The Corticospinal Tract
*corticospinal tract, rubrospinal tract, reticulospinal tract, vestibulospinal tract*
MEPs are mediated primarily by the corticospinal (pyramidal) tract which synapses on the anterior horn
The scalp/skull/dura/csf interface acts as a spatial low pass filter, most of the current flows through the scalp, temporal muscles are directly excited
Where does the MEP stimulus act? the lowest threshold is at the axon hillock, so intracranial currents activate the corticospinal tract directly, bypassing cortical synapses
Topogrophy of the anterior horn; F. Netter
Termination of major descending tracts; corticospinal tract crossed
the corticospinal tract terminates primarily on distal muscles; lateral (crossed) tracts project to distal muscles, anterior (uncrossed) tracts project to proximal muscles, most fibers cross, so distal muscles are favored.
monkey twisting the device, pyramidal tract neurons fire in high frequency bursts, multipulse stimulation mimics normal firing pattern
vascular anatomy of the spinal cord; there is a separate blood supply for sensory and motor pathways
true MEPs; activate the corticospinal tract at the cortical level; motor cortex
no antidromic activation of sensory pathways (cannot pass synapses in thalamus and medulla)
magnetic stimulation; useful in clinic but not in or
electrical stimulation; useful in or but not in clinic
responses seen in single trials, no averaging delays
issues with anesthesia, patient movement
Corticospinal Tract - Descending
MEP Considerations and Risks
pre-existing neurological deficits
history of seizure disorder
previous skull injury or craniotomy
unstable neck
age factors
most common complication: TONGUE BITES, TONGUE LACERATION/TEETH BREAKAGE
MUST USE BITE BLOCK! BILATERAL BITE BLOCKS ARE BEST! SAFETY TOOTH GUARDS! ARMORED ENDOTRACHEAL TUBE!
SEAT IN THE MOLARS, CHECK FOR HEAD TO FOLLOW WITH MICRO-MOVEMENTS OF THE BITE-BLOCK FOR CORRECT PLACEMENT! RECHECK AFTER ROTATION TO PRONE
Necessary features of stimulator; high voltage, current capacity, multipulses, variable ISI, provision for reversing polarity, no artifact introduced when off
minimum of 4 channels, more is better, amplifiers must recover rapidly from stimulus artifact, little or no signal averaging is necessary, facilitation sometimes needed
store/compare/measure/trend/print functions all desireable, isolate muscle groups for concise coverage of nerve root innervation; i.e. biceps and triceps versus biceps/triceps
Use corkscrews in quadripolar stimulation setup with jumpers or cadwell combination cable
communicate with anesthesiologist and surgeon, no muscle relaxants, suggamadex for complete reversal, intravenous agents are best, no neuromuscular blockade, keep an eye on the MAP, typically above 80 yields excellent results, below 70 poor results
stimulus may cause movement in the field, depends on the patient and electrode placement, watch for opportunities to run, should keep an eye on timing, run at least every 10 to 15 minutes and in between major introduction of instrumentation. Ask permission in case the surgeon doesn't want you to, communicate effectively. pay attention to EMG activity which is the first line of defense.
patient's with 3/5 or lower strength may have weak or absent MEPs at baseline
review history and physical carefully
place electrodes in stronger muscles
MEP is unlikely to trigger a seizure as compared with direct cortical stimulation
unstable neck high risk group, particularly during positioning, advantages of MEPs probably outweigh the risks
pre-position baseline necessary
time is of the essence
confirm intact signals before draping
MEPs are mediated primarily by the corticospinal (pyramidal) tract which synapses on the anterior horn
The scalp/skull/dura/csf interface acts as a spatial low pass filter, most of the current flows through the scalp, temporal muscles are directly excited
Where does the MEP stimulus act? the lowest threshold is at the axon hillock, so intracranial currents activate the corticospinal tract directly, bypassing cortical synapses
Topogrophy of the anterior horn; F. Netter
Termination of major descending tracts; corticospinal tract crossed
the corticospinal tract terminates primarily on distal muscles; lateral (crossed) tracts project to distal muscles, anterior (uncrossed) tracts project to proximal muscles, most fibers cross, so distal muscles are favored.
monkey twisting the device, pyramidal tract neurons fire in high frequency bursts, multipulse stimulation mimics normal firing pattern
vascular anatomy of the spinal cord; there is a separate blood supply for sensory and motor pathways
true MEPs; activate the corticospinal tract at the cortical level; motor cortex
no antidromic activation of sensory pathways (cannot pass synapses in thalamus and medulla)
magnetic stimulation; useful in clinic but not in or
electrical stimulation; useful in or but not in clinic
responses seen in single trials, no averaging delays
issues with anesthesia, patient movement
Corticospinal Tract - Descending
- origin of the pathway is in the motor cortex
- type of information relayed is motor
- decussates in the medulla
MEP Considerations and Risks
pre-existing neurological deficits
history of seizure disorder
previous skull injury or craniotomy
unstable neck
age factors
most common complication: TONGUE BITES, TONGUE LACERATION/TEETH BREAKAGE
MUST USE BITE BLOCK! BILATERAL BITE BLOCKS ARE BEST! SAFETY TOOTH GUARDS! ARMORED ENDOTRACHEAL TUBE!
SEAT IN THE MOLARS, CHECK FOR HEAD TO FOLLOW WITH MICRO-MOVEMENTS OF THE BITE-BLOCK FOR CORRECT PLACEMENT! RECHECK AFTER ROTATION TO PRONE
Necessary features of stimulator; high voltage, current capacity, multipulses, variable ISI, provision for reversing polarity, no artifact introduced when off
minimum of 4 channels, more is better, amplifiers must recover rapidly from stimulus artifact, little or no signal averaging is necessary, facilitation sometimes needed
store/compare/measure/trend/print functions all desireable, isolate muscle groups for concise coverage of nerve root innervation; i.e. biceps and triceps versus biceps/triceps
Use corkscrews in quadripolar stimulation setup with jumpers or cadwell combination cable
communicate with anesthesiologist and surgeon, no muscle relaxants, suggamadex for complete reversal, intravenous agents are best, no neuromuscular blockade, keep an eye on the MAP, typically above 80 yields excellent results, below 70 poor results
stimulus may cause movement in the field, depends on the patient and electrode placement, watch for opportunities to run, should keep an eye on timing, run at least every 10 to 15 minutes and in between major introduction of instrumentation. Ask permission in case the surgeon doesn't want you to, communicate effectively. pay attention to EMG activity which is the first line of defense.
patient's with 3/5 or lower strength may have weak or absent MEPs at baseline
review history and physical carefully
- note any asymmetric deficits
- note deficits in specific muscle groups
place electrodes in stronger muscles
MEP is unlikely to trigger a seizure as compared with direct cortical stimulation
unstable neck high risk group, particularly during positioning, advantages of MEPs probably outweigh the risks
pre-position baseline necessary
time is of the essence
confirm intact signals before draping
CONTROL PATHWAYS
Two parallel pathways, the cerebellar and the basal ganglia pathways, control and modify motor activity. The cerebellum and basal ganglia both receive input from several motor and sensory cortical areas and send information back to the cortex through different nuclei of the thalamus.
