Functional Anatomy of the Motor Systems and the Descending Motor Pathway
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Spinal cord lesions produce particular patterns of somatic sensory loss. For example, unilateral spinal injury produces ipsilateral loss of mechanical sensations (eg, touch) and contralateral loss of pain, temperature and itch sensations below the level of the lesion (see figure 5-9). Spinal damage that interrupts the decussating axons of the anterolateral system produces bilateral loss of pain, temperature, and itch sensations.
The location of the anterolateral system is revealed by examining the degenerted area in the lateral column in figure 5-8A. Although the anterolateral system is also somatotopically oranized it is not as prcese as that for the dorsl column and only a trend is apparent. Axons transmitting sensory information from more caudal segments are located lateral to those from more rostral segments.
Spinal cord lesions produce particular patterns of somatic sensory loss. For example, unilateral spinal injury produces ipsilateral loss of mechanical sensations (eg, touch) and contralateral loss of pain, temperature and itch sensations below the level of the lesion (see figure 5-9). Spinal damage that interrupts the decussating axons of the anterolateral system produces bilateral loss of pain, temperature, and itch sensations.
The location of the anterolateral system is revealed by examining the degenerted area in the lateral column in figure 5-8A. Although the anterolateral system is also somatotopically oranized it is not as prcese as that for the dorsl column and only a trend is apparent. Axons transmitting sensory information from more caudal segments are located lateral to those from more rostral segments.
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Diverse Central Nervous System Structures Comprise the Motor Systems
Four separate components of the central nervous system comprise the systems for controlling skeletal muscles that steer movements of the limbs and trunk
1 descending motor pathways work together with their associated cortical areas and subcortical nuclei
2 motor neurons and interneurons
3 basal ganglia, and
4 the cerebellum
The regions of the cerebral cortex and brain stem that contribute to the descending motor pathways are organized much like the ascending sensory pathways but in reverse: from the cerebral cortex toward the periphery. The brain stem motor pathways engage in relatively automatic control, such as rapid postural adjustments and correction of misdirected movements. By contrast, the cortical motor pathways participate in more refined and adaptive control, such as reaching to objects and grasping. The motor pathways synapse directly on motor neurons as well as on interneurons that in turn synapse on motor neurons.
These motor neurons and interneurons comprise the second component of the motor systems. For muscles of the limbs and trunk, motor neurons and most interneurons are found in the ventral horn and intermediate zone of the spinal cord. (The intermediate zone corresponds primarily to the spinal gray matter lateral to the central canal. It is sometimes included within the ventral horn.) For muscles of the head, interneurons are located in the cranial nerve motor nuclei and the reticular formation, respectively.
The third and fourth components of the motor systems, the cerebellum and the basal ganglia do not contain neurons that project directly to motor neurons. Nevertheless, these structures have a powerful regulatory influence over motor behavior. They act indirectly in controlling motor behavior through their effects on the descending brain stem pathways and, via the thalamus, cortical pathways.
Many Cortical Regions are recruited into action during visually guided movements
Other areas of the brain provide the motor systems with information essential for accurate movement control. For example, during visually guided behaviors - such as reaching to grasp a cup - the process of translating thoughts and sensations into action begins with the initial decision to move. This process is dependent on the limbic and pre-frontal association areas, which are involved in emotions, motivation, and cognition.
The magnocellular visual system processes visual information for guiding movements. as discussed in Chapter 7, the magnocellular visual system projects to the posterior parietal lobe, a cortical area important for identifying the location of salient objects in the environment and for attention. visual information next is distributed to premotor areas of the frontal lobe, where the plan of action to reach to the cup is formed. This plan specifies the path the hand takes to reach the target and prepares the hand for grasping or manipulating once contact with the object occurs. Planning requires determining which muscles to contract, and when. Whereas the decision to move is a conscious experience, much of the planning of a movement is not.
The next step in translating into action the decision to reach is directing the muscles to contract. This step in translating into action the decision to reach is directing the muscles to contract. This step involves the cortical motor areas and the corticospinal tract. The primary motor cortex is the most important area because it has the largest projection to the spinal cord. The corticospinal tract transmits control signals to motor neurons and to interneurons. As discussed below, most of the premotor areas also contribute axons to the corticospinal tract. The cortical motor pathways also recruit brain stem pathways for coordinating voluntary movements with postural adjustments, such as maintaining balance when lifting a heavy object.
