A motor pool consists of all individual

motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly ...

s that innervate a single

muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other ...

. Each individual

muscle fiber A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a m ...

is innervated by only one motor neuron, but one motor neuron may innervate several muscle fibers. This distinction is physiologically significant because the size of a given motor pool determines the activity of the muscle it innervates: for example, muscles responsible for finer movements are innervated by motor pools consisting of higher numbers of individual motor neurons. Motor pools are also distinguished by the different classes of motor neurons that they contain. The size, composition, and anatomical location of each motor pool is tightly controlled by complex developmental pathways.Carp, J. S. and Wolpaw, J. R. 2010. Motor Neurons and Spinal Control of Movement. eL
DOI: 10.1002/9780470015902.a0000156.pub2
/ref>

Anatomy

Distinct

skeletal muscles Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...

are controlled by groups of individual motor units. Such motor units are made up of a single

and the muscle fibers that it innervates. The cell bodies of motor neurons are located in the ventral horn of the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spin ...

and the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...

. These neurons innervate skeletal muscle fibers through the propagation of

action potentials An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

down their

axons An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...

(through ventral roots and

cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and ...

), and they stimulate skeletal muscle fibers at

neuromuscular junctions A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation to ...

where they

synapse In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses fr ...

with the motor end plates of muscle fibers. In humans, these axons can be as long as one meter. Motor neurons themselves fall into three main classes: alpha-motor neurons control

extrafusal muscle fibers Extrafusal muscle fibers are the standard skeletal muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement. They make up the large mass of skeletal striated muscle tis ...

, meaning that they innervate skeletal muscles leading to movement; gamma-motor neurons innervate

intrafusal muscle fibers Intrafusal muscle fibers are skeletal muscle fibers that serve as specialized sensory organs ( proprioceptors). They detect the amount and rate of change in length of a muscle.Casagrand, Janet (2008) ''Action and Movement: Spinal Control of ...

, controlling the sensitivity of

muscle spindles Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be ...

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. Motor pools in the

are clustered in distinct columns of motor neurons extending over multiple spinal cord segments; although, there is significant overlap. Motor pools that control proximal muscles are generally located medial to the ventral horn, while those that control distal muscles are located laterally. Motor pools that control flexor muscles are located dorsally to the ventral horn while those that control extensor muscles are located ventrally. The number of motor neurons in an individual motor pool is highly variable and can generally be predicted by the level of nuanced control that a specific muscle requires. For example, some muscles have relatively low numbers of motor units in their respective motor pools while others, with highly nuanced control (such as the muscles in the human

hand A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each " ...

) have higher densities of motor units.

Function

Motor pools function primarily to integrate synaptic input from higher CNS centers into precise and consistent contraction patterns. 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 Contraction may refer to: Linguistics * Contraction (grammar), a shortened word * Poetic contraction, omission of letters for poetic reasons * Elision, omission of sounds ** Syncope (phonology), omission of sounds in a word * Synalepha, merged ...

of a terminal muscle fiber, the motor units containing the smallest motor neurons fire first. As excitatory signalling 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.Henneman E and Mendell LM (1981) Functional organization of motoneuron pool and inputs. In: BrooksVB (ed.), Handbook of Physiology, sect. I: The Nervous System, vol. II: Motor Control, part I, pp. 423–507. Baltimore: Williams and Wilkins. The size principle has important functional benefits. Primarily, this system frees higher centers of the CNS from having to signal specific contraction patterns for distinct levels of muscle contraction. The level of synaptic input that higher centers provide to a given motor pool must determine the contraction strength, and this simplifies the process of contraction strength modulation. This system allows for very precise and consistent modulation of contraction strength from just increased or decreased levels of synaptic input: with additional motor units of increasing size, there will be a consistent and precise effect on the force of contraction. Another key benefit derived from the size principle is that smaller neurons will be fired more regularly and for a longer duration of time compared to larger neurons. These smaller motor units are more resistant to fatigue, and as such, are better suited to this role.Burke RE (1981) Motor units: anatomy, physiology, and functional organization. In: BrooksVB (ed.) Handbook of Physiology, sect. I: The Nervous System, vol. II: Motor Control, part I, pp. 345–422. Baltimore: Williams and Wilkins.

Specialization and development

There are several layers of differentiation and specialization to consider the complicated development of motor pools.

Alpha- and gamma-motor neuron differentiation

Alpha motor neurons Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their c ...

and

gamma motor neurons A gamma motor neuron (γ motor neuron), also called gamma motoneuron, or fusimotor neuron, is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of ( Aγ) fibers going to the muscle. Like al ...

do not merely differ in their postsynaptic targets. The physiological differences between these two classes are significant. The axonal diameter of gamma-motor neurons is half of that of alpha-motor neurons, resulting in a higher cytoplasmic resistance and therefore a slower signal propagation velocity. Additionally, gamma-motor neurons display far simpler branching patterns than that of their alpha- counterparts. The differentiation into these two classes is regulated by complex interactions between several

neurotrophic factors Neurotrophic factors (NTFs) are a family of biomolecules – nearly all of which are peptides or small proteins – that support the growth, survival, and differentiation of both developing and mature neurons. Most NTFs exert their trop ...