These systems are organized into several parallel loops: cerebral cortex-basal ganglia-thalamus-cerebral cortex. They integrate and modulate motor activity primarily through the cerebral cortex and corticospinal tracts. However, the cerebellum and basal ganglia also send information to the brainstem and the extrapyramidal pathways.
The functions and connections of the basal ganglia and cerebellar control circuits are different despite the general features they have in common. The basal ganglia are concerned with selective activation and inhibition of specific motor programs necessary for automatic performance of learned movements and postural adaptations. There cerebellum is involved in the control of the execution of motor acts, including maintenance of balance and posture, planning and execution of coordinated limb movements, adjustments of motor performance, and learning of new motor tasks. Abnormalities of the control circuits result in disorders of posture and coordination, at times accompanied by tremor or other abnormal involuntary movements. Control circuit damage does not produce weakness.
The basal ganglia are concerned primarily with learned, automatic behavior and with preparing and maintaining the background support, or posture, needed for voluntary motor activity. Components of the basal ganglia include the striatum, globus pallidus, subthalamic nucleus, and substantia nigra. The striatum is the receptive component of the basal ganglia and receives input from the cerebral cortex. The striatum receives three main inputs: the cerebral cortex, the intralaminar thalamic nuclei, and the substantia nigra pars compacta. The most important of output projection is to the thalamus nuclei that relay information to the premotor and supplementary motor areas and to other regions of the frontal lobe. The output of the basal ganglia affects both the corticospinal and the brainstem motor pathways.
The cerebellum accomplishes the coordination and correction of movement errors of muscles during active movements. The cerebellum and its connections compose the second major control circuit. This control is concerned with the planning and execution of movements, adaptation of motor performance, and motor learning. Its functions include control of posture, balance and eye movements necessary for maintaining equilibrium, adjustment of ongoing execution of movement; initiation, timing and planning of coordinated limb movements; and learning new motor tasks.
Cerebellar inputs and outputs are side loops of pathways from the motor cortex, subcortical nuclei and spinal cord. Information about motor plans is provided to the cerebellum by collateral projections from premotor cortex and motor cortex (relayed to the cerebellum via the pontine nuclei) and from brainstem motor regions. Information about motor performance, or external feedback, is provided by inputs from peripheral receptors via the dorsal spinocerebellar tracts. The cerebellum is subdivided into floccuonodular lobe and the body of the cerebellum which includes the anterior lobe and the posterior lobe. The midline portion of the anterior and posterior lobe is called the vermis and the lateral portions of the cerebellar hemispheres. Nerve fibers enter or leave the cerebellum in the three cerebellar peduncles. The inferior cerebellar peduncle (or restiform body) connects the cerebellum with the medulla and spinal cord; the middle cerebellar peduncle (or brachium pontis) connects the cerebellum with the pons; and the superior cerebellar peduncle (or brachium conjunctivum) connects the cerebellum with the midbrain and cerebral hemispheres.
The cerebellum corrects motor performance through its output to brainstem nuclei and to premotor cortex and motor cortex. The cerebellar output to the motor cortex is relayed through the ventral lateral nucleus of the thalamus. The cerebellum controls the ipsilateral limbs. Therefore, it processes input from the ipsilateral spinal cord and vestibular nuclei and the contralateral cerebral hemisphere and red nucleus. The cerebellar projections to the motor cortex and red nucleus travel in the superior cerebellar peduncles, which decussate in the midbrain. Descending input from the cerebral hemispheres provides information to the contralateral cerebellum via the pontine nuclei; the crossed ponto-cerebellar axons form the entire middle cerebellar peduncle. Projections from the dentate nucleus to the thalamus close an important feedback loop between the cerebralcortex and the cerebellum; the corticocerebellar-dentothalmo-cortical loop. This loop is thought to be critical for the initiation, planning and timing of motor acts, including specification of the direction, pattern and intensity of movement of the upper extremity. The posterior lobes, particularly the large lateral hemispheres form a servomechanism for coordination of skilled action. Lesions of the posterior lobes produce irregular movements of the limbs, loss of muscle coordination, loss of ability to measure range of motion, irregularity in alternate motion rate, and tremor with voluntary activity (intention tremor). All these manifestations are ipsilateral to the side of the lesion.
Two parallel pathways, the cerebellar and the basal ganglia pathways, control and modify motor activity. The cerebellum and basal ganglia both receive input from several motor and sensory cortical areas and send information back to the cortex through different nuclei of the thalamus.
These systems are organized into several parallel loops: cerebral cortex-basal ganglia-thalamus-cerebral cortex. They integrate and modulate motor activity primarily through the cerebral cortex and corticospinal tracts. However, the cerebellum and basal ganglia also send information to the brainstem and the extrapyramidal pathways.
The functions and connections of the basal ganglia and cerebellar control circuits are different despite the general features they have in common. The basal ganglia are concerned with selective activation and inhibition of specific motor programs necessary for automatic performance of learned movements and postural adaptations. There cerebellum is involved in the control of the execution of motor acts, including maintenance of balance and posture, planning and execution of coordinated limb movements, adjustments of motor performance, and learning of new motor tasks. Abnormalities of the control circuits result in disorders of posture and coordination, at times accompanied by tremor or other abnormal involuntary movements. Control circuit damage does not produce weakness.
The basal ganglia are concerned primarily with learned, automatic behavior and with preparing and maintaining the background support, or posture, needed for voluntary motor activity. Components of the basal ganglia include the striatum, globus pallidus, subthalamic nucleus, and substantia nigra. The striatum is the receptive component of the basal ganglia and receives input from the cerebral cortex. The striatum receives three main inputs: the cerebral cortex, the intralaminar thalamic nuclei, and the substantia nigra pars compacta. The most important of output projection is to the thalamus nuclei that relay information to the premotor and supplementary motor areas and to other regions of the frontal lobe. The output of the basal ganglia affects both the corticospinal and the brainstem motor pathways.
The cerebellum accomplishes the coordination and correction of movement errors of muscles during active movements. The cerebellum and its connections compose the second major control circuit. This control is concerned with the planning and execution of movements, adaptation of motor performance, and motor learning. Its functions include control of posture, balance and eye movements necessary for maintaining equilibrium, adjustment of ongoing execution of movement; initiation, timing and planning of coordinated limb movements; and learning new motor tasks.
