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Lecture

HMB320H1 Lecture Notes - Resting Potential, Extracellular Fluid, Ion Transporter


Department
Human Biology
Course Code
HMB320H1
Professor
Joordens

Page:
of 6
NROB60 Chapter 3
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
o Substances with uneven electrical charges will dissolve in water
These substances are hydrophilic
o Compounds whose atoms are bonded by non-polar covalent bonds have no
basis for chemical interactions with water
Such compounds will not dissolve in water and are said to be
hydrophobic
One familiar example are lipids which are important to the structure of cell
membranes
The lipids of the neuronal membrane contribute to the resting and
action potentials by forming a barrier to water-soluble ions and in
water itself
o The Phospholipid Bilayer
The main chemical building blocks of cell membranes are phospholipids
A phospholipid has a polar phosphate group attached to one end of the
molecule
Thus phospholipids are said to have a polar “head that is
hydrophilic and a non-polar “tail” that is hydrophobic
The neuronal membrane consists of a sheet of phospholipids, two
molecules thick
Hydrophilic heads face each other
This stable arrangement is called a phospholipid Bilayer and it
effectively isolates the cytosol of the neuron from the extracellular
fluid
Protein
o The type and distribution of protein molecules distinguish neurons from other
types of cells (e.g. enzymes, cytoskeleton, and receptors)
o Protein Stucture
All amino acids have a central carbon atom (alpha carbon) which is
covalently bonded to four molecular groups a hydrogen, carboxyl and
amino groups and also a variable R group
The differences between amino acids result from differences in the size
and nature of these R groups
The properties of the R groups determine the chemical
relationships in which each amino acid can take part in
Amino acids assemble into a chain connected by peptide bonds, which
join the amino group of one amino acid to the carboxyl group of the next
The four levels of protein structure are:
Primary: a chain in which the amino acids are linked together by
peptide bonds
Secondary: the linear chain may coil up into an alpha helix
Tertiary: Interactions among the R groups can cause the molecule
to change its 3D conformation even further and assume a globular
shape
Quaternary: different polypeptides chains can bond together to
form a larger molecule
o Each polypeptide contributing to the protein’s structure is
called a subunit
Channel Proteins
Ion channels are made from just these sorts of membrane-spanning
protein molecules.
Typically, a functional channel across the membrane requires that
4-6 similar protein molecules assemble to form a pore between
them
One important property of most ion channels, specified by the
diameter of the pore and the nature of the R groups lining it, is ion
selectivity.
Another important property is gating.
o Channels can be opened and closed by changes in the
local microenvironment of the membrane
o Ion Pumps
Other membrane-spanning proteins come together to form ion pumps
Ion pumps are enzymes that use energy released by the breakdown of
ATP to transport certain ions across the membrane
The Movement of Ions
Ionic movements through channels are influenced by two factors: diffusion and electricity
Diffusion
o Ions and molecules dissolved in water are in constant motion.
o This temperature-dependent, random movement will tend to distribute the ions
evenly throughout the solution
o There will be a net movement of ions from regions of high concentration to
regions of low concentration diffusion
o Diffusion will cause ions to be pushed through channels in the membrane
For example, NaCl is dissolved in fluid on one side of a permeable
membrane
The Na+ and Cl- ions will cross until they are evenly distributed in the
solutions on both sides
The difference in concentration on both sides is called a concentration
gradient
Thus, it is said that ions flow down a concentration gradient
o Driving ions across the membrane by diffusion happens when:
1) The membrane possess channels permeable to the ions, and
2) There is a concentration gradient across the membrane
Electricity
o Another way to induce a net movement of ions in a solution is to use an electrical
field
o Since opposite charges attract and like charges repel, there will be a net
movement of Na+ toward the negative terminal and of Cl- toward the positive
terminal.
o The movement of electrical charge is called electrical current (I) and is
measured in amperes
o Two important factors determine how much current will flow: electrical potential
and electrical conductance
o Electrical potential (voltage V) is the force exerted on a charged particle and it
reflects the difference in charge between the anode and the cathode
More current will flow as this difference is increased