Movement Disorders in Children & Adolescents GP Mathur, Sarla Mathur
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Movement and Its Control1

Sarla Mathur,
GP Mathur
Movement and its control is carried out through the integrated approach of pyramidal system, extrapyramidal system, cerebellum, vestibular fibers of eight cranial nerve, sensory system reflex activity and receptors.1-5 Movements are achieved by contraction and relaxation of skeletal muscles.
 
MOTOR UNIT
The motor unit is the final common pathway through which spinal, cerebral and cerebellar control systems act to co-ordinate movement. The motor unit (Fig. 1.1) comprises a spinal anterior horn cell (or cranial nerve motor neuron), its motor axon and the muscle fibers it supplies. The number of muscle fibers innervated by one axon varies from 10 to 1000 or more in different muscles. The motor axons terminate in specialized end plates on the muscle surface. There is release of acetylcholine at the end plate on the nerve activity, which causes depolarization of the fiber and activation of the contractile process. There are two main types (slow and fast) of muscle fibers. Most muscles contain a mixture though one type may predominate (Table 1.1).
Table 1.1   Main type of muscle fibers
Type
Contraction
Fatigue
Enzymes
Nerve activity
I
Slow
Resistant
Oxidative
Tonic
II a
Fast
Resistant
Glycolytic+Oxidative
Phasic
II b
Fast
Prone
Glycolytic
Phasic
The activity of motor units is governed by both local spinal reflex activity and by descending pathways from the cerebrum and cerebellum. Spinal reflexes may be monosynaptic as in the stretch reflex or polysynaptic as with flexor withdrawal and extensor plantar responses. Damage of the motor unit results in weakness or paralysis of muscles (Table 1.2).
 
SUPRASPINAL MECHANISM
The important supraspinal mechanism involved in movement control are the pyramidal, extrapyramidal and the cerebellar systems. These descending pathways interact via excitation and inhibition of the motor neurons and with one another within the brain.
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Fig. 1.1: Diagram showing essential elements of a reflex arc. (R) Receptor represented by a free nerve ending in the skin; (AFF. C.) Afferent conductor; (EFF.C.) Efferent conductor; (E) Effector represented by a striated muscle fiber; (INT.) Intercalated neuron
Table 1.2   Signs of damage to the motor unit (Lower motor neuron)
  • Weakness or paralysis of muscles supplied by affected motor neuron or axons
  • Hypotonia
  • Reduced or absent tendon reflexes
  • Flexor or absent plantar response
  • Wasting and fasciculation of affected muscle
 
