Flashcards in Efferent Motor Pathways - Lower Motor And Local Circuit Neurons (10A) Deck (52)
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What do local circuit neurons do?
Are the major source of synaptic input for lower motor neurons - serve to integrate info coming in from sensory systems and also descending projections from higher brain centers
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What does the input of local circuit neurons ultimately allow for lower motor neuron coordination of?
Provide coordination between muscle groups that is essential for coordinated movements, such as walking, chewing, etc.
* these activities are "preprogrammed" events that may be initiated independent of higher brain input (local circuit/lower motor neuron interactions literally responsible for a chicken running around w/ it's head cut off)
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What is the function of lower motor neurons?
Final common pathway for movement - all commands for movement, whether reflexive or involuntary are ultimately conveyed to the muscles by lower motor neurons
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Where are cell bodies of upper motor neurons found? (2 places)
In the brainstem and cerebral cortex
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What is the main different feature of an upper motor neuron compared to lower motor neurons?
Upper motor neurons are entirely contained w/in the CNS(no part leaves it), while the axons of lower motor neurons project into the periphery
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What do upper motor neurons synapse with?
Usually with local circuit neurons (less frequently may directly synapse with lower motor neurons)
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Upper motor pathways arising from where are responsible for initiation of voluntary movement and complex skilled movements like fine motor control?
Cerebral cortex
* projections from primary motor cortex, promotor cortex! and supplementary motor cortex are essential for planning, initiating, and directing the sequences of voluntary movement
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Upper motor neurons that regulate muscle tone, and orient the eyes head and body with respect to incoming sensory info originate from where?
Brainstem
* these are critical for basic navigational movement and control of posture
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How do the cerebellum and the basal ganglia alter the output of motor function?
They modulate the output of the upper motor neurons to the lower motor neurons (don't influence lower motor neurons directly)
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Where are the cell bodies of the 1st order neurons in the Corticospinal (pyramidal) Tracts located? What type do motor neurons are these?
In the precentral gyrus ; these are upper motor neurons
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What structure do fibers in the corticospinal (pyramidal) tract form after leaving the primary motor cortex (precentral gyrus)?
Corona radiata; they then converge to pass through the posterior limb of the internal capsule
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What structure do fibers in the corticospinal (pyramidal) tract pass through in the anterior midbrain?
Crus cerebri (basis pedunculi)
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What structure do fibers of the corticospinal tract form in the anterior medulla?
Pyramids
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Where do fibers of the corticospinal tract decussate? What are they called afterwards?
At the junction of the medulla and spinal cord - now known as the lateral corticospinal tract
*85-90% of the fibers decussate; the rest remain on the ipsilateral side
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What are the few fibers of the corticospinal tract that don't decussate at the pyramids called as they progress caudally?
Ventral (anterior) corticospinal tract
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Where do the ventral (anterior) corticospinal tract neurons decussate? Via what structure do they cross over?
At the level they synapse on the lower motor neurons in the ventral horn of the spinal cord
* fibers that decussate cross over via the anterior white commisure
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Which corticospinal tract innervates limb (distal) musculature? Do they do this ipsilaterally or bilaterally?
Lateral corticospinal tract upper motor neurons; synapse only ipsilaterally to innervate limb musculature
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Which corticospinal tract neurons innervate axial (proximal) musculature lower motor neurons? Do they do so ipsilaterally or bilaterally?
Anterior (ventral) corticospinal tract upper motor neurons synapse bilaterally on lower motor neurons to axial musculature
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What lamellae in spinal cord grey matter are local circuit interneurons found?
VII-VIII
*in between the intermediate zone(VII) and ventral horn(VIII)
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What lamella are the lower motor neurons found in?
IX (ventral horn)
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Where in the ventral horn spinal cord grey matter are neurons innervating distal structures located? Axial structures? (Somatotropin distribution)
Distal structures - lateral portions
Axial structures - medial portions
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What is a motor unit? What kind of motoneuron is involved?
An alpha-motoneuron and the several muscle fibers in a single muscle it innervates
*activation of a single motor unit represents the smallest unit of force that can be generated by a single muscle fiber
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What does the size of the alpha-motoneuron cell body have to do with its function?
The greater the size of the motor neuron the greater number of fibers it synapses with.
* alpha-motoneurons are the same as lower motor neurons
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How is more precise control achieved for fine motor movements, like those in the fingers or eyes?
By having a single motor unit innervating fewer fibers.
(But having more fibers per motor unit gives more power)
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Which of the three types of muscle fiber types Is dark meat composed of? Do they have a high or low ratio of muscle fibers per neuron?
Slow, fatigue resistant fibers - are red due to increased blood supply and ample myoglobin; tire slowly due to aerobic metabolism
* have a low number of muscle fibers per neuron(thus smaller motoneuron cell bodies)
Aka Type I fibers
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What type of muscle fiber types are characteristic of white meat? Why are they pale? What is the ratio of muscle fiber types per neuron in these fibers?
Fast, fatiguable fibers - are paler due to less blood supply and less myoglobin; have a high ratio of fibers to neurons (so large motoneuron cell bodies, and more powerful, less control)
Aka Type II fibers
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What neuron subtype send afferents from muscle spindle cells to the spinal cord to synapse on alpha-motoneurons as part of the muscle stretch (monosynaptic) reflex? Why do they also synapse on inhibitory interneurons?
1A (sensory from muscle spindle)
Activation of interneurons inhibit the lower motoneurons of the antagonistic muscle set
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What is the name for the connection made when a sensory afferent nerve (1A) provides excitatory information to an alpha-motoneuron?
Monosynaptic connection
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What kind of amino acids are common excitatory NTs for motoneurons?
Acidic a.a.
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