Motor Unit Firing Pattern Variation between the Upper and Lower Limbs in Humans

Advisor(s)

Dr. Obaid Ul Haq Khurram; Northwestern University, Feinberg School of Medicine

Dr. Gregory E. Pearcey; Northwestern University, Feinberg School of Medicine

Dr. Charles J. Heckman; Northwestern University, Feinberg School of Medicine

Discipline

Biology

Start Date

21-4-2021 9:30 AM

End Date

21-4-2021 9:45 AM

Abstract

The basic unit for motor control is the motor unit which consists of the motor neuron, its axon, the muscle fibers, the axon innervates in the target muscle. Due to the one-to-one spike ratio between motor neurons and the muscle fibers they innervate, motor unit firing patterns can be easily measured to better understand motor control mechanisms and the structure of the musculoskeletal system. An important mediator of motor unit firing patterns is the persistent inward current (PIC) which is a lasting, inward flow of ions that elucidates large magnitude changes in output from the motor neuron. An estimate of the PIC can be derived from the well-established paired motor unit analysis technique, which compares the onset and offset of a high-threshold motor unit (ΔF). Preliminary lab studies have shown that proximal muscles in the arms generate motor unit firing patterns consistent with high neuromodulatory drive (i.e. ΔF = ~4–6 spikes/s), whereas distal muscles show lower neuromodulatory drive (i.e. ΔF = ~2–3 spikes/s). However, this proximal to distal gradient is not found within the lower limbs as the activation of the quadriceps generates motor unit firing patterns consistent with moderate neuromodulatory drive (i.e. ΔF = ~3.5 spikes/s), which is similar to the more distal muscles in the upper limbs. These findings reflect the differences in motor tasks and the functional organization between the arms and legs in bipeds.

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Apr 21st, 9:30 AM Apr 21st, 9:45 AM

Motor Unit Firing Pattern Variation between the Upper and Lower Limbs in Humans

The basic unit for motor control is the motor unit which consists of the motor neuron, its axon, the muscle fibers, the axon innervates in the target muscle. Due to the one-to-one spike ratio between motor neurons and the muscle fibers they innervate, motor unit firing patterns can be easily measured to better understand motor control mechanisms and the structure of the musculoskeletal system. An important mediator of motor unit firing patterns is the persistent inward current (PIC) which is a lasting, inward flow of ions that elucidates large magnitude changes in output from the motor neuron. An estimate of the PIC can be derived from the well-established paired motor unit analysis technique, which compares the onset and offset of a high-threshold motor unit (ΔF). Preliminary lab studies have shown that proximal muscles in the arms generate motor unit firing patterns consistent with high neuromodulatory drive (i.e. ΔF = ~4–6 spikes/s), whereas distal muscles show lower neuromodulatory drive (i.e. ΔF = ~2–3 spikes/s). However, this proximal to distal gradient is not found within the lower limbs as the activation of the quadriceps generates motor unit firing patterns consistent with moderate neuromodulatory drive (i.e. ΔF = ~3.5 spikes/s), which is similar to the more distal muscles in the upper limbs. These findings reflect the differences in motor tasks and the functional organization between the arms and legs in bipeds.