Muscle spindles are sensory receptors within the belly of a muscle that
primarily detect changes in the length of this muscle. They convey length
information to the central nervous system via sensory neurons. 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
motor neurons via the stretch reflex to resist muscle stretch.
Muscle spindles are found within the belly of muscles, embedded in extrafusal
muscle fibers. 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 fibers static nuclear bag fibers
nuclear chain fibers and the axons of sensory neurons.
Axons of gamma motoneurons also terminate in muscle spindles; they make
synapses 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 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.
Primary and secondary 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 innervate only 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.
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 acetylcholine
by the active gamma motoneuron, the end portions of the intrafusal muscle fibers
contract, thus elongating the non-contractile central portions. This
opens stretch-sensitive ion channels of the sensory endings, leading to an
influx of sodium ions. 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 action,
go to: Arthur Prochazka's Lab, University of Alberta
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 of the muscle spindle
respond to both changes in muscle length and velocity and transmit this activity
to the spinal cord in the form of changes in the rate of action
potentials. Likewise, secondary sensory fibers respond to muscle length changes
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,
which inhibit alpha motoneurons of antagonist muscles, causing them to
relax. After stroke or spinal cord injury in
humans, spastic hypertonus 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
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.
Development It is also believed that muscle spindles
play a critical role in sensorimotor development.
Additional images See also
Type Ia sensory fiber Type II sensory fiber
Alpha motor neuron Gamma motor neuron
Beta motor neuron Intrafusal muscle fibre
Extrafusal muscle fiber References
External links Muscle Spindles at the US National
Library of Medicine Medical Subject Headings