The specific contributions of the cerebellum and basal ganglia to movement control are surprisingly elusive. The cerebellum is part of a set of neural circuits that compare intention to move with the actual movement that took place. When a disparity between intent and action is detected, the cerebellum can generate an error-correcting control signal. This signal is then transmitted to the cortical and brain stem motor pathways. However, this description only captures a small part of cerebellar function. Even less is known about basal ganglia function. The specific contribution of the basal ganglia to motor action plan formulation is unknown, but movement become disordered when the basal ganglia are damaged. For example, in patients with Parkinson disease, a neurodegenerative disease that primarily affects the basal ganglia, movements are slow or fail to be initiated and patients have significant tremors.
Diverse Central Nervous System Structures Comprise the Motor Systems
Four separate components of the central nervous system comprise the systems for controlling skeletal muscles that steer movements of the limbs and trunk
1 descending motor pathways work together with their associated cortical areas and subcortical nuclei
2 motor neurons and interneurons
3 basal ganglia, and
4 the cerebellum
The regions of the cerebral cortex and brain stem that contribute to the descending motor pathways are organized much like the ascending sensory pathways but in reverse: from the cerebral cortex toward the periphery. The brain stem motor pathways engage in relatively automatic control, such as rapid postural adjustments and correction of misdirected movements. By contrast, the cortical motor pathways participate in more refined and adaptive control, such as reaching to objects and grasping. The motor pathways synapse directly on motor neurons as well as on interneurons that in turn synapse on motor neurons.
These motor neurons and interneurons comprise the second component of the motor systems. For muscles of the limbs and trunk, motor neurons and most interneurons are found in the ventral horn and intermediate zone of the spinal cord. (The intermediate zone corresponds primarily to the spinal gray matter lateral to the central canal. It is sometimes included within the ventral horn.) For muscles of the head, interneurons are located in the cranial nerve motor nuclei and the reticular formation, respectively.
The third and fourth components of the motor systems, the cerebellum and the basal ganglia do not contain neurons that project directly to motor neurons. Nevertheless, these structures have a powerful regulatory influence over motor behavior. They act indirectly in controlling motor behavior through their effects on the descending brain stem pathways and, via the thalamus, cortical pathways.
Many Cortical Regions are recruited into action during visually guided movements
Other areas of the brain provide the motor systems with information essential for accurate movement control. For example, during visually guided behaviors - such as reaching to grasp a cup - the process of translating thoughts and sensations into action begins with the initial decision to move. This process is dependent on the limbic and pre-frontal association areas, which are involved in emotions, motivation, and cognition.
The magnocellular visual system processes visual information for guiding movements. as discussed in Chapter 7, the magnocellular visual system projects to the posterior parietal lobe, a cortical area important for identifying the location of salient objects in the environment and for attention. visual information next is distributed to premotor areas of the frontal lobe, where the plan of action to reach to the cup is formed. This plan specifies the path the hand takes to reach the target and prepares the hand for grasping or manipulating once contact with the object occurs. Planning requires determining which muscles to contract, and when. Whereas the decision to move is a conscious experience, much of the planning of a movement is not.
The next step in translating into action the decision to reach is directing the muscles to contract. This step in translating into action the decision to reach is directing the muscles to contract. This step involves the cortical motor areas and the corticospinal tract. The primary motor cortex is the most important area because it has the largest projection to the spinal cord. The corticospinal tract transmits control signals to motor neurons and to interneurons. As discussed below, most of the premotor areas also contribute axons to the corticospinal tract. The cortical motor pathways also recruit brain stem pathways for coordinating voluntary movements with postural adjustments, such as maintaining balance when lifting a heavy object.
The specific contributions of the cerebellum and basal ganglia to movement control are surprisingly elusive. The cerebellum is part of a set of neural circuits that compare intention to move with the actual movement that took place. When a disparity between intent and action is detected, the cerebellum can generate an error-correcting control signal. This signal is then transmitted to the cortical and brain stem motor pathways. However, this description only captures a small part of cerebellar function. Even less is known about basal ganglia function. The specific contribution of the basal ganglia to motor action plan formulation is unknown, but movement become disordered when the basal ganglia are damaged. For example, in patients with Parkinson disease, a neurodegenerative disease that primarily affects the basal ganglia, movements are slow or fail to be initiated and patients have significant tremors.
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There are three functional classes of descending pathways
Descending pathways can be classified as
1 motor control pathways
2 pathways that regulate somatic sensory processing
3 pathways that regulate the functions of the autonomic nervous system.
