Lower Motor Neuron Pool

Contraction of skeletal muscle can occur only when a nervous impulse is conducted down the axons of the motor

neuron. The motor neurons lie in the ventral horn of the spinal cord and are known as lower motor neurons. There are two types of motor neurons that provide a parallel system of innervation of the muscle. The larger alpha motor neurons innervate the extrafusal muscle fibers, and the smaller gamma motor neurons innervate the intrafusal muscle fibers of the muscle spindle (Fig. 15-3 (Figure Not Available) ). The axons of the motor neurons leave the spinal cord through the ventral roots and continue into the ventral and dorsal branches (rami) of the spinal nerves. The dorsal rami innervate the paravertebral muscles of the neck and trunk, and the ventral rami innervate the rest of the trunk and the extremities. In the brain stem, the motor neurons in the cranial nerve motor nuclei send axons to innervate the muscles of the tongue, pharynx, larynx, palate, face, and extraocular muscles. Because signals to skeletal muscle from any part of the motor system rely on activation of the lower motor neuron pool, Sir Charles Sherrington coined the term final common pathway to describe the function of the lower motor neurons. The activation of different numbers and combinations of alpha motor neurons by other motor centers determines the force and speed of the resulting movement. The accuracy of a movement depends on sensory feedback. The parallel gamma motor neuron system provides a way for the motor system to ensure the accuracy of the sensory information it is receiving.

ALPHA MOTOR NEURON

Nerve impulses generated by the alpha motor neuron cause contraction of extrafusal muscle fibers, resulting in the generation of force and movement. The axon of the alpha motor neuron divides into numerous branches when it enters the muscle, and each terminal branch makes synaptic contact with one muscle cell. The number of terminal branches and muscle cells innervated by an individual alpha motor neuron may vary greatly but generally corresponds to the size of the motor neuron. An alpha motor neuron and all the muscle cells innervated by it are called a motor unit, another term introduced by Sherrington. Therefore, all muscle cells innervated by that alpha motor neuron contract simultaneously when an action potential is generated by that motor neuron. The size of a motor unit may range from as few as ten muscle cells innervated by one motor neuron to more than 1000. The smallest motor units are found in muscles that are used for delicate precise movements such as the extraocular muscles, the larynx, and the intrinsic muscles of the hand. The largest motor units are found in large powerful muscles involved in more forceful movements the require less precise control like the back and thigh muscles. y

The myofibers belonging to the same motor unit may be scattered throughout the muscle, but they are all of the same fiber type. Therefore, all of the myofibers in a given motor unit have the same characteristics with regard to maximal force, velocity of contraction, and endurance. Also, there is a correlation between the size of the motor units and the fiber type. The smallest motor units consist of type I fibers, whereas the largest ones consist of type IIB fibers.

The difference in fiber types amplifies the difference in the maximal force that can be generated by the large and small motor units. Therefore, in the largest motor units, where type II fibers predominate, not only are there more myofibers but each one can also generate more force.

Figure 15-3 (Figure Not Available) The parallel organization of the alpha and gamma lower motor neurons (black cell bodies). The alpha motor neurons innervate extrafusal skeletal muscle, the gamma motor neuron innervates the intrafusal muscle fibers to ensure proper sensory feedback from the muscle spindle. The activity of both motor neurons is modulated by multiple segmental and suprasegmental ir(From Snell RS: Clinical Neuroanatomy for Medical Students. Boston, Little, Brown & Co., Inc., 1987.)

Smaller motor units made up of type I fibers are found in muscles used for finely graded changes of force needed for more precise control.

The force of a muscle contraction can be increased in a graded manner by two means. One is a gradual increase in the number of alpha motor neurons activated, which results in the recruitment of more and more motor units and thus more myofibers. When a muscle contraction is initiated, the smallest, most fatigue-resistant, and slowest motor units are recruited first; as the force is increased, larger and faster-contracting motor units that are more susceptible to fatigue are recruited successively. This concept was demonstrated by Henneman and colleagues,[9] who referred to it as the "size" principle. They also described the relationship between the size of a motor unit (i.e., the number of myofibers innervated) and the size of its motor neuron. The smallest motor neurons are more excitable and thus are activated first. The largest motor neurons are not as easily excited but have the highest maximum firing frequencies and can generate the quickest forceful movements. Therefore, the size principle ensures selection of the motor units that are best suited for a particular kind of movement.

