Motor unit
Encyclopedia
”A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type (either fast twitch or slow twitch). When a motor unit is activated, all of its fibers contract. Groups of motor units often work together to coordinate the contractions of a single muscle
; all of the motor units that subserve a single muscle are considered a motor unit pool. Larger motor units have stronger twitch tensions.http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.1000050&ct=1
The number of muscle fibers within each unit can vary: thigh
muscles can have a thousand fibers in each unit, eye
muscles might have ten. In general, the number of muscle fibers innervated by a motor unit is a function of a muscle's need for refined motion. The smaller the motor unit, the more precise the action of the muscle. Muscles requiring more refined motion are innervated by motor units that synapse
with fewer muscle fibers.
Nerve cell axons are very thin, about 1 micrometer. However, they are extraordinarily long. For many motor neurons the axon is over a meter long, extending from the spinal column to a muscle cell. They stretch the spinal column to increase height.
In medical electrodiagnostic testing for a patient with weakness
, careful analysis of the motor unit action potential (MUAP) size, shape, and recruitment pattern can help in distinguishing a myopathy
from a neuropathy.
Generally, three or four groups are identified, based upon several factors:
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The typing of motor units has thus gone through many stages and reached a point where it is recognized that muscle fibers contain varying mixtures of several mysosin types that can not easily be classified into specific groups of fibers. The three (or four) classical fiber types represent peaks in the distribution of muscle fiber properties, each determined by the overall biochemistry of the fibers.
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...
; all of the motor units that subserve a single muscle are considered a motor unit pool. Larger motor units have stronger twitch tensions.http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.1000050&ct=1
The number of muscle fibers within each unit can vary: thigh
Thigh
In humans the thigh is the area between the pelvis and the knee. Anatomically, it is part of the lower limb.The single bone in the thigh is called the femur...
muscles can have a thousand fibers in each unit, eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
muscles might have ten. In general, the number of muscle fibers innervated by a motor unit is a function of a muscle's need for refined motion. The smaller the motor unit, the more precise the action of the muscle. Muscles requiring more refined motion are innervated by motor units that 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...
with fewer muscle fibers.
Nerve cell axons are very thin, about 1 micrometer. However, they are extraordinarily long. For many motor neurons the axon is over a meter long, extending from the spinal column to a muscle cell. They stretch the spinal column to increase height.
In medical electrodiagnostic testing for a patient with weakness
Weakness
Weakness is a symptom represented, medically, by a number of different conditions, including: lack of muscle strength, malaise, dizziness, or fatigue. The causes are many and can be divided into conditions that have true or perceived muscle weakness. True muscle weakness is a primary symptom of a...
, careful analysis of the motor unit action potential (MUAP) size, shape, and recruitment pattern can help in distinguishing a myopathy
Myopathy
In medicine, a myopathy is a muscular disease in which the muscle fibers do not function for any one of many reasons, resulting in muscular weakness. "Myopathy" simply means muscle disease...
from a neuropathy.
Motor unit types (mammalian)
Motor units possess a range of properties and are generally placed into groups based upon the similarities between those properties.Generally, three or four groups are identified, based upon several factors:
- Physiological
- Contraction speed in Isometric contractionsIsometric exerciseIsometric exercise or isometrics are a type of strength training in which the joint angle and muscle length do not change during contraction...
- Rate of rise of force
- Time to peak of a twitch contraction (re
- Contraction speed in Isometric contractions
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- FF - Fast fatigable - high force, fast contraction speed but fatigue in a few seconds.
- FR - Fast fatigue resistant - intermediate force, fatigue resistant - fast contraction speed and resistant to fatigue.
- FI - Fast intermediate - intermediate between FF and FR.
- S or SO - Slow (oxidative) - low force, slower contraction speed, highly fatigue resistant.
- Biochemical
- Histochemical (the oldest form of biochemical fiber typing)
- Glycolytic enzyme activity (e.g. glycerophosphate dehydrogenase (GPD)Glyceraldehyde 3-phosphate dehydrogenaseGlyceraldehyde 3-phosphate dehydrogenase is an enzyme of ~37kDa that catalyzes the sixth step of glycolysis and thus serves to break down glucose for energy and carbon molecules...
) - Oxidative enzyme activity (e.g. succinate dehydrogenase -SDH )
- Sensitivity of MyosinMyosinMyosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...
ATPase to acid and alkali
- Glycolytic enzyme activity (e.g. glycerophosphate dehydrogenase (GPD)
- Histochemical (the oldest form of biochemical fiber typing)
-
- These generally designate fibers as:
- I (Slow oxidative, SO)- Low glycolytic and high oxidative presence. Low(er) myosin ATPase, sensitive to alkali.
- IIa (Fast oxidative/glycolytic, FOG) - High glycolytic, oxidative and myosin ATPase presence,sensitive to acid.
- IIb (Fast glycolytic, FG) - High glycolytic and myosin ATPase presence, sensitive to acid. Low oxidative presence.
- IIi - fibers intermediate between IIa and IIb.
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- Histochemical and Physiological types correspond as follows:
- S and Type I, FR and type IIa, FF and type IIb, FI and IIi.
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- ImmunohistochemicalImmunohistochemistryImmunohistochemistry or IHC refers to the process of detecting antigens in cells of a tissue section by exploiting the principle of antibodies binding specifically to antigens in biological tissues. IHC takes its name from the roots "immuno," in reference to antibodies used in the procedure, and...
