Muscle spindle
Encyclopedia
Muscle spindles are sensory receptor
s within the belly of a muscle
, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system
via sensory neuron
s. This information can be processed by the brain to determine the position
of body parts. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motoneurons via the stretch reflex
to resist muscle stretch.
]
s, embedded in extrafusal muscle fiber
s. Note that "fusus" is the Latin word for spindle. Muscle spindles are composed of 3-12 intrafusal muscle fibers, of which there are three types:
Axons of gamma motoneurons
also terminate in muscle spindles; they make synapse
s at either or both of the ends of the intrafusal muscle fibers and regulate the sensitivity of the sensory afferents, which are located in the non-contractile central (equatorial) region.
Muscle spindles are encapsulated by connective tissue, and are aligned parallel
to extrafusal muscle fibers, unlike Golgi tendon organs, which are oriented in series.
The muscle spindle has both sensory and motor components.
Fusimotor neurons are classified as static or dynamic according to the type of intrafusal muscle fibers they innervate and their physiological effects on the responses of the Ia and II sensory neurons innervating the central, non-contractile part of the muscle spindle.
by the active gamma motoneuron, the end portions of the intrafusal muscle fibers contract, thus elongating the non-contractile central portions (see "fusimotor action" schematic below). This opens stretch-sensitive ion channels of the sensory endings, leading to an influx of sodium
ion
s. This raises the resting potential
of the endings, thereby increasing the probability of action potential
firing, thus increasing the stretch-sensitivity of the muscle spindle afferents. For an interactive animation created by Jan Kowalczewski at the University of Alberta, demonstrating spindle afferent responses to muscle stretch with and without gamma (fusimotor) action, go to: http://www.ualberta.ca/~aprochaz/research_interactive_receptor_model.html.
How does the central nervous system control gamma fusimotor neurons? It has been difficult to record from gamma motoneurons during normal movement because they have very small axons. Several theories have been proposed, based on recordings from spindle afferents.
in the form of changes in the rate of action potentials. Likewise, secondary sensory fibers (Group II afferent neurons) respond to muscle length changes (but with a smaller velocity-sensitive component) and transmit this signal to the spinal cord. The Ia afferent signals are transmitted monosynaptically to many alpha motor neurons of the receptor-bearing muscle. The reflexly-evoked activity in the alpha motoneurons is then transmitted via their efferent axons to the extrafusal fibers of the muscle, which generate force and thereby resist the stretch. The Ia afferent signal is also transmitted polysynaptically through interneurons (Renshaw cell
s) which inhibit alpha motoneurons of antagonist muscles, causing them to relax.
After stroke
or spinal cord injury in humans, spastic hypertonus (spastic paresis or spatic paralysis) often develops, whereby the stretch reflex in flexor muscles of the arms and extensor muscles of the legs is overly sensitive. This results in abnormal postures, stiffness and contractures. Hypertonus (Hypertonia
) may be the result of over-sensitivity of alpha motoneurons and interneurons to the Ia and II afferent signals.
PNF stretching
, or proprioceptive neuromuscular facilitation, is a method of flexibility training that can reduce hypertonus, allowing muscles to relax and lengthen.
.
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
s within the belly of a muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
via sensory neuron
Sensory neuron
Sensory neurons are typically classified as the neurons responsible for converting external stimuli from the environment into internal stimuli. They are activated by sensory input , and send projections into the central nervous system that convey sensory information to the brain or spinal cord...
s. This information can be processed by the brain to determine the position
Proprioception
Proprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...
of body parts. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motoneurons via the stretch reflex
Stretch reflex
The stretch reflex is a muscle contraction in response to stretching within the muscle. It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length....
to resist muscle stretch.
]
Anatomy
Muscle spindles are found within the belly of muscleMuscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s, embedded in extrafusal muscle fiber
Extrafusal muscle fiber
Extrafusal muscle fiber is a term given to standard muscle fibers as to distinguish them from intrafusal muscle fibers. Extrafusal muscle fibers are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement...
s. Note that "fusus" is the Latin word for spindle. Muscle spindles are composed of 3-12 intrafusal muscle fibers, of which there are three types:
- dynamic nuclear bag fiberNuclear bag fiberA nuclear bag fiber is a type of intrafusal muscle fiber that lies in the center of a muscle spindle. Each has a large number of nuclei concentrated in bags and they cause excitation of both the primary and secondary nerve fibers....
s (bag1 fibers) - static nuclear bag fibers (bag2 fibers)
- nuclear chain fiberNuclear chain fiberA nuclear chain fiber is a specialized sensory organ contained within a muscle. Nuclear chain fibers are intrafusal fibers which, along with nuclear bag fibers, make up the muscle spindle responsible for the detection of changes in muscle length....
s and the axons of sensory neurons.
