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Archive for the ‘neurons’ Category

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A three-dimensional, reconstructed magnetic resonance image (upper) shows a cavity caused by a spinal injury nearly filled with grafted neural stem cells, colored green. The lower image depicts neuronal outgrowth from transplanted human neurons (green) and development of putative contacts (yellow dots) with host neurons (blue).

A new study has found that a single injection of human neural stem cells produced neuronal regeneration and improvement of function and mobility in rats impaired by an acute spinal cord injury (SCI).
The human stem cells appeared to vigorously take root at the injury site and produced an array of therapeutic benefits.

A 3-D, reconstructed MRI (upper image) shows a cavity caused by a spinal injury nearly filled with grafted neural stem cells (green). The lower image depicts neuronal outgrowth from transplanted human neurons (green) and development of putative contacts (yellow dots) with host neurons (blue).

Read more: http://bit.ly/16mQaOH
Image credit: UC San Diego School of Medicine
Journal article: Amelioration of motor/sensory dysfunction and spasticity in a rat model of acute lumbar spinal cord injury by human neural stem cell transplantation. Stem Cell Research & Therapy, 2013 DOI: 10.1186/scrt209

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A new finding by Harvard stem cell biologists turns one of the basics of neurobiology on its head — demonstrating that it is possible to turn one type of already differentiated neuron into another within the brain.

More info: http://bit.ly/Wd6hHh

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Action potentials are most commonly initiated by excitatory postsynaptic potentials from a presynaptic neuron. Typically, neurotransmitter molecules are released by the presynaptic neuron. These neurotransmitters then bind to receptors on the postsynaptic cell. This binding opens various types of ion channels. This opening has the further effect of changing the local permeability of the cell membrane and, thus, the membrane potential. If the binding increases the voltage (depolarizes the membrane), the synapse is excitatory.

Action potentials are generated by special types of voltage-gated ion channels embedded in a cell’s plasma membrane. These channels are shut when the membrane potential is near the resting potential of the cell, but they rapidly begin to open if the membrane potential increases to a precisely defined threshold value. When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential. This then causes more channels to open, producing a greater electric current, and so on. The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential. The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate. As the sodium channels close, sodium ions can no longer enter the neuron, and they are actively transported out of the plasma membrane. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state. After an action potential has occurred, there is a transient negative shift, called the afterhyperpolarization or refractory period, due to additional potassium currents. This is the mechanism that prevents an action potential from traveling back the way it just came.

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Neural Cell Adhesion Molecule (NCAM, also the cluster of differentiation CD56) is a homophilic binding glycoprotein expressed on the surface of neurons, glia, skeletal muscle and natural killer cells. NCAM has been implicated as having a role in cell–cell adhesion,neurite outgrowth, synaptic plasticity, and learning and memory.

NCAM is a glycoprotein of Immunoglobulin(Ig)superfamily. At least 27 alternatively spliced NCAM mRNAs are produced, giving a wide diversity of NCAM isoforms. The three main isoforms of NCAM vary only in their cytoplasmic domain:

  • NCAM-120kDa (GPI anchored)
  • NCAM-140kDa (short cytoplasmic domain)
  • NCAM-180kDa (long cytoplasmic domain)

The extracellular domain of NCAM consists of five immunoglobulin-like (Ig) domains followed by two fibronectin type III (FNIII) domains. The different domains of NCAM have been shown to have different roles, with the Ig domains being involved in homophilic binding to NCAM, and the FNIII domains being involved signaling leading to neurite outgrowth.

Homophilic binding occurs between NCAM molecules on opposing surfaces (trans-) and NCAM molecules on the same surface (cis-)1. There is much controversy as to how exactly NCAM homophilic binding is arranged both in trans- and cis-. Current models suggest trans- homophilic binding occurs between two NCAM molecules binding antiparallel between all five Ig domains or just IgI and IgII. cis- homophilic binding is thought to occur by interactions between both IgI and IgII, and IgI and IgIII, forming a higher order NCAM multimer. Both cis- and trans- NCAM homophilic binding have been shown to be important in NCAM “activation” leading to neurite outgrowth.

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The neuron

A neuron (play /ˈnjʊərɒn/ newr-on; also known as a neurone or nerve cell) is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form neural networks. Neurons are the core components of the nervous system, which includes the brain, spinal cord, and peripheral ganglia. A number of specialized types of neurons exist: sensory neurons respond to touch, sound, light and numerous other stimuli affecting cells of the sensory organs that then send signals to the spinal cord and brain. Motor neurons receive signals from the brain and spinal cord, cause muscle contractions, and affect glands. Interneurons connect neurons to other neurons within the same region of the brain or spinal cord.

A typical neuron possesses a cell body (often called the soma), dendrites, and an axon. Dendrites are thin structures that arise from the cell body, often extending for hundreds of micrometres and branching multiple times, giving rise to a complex “dendritic tree”. An axon is a special cellular extension that arises from the cell body at a site called the axon hillock and travels for a distance, as far as 1 m in humans or even more in other species. The cell body of a neuron frequently gives rise to multiple dendrites, but never to more than one axon, although the axon may branch hundreds of times before it terminates. At the majority of synapses, signals are sent from the axon of one neuron to a dendrite of another. There are, however, many exceptions to these rules: neurons that lack dendrites, neurons that have no axon, synapses that connect an axon to another axon or a dendrite to another dendrite, etc.

All neurons are electrically excitable, maintaining voltage gradients across their membranes by means of metabolically driven ion pumps, which combine with ion channels embedded in the membrane to generate intracellular-versus-extracellular concentration differences of ions such as sodium, potassium, chloride, and calcium. Changes in the cross-membrane voltage can alter the function of voltage-dependent ion channels. If the voltage changes by a large enough amount, an all-or-none electrochemical pulse called an action potential is generated, which travels rapidly along the cell’s axon, and activates synaptic connections with other cells when it arrives.

With the exception of neural stem cells and a few other types of neurons, neurons do not undergo cell division. In most cases, neurons are generated by special types of stem cells. Astrocytes, a type of glial cell, have also been observed to turn into neurons by virtue of the stem cell characteristic pluripotency. In humans, neurogenesis largely ceases during adulthood—only for two brain areas, the hippocampus and olfactory bulb, is there strong evidence for generation of substantial numbers of new neurons.

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