(more recent form of fiber typing)- Myosin Heavy Chain (MHC)
- Myosin Light Chain - alkali (MLC1)
- Myosin Light Chain - regulatory (MLC2)
- Immunohistochemical
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- The Immunohistochemical types are as follows, with the type IIa, IIb and slow corresponding to IIa, IIb and slow (type I) histochemical types:
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- Gene characterization of myosinMyosinMyosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...
s
- Gene characterization of myosin
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- There are currently about 15 known different types of MHC genes recognized in muscle, only some of which may be expressed in a single muscle fiber. These genes form one of ~18 classes of myosin genes, identified as class II which should not be confused with the type II myosins identified by immunohistochemistry. The expression of multiple MHC genes in a single muscle fiber is an example of polymorphismPolymorphism (biology)Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
. The relative expression of these myosin types is determined partly by genetics and partly by other biological factors such as activity, innervation and hormones.
- There are currently about 15 known different types of MHC genes recognized in muscle, only some of which may be expressed in a single muscle fiber. These genes form one of ~18 classes of myosin genes, identified as class II which should not be confused with the type II myosins identified by immunohistochemistry. The expression of multiple MHC genes in a single muscle fiber is an example of polymorphism
The typing of motor units has thus gone through many stages and reached a point where it is recognized that muscle fibers contain varying mixtures of several mysosin types that can not easily be classified into specific groups of fibers. The three (or four) classical fiber types represent peaks in the distribution of muscle fiber properties, each determined by the overall biochemistry of the fibers.
See also
- MyosinMyosinMyosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...
- MYH1MYH1Myosin-1 is a protein that in humans is encoded by the MYH1 gene.-Further reading:...
, MYH2MYH2Myosin-2 is a protein that in humans is encoded by the MYH2 gene.-Further reading:...
, MYH3MYH3Myosin-3 is a protein that in humans is encoded by the MYH3 gene.-Further reading:...
, MYH4MYH4Myosin-4 also known as myosin, heavy chain 4 is a protein which in humans is encoded by the MYH4 gene.-Further reading:...
, MYH6MYH6Myosin-6 is a protein that in humans is encoded by the MYH6 gene.- Function :Cardiac muscle myosin is a hexamer consisting of two heavy chain subunits, two light chain subunits, and two regulatory subunits. This gene encodes the alpha heavy chain subunit of cardiac myosin...
, MYH7MYH7MYH7 is a gene encoding a myosin heavy chain beta isoform expressed primarily in the heart.Changes in the relative abundance of MHC-β and MHC-α correlate with the contractile velocity of cardiac muscle...
, MYH7BMYH7BMyosin-7B also known as myosin, heavy chain 7B is a protein that in humans is encoded by the MYH7B gene....
, MYH8MYH8Myosin-8 is a protein that in humans is encoded by the MYH8 gene.-Further reading:...
, MYH9MYH9Myosin, heavy chain 9, non-muscle is a protein which in humans is encoded by the MYH9 gene.-Clinical significance:MYH9 polymorphisms have been shown to associate with glomerulosclerosis and non-diabetic end stage renal disease in African Americans and in Hispanic Americans, though it was later...
, MYH10MYH10Myosin-10 is a protein that in humans is encoded by the MYH10 gene.-Further reading:...
, MYH11MYH11Myosin-11 is a protein that in humans is encoded by the MYH11 gene.- Function :Myosin-11 is a smooth muscle myosin belonging to the myosin heavy chain family...
, MYH13MYH13Myosin-13 also known as myosin, heavy chain 13 is a protein which in humans is encoded by the MYH13 gene.- Function :MYH13 is a myosin whose expression is restricted primarily to the extrinsic eye muscles which are specialized for function in eye movement....
, MYH14MYH14Myosin-14 is a protein that in humans is encoded by the MYH14 gene.-Further reading:...
, MYH15MYH15Myosin-15 also known as myosin, heavy chain 15 is a protein that in humans is encoded by the MYH15 gene....
, MYH16 - motor unit recruitmentMotor unit recruitmentMotor unit recruitment is the progressive activation of a muscle by successive recruitment of contractile units to accomplish increasing gradations of contractile strength....
- tetanic contractionTetanic contractionA tetanic contraction occurs when a motor unit has been maximally stimulated by its motor neuron. This occurs when a muscle's motor unit is stimulated at a sufficiently high frequency of multiple impulses. Each stimulus causes a twitch. If stimuli are delivered slow enough, the tension in the...
- myopathy (muscle pathology/diseases)MyopathyIn medicine, a myopathy is a muscular disease in which the muscle fibers do not function for any one of many reasons, resulting in muscular weakness. "Myopathy" simply means muscle disease...
- motor unit number estimationMotor unit number estimationMotor Unit Number Estimation is a technique that uses electromyography to estimate the number of motor units in a muscle.- Principles :A motor unit consists of one alpha motoneuron and all the muscle fibres it innervates....
- Tonic and Phasic Inputs
- Motor poolMotor poolIn neuroscience, a motor pool refers to a group of motor spinal neurons that innervate the same muscle. The biological significance of motor pool organization is in the fact that motor pools with many neurons produce finer movements...