The other method of increasing the force of muscle contraction and movement is to increase the firing frequency of the alpha motor neurons already recruited, thereby increasing the force developed by each motor unit. A single action potential from a motor neuron produces only a very brief contraction of the myofiber, or twitch. If no other action potentials arrive within a tenth of a second, no movement is produced. However, if action potentials arrive at closer and closer intervals, the individual twitches become a sustained contraction. As the firing frequency of the motor neuron increases, the tension and force generated increase also; this is referred to as summation. The maximum muscle contraction at the maximum firing frequency is known as fused tetanus. Unfused tetanus occurs at submaximum firing frequencies.

Motor neurons are somatotopically organized in the ventral gray matter of the spinal cord. Within the ventral gray, the motor neurons are grouped in longitudinally oriented columns (.Fig 15-4 ). Each column contains the alpha and gamma motor neurons to one or a few functionally similar muscles. Generally, each column extends through more than one segment of the cord. Therefore, each muscle receives motor fibers through more than one ventral root and spinal nerve. This explains why destruction of one root or spinal nerve by a lumbar disc protrusion, for example, produces weakness but not complete paralysis of a muscle.

Another level of organization is seen in the distribution of the neurons supplying the axial muscles versus those supplying the extremities. The motor neurons innervating the paravertebral muscles are located most medially in the ventral gray, whereas those supplying the muscles of the extremities are located more laterally. This arrangement can be seen most dramatically on cross-sections of segments of the cord that send fibers to the extremities; the ventral horn is much larger and extends more laterally in the cervical and lumbar enlargements. There is also a dorsal-ventral organization in which the motor neurons supplying the proximal muscles of the extremities lie more ventrally in the ventral gray than those supplying the distal muscles of the hand and foot.

The spinal cord is divided into segments, each giving rise to a dorsal and ventral root, which combine to form a spinal nerve. The axons of all the motor neurons located in one spinal segment leave the spinal cord through one ventral root and continue into the spinal nerves.

These spinal nerves conform to the embryological myotomes. This myotomal organization can still be seen in the rostral caudal distribution of innervation from the cervical and lumbar cord segments innervating the upper and lower extremities, respectively. The proximal muscles of the shoulder and hip girdle are innervated by the more rostral segments of the cervical cord (C5-C6) and the lumbar cord (L2-L4), whereas the intrinsic muscles of the hand and the more distal leg and foot muscles are innervated by the lowermost segments of the cervical (C8-T1) and lumbar (L5-S1) segments. There are the same number of thoracic (12), lumbar (5), and sacral (5) spinal roots as vertebral bodies. However, there are eight cervical spinal roots, whereas there are only seven cervical vertebral bodies. The first pair of cervical nerves exits from the vertebral column between the atlas and the skull. Each of the remaining cervical nerves from C2 to C7 leaves the spinal column above the vertebra of the corresponding number. Because there are eight pairs of cervical nerves and only seven cervical vertebrae, the eighth cervical nerve leaves the spinal column between C7 and T1. The first thoracic nerve and all the remaining spinal nerves pass out from the vertebral column below the vertebra of like number.

After leaving the spinal column, the cervical and lumbar nerves combine to form the brachial plexus and the lumbosacral plexus from which the peripheral nerves to the arm and leg are formed. As a result, motor axons from one spinal segment are distributed to several peripheral nerves. Looking at it from the standpoint of the muscle, each limb muscle receives motor axons from more than one spinal segment. As noted earlier, destruction of one spinal nerve therefore produces only a weakness rather than a paralysis of a muscle. Destruction of the peripheral nerve innervating the muscle, however, interrupts all of the motor innervation for that muscle, resulting in paralysis and atrophy of the muscle. Knowledge of the peripheral nerves, the spinal nerves contained in them, and the muscles innervated by them is crucial in localizing lesions in the subdivisions of the lower motor neuron, whether in the spinal nerve, plexus, or peripheral nerve. Diagrams of the important relationships are contained in Figures 15-5 15.-6. 15.-7..

The cervical, brachial, and lumbosacral plexuses are formed by the anterior primary rami of the spinal nerve roots (the posterior rami innervate the paravertebral muscles). In the plexuses the roots become reorganized into peripheral nerves, which then contain contributions from two or more spinal roots. Therefore, to accurately localize a lesion to a specific part of a plexus, one has to know the motor and sensory supply of all peripheral nerve components supplied by that division.