Cerebellar inputs and outputs are side loops of pathways from the motor cortex, subcortical nuclei and spinal cord. Information about motor plans is provided to the cerebellum by collateral projections from premotor cortex and motor cortex (relayed to the cerebellum via the pontine nuclei) and from brainstem motor regions. Information about motor performance, or external feedback, is provided by inputs from peripheral receptors via the dorsal spinocerebellar tracts. The cerebellum is subdivided into floccuonodular lobe and the body of the cerebellum which includes the anterior lobe and the posterior lobe. The midline portion of the anterior and posterior lobe is called the vermis and the lateral portions of the cerebellar hemispheres. Nerve fibers enter or leave the cerebellum in the three cerebellar peduncles. The inferior cerebellar peduncle (or restiform body) connects the cerebellum with the medulla and spinal cord; the middle cerebellar peduncle (or brachium pontis) connects the cerebellum with the pons; and the superior cerebellar peduncle (or brachium conjunctivum) connects the cerebellum with the midbrain and cerebral hemispheres.
The cerebellum corrects motor performance through its output to brainstem nuclei and to premotor cortex and motor cortex. The cerebellar output to the motor cortex is relayed through the ventral lateral nucleus of the thalamus. The cerebellum controls the ipsilateral limbs. Therefore, it processes input from the ipsilateral spinal cord and vestibular nuclei and the contralateral cerebral hemisphere and red nucleus. The cerebellar projections to the motor cortex and red nucleus travel in the superior cerebellar peduncles, which decussate in the midbrain. Descending input from the cerebral hemispheres provides information to the contralateral cerebellum via the pontine nuclei; the crossed ponto-cerebellar axons form the entire middle cerebellar peduncle. Projections from the dentate nucleus to the thalamus close an important feedback loop between the cerebralcortex and the cerebellum; the corticocerebellar-dentothalmo-cortical loop. This loop is thought to be critical for the initiation, planning and timing of motor acts, including specification of the direction, pattern and intensity of movement of the upper extremity. The posterior lobes, particularly the large lateral hemispheres form a servomechanism for coordination of skilled action. Lesions of the posterior lobes produce irregular movements of the limbs, loss of muscle coordination, loss of ability to measure range of motion, irregularity in alternate motion rate, and tremor with voluntary activity (intention tremor). All these manifestations are ipsilateral to the side of the lesion.
THE CORTICOSPINAL TRACT
The largest, best-defined motor pathway is a single neuron pathway that extends from the cerebral cortex to the spinal cord called the corticospinal tract. This pathway provides a direct route by which information can travel from the cerebral cortex to the brainstem and spinal cord without an intervening synapse. Its major function is to effect voluntary activity, in particular, skilled movements under conscious control. the corticospinal tract descends from the cerebral cortex through the white matter of the cerebral hemispheres, the pyramids in the brainstem, and the spinal cord to end on ventral horn cells. The axons cross the midline at the junction of the brainstem and spinal cord to end on the opposite side. The effectiveness of this pathway depends on an intact final common pathway to carry information to the muscles.
The corticospinal tract is the route by which the motor areas of the cerebral cortex in each hemisphere control motor neurons in the ventral horn on the opposite side of the spinal cord and in the motor nuclei of the brainstem. The fibers in the corticospinal tract are corticospinal and corticobulbar. those traveling to the spinal cord are called corticospinal or pyramidal, tract. Those ending on brainstem nuclei are corticobulbar fibers. The neurons from which these tracts arise are known as upper motor neurons. The major function of the corticospinal pathway is to initiate and control skilled voluntary activity.
Each corticospinal tract arises primarily from cells in the cortex of the frontal lobe of one hemisphere and descends through the corona radiata into the internal capsule. The tract passes from the internal capsule via the cerebral peduncles to the base of the brainstem where it forms the medullary pyramids. At the junction between the medulla and spinal cord, most of the fibers in each pyramid cross the midline (the corticospinal decussation) to lie in the lateral funiculus of the opposite half of the spinal cord. These crossed fibers form the lateral corticospinal tract of the cord.
The corticospinal tract is formed by axons of neurons located in the primary motor cortex, the lateral premotor cortex, the supplementary motor area (or medial premotor cortex, and the anterior cingulate motor area (on the medial surface of the hemisphere). All these areas are closely interconnected and project to the ventral horn. The primary motor cortex occupies the anterior lip of the central sulcus of Rolando and the adjacent precentral gyrus (area 4). The primary motor cortex integrates input from multiple sources and has a somatotopic organization, with the contralateral body represented upside down: the head area is located above the fissure of Sylvius, the upper extremity next (with the thumb and index finger in proximity to the face), the trunk interposed between the shoulder and hip areas high on the convexity, and the lower limb representation extending on o the paracentral lobule in the longitudinal fissure. the frontal eye field (area 9) contains neurons involved in the generation of spontaneous and visually guided rapid eye movements. Broca's area is immediately ventral to the motor area of the left cerebral hemisphere near where the face is represented. Neurons in Broca's area participate in the motor programming for speech.
Axons from the motor cortex converge in the corona radiata toward the internal capsule, where they are compactly gathered in a topographic localization. The corticobulbar fibers occupy a more anterior location in the posterior limb of the internal capsule than the corticospinal fibers.
The pyramidal fibers remain grouped together as they pass from the internal capsule to the cerebral peduncle in the midbrain. In the midbrain, the corticospinal and corticobulbar fibers occupy the middle two-thirds of the cerebral peduncle, with the corticobulbar fibers being more medial. During their course in the brainstem, the corticobulbar fibers leave the pyramidal pathway at several levels, some crossing the midline and some remaining uncrossed. These fibers synapse in the motor centers and nuclei of the cranial nerves - trigeminal, facial, vagus, spinal accessory and hypoglossal. The fibers in the medulla form the medullary pyramids. At the lower border of the medulla, the main pyramidal decussation occurs with about 80% of the fibers crossing to the opposite side of the spinal cord.
In the spinal cord, the crossed pyramidal fibers occupy the lateral column (the lateral corticospinal tract). Because of the decussation of most of the fibers of the pyramidal tracts, the voluntary movmeents of one side of the body are under the control of the opposite cerebral hemisphere.
More diffuse, extrapyramidal pathways act indirectly on the final common pathway, mediating the enormous number of automatic activities involved in normal motor function. For example, the maintenance of erect posture when sitting or standing requires the corordinated contraction of many muscles This coordination is under subconscious control and is mediated by the reticulospinal, vestibulospinal and rubrospinal tracts.
