on 14-Sep-2020 (Mon)

Motor and sensory systems, although separated for practical clinical purposes, are not independent entities but are closely integrated. Without sensory feedback, motor control is ineffective. Furthermore, at the higher cortical levels of motor control, motivation, planning, and other frontal lobe activities that subserve volitional movement are preceded and modulated by activity in the parietal sensory cortex

The motor neurons in the frontal cortex adjacent to the rolandic fissure (motor strip) connect with the spinal motor neurons by a system of fibers known, because of the shape of its fasciculus in the medulla, as the pyramidal tract. Because the motor fibers that extend from the cerebral cortex to the spinal cord are not confined to the pyramidal tract, they are more accurately designated as the corticospinal tract, or, alternatively, as the upper motor neurons, to distinguish them from the lower motor neurons.

All variations in the force, range, rate, and type of movement are determined by the number and size of motor units called into action and the frequency and sequence of firing of each motor unit. Much of the sequence and coordination of firing is modulated by subcortical structures or the basal ganglia and cerebellum

Although the muscles are innervated in patterns largely corresponding to segments of the spinal cord (a myotome), each large muscle is usually supplied by two or more roots. In contrast, a single peripheral nerve usually provides the complete motor innervation of a muscle or group of muscles. For this reason, paralysis caused by disease of the anterior horn cells or anterior roots has a different topographic pattern than paralysis fol- lowing interruption of a peripheral nerve

The coordination of agonists, antagonists, synergists, and fixators is effected mainly by segmental spinal reflexes under the guidance of proprioceptive sensory stimuli. In general, the more delicate the movement, the more precise must be the coordination between agonist and antagonist muscles.

Motor activities include not only those that alter the position of a limb or other part of the body (isotonic con- traction) but also those that stabilize posture (isometric contraction). Movements that are performed slowly are called ramp movements. Very rapid movements, which are too fast for sensory control, are called ballistic (also termed phasic)

Some gamma motor neurons are toni- cally active at rest, keeping the intrafusal muscle fibers taut and more sensitive to active and passive changes in muscle length

A tap on a tendon stretches causes a vibratory wave of the spindle and activates its nuclear bag fibers. Afferent projections from these fibers synapse directly with alpha motor neurons in the same and adjacent spinal segments; these neurons, in turn, send impulses to the skeletal mus- cle fibers, resulting in the familiar monosynaptic muscle contraction or monophasic (myotatic) stretch reflex, com- monly referred to as the tendon reflex or “tendon jerk” (Fig. 3-1), more correctly called the muscle stretch or proprioceptive reflex

Thus the tension in the spindle fibers and the state of excitability of the alpha and gamma neurons (influenced greatly by descending fiber systems) determines the level of activity of the tendon reflexes and muscle tone (the responsiveness of muscle to stretch).

Both groups of alpha neurons receive projections from the propriospinal neurons via the large fasciculi proprii from adjacent spinal segments. All the facilitatory and inhibitory influences supplied by cutaneous and proprioceptive afferent and descending suprasegmental neurons are coordinated at segmental levels. For further details the reader may con- sult Burke and Lance and also Davidoff (1992)

The large neurons of the anterior horns of the spinal cord contain high concentra- tions of choline acetyltransferase and use acetylcholine as their transmitter at the neuromuscular junction. The main neurotransmitters of the descending corticospinal tract, in so far as can be determined in humans, are aspartate and glutamate. Glycine is the neurotransmitter released by Renshaw cells, which are responsible for recurrent inhibition, and by interneurons that mediate reciprocal inhibition during reflex action. Gamma-aminobutyric acid (GABA) serves as the inhibitory neurotransmitter of inter - neurons in the posterior horn.

The denervated muscle undergoes extreme atrophy, being reduced to 20 or 30 percent of its original bulk within 3 to 4 months.

Damage restricted to only a portion of the motor fibers supplying the muscle results in partial paralysis, or paresis, and a proportion- ate diminution in the force and speed of contraction. The atrophy will be less and the tendon reflex reduced instead of lost

The electrodiagnosis of denervation depends upon finding fibrillations, fasciculations, and other abnormali- ties on needle electrode examination as mentioned in the previous chapter. However, some of these abnormalities do not appear until several days or a week or two after nerve injury

Lower motor neuron (infranuclear) paralysis is the direct result of loss of function or destruction of anterior horn cells or their axons in anterior roots and nerves. The signs and symptoms vary according to the location of the lesion. In any individual case, the most important clinical question is whether sensory changes coexist

However, acute and profound spinal cord lesions and, to a lesser extent, corticospinal lesions in the brainstem and cerebrum, may temporar- ily abolish spinal reflexes (“spinal shock”; see Chap. 42). This is probably caused by the interruption of descend- ing tonic excitatory impulses, which normally maintain a sufficient level of excitation in spinal motor neurons to permit the peripheral activation of segmental reflexes. The attenuation of spinal shock by opiate antagonists, such as naloxone, suggests that the phenomenon is at least in part mediated by the release of previously stored endogenous opiates from the distal terminals of neurons in the periaq- ueductal gray matter. Once the stored opiates are depleted, the presynaptic inhibition of motor neurons ceases, her- alding the end of spinal shock and the beginning of the period of spasticity

The pyramidal tract, strictly speaking, designates only those fibers that course longitu- dinally in the pyramid of the medulla oblongata. Of all the fiber bundles in the brain, the pyramidal tract has been known for the longest time, the first accurate description having been given by Türck in 1851

