BIOL 4503 Lecture Notes - Alpha Helix, Chemical Polarity, Resting Potential

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2 Feb 2013
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Introduction
The neuron solves the problem of conducting information over a distance by using
electrical signals that sweep along the axon.
Axons act like telephone wires, however, the type of signal used by the neuron is
constrained by the special environment of the nervous system
Electrical charge in the cytosol of the axon is carried by electrically charge atoms (ions)
instead of free electrons
o This makes the cytosol far less conductive
o Also, the axon is not especially well insulated and is bathed in salty extracellular
fluid, which conducts electricity
Fortunately, the axonal membrane has properties that enable it to conduct a special type
of signal, action potential, that overcomes these biological constraints
Action potentials do not diminish over distance; they are signals of fixed size and
duration
o Information is encoded in the frequency of action potentials of individual neurons,
as well as in the distribution and number of neurons firing action potentials in a
given nerve
Cells capable of generating and conducting action potentials are said to have excitable
membrane. Thus the “action” in action potentials occurs at the cell membrane
When a cell is not generating impulse, it is said to be at rest
o In resting, the cytosol along the inside surface of the membrane has a negative
electrical charge compared to the outside
o This difference in electrical charge across the membrane is called the resting
membrane potential
The action potential is simply a brief reversal of this condition and for an instant, the
inside of the membrane becomes positively charge with respect to the outside
The Cast of Chemicals
Cytosol and Extracellular Fluid
o Water is the main ingredient of the fluid inside the neuron, the cytosol, and the
extracellular
o Electrically charged atoms are dissolved in this water
They are responsible for the resting and action potentials
o Water
The most important property of the water molecule is its uneven
distribution of electrical charge. Because of this property, the oxygen
atoms in water acquire a net negative charge
Water is said to be a polar molecule, held together by polar covalent
bonds
o Ions
Atoms or molecules that have a net electrical charge are known as ions
Ions are held together by ionic bonds
Ions with a net positive charge are called cations
Ions with a negative charge are called anions
Ions are the major charge carriers involved in the conduction of electricity
in biological systems
The Phospholipid Membrane
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