PYB102 – Week Three Revision
Cells in the human nervous system:
Neurons – these are the basic units of the human nervous system. They have three
main functions: taking in information from other neurons (reception), integrating
these signals (conduction) and passing these signals on to other neurons
Glial cells – these nourish, protect and physically support the neurons and are critical
to brain development. There are many different types; however one particular type is
responsible for covering neurons with myelin – a substance which is essential for the
brain to function.
Parts of a Basic Neuron
Dendrites – the dendrites branch out from the soma and receive the messages from
other neurons. Once they retrieve this information, they relay it to the soma.
Cell Body (Soma) – the soma or cell body is the main part of a neuron. One of its
functions is the maintenance and responsibility for metabolical functions of the
Axon – the axon then carries these messages away from the soma and towards the
terminal buttons. These messages are called ‘action potential’.
Terminal Buttons – Terminal buttons branch off from the end of the axon. The
messages are then transmitted via the terminal buttons to the dendrites of the next
neuron, thus completing the communication of a message through the entire neuron
Myelin – a substance that in neurons is used to insulate the axon and promotes
efficient and timely transmission of the message along the axon.
When neurons transmit information from one to another, there is no actual physical
interaction between the two. In fact, there are miniscule gaps between the terminal button
of one neuron and the dendrite of another, and these gaps are called synapses or the
Because of this, we describe neurons as either pre-synaptic or post-synaptic neurons,
according to where they are in relation to the synapse. Those who are transmitting the
message are pre-synaptic and those receiving it are described as post-synaptic.
The Cell Membrane
The cell membrane is essentially the outside or ‘skin’ of the neuron. It is made up of two
fatty layers – called a lipid bilayer – and many proteins float inside it. The interior of the
membrane is high in negatively-charged proteins. These protein molecules form pores or
channels called ion channels which control the material that moves in and out of the cell.
These ion channels are usually closed when the neuron is at rest (not sending or receiving
messages). When the neuron is at rest, there are usually more Potassium (K+) ions on the inside of the cell membrane, and more Sodium ions (Na+) on the outside of the cell
membrane. Both kinds have a single positive charge.
Materials inside the neuron are referred to as intracellular while those on the outside of the
neuron are called extracellular. Potassium has a higher concentration while it is
intracellular, whilst sodium has a higher concentration while it is extracellular.
The Resting Membrane Potential (RMP)
The RMP is the difference in voltage between the inside and outside of the neuron. The
average RMP of a neuron is -70 mV (millivolts) – which means that the inside of the neuron
is 70 mV less than the outside.
A good way to measure the RMP of a neuron is by placing the neuron in a Petri dish and
comparing to voltage of an electrode both when it is outside of the neuron (in the
extracellular substance) and inside. An amplifier will show when the voltage has changed.
The Action Potential
The Action Potential, as described above, is the opposite or reversal of the resting
membrane. This means that the inside of the neuron goes from relatively negative to
relatively positive as a result of the ion channels opening again. It occurs when the neuron
exchanges ions (sends a message) down the axon away from the soma (cell body).
In terms of voltage, the action potential occurs in response to the neuron becoming
depolarised. This means that the intracellular voltage begins to move more positively
towards 0 mV. At around -55 mV the depolarisation reaches a threshold and the neuron
begins to fire the action potential. If this threshold is not reached, the action potential will
not fire and the neuron goes back to its resting potential.
The All-or-None Principle
It is important to remember that no matter the size of the neuron, the action potential fired
will always be the same size. There is no such thing as a part action potential or a little bit
being fired. It is all or nothing.
Depolarisation, Repolarisation and Hyperpolarisation
Depolarisation as described