This is the only direct long-fiber connection between the cerebral cortex and the spinal cord. The indirect path- ways through which the cortex influences spinal motor neurons are the rubrospinal, reticulospinal, vestibulospi- nal, and tectospinal; these tracts do not run in the pyra- mid. All these pathways, direct and indirect, are embraced by the term upper motor neuron or supranuclear, meaning above the anterior horn cells

A major source of confusion about the pyramidal tract stems from the traditional view, formulated at the turn of the twentieth century, that it originates entirely from the large motor cells of Betz in the fifth layer of the precen- tral convolution (the primary motor cortex, or area 4 of Brodmann 1 ) (Figs. 3-3 and 21-1). However, there are only some 25,000 to 35,000 Betz cells, whereas the medullary pyramid contains about 1 million axons (Lassek). Thus most of the fibers of the pyramidal tract arise from cortical neurons other than Betz cells, particularly in Brodmann areas 4 and 6 (the frontal cortex immediately rostral to area 4, including the medial portion of the superior fron- tal gyrus, that is, the supplementary motor area); in the primary somatosensory cortex (Brodmann areas 3, 1, and 2); and in the superior parietal lobule (areas 5 and 7)

As the corticospinal tracts descend in the cerebrum and brainstem, they send col- laterals to the striatum, thalamus, red nucleus, cerebellum, and reticular formations. Accompanying the corticospinal tracts in the brainstem are the corticobulbar tracts, which are distributed to motor nuclei of the cranial nerves ipsilat- erally and contralaterally (see Fig. 3-2)

The corticospinal tracts decussate at the lower end of the medulla, although some of their fibers may cross above this level. The fibers destined for the upper limb neurons cross first (more rostrally). The proportion of crossed and uncrossed fibers varies to some extent from one person to another (Nyberg). About 75 to 80 percent of the fibers cross and the remaining fibers descend ipsilater- ally, mostly in the uncrossed ventral corticospinal tract.

The corticospinal tract is phylogenetically relatively new, being found only in mammals, which probably accounts for its variability between individuals as com- pared to the older vestibulospinal, rubrospinal, and reticulospinalparapyramidal systems, which are invariant among persons. Uncrossed fibers in the corticospinal tract account for mirror movements that are seen during efforts at fine motor tasks, particularly in children, and also in some disorders of the nervous system such as the Klippel- Feil syndrome and the Kallmann syndrome

For a more complete discussion of the crossing of the various tracts of the nervous system, the reader is referred to the review by Vulliemoz et al.

The axons subserving facial movement are situ- ated anterior in the posterior limb of the capsule, those for hand and arm in the central portion and those for the foot and leg, posteriorly (as detailed by Brodal).

Restricted pontine lesions may cause a pure motor hemiplegia that is indistinguishable from the syndrome of the internal capsule. However, a study conducted by Marx and colleagues using MRI mapping techniques of patients with hemiplegia due to brainstem lesions suggests that the usual somatotopic organization breaks down in the base of the pons, and there is a con- centration of fibers innervating proximal muscles lying more dorsally and those exciting distal parts of the limbs, more ventrally

The descending pontine bundles, now devoid of their corticopontine fibers, reunite to form the medullary pyra- mid. The brachial–crural pattern may persist in the pyra- mids and is certainly reconstituted in the lateral columns of the spinal cord (see Fig. 7-3), but it should be empha- sized that the topographic separation of motor fibers at cervical, thoracic, lumbar, and sacral levels is not as dis- crete as usually shown in schematic diagrams of the spinal cord

The corticospinal tracts and other upper motor neu- rons terminate mainly in relation to nerve cells in the intermediate zone of spinal gray matter (internuncial neu- rons), from which motor impulses are then transmitted to the anterior horn cells. Only 10 to 20 percent of corti- cospinal fibers (presumably the thick, rapidly conducting axons derived from Betz cells) establish direct synaptic connections with the large motor neurons of the anterior horns.

The motor area of the cerebral cortex is defined physi- ologically as the region of electrically excitable cortex from which isolated movements can be evoked by stimuli of minimal intensity

Thus the prefrontal cortex, supplementary motor cor- tex, premotor cortex, and motor cortex are all responsive to afferent stimuli and are involved prior to, and in coor- dinated fashion with, a complex movement. As remarked later on, the striatopallidum and cerebellum, which proj- ect to these cortical areas, are also activated prior to or concurrently with the discharge of corticospinal neurons (see Thach and Montgomery for a critical review of the physiologic data)

The corticospinal and corticobulbar tracts, which proj- ect to all levels of the spinal cord and brainstem, terminat- ing diffusely throughout the nucleus proprius of the dorsal horn and the intermediate zone. A portion of these con- nect directly with the large motor neurons that innervate the muscles of the fingers, face, and tongue; this system provides the capacity for a high degree of fractionation of movements, as exemplified by independent finger move- ments

A ventromedial pathway, which arises in the tectum (tectospinal tract), vestibular nuclei (vestibulospinal tract), and pontine and medullary reticu- lar cells (reticulospinal tract) and terminates principally on the internuncial cells of the ventromedial part of the spinal gray matter. This system is mainly concerned with axial movements—the maintenance of posture, integrated movements of body and limbs, and total limb movements

A lateral pathway, which is derived mainly from the magnocellular part of the red nucleus and terminates in the dorsal and lateral parts of the internuncial zone. This pathway adds to the capacity for independent use of the extremities, especially of the hands

Usually, when hemiplegia is severe and permanent as a consequence of disease, much more than the long, direct corticospinal pathway is involved

