Muscle Innervation

Cross references:  Human Neuroanatomy    Human Muscles    
Medial Motor Column    Amphioxus Muscles    
Amphioxus Locomotion       Motor Neuron Evolution   
Motor Nerve Organization     

The quote in Human Muscles of the extremely good article from Wikipedia confirms that we humans have both slow-twitch and fast-twitch muscle fibers, but it doesn't discuss their innervation. 

Searching Google for "muscle innervation" yields 2,280,000 claimed hits.
     Two references located by this search which did not differentiate between slow and fast twitch have been summarized in Human Muscles

My interest here is in the possibility that human slow and fast twitch muscles are separately innervated as is the case in the amphioxus.  See:  Amphioxus Locomotion .     
  
Motor Units and Muscle Receptors (Section 3, Chapter 1) Neuroscience Online: An Electronic Textbook for the Neurosciences
http://neuroscience.uth.tmc.edu/s3/chapter01.html   
    "When a signal is sent to the motor neurons to execute a movement, motor neurons are not all recruited at the same time or at random. The motor neuron size principle states that, with increasing strength of input onto motor neurons, smaller motor neurons are recruited and fire action potentials before larger motor neurons are recruited." 
    "Because of Ohm’s Law, a small amount of synaptic current will be sufficient to cause the membrane potential of a small motor neuron to reach firing threshold, while the large motor neuron stays below threshold. As the amount of current increases, the membrane potential of the larger motor neuron also increases, until it also reaches firing threshold."   

Note:  I was unable to copy-and-paste Figure 1.6 mentioned below.  If you wish to see it, click on the link above.  It provides a helpful animation.   

    "Figure 1.6 demonstrates how the size principle governs the amount of force generated by a muscle. Because motor units are recruited in an orderly fashion, weak inputs onto motor neurons will cause only a few motor units to be active, resulting in a small force exerted by the muscle (Play 1). With stronger inputs, more motor neurons will be recruited, resulting in more force applied to the muscle (Play 2 and Play 3).
    Moreover, different types of muscle fibers are innervated by small and larger motor neurons. Small motor neurons innervate slow-twitch fibers; intermediate-sized motor neurons innervate fast-twitch, fatigue-resistant fibers; and large motor neurons innervate fast-twitch, fatigable muscle fibers.   
    The slow-twitch fibers generate less force than the fast-twitch fibers, but they are able to maintain these levels of force for long periods. These fibers are used for maintaining posture and making other low-force movements. Fast-twitch, fatigue-resistant fibers are recruited when the input onto motor neurons is large enough to recruit intermediate-sized motor neurons. These fibers generate more force than slow-twitch fibers, but they are not able to maintain the force as long as the slow-twitch fibers. Finally, fast-twitch, fatigable fibers are recruited when the largest motor neurons are activated. These fibers produce large amounts of force, but they fatigue very quickly. They are used when the organism must generate a burst of large amounts of force, such as in an escape mechanism.   
    Most muscles contain both fast- and slow-twitch fibers, but in different proportions. Thus, the white meat of a chicken, used to control the wings, is composed primarily of fast-twitch fibers, whereas the dark meat, used to maintain balance and posture, is composed primarily of slow-twitch fibers."   
My comment:
    This article does not discuss the origin of the small, intermediate-sized and large motor neurons.  It is clear that I need to know more about motor neurons.


Searching Google for "motor neurons" yielded 4,720,000 claimed hits.  

