14.1.1 Tranismission of Signals

Topic 14: Transmission of Signals
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT Point(s)
 identify neurones as nerve cells that are the transmitters of
signals by electro-chemical changes in their membranes
 present information from secondary sources to graphically
represent a typical action potential
 define the term ‘threshold’ and explain why not all stimuli
generate an action potential
Signals are transmitted by nerves in the form of electricity along
the neuronal membrane, and by chemicals from one neurone to
the next. This is why its called an electrochemical change.
Neurones Contain Ions
 Nerve impulses are electrical signals which are produced by
the plasma membrane (cell membrane) of the nerve cell.
The human body as a whole is electrically neutral, having almost
the same number of positive charges as negative charges.
Neurones Contain Ions
It may help to think of a torch and how it works. Its battery
separates electrical charges between two poles, the positive and
the negative. This gives the two charges potential energy. The
measure of this potential energy is called voltage.
Neurones Contain Ions
Voltage is always measured between two points and is called
potential difference or simply potential. The flow of electricity
from one point to another is called a current and it can be made
to do work.
Neurones Contain Ions
 A potential difference exists across every cell’s plasma
The side of the membrane exposed to the cytoplasm is
negative while the side exposed to the extracellular fluid is
Neurones Contain Ions
 The differences on either side
of the membrane result in a
cellular voltage, which is
called the resting membrane
potential and measures about
70 millivoilts.
It is written as -70mV, which
indicates that the inside of the
membrane is negative. The
membrane is then said to be
Neurones Contain Ions
This arises because neurones
contain ions (charged particles).
 Positively charged ions are called
 Negative charged ions are called
When either of these is found in a
solution, the solution is then an
electrolyte solution. The cytoplasm
of a cell and the extracellular fluid
are both electrolyte solutions, but
they differ greatly from each other.
Neurones Contain Ions
Cell membranes are
impermeable to proteins and
most organic phosphates, so
these are kept inside the
cytoplasm. The membranes are
selectively permeable to Na+,
K+ and Cl- because of ion
Neurones Contain Ions
When the ion channel pores are open, ions can move from one
side of the membrane to the other. Each channel allows only a
specific type of ion to diffuse through it.
Depolarisation and Action Potential
Changes in the environment of
a neurone can affect the
permeability of the plasma
membrane to ions and therefore
change the membrane’s
 Any environmental factor
which causes such a change is
called a stimulus and
neurones are highly reactive
to stimuli.
Depolarisation and Action Potential
 A cell’s membrane potential
can change in response to
appropriate stimulation.
A positive shift in membrane
potential, from -70mV to 10mV, for example, is called a
 If the depolarisation is strong
enough, this flow of ions
causes the neurone to
generate a nerve impulse or
action potential.
Depolarisation and Action Potential
 The action potentials are
transmitted from neurone to
neurone across small gaps
called synapses (the junctions
between the end of one axon and
the dendrites of the cell body of a
receiving neurone). The
movement is in one direction
Depolarisation and Action Potential
 At the synapse, chemicals known as neurotransmitters diffuse
across the gap from one neurone to the membrane of the
receiving neurone, causing an electrical response.
Depolarisation and Action Potential
 The threshold is the
amount of positive change in
membrane potential which is
required before an action
potential is produced.
 The depolarisation must
reach a threshold, which is at
least 15mV more positive
than the resting potential of 70mV.
Depolarisation and Action Potential
 No action potential is
produced if the depolarisation
is below this level.
This is one of the reasons why
not all stimuli generate an action
potential. Also, Each stimuli
produces either a full action
potential or none at all. Each
action potential is a separate
event. Therefore, a cell cannot
produce another action potential
until the previous one is
-Handout Table 7.3 Typical changes in membrane potential
-Handout DOT Point 7.8 Typical Action Potential
-Students to create a graph to convey information in table.

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