, and all of these interactions are not yet well understood. Glial cell line-derived neurotrophic factor ( GDNF) has been discovered to play an especially important role in all layers of motor pool development. In the case of alpha- and gamma- differentiation, it has been shown that gamma motor neurons express significantly higher levels of certain GDNF receptor subunits.Kanning, Kevin C., Artem Kaplan, and Christopher E. Henderson. "Motor Neuron Diversity in Development and Disease." ''Annual Review of Neuroscience'' 33.1 (2010): 409-40. Print.

Alpha-motor neuron muscle fiber targets

Alpha motor neurons are also grouped into subclasses based on the types of muscle fibers they target, which also determines their functional characteristics. Motor neurons that target fast-twitch, fatigable (FF) muscle fibers are the largest (and therefore the fastest in propagating signals); those that target fast-twitch, fatigue-resistant (FR) fibers are of intermediate size; and those that target slow-twitch, fatigue-resistant (S) fibers are the smallest. In addition to signaling velocity, the range of sizes across these subclasses also forms the physiological basis of the size principle. Due to their relatively small axonal diameter, neurons that target S type fibers require a smaller input current to reach

threshold potential In electrophysiology, the threshold potential is the critical level to which a membrane potential must be depolarized to initiate an action potential. In neuroscience, threshold potentials are necessary to regulate and propagate signaling in both ...

. Conversely, the largest neurons, which target FF type fibers, require a greater input current to reach threshold. Therefore, the axonal diameter of the three subclasses of

alpha motor neurons Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their c ...

clearly determines the patterns of the recruitment of motor units predicted by the size principle. The specific regulatory mechanisms that determine the size of these three alpha-motor neuron subclasses are not well known.

Pool specificity and spatial orientation

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

. These motor columns are motor pools, and each has its own unique identity: each

neuron A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...

within the pool (regardless of its class) expresses a unique profile of

transcription factors In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The f ...

, cell-surface proteins (such as axonal guidance receptors and

adhesion molecules Cell adhesion molecules (CAMs) are a subset of cell surface proteins that are involved in the binding of cells with other cells or with the extracellular matrix (ECM), in a process called cell adhesion. In essence, CAMs help cells stick to each ...

), and

neurotransmitter receptor A neurotransmitter receptor (also known as a neuroreceptor) is a membrane receptor protein that is activated by a neurotransmitter. Chemicals on the outside of the cell, such as a neurotransmitter, can bump into the cell's membrane, in which the ...

s. These pool-specific biochemical markers provide the developmental framework for muscle-specific synapsing. Among this pool-specific combination of transcription factors, Hox factors play an especially important role. Hox transcription factors play a central role in the spatial orientation of the motor pool within the spine, and in the site-specific synapsing of the motor pool on to its down-stream

muscle fibers A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a mu ...

.Song, Mi-Ryoung, and Samuel L. Pfaff. "Hox Genes: The Instructors Working at Motor Pools." ''Cell'' 123.3 (2005): 363-65. Print.

Pool size

The number of motor neurons within a specific motor pool is a crucial developmental step. Again, little is known about the precise mechanisms and molecules involved in this process of specialization. However, it is known that the mechanism involves an initial generation of large numbers of motor neurons, followed by a pruning process that is mediated by

cell death Cell death is the event of a biological cell ceasing to carry out its functions. This may be the result of the natural process of old cells dying and being replaced by new ones, as in programmed cell death, or may result from factors such as di ...

mechanisms and survival factors. Some researchers have speculated that the number of neurons within a given pool is determined by competitive interactions between different

Hox genes Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of 'position', ensuring that the co ...

Evolution

Muscles responsible for finer, more nuanced movements are innervated by motor pools consisting of higher numbers of individual motor neurons. This principle is highlighted by examining the evolution of the human

tongue The tongue is a muscular organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive process, and is the primary organ of taste. The tongue's upper surface (dorsum) is covered by taste bu ...

and

motor pools.

Human tongue

Highly refined and coordinated movements of the tongue are responsible for the intricacies of human

speech Speech is a human vocal communication using language. Each language uses phonetic combinations of vowel and consonant sounds that form the sound of its words (that is, all English words sound different from all French words, even if they are th ...

. The evolutionary analysis would predict an abnormally large motor pool innervating the muscle of the human tongue, relative to those of other mammals. Such a large motor pool would allow for region-specific innervation of the tongue muscle, motor neurons with task-specificity, motor neuron specializations that allow for quick movements, and various other motor nuances necessary for producing complex speech. Anatomical analysis has validated this evolutionary prediction: in the average adult human, the motor pool for the tongue contains between 7,093 and 8,817 motor neurons. This neuron density far exceeds that measured in other mammals, and even exceeds the motor pool size for many muscles in the human body:

biceps brachii The biceps or biceps brachii ( la, musculus biceps brachii, "two-headed muscle of the arm") is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both heads of the muscle arise on the scapula and join t ...