Motor neuron (Wiki)   
http://en.wikipedia.org/wiki/Motor_neuron    
    "Motor neurones are neurones that carry signals from the spinal cord to the muscles to produce movement. [1]   A single motor neuron may innervate many muscle fibres ( muscle cells ), and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch. As a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another>"  
    "Somatic motoneurons are further subdivided into two types: alpha efferent neurons and gamma efferent neurons. (Both types are called efferent to indicate the flow of information from the central nervous system (CNS) to the periphery .)
    Alpha motoneurons innervate extrafusal muscle fibers (typically referred to simply as muscle fibers) located throughout the muscle. Their cell bodies are in the ventral horn of the spinal cord and they are sometimes called ventral horn cells.
    Gamma motoneurons innervate intrafusal muscle fibers found within the muscle spindle.
    "A single motor neuron may synapse with one or more muscle fibers. The motor neuron and all of the muscle fibers to which it connects is a motor unit. Motor units are split up into 3 categories: slow motor units, fast fatiguing motor units, and fast fatigue-resistant motor units.
    Slow motor units are used to stimulate small muscle fibres which contract very slowly and provide small amounts of energy but are very resistant to fatigue, so they are used to sustain muscular contraction such as keeping the body upright.
    Fast fatiguing motor units are used to stimulate larger muscle groups which apply large amounts of force but fatigue very quickly. They are used for tasks that requires large brief bursts on energy such as jumping or running.
    Fast fatigue-resistant motor units stimulate moderate-sized muscles groups that don't react as fast as the FF motor units, but can be sustained much longer, as implied by the name, and provide more force than S motor units. [4]"  
My comment:   
    No discussion of the origin of the motor neurons other than that they originate in the spinal cord.  Although it still isn't enough, a more extensive discussion of alpha motoneurons is given in the next reference. 

Alpha motor neuron (Wiki)     
http://en.wikipedia.org/wiki/Alpha_motoneurons   
    "Alpha motor neurons (?-MNs) 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. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.

While their cell bodies are found in the central nervous system (CNS), alpha motor neurons are also considered part of the somatic nervous system—a branch of the peripheral nervous system (PNS)—because their axons extend into the periphery to innervate skeletal muscles.

An alpha motor neuron and the muscle fibers it innervates is a motor unit. A motor neuron pool contains the cell bodies of all the alpha motor neurons involved in contracting a single muscle. 

Contents

"  
My comment
    This reference does not acknowledge that there is more than one size of alpha motoneuron.  To quote the Neuroscience Online chapter, above: 
    "Small motor neurons innervate slow-twitch fibers; intermediate-sized motor neurons innervate fast-twitch, fatigue-resistant fibers; and large motor neurons innervate fast-twitch, fatigable muscle fibers."      
    So, if I'm going to distinguish between slow and fast twitch fiber systems, I need to be able to distinguish between small and large motoneuron systems. 


Searching Google for "alpha motor neuron size principle" yielded 1,780,000 claimed hits. 

Motor pool (Wiki) 
http://en.wikipedia.org/wiki/Motor_pool   
    "A motor pool refers to all of the individual motor neurons that innervate a single muscle. Each individual muscle fiber is innervated by only one motor neuron, but one motor neuron may innervate several muscle fibers."   
    "Motor pools are also distinguished by the different classes of motor neurons that they contain." 
    "Distinct skeletal muscles are controlled by groups of individual motor units. Such motor units are made up of a single motor neuron and the muscle fibers that it innervates."      
    "Motor neurons themselves fall into three main classes: alpha-motor neurons control extrafusal muscle fibers, meaning that they innervate skeletal muscles leading to movement; gamma-motor neurons innervate intrafusal muscle fibers, controlling the sensitivity of muscle spindles to stretch; beta-motor neurons are capable of synapsing on either type of muscle fiber.
    Alpha-motor neurons can further be divided into three separate subclasses, distinguished according to the contractile properties of the motor units that they form: fast-twitch fatigable (FF), fast-twitch fatigue-resistant (FR), and slow-twitch fatigue-resistant (S). The composition of a motor pool may consist of multiple classes and subclasses of motor neurons.[1]"      
    "Motor pools in the spinal cord are clustered in distinct columns of motor neurons extending over multiple spinal cord segments; although, there is significant overlap."      
    "Individual motor neurons within a given motor pool fire in accordance with what is known as the 'size principle'. The size principle was proposed by Elwood Henneman and his group in the 1960s as an explanation of the characteristic pattern with which individual motor neurons in a motor pool fire.
    The size principle stipulates that when the motor neurons of a motor pool fire, leading to the contraction of a terminal muscle fiber, the motor units containing the smallest motor neurons fire first. As excitatory signaling increases, larger motor neurons are subsequently recruited and contraction strength increases. Further, this differential recruitment of motor neurons occurs in instances of both increasing and decreasing contraction strength. As contraction strength is increased, the smallest motor units fire first and are also the last to stop firing as the contraction strength decreases.[2]"  
    "At a further level of specialization, specific combinations of motor neuron classes and subclasses are grouped spatially in clusters known as motor columns throughout the spinal cord."  

