femme Neuroscience

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  • Neurons are the Cells of the Mind

    Neurons and glial cells are the brain cells that a manifest all the properties of mind. The study of neurons could be considered ne plus ultra, the quantum mechanics of biology. Neurons come in different shapes and sizes but have the common property of receiving and sending information. Neurons conduct discrete signals as electro-chemical pulses, known as action potentials or “spikes.” The signal passes from one neuron to another by the secretion of chemical neurotransmitters in synapses.

    There are trillions of synaptic junctions in the human brain. Learning occurs at least in part by changes in the number, strength and kind of synaptic connections. Early studies of neurons focused on the on-off characteristic of action potentials and a misleading comparison has been made with the transistor binary switch in digital circuits.

    Neurons have root-like inputs, dendrites, and tree- like outputs, axons that transmit signals. Spines on the dendrites make contact with axons from other neurons. Signals are transmitted along axons and dendrites by the movement of sodium and potassium ions across cell membranes. The movement of ions creates a wave of electrical charge something like the wavy motions of electrons in copper wire. Where axons contact other neurons, the signal is transmitted across synapses by neurotransmitters such as glutamate, acetylcholine, serotonin and dopamine.

    The sending side of the synapse is called the presynaptic membrane and the receiving side is postsynaptic. Neurotransmitters are chemicals stored in packets or vesicles on the presynaptic side and are released in clusters to cross the synapse and dock with postsynaptic receptors. The postsynaptic receptor is activated and conveys its signal to chemical devices inside the cell that can propagate the activity started at the receptor surface.

    When enough neurotransmitters activate enough receptors, the receiving neuron sends an action potential along its axon to other neurons downstream. You could argue that much of the computation in the brain is done by adding and subtracting voltage fluctuations on the surface of neurons and the action potentials or pulses carry the results over longer distances to other neurons. Neuronal computation cannot be understood by looking at single neurons but may be understood by examining neuronal networks that receive and send pulse-encoded information.

    Waves and Chemicals

    There are at least two kinds of phenomena at work in brains, waves and chemicals. The wavy stuff enters the brain from the world outside and ends up as packets of chemical or quanta. I refer to all the mechanisms of chemical neurotransmission as neuronal quantum mechanics. Neuronal signals are sent along axons by a waveform and then converted into quantum signals using neurotransmitters. The waveform is usually described as ‘electrical or electronic” although the transmission is different from the oscillation and flow of electrons along a wire. A human who takes Prozac to modify his or her brain function is changing the neuronal quantum mechanics of the brain. The mystic who goes to the forest to meditate and enjoy bird songs is modifying his or her brain function by changing the wave mechanics of the brain. A change in quantum mechanics, of course will lead to a change in wave mechanics and visa versa. The overall activity of signals passing through neuronal networks can be compared with information processed by electronic circuits. The waveform is produced by charged ions moving across the cell membrane. A wave of changing outer charge is described as the action potential . The effect of the action potential is to trigger the release of neurotransmitters in the synapses that connect the axon to the receiving cells at the contact points. The wave activates the release of packets of molecules or quanta. There is no easy comparison between the quantum mechanics of the brain and the operation of any other device or system. The descriptions of synapses have grown increasingly detailed and the complexity of neural systems is daunting even to the brightest, best informed researchers.

    The strength of the quantum-transmitted signal is influenced by the amount of neurotransmitter released and by how long it remains attached to postsynaptic receptors. Two classes of molecules on the surface of the releasing cell influence quanta activity in the synapse, auto receptors and transporters. Neurotransmitter science has continued to grow in complexity. When I was first studying neurotransmitters, there were five chemicals to consider: acetylcholine, epinephrine, norepinephrine, dopamine and serotonin. Acetylcholine was the first to be described. Histamine was soon added to the list, then several peptides and amino acids such as glycine and glutamine joined the group. Three gases, nitric oxide, carbon monoxide and hydrogen disulphide have been recognized as neurotransmitters. The discoveries of three gases at work in the nervous system was surprising, especially since all three gases are considered to be air pollutants and potentially toxic. We now know that amino acids are the most abundant neurotransmitters in the brain. Nichols suggested: “amino acids synapses exceed those of all the other neurotransmitters combined…amino acids are responsible for almost all the fast signaling between neurons, leaving predominantly modulator roles for the other transmitters.
    Peptides have dual roles as neurotransmitters within the nervous system and hormones that carry messages from peripheral organs to the brain. Peptides are pieces of proteins and the oldest signaling molecules that occur in all animals and plants. Peptides have their own receptors and their own second messenger systems. You might guess that peptides are the oldest communication system that now coexists with more recent and specific neurotransmitter systems. Peptides are more like hormones that are often broadcast in the blood, acting more slowly and diffusely and playing a more pervasive modulator role in the brain rather than sending specific information within computational networks. The hypothalamus produces a series of peptides that control all the endocrine organs in the body. Two peptides produced by the hypothalamus and released by the pituitary gland act as both hormones and neurotransmitters: oxytocin is involved in reproductive functions, participating in pair bonding and initiating labor; vasopressin is a regulator of water and electrolyte balance. The hypothalamic releasing peptide, TRH is used diagnostically to asses both thyroid and pituitary function.

