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Download SPHS 4050, Neurological Bases, PP 08b
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08b Direct (Pyramidal) Motor Systems (Associated with material in Chapter 16) Overview of the motor systems • Extrapyramidal systems – Brainstem to spinal cord (indirect activation) – Feedback loop to direct system via basal ganglia • Cerebellar system – Feedback loop to direct system via cerebellum • Pyramidal systems (direct activation) – corticospinal – corticonuclear (corticobulbar) Structural components of motor system • Cerebral cortex • Subcortical nuclei of the cerebrum: Basal ganglia – putamen, caudate, globus pallidus, subthalamic nucleus and substantia nigra • Brainstem ( PNS) • Cerebellum • Spinal cord ( PNS) • PNS Peripheral mechanism • Pre-frontal association cortex (“executive • • • • function”= planning, reasoning, problem solving) Responsible for deciding when to move/act, and when not to. Limbic association cortex (emotion) provides input to pre-frontal cortex Posterior parietal cortex synthesizes two inputs, to make sense of where body is relative to other objects. Input from – Primary somato-sensory cortex – Visual cortex of occipital lobe Output of posterior parietal cortex and prefrontal cortex sent to pre-motor cortex, so details of the desired movement can be programmed there. Output of pre-motor cortex goes to primary motor cortex, where the direct motor pathway starts (cell bodies of start of efferent pathway are here) Component of motor system: Cortex Component of motor system: Brainstem and associated cranial nerves Cranial nerves (PNS) have associated nuclei in the brainstem (CNS), which are organized in columns/clusters in the brainstem. Nuclei are groups/clusters of _____________ in the ______________. This is where _____________ occur. Cranial nerves are made of axons. In the motor system, the cell bodies associated with these axons are found in the _________________ of the brainstem. The motor neuron is the final common pathway for motor impulses to the muscles (effectors) of the face/head. Components of the motor system: Spinal cord and associated spinal nerves The central gray matter of the spinal cord is made up of _____________. This is where _____________ occur. Spinal nerves are made of axons. In the motor system, the cell bodies associated with these motor neurons are found in the _________________ of the spinal cord, ____________ horns. The motor neuron (alpha motor neuron) is the final common pathway for motor impulses to the muscles (effectors). Now that we’ve looked at (revisited) the structural components that make up the direct, pyramidal motor systems, let’s look more closely at these systems. Remember: Contrast the direct system with the indirect pathways and basal ganglia feedback look (which are extra pyramidal) and the cerebellar feedback loop, which were covered in the previous lecture. Pyramidal systems (direct) • Corticospinal • Corticonuclear (corticobulbar) Do you see the locations of synapses? Where is the efferent (motor) pathway? Example of pathways Whole bundles of axons course together in pathways, like those you see in the illustration above. How many synapses are there in this pathway? How many bundles/groupings of neurons make up the pathway? In pyramidal (direct activation) pathways , the pathway is generally made up of two sets of neurons that convey the efferent impulses caudally • Upper motor neurons (1st order neurons of the motor pathway) – multipolar neuron which functions as interneuron – cell bodies are in primary motor cortex (pre-central gyrus) – axons descend • ….synapse to lower motor neurons (2nd order neurons of the motor pathway): Multipolar neurons which function as motor neuron (alpha motor neuron) – In efferent pathways to head, pharynx, larynx, and muscles of shoulder shrugging and head turning, cell bodies are in nuclei of the brainstem, and axons are in the cranial nerves – In efferent pathways to body, cell bodies are in the central gray matter, and axons are in the spinal nerves Major motor system/group: Pyramidal system • Pyramidal systems – Primary, direct activation pathway from cortex to muscles, under voluntary control • Corticospinal system • Corticonuclear system (formerly corticobulbar system) – Precise control for small groups of muscles *, primarily contralateral (some ipsilateral) – Rapid and precise movement (direct innervation) * Small group of muscles does not necessarily mean that the muscles in that group are small! Cortical origins of pyramidal system • Majority of fibers begin in • primary motor cortex (precentral gyrus) Location in the primary motor cortex depends on which structure they control (see homunculus), e.g. – legs/feet – hands – face and tongue Pyramidal tracts originate in cortex and consist of upper motor neurons (1st order) and lower motor neurons (2nd order) Do you see how fibers of upper motor neuron descend in corona radiata, then through internal capsule? Example of corticospinal Example of Corticonuclear (cranial nerve VII=facial nerve) The facial nerve (cranial nerve VII) contains the fibers of the lower motor neuron of the corticonuclear (cortical bulbar) pathway for facial movement Pyramidal tracts originate in cortex and consist of upper motor neurons (1st order) and lower motor neurons (2nd order) During the descent, the axons of the upper motor neuron send off collaterals to the cerebellum (via the pons), as input for coordination of movement. In medulla, corticospinal UMN courses through the pyramids (in medulla), crossing at pyramidal (motor) decussation (to cerebellum) Example of corticospinal Example of Corticonuclear (cranial nerve VII =facial nerve) Pyramidal innervation primarily contralateral, with some ipsilateral innervation Fibers cross Fibers don’t cross Upper face: Mostly contralateral, some ipsilateral Contralateral Ipsilateral Lower face: Contralateral only