In the cerebral white matter (corona radiata) and internal capsule, the corticospinal fibers are intermingled with corticostriate, corticothalamic, corticorubral, cortico- pontine, cortico-olivary, and corticoreticular fibers. It is noteworthy that thalamocortical fibers, which are a vital link in an ascending fiber system from the basal ganglia and cerebellum, also pass through the internal capsule and cerebral white matter. Thus lesions in these parts can simultaneously affect both corticospinal and extra- pyramidal systems. Attribution of a capsular hemiple- gia solely to a lesion of the corticospinal or pyramidal pathway is therefore not entirely correct. The term upper motor neuron (supranuclear) paralysis, which recognizes the involvement of several descending fiber systems that influence and modify the lower motor neuron, is more appropriate

The one place where corticospinal fibers are entirely isolated is the pyramidal tract in the medulla. In humans, there are a few documented cases of a lesion more or less confined to this location. The result of such lesions has been an initial flaccid hemiplegia (with sparing of the face), from which there is considerable recovery

Also, the cerebral peduncle had in the past been sectioned in patients in an effort to abol- ish involuntary movements (Bucy et al). In some of these patients, a slight degree of weakness or only a Babinski sign was produced but no spasticity developed. These observations indicate that a pure pyramidal tract lesion does not result in spasticity.

The distribution of the paralysis caused by upper motor neuron (supranuclear) lesions varies with the locale of the lesion, but certain features are characteristic of all of them. A group of muscles is always involved, never individual muscles, and if any movement is possible, the proper relationships between agonists, antagonists, syn- ergists, and fixators are preserved.

On careful inspection, the paralysis never involves all the muscles on one side of the body, even in the severest forms of hemiplegia. Movements that are invariably bilateral—such as those of the eyes, jaw, pharynx, upper face, larynx, neck, thorax, diaphragm, and abdomen—are affected little or not at all. This occurs because these muscles are bilaterally inner- vated; that is, stimulation of either the right or left motor cortex results in contraction of these muscles on both sides of the body.

Upper motor neuron lesions are characterized fur- ther by certain peculiarities of retained movement. There is decreased voluntary drive on spinal motor neurons (fewer motor units are recruitable and their firing rates are slower), resulting in a slowness of movement. There is also an increased degree of cocontraction of antagonistic muscles, reflected in a decreased rate of rapid alternating movements. These abnormalities probably account for the greater sense of effort and the manifest fatigability in effect- ing voluntary movement of the weakened muscles.

Another phenomenon is the activation of paralyzed muscles as parts of certain automatisms (synkinesias). For example, the paralyzed arm may move suddenly during yawning and stretching. Attempts by the patient to move the hemiplegic limbs may also result in a variety of associated movements. Thus, flexion of the arm may result in involuntary pronation and flexion of the leg or in dorsiflexion and eversion of the foot. Also, volitional movements of the paretic limb often evoke imitative (mirror) movements in the normal one or vice versa.

If the upper motor neurons are interrupted above the level of the facial nucleus in the pons, hand and arm muscles are affected most severely and the leg muscles to a lesser extent; of the cranial musculature, only muscles of the tongue and lower part of the face are involved to any significant degree (Fig. 3-5). Because Broadbent was the first to call attention to this distribution of facial paralysis that relatively spares the forehead muscles, it has been referred to as “Broadbent’s law.”

The precise course taken by fibers that innervate the facial nucleus is still somewhat uncertain; however, the majority crosses in the mid-pons to innervate the contralateral facial nerve nucleus. Some fibers may descend to the upper medulla and then ascend recurrently to the pons (Pick’s bundle), accounting for the rare, mild facial weakness that may be seen with lesions of the lower pons and upper medulla

This is the condition referred to earlier as spinal shock, a state of acute flaccid paralysis that is replaced later by spasticity. A comparable state of areflexia and hypotonia may occur with acute cerebral lesions but is less sharply defined than is the spinal state

With some acute cerebral lesions, spasticity and paralysis develop together; in others, especially with parietal lesions, the limbs remain flaccid but reflexes are retained.

At rest, with the muscles shortened to midposition, they are flaccid to palpation and electromyographically silent. If the arm is extended or the leg flexed very slowly, there may be little or no change in muscle tone. By con- trast, if the muscles are briskly stretched, the limb moves freely for a very short distance (free interval), beyond which there is an abrupt catch and then a rapidly increas- ing muscular resistance up to a point; then, as passive extension of the arm or flexion of the leg continues, the resistance melts away. This velocity-dependent tone con- stitutes the “clasp-knife” phenomenon of spasticity. With the limb in the extended or flexed position, a new passive movement may not encounter the same sequence

Clinicians have known for some time that there is not a constant relationship between spasticity and weakness. Severe weakness may be associated with only the mildest signs of spasticity; in contrast, the most extreme degrees of spasticity, observed in certain patients with cervical spinal cord disease, may seem disproportionate to the extent of weakness, signifying that these two states depend on sepa- rate mechanisms. Indeed, the selective blocking of small gamma neurons abolishes spasticity as well as hyperactive segmental tendon reflexes but leaves power unchanged

The heightened stretch reflexes (tendon jerks) of the spastic state may be a “release” phenomenon—the result of interruption of descending inhibitory pathways. Animal experiments have demonstrated that this aspect of the spastic state is mediated through disinhibition of spindle efferents (increased tonic activity of gamma motor neu- rons) and through loss of the influence of reticulospinal and vestibulospinal pathways that act on alpha motor neurons. The clasp-knife phenomenon appears to derive at least partly from a lesion (or presumably a change in central control) of a specific portion of the reticulospinal system

The pathophysiology of spasticity is further dependent on two more refined descending tracts: (1) the dorsal reticulospinal tract, which has inhibitory effects on stretch reflexes; and (2) the medial reticulospinal and vestibulospinal tracts, which together facilitate extensor tone. In cerebral and capsular lesions, cortical inhibition from these pathways is reduced, resulting in spastic hemiplegia.