PYRAMIDAL SYSTEM
The neurons of the pyramidal tract lie in the cerebral cortex just anterior to the central sulcus, the precentral gyrus. Fibers for the pyramidal cell lie close together in the internal capsule, which passes close to the thalamus and basal ganglia deep in the cerebral hemisphere, and descend into the brain stem. As these pyramidal fibers descend in the brainstem, they innervate contralateral cranial nerve motor nuclei and send fibers to the cerebellum via the middle cerebellar peduncle. At the lower level of the medulla, the tract decussates, most of its fibers crossing over and descending the contralateral side of the spinal cord. The pyramidal fibers have both excitatory and inhibitory effects on the anterior horn cells. The specific area of the precentral gyrus control movements of different parts of the contralateral side of the body. The larynx and pharynx are represented at the most inferior end of the gyrus, followed by relatively large contiguous areas for movements of the face, mouth and hand. The trunk is represented by a smaller area near the midline and the foot on the medial aspect of the hemisphere. The pyramidal tract lesion causes increased muscle tone of spastic type, hyperactivity of the tendon (stretch) reflexes and extensor plantar response in addition to causing weakness and impairment of the movements (Table 1.3). These changes occur due to the removal of inhibitory effects of the pyramidal system on spinal neurons. The distribution of the weakness and the presence or absence of cranial nerve deficits are helpful in localizing the site of a lesion involving the pyramidal tract (Table 1.4).
Table 1.3   Signs of pyramidal tract lesions
  • Loss of voluntary movements
  • Impaired fine movements
  • Increased muscle tone (spasticity)
  • Exaggerated tendon reflexes and clonus
  • Extensor plantar response
  • Little or no muscular wasting
  • Absent abdominal reflexes
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Table 1.4   Movements affected in pyramidal tract lesions
Most affected
Least affected
Upper limb
Shoulder abduction
Elbow extension
Finger extension
Finger abduction
Shoulder adduction
Elbow flexion
Wrist flexion
Finger flexion
Lower limb
Hip flexion
Knee flexion
Foot dorsiflexion
Foot eversion
Hip extension
Knee extension
Foot plantarflexion
Foot inversion
The motor cortex is considered a storehouse of basic instructions for eliciting movements. Activation of the motor cortex causes several muscles to interact to produce movements. Whereas, motor unit activation of the spinal or peripheral nerve level causes only single muscle to contract. Lesion to the pyramidal tract causes weakness or paralysis of voluntary movements on the contralateral side of the body. If the damage is in the cortex, weakness may be limited to one limb or part of it. But if the lesion is at a site where the pyramidal fibers are closely compacted (e. g. internal capsule), then weakness of the whole of one side of the body (hemiparesis or hemiplegia) results. Lesions to the pyramidal tract in the brainstem result in cranial nerve deficit(s) with contralateral hemiparesis. Damage to the pyramidal tract below the medulla causes ipsilateral weakness. Pyramidal tract lesions in the spinal cord cause weakness below the medulla causes ipsilateral weakness. Pyramidal tract lesions in the spinal cord cause weakness below the lesion, which is often bilateral.
 
EXTRAPYRAMIDAL SYSTEM
Neurons and fibers of extrapyramidal system have reciprocal connections with the cerebral cortex, thalamus, cerebellar and brainstem nuclei and spinal cord. The main collections of neurons are the caudate nucleus, putamen, globus pallidus, substantia nigra, and subthalamic and red nuclei. Varieties of movement disorders result depending upon the site of lesion. The role of extrapyramidal system is the activation of voluntary movements programmed in the motor cortex, and also in the involuntary adjustments of posture and muscle tone which enabled willed movements to take place. Lesions of the extrapyramidal system may cause various problems depending on their site (Table 1.5).
Table 1.5   Clinical features of extrapyramidal lesions
Signs
Usual site of lesion
Resting tremor
Substantia nigra, red nucleus
Muscular rigidity
Substantia nigra, putamen
Hypokinesia
Substantia nigra, putamen, globus pallidus
Chorea
Caudate nucleus
Hemiballismus
Subthalamic nucleus
Dystonia, athetosis
Putamen
Slowness in initiating movements 4(hypokinesis) and difficulty in fine tasks are common, but muscular weakness is absent. Tendon reflexes are usually normal and plantar reflexes are flexor. Muscle tone may be either decreased (e.g. Chorea) or increased in either smooth fashion (lead pipe) or have a phasic component as in the cogwheel rigidity of parkinsonism.
 
CEREBELLUM
The cerebellum is concerned with the control of voluntary movements and the maintenance of posture and balance (Table 1.6). The vermis and the anterior lobe are concerned chiefly with maintenance of posture and balance, while the posterolateral lobes are responsible for coordination of the limbs. The lateral parts of the cerebellum interconnect with contralateral thalamus and cerebral cortex thus controlling movements of the ipsilateral limbs. It is closely connected to the vestibular system, and receives further proprioceptive input from the spinocerebellar tracts of the spinal cord, which sends fibers through the inferior cerebellar peduncle. Information about cortical motor instructions is relayed from the pyramidal tracts via the middle cerebellar peduncle to the cerebellum. The outputs are mainly through the superior cerebellar peduncle to the ventrolateral thalamus and then to the cerebral cortex, and also to the spinal motor neurons via the reticular formation, red nucleus and vestibular nuclei.
 