Motor unit (Wiki) 
http://en.wikipedia.org/wiki/Motor_unit   
    "A motor unit is made up of a motor neuron and the skeletal muscle fibers innervated by that axon.[1] "
    "All muscle fibers in a motor unit will be of the same fiber type. When a motor unit is activated, all of its fibers contract."  
My comment:   
    What I'm looking for is the origin and location of the neurons which activate each fiber type.  In the amphioxus, three fiber types and their locations are clearly identified.  Can ventral and dorsal compartment motor neurons be identified in higher animals?  Do they constitute three different identifiable systems as they do in the amphioxus?  If so, can they be related to the phenomena of Human Asymmetry ?  At least on the surface, fast twitch neurons sound left hemispheric, and slow twitch neurons sound right hemispheric.      
    From  Amphioxus Locomotion :   
    "Amphioxus has three types of motoneurons, ventral compartment (VC) motoneurons, which innervate the fast fibers of the myotome, dorsal compartment (DC) ones, which innervate the slow fibers, and visceral motoneurons, which innervate all other body muscles "  
          
Clicking on "fiber type", above, directs the browser to: 

Skeletal striated muscle (Wiki)   
http://en.wikipedia.org/wiki/Skeletal_muscle#Muscle_fibers   
    "There are two principal ways to categorize muscle fibers: the type of myosin (fast or slow) present, and the degree of oxidative phosphorylation that the fiber undergoes. Skeletal muscle can thus be broken down into two broad categories: Type I and Type II.
    Type I fibers appear red due to the presence of the oxygen binding protein myoglobin. These fibers are suited for endurance and are slow to fatigue because they use oxidative metabolism to generate ATP.
    Type II fibers are white due to the absence of myoglobin and a reliance on glycolytic enzymes. These fibers are efficient for short bursts of speed and power and use both oxidative metabolism and anaerobic metabolism depending on the particular sub-type. These fibers are quicker to fatigue."  
    "Type I Red fibers. Slow oxidative (also called slow twitch or fatigue resistant fibers)."  
    "Type IIa Red fibers. Fast oxidative (also called fast twitch A or fatigue resistant fibers)."  
    "Type IIx / IIb (dependent upon species) White. Fast glycolytic (also called fast twitch B or fatigable fibers)."  
My comment:   
    The "Motor unit (Wiki)  " reference, above, says that: 
    "Alpha-motor neurons can further be divided into three separate subclasses, distinguished according to the contractile properties of the motor units that they form: fast-twitch fatigable (FF), fast-twitch fatigue-resistant (FR), and slow-twitch fatigue-resistant (S)."  
    This seems to imply that the fast or slow nature of the motor unit is determined by the type of muscle fiber and not by any characteristic of the associated neuron.  In contrast, the Neuroscience Online reference, above, says: 
    "The motor neuron size principle states that, with increasing strength of input onto motor neurons, smaller motor neurons are recruited and fire action potentials before larger motor neurons are recruited."  and: 
    "Small motor neurons innervate slow-twitch fibers; intermediate-sized motor neurons innervate fast-twitch, fatigue-resistant fibers; and large motor neurons innervate fast-twitch, fatigable muscle fibers."   
    This appears to be a contradiction.  However, it's possible that the neurons and the fibers they innervate develop at the same time.  This would be expected if the three different skeletal neuron-fiber types are three separate systems. 
    If the three different skeletal neuron-fiber systems really are three distinct systems, which is my hypothesis, then it's possible that they developed in three different evolutionary stages.  Amphioxus Locomotion describes only two different skeletal systems, while we humans apparently have three. 
    Although I don't have a reference handy, I'm quite sure that the amphioxus doesn't have hemoglobin.  Does it have myoglobin?  Are there clear biochemical differences between its fast ventral fibers and its slow dorsal fibers?          
    Still no information on whether the neurons innervating human fast and slow muscle fibers have different origins, and, if they do, what those origins are.  The "Motor pool (Wiki)" reference, above, says: 
    "At a further level of specialization, specific combinations of motor neuron classes and subclasses are grouped spatially in clusters known as motor columns throughout the spinal cord."  
    So, maybe I need to look at the motor columns in the spinal cord.             
    See:  Medial Motor Column  . 


