Movement Control Case File

Movement Control Case File

EUGENE C.TOY, MD, RAHUL JANDIAL, MD, PhD, EVAN YALE SNYDER, MD, PhD, MARTIN T. PAUKERT, MD

CASE 24

A 65-year-old man develops sudden weakness in his right arm and leg. His wife notes that the lower right half of his face appears to droop as well. He is taken to the emergency room, where he is noted to have a medical history significant for diabetes and poorly controlled hypertension. On physical examination he is noted to have a flaccid paralysis of the right side, and paralysis of the right lower side of his face. An MRI is immediately obtained which reveals an acute, left-sided ischemic infarct.

• Which infarcted area of the brain is responsible for the patient’s symptoms?
• What treatment is available?
• High levels of what lipid greatly increases the risk of stroke?

ANSWERS TO CASE 24: MOVEMENT CONTROL

Summary: A 65-year-old man with diabetes and poorly controlled hypertension develops acute-onset right hemiparesis and a partial right facial droop. The MRI is consistent with a left-sided ischemic stroke.

• Affected brain area responsible for patient’s symptoms: The left internal capsule contains the corticobulbar and corticospinal fibers innervating the patient’s contralateral face, arm, and leg as they travel to the brainstem.
• Treatment options: If an ischemic stroke is diagnosed within a 3-hour window, tissue plasminogen activator can be used to attempt to dissolve the clot in the blood vessels perfusing the internal capsule.
• Stroke risk factors: High levels of cholesterol increase risk for ischemic stroke. Other risk factors include advanced age, hypertension, smoking, heart disease, and diabetes.

CLNICAL CORRELATION

Cerebral stroke is characterized by the sudden loss of blood supply to an area of the brain, resulting in a loss of corresponding neurological function. The majority of strokes are ischemic, caused by a thrombosis or embolism to a cerebral blood vessel. They can present as hemorrhagic in quality as well, and an ischemic stroke can convert to a hemorrhagic strokes. The symptoms of a stroke will depend on the area of the brain that is affected by the lost blood supply. The most common location for a stroke is the posterior limb of the internal capsule which carries the descending corticospinal and corticobulbar fibers and results in purely motor symptoms.

APPROACH TO CONTROL OF MOVEMENT

Objectives

1. Know the structures of the central nervous system involved in motor control.
2. Describe the descending motor pathways and their functions.
3. Be aware of the interactions and influences of the sensory systems on motor function.

Definitions

Homunculus: A figure of the human body superimposed on the cortical surface of the brain to represent the motor or sensory regions of the body represented there.

Upper motoneurons: Upper level neurons from the cerebral cortex, cerebellum, and basal ganglia that control descending motor pathways either directly or indirectly.

Lower motoneurons: Motor neurons originating from nuclei of the spinal cord and brain stem that innervate skeletal muscle and provide the final direct link from the nervous system through the neuromuscular junctions.

Premotor cortex: An area of motor cortex, located between the primary motor cortex and the prefrontal cortex, responsible for sensory guidance of movement and control of proximal and trunk musculature of the body.

Betz cells: Large pyramidal cells located within the primary motor cortex, which give rise to the portion of the corticospinal tract which synapses directly with lower motoneurons in the anterior horn cells of the spinal cord.

Somatotopic: The maintenance of spatial organization within the central nervous system, for instance, fibers innervating the foot will travel next to fibers innervating the lower leg.

DISCUSSION

The motor system is organized in a functional hierarchy, with each level responsible for a specific task. Movement must be planned, have a purpose, respond to sensory input, and function with coordination using spatiotemporal details of muscle positions. There are several anatomical pathways that project to the spinal cord from higher motor centers. Most of these are organized somatotopically, with movements of adjacent body parts being controlled by contiguous areas of the brain at each level within the motor hierarchy.

The primary motor cortex lies in the precentral gyrus and paracentral lobule of the frontal lobe, and is responsible for controlling simple movement. It extends from the lateral fissure upward to the dorsal border of the hemisphere and beyond to the paracentral lobule. The left motor strip controls the right side of the body, and the right strip controls the left side. Neurons in the lowest lateral part of the motor strip influence the larynx and tongue, followed in upward sequence by neurons affecting the face, thumb, hand, arm, thorax, abdomen, thigh, leg, foot, and perineal muscles. The areas for the hand, tongue, and larynx are disproportionately large, given the elaborate motor control needed for these muscle groups. This functional representation is called a homunculus.