EIGHTH (VESTIBULOCOCHLEAR) CRANIAL NERVE
This nerve consists of two sets of fibers. One supplies the cochlea and subserves hearing. The other (vestibular fibers) supplies the labyrinth and semicircular canals and subserves equilibrium, balance and sensations of bodily displacement. The vestibular fibers originate in the vestibular ganglion and terminate in a group of nuclei in the pons and medulla. The vestibular nerve is closely connected with the cerebellum. It also has a cerebral projection to the temporal lobe.
Table 1.6   Signs of lesions of cerebellum
  • Incoordination (ataxia)
  • No paralysis
  • Tendon reflexes —not increased
 
SENSORY SYSTEM
Superficial sensations of touch, warmth, cold, pain and deep sensations of position, pressure and pain, begin peripherally in specialized receptor organs which transduce physical modalities into sensory nerve impulses. Sensory nerve fibers are elongated processes of the bipolar neurons of the dorsal root ganglions; their central processes pass in the dorsal root to the spinal cord and either synapse there or pass directly towards the brain. Two main relay systems for sensory information, are the lemniscal and the spinothalamic. The lemniscal system is concerned with the transmission of proprioceptive and well-localized touch information. Modalities such as joint position, two-point discrimination and vibration sense passes through this pathway. The peripheral fibers are large, myelinated and fast conducting. They enter the dorsal horn of the spinal cord and without synapsing pass up the dorsal (posterior) columns to the gracile and cuneate nuclei in the lower medulla. Here they synapse and second order fibers cross the midline and ascend the brainstem in the medial lemniscus 5and then to the thalamus. The sensations from the lower limb goes in the nucleus gracilis and of the upper limb in the nucleus cuneatus and are external to the sensory fibers coming from the lower limb.
The spinothalamic system is the pathway for pain, temperature and poorly localized touch. The peripheral fibers for these modalities are smaller, slower conducting and sometimes unmyelinated axons. They enter the spinal cord in the dorsal root, and synapse in the dorsal horn. Most of the second order fibers cross the midline of the cord over the distance of several segments and ascend in the contralateral spinothalamic tract, joining the medial lemniscus fibers to synapse in the thalamus. The spinothalamic tract fibers from the lower limb come to lie outermost and those from the upper limb innermost. Both the main sensory pathways synapse in the ventroposteriolateral nucleus of the thalamus. The post-synaptic fibers lie closely packed together in the internal capsule, immediately posterior to the pyramidal tract fibers. They project up to the post-central gyrus (primary sensory cortex) in a topographical arrangement, similar to that on the motor cortex. Some appreciation of sensation and pain probably occurs at the thalamus, but the sensory cortex is necessary for localization of sensations from different parts of the body and provides the accuracy needed for two-point discrimination and joint position sense. Further analysis goes on in the adjoining parietal lobe, which is responsible for the perception of pattern, shape, size, texture and weight. Collateral fibers from the ascending sensory pathway also make contact with the reticular formation. This is a chain of interconnecting short fiber neurons, which lie in the centre of the brainstem and projects up to the midline thalamus nuclei, which in turn send fibers widely over the cortex. These non-specific projections (the reticular activating system) are important for maintaining awareness.
 
REFLEX ACTIVITY
Reflex represents the simplest from of integrated activity in the nervous system. The functional components of a reflex arc are a peripheral receptor, a sensory nerve (or autonomic afferent), a central neuron or chain of neurons (spinal, cranial or autonomic), a motor nerve or autonomic efferent and an effective organ (skeletal, smooth muscle, gland). The activity of a given reflex may be altered by local descending neural influences acting on the central neurons. For example tendon reflexes become exaggerated when there is a damage to the pyramidal tract, depressed when there is a cerebellar lesion. Many reflexes involve not only do activation of an effected muscle but also do relaxation of antagonists. Reflexes can be conveniently divided into monosynaptic (or oligosynaptic) and polysynaptic types.
 