Axons of gamma motoneurons
Gamma motoneurons
Gamma motoneurons , also called gamma motor neurons, are the efferent component of the fusimotor system, the system by which the central nervous system controls and modifies muscle spindle sensitivity. The fusimotor system refers to the combination of muscle spindles and γ-motoneurons...
also terminate in muscle spindles; they make synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
s at either or both of the ends of the intrafusal muscle fibers and regulate the sensitivity of the sensory afferents, which are located in the non-contractile central (equatorial) region.
Muscle spindles are encapsulated by connective tissue, and are aligned parallel
Parallel (geometry)
Parallelism is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The assumed existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not...
to extrafusal muscle fibers, unlike Golgi tendon organs, which are oriented in series.
The muscle spindle has both sensory and motor components.
- PrimaryType Ia sensory fiberType Ia Sensory Fiber also called Primary Afferent Fiber is a type of sensory fiber. It is a component of a muscle fiber's muscle spindle which keeps track of how fast a muscle stretch changes .-Function of muscle spindles:...
and secondaryType II sensory fiberType II sensory fiber is a type of sensory fiber, the second of the two main groups of stretch receptors. They are non-adapting, meaning that they keep responding even when the muscle has stopped changing its length. They are the second most highly myelinated fibers in the body.Their firing rate...
sensory nerve fibers spiral around and terminate on the central portions of the intrafusal muscle fibers, providing the sensory component of the structure via stretch-sensitive ion-channels of the axons. - In mammals including humans, the motor component is provided by up to a dozen gamma motoneurons and to a lesser extent by one or two beta motoneurons. Gamma and beta motoneurons are called fusimotor neurons, because they activate the intrafusal muscle fibers. Gamma motoneurons only innervate intrafusal muscle fibers, whereas beta motoneurons innervate both extrafusal and intrafusal muscle fibers and so are referred to as skeletofusimotor neurons.
- Fusimotor drive causes a contraction and stiffening of the end portions of the intrafusal muscle fibers.
Fusimotor neurons are classified as static or dynamic according to the type of intrafusal muscle fibers they innervate and their physiological effects on the responses of the Ia and II sensory neurons innervating the central, non-contractile part of the muscle spindle.
- The static axons innervate the chain or bag2 fibers. They increase the firing rate of Ia and II afferents at a given muscle length (see schematic of fusimotor action below).
- The dynamic axons innervate the bag1 intrafusal muscle fibers. They increase the stretch-sensitivity of the Ia afferents by stiffening the bag1 intrafusal fibers.
Sensitivity modification
The function of the gamma motoneurons is not to supplement the force of muscle contraction provided by the extrafusal fibers, but to modify the sensitivity of the muscle spindle sensory afferents to stretch. Upon release of acetylcholineAcetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
by the active gamma motoneuron, the end portions of the intrafusal muscle fibers contract, thus elongating the non-contractile central portions (see "fusimotor action" schematic below). This opens stretch-sensitive ion channels of the sensory endings, leading to an influx of sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s. This raises the resting potential
Resting potential
The relatively static membrane potential of quiescent cells is called the resting membrane potential , as opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane potential....
of the endings, thereby increasing the probability of action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
firing, thus increasing the stretch-sensitivity of the muscle spindle afferents. For an interactive animation created by Jan Kowalczewski at the University of Alberta, demonstrating spindle afferent responses to muscle stretch with and without gamma (fusimotor) action, go to: http://www.ualberta.ca/~aprochaz/research_interactive_receptor_model.html.
How does the central nervous system control gamma fusimotor neurons? It has been difficult to record from gamma motoneurons during normal movement because they have very small axons. Several theories have been proposed, based on recordings from spindle afferents.
