Direct Activation Pathways




Learning Objectives



  1. Explain the difference between the direct and indirect pathways of motor control.



  2. Discuss the differences and similarities between and UMN and LMN.



  3. Discuss the motor somatotopy of the cortex and the spinal cord.



  4. Identify the three major motor tracts of the direct pathway.



  5. Describe the pathway of the corticospinal and corticobulbar tracts.



  6. Discuss a lesion of the corticobulbar tract involving CN VII and CN XII.



  7. Explain the difference between a UMN and a LMN lesion.



  8. List the clinical signs of upper motor neuron syndrome and lower motor neuron syndrome.




Overview of Direct Motor Pathways


Motor control is a complex system composed of several individual components that together produce movement. These include motor neurons, descending motor pathways and associated cortical areas, basal ganglia, and cerebellum. Pathways that are directly responsible for the voluntary activity of the muscles of the head, neck, and limbs are referred to as the direct activation pathway. The direct motor pathway is monosynaptic. Cortical upper motor neuron (UMN) project to and synapses on a lower motor neuron (LMN) located in the brainstem and spinal cord. Fibers from cortical neurons connect the UMN, LMN, and the skeletal muscle they innervate forming the descending pathways. The corticobulbar tract involves cortical UMNs that project to cranial nerve motor nuclei (LMN) in the brainstem. The corticospinal tract consists of UMN in the cortex projecting to the ventral gray matter of the spinal cord where the LMN cell bodies are located. The axons of LMNs synapse on skeletal muscle to initiate movement. Although not part of the direct motor pathways, the basal ganglia and cerebellum have profound regulatory influence on movement (see Chapter 17). Other descending pathways involved in movement but not originating in the cortex and involving multiple synapses are the rubrospinal tract, the tectospinal tract, reticulospinal tract, and the vestibulospinal tract (see Chapter 17). These four indirect pathways originate in brainstem nuclei. In addition to movement, motor pathways are important components of reflex pathways. Reflexes are involuntary (efferent) responses to sensory (afferent) input.



Motor Neurons




  • UMN (pyramidal cells) reside within the cerebral cortex. There are four specific cortical areas where the corticospinal and corticobulbar tracts originate ().




    • Precentral gyrus (primary motor cortex)




      • Located in frontal lobe.



      • Receives input from the thalamus, supplemental motor area, cingulate gyrus, and primary somatosensory gyrus (postcentral gyrus).



      • Involved in the execution of voluntary movements.



    • Postcentral gyrus (primary somatosensory cortex)




      • Located in the parietal lobe.



      • Some fibers from the dorsal column medial lemniscus tract in the dorsal column carry proprioceptive and tactile input to the primary motor and premotor cortex.



    • Supplemental motor areas (SMA)




      • Located in medial aspect of frontal lobe, rostral to precentral gyrus.



      • Involved in programming complex sequences of movements and coordinating bilateral movement.



    • Premotor cortex (PMC)




      • Located in frontal lobe, rostral to precentral gyrus.



      • Mediates the selection of movements by utilizing information from other areas of the cortex.



  • The precentral gyrus is somatotopically organized.




    • Stimulation of motor neurons in a specific area of the primary motor cortex results in the contraction of specific muscles. In other words, there is a direct relationship between areas of the motor cortex and regions of the body creating a cortical map, known as a homunculus ().



    • The proportions of the motor homunculus represent the relative number of motor units involved in control of that particular region. Thus, the homunculus indicates the amount of fine motor control in a given area. For example, the hands, face, and tongue are depicted as being quite large compared to the trunk.



  • UMN cells in the cortex project to LMN in the brainstem and spinal cord ().




    • The axons of the UMNs that innervate cranial nerve nuclei (LMN) exit the pathway at the appropriate level forming the corticobulbar tract.



    • The remainder of the axons continue through the pyramids. Most will cross or decussate forming the decussation of the pyramids. All will enter the spinal cord as the corticospinal tract.



    • LMNs are the target of UMN.




      • Axons of UMN terminate directly on LMN or on interneurons, which in turn will synapse on LMN.



  • LMN initiates skeletal muscle contraction.




    • LMN cell bodies are found within the gray matter of the cranial nerve nuclei in the brainstem and the ventral horn of the spinal cord.



    • Their axons exit the gray matter/nuclei and innervate skeletal muscle via peripheral nerves or cranial nerves.




      • Not all cranial nerves have LMN components. Some cranial nerves contain only sensory fibers (CNs I, II, and VIII).



    • LMNs are somatotopically arranged ( ).



  • Skeletal muscles receive innervation from a motor pool.




    • A motor pool consists of all a-motor neurons that innervate a single muscle.



    • Each a-motor neuron within a motor pool sends an efferent (motor) axon toward the periphery, which branches to form synapses called neuromuscular junctions (NMJs) on specific groups of individual muscle fibers.



    • Innervation is achieved by release of a neurotransmitter (acetylcholine) that is released from the axon of the LMN. Receptors present in the membrane of the plasma membrane of the muscle fiber bind the acetylcholine, which will facilitate the transmission of the impulse resulting in contraction of the muscle fiber.



  • The group of individual muscle fibers innervated by a single a-motor neuron and its efferent axon constitutes a motor unit. An action potential generated by a motor neuron results in the simultaneous contraction of all muscle fibers in the motor unit.



  • There are three different types of motor units ().




    • S or slow-twitch




      • Produces small amounts of force over prolonged periods of time.



    • FF or fast-twitch




      • Produces large amounts of force for short periods of time.



    • FR or fast-twitch, fatigue resistant




      • Produces moderate amounts of force that can be sustained for moderate amounts of time.