Monosynaptic or Oligosynaptic Reflexes
Monosynaptic or oligosynaptic reflexes are the tendon stretch reflexes (e.g. knee, ankle, biceps, triceps and supinator jerks) and cranial nerve reflexes (e.g. jaw jerk, corneal reflex). A tendon stretch reflex serves to maintain muscle tone and to adjust it during movement. By eliciting tendon reflexes, one can test the integrity of the reflex arc components at different spinal levels and can also judge changes in the descending (especially pyramidal) systems.
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Polysynaptic Reflexes
Polysynaptic reflexes are plantar reflex, flexion withdrawal reflex and abdominal reflexes. These polysynaptic reflexes involve chains of interconnected neurons within the spinal cord which activate several muscle groups to produce coordinated movements in response to usually noxious stimuli.
 
RECEPTORS
Receptors concerned with locomotor system are exteroceptors and proprioceptors.
 
General or Cutaneous Sense Organs
They respond to external stimuli and are at or close to surfaces and are known as exteroceptors. The general sensory receptors include the ‘free’ and the encapsulated terminals in skin and near hairs.
Table 1.7   Receptors concerned with locomotor system
  • Exteroceptors
    • (i) General or cutaneous sense organs
  • Proprioceptors
    • (i) Neurotendinous organs of Golgi
    • (ii) Neuromuscular spindles
    • (iii) Pacinian corpuscles
    • (iv) Other endings in joints
    • (v) Vestibular receptors system
 
Proprioceptors
They respond to stimuli proper to deeper tissues, especially of the locomotor system and are concerned with detecting movements, mechanical stresses and position. They include the neurotendinous organs of Golgi, neuromuscular spindles, pacinian corpuscles, other endings in joints and vestibular receptors. Proprioceptors are stimulated by the contraction of muscles, movements of joints and changes in the position of the body or of its parts. They are essential to the coordination of muscles, the grading of muscular contraction and maintenance of equilibrium.
 
PHYSIOLOGICAL RESPONSES IMPORTANT IN STANDING AND WALKING
The upright posture is maintained from actions of a number of postural reflex responses (Table 1.8).
Table 1.8   Postural reflex responses
  • Local static reactions acting on individual limbs-stretch reflex and positive supporting reaction
  • Segmental static reactions linking the extremities together — crossed extension reflex and interlimb coordination
  • General static reactions resulting from the position of the head in space tonic neck and labyrinthine reflexes (function together), righting reflex, placing and hopping reactions
 
CLINICAL APPROACH TO A CHILD WITH DISORDERS OF EQUILIBRIUM AND GAIT
When evaluating a child with disorder of gait the physician should enquire the following questions:
  • Whether the disturbance occurs more in the dark than in the light?
  • Whether vertigo, giddiness or light-headedness accompanies the disorder?
  • Whether there is pain, tenderness or tingling of the limbs?
  • Is there weakness, limb stiffness or rigidity?
  • 7Is there bladder and bowel dysfunction?
  • Whether there is initiation or termination of walking?
  • Whether walking up or down stairs or on uneven surfaces worsens the disturbance in walking?
Observe the gait initially as the patient comes into the examining room, when the patient is unaware that gait and stance are being examined. The physician should then observe the patient from the front, back, and sides while walking. The patient should rise quickly from a chair (or from the ground), walk normally at a slow pace, then more rapidly, and then turn around. The patient should walk on the heels, on the toes and in tandem, placing the heel of one foot directly in front of the toes of the opposite foot, attempting to go ahead in a straight line. Ask the patient to stand erect with the feet together and the head straight, first with eyes open and then with eyes closed to determine whether balance can be maintained (Romberg’s sign). Each person walks in a characteristic fashion that is often familial. A person’s gait is often a reflection of personality traits and can reflect shyness and timidity or aggressiveness or self-confidence.3,6
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