Damage to the motor pathways result in characteristic clinical patters. There is weakness or paralysis of muscles, especially the distal muscles, especially the distal muscles. The impairment is greatest for fine motor movements, skilled movements and movements under voluntary control. The distribution of the weakness is a function of the site of the lesion. If the lesion os localized in a limited area of cortex, then a single limb or one side of the face only may be involved. If the lesion involves only the pyramidal tract fibers in the pyramids of the medulla, one side of the body below the level of the lesion is affected. Spasticity and hyper-reflexia are the result of the loss of activity of inhibitory interneurons (reticulospinal, the lateral vestibulospinal, and pontine reticulospinal). A lesion at any corticospinal level produces the upper motor neuron syndrome of distal weakness, loss of cutaneous reflexes, and Babinski's sign with increased muscle tone and reflexes
The largest, best-defined motor pathway is a single neuron pathway that extends from the cerebral cortex to the spinal cord called the corticospinal tract. This pathway provides a direct route by which information can travel from the cerebral cortex to the brainstem and spinal cord without an intervening synapse. Its major function is to effect voluntary activity, in particular, skilled movements under conscious control. the corticospinal tract descends from the cerebral cortex through the white matter of the cerebral hemispheres, the pyramids in the brainstem, and the spinal cord to end on ventral horn cells. The axons cross the midline at the junction of the brainstem and spinal cord to end on the opposite side. The effectiveness of this pathway depends on an intact final common pathway to carry information to the muscles.
The corticospinal tract is the route by which the motor areas of the cerebral cortex in each hemisphere control motor neurons in the ventral horn on the opposite side of the spinal cord and in the motor nuclei of the brainstem. The fibers in the corticospinal tract are corticospinal and corticobulbar. those traveling to the spinal cord are called corticospinal or pyramidal, tract. Those ending on brainstem nuclei are corticobulbar fibers. The neurons from which these tracts arise are known as upper motor neurons. The major function of the corticospinal pathway is to initiate and control skilled voluntary activity.
Each corticospinal tract arises primarily from cells in the cortex of the frontal lobe of one hemisphere and descends through the corona radiata into the internal capsule. The tract passes from the internal capsule via the cerebral peduncles to the base of the brainstem where it forms the medullary pyramids. At the junction between the medulla and spinal cord, most of the fibers in each pyramid cross the midline (the corticospinal decussation) to lie in the lateral funiculus of the opposite half of the spinal cord. These crossed fibers form the lateral corticospinal tract of the cord.
The corticospinal tract is formed by axons of neurons located in the primary motor cortex, the lateral premotor cortex, the supplementary motor area (or medial premotor cortex, and the anterior cingulate motor area (on the medial surface of the hemisphere). All these areas are closely interconnected and project to the ventral horn. The primary motor cortex occupies the anterior lip of the central sulcus of Rolando and the adjacent precentral gyrus (area 4). The primary motor cortex integrates input from multiple sources and has a somatotopic organization, with the contralateral body represented upside down: the head area is located above the fissure of Sylvius, the upper extremity next (with the thumb and index finger in proximity to the face), the trunk interposed between the shoulder and hip areas high on the convexity, and the lower limb representation extending on o the paracentral lobule in the longitudinal fissure. the frontal eye field (area 9) contains neurons involved in the generation of spontaneous and visually guided rapid eye movements. Broca's area is immediately ventral to the motor area of the left cerebral hemisphere near where the face is represented. Neurons in Broca's area participate in the motor programming for speech.
Axons from the motor cortex converge in the corona radiata toward the internal capsule, where they are compactly gathered in a topographic localization. The corticobulbar fibers occupy a more anterior location in the posterior limb of the internal capsule than the corticospinal fibers.
The pyramidal fibers remain grouped together as they pass from the internal capsule to the cerebral peduncle in the midbrain. In the midbrain, the corticospinal and corticobulbar fibers occupy the middle two-thirds of the cerebral peduncle, with the corticobulbar fibers being more medial. During their course in the brainstem, the corticobulbar fibers leave the pyramidal pathway at several levels, some crossing the midline and some remaining uncrossed. These fibers synapse in the motor centers and nuclei of the cranial nerves - trigeminal, facial, vagus, spinal accessory and hypoglossal. The fibers in the medulla form the medullary pyramids. At the lower border of the medulla, the main pyramidal decussation occurs with about 80% of the fibers crossing to the opposite side of the spinal cord.
In the spinal cord, the crossed pyramidal fibers occupy the lateral column (the lateral corticospinal tract). Because of the decussation of most of the fibers of the pyramidal tracts, the voluntary movmeents of one side of the body are under the control of the opposite cerebral hemisphere.
More diffuse, extrapyramidal pathways act indirectly on the final common pathway, mediating the enormous number of automatic activities involved in normal motor function. For example, the maintenance of erect posture when sitting or standing requires the corordinated contraction of many muscles This coordination is under subconscious control and is mediated by the reticulospinal, vestibulospinal and rubrospinal tracts.
Damage to the motor pathways result in characteristic clinical patters. There is weakness or paralysis of muscles, especially the distal muscles, especially the distal muscles. The impairment is greatest for fine motor movements, skilled movements and movements under voluntary control. The distribution of the weakness is a function of the site of the lesion. If the lesion os localized in a limited area of cortex, then a single limb or one side of the face only may be involved. If the lesion involves only the pyramidal tract fibers in the pyramids of the medulla, one side of the body below the level of the lesion is affected. Spasticity and hyper-reflexia are the result of the loss of activity of inhibitory interneurons (reticulospinal, the lateral vestibulospinal, and pontine reticulospinal). A lesion at any corticospinal level produces the upper motor neuron syndrome of distal weakness, loss of cutaneous reflexes, and Babinski's sign with increased muscle tone and reflexes
The Corticospinal Tract
The dorsal column-medial lemnisal system, the principal pathway for touch, and the corticospinal tract, the key pathway for voluntary movement, each have a longitudinal organization, spanning virtually the entire neuraxis. These two pathways are good examples of how particular patters of connetions between structures at different levels of the neruaxis produce a circuit with a limited number of functions. The dorsal column-medial lemniscal system is termed an ascending pathway because it brings informtion from the sensory receptors in the periphery to lower levels of the central nervous system, such as the brain stem, and then to higher levels, such as the thalamus and cerebral cortex. In contrast, the corticospinal tract, a descending pathway, carries information from the cerebral cortex to a lower level of the central nervous system, the spinal cord.