In spinal cord lesions that involve the corticospinal tract, the dorsal reticulospinal tract is usually involved as well. If the latter tract is spared, only paresis, loss of support reflexes, and possibly release of flexor reflexes (Babinski phenomenon) occur. In some instances, flaccidity persists after hemiplegic stroke, possibly as a result of primary involvement of the lenticular nucleus of the basal ganglia and the thalamus as suggested by Pantano and colleagues

In its essential form, the sign consists of extension of the large toe and extension and fanning of the other toes during and immediately after stroking the lateral plantar surface of the foot. The stimulus is applied along the dor- sum of the foot from the lateral heel and sweeping upward and across the ball of the foot. The stimulus must be firm but not necessarily painful. Several dozen surrogate responses (with numerous eponyms) have been described over the years, most utilizing alternative sites and types of stimulation, but all have the same significance as the Babinski response

Clinical and electrophysiologic observations indicate that the extension movement of the great toe is a com- ponent of a larger synergistic flexion or shortening reflex of the leg—that is, toe extension when viewed from a physiologic perspective is a protective (nocifensive, or defensive) response. The most characteristic of these is the “triple flexion response,” in which the hip, thigh, and ankle flex (dorsiflex) slowly, following an appropriate stimu- lus. These spinal flexion reflexes, of which the Babinski sign is the most characteristic, are common accompani- ments to—but not essential components of—spasticity.

They are present because of disinhibition or release of motor programs of spinal origin. Important characteris- tics of these responses are their capacity to be induced by weak superficial stimuli (such as a series of pinpricks) and their tendency to persist for a few moments after the stimulation ceases. With incomplete suprasegmental lesions, the response may be fractionated; for example, the hip and knee may flex but the foot may not dorsiflex, or vice versa

Clonus requires an appro- priate degree of muscle relaxation, integrity of the spinal stretch reflex mechanisms, sustained hyperexcitability of alpha and gamma motor neurons (suprasegmental effects), and synchronization of the contraction–relaxation cycle of muscle spindles.

The hyperreflexic state that characterizes spasticity may take the form of clonus, a series of rhythmic involun- tary muscular contractions occurring at a frequency of 5 to 7 Hz in response to an abruptly applied and sustained stretch stimulus

Spread, or radiation of reflexes, is regularly associated with spasticity, although it may be observed to a slight degree in normal persons with brisk tendon reflexes. Tap- ping of the radial periosteum, for example, may elicit a reflex contraction not only of the brachioradialis but also of the biceps, triceps, or finger flexors. This spread of reflex activity is probably not the result of radiation of impulses in the spinal cord, but a result of the propagation of a vibra- tion wave from bone to muscle, stimulating the excitable muscle spindles in its path (Lance).

Other manifestations of the hyperreflexic state are the Hoffmann sign and the crossed adductor reflex of the thigh muscles. Also, reflexes may be “inverted,” as in the case of a lesion of the fifth or sixth cervical segment. Here the biceps and brachiora- dialis reflexes are abolished and in response to a tap over the distal radius or the biceps tendon, only the remaining triceps and finger flexor reflex arcs are engaged

With bilateral cerebral lesions, exaggerated stretch reflexes may be elicited in cranial as well as limb and trunk muscles because of interruption of the corticobulbar pathways. These are seen as easily triggered masseter con- tractions in response to a brisk downward tap on the chin (“jaw jerk”) and brisk contractions of the orbicularis oris muscles in response to tapping the philtrum or corners of the mouth. In advanced cases, weakness or paralysis or slowness of voluntary movements of the face, tongue, larynx, and pharynx are added (bulbar spasticity or “pseu- dobulbar” palsy; see also Chap. 24)

Pause and colleagues have described the motor disturbances caused by lesions of the parietal cor- tex. The patient is unable to maintain stable postures of the outstretched hand when his eyes are closed and cannot exert a steady contraction. Exploratory movements and manipulation of small objects are impaired, and the speed of tapping is diminished.

Posterior parietal lesions (involving areas 5 and 7 in Fig. 3-3) are more detrimental in this respect than anterior ones (areas 1, 3, and 5), but both regions are affected in patients with the most severe deficits

The term apraxia denotes a disorder in which an attentive patient loses the ability to execute previously learned activities in the absence of weakness, ataxia, sensory loss, or extrapyramidal derangement that would be adequate to explain the deficit. All the elements of the activity may be demonstrated in circumstances other than in response to the command to execute the activity or ges- ture. This was the meaning given to apraxia by Liepmann, who introduced the term in 1900, and discussed further by Denny-Brown in 1958

Lesions of the frontal lobes have the effect of impeding the organization of motor sequences in the contralateral limbs so that a complex activity will not be initiated or sustained long enough to permit its completion, or it may be performed awkwardly. However, clinical and functional imaging data indicate that planned or commanded action is formulated not in the frontal lobe, where the impulse to action arises, but in the parietal lobe of the language-dom- inant hemisphere, where visual, auditory, and somesthetic information is integrated.

The failure to conceive or formulate an action to command was referred to by Liepmann as ideational apraxia. Sensory areas 5 and 7 in the dominant parietal lobe, the supplementary and premotor cortices of both cerebral hemispheres and their integral connections are involved collectively to accomplish these actions.