The premotor cortex lies immediately rostral to the primary motor area on the lateral surface of the hemispheres. The premotor area also contains a homunculus representation. The medial aspect contains the supplementary motor area. The postcentral gyrus and the secondary motor cortex located where the pre- and postcentral gyri are continuous at the base of the central sulcus are also cortical regions that influence movement. The frontal eye fields, located in the middle frontal gyrus, initiate voluntary saccades and contain neurons that influence eye movement.

Movements result from the actions of neuronal networks at many different levels of the nervous system. The brain stem and spinal cord contain pattern generator for rhythmic activities and complex movements such as locomotion. The descending pathways interact with and control the lower level neuronal patterns of discharge in a hierarchical manner (see Figure 24-1). The cerebral cortex can control contractions of individual muscles and can determine the force of these contractions. Populations of motor cortical neurons, however, must act together to specify the direction and force of movements. The premotor and supplementary motor areas are important in planning movements. The supplementary motor area also functions to integrate movements performed simultaneously on both sides of the body.

Figure 24-1. Schematic illustration of upper motor neuron pathways. (With permission from Adam and Victor’s Principles of Neurology. 7th ed. Figure 3-2, page 51.)

The lower motoneurons innervate skeletal muscle through neuromuscular junctions. Their cell bodies reside in the spinal cord and cranial nerve nuclei of the central nervous system, where they are influenced by the higher nervous system structures to stimulate or inhibit voluntary movement. The lower levels of the motor system coordinate simple reflexes and control the amount of force and velocity generated by a single muscle. They coordinate movements and changes in posture. The upper motoneurons technically include the cerebral cortex, cerebellum, and basal ganglia which all regulate lower motoneuron activity either directly or indirectly through interneurons. The higher levels of the motor systems are involved in more global tasks and coordinate and calculate the activity of many limbs or muscle groups and evaluate the appropriateness of a particular action.

The corticospinal tract, or pyramidal tract, controls skilled movements of the distal limbs and influences the distal flexor muscles. Onethird of the axons in the corticospinal tract originate in the primary motor cortex, one-tenth of these cells originate from Betz cells, which are large pyramidal cells located in the fifth cortical layer. Another third of the corticospinal tract axons arise from the premotor and supplementary motor regions and one-third of the fibers arise from the parietal lobe, primarily the postcentral gyrus. The areas of the cortex that contribute to the corticospinal tract are collectively termed the sensorimotor cortex. After passing through the posterior limb of the internal capsule and the middle of the cerebral pedicle, or crus cerebri, the corticospinal tract splits into bundles in the pons prior to reforming as a discrete bundle to form the medullary pyramid. Approximately 90% of the fibers cross to the other side at the level of the pyramidal tract decussation in the lower medulla and continue descending as the lateral corticospinal tract. This tract travels to all levels of the spinal cord, synapsing in the lateral aspects of laminae IV through VIII. Most of the synapses in these layers are onto inerneurons which then synapse directly onto motoneurons in lamina IX. Some fibers in the lateral corticospinal tract, however, synapse directly onto the motoneurons in lamina IX. The remaining 10% of corticospinal fibers that do not decussate in the medulla descend in the anterior funiculus of the cervical and upper thoracic cord as the ventral corticospinal tract. Most of these fibers decussate through the ventral white commissure at their level of termination prior to synapsing on interneurons and motoneurons of the contralateral side. The number of fibers in both lateral and ventral corticospinal tracts successively decreases in lower spinal cord segments as more and more fibers reach their terminations.

The corticospinal tract fibers that synapse on the interneurons of the dorsal horn influence both local reflex arcs and originating cells of ascending sensory pathways. This system allows the cerebral cortex to control reflex motor output and to modify the sensory input that reaches the brain. Cortical excitatory signals usually result from monosynaptic connections with motoneurons and are facilitated by the neurotransmitter glutamate. Inhibitory signals occur through synaptic connections in inhibitory interneurons and are mediated by glycine. The activation of the corticospinal tract generally results in excitatory input to motoneurons of flexor muscles and inhibitory input to those of extensor muscles.