The cervical plexus is formed by the anterior primary rami of C1 through C4 behind the sternocleidomastoid and in front of the scalenus medius and levator scapulae muscles. The motor branches of the plexus supply the muscles of the neck. Injuries to the cervical plexus are infrequent,

Figure 15-4 Functional organization of the lower motor neurons in the spinal cor(From Bossy: Atlas of Neuroanatomy and Special Sense Organs. Philadelphia, W. B. Saunders, 1970.)

but any of its branches can be injured by penetrating wounds, surgery, or enlarged lymph nodes (see Fig 15-6 A). Involvement of the cervical plexus may also compromise the closely associated cranial nerves XI and XII. In patients with lesions of the cervical plexus, sensory symptoms in the form of head and neck dysesthesia and pain may predominate over motor signs.

Injuries are more common to the brachial plexus, which is formed from the anterior primary rami of C5 through T1. It extends from the spinal column to the axilla and is divided into five major components starting proximally and proceeding distally. The first components are the roots, which recombine to form trunks, and then divisions, cords, and finally branches. EiguieJ..^ shows a diagram of the anatomy of the brachial plexus.

The lumbosacral plexus is derived from the anterior primary rami of the twelfth thoracic through the fourth sacral levels and is contained within the psoas major muscle. Although many more roots contribute to the lumbosacral plexus, it is somewhat simpler than the brachial plexus. Two major nerves, the femoral nerve and the sciatic nerve, are formed from the plexus (see Fig 15-7 ).

GAMMA MOTOR NEURON

Like the alpha motor neurons, the gamma motor neurons lie in the ventral horn of the spinal cord interspersed among the alpha motor neurons innervating the same muscle. Gamma motor neurons innervate the intrafusal muscle fibers of a specialized sensory organ, the muscle spindle. y The muscle spindle consists of a small bag of muscle fibers that lie in parallel with the extrafusal skeletal muscle fibers. Therefore, when the muscle lengthens or shortens, the intrafusal muscle spindle fibers are stretched or relaxed correspondingly. The intrafusal fibers are surrounded by

Figure 15-5 The axons of the lower motor neurons form the spinal roots, one for each spinal segmfAj. The spinal roots (containing both sensory and motor axons) recombine to form the brachial plefB), from which the major peripheral nerves are formed (C). (Adapted from Patton HD, Sundsten JW, Crill W, Swanson PD: Introduction to Basic Neurology. Philadelphia, W. B. Saunders, 1976.)

sensory nerve endings that become 1a afferents to the dorsal root ganglion. When the intrafusal fibers are stretched as the muscle is lengthened, the sensory fibers are activated, providing sensory feedback about the degree of lengthening that has occurred. When the muscle contracts and shortens, the intrafusal spindle fibers also shorten. If this were a completely passive system, the intrafusal fibers would relax and the sensory endings would become silent, providing no helpful feedback information about the state of the muscle. To prevent this inefficient circumstance, gamma motor neurons are activated, maintaining tension in the intrafusal fibers to continue to provide precise sensory information.

This phenomenon can be observed experimentally if alpha motor neurons are excited in isolation. When the muscle contracts, there is a pause in volleys from the 1a afferent. When the gamma motor neuron is excited at the same time, the afferent volleys do not pause. This simultaneous activation of alpha and gamma motor neurons during muscle contraction is called alpha-gamma co-activation; it is an excellent example of sensory motor integration in the nervous system. By this means, innervation of the muscle spindle by an independent system of gamma motor neurons allows the central nervous system to adjust the sensitivity of the spindle and fine-tune the information it receives (. ..Fig 15.-8 ).

Another sensory organ, the Golgi tendon organ, also conveys information about the state of the muscle. The Golgi tendon organs lie in the muscle tendon and, unlike the muscle spindle, are coupled in series with the extrafusal muscle fibers. Therefore, both passive stretch and active contraction of the muscle increase the tension of the tendon and activate the tendon organ receptor. In contrast to

Figure 15-6 The organization of the brachial plexus. The spinal roots C5-T1 recombine to form trunks (upper, middle, and lower), divisions (anterior and posterior), and cords (lateral, posterior, and m(From Haymaker N, Woodhall B: Peripheral Nerve Injuries. Philadelphia, W. B. Saunders, 1953.)

