary sensory endings of the muscle spindles to the CNS. It is produced in response to the rapid stretch or unstretch. When a muscle is suddenly stretched or unstretched, a strong signal is transmitted to the anterior column of the spinal cord, which causes an instantaneous strong reflex contraction (or decrease in contraction) of the same muscle from which the signal originated. Thus, this reflex action functions to oppose sudden changes in the length of the muscle. The dynamic stretch reflex is fast in onset, short-lived, and over within a fraction of a second. When the dynamic reflex is over, a weaker static stretch reflex continues for a prolonged period thereafter. This reflex occurs due to the continuous static receptor signals transmitted by both primary and secondary sensory nerve endings. The importance of the static stretch reflex is that it provides a small degree of constant background signaling to help to maintain the muscle tone at a constant level unless one wills otherwise.
“Damping” function of the dynamic and static stretch reflexes
An important function of the muscle stretch reflex is its ability to prevent the oscillation or jerkiness of muscular movements, which is called damping, or smoothing. Motor signals arising from the spinal cord are often transmitted to a muscle in an unsmooth or unstable form such as increasing in intensity for a few milliseconds, then decreasing in intensity, then changing to another intensity level, and so forth. When the muscle spindle apparatus is not functioning properly, the muscle contraction is jerky and unstable due to such unstable and nonuniform signals. Muscle spindle reflexes damp these intensity changes of the signals, enabling uniform muscular movements.
Role of the muscle spindle in voluntary motor activity
The small type A gamma efferent fibers, stimulating the muscle spindles make up 31 percent of all the motor nerve fibers to the skeletal muscles. When the signals are transmitted from the motor cortex or from any other brain area to the alpha motor neurons, in most instances, the gamma motor neurons are stimulated simultaneously, an effect known as the coactivation of the alpha and gamma motor neurons. This effect is responsible for the simultaneous contraction of extrafusal and intrafusal muscle fibers. The benefits of this simultaneous contraction of the muscle spindle intrafusal fibers and the large extrafusal muscle fibers are twofold:
- First, it keeps the length of the receptor portion of the intrafusal muscle fibers from changing during the time of the whole muscle contraction. Therefore, this coactivation prevents the muscle spindle reflex from opposing the normal muscle contraction.
- Secondly, it maintains the proper damping function of the muscle spindle, during the course of movement. For instance, if the muscle spindle fibers don’t contract and relax along with the large extrafusal muscle fibers, the receptor portion of the spindle would sometimes be relaxed and sometimes be overstretched, in neither instance operating under normal conditions for spindle function.
The muscle spindle system and stabilization of body position
One of the critical functions of the muscle spindle system is to stabilize body position at the time of tense motor actions. To perform this function, the bulboreticular facilitatory region of the brain stem and its allied areas of the brain stem send excitatory signals through the gamma nerve fibers to the intrafusal fibers of the muscle spindles. This stimulation causes fiber contraction at the ends of the spindles and stretches the central receptor regions. As a result, the signal output of the receptor increases. If the receptors of the spindles on both sides of a joint are activated at the same time, reflex excitation of the skeletal muscles on both sides of the joint occurs due to increased receptor firing, producing tight and tense muscle contractions opposing each other at the joint. The net effect of this phenomenon is that the position of the joint becomes strongly stabilized, and any force that tends to move the joint from its stabilized position is opposed by highly sensitized stretch reflexes working on both sides of the joint. Any time a person wants to perform a muscle function that requires a high degree of accuracy and exact positioning, excitation of the appropriate muscle spindles fibers by signals from the bulboreticular facilitatory region of the brain stem stabilizes the positions of the major joints by virtue of the opposing reflex actions at the joints. This stabilization is of great help in performing the detailed voluntary movements of fingers or other body parts for intricate motor functions.
Summary
Muscle spindles are the mechanoreceptors present in skeletal muscles which primarily detect the stretch stimulus. They provide feedback information to regulate muscle contractility.
They are made up of small intrafusal fibers present in the muscle belly and are surrounded by large extrfusal fibers. Intrfusal fibers don’t have contractile proteins in their central portion which acts as the sensory receptor.
These fibers are classified into nuclear bag-like fibers having many nuclei aggregated in the center of the cell and nuclear chain-like fibers in which nuclei are arranged in the form of a chain in the receptor portion of the cell.
These fibers are supplied by primary nerve endings which are large rapidly conducting type Ia fibers and secondary nerve endings which are small type II fibers.
The motor supply of muscle spindles comes from the gamma motor neurons of the spinal cord which may be stimulated by bulboreticular facilitatory region of the brain stem.
Primary sensory endings are important in dynamic response when the length of muscle spindles is actively changing while secondary nerve endings are important in static response transferring a constant background level of the impulses.
Muscle spindles play an important role in protecting the stretch-induced damage to fibers. When a skeletal muscle is stretched, the muscle spindle produces the reflex contraction of the muscle and prevents excessive stretching. Moreover, it dampens the jerkiness of contractility due to unsmooth motor signals of the spinal cord. It’s also involved in maintaining normal muscle tone and stabilizing the joints.