, for example, is innervated by a motor pool that averages 441.5 motor neurons in size.

Human hand

The evolutionary analysis also predicts large motor pools innervating the muscles of the human hand. The human innovations of tool-making, throwing motions, and clubbing motions required unprecedented manual dexterity, only made possible by extensive networks of large motor pools innervating the human hand. As Richard Young explains:

It has been proposed that the hominid lineage began when a group of chimpanzee-like apes began to throw rocks and swing clubs at adversaries and that this behaviour yielded reproductive advantages for millions of years, driving natural selection for improved throwing and clubbing prowess. This assertion leads to the prediction that the human hand should be adapted for throwing and clubbing… thereby providing an evolutionary explanation for the two unique grips, and the extensive anatomical remodelling of the hand that made them possible.Young, Richard W. "Evolution of the Human Hand: The Role of Throwing and Clubbing." ''Journal of Anatomy'' 202.1 (2003): 165-74. Print.

Precise neuromuscular control allows for the sub-millisecond release times necessary for throwing. Effective clubbing requires the recruitment of several motor units to produce a secure grip through impact. These evolutionary predictions have been verified by comparative anatomical studies: the motor pools innervating the human hand are significantly larger than those innervating related primate hands.

Relevance to disease

ALS

Amyotrophic lateral sclerosis Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease (MND) or Lou Gehrig's disease, is a neurodegenerative disease that results in the progressive loss of motor neurons that control voluntary muscles. ALS is the most comm ...

(ALS) is a

neurodegenerative disease A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...

that differentially affects specific motor pools and classes of motor neurons. ALS modelled in mice, for example, was shown to lead first to rapid FF motor neuron loss, followed by a delayed loss of FR neurons, leaving S type neurons largely intact in the late stages of the disease. Further, in late-stage ALS patients, death of motor neurons is pool-specific. Motor neuron death all throughout the

leads to a nearly complete loss of voluntary movement; however, ocular control and voluntary control of excretory functions remain mostly unaffected. These movements are governed by motor pools in the

midbrain The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal ( alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...

and in

Onuf's nucleus Onuf's nucleus is a distinct group of neurons located in the ventral part ( laminae IX) of the anterior horn of the sacral region of the human spinal cord involved in the maintenance of micturition and defecatory continence, as well as muscular ...

in the lumbosacral spinal cord, respectively. Currently, research is being conducted on these ALS-resistant motor pools, and their pool-specific molecular identities are being examined for potential

neuroprotective Neuroprotection refers to the relative preservation of neuronal structure and/or function. In the case of an ongoing insult (a neurodegenerative insult) the relative preservation of neuronal integrity implies a reduction in the rate of neuronal los ...

qualities.

Parallels in SMA and ageing

Spinal muscular atrophy Spinal muscular atrophy (SMA) is a rare neuromuscular disorder that results in the loss of motor neurons and progressive muscle wasting. It is usually diagnosed in infancy or early childhood and if left untreated it is the most common genet ...

(SMA) and ageing-related motor degeneration share clear parallels with

ALS Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease (MND) or Lou Gehrig's disease, is a neurodegenerative disease that results in the progressive loss of motor neurons that control voluntary muscles. ALS is the most comm ...

in the patterns and specificity of motor neuron loss. Though it is completely distinct from ALS in its

pathogenesis Pathogenesis is the process by which a disease or disorder develops. It can include factors which contribute not only to the onset of the disease or disorder, but also to its progression and maintenance. The word comes from Greek πάθος ''pat ...

, SMA leads to a similar rapid death of FF motor neurons, with S type neurons being generally spared. Further, motor pools controlling

facial muscles The facial muscles are a group of striated skeletal muscles supplied by the facial nerve (cranial nerve VII) that, among other things, control facial expression. These muscles are also called mimetic muscles. They are only found in mammals, al ...

(including those of the

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

) and voluntary excretory muscles are spared. Motor neuron degeneration caused by ageing similarly affects FF types but not S types; ageing also seems to spare ocular motor pools.Enoka, R. "Mechanisms That Contribute to Differences in Motor Performance between Young and Old Adults." ''Journal of Electromyography and Kinesiology'' 13.1 (2003): 1-12. Print. These similar patterns of

neurodegeneration A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...

in three different diseases have led researchers to speculate that slow-twitch motor neurons, along with motor pools of the eyes and excretory muscles, have intrinsic neuroprotective properties that are not disease-specific. Research is currently being conducted to discover a possible molecular basis for

neuroprotection Neuroprotection refers to the relative preservation of neuronal structure and/or function. In the case of an ongoing insult (a neurodegenerative insult) the relative preservation of neuronal integrity implies a reduction in the rate of neuronal los ...

in these cell types.

References

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