Axons of the corticospinal tract descend from the cerebral cortex to terminate on motor neurons in the spinal cord. in contrast to the dorsal column-medial lemniscal system, in which fast transmission lines are interrupted by a series of relay nuclei in the brain stem and thalamus, the corticospinal tract consists of single neurons that link the cortex directly with the spinal cord. The cell bodies of many cortico-spinal tract neurons are located in the primary motor cortex on the precentral gyrus of the frontal lobe, just rostral to the primary somatic sensory cortex. the axons of these neurons leave the motor cortex and travel down in the internal capsule, near the thalamic axons transmitting information to the somatic sensory cortex.
The corticospinal tract emerges from the internal capsule in the cerebral hemisphere to course ventrally within the brain stem. In the medulla the corticospinal axons form the pyramid, a prominent landmark on the ventral surface. in the caudal medulla, most corticospinal axons decussate (pyramidal, or motor, decussation) and descend into the spinal cord. These cortical axons travel within the white matter before terminating on motor neurons in the gray matter. These motor neurons innervate skeletal muscle; hence, the motor cortex can directly control limb and trunk movements. For example, patients with corticospinal tract damage, commonly caused by interruption of the blood supply to the internal capsule, demonstrate arm muscle weakness and impaired fine motor skills.
The corticospinal tract emerges from the internal capsule in the cerebral hemisphere to course ventrally within the brain stem. In the medulla the corticospinal axons form the pyramid, a prominent landmark on the ventral surface. in the caudal medulla, most corticospinal axons decussate (pyramidal, or motor, decussation) and descend into the spinal cord. These cortical axons travel within the white matter before terminating on motor neurons in the gray matter. These motor neurons innervate skeletal muscle; hence, the motor cortex can directly control limb and trunk movements. For example, patients with corticospinal tract damage, commonly caused by interruption of the blood supply to the internal capsule, demonstrate arm muscle weakness and impaired fine motor skills.
Martin 235
Regional Anatomy of the Motor Systems and the Descending Motor Pathways
The rest of this chapter examines the brain and spinal cord with the aim of understanding the motor pathways and their spinal terminations. This discussion begins with the cerebral cortex-the highest level of the movement control hierachy and proceeds caudally to the spinal cord, following the natural flow of information processing in the motor systems.
The Cortical Motor Areas are located in the Frontal Lobe
Similar to each sensory modality, multiple cortical sites serve motor control functions. four separate motor areas have been identifies in the frontal lobe: the primary motor cortex, the supplementary motor area, the premotor cortex, and the cingulate motor area. Many of these areas have distinct subregions.
The primary motor cortex gives rise to most of the fibers in the corticospinal tracts and has complete and somatotopically organized body representation. It plays a key role in the execution of skilled movement. The primary motor cortex is found in the caudal part of the pre-central gyrus, extending from the lateral sulcus to the medial surface of the cerebral hemisphere. The premotor cortical regions are rostral to the primary motor cortex and consist of the supplementary motor area, the premotor cortex, and the cingulate motor areas. Collectively the premotor cortical regions receive information from the associate cortex - the prefrontal cortex, posterior parietal cortex, and limbic cortex - and participate in movement planning. They have dense projections to the primary motor cortex. They also may participate in movement execution because many premotor cortical regions have direct spinal projections, via the corticospinal tract.
The Premotor Cortical Regions Integrate Information from Diverse Sources
The Primary Motor Cortex Gives Rise to Most of the Fibers of the Corticospinal tract
The primary motor cortex, which corresponds to cytoarchitectonic area 4, receives input from three major sources: the premotor cortical regions, the somatic sensory areas (in the parietal lobe) and the thalamus. These input pathways transmit neural control signals that motor cortical neurons integrate to produce accurately directed voluntary movements. As discussed earlier, premotor cortical regions receive input from diverse cortical and subcortical sources. The somatic sensory cortical areas (primary, secondary, and higher-order areas) have privileged access to the primary motor cortex, whereas other sensory cortical areas do not. This may be because somatic sensory information from the limbs and trunk is essential for coordinating all movements. Thalamic input to the primary motor cortex comes primarily from the ventrolateral nucleus, the principal thalamic relay nucleus for the cerebellum. The primary motor cortex also receives a smaller input from the basal ganglia, relayed by the ventral anterior nucleus. Thus, the cerebellum and basal ganglia can influence the primary motor cortex by two separate routes: through direct thalamic projections to the primary motor cortex and through cortico-cortical projections from the premotor cortex and the supplementary motor areas, respectively.
The cytoarchitecture of the primary motor cortex is different from that of sensory areas in the parietal, temporal, and occiptial lobes. Whereas the sensory areas have a thick layer IV and a thin layer V, the primary motor cortex has a think layer IV and a thick layer V. Recall that layer IV is the principal input layer of the cerebral cortex, where most of the axons from the thalamic relay nuclei terminate, and that layer V is the layer from which descending projections originate. In the motor areas, thalamic terminations have a wider laminar distribution than sensory areas.
The primary motor cortex, like the somatic sensory cortex is somatotopically organized. In the primary motor cortex, somatotopy can be revealed by electrical stimulation of the cortical surface, a procedure often used during neurosurgery or by functional imaging, such as functional magnetic resonance imaging. Regions controlling facial muscles (from projections to the cranial nerve motor nuclei) are located in the lateral portion of the pre-central gyrus, close to the lateral sulcus. Regions controlling other body parts are - from the lateral side of the cerebral cortex to the medial side - neck, arm, and trunk areas. The leg and foot areas are found on and close to the medial surface of the brain. Figure 10-8 shows an example of two fMRI scans of the primary motor cortex while the person made simple movements of the arm and leg. Arm movement activate the lateral part of the primary motor cortex, whereas leg movement activated the medial part. The motor representation in the pre-central gyrus forms the motor homunculus; it is distorted in a similar way as the sensory homunculus of the postcentral gyrus.
Within the representations of the individual major body parts in the primary motor cortex - face, arm, trunk, and leg - the somatotopic organization does not seem to be as precise as that in the primary somatic sensory cortex. Functional imaging in humans as well as studies in laboratory animals suggest that although the primary motor cortex has an overall somatotopic organization - with distinct face, arm, trunk, and leg zones - within a given zone the organization is more complex. For example, muscles of a body part that act together to produce a particular motor behavior, such as reaching, appear to represented together within a somatotopic zone.
Somatotopic organization means that different mediolateral regions of the primary motor cortex contribute differently to the three descending corticospinal pathways. As described above, limb muscles are preferentially controlled by the lateral corticospinal tract, and girdle and axial muscles are controlled by the ventral corticospinal tract. It follows tha arm and leg areas contribute preferentially to the lateral corticospinal tract, and neck, shoulder, and trunk regions to the ventral corticospinal tract. The face area of the primary motor cortex projects to the cranial nerve motor nuclei and thus contributes axons to the corticobulbar projection.
