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Diffusion01:12

Diffusion

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Diffusion is the passive movement of substances down their concentration gradients—requiring no expenditure of cellular energy. Substances, such as molecules or ions, diffuse from an area of high concentration to an area of low concentration in the cytosol or across membranes. Eventually, the concentration will even out, with the substance moving randomly but causing no net change in concentration. Such a state is called dynamic equilibrium, which is essential for maintaining overall...
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Diffusion01:21

Diffusion

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Diffusion is a type of passive transport. In passive transport, a substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space. For example, take the diffusion of substances through the air. When someone opens a perfume bottle in a room filled with people, the perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room. The perfume vapor will diffuse, or spread away, from the...
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Water and Mineral Acquisition02:34

Water and Mineral Acquisition

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Specialized tissues in plant roots have evolved to capture water, minerals, and some ions from the soil. Roots exhibit a variety of branching patterns that facilitate this process. The outermost root cells have specialized structures called root hairs that increase the root surface, thus increasing soil contact. Water can passively cross into roots, as the concentration of water in the soil is higher than that of the root tissue. Minerals, in contrast, are actively transported into root cells.
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Nervous Tissue: Myelin01:25

Nervous Tissue: Myelin

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The myelin sheath is a multilayered lipid and protein covering that insulates the axon of a neuron, enhancing the speed of nerve impulse conduction. Axons without this sheath are referred to as unmyelinated. Two types of neuroglia, Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS) are responsible for producing myelin sheaths.
Schwann cells begin to form myelin sheaths around axons during fetal development. They wrap around a small...
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Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion03:48

Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion

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Although gaseous molecules travel at tremendous speeds (hundreds of meters per second), they collide with other gaseous molecules and travel in many different directions before reaching the desired target. At room temperature, a gaseous molecule will experience billions of collisions per second. The mean free path is the average distance a molecule travels between collisions. The mean free path increases with decreasing pressure; in general, the mean free path for a gaseous molecule will be...
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States of Water01:23

States of Water

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Water exists in any one of the three classical states: solid (ice), liquid (water), and gas (steam or water vapor). The state of water depends on i) the intermolecular forces that draw molecules together and ii) the kinetic energy that leads to movements that pull them apart.
Water freezes when the intermolecular forces are greater than the kinetic energy. Unlike most other substances, water is less dense in its solid state than in its liquid state. This is because each water molecule can form...
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Video Experimental Relacionado

Updated: Feb 15, 2026

Spectral Reflectometric Microscopy on Myelinated Axons In Situ
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Spectral Reflectometric Microscopy on Myelinated Axons In Situ

Published on: July 2, 2018

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Mapeo del Diámetro del Axón mediante RM de Difusión de Agua de Mielina

Hong-Hsi Lee, Kwok-Shing Chan, Dmitry S Novikov

    IEEE transactions on medical imaging
    |February 13, 2026
    PubMed
    Resumen

    Este estudio introduce un nuevo método de RM de difusión para medir el diámetro del axón mielinizado de forma no invasiva. Las simulaciones confirman su precisión para aplicaciones in vivo, allanando el camino para su uso clínico.

    Palabras clave:
    RM de difusióndiámetro del axónmielinaneuroimagenmicroestructura

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    Área de la Ciencia:

    • Neuroimagen
    • Biofísica
    • Física Médica

    Sus antecedentes:

    • La RM de difusión mide la difusión del agua dentro de los tejidos.
    • La difusión del agua de mielina es sensible al entorno microestructural de los axones mielinizados.
    • La medición precisa in vivo del diámetro del axón es crucial para comprender las afecciones neurológicas.

    Objetivo del estudio:

    • Desarrollar y validar un método de RM de difusión para la medición no invasiva del diámetro del axón mielinizado.
    • Establecer el marco teórico para la difusión del agua de mielina bajo secuencias de pulsos de RM específicas.
    • Evaluar la viabilidad de la aplicación in vivo utilizando simulaciones numéricas.

    Principales métodos:

    • Se propuso una teoría para la difusión del agua de mielina utilizando RM de difusión con pulsos de gradiente amplios y la aproximación de fase gaussiana.
    • Se emplearon simulaciones de Monte Carlo en modelos cilíndricos que imitan las vainas de mielina.
    • Se evaluaron las señales de difusión utilizando la difusión media esférica y se evaluó el rendimiento en varias relaciones señal/ruido (SNR).

    Principales resultados:

    • La teoría desarrollada estima con precisión los diámetros axonales, con un peso hacia los calibres exteriores.
    • Las simulaciones demostraron la aplicabilidad del método a una SNR > 20 en un escáner de RM de alto rendimiento (Connectome 2.0).
    • La técnica mide la difusión restringida del agua de mielina para inferir el diámetro del axón.

    Conclusiones:

    • El protocolo de RM de difusión propuesto permite la medición no invasiva in vivo de los diámetros de los axones mielinizados.
    • El método muestra la promesa de adaptación a escáneres de RM de alto gradiente disponibles clínicamente.
    • Esta técnica ofrece una nueva herramienta para el estudio de la microestructura de la sustancia blanca en la salud y la enfermedad.