The corticobulbar tract arises primarily from the ventral portion of the sensorimotor cortex on the lateral surface of the hemisphere and from the frontal eye fields. The axons diverge from the corticospinal tract at the level of the midbrain and terminate in the brain stem nuclei of cranial nerves III, IV, V, VI, VII, IX, X, XI, and XII. Fibers projecting from the frontal eye fields indirectly influence eye movements by synapsing on cells in the pontine reticular formation that then project to the nuclei of cranial nerves III, IV, and VI. The cranial nerve motor nuclei receive innervation from both cerebral hemispheres, creating symmetric movements on both sides of the face. The lower facial nucleus and hypoglossal nucleus receive far heavier innervation from the ipsilateral cortex, allowing muscles controlled by these groups (lower face and tongue) to be controlled independently on the two sides. Similar to the corticospinal tract, the corticobulbar tract contains fibers that terminate on ascending sensory neurons, allowing for mediation of sensory information from the nucleus gracilis, nucleus cuneatus, sensory trigeminal nuclei, and nucleus of the solitary tract.

The corticotectal tract contains fibers that project from cortical areas of the occipital and inferior parietal lobes to the superior colliculus, the interstitial nucleus of Cajal, and the nucleus of Darkschewitsch. Axons project from here to the pontine reticular formation and from there to the medial longitudinal fasciculus (MLF) to synapse on the oculomotor, trochlear, and abducens nuclei. This input allows for cortical influence on extraocular muscle activity. The corticotectal tract also connects with neurons in the superior colliculus that give rise to the tectospinal tract. This tract crosses at the dorsal tegmental decussation and descends to the cervical spinal cord where the fibers become incorporated with the MLF. Input from the tectospinal tract influences neurons innervating the muscles of the neck, and are concerned with reflexive turning movements of the head and eyes.

The cortical areas that give rise to the corticospinal tract also form the corticorubral tract. These neurons project to the ipsilateral red nucleus in the tegmentum of the midbrain. Neurons of the red nucleus then give rise to the rubrospinal tract, which crosses at the ventral tegmental decussation and descends through the lateral tegmentum of the pons, midbrain, and medulla. Descending just anterior to the lateral corticospinal tract, the fibers synapse in laminae V, VI, and VII of the spinal gray matter. The rubrospinal tract facilitates flexors and inhibits extensor motoneurons.

The corticoreticular fibers travel with the corticospinal and corticobulbar tracts to the reticular formation. The reticular formation of the brain stem receives sensory information from numerous systems and communicates heavily with the cerebellum and limbic systems. Two corticoreticular fibers originate from both the sensorimotor cortex, and other regions of the brain such as the medial prefrontal cortex, the limbic lobe, and the amygdala. These cortical areas integrate somatic and visceral components of complex reflex systems such as micturition and genital function, as well as controlling the complex emotional and social behaviors associated with them.

The pontine reticulospinal tract is important for the control of both posture and locomotion. It originates from the pontine reticular formation and travels with the MLF through the medulla and cervical spinal cord before terminating in the thoracic spinal cord. Its fibers synapse in laminae VII and VIII with excitatory signals for extensor motoneurons innervating the midline musculature of the body and the proximal extremities.

Several raphe nuclei within the reticular formation have an important function in the modulation of the responsiveness of the motor system to reflex or corticospinal inputs. The caudal raphe nuclei in the reticular formation give rise to fibers that project to the spinal cord where they influence incoming sensory signals as well as motor responsiveness. Fibers from the nucleus raphe magnus exert important influences on the transmission of pain stimuli from peripheral nerves. The nucleus locus ceruleus and nucleus subceruleus give rise to noradrenergic projections to the spinal cord through the ventrolateral funiculus. While these raphe-spinal connections do not evoke movement, they are important in producing excitatory or general inhibitory effects that influence the overall motoneuron responsiveness and modulate the motor system in different phases of sleep–wake cycles and with changing emotional states.

The vestibulospinal tracts are important pathways for the control of postural tone and postural adjustments of the body that accompany head movements. They arise from neurons in the vestibular nuclei of the medulla and descend as the lateral vestibulospinal tract over the entire length of the cord, and the medial vestibulospinal tract through the upper thoracic levels. They descend in the anterior funiculus and synapse on lamina VII and VIII of the spinal gray matter to evoke excitatory postsynaptic potentials in extensor motoneurons innervating the neck, back, forelimb, and hindlimb muscles.

The integration of sensory input with movement allows us to continually interact with our environment through varied and purposeful motor behaviors. Motor systems are continuously refined by repetition and learning because of constant influences from the complex cortical association areas. Lesions along the motor hierarchy can lead to both negative (paralysis) and positive (spasticity) sequelae caused by the combination of excitatory and inhibitory input to lower motor systems.

