Organization of motor neurons
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Organization of motor neurons
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[post_date] => 2024-12-23 18:11:52
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[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
The organization of upper motor neurons (UMNs), lower motor neurons (LMNs), and sensory neurons creates a feedback system that modulates the strength of muscle contraction, maintains passive muscle tone, and mitigates stretch and spinal reflexes. UMNs are motor neurons that have cell bodies in the precentral gyrus of the brain and project their axons from the brain to synapses on lower motor neurons (LMNs) in the ventral horn of the spinal cord. LMNs are motor neurons that project from their respective spinal segments onto muscles at neuromuscular junctions to promote muscle contraction. LMNs in the ventral horn also receive input from spinal interneurons, as well as sensory input from type Ia sensory axons. This organization is summarized in Figure 1.
Figure 1 Hierarchical organization of somatic motor systems
During a reflex contraction, such as a knee-jerk or patellar reflex, specialized cells called muscle spindles sense increased tension in a muscle or tendon; when stretched, they trigger type Ia sensory axons to fire action potentials, which then excite alpha motor neurons in the ventral horn, causing the muscle to contract. This contraction is modulated by UMNs, which can send inhibitory signals to suppress or dampen the activity of spinal reflex arcs, or excitatory signals that can enhance the responsiveness of reflex movements when appropriate, allowing for coordinated motor responses.
Conversely, when a muscle becomes increasingly relaxed, slackened muscle spindles cease firing. A special type of LMN, called a gamma motor neuron, responds to this decreased firing and causes the muscle spindle to contract. This is known as a gamma loop and maintains passive muscle tone.
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease caused by progressive degeneration of both UMNs and LMNs. It can be distinguished from other neurological disorders that only involve UMNs or LMNs by its unique set of symptoms. Specifically, symptoms of muscle atrophy, weakness, either increased or decreased muscle tone, and concurrent symptoms of spasticity and exaggerated deep tendon reflexes, are hallmarks of the disease.
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[question] => Would a person’s sense of proprioception be impacted by ALS? Why or why not?
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[answer] => 4
[description] => Reason for Correct Answer:
Proprioception is the sense of the body’s position in space.
ALS does not affect sensory (afferent) nerve cells.
Proprioceptive information is carried by sensory (afferent) nerve cells, independent of spinal reflex arcs and motor neurons.
Therefore, a person’s sense of proprioception would not be impacted by ALS, because sensory neurons are unaffected.
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[each_answer] => A. Yes, because both central and peripheral motor neurons are affected
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[each_answer] => B. Yes, because the spinal reflex arc would be interrupted
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[each_answer] => C. No, because both central and peripheral motor neurons are affected
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[each_answer] => D. No, because sensory neurons are unaffected
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[quiz_unique_key] => 1403770772
[question] => Where are the dendrites of the primary afferent Ia nerve cells that are involved in the spinal reflex arc located?
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[answer] => 3
[description] => Reason for Correct Answer:
The ventral (anterior) horn of the spinal cord is associated with the cell bodies of motor neurons and interneurons.
Paragraph 2 explains that type Ia sensory neurons excite alpha motor neurons in the ventral horn. In Figure 1, you can see type Ia sensory neurons originating outside the spinal cord, then synapsing on motor neurons in the spinal cord.
Paragraph 2 also states that type Ia sensory neurons are triggered by muscle spindles, which sense increased tension.
This means that type Ia sensory neurons have axon terminals in the ventral horn (where they form synapses with motor neurons) but dendrites in the muscle spindles (where they are activated).
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[each_answer] => A. Dorsal root ganglion
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[each_answer] => B. Ventral horn
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[each_answer] => C. Muscle spindles
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[each_answer] => D. Amyotrophic lateral root
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[quiz_unique_key] => 1403770772
[question] => he destruction of which level of motor neuron produces the exaggerated deep tendon reflex observed in ALS? Why?