Motor control pathways mediate the voluntary and involuntary (or automatic) control of movement. As described earlier, they originate in the cerebral cortex and brain stem and synapse on motor neurons and interneurons.
The descending pathways that regulate somatic sensory processing also originate in the cerebral cortex and brain stem but terminate primarily on dorsal horn neurons and in brain stem somatic sensory relay nuclei. These pathways are important for controlling the flow of somatic sensory information into the central nervous system, which has an important effect on perception. For example, the raphespinal pathway suppresses pain transmission.
The descending pathways that regulate the autonomic nervous system originate from the cerebral cortex, amygdala, hypothalamus, and brain stem. The ganglionic autonomic neurons in the brain stem and spinal cord. The autonomic pathways, together with the autonomic nervous system itself, are examined along with the hypothalamus in Chapter 15.
There are three functional classes of descending pathways
Descending pathways can be classified as
1 motor control pathways
2 pathways that regulate somatic sensory processing
3 pathways that regulate the functions of the autonomic nervous system.
Motor control pathways mediate the voluntary and involuntary (or automatic) control of movement. As described earlier, they originate in the cerebral cortex and brain stem and synapse on motor neurons and interneurons.
The descending pathways that regulate somatic sensory processing also originate in the cerebral cortex and brain stem but terminate primarily on dorsal horn neurons and in brain stem somatic sensory relay nuclei. These pathways are important for controlling the flow of somatic sensory information into the central nervous system, which has an important effect on perception. For example, the raphespinal pathway suppresses pain transmission.
The descending pathways that regulate the autonomic nervous system originate from the cerebral cortex, amygdala, hypothalamus, and brain stem. The ganglionic autonomic neurons in the brain stem and spinal cord. The autonomic pathways, together with the autonomic nervous system itself, are examined along with the hypothalamus in Chapter 15.
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Multiple Parallel Motor Pathways Originate from the Cortex and Brain Stem
Seven major descending motor control pathways terminate in the brain stem and spinal cord. Three of these pathways originate in layer V of the cerebral cortex, primarily in the frontal lobe
1 the lateral corticospinal tract
2 the ventral (or anterior) corticospinal tract
3 the corticobulbar tract
The corticobulbar tract terminates primarily in cranial motor nuclei in the pons and medulla and is considered in deatil in Chapter 11
The remaining four pathways originate from brain stem nuclei
4 the rubrospinal tract
5 the vestibulospinal tracts.
6 the tectospinal tract
7 the vestibulospinal tracts
Similar to the ascending somatic sensory pathways, the various parallel motor control pathways serve separate but overlapping functions, which are discussed below. In addition to these seven descending motor pathways, neurotransmitter-specific systems - including the raphe nuclei, locus ceruleus and midbrain dopaminergic neurons- have diffuse projections to the spinal cord intermediate zone and ventral horn. Other than pain regulation, the functions of these descending pathways are not well understood.
Multiple Parallel Motor Pathways Originate from the Cortex and Brain Stem
Seven major descending motor control pathways terminate in the brain stem and spinal cord. Three of these pathways originate in layer V of the cerebral cortex, primarily in the frontal lobe
1 the lateral corticospinal tract
2 the ventral (or anterior) corticospinal tract
3 the corticobulbar tract
The corticobulbar tract terminates primarily in cranial motor nuclei in the pons and medulla and is considered in deatil in Chapter 11
The remaining four pathways originate from brain stem nuclei
4 the rubrospinal tract
5 the vestibulospinal tracts.
6 the tectospinal tract
7 the vestibulospinal tracts
Similar to the ascending somatic sensory pathways, the various parallel motor control pathways serve separate but overlapping functions, which are discussed below. In addition to these seven descending motor pathways, neurotransmitter-specific systems - including the raphe nuclei, locus ceruleus and midbrain dopaminergic neurons- have diffuse projections to the spinal cord intermediate zone and ventral horn. Other than pain regulation, the functions of these descending pathways are not well understood.
