Current Opinion in Neurobiology 16 , — Poliak, S. The local differentiation of myelinated axons at nodes of Ranvier. Nature Reviews Neuroscience 4 , — Sherman, D. Mechanisms of axon ensheathment and myelin growth. Nature Reviews Neuroscience 6 , — Siffrin, V. Multiple sclerosis — candidate mechanisms underlying CNS atrophy. Trends in Neurosciences 33 , — Susuki, K. Molecular mechanisms of node of Ranvier formation. Current Opinion in Cell Biology 20 , — Cell Signaling.
Ion Channel. Cell Adhesion and Cell Communication. Aging and Cell Division. Endosomes in Plants. Ephs, Ephrins, and Bidirectional Signaling. Ion Channels and Excitable Cells. Signal Transduction by Adhesion Receptors. Citation: Susuki, K.
Nature Education 3 9 How does our nervous system operate so quickly and efficiently? The answer lies in a membranous structure called myelin. Aa Aa Aa. Information Transmission in the Body. Figure 1. Figure Detail. Axonal Signaling Regulates Myelination. Figure 2: The fate of demyelinated axons. The case in the CNS is illustrated.
Research in Myelin Biology and Pathology. Figure 3. References and Recommended Reading Brinkmann, B. Waxman, S. The Axon: Structure, Function and Pathophysiology. New York: Oxford University Press, Article History Close.
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The Success Code. Why Science Matters. The Beyond. Each initiates nerve impulses in sensory neurons when it is physically deformed by an outside force such as:. Touch Light touch is detected by receptors in the skin. These are often found close to a hair follicle so even if the skin is not touched directly, movement of the hair is detected. In the mouse, light movement of hair triggers a generator potential in mechanically-gated sodium channels in a neuron located next to the hair follicle.
This potential opens voltage-gated sodium channels and if it reaches threshold, triggers an action potential in the neuron. Touch receptors are not distributed evenly over the body. The fingertips and tongue may have as many as per cm 2 ; the back of the hand fewer than 10 per cm 2. This can be demonstrated with the two-point threshold test. With a pair of dividers like those used in mechanical drawing, determine in a blindfolded subject the minimum separation of the points that produces two separate touch sensations.
The ability to discriminate the two points is far better on the fingertips than on, say, the small of the back. The density of touch receptors is also reflected in the amount of somatosensory cortex in the brain assigned to that region of the body.
Proprioception is our "body sense". It enables us to unconsciously monitor the position of our body. It depends on receptors in the muscles, tendons, and joints. If you have ever tried to walk after one of your legs has "gone to sleep", you will have some appreciation of how difficult coordinated muscular activity would be without proprioception.
The Pacinian Corpuscle Pacinian corpuscles are pressure receptors. They are located in the skin and also in various internal organs. Each is connected to a sensory neuron. Pacinian corpuscles are fast-conducting, bulb-shaped receptors located deep in the dermis.
They consist of the ending of a single neurone surrounded by lamellae. They are the largest of the skin's receptors and are believed to provide instant information about how and where we move. They are also sensitive to vibration.
Pacinian corpuscles are also located in joints and tendons and in tissue that lines organs and blood vessels. Pressure on the skin changed the shape of the Pacinian corpuscle. This changes the shape of the pressure sensitive sodium channels in the membrane, making them open. Sodium ions diffuse in through the channels leading to depolarisation called a generator potential.
The greater the pressure the more sodium channels open and the larger the generator potential. If a threshold value is reached, an action potential occurs and nerve impulses travel along the sensory neurone. The frequency of the impulse is related to the intensity of the stimulus. When pressure is first applied to the corpuscle, it initiates a volley of impulses in its sensory neuron. However, with continuous pressure, the frequency of action potentials decreases quickly and soon stops.
This is the phenomenon of adaptation. Adaptation occurs in most sense receptors. It is useful because it prevents the nervous system from being bombarded with information about insignificant matters like the touch and pressure of our clothing. Stimuli represent changes in the environment. If there is no change, the sense receptors soon adapt. But note that if we quickly remove the pressure from an adapted Pacinian corpuscle, a fresh volley of impulses will be generated.
The speed of adaptation varies among different kinds of receptors. Receptors involved in proprioception - such as spindle fibres - adapt slowly if at all. Check Point g The Pacinian Corpuscle Deforming the corpuscle creates a generator potential in the sensory neuron arising within it. This is a graded response: the greater the deformation, the greater the generator potential.
It relies on interactions across complex networks of neurons distributed throughout the peripheral and central nervous systems. Researchers can use imaging techniques, such as functional magnetic resonance imaging and electroencephalography , to see what areas of the nervous system are active during different thought processes, and how information flows through the nervous system.
Many scientists consider the best proxy measure of the speed or efficiency of thought processes to be reaction time — the time from the onset of a specific signal to the moment an action is initiated. Indeed, researchers interested in assessing how fast information travels through the nervous system have used reaction time since the mids.
This approach makes sense because thoughts are ultimately expressed through overt actions. Reaction time provides an index of how efficiently someone receives and interprets sensory information, decides what to do based on that information, and plans and initiates an action based on that decision. The time it takes for all thoughts to occur is ultimately shaped by the characteristics of the neurons and the networks involved.
Many things influence the speed at which information flows through the system, but three key factors are:. Distance — The farther signals need to travel, the longer the reaction time is going to be. Reaction times for movements of the foot are longer than for movements of the hand, in large part because the signals traveling to and from the brain have a longer distance to cover.
The key observation for the present purpose is that the same reflexes evoked in taller individuals tend to have longer response times than for shorter individuals. By way of analogy, if two couriers driving to New York leave at the same time and travel at exactly the same speed, a courier leaving from Washington, DC will always arrive before one leaving from Los Angeles. Neuron characteristics — The width of the neuron is important.
Signals are carried more quickly in neurons with larger diameters than those that are narrower — a courier will generally travel faster on wide multi-lane highways than on narrow country roads.
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