- 1) Alpha-gamma coactivation. Here it is posited that gamma motoneurons are activated in parallel with alpha motoneurons to maintain the firing of spindle afferents when the extrafusal muscles shorten.
- 2) Fusimotor set: gamma motoneurons are activated according to the novelty or difficulty of a task. Whereas static gamma motoneurons are continuously active during routine movements such as locomotion, dynamic gamma motoneoruns tend to be activated more during difficult tasks, increasing Ia stretch-sensitivity.
- 3) Fusimotor template of intended movement. Static gamma activity is a "temporal template" of the expected shortening and lengthening of the receptor-bearing muscle. Dynamic gamma activity turns on and off abruptly, sensitizing spindle afferents to the onset of muscle lengthening and departures from the intended movement trajectory.
Stretch reflex
When a muscle is stretched, primary sensory fibers (Group Ia afferent neurons) of the muscle spindle respond to both changes in muscle length and velocity and transmit this activity to the spinal cordSpinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
in the form of changes in the rate of action potentials. Likewise, secondary sensory fibers (Group II afferent neurons) respond to muscle length changes (but with a smaller velocity-sensitive component) and transmit this signal to the spinal cord. The Ia afferent signals are transmitted monosynaptically to many alpha motor neurons of the receptor-bearing muscle. The reflexly-evoked activity in the alpha motoneurons is then transmitted via their efferent axons to the extrafusal fibers of the muscle, which generate force and thereby resist the stretch. The Ia afferent signal is also transmitted polysynaptically through interneurons (Renshaw cell
Renshaw cell
Renshaw cells are inhibitory interneurons found in the gray matter of the spinal cord, and are associated in two ways with an alpha motor neuron....
s) which inhibit alpha motoneurons of antagonist muscles, causing them to relax.
After stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
or spinal cord injury in humans, spastic hypertonus (spastic paresis or spatic paralysis) often develops, whereby the stretch reflex in flexor muscles of the arms and extensor muscles of the legs is overly sensitive. This results in abnormal postures, stiffness and contractures. Hypertonus (Hypertonia
Hypertonia
Hypertonia a condition marked by an abnormal increase in muscle tension and a reduced ability of a muscle to stretch. It is caused by lesions to upper motor neurons in the central nervous system, which carry information from the central nervous system to the muscles and control posture, muscle...
) may be the result of over-sensitivity of alpha motoneurons and interneurons to the Ia and II afferent signals.
PNF stretching
PNF stretching
PNF stretching, or proprioceptive neuromuscular facilitation stretching, is an occupational therapy and physical therapy procedure designed in the 1940s and 1950s to rehabilitate patients with paralysis. It is often a combination of passive stretching and isometrics contractions. In the 1980s,...
, or proprioceptive neuromuscular facilitation, is a method of flexibility training that can reduce hypertonus, allowing muscles to relax and lengthen.
Development
It is also believed that muscle spindles play a critical role in sensorimotor developmentDevelopmental psychology
Developmental psychology, also known as human development, is the scientific study of systematic psychological changes, emotional changes, and perception changes that occur in human beings over the course of their life span. Originally concerned with infants and children, the field has expanded to...
.
See also
- Type Ia sensory fiberType Ia sensory fiberType Ia Sensory Fiber also called Primary Afferent Fiber is a type of sensory fiber. It is a component of a muscle fiber's muscle spindle which keeps track of how fast a muscle stretch changes .-Function of muscle spindles:...
- Type II sensory fiberType II sensory fiberType II sensory fiber is a type of sensory fiber, the second of the two main groups of stretch receptors. They are non-adapting, meaning that they keep responding even when the muscle has stopped changing its length. They are the second most highly myelinated fibers in the body.Their firing rate...
- Alpha motor neuronAlpha motor neuronAlpha motor neurons are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction...
- Gamma motor neuron
- Beta motor neuronBeta motor neuronA beta motor neuron is a kind of lower motor neuron, along with alpha motor neuron and gamma motor neuron. These motor neurons innervate intrafusal fibers of muscle spindles with collaterals to extrafusal fibers . Axons of beta motor neurons are myelinated...
- Intrafusal muscle fibre
- Extrafusal muscle fiberExtrafusal muscle fiberExtrafusal muscle fiber is a term given to standard muscle fibers as to distinguish them from intrafusal muscle fibers. Extrafusal muscle fibers are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement...