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BISC 104 - Physio Ex_Exercise 3 - Neurophysiology of

BISC 104 - Physio Ex_Exercise 3 - Neurophysiology of -...

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T he nervous system is responsible for most of the functions that character- ize higher organisms, such as muscular movement, awareness, thought, learning, and memory. Neurons are the functional cellular units of the nervous system. They are “excitable” cells that communicate by transmitting electrical impulses (“excitable” means that they are capable of producing large, rapid electrical signals called action potentials ). Neurons are specialized for re- ceiving, integrating, and transmitting information to other neurons and/or effec- tor cells. A typical neuron consists of a cell body, containing its nucleus and or- ganelles; dendrite(s), responsible for carrying nerve impulses toward the cell body; and an axon, responsible for carrying nerve impulses away from the cell. Junctions between cells are called synapses, where one cell (the presynaptic cell) releases a chemical messenger called a neurotransmitter that communicates with the dendrite or cell body of a postsynaptic cell. While synaptic transmission is usually thought of as being excitatory (inciting an action potential in the postsy- naptic cell), some are inhibitory. This is accomplished by causing the postsynap- tic cell to become hyperpolarized, or having a resting membrane potential that is more negative than the normal resting membrane potential. Neurons have two major physiologic properties: irritability, the ability to re- spond to stimuli and convert them into nerve impulses, and conductivity, the ability to transmit an impulse (in this case, to take the neural impulse and pass it along the cell membrane). In the resting neuron (that is, a neuron that is neither receiving nor transmitting any signals), the exterior of the cell membrane is positively charged and the interior of the neuron is negatively charged. This difference in electrical charge across the plasma membrane is referred to as the resting membrane potential, and the membrane is said to be polarized. The sodium-potassium pump in the mem- brane maintains the difference in electrical charge established by diffusion of ions. This active transport mechanism moves three sodium ions out of the cell while mov- ing in two potassium ions. Therefore, the major cation in the extracellular fluid out- side of the cell is sodium, while the major cation inside of the cell is potassium. The inner surface of the cell membrane is more negative than the outer surface, mainly due to intracellular proteins, which, at body pH, tend to be negatively charged. The resting membrane potential can be measured with a voltmeter by putting a recording electrode just inside the cell membrane and by placing a reference, or ground, electrode outside of the membrane. In the giant squid axon (where most early neural research was conducted), and in the frog axon that will be used in Neurophysiology of Nerve Impulses 3 E X E R C I S E O B J E C T I V E S 1. To define the following: irritability, conductivity, resting membrane potential, polarized, sodium-potassium pump, threshold stimulus,
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