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[answer] => 2
[description] => Reason for Correct Answer:
According to the passage, spinal reflex arcs consist of primary afferent Ia (sensory) nerves from muscle spindles, which form synapses with LMNs in the ventral horn. This single synaptic system is responsible for the “stretch reflex”. This reflex is normally modulated and limited by neural input from UMNs, according to Paragraph 2.
Loss of lower motor neurons would result in an inability to generate potentials at the neuromuscular junction, and therefore an inability to induce muscle contraction.
The destruction of UMNs, on the other hand, would produce an exaggerated deep tendon reflex observed in ALS, because of a loss of feedback to modulate the spinal reflex arc.
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[each_answer] => A. UMN, because gamma motor neurons would not be able to restore muscle tension
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[each_answer] => B. UMN, because of a loss of feedback to modulate the spinal reflex arc
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[each_answer] => C. LMN, because afferent fibers from muscle spindles would have nothing to form a synapse with in the ventral horn
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[each_answer] => D. LMN, because there would be no efferent signal to the neuromuscular junction
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[quiz_unique_key] => 1403770772
[question] => What symptom would follow damage to gamma motor neurons and why?
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[answer] => 3
[description] => Reason for Correct Answer:
According to the passage, the gamma loop regulates passive muscle tone, the degree to which a muscle is passively contracted.
Gamma motor neurons sense the decreased firing rates of muscle spindles when muscles become relaxed, and then restore them to a firing state by shortening them.
A damaged gamma motor neuron would not be able to reactivate relaxed muscle spindles, therefore the firing rate of spindles would remain low.
Sustained decreases in the firing rates of muscle spindles would then cause relaxation and flaccidity in muscles.
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[each_answer] => A. Tonic contraction of affected muscles because of sustained decreases in the firing rates of muscle spindles
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[each_answer] => B. Tonic contraction of affected muscles because of sustained increases in the firing rates of muscle spindles
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[each_answer] => C. Flaccidity of affected muscles because of sustained decreases in the firing rates of muscle spindles
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[each_answer] => D. Flaccidity of affected muscles because of sustained increases in the firing rates of muscle spindles
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[quiz_unique_key] => 1403770772
[question] => In terms of their electrical activity, how do motor neurons produce more forceful muscle contractions?
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[answer] => 4
[description] => Reason for Correct Answer:
Action potentials propagate in an “all or none” fashion.
Muscle contraction is increased chiefly by two mechanisms: increasing the firing rate of action potentials, and increasing the number of neurons firing action potentials.
Motor neurons produce more forceful muscle contractions, relative to weaker muscle contractions by increasing action potential firing rate.
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[answers] => Array
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[0] => Array
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[each_answer] => A. By increasing the duration of voltage-gated sodium channel activity
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[1] => Array
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[each_answer] => B. By increasing the duration of nerve cell potassium channel activity
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[each_answer] => C. By increasing action potential amplitude
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[each_answer] => D. By increasing action potential firing rate
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[quiz_unique_key] => 1325138223
[question] => Where are the cell bodies of upper motor neurons primarily located?
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[answer] => 1
[description] => Reason for Correct Answer:
Upper motor neurons originate in the brain and lower motor neurons originate in the spinal cord.
Ganglia are collections of cell bodies of the peripheral nervous system – specifically of autonomic and sensory nerve fibers. The cell bodies of peripheral neurons, on the other hand, are found in the anterior horn of the spinal cord.
The amygdala’s role is central to the emotional and social aspects of our behavior, helping us navigate and respond appropriately to the world around us based on emotional cues and experiences.
The cerebral cortex, specifically the precentral gyrus in the frontal lobe, is the primary location for the cell bodies of upper motor neurons that are responsible for controlling voluntary movements, in addition to modulating reflexes. These neurons are part of the corticospinal tract, which is the main descending pathway that carries signals from the brain to the spinal cord.
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[each_answer] => A. Cerebral cortex
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[each_answer] => B. Spinal cord
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[each_answer] => C. Dorsal root ganglia
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[each_answer] => D. Amygdala
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Figure 1
