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Motor Units

Motor Units

A motor unit consists of two main components: a single efferent motor neuron (i.e., a neuron that carries impulses away from the central nervous system) and all of the muscle fibers it innervates. The motor neuron may innervate multiple muscle fibers, which are single cells, but only one motor neuron innervates a single muscle fiber.

Motor Units

Motor Units

The motor unit is a fundamental component of the neuromuscular system and plays a crucial role in coordinating muscle contractions. It consists of a somatic motor neuron, which connects and controls multiple skeletal muscle fibers, forming a single functional segment. The axon of the motor neuron branches out and establishes synaptic connections known as neuromuscular junctions with individual muscle fibers within the motor unit.
Motor units come in different sizes, with smaller units...

Classification of Skeletal Muscle Fibers

Classification of Skeletal Muscle Fibers

Skeletal muscles continuously produce ATP to provide the energy that enables muscle contractions. Skeletal muscle fibers can be categorized into three types based on differences in their contraction speed and how they produce ATP, as well as physical differences related to these factors. Most human muscles contain all three muscle fiber types, albeit in varying proportions.
Slow-Twitch Muscle Fibers
Slow oxidative, muscle fibers appear red due to large numbers of capillaries and high levels of...

Hierarchy of Motor Control

Hierarchy of Motor Control

The hierarchy of motor control refers to the different levels of organization and processing involved in controlling movement in the body. These levels range from higher cortical areas involved in planning and decision-making to lower spinal cord reflexes that respond automatically to external stimuli.

Pleiotropy

Pleiotropy

Pleiotropy is the phenomenon in which a single gene impacts multiple, seemingly unrelated phenotypic traits. For example, defects in the SOX10 gene cause Waardenburg Syndrome Type 4, or WS4, which can cause defects in pigmentation, hearing impairments, and an absence of intestinal contractions necessary for elimination. This diversity of phenotypes results from the expression pattern of SOX10 in early embryonic and fetal development. SOX10 is found in neural crest cells that form melanocytes,...

Methods of Documentation II: POMR

Methods of Documentation II: POMR

The Problem-Oriented Medical Record (POMR) revolutionized medical record-keeping by introducing a systematic approach focusing on the patient's problems rather than merely listing symptoms. Dr. Lawrence Weed's introduction of this method in the 1960s marked a significant advancement in medical documentation. The POMR framework consists of four key components: the database, problem list, plan of care, and progress notes.

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Related Experiment Video

Updated: Mar 19, 2026

Author Spotlight: Generating Neuronal Phenotypic Profiles - A Protocol to Culture and Image Human Midbrain Dopaminergic Neurons

Author Spotlight: Generating Neuronal Phenotypic Profiles - A Protocol to Culture and Image Human Midbrain Dopaminergic Neurons

Published on: July 7, 2023

ALS motor phenotypes: a revised 'OPM' classification.

Thomas Meyer^1,2, Nicola Ticozzi^3,4, Markus Weber⁵

¹Center for ALS and other Motor Neuron Disorders, Department of Neurology, Charité - Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany.

Amyotrophic Lateral Sclerosis & Frontotemporal Degeneration

|March 17, 2026

Summary

This summary is machine-generated.

A revised Amyotrophic Lateral Sclerosis-Onset-Propagation-Motor Neuron Dysfunction (ALS-OPM) classification refines motor phenotyping. This updated system aids in personalized ALS care and clinical trial design for better outcomes.

Keywords:

ALS motor phenotypes Amyotrophic lateral sclerosis OPM classification revision

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Last Updated: Mar 19, 2026

Author Spotlight: Generating Neuronal Phenotypic Profiles - A Protocol to Culture and Image Human Midbrain Dopaminergic Neurons

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Evaluation of Motor Impairment in C. elegans Models of Amyotrophic Lateral Sclerosis

Evaluation of Motor Impairment in C. elegans Models of Amyotrophic Lateral Sclerosis

Published on: September 2, 2021

Area of Science:

Neurology
Clinical Phenotyping
Motor Neuron Diseases

Background:

Accurate motor phenotyping in Amyotrophic Lateral Sclerosis (ALS) is crucial for personalized patient care and optimizing clinical trial design.
The existing ALS-Onset-Propagation-Motor Neuron Dysfunction (ALS-OPM) classification system categorizes ALS based on symptom onset, propagation, and motor neuron involvement.

Purpose of the Study:

To revise and refine the ALS-OPM classification system.
To enhance the practicality and feasibility of ALS phenotyping in routine clinical practice and therapeutic trials.

Main Methods:

An international focus group of ALS experts convened in September 2025.
A consensus process was employed to finalize revisions to the OPM classification.

Main Results:

The revised classification details onset regions (O1-4) including head, arm, trunk, or leg.
Propagation (P0(n) or P1(n)) and the degree of upper motor neuron (UMN) and/or lower motor neuron (LMN) dysfunction (M0-M3) are precisely defined.
Specific criteria for UMN (e.g., spasticity, hyperreflexia) and LMN (e.g., weakness, atrophy) dysfunction are established.

Conclusions:

The updated ALS-OPM classification provides a standardized and practical framework for capturing ALS phenotypes.
This revised system is expected to improve the routine assessment of ALS patients in clinical settings and facilitate more effective therapeutic trials.