    Peptide neurotransmitters coexist with at least one classic transmitter and often with other peptides. For example, serotonin neurons in the raphe nuclei and noradrenaline neurons in the rat locus coeruleus synthesize galanin which inhibits neuron firing. The opioid peptides relieve pain. Multiple binding sites for a variety of opioid peptides have been discovered. Peptides are important in the control of feeding and body weight: they include CCK, NPY and galanin as well as the orexins, melanocortins, MCH, AGRP and CART peptides. Despite an explosive increase in knowledge about neurons and neurotransmitters, no-one has understood the fundamental principles of signal processing in the brain. Mel suggested: ‘After more than a century of neuroscience research, a remarkably simple question remains unanswered: What do nerve cells in the brain do? Most of these nerve cells (neurons) have dendritic roots -finely branched extensions emanating from the cell body--that receive tens of thousands of synaptic inputs from other neurons. At any given time, tens or hundreds of these synapses may be "firing" at a rate of tens or hundreds of times per second all across the dendritic tree. How does the neuron integrate these widely scattered synaptic inputs to generate an overall response? Neurophysiologists have frequently approached this larger question by first tackling a smaller question: What is the effect on a target neuron when one synapse on one dendrite is activated once? The great majority of synaptic contacts on pyramidal cells (the most common neurons in the cerebral cortex) are excitatory. The stereotyped electrical response elicited by an excitatory synapse is called an excitatory postsynaptic potential (EPSP). Interest in the brain's 1015 excitatory synapses runs high, in part because they make up the bulk of the brain's massive interconnection network, in part because changes in excitatory connections between neurons are believed to mediate most forms of neural learning and memory.”

    Glial Cells

    The human brain contains roughly 100 billion neurons, it contains billions more cells called glia. All major glial cell types in the brain — oligodendrocytes, microglia and astrocytes — communicate with each other and with neurons by using chemical neurotransmitters and gap junctions, channels that permit the direct transfer between cells of ions and small molecules.

    Glial cells intermingle with and closely embrace neurons. Glial cells are caretakers and custodians for neurons. Oligodendrocytes form myelinated conduction pathways that facilitate signal conduction. Some have immune cell activity. Neurons require a steady supply of energy in the form of glucose and/or lactate. Astrocytes extract nutrients from the blood and feed neurons. Blood vessels in the brain regulate flow to match oxygen and glucose delivery with metabolic demands determined by neural activity. Astrocytes connect neuronal synapses and blood vessels regulating blood flow in terms of synaptic activity. Glial cells form a slow conduction and biasing network that regulates brain function overall, but little is known about their role in detailed signal processing tasks.

    Glial cells have structural functions and provide nurturing and defense services. Glial cells help to create the blood-brain barrier that limits access to neurons from the blood. Glial cells are active after injury in brain repair, but may contribute to neurodegenerative diseases. Microglial cells are resident macrophages in the brain. Microglia are trigged by foreign antigens and activate a variety of immune responses. Like macrophages in other tissues, their sensing and reacting ability is both defensive and destructive. Inherited forms of neurodegenerative diseases, such as amyotrophic lateral sclerosis, have been explained as death of neurons because of mutant proteins produced internally; however, more complexity is always revealed by more research. The disease process involves interactions of glial cells with neurons. Microglia may initiate or at least contribute to disease progression. Proliferation of microglia is often observed in many brain diseases. Bahareh et al suggest that resident progenitor cells give rise to new microglia. Unchecked proliferation of astrocytes produces a common form of brain cancer. The most malignant is Glioblastoma multiforme.

    Fields stated: "glia operate in diverse mental processes, for instance, in the formation of memories. They have a central role in brain injury and disease, and they are even at the root of various disorders — such as schizophrenia and Alzheimer's — previously presumed to be exclusively neuronal... neurons working alone provide only a partial explanation for complex cognitive processes, such as the formation of memories. The complex branching structure of glial cells and their relatively slow chemical (as opposed to electrical) signaling make them better suited than neurons to certain cognitive processes. These include processes requiring the integration of information from spatially distinct parts of the brain, such as learning or the experiencing of emotions, which take place over hours, days and weeks, not in milliseconds or seconds. Nearly all cancers originating in the brain derive from glia (which, unlike mature neurons, undergoes cell division). In multiple sclerosis, the myelin sheaths around axons become damaged. In HIV-associated neurological conditions, the virus infects astrocytes and microglia, not neurons.( R. Douglas Fields. Map the other brain. Nature 501,25–27(05 September 2013)


    • Neuroscience Notes

    • This book places the human brain at the center of the universe. Since the brain is the organ of the mind, consciousness and all knowledge is contained within the brain. Everyone needs to know something about neuroscience. The brain has become a popular topic in all media, but confusions arise when the brain becomes an abstract fantasy in the minds of journalists and product promoters. While it is true that brain is the organ of the mind, our language makes it difficult to speak correctly at different levels of meaning. Neuroscience notes will give the intelligent reader and understanding of how the brain actually works.

    • Neuroscience Notes is part of the Persona Digital Psychology and Philosophy Series of related books. The closely related volumes are the Human Brain, Language and Thinking, Emotions and Feelings, Intelligence and Learning. Neuroscience notes is available as eBook download from Alpha Online.
    • The author is Stephen Gislason MD The latest date of publication is 2018. 306 Pages

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