In ideomotor apraxia, the patient may know and remem- ber the planned action, but because these areas or their connections are interrupted, he cannot actually execute it with either hand. Certain tasks are said to differentiate ideomotor from ideational apraxia, as discussed further on, but the distinction may be quite subtle. Nonetheless, ideational apraxia has been said to be characterized by difficulty in “what to do,” whereas ideomotor apraxia is a block in “how to do” as a result of an inability to transmit the gesture to executive motor centers

A third disorder, opaque to many neurologists, is limb-kinetic apraxia (or kinetic-limb apraxia). It is a clum- siness and maladroitness that is the result of an inability to fluidly connect or to isolate individual movements of the hand and arm as described by Kleist. In the originally conceived form, a hand displays awkwardness that is dis- proportionate to weakness or sensory loss, yet gestures and complex movements can be accomplished, unlike the case in ideomotor apraxia

The term limb-kinetic apraxia has also been applied to cases of paralysis that obscures the apraxia on one side but causes a breakdown of fine finger movements on the opposite side. This is more properly termed “sympathetic apraxia.” In particular, in a right-handed person, a lesion in the left frontal lobe that includes Broca’s area, the left motor cortex, and the deep underlying white matter may cause left-limb apraxia. Clinically, there is a nonfluent aphasia, a right hemiparesis, and clumsiness of the non- paralyzed left hand

Of a somewhat different nature is an oral-buccal- lingual apraxia, which is probably the most commonly observed of all apraxias in practice. It may occur with lesions that undercut the left supramarginal gyrus or the left motor association cortex and may or may not be associated with the apraxia of the limbs described above. Such patients are unable to carry out facial movements on command (lick the lips, blow out a match, etc.), although they may do better when asked to imitate the examiner or when confronted with a lighted match. With lesions that are restricted to the facial area of the left motor cortex, the apraxia will be limited to the facial musculature bilat- erally and may be associated with a verbal apraxia or corti- cal dysarthria (namely, Broca’s aphasia, see Chap. 22)

In the authors’ opinion, the time-honored division of apraxia into ideational, ideomotor, and kinetic types is not entirely satisfactory because of the difficulty separating them in practice. We have sometimes been unable to con- fidently separate ideomotor from ideational apraxia. The patient with a severe ideomotor apraxia nearly always has difficulty at the ideational level and, in any case, similarly situated left parietal lesions give rise to both types. Fur- thermore, in view of the complexity of the motor system, we have frequently been uncertain whether the clumsiness or ineptitude of a hand in performing a motor skill repre- sents a kinetic apraxia or some other subtle fault in hand control by the corticospinal or one of the other parallel motor systems

Testing for apraxia is carried out in several ways. First, one observes the actions of the patient as he engages in simulated tasks as dressing, washing, shaving, and using eating utensils. Second, the patient is asked to carry out familiar symbolic acts—wave goodbye, salute the flag, shake a fist as though angry, or blow a kiss. If he fails, he is asked to imitate such acts made by the examiner. Finally, he is asked to show how he would hammer a nail, brush his teeth, take a comb out of his pocket and comb his hair, or to execute a more complex act, such as lighting and smoking a cigarette or opening a bottle of soda, pouring some into a glass, and drinking it. These latter actions, involving more complex sequences, are said to be tests of ideational apraxia; the simpler and familiar acts are considered tests of ideomotor apraxia

Muscle spindles maintain their sensitivity to muscle length, even when the muscle is contracting to a shorter length, via a mechanism known as alpha-gamma coactivation. As the alpha motoneurons stim- ulate the extrafusal muscle fibers to contract, simultaneously the gamma motoneurons stimulate the contractile portions of the intrafusal fibers to contract.

The corticonuclear fibers diverge from the corticospinal fibers at various brainstem levels to terminate in their target cranial nerve somatic motor or branchiomotor nuclei.

Sim- ilar to the fibers of the corticospinal tracts, some of the corti- conuclear fibers synapse directly with motoneurons, but the majority of fibers synapse with interneurons housed within the nucleus of termination or with local interneurons of the brainstem reticular formation

The corticonuclear fibers affect the mo- tor nuclei of the following cranial nerves: trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX), vagus (CN X), accessory (XI), and hypoglossal (XII)

In general, most cra- nial nerve motor nuclei receive bilateral projections from the corticonuclear tracts

The corticonuclear tract fibers do not project to the motor nuclei of the oculomotor, trochlear, and abducent nerves. These nuclei receive motor signals from the cerebral cor- tex indirectly, and do so via a different group of fibers that takes a different route.

Signals arising from the frontal and parietal motor eye fields result in conjugate eye movements away from the side of origin of the cortical signals

A number of cor- ticonuclear fibers terminate directly and bilaterally in the motor nuclei of the trigeminal and facial nerves

A number of the corticonuclear fibers terminate in the hypoglossal nuclei that house the lower motoneurons that innervate the tongue musculature. The corticonuclear fibers project mainly contralaterally on the lower motoneurons that innervate the genioglossus muscle, whereas the remaining lower motoneurons that innervate all other tongue mus- culature receive bilateral projections (with a contralateral predominance)

A small bundle of corticonuclear fibers, Pick’s bundle, proceeds inferiorly along with the corticospinal tract to the level of the pyramidal decussation where they cross, then recur and ascend to terminate in the nucleus ambiguus of the opposite side

The deep layers of the superior colliculus gives rise to the fibers of the tectospinal tract (Fig. 13.14; Table 13.1), which decussates at the level of the red nucleus in the mid- brain, and then descends to the medulla, in the medial lon- gitudinal fasciculus. The tectospinal fibers continue their descent in the anterior funiculus of the spinal cord to end at cervical and upper thoracic spinal cord levels where they synapse with interneurons. The tectospinal tract is involved in the coordination of head movements with eye movements elicited by visual, auditory, or vestibular stimuli.