Figure 15-7 A, The lumbar plexus.B, The sacral plexus. The lumbosacral trunk is the connection between the lumbar and sacral plexfFrom Haymaker N, Woodhall B: Peripheral Nerve Injuries. Philadelphia, W. B. Saunders, 1953.)

Figure 15-8 The effect of gamma activation on maintenance of tone in the muscle spincA The extrafusal fibers innervated by the alpha motor neuron and the intrafusal muscle fibers of the muscle spindle innervated by the gamma motor neuron lie in parallel.

B, Condition that would exist if the intrafusal fibers of the muscle spindle did not have a separate innervation to maintain tone when the alpha motor neuron fires causing contraction and shortening of the extrafusal skeletaC, Firing of the gamma motor neurons activates the muscle spindle to maintain normal tone and responsivity of the 1a afferent (alpha-gamma coactivation).

the activity of the muscle spindle, which depends on muscle length, the Golgi tendon organ conveys information about muscle tension. Together they convey precise information about the length, tension, velocity, and force of the muscle contraction, thus allowing greater precision of movement. y

Changes in muscle tone, defined as resistance to passive stretch of muscle, are an important feature of diseases of the motor system. At one time, an important component of normal resting muscle tone was thought to be the result of low-level background alpha-gamma co-activation. However, more recent studies in fully relaxed individuals indicate that the viscoelastic properties of muscle and tendons account for normal resting tone. y Common experience indicates that resting muscle tone can vary considerably depending on the state of relaxation of the muscle as well as its viscoelastic properties. Some studies have shown that the passive viscoelastic properties of muscle may contribute to increased tone in patients with chronic spasticity and rigidity, but the most important determinants of pathological alterations in tone are the result of alterations in stretch reflexes.

ALPHA AND GAMMA MOTOR NEURONS AND REFLEX ARCS

Reflex actions are the simplest form of coordinated movement. A reflex action is a stereotyped response to a specific sensory stimulus. The reflex elicited depends on the site of the stimulus, and the strength of the stimulus determines the amplitude of the response. Reflex responses are used by higher motor centers to generate more complex movements and behaviors. The neural circuitry responsible for reflex actions is present at different levels of the motor system, and disturbances in these reflexes are important for localizing lesions in the motor system. Some reflexes, especially spinal and brain stem reflexes, are normally observed or elicited only in the developing nervous system. As the nervous system and higher motor centers mature, these reflexes are suppressed, only to reemerge if damage to the higher motor centers modulates the reflex. The unmasking of spinal reflexes in particular is a good example of the hierarchical organization of the motor system.

The monosynaptic stretch reflex is the simplest spinal reflex. As the name implies, the reflex muscle contraction is elicited by lengthening or stretching the muscle. To evoke this reflex, the muscle must be stretched rapidly, which produces a short phasic contraction. Thus, this reflex is termed a phasic stretch reflex.[Ki In the human subject, sudden stretch is produced by tapping a tendon with a reflex hammer.

Sherrington demonstrated that this reflex could be eliminated by cutting either the dorsal or the ventral root, thus establishing that this reflex requires both sensory input from the muscle to the spinal cord and motor output from the motor neurons to the muscle. [2] The sensory receptor for the afferent limb of the reflex is the muscle spindle.y Sudden lengthening of the muscle by the tap on the tendon causes an increase in the discharge rate of type 1a nerve fibers from the muscle spindle. The 1a afferents entering the spinal cord excite motor neurons to both agonist and

Figure 15-9 (Figure Not Available) The two neuron monosynaptic phasic stretch reflex. A rapid stretch of a muscle increases the discharge rate of the 1a afferent fibers from the muscle spindle. The 1a afferent entering the spinal cord directly excites the alpha motor neuron, causing contraction of agonist muscles to oppose the lengthenirfF/om Gardiner E: Fundamentals of Neurology. Philadelphia, W. B. Saunders, 1968.)

synergist muscles, causing contractions that oppose the lengthening (Fig. 15-9 (Figure Not Available) ). Branches from the 1a afferents also excite interneurons that inhibit antagonist motor neurons, causing the antagonist muscles to relax. Thus, this reflex can be seen as a negative feedback loop that resists changes in muscle length. Note that although contraction of the agonist muscles results from a simple monosynaptic connection between the 1a afferent and the alpha motor neuron, relaxation of the antagonist muscles requires an inhibitory interneuron and a phenomenon called reciprocal inhibition (Fig. 15-10 (Figure Not Available) ).

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