Level 2 Unit 1 Part 2: Neuron membrane potentials

The function of neurons is to process and transmit information. Without input, most neurons have a stable electrical charge difference across the cell membrane called the resting potential. Neurons receive excitatory or inhibitory input information, either from other cells or from stimuli such as airborne molecules we perceive as odors. Input information from other cells usually enters through the dendrites, or less often the soma or parts of the axon. The information from inputs to the dendrites or soma is transmitted to the axon with membrane potential changes called graded potentials, which are usually small in size and brief in duration, and which travel short distances. The size and duration of a graded potential is usually proportional to the size and duration of the input. At any moment in time, summation, or combining, of all the excitatory and inhibitory graded potentials occurs at a place called the axon trigger zone. This is usually at, or close to, a structure called the axon hillock, which is where the axon connects to the soma. Summation of graded potentials at the trigger zone is the main way neurons process information from inputs.

If enough excitatory graded potentials summate to bring the membrane potential at the trigger zone over a value called the threshold potential, a different type of membrane potential change called an action potential usually occurs. This is then conducted the entire length of the axon. An action potential is large in size and brief in duration, and it may travel a long distance if the axon is long. Action potentials are usually the same size and duration for any given neuron. Action potentials are conducted faster along thicker axons, and those with a myelin sheath. When an action potential reaches an axon terminal, neurotransmitter is usually released at a synapse to bind to receptors on the target cell, which may change its behavior. The input information contained in the size and duration of graded potentials is converted into the temporal pattern, or timing, of action potentials conducted along the axon. That information is then usually converted into the amount and temporal pattern of neurotransmitter release at the synapse. Patterns of neurotransmitter release, determined by patterns of action potentials, are the main way neurons transmit information to their target cells.


Level 2 Unit 1 Part 3: Synapses

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