Still other rubrobulbar fibers terminate in the lateral reticular nucleus, as well as the nucleus gracilis and nucleus cuneatus in the medulla. The fibers that terminate in the nuclei gracilis and cuneatus modulate the transmission of afferent sensory information to the spinal cord via presyn- aptic inhibition

The red nucleus facilitates the alpha, beta, and gamma motoneurons that innervate the contralateral upper limb flexor muscles, whereas it simultaneously inhibits those of the extensors, specifically the nerve cells that innervate the distal muscles of the upper limbs. This facilitation and inhibition is medi- ated by the rubrospinal tract terminating in the spinal cord, and by the rubrobulbar tract, which terminates in the flexor region of the medullary reticular formation. The rubrospinal tract (along with the corticospinal tract) functions in con- trolling the movement of the hand and digits, by facilitating flexor muscle tone and inhibiting the extensor musculature of the upper limb

Neurons in the pontine reticular nuclei (reticularis pontis oralis and caudalis) give rise to fibers that form the medial (pontine) reticulospinal tract, which descends ipsilaterally in the anterior funiculus of the spinal cord. These fibers terminate and synapse with spinal cord interneurons and gamma motoneurons at all levels of the spinal cord. The pontine reticular fibers stimulate extensor muscle and inhibit flexor muscle movements

The lateral (medullary) reticulospinal tract arises from the nucleus reticularis gigantocellularis and descends bilat- erally in the lateral funiculus of the spinal cord. Nerve fibers terminate at all spinal cord levels, where they synapse mainly with interneurons in the intermediate zone gray matter of the spinal cord. The medullary reticular fibers have an inhibitory effect on extensors and an excitatory affect on flexors. The lateral reticulospinal tract also relays an autonomic input to the sympathetic and parasympathetic neurons of the spinal cord, which mediate autonomic functions such as pupillary dilation, heart rate modulation, and sweating (Fig. 13.15)

The lateral vestibulospinal tract arises mainly from the lateral vestibular nucleus (Fig. 13.16). This tract con- tains ipsilateral fibers that descend in the anterior funiculus to end at all spinal cord levels, to synapse mainly with excit- atory interneurons. These interneurons stimulate motoneu- rons that innervate axial (trunk) and proximal limb extensor muscles, and simultaneously inhibit lower motoneurons that innervate limb flexor muscles. This tract is involved in the maintenance of posture and balance by specifically facili- tating motoneurons that innervate the antigravity muscles (extensor muscle tone of the antigravity muscles). This tract also mediates head and neck movements in response to ves- tibular sensory input

The medial vestibulospinal tract arises from the medial vestibular nucleus (Fig. 13.16). This tract descends mostly ipsilaterally in the descending medial longitudinal fascic- ulus of the brainstem and then in the anterior funiculus of the cervical and upper half of the thoracic spinal cord. Its fibers synapse with interneurons that synapse with alpha and gamma motoneurons. Some fibers terminate on alpha motoneurons directly. These fibers exert their influence on neurons of the cervical spinal cord (maintaining equilibrium elicited by vestibular input) and mediating head movement while maintaining gaze fixation on an object.

The descending motor pathways are classified into three func- tional categories: the ventromedial (anteromedial) group, the lateral group, and the cortical group. In general, the fibers of each of these functional groups synapse in the gray matter that is in close proximity to their position in the white mat- ter

Furthermore, the lower motoneurons that inner- vate the flexor muscles occupy a region of gray matter that is posterior to the lower motoneurons innervating the extensor muscles

The ventromedial group consists of the anterior cortico- spinal tract, the medial and lateral vestibulospinal tracts, the medial and lateral reticulospinal tracts, and the tectospinal tract, which are all located in the anterior funiculus of the spinal cord and synapse in the medial aspect of the anterior horn and intermediate zone (Figs 13.9, 13.17)

The lateral group consists of the lateral corticospinal and the rubrospinal tracts, located mostly in the lateral funiculus of the spinal cord. The fibers of these two tracts synapse in the lateral aspect and intermediate zone of the ventral horn of the spinal cord (Figs 13.10A, 13.17)

The cortical group consists of lateral corticospinal tract fibers that synapse directly with lower motoneurons, partic- ularly the neurons whose fibers innervate the distal muscles of the limbs, such as the intrinsic muscles of the hand

A lesion involving the medullary pyramid may damage not only corticospinal and corticonuclear fibers, but also reticulospinal fibers.

The clasp-knife response is believed to be caused by activation of the Golgi tendon organs (GTOs) as muscles are stretched during the limb manipulation. This in turn stimulates the GTOs Ib sensory afferent fibers that relay excitatory messages to inhibitory spinal cord interneurons. These interneurons inhibit the alpha motor neurons that overstimulated the skeletal muscles to create the hypertonia and increased resistance to passive stretch

Hypertonia, hyperreflexia and spasticity are not well understood. There are several hypotheses that have offered an explanation of the cause of spasticity and related hypertonia and hyperreflexia

One hypothesis suggests that an upper motor neuron (UMN) lesion diminishes or eliminates descending inhibitory influences on the dynamic gamma motor neurons. This in turn causes overactivity of the gamma motor neurons resulting in contraction of the contractile portions of intrafusal muscle fibers and stretching of the central noncontractile portion of the intrafusal fibers. This overstimulates the type Ia annulospiral afferent fibers from the muscle spindles whose central processes synapse with alpha motor neurons, which are overstimulated causing an abnormal increase in skeletal muscle tone.