COMPREHENSION QUESTIONS

[24.1] A 68-year-old woman presents to your clinic with the complaint of gradually worsening weakness in her right leg and new-onset weakness of her left leg. She states that the weakness in her right leg is now so profound that she is nearly incapable of moving it. On examination

she has normal muscle bulk in both her lower extremities, hyperreflexia of her right patellar and Achilles reflexes, and 1/5 strength in the right lower extremity. You suspect a brain tumor could be causing her symptoms, and send her for an MRI, which most likely shows a tumor in which of the following locations?

A. On the convexity of the left hemisphere over the fronto-parietal region

B. On the convexity of the right hemisphere over the fronto-parietal region

C. In the midsagittal plane near the paracentral lobule

D. On the convexity of the left hemisphere over the parieto-temporal region.

Refer to the following case scenario to answer questions 24.2–24.3:

A 25-year-old man is hospitalized following a motorcycle accident in which he was not wearing a helmet. He remains unconscious in critical condition in the ICU for several weeks, but eventually regains consciousness. When he does so, it is noted that while he is able to move all of his muscle groups with full strength while testing them individually, and can voluntarily make simple movements without problem, he has great difficulty performing complex movements.

[24.2] Which of following areas in the motor system is most likely damaged in this patient?

A. Primary motor cortex

B. Premotor and supplemental motor areas

C. Internal capsule

D. Anterior horn cells

[24.3] At what level in the nervous system do the majority of the descending fibers of the corticospinal tract decussate to become contralateral?

A. Internal capsule

B. Medullary pyramids

C. Ventral white commissure of the spinal cord

D. Corticospinal neurons do not decussate

Answers

[24.1] C. The lesion described is a parasagittal meningioma, a generally benign tumor of the meninges. In this case, it is growing in the midsagittal plane, and causing compression of the left paracentral lobule and also the right paracentral lobule to a lesser extent. Recall that the somatotopic organization of the primary motor cortex places control of the contralateral leg and foot in the paracentral lobule, and control of more superior body parts in progressively more inferior aspects of the precentral gyrus as you progress toward the sylvian fissure. A lesion over the lateral convexity of the fronto-parietal region would compress one of the primary motor strips, located in the precentral gyrus, resulting in weakness of the contralateral hand or arm or face, depending on the exact location of the tumor.

[24.2] B. The deficit in executing complex movements can be attributed to a dysfunction in his supplemental and premotor areas. The motor system is arranged in a hierarchical fashion, with each higher level adding complexity to possible movements. At the top of this hierarchy are the supplemental motor area and premotor cortex, which are involved in planning and execution of complex motor behaviors. Neurons in the primary motor cortex are involved in simple movements, and can determine the speed and strength with which muscle groups contract. Descending axons from both these areas travel through the internal capsule and the corticospinal tracts to synapse on anterior horn cells, which control the actual contraction of individual muscle groups. Since this man has full voluntary movement and strength in all of his muscle groups, his primary motor cortex seems to be intact.

[24.3] B. Approximately 90% of the descending axons of the corticospinal tract decussate at the level of the medullary pyramids in the pyramidal decussation. These fibers then travel in the contralateral lateral corticospinal tract in the lateral column of the spinal cord. They end on alpha motor neurons and interneurons at the appropriate spinal level, resulting in control of movement by the contralateral motor cortex. The remaining 10% of the corticospinal neurons do not cross in the medullary pyramids, and travel down the ipsilateral cord in the anterior corticospinal tract, and eventually cross in the ventral white commissure of the spinal cord at their target spinal level.

NEUROSCIENCE PEARLS ❖ The motor system is somatotopically organized in a functional, hierarchical pattern. ❖ Cortical motor areas are important for planning movements and integrating motor output with sensory input. ❖ Descending motor pathways carry excitatory and inhibitory input to the spinal cord, allowing for purposeful, controlled movements.

REFERENCES

Ghez C, Krakauer J. The organization of movement. In: Kandel ER, Schwartz JH, Jessell TM, eds. Principles of Neural Science. 4th ed. New York: McGraw-Hill; 2000. 

Martin JH. Descending projection systems and the motor function of the spinal cord. In: Neuroanatomy, Text and Atlas. 2nd ed. Stamford, CT: Appleton & Lange; 1996. 

Ropper AH, et al. Disorders of motility. In: Adam’s and Victor’s Principles of Neurology. 8th ed. New York: McGraw-Hill; 2005.

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