Motor Units | Brainy Info   

   

Motor Units

Definition:
    A motor unit is an alpha-motoneuron, its axon and all muscle fibres to which it connects.
Summary:
    Motor Units are the final connection between CNS and muscle. They are formed by lower motor neurons (alpha-motoneurons) and create three different types of pathways. S (slow twitch) is the weakest and is able to be sustained for the longest amount of time. FF (fast twitch, fatiguable) fibres are strong and fast but fatigues quickly. FR (fast twitch, fatigue resistant) is a blend between FF and S, providing a middle ground of both strength and sustainability. 

    "Found in the spinal cord and brainstem, Lower Motor Neurons are bundled into groups which form motor units. Each motor unit innervates only one type of skeletal muscle, and as there are three types of skeletal muscle there are three types of motor units."    
My comment:
    The above is inconsistent with how it uses the term "motor unit".  In the "Definition" section, it seems to imply that a motor unit consists of just a single neuron.  In the "Summary" section it seems to speak of multiple neurons being bundled together into a single motor unit.   

    "Muscles contain varying amounts of each type of muscle fibre, and therefore varying amounts of each motor unit. A quadricep muscle, for example, which is used for standing, and walking (etc.) will contain mostly red and intermediate fibres whereas a hand which is used for fine, highly coordinated activities such as writing will contain mostly white fibres."  
My comment:   
    This is an important clue in my search for the differing origins of the neurons innervating fast and slow twitch fibers.     


Chapter 12:  Muscle Mechanisms of Contraction and Neural Control 
http://highered.mcgraw-hill.com/sites/dl/free/0072852933/221304/foxch12.pdf    
    This is a 42 page PDF available online for free.  It has many very good illustrations. 
    "A muscle such as the gastrocnemius contains both fast and slow-twitch fibers, although fast-twitch fibers predominate.  A given somatic motor axon, however, innervates muscle fibers of one type only."  
My comment
    It does not differentiate between the sources of the neurons innervating the fast and slow twitch fibers.     

Motor Control (Goog)   
21 page PDF available online for free. 

Neuroscience: Motor Systems (Goog) 
Long, informative outline available online for free.    


Searching Google for "retrograde tracing motor neuron" yielded 115,000  claimed references. 


Searching PubMed for "retrograde tracing motor neuron" yielded 531  claimed references. 
Working in chronological order, I scanned the abstracts of the articles which seemed possibly relevant from 531 to 1.  

484<53
1  
From:  Searching PubMed for "retrograde tracing motor neuron" ... 
Morphology of motoneurons in a mixed motor pool of the cat facial nucleus that innervate orbicularis oculis and quadratus labii superioris, stained intracellularly with horseradish peroxidase. - 1985  (PubMed)      
    See:  Mammalian Muscles

386<53
1
Metabolic profiles of white and red-intermediate spinal motoneurons in the zebrafish.  - 1993 (PubMed)   
    See:  Teleost Muscles .  
My comment:   
    "identified by retrograde tracing ... and classified on the basis of their location in the motor column."  This is very important.  How were they "classified on the basis of their location in the motor column"?  Unfortunately, I don't seem to have access to the full article.  All I have is the abstract.     


139<531
 
  From:  Searching PubMed for "retrograde tracing motor neuron" ...   
Horizontal eye movement networks in primates as revealed by retrograde transneuronal transfer of rabies virus: differences in monosynaptic input to "slow" and "fast" abducens motoneurons. - 2006  (PubMed)      
    See:  Mammalian Muscles  

Related citation
:
From:  Searching PubMed for "retrograde tracing motor neuron" ...    
Motoneurons of twitch and nontwitch extraocular muscle fibers in the abducens, trochlear, and oculomotor nuclei of monkeys.  
    See:  Mammalian Muscles .  

37>53
  From:  Searching PubMed for "retrograde tracing motor neuron" ...   
Characterization of last-order premotor interneurons by transneuronal tracing with rabies virus in the neonatal mouse spinal cord. - 2011 (PubMed)      
    See:  Mammalian Muscles   

36<531 
  From:  Searching PubMed for "retrograde tracing motor neuron" ...    
The red nucleus and the rubrospinal projection in the mouse.   
    See:   Red Nucleus



















CotA - Muscle Innervation
130622 - 1702
modified
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