Another hypothesis suggests that an UMN lesion may result in diminution of cortical stimulation of spinal cord inhibitory interneurons, resulting in hypertonia and spasticity

Cortical UMN axons stimulate an inhibitory interneuron, the Renshaw cell. This interneuron is also stimulated by a collateral branch of a lower motor neuron and in turn inhibits the lower motor neuron. These neural connections inhibit the reflex stimulation of antagonist muscles when the agonists are contracting. An UMN lesion involving cortical neurons would eliminate the inhibitory effect on antagonist muscles. This causes alternating contraction of agonists and antagonist muscles known as clonus which accompanies spasticity and hyperreflexia

Normally, stimulation of the frontal eye field results in deviation of both eyes toward the contralateral side. A unilateral lesion in the frontal eye field (Brodmann’s area 8) results in both eyes deviating to the side ipsilateral to the lesion. In addition, the affected individual will be unable to turn the eyes contralateral to the lesion. The effects of frontal eye field lesions are not permanent.

Two principal types of α motor neurons are recognised: tonic and phasic. Tonic α motor neurons innervate slow, oxidative–glycolytic (red) muscle fibres; they are readily depolarised and have relatively slowly conducting axons with small spike amplitudes. Phasic α motor neurons innervate fast, oxidative–glycolytic (white) muscle fibres. The phasic neurons are larger, have higher thresholds, and have rapidly conducting axons with large spike amplitudes. Tonic neurons are usually the first recruits when voluntary move- ments are initiated, even if the movement is to be fast

Renshaw cells The axons of the α motor neurons give off recurrent branches, which form excitatory cholinergic synapses upon inhibitory interneurons called Renshaw cells in the medial part of the ventral horn. The Renshaw cells form inhibitory, glycinergic synapses upon the α motor neurons. This is a classic example of negative feedback,orrecurrent inhibition, through which the discharges of α motor neurons are self-limiting (cf. Clinical Panel 8.1)

At each segmental level, α motor neurons receive powerful inputs from muscle spindles, Golgi tendon organs, and joint capsules. Note that any inhibitory effect produced by activity in dorsal nerve root fibres requires interpolation of inhibitory interneurons because all primary afferent neurons are excitatory in nature

Half of these fibres that do not decussate enter the ventral corticosp- inal tract/anterior corticospinal tract , which occupies the ventral/ anterior funiculus at cervical and upper thoracic levels. These fibres cross in the white commissure and supply motor neurons serving muscles in the anterior and posterior abdominal walls

A unique property of these corticomotoneuronal fibres of the lateral corticospinal tract is the concept of fractionation, relating to the vari- able activity of interneurons, whereby small groups of neurons can be selectively activated to perform a specific function. It is most obvious in the case of the index finger, which can be flexed or extended quite inde- pendently, although three of its long tendons arise from muscle bellies devoted to all four fingers. Fractionation is essential for the execution of skilled movements such as buttoning a coat or tying shoelaces. Follow- ing damage to the corticomotoneuronal system anywhere from the motor cortex to the spinal cord, skilled movements are lost and seldom recover completely

Cocontraction is achieved by the inactivation of Ia inhibitory interneurons by Renshaw cells

In the intermediate grey matter and the base of the ventral horn, motor neurons supplying axial (vertebral) and proximal limb muscles are mainly recruited indirectly by the lateral corticospinal tract, by way of excitatory interneurons

Also located in the intermediate grey matter are the Ia inhibitory interneurons, and these are the first neurons to be activated by the lateral corticospinal tract during voluntary move- ments. Activity of the Ia interneurons causes the antagonist muscles to relax before the prime movers (agonists) contract. In addition, it renders the antagonists’ motor neurons refractory to stimulation by spindle affer- ents passively stretched by the movement

In the dorsal grey horn there is some suppression of sensory transmission into the spi- nothalamic tract during voluntary movement. This is brought about by the activation of inhibitory interneurons synapsing upon primary afferent nerve terminals. Modulation is more subtle at the level of the gracile and cuneate nuclei, where pyramidal tract fibres (after crossing) are capable of either enhancing sensory transmission during slow, exploratory movements or reducing it during rapid movements

The pontine reticulospinal tract descends ipsilaterally in the ante- rior funiculus, and the medullary reticulospinal tract descends, partly crossed, in the lateral funiculus (Figure 16.9). Both tracts act, via inter- neurons shared with the corticospinal tract, upon motor neurons sup- plying axial (trunk) and proximal limb muscles. Information from animal experiments indicates that the pontine reticulospinal tract acts upon extensor motor neurons and the medullary reticulospinal tract acts upon flexor motor neurons. Both pathways exert reciprocal inhibition

Locomotion is initiated from a locomotor centre located in the lower midbrain of humans and in the pons in laboratory animals. In anaesthetised cats, electrical stimulation of the locomotor centre with pulses of increasing frequency produces walking movements, then trot- ting, and finally galloping

Although the basic locomotor patterns are inbuilt, they are modu- lated by sensory feedback from the terrain. Overall control of the motor output resides in the premotor cortex, which has direct projections to the brainstem neurons that give rise to the reticulospinal tracts

Human locomotion is less ‘spinal’ than that of quadrupeds. However, the general neuroanatomic framework has been conserved during higher evolution, and the basic physiology seems to be in place as well. In par- ticular, a bilaterally organised motor system controlling proximal and axial muscles must exist to account for the return of near-perfect loco- motor function following removal of an entire cerebral hemisphere dur- ing childhood or adolescence. Such people never recover manual skill on the contralateral side, and this reinforces the belief among physical ther- apists that two distinct pathways are involved in motor control: pyrami- dal and ‘extrapyramidal’. The latter term denotes the reticulospinal tract and its controls upstream in the cerebral cortex and basal ganglia