  • Any given muscle contains multiple types of motor units that are interspersed throughout different regions of the muscle and provides a selective level of motor unit activation and control. Difference in motor unit fiber distribution reflects difference in muscle function.



  • The relative proportion of a specific type of motor unit along with the innervation ratio (motor unit size), the motor unit distribution within a muscle, and the overall number of motor units found in a muscle varies between different muscles and reflects muscle function.



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Fig. 16.1 Sensory and motor systems. The sensory system and motor system are so functionally interrelated they may be described as one (sensorimotor system). Cortical areas of the sensorimotor system. Lateral view of the left hemisphere. (Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM. Atlas of Anatomy. Second Edition. © Thieme 2012. Illustrations by Markus Voll and Karl Wesker.)



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Fig. 16.2 (a) Motor homunculus. (b) Sensory homunculus. The relationship between the motor cortex and the rest of the body can be illustrated by the homunculus. This cortical map represents the area of the brain that is responsible for motor function in the rest of the body, as well as the relative number of motor units as seen by the proportions of the structures.



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Fig. 16.3 UMNs located in the cortex project to LMNs in the brainstem and spinal cord. LMN, lower motor neuron; UMN, upper motor neuron.



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Fig. 16.4 LMNs in the brainstem and spinal cord are also somatotopically positioned. LMN, lower motor neuron.



Classification of motor neurons


















































Classification of motor neurons

Motor Unit Classification


Size of Motor Neuron


Activation Threshold of Neuron


Type of Muscle Fibers


Contraction (Twitch) Speed


Contractile Force of Muscle Fiber


Resistance to Fatigue


Example of Muscle


Type I


Small neuron


Discharge at slower rates


Low threshold—responds to weak stimulus


Recruited first


Type I fibers—


slow oxidative, fatigue resistant


Small fibers with high amount of mitochondria


Myosin isoform


MHC-β


Slow twitch


Low tension


Remain tonically active during movements requiring sustained activation


Fatigue resistant


Type IIa


Intermediate neuron


Discharge at intermediate rates


Intermediate threshold—requires higher stimulus than


Type II fibers; fast oxidative/glycolytic, fatigue resistant


Moderate fiber size Moderate glycogen and mitochondria


Myosin isoform MHC-IIa


Moderate Fast twitch


Moderate amount of force


Fatigue resistant


Type IIx


Large neuron


Discharge at faster rates


High activation threshold—requires strong stimulus


Recruited last


Type IIx fibers


Fast glycolytic, fatigue quickly


Large fiber size


High glycogen


Myosin isoform MHC IIx/b


Fastest twitch


High


Maximal contractile force


Fatigue easily



Corticospinal Tract




  • Multiple cortical areas contribute to motor control of the head and the rest of the body.



  • The axons of the UMN in the cortex project to both the spinal cord and brainstem.



  • There are three major descending motor tracts that innervate skeletal muscles of the head and body ().




    • Lateral corticospinal tract




      • Predominantly controls muscles of the limbs.



    • Anterior (ventral) corticospinal tract




      • Controls girdle and axial muscles.



    • Corticobulbar tract




      • Innervates muscles of the head.



  • The axons of cortical UMNs exit the subcortical white matter and form the corona radiata ().




    • The corona radiata also contains ascending thalamocortical axons.



    • The corona radiata eventually forms the internal capsule.



  • Thus, the internal capsule contains the same axons of the corona radiata ().




    • It is flanked by the basal ganglia and the thalamus.



    • The internal capsule is made up of an anterior and a posterior limb.




      • The genu is between the two limbs and therefore connects the anterior and posterior limbs.



      • The descending fibers from the motor cortex primarily run in the posterior limb.



  • The motor fibers continue to descend through the brainstem (a). As they approach the crus cerebri (the anterior portion of the cerebral peduncle) (b), they reorganize such that the fibers associated with the head are more medial (corticobulbar) and those associated with the legs are more lateral (corticospinal). The fibers that will innervate the upper limbs are in between (corticospinal).



  • As the fibers approach the lower brainstem (medulla), the corticospinal and corticobulbar fibers form the medullary pyramids (a, b).




    • Corticobulbar fibers leave the pyramids along the entire length of the medulla to terminate in the medullary reticular formation and cranial nerve nuclei.



  • When the corticospinal fibers approach the spinal cord, most will cross in the midline to form the pyramidal decussation.




    • From the decussation, the corticospinal tract continues into the spinal cord (a, b).



    • Approximately 85% of the corticospinal fibers will cross as the lateral corticospinal tract. They descend in the lateral funiculus of the spinal cord and will ultimately innervate the more distal musculature.



    • The remaining 15% of corticospinal fibers that descend uncrossed are called the anterior (ventral) corticospinal tract. These fibers run in the anterior funiculus of the spinal cord and will innervate the more proximal musculature.




      • Most of the fibers of the anterior corticospinal tract will decussate in the spinal cord via the anterior white commissure and then synapse on the ventral horn of the gray matter.



  • The majority of the corticospinal fibers will terminate on interneurons rather than directly on motor neurons in the ventral horn of the spinal cord ().



  • There are functional differences between the different components of corticospinal tract.




    • The lateral corticospinal tract innervates the cervical and lumbar part of the spinal cord and controls fine movements of the extremities.



    • The anterior corticospinal tract mediates postural mechanisms.



    • Some fibers from the corticospinal tract project to the dorsal horn in order to modulate sensory information that travels to the cortex.


Sep 13, 2022 | Posted by in NEUROLOGY | Comments Off on Direct Activation Pathways
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