Regional Anatomy of the Motor Systems and the Descending Motor Pathways
The rest of this chapter examines the brain and spinal cord with the aim of understanding the motor pathways and their spinal terminations. This discussion begins with the cerebral cortex-the highest level of the movement control hierachy and proceeds caudally to the spinal cord, following the natural flow of information processing in the motor systems.
The Cortical Motor Areas are located in the Frontal Lobe
Similar to each sensory modality, multiple cortical sites serve motor control functions. four separate motor areas have been identifies in the frontal lobe: the primary motor cortex, the supplementary motor area, the premotor cortex, and the cingulate motor area. Many of these areas have distinct subregions.
The primary motor cortex gives rise to most of the fibers in the corticospinal tracts and has complete and somatotopically organized body representation. It plays a key role in the execution of skilled movement. The primary motor cortex is found in the caudal part of the pre-central gyrus, extending from the lateral sulcus to the medial surface of the cerebral hemisphere. The premotor cortical regions are rostral to the primary motor cortex and consist of the supplementary motor area, the premotor cortex, and the cingulate motor areas. Collectively the premotor cortical regions receive information from the associate cortex - the prefrontal cortex, posterior parietal cortex, and limbic cortex - and participate in movement planning. They have dense projections to the primary motor cortex. They also may participate in movement execution because many premotor cortical regions have direct spinal projections, via the corticospinal tract.
The Premotor Cortical Regions Integrate Information from Diverse Sources
The Primary Motor Cortex Gives Rise to Most of the Fibers of the Corticospinal tract
The primary motor cortex, which corresponds to cytoarchitectonic area 4, receives input from three major sources: the premotor cortical regions, the somatic sensory areas (in the parietal lobe) and the thalamus. These input pathways transmit neural control signals that motor cortical neurons integrate to produce accurately directed voluntary movements. As discussed earlier, premotor cortical regions receive input from diverse cortical and subcortical sources. The somatic sensory cortical areas (primary, secondary, and higher-order areas) have privileged access to the primary motor cortex, whereas other sensory cortical areas do not. This may be because somatic sensory information from the limbs and trunk is essential for coordinating all movements. Thalamic input to the primary motor cortex comes primarily from the ventrolateral nucleus, the principal thalamic relay nucleus for the cerebellum. The primary motor cortex also receives a smaller input from the basal ganglia, relayed by the ventral anterior nucleus. Thus, the cerebellum and basal ganglia can influence the primary motor cortex by two separate routes: through direct thalamic projections to the primary motor cortex and through cortico-cortical projections from the premotor cortex and the supplementary motor areas, respectively.
The cytoarchitecture of the primary motor cortex is different from that of sensory areas in the parietal, temporal, and occiptial lobes. Whereas the sensory areas have a thick layer IV and a thin layer V, the primary motor cortex has a think layer IV and a thick layer V. Recall that layer IV is the principal input layer of the cerebral cortex, where most of the axons from the thalamic relay nuclei terminate, and that layer V is the layer from which descending projections originate. In the motor areas, thalamic terminations have a wider laminar distribution than sensory areas.
The primary motor cortex, like the somatic sensory cortex is somatotopically organized. In the primary motor cortex, somatotopy can be revealed by electrical stimulation of the cortical surface, a procedure often used during neurosurgery or by functional imaging, such as functional magnetic resonance imaging. Regions controlling facial muscles (from projections to the cranial nerve motor nuclei) are located in the lateral portion of the pre-central gyrus, close to the lateral sulcus. Regions controlling other body parts are - from the lateral side of the cerebral cortex to the medial side - neck, arm, and trunk areas. The leg and foot areas are found on and close to the medial surface of the brain. Figure 10-8 shows an example of two fMRI scans of the primary motor cortex while the person made simple movements of the arm and leg. Arm movement activate the lateral part of the primary motor cortex, whereas leg movement activated the medial part. The motor representation in the pre-central gyrus forms the motor homunculus; it is distorted in a similar way as the sensory homunculus of the postcentral gyrus.
Within the representations of the individual major body parts in the primary motor cortex - face, arm, trunk, and leg - the somatotopic organization does not seem to be as precise as that in the primary somatic sensory cortex. Functional imaging in humans as well as studies in laboratory animals suggest that although the primary motor cortex has an overall somatotopic organization - with distinct face, arm, trunk, and leg zones - within a given zone the organization is more complex. For example, muscles of a body part that act together to produce a particular motor behavior, such as reaching, appear to represented together within a somatotopic zone.
Somatotopic organization means that different mediolateral regions of the primary motor cortex contribute differently to the three descending corticospinal pathways. As described above, limb muscles are preferentially controlled by the lateral corticospinal tract, and girdle and axial muscles are controlled by the ventral corticospinal tract. It follows tha arm and leg areas contribute preferentially to the lateral corticospinal tract, and neck, shoulder, and trunk regions to the ventral corticospinal tract. The face area of the primary motor cortex projects to the cranial nerve motor nuclei and thus contributes axons to the corticobulbar projection.
Martin 243
The Projection From Cortical Motor Regions Passes Through the Internal Capsule En Route to the Brain Stem and Spinal Cord
The corona radiata is the portion of the subcortical white matter that contains descending cortical axons and ascending thalamocortical axons. The corona rdiata is superficial to the internal capsule, which contains approxiately the same set of axons but is flanked by the deep nuclei of the basal ganglia and thalamus. The internal capsule is shaped like a curfed fan with three major parts
1 the rostral component, thermed the anterior limb
2 the caual componnent, termed the posterior limb
3 the genu (latin for knee), which joins the two limbs. The anterior limb is rostral to the thalamus, and the posterior limb is lateral to the thalamus.
Each cortical motor area sends its axons into a different part of the corona radiata and internal capsule. The descending motor projection from the primary motor cortex to the spinal cord courses in the peosterior opat of the posterior limb. The location of this projection is revealed in an MRI scan from a patient with a small lesion confined to the posterior limb of the internal capsule. The pathway can be seen in this scan because degernerating axons produce a different magnetic resonance signal from that of normal axons. Retrograde degerneration can be followe d back toward the cortex, and anterograde degeneration can be followed toward the brain stem. The approximate location of the corticospinal projection in the posterior limb is shown in Figure 10-10A (lavbeled A, T, and L, for projections controlling muscles of the arm, trunk, and leg.) The projection to the caudal brain stem, via the corticobulbar tract descends rostrally to thecorticospinal fibers, in the genu as well as to part of the posterior limbe of the internal capsule. Most of the path of the descending motor projectyion within the brain can be followed in a coronal section through the cerebral hemispheres, diencephalon, and brain stem, and in an MRI from another patient who had a stroke in the posterior limb of the internal capsule (figure 10-10C)
The descending projections from the premotor areas also course within ghe internal capsule but rostral to those from the primary motor cortex. This separation of the projections from pirimary and premotor cortical areas is clinically significant. Patients with a small posterior limb stroke can exhibit sever signs obecause of the high density of corticospinal axons. Typically, nowever, they can recover some function, such as strength. This recover is mediated in part by the spinal projections from the premotor cortical regions that are rostral to the injury. Small strokes tend to damage one or the other contingent of descedning axxonds because of the particluar vascula distribution s in the region of the internal capsule. The anterior choroidal artery supplies the posterior limb, wher ethe projections from the primary motor cortex are located. Branches form the anterior cerebral artery or the lenticulosriate branches supply the anterior limb and genu.
The internal capsule also contains ascending axons as well as other descending axons. The thalamic radioations are the ascending thalamocortical projections located in the internal capsule. The projections from the ventral anterior and ventrolateral nuclei of the thalamus course here. corticopontine axons, which carry information to the crerbvellum for controlling movements and corticoreticular axons which affect the reticular formation and reticulospinal tracs, are also located in the internal capsule. The entire internal capsule appears to condense to form the basis pedunculi o the midbrain. The basis peduculi contains onl descening fibers and therefore appears more compatct than the internal capusle which contains bioth asceding thalaocortical fibers and desceding cortical figers.
The Projection From Cortical Motor Regions Passes Through the Internal Capsule En Route to the Brain Stem and Spinal Cord
The corona radiata is the portion of the subcortical white matter that contains descending cortical axons and ascending thalamocortical axons. The corona rdiata is superficial to the internal capsule, which contains approxiately the same set of axons but is flanked by the deep nuclei of the basal ganglia and thalamus. The internal capsule is shaped like a curfed fan with three major parts
1 the rostral component, thermed the anterior limb
2 the caual componnent, termed the posterior limb
3 the genu (latin for knee), which joins the two limbs. The anterior limb is rostral to the thalamus, and the posterior limb is lateral to the thalamus.
Each cortical motor area sends its axons into a different part of the corona radiata and internal capsule. The descending motor projection from the primary motor cortex to the spinal cord courses in the peosterior opat of the posterior limb. The location of this projection is revealed in an MRI scan from a patient with a small lesion confined to the posterior limb of the internal capsule. The pathway can be seen in this scan because degernerating axons produce a different magnetic resonance signal from that of normal axons. Retrograde degerneration can be followe d back toward the cortex, and anterograde degeneration can be followed toward the brain stem. The approximate location of the corticospinal projection in the posterior limb is shown in Figure 10-10A (lavbeled A, T, and L, for projections controlling muscles of the arm, trunk, and leg.) The projection to the caudal brain stem, via the corticobulbar tract descends rostrally to thecorticospinal fibers, in the genu as well as to part of the posterior limbe of the internal capsule. Most of the path of the descending motor projectyion within the brain can be followed in a coronal section through the cerebral hemispheres, diencephalon, and brain stem, and in an MRI from another patient who had a stroke in the posterior limb of the internal capsule (figure 10-10C)
The descending projections from the premotor areas also course within ghe internal capsule but rostral to those from the primary motor cortex. This separation of the projections from pirimary and premotor cortical areas is clinically significant. Patients with a small posterior limb stroke can exhibit sever signs obecause of the high density of corticospinal axons. Typically, nowever, they can recover some function, such as strength. This recover is mediated in part by the spinal projections from the premotor cortical regions that are rostral to the injury. Small strokes tend to damage one or the other contingent of descedning axxonds because of the particluar vascula distribution s in the region of the internal capsule. The anterior choroidal artery supplies the posterior limb, wher ethe projections from the primary motor cortex are located. Branches form the anterior cerebral artery or the lenticulosriate branches supply the anterior limb and genu.
The internal capsule also contains ascending axons as well as other descending axons. The thalamic radioations are the ascending thalamocortical projections located in the internal capsule. The projections from the ventral anterior and ventrolateral nuclei of the thalamus course here. corticopontine axons, which carry information to the crerbvellum for controlling movements and corticoreticular axons which affect the reticular formation and reticulospinal tracs, are also located in the internal capsule. The entire internal capsule appears to condense to form the basis pedunculi o the midbrain. The basis peduculi contains onl descening fibers and therefore appears more compatct than the internal capusle which contains bioth asceding thalaocortical fibers and desceding cortical figers.
Martin 247
The Corticospinal Tract Courses in the Base of the Midbrain
Each division of the brain stem contains three three regions from its dorsal to ventral surfaces; tectum, tegmentum and base. ..
Descending Cortical Fibers Separate Into Small Fascicles in the Ventral Pons
The Pontine and Medullary Reticular Formation Gives Rise to the Reticulospinal Tracts
The Lateral Corticospinal Tract Decussates in the Caudal Medulla
The Intermediate Zone and Ventral Horn of the Spinal Cord Receive Input From the Descending Pathways
Lesions of the Desending Cortical Pathway in the Brain and Spinal Cord Produce Flaccid Paralysis Followed by Spasticity
...In additon to producing spasticity, lesions of the descending cortical projection pathway result in the emergence of abnormal reflexes, the most notable of which is Babinski's sign. This sign involves extension (dorsiflexion) of the big toe in response to scratching the lateral margin and then the ball of the foot (but not the toes). Babinsii's sign is though to be a withdrawal refelx. Normally such withdrawal of the big toe is prodeced by scratching the toe's ventral surface. After damage to the descending cortical fibers, the reflex can be evoked from a much larger area than normal. Hoffmann's sign, which is thumb adduction in response to the flexion of the distal phalnx of the third digt, is an example of an abnormal upper limb relex caused by damage to the descending cortical fibers.
Spinal Cord Hemiseciton Produces Ipilateral Limb Motor Signs
The Corticospinal Tract Courses in the Base of the Midbrain
Each division of the brain stem contains three three regions from its dorsal to ventral surfaces; tectum, tegmentum and base. ..
Descending Cortical Fibers Separate Into Small Fascicles in the Ventral Pons
The Pontine and Medullary Reticular Formation Gives Rise to the Reticulospinal Tracts
The Lateral Corticospinal Tract Decussates in the Caudal Medulla
The Intermediate Zone and Ventral Horn of the Spinal Cord Receive Input From the Descending Pathways
Lesions of the Desending Cortical Pathway in the Brain and Spinal Cord Produce Flaccid Paralysis Followed by Spasticity
...In additon to producing spasticity, lesions of the descending cortical projection pathway result in the emergence of abnormal reflexes, the most notable of which is Babinski's sign. This sign involves extension (dorsiflexion) of the big toe in response to scratching the lateral margin and then the ball of the foot (but not the toes). Babinsii's sign is though to be a withdrawal refelx. Normally such withdrawal of the big toe is prodeced by scratching the toe's ventral surface. After damage to the descending cortical fibers, the reflex can be evoked from a much larger area than normal. Hoffmann's sign, which is thumb adduction in response to the flexion of the distal phalnx of the third digt, is an example of an abnormal upper limb relex caused by damage to the descending cortical fibers.
Spinal Cord Hemiseciton Produces Ipilateral Limb Motor Signs
Martin 255
Summary
Descending Pathways
Lateral Descending Pathways
Medial Descending Pathways
Summary
Descending Pathways
Lateral Descending Pathways
Medial Descending Pathways
Diamond
Descending Pathways: Corticospinal Tract
The corticospinal tract is the major cortically derived descending pathway affecting the motor neurons as well as the sensory interneurons of the spinal cord. Found in most mammals, it reaches its greatest size and importance in humans, where much of its function deals with voluntary control of the upper limbs (manipulation of objects) and lower limbs (locomotion) as well as modifying sensory impulses to regulate ascending information. Like all tracts that descend into the spinal cord, the corticospinal pathway may be termed a suprasegmental or supraspinal tract and is considered a part of the upper motor neuron system.
The neurons arise in the cortex or subcortical nuclei and descend to the lower motor (anterior horn) neurons without synapsing. However, there may be several nuclei that contribute axons to a particular tract.
Approximately one million neurons located in extensive areas on each side of the cerebral cortex (see upper brain dot density) contribute axons to the corticospinal tract. Those projecting to the anterior horn are among the cells called upper motor neurons. Many of their axons are unmyelinated, and their function remains uncertain; of those axons that are myelinated, 90 percent are very small (1-4 um) and only 2 to 4 percent are large (10-20 um). these large, rapidly conducting axons are derived from the giant pyramidal cells of Betz located in the primary motor cortex of the frontal lobe. Their action briefly inhibits the antigravity tone of extensor muscles preceding a motor act.
These axons descend from the cortex to become part of the vast corona radiata, which narrows just lateral to the diencephalon to become the internal capsule. Tightly compacted here, these axons are especially vulnerable to a small hemorrhage or thrombosis, which can result in serious neurological consequences. continuing in result in serious neurological consequences. Continuing in their decent, giving off collaterals and terminals en route, the corticospinal tract traverses the base of the route, the corticospinal tract traverses the base of the midbrain, making up the middle two-thirds of the crus cerebri. The tract reaches the anterior pons and is broken up into bundles by clusters of poontine cells. The bundles reassembly caudally as they approach the medulla, giving off collaterals and terminals there and forming pyramids on the medullary anterior surface.
At the medullospinal junction, the greatest part of the tract (75 to 90 percent) on each side crosses to the contralateral side and forms the decussation of the pyramids. After crossing, each tract takes up a position in the lateral funiculus of the spinal cord as the lateral corticospinal tract. The uncrossed fibers continue caudally in the anterior funiculus as the anterior corticospinal tract , but usually no further than the caudal thoraci segments. These fibers will decussate at the cord segment within which they terminate.
Almost one-half of the corticospinal fibers terminate on interneurons in the base of the posterior horn of the cord, modifying sensory input. The remainder terminate on the anterior horn motor neurons in lamina IX and on adjacent interneurons. The anterior horn motor cell, with its cranial nerve motor nuclei counterparts and their axons, constitutes the final common pathway for the expression of voluntary motor activity and terminates at the neuromuscular junction of the muscle.
Descending Pathways: Corticospinal Tract
The corticospinal tract is the major cortically derived descending pathway affecting the motor neurons as well as the sensory interneurons of the spinal cord. Found in most mammals, it reaches its greatest size and importance in humans, where much of its function deals with voluntary control of the upper limbs (manipulation of objects) and lower limbs (locomotion) as well as modifying sensory impulses to regulate ascending information. Like all tracts that descend into the spinal cord, the corticospinal pathway may be termed a suprasegmental or supraspinal tract and is considered a part of the upper motor neuron system.
The neurons arise in the cortex or subcortical nuclei and descend to the lower motor (anterior horn) neurons without synapsing. However, there may be several nuclei that contribute axons to a particular tract.
Approximately one million neurons located in extensive areas on each side of the cerebral cortex (see upper brain dot density) contribute axons to the corticospinal tract. Those projecting to the anterior horn are among the cells called upper motor neurons. Many of their axons are unmyelinated, and their function remains uncertain; of those axons that are myelinated, 90 percent are very small (1-4 um) and only 2 to 4 percent are large (10-20 um). these large, rapidly conducting axons are derived from the giant pyramidal cells of Betz located in the primary motor cortex of the frontal lobe. Their action briefly inhibits the antigravity tone of extensor muscles preceding a motor act.
These axons descend from the cortex to become part of the vast corona radiata, which narrows just lateral to the diencephalon to become the internal capsule. Tightly compacted here, these axons are especially vulnerable to a small hemorrhage or thrombosis, which can result in serious neurological consequences. continuing in result in serious neurological consequences. Continuing in their decent, giving off collaterals and terminals en route, the corticospinal tract traverses the base of the route, the corticospinal tract traverses the base of the midbrain, making up the middle two-thirds of the crus cerebri. The tract reaches the anterior pons and is broken up into bundles by clusters of poontine cells. The bundles reassembly caudally as they approach the medulla, giving off collaterals and terminals there and forming pyramids on the medullary anterior surface.
At the medullospinal junction, the greatest part of the tract (75 to 90 percent) on each side crosses to the contralateral side and forms the decussation of the pyramids. After crossing, each tract takes up a position in the lateral funiculus of the spinal cord as the lateral corticospinal tract. The uncrossed fibers continue caudally in the anterior funiculus as the anterior corticospinal tract , but usually no further than the caudal thoraci segments. These fibers will decussate at the cord segment within which they terminate.
Almost one-half of the corticospinal fibers terminate on interneurons in the base of the posterior horn of the cord, modifying sensory input. The remainder terminate on the anterior horn motor neurons in lamina IX and on adjacent interneurons. The anterior horn motor cell, with its cranial nerve motor nuclei counterparts and their axons, constitutes the final common pathway for the expression of voluntary motor activity and terminates at the neuromuscular junction of the muscle.
Question: What loss of function would result if a unilateral lesion of the corticospinal tract above the decussation, what loss of functions would result and which side of the body would be impaired?