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Neurobiology is the study of the CNS (Central Nervous System) cells and the structure in which these cells create brain function. Neurobiology is an interdisciplinary field that derives both from biology and neuroscience. Neurobiology is a relatively narrow field of research when compared with neuroscience, a field of study that includes every aspect of the nervous system.

Neurons are cells that are designed to receive, propagate, and pass electrochemical impulses. An ordinary human brain contains over 100 billion neurons. Neurons differ in shape and function. One important trait of most neurons is excitability. Neurons generate electrical impulses in two ways: graded potentials and action potentials. Graded potentials happen when the membrane potential depolarizes and hypolarizes in a graded fashion relative to the amount of stimulus that is applied to the neuron, while an action potential is a "go or no-go" electrical impulse. Action potential is slower, but runs long distances with negligible decrement. Much of the current knowledge of action  potentials comes from squid axon experiments conducted by A. Lloyd Hodgkin and Andrew Huxley.  

Credit: Graham Johnson, - Graham Johnson Medical Media, Boulder, Colorado.

Action Potential is generated via two ions: Na+ and K+. An action potential can be divided into these sequential phases: rising, falling, undershoot,threshold and recovery. Following some local graded depolarizations of the membrane potential, the excitation threshold is reached, and voltage gated sodium pathways are opened, which causes an influx of Na+ ions.
As Na+ ions reach the cell, the membrane potential is further depolarized, and extra voltage gated sodium pathways are opened. This mechanism is also called a positive-feedback loop. As the rising phase reaches its climax, voltage gated Na+ pathways are inhibited whereas voltage gated K+ pathways open up, resulting in an aggregate outward flow of K+ ions, which repolarizes the membrane potential towards the resting membrane potential. Repolarization of the membrane potential keeps on going, resulting in an undershoot phase or absolute refractory period. The undershoot phase happens because unlike voltage gated sodium pathwys, voltage gated potassium channels are inhibited in a slower pace. And yet, as more voltage gated K+ pathways become inhibited, the membrane potential returns to its usual resting state.

Neurons communicate through synapses. Synapses are unique crossways between two approximate cells. The neuron that sends the signal is called the presynaptic neuron and the cell that receives the signal is the postsynaptic neuron. Synapses can be either electrical or chemical. Electrical synapses form gap junctions that let ions and other organic compounds pass between cells. Chemical synapses release neurotransmitters into the synaptic gap.
A neurotransmitter is a chemical messenger.
A receptor is a transmembrane protein molecule that a neurotransmitter binds with. Chemical synapses are substantially slower than electrical synapses.


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