Definitions of posture vary with the context in which the term is used. In the general context of standing, sitting, and recumbency, posture may be defined as the position held between movements. In the local context of a single hand or foot, the term signifies postural fixa- tion—the immobilisation of proximal limb joints by cocontraction of the surrounding muscles, leaving the distal limb parts free to do volun- tary business. As will be noted in Chapter 29, there is reason to believe that the human premotor cortex is programmed to select appropriate proximal muscle groups by way of the reticulospinal tracts, to set the stage for any particular movement of the hand or foot

The interpolation of interneurons between the two main motor pathways acting upon motor neurons serving axial and proximal limb muscles means that either pathway may be in command for a particular movement sequence—the extrapyramidal (reticulospinal) pathway for routine tasks such as walking along a clear path and the pyramidal pathway for tasks requiring close attention such as picking one’s way along a path strewn with rubble

The tectospinal tract is a crossed pathway descending from the tectum of the midbrain to the medial part of the ventral grey horn at cervical and upper thoracic levels. It is strategically placed for access to axial motor neurons (Figure 16.9). This tract is an important motor pathway in the reptilian brain, being responsible for orienting the head/trunk towards sources of visual stimulation (superior colliculus) or auditory stimulation (infe- rior colliculus). It is likely to have similar automatic functions in humans

The vestibulospinal tract is an important uncrossed pathway whereby the tone of appropriate antigravity muscles is automatically increased when the head is tilted to one side. It descends in the anterior funiculus (Figure 16.9), and its function is to keep the centre of gravity between the feet. It originates in the vestibular nucleus in the medulla oblongata. (Note: As explained in Chapter 19, there are in fact two vestibulospinal tracts on each side. The unqualified term refers to the lateral vestibu- lospinal tract.)

The raphespinal tract originates in and beside the raphe nucleus sit- uated in the midline in the medulla oblongata. It descends on both sides within the dorsolateral tract of Lissauer. Its function is to mod- ulate sensory transmission between first-order and second-order neu- rons in the dorsal grey horn—particularly with respect to pain (see Chapter 24)

Note on the rubrospinal tract In humans, the rubrospinal tract is one of several motor control path- ways and it has fewer axons than the corticospinal tract. The tract is thought to be responsible for large muscle movement, and it extends primarily into the cervical spinal cord, suggesting that it functions in upper limb, but not in lower limb, control. It is believed to primarily facilitate extensor motor neurons and inhibit flexor motor neurons in the upper extremities. It is small and rudimentary in humans, but in some primates over time, the rubrospinal tract can assume almost all the duties of the corticospinal tract when the corticospinal tract is cut

After several days or weeks, some return of voluntary motor function can be expected. At the same time, muscle tone increases progressively. The typical long-term effect on muscle tone is one of spasticity, with abnormally brisk reflexes (hyperreflexia). Classically, spasticity in the leg is ‘ claspknife’ in character; after initial strong resistance to passive flexion of the knee, the joint gives way

As illus- trated in Clinical Panel 35.3, the spasticity following a stroke characteristically affects the antigravity muscles. In the lower limb, these are the extensors of the knee and the plantar flexors of the foot; in the upper limb, they are the flexors of the elbow and of the wrist and fingers. Following complete transection of the spinal cord, on the other hand, there may be a paraplegia in flexion of the lower limbs, owing to concurrent interruption of the vestibulospinal tract (Clinical Panel 16.3)

The ‘positive’ signs listed in 2, 3, and 4 above cannot be explained on the basis of interruption of the corticospinal tract alone. In the rare cases in which the human pyramid has been transected surgically, spasticity and hyperreflexia have not been prominent later on, although a Babinski sign has been present

Spasticity and hyperreflexia are largely explained by the fact that stretch reflexes in spastic muscle groups are hyperactive. Electromyography (EMG) records of spastic muscles show enhanced motor unit activity in response to rel- atively slow rates of stretch, such as slow passive elbow extension. However, this is not the sole basis of explanation. In patients with spastic hemiparesis, the ankle flexors show increased tone (resistance to passive dorsiflexion) even with very slow rates of stretch—too slow to elicit any EMG response. The resis- tance takes several weeks to become pronounced. It is called passive stiffness and may be caused by progressive accumulation of collagen within the muscles affected. In addition, biochemical changes within paretic muscle lead to increas- ing change of fast-twitch to slow-twitch fibres, accounting for progressively greater difficulty in execution of rapid movements

Wasting. Wasting is not merely disuse atrophy, but it results from loss of a trophic (nourishing) factor produced by motor neurons and conveyed to muscle by axonal transport

Fasciculations, which are visible twitches of small groups of muscle fibres in the early stage of wasting. They arise from spontaneous discharge of motor neurons with activation of motor units, as described in the context of EMG in Clinical Panel 12.14.

Spinal shock is currently attributed to a generalised hyperpolarisation of spinal neurons below the level of the lesion, perhaps because of large-scale release of the inhibitory transmitter glycine. In addition, the patient develops postural hypo- tension when raised from the recumbent position, owing to interruption of the baroreceptor reflex. (Wearing an abdominal binder may be sufficient to compen- sate for the lost reflex.)

If extensor spasticity in the lower limbs is dominant, the patient develops para- plegia in extension; if flexor spasticity is dominant, the patient develops paraple- gia in flexion. An extended posture may permit spinal standing; it is promoted by appropriate passive placement of the limbs, and it is the rule following cord injury that is either incomplete or low. A flexed posture is promoted by repetitive mass flexor reflexes involving the ankles, knees, and hips; mass reflexes can follow any cutaneous stimulation of the legs if the flexor reflex interneurons of the cord are already sensitised by afferent discharges from a pressure sore or from an infected bladder