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Encyclopedia > Muscle spindle

A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. Its functions are to send proprioceptive information about the muscle to the central nervous system, and to respond to muscle stretching. A top-down view of skeletal muscle Muscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. ... Sensory neurons are nerve cells within the nervous system responsible for converting external stimuli from the organisms environment into internal electrical impulses. ... In vertebrates, motoneurons (also called motor neurons) are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. ... Proprioception (from Latin proprius, meaning ones own and perception) is the sense of the position of parts of the body, relative to other neighbouring parts of the body. ... A diagram showing the CNS: 1. ...

Contents


Anatomy

Muscle spindles are found within the fleshy portions of muscles, embedded in so-called extrafusal muscle fibers. They are composed of 3-10 intrafusal muscle fibers, of which there are three types, dynamic nuclear bag fibers (bag1 fibres), static nuclear bag fibers (bag2 fibres) and nuclear chain fibers and the axons of sensory neurons. Axons of motoneurons also terminate in muscle spindles; they make synapses at either or both of the ends of the intrafusal muscle fibers and regulate spindle sensitivity. Muscle spindles are encapsulated by connective tissue, and are aligned parallel to extrafusal muscle fibers, unlike Golgi tendon organs, which are oriented in series. A top-down view of skeletal muscle Muscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. ... extrafusal muscle fibers - served by axons of the alpha motor neurons (serve multiple muscle fibers); contraction of these muscles provides movement - extrafusal muscle fibers and associated alpha motor neurons are called a motor unit ... Intrafusal muscle fibers are muscle fibers that comprise the muscle spindle. ... Brief Outline: 1-3 nuclear bag fibres lie in the centre of each intrafusal muscle fibre of a muscle spindle. ... Brief Outline: There are 3-9 nuclear chain fibres per muscle spindle that are half the size of the nuclear bag fibres. ... An axon, or nerve fiber, is a long slender projection of a nerve cell, or neuron, which conducts electrical impulses away from the neurons cell body or soma. ... An axon, or nerve fiber, is a long slender projection of a nerve cell, or neuron, which conducts electrical impulses away from the neurons cell body or soma. ... In vertebrates, motoneurons (also called motor neurons) are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. ... Illustration of the major elements in a prototypical synapse. ... intrafusal muscle fibers - specialized sensory organs served by 2 axons, one sensory and one motor - also called muscle spindles ... Parallel 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. ... extrafusal muscle fibers - served by axons of the alpha motor neurons (serve multiple muscle fibers); contraction of these muscles provides movement - extrafusal muscle fibers and associated alpha motor neurons are called a motor unit ... Golgi tendon organs are proprioceptive sense organs that are located at the insertion of skeletal muscle fibres into the tendons of skeletal muscle. ...


The muscle spindle has both sensory and motor components. Primary and secondary sensory fibers spiral around and terminate on the central portions of intrafusal fibers, providing the sensory component of the structure via stretch-sensitive ion-channels of the axons. The motor component is provided by gamma motoneurons and beta motoneurons that innervate the spindle and cause a slight contraction of the end portions of the intrafusal muscle fibers when activated. The gamma, or fusimotor, axons only innervate the intrafusal muscle fibres whereas the beta, or skeletofusimotor, axons innervate both extrafusal and intrafusal muscle fibres. These motornurons are classified as static or dynamic according to their pattern of innervation and their physiological effects. The static axons innervate the chain or bag2 fibres while the dynamic axons innervate the bag1 fibres and increase the velocity sensitivity of the Ia afferents. Type Ia Sensory Fiber also called Primary Afferent Type 1A Fiber is a neuron component of the peripheral sensory system which innervates the muscle spindles, a kind of specialized muscle fiber which is sensitive to muscle lenght. ... γ-motoneurons (gamma-motoneurons or motor neurons) are a component of the fusimotor system, the system by which the CNS controls muscle spindle sensitivity. ... intrafusal muscle fibers - specialized sensory organs served by 2 axons, one sensory and one motor - also called muscle spindles ...


Sensitivity Modification

The function of the gamma motoneuron neuromuscular junction is not to supplement the general muscle contraction provided by extrafusal fibers, but to modify the sensitivity of the muscle spindle to stretch. Upon release of acetylcholine by the gamma neuron, the end portions of the intrafusal muscle fibers contract, thus deliberately elongating the non-contractile central portions of intrafusal muscle fibers. This opens stretch-sensitive ion channels of the centrally-positioned sensory axons, leading to an influx of sodium ions. This raises the resting potential of these axons, thereby increasing the probability of action potential firing, thus increasing the sensitivity of the muscle spindle. A neuromuscular junction is the junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscles surface. ... The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... intrafusal muscle fibers - specialized sensory organs served by 2 axons, one sensory and one motor - also called muscle spindles ... intrafusal muscle fibers - specialized sensory organs served by 2 axons, one sensory and one motor - also called muscle spindles ... Ion channels are present in the membranes that surround all biological cells. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... ... The resting potential of a cell is the membrane potential that would be maintained if there were no action potentials, synaptic potentials, or other active changes in the membrane potential. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ...


Stretch reflex

When a muscle is stretched, primary sensory fibers (Group Ia afferent neurons) of the muscle spindle respond to both the velocity and the degree of stretch, and send this information to the spinal cord. Likewise, secondary sensory fibers (Group II afferent neurons) detect and send information about the degree of stretch (but not the velocity thereof) to the CNS. This information is transmitted monosynaptically to an alpha efferent motor fiber, which activates extrafusal fibers of the muscle to contract, thereby reducing stretch, and polysynaptically through an interneuron to another alpha motoneuron, which inhibits contraction in opposing muscles. Cross-section through cervical spinal cord. ... Interneuron: noun An interneuron is a neuron that communicates only to other neurons. ...


PNF stretching, or proprioceptive neuromuscular facilitation, is a method of flexibility training that reduces the automatic reflex action in order allow muscles to lengthen. PNF stretching (“proprioceptive neuromuscular facilitation”) is a form of stretching in which a muscle is alternatingly stretched passively and contracted. ...


Development

It is also believed that muscle spindles play a critical role in sensorimotor development. Developed first by Jean Piaget, the theory of cognitive development is based on schemas, or schemes of how one perceives the world, in critical periods -- times during which one is particularly susceptible to certain information. ... Developmental psychology is the scientific study of progressive psychological changes that occur in human beings as they age. ...


See also

Nervous system - Sensory system - Somatosensory system - edit
Spinal pathway: Somatosensory information
Medial lemniscusTouch (Pressure & Vibration) | Proprioception
Spinothalamic tractPain | Temperature
Receptors
Touch: Pacinian corpuscles | Meissner's corpuscles | Merkel's discs | Ruffini endings | Free nerve endings | Hair follicle receptors
Proprioception: Golgi organ | Muscle spindle (Intrafusal muscle fiber)
Pain: Nociceptors    Temperature: Thermoreceptors

  Results from FactBites:
 
NationMaster - Encyclopedia: Muscle spindle (1844 words)
Muscle spindles are found within the fleshy portions of muscles, embedded in so-called extrafusal muscle fibers.
Muscle spindles are encapsulated by connective tissue, and are aligned parallel to extrafusal muscle fibers, unlike Golgi tendon organs, which are oriented in series.
When a muscle is stretched, primary sensory fibers (Group Ia afferent neurons) of the muscle spindle respond to both the velocity and the degree of stretch, and send this information to the spinal cord.
  More results at FactBites »

 
 

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