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Motor pathways of the Spinal Cord have a hierarchical organization
Each of the descending motor pathways influences skeletal muscle via monosynaptic, disynaptic, and polysynaptic connections between descending projection neurons and motor neurons. Typically the axon of a descending projection neuron makes all types of connections with motor neuron. Whether disynatpic or polysynapitc, the connections are mediated by two kinds of spinal cord interneurons: segmental interneurons and propriospinal neurons. Segmental interneurons have a short axon that distributes branches within a single spinal cord segment to synapse on motor neurons. In addition to receiving input from the descending motor pathways, segmental interneurons receive convergent input from different classes of somatic sensory receptors for the reflex control of movement. For example particular interneurons receive input from nociceptors and mediate limb withdrawal reflexes in response to painful stimuli, such as when you jerk your hand away from a hot stove. Segmental interneurons are located primarily in the intermediate zone and the ventral horn on the same side (ipsilateral) as the motor neurons on which they synapse. Propriospinal neurons (sometimes termed intersegmental neurons have an axon that projects for multiple spinal segments before synapsing on motor neurons.
The hierarchical organization of the motor pathways reflects the fact that there are monosynaptic and polysynaptic pathways to the motor neurons. Three basic hierarchical motor pathways exist:
1 monosynaptic corticospinal projections to motor neurons
2 disynaptic corticospinal projections to motor neurons, via segmental and propriospinal interneurons
3 polysynaptic pathways to complex spinal interneuronal circuits.
In addition to the direct corticospinal pathways to the spinal cord, there are indirect cortical pathways that route through the brain stem, such as the cortico-reticulo-spinal pathway. The first leg of this type of pathway consists of a cortical projection to the brain stem; the second leg is simply the brain stem motor pathway, such as the reticulospinal tract. considering all combinations of pathways, the projection neurons of the cerebral cortex constitute the highest level in the hierarchy, the brain stem projection neurons comprise the next lower level, and spinal interneurons and the motor neurons are the two lowest levels. Can brain stem motor pathways work independent of the cortical pathways? The answer is probably yes, based on laboratory animal studies. Both the cerebellum and basal ganglia have direct brain stem projections, which could influence brain stem motor pathway function without cortical pathway involvement.
Motor pathways of the Spinal Cord have a hierarchical organization
Each of the descending motor pathways influences skeletal muscle via monosynaptic, disynaptic, and polysynaptic connections between descending projection neurons and motor neurons. Typically the axon of a descending projection neuron makes all types of connections with motor neuron. Whether disynatpic or polysynapitc, the connections are mediated by two kinds of spinal cord interneurons: segmental interneurons and propriospinal neurons. Segmental interneurons have a short axon that distributes branches within a single spinal cord segment to synapse on motor neurons. In addition to receiving input from the descending motor pathways, segmental interneurons receive convergent input from different classes of somatic sensory receptors for the reflex control of movement. For example particular interneurons receive input from nociceptors and mediate limb withdrawal reflexes in response to painful stimuli, such as when you jerk your hand away from a hot stove. Segmental interneurons are located primarily in the intermediate zone and the ventral horn on the same side (ipsilateral) as the motor neurons on which they synapse. Propriospinal neurons (sometimes termed intersegmental neurons have an axon that projects for multiple spinal segments before synapsing on motor neurons.
The hierarchical organization of the motor pathways reflects the fact that there are monosynaptic and polysynaptic pathways to the motor neurons. Three basic hierarchical motor pathways exist:
1 monosynaptic corticospinal projections to motor neurons
2 disynaptic corticospinal projections to motor neurons, via segmental and propriospinal interneurons
3 polysynaptic pathways to complex spinal interneuronal circuits.
In addition to the direct corticospinal pathways to the spinal cord, there are indirect cortical pathways that route through the brain stem, such as the cortico-reticulo-spinal pathway. The first leg of this type of pathway consists of a cortical projection to the brain stem; the second leg is simply the brain stem motor pathway, such as the reticulospinal tract. considering all combinations of pathways, the projection neurons of the cerebral cortex constitute the highest level in the hierarchy, the brain stem projection neurons comprise the next lower level, and spinal interneurons and the motor neurons are the two lowest levels. Can brain stem motor pathways work independent of the cortical pathways? The answer is probably yes, based on laboratory animal studies. Both the cerebellum and basal ganglia have direct brain stem projections, which could influence brain stem motor pathway function without cortical pathway involvement.
Martin 234 Unedited
The functional organization of the descending pathways parallels the somatotopic organization of the motor nuclei in the ventral horn
The laterally descending pathways control limb muscles and regulate voluntary movement
The medially descending pathways control axial and girdle muscles and regulate posture
Pickup on Corticospinal page...
The functional organization of the descending pathways parallels the somatotopic organization of the motor nuclei in the ventral horn
The laterally descending pathways control limb muscles and regulate voluntary movement
The medially descending pathways control axial and girdle muscles and regulate posture
Pickup on Corticospinal page...