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( Neurobiology)
Neurobiology is the study of cells of the nervous system and the organization of these cells into functional circuits that process information and mediate behavior.[1] It is a subdiscipline of both biology and neuroscience. Neurobiology differs from neuroscience, a much broader field that is concerned with any scientific study of the nervous system. Neurobiology should also not be confused with other subdisciplines of neuroscience such as computational neuroscience, cognitive neuroscience, behavioral neuroscience, biological psychiatry, neurology, and neuropsychology despite the overlap with these subdisciplines. Scientists that study neurobiology are called neurobiologists. Another major class of cells found in the nervous system are glial cells. Despite the abundance of glial cells relative to neurons in the nervous system (there are ten glial cells for every single neuron), glial cells are only recently beginning to receive attention from neurobiologists for being involved not just in nourishment and support of neurons, but also in modulating synapses. For example, Schwann cells, which are a type of glial cell found in the peripheral nervous system, modulate synaptic connections between presynaptic terminals of motor neuron endplates and muscle fibers at neuromuscular junctions. One prominent characteristic of many neurons is excitability. Neurons generate electrical impulses or changes in voltage of two types graded potentials and action potentials. Graded potentials occur when the membrane potential depolarizes and hyperpolarizes in a graded fashion relative to the amount of stimulus that is applied to the neuron. An action potential on the other hand is an all-or-none electrical impulse. Despite being slower than graded potentials, action potentials have the advantage of traveling long distances in axons with little or no decrement. Much of the current knowledge of action potentials comes from squid axon experiments by Sir Alan Lloyd Hodgkin and Sir Andrew Huxley. The Hodgkin-Huxley Model of an action potential in the squid giant axon has been the basis for much of the current understanding of the ionic bases of action potentials. Briefly, the model states that the generation of an action potential is determined by two ions Na+ and K+. An action potential can be divided into several sequential phases threshold, rising phase, falling phase, undershoot phase, and recovery. Following several local graded depolarizations of the membrane potential, the threshold of excitation is reached, voltage-gated sodium channels are activated, which leads to an influx of Na+ ions. As Na+ ions enter the cell, the membrane potential is further depolarized, and more voltage-gated sodium channels are activated. Such a process is also known as a positive-feedback loop. As the rising phase reaches its peak, voltage-gated Na+ channels are inactivated whereas voltage-gated K+ channels are activated, resulting in a net outward movement of K+ ions, which repolarizes the membrane potential towards the resting membrane potential. Repolarization of the membrane potential continues, resulting in an undershoot phase or absolute refractory period. The undershoot phase occurs because unlike voltage-gated sodium channels, voltage-gated potassium channels inactivate much more slowly. Nevertheless, as more voltage-gated K+ channels become inactivated, the membrane potential recovers to its normal resting steady state.
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