Biological Bases of Behaviour.
Lecture 3: Brain Cells
and Neural Communication
Learning Outcomes.
• At the end of this lecture you should be able to:
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1. Describe the key elements of a nerve cell (neuron).
2. Describe the main support cells of the CNS.
3. Explain what is meant by the term 'membrane potential'.
4. Explain how an action potential is initiated and
conducted down the axon
1. Neurons.
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According to Williams &
Herrup (1988) the adult
human brain contains around
100 billion neurons (nerve
cells).
These are specialised cells
which receive and transmit
information.
They vary in size and shape
but all consist of the same
basic structures:
Structure of a Neuron.
• a) Cell body (soma).
• Contains the nucleus,
which houses the
chromosomes.
• The bulk of the cell
consists of cytoplasm - a
jelly like substance
containing structures
which carry out certain
functions:
soma
nucleus
Structures in the Cytoplasm.
• Mitochondria: Extract energy from the breakdown of
nutrients and provide energy in the form of adenosine
triphosphate (ATP).
• Endoplasmic reticulum: Store and transport chemicals
through the cytoplasm. 2 forms:
• i) Rough endoplasmic reticulum contain ribosomes which
are involved in protein synthesis.
• ii) Smooth endoplasmic reticulum transport substances
around the cytoplasm and produce lipid (fat).
• Golgi apparatus: A special type of endoplasmic reticulum
breaks down substances no longer required by the cell.
• The plasma membrane separates the inside of the cell from
the outside, it is selectively permeable with charged ions
only able to pass through protein channels.
b) Dendrites.
• These are the informationreceiving parts of a neuron.
• Dendrites receive chemical
information across a tiny gap
called a synapse.
• The surface of a dendrite is
lined with synaptic receptors.
• Outgrowths called dendritic
spines increase the surface
area available for synaptic
communication.
dendrites
Dendritic
spines
c) Axon.
• This is the informationsending part of the neuron
• A neural impulse (action
potential) flows along the
axon.
• Many vertebrate axons are
covered with an insulating
substance called a myelin
sheath.
• This consists of segments
separated by unmyelinated
regions called nodes of
myelin
Ranvier.
Axon
Node of Ranvier
The Information Flow.
• Action potentials flow along the axon to the presynaptic
terminals.
• Axons that send information to the periphery are called
efferent axons (e.g. motor neurons).
Presynaptic
terminals
A motor neuron
Flow of
information
Muscle fibre
The Information Flow, continued.
• Axons that receive information from the periphery are
called afferent axons (e.g. sensory endings in the skin).
• Thus, motor neurons act as efferents from the nervous
system, sensory neurons act as afferents into the nervous
system.
• So, efferent out, afferent in.
Sensory endings
A sensory neuron
Information
flow
Cross section of skin
d) Presynaptic Terminals.
• At the end of an axon are the presynaptic terminals (or
terminal buttons).
• When an action potential reaches the terminal buttons they
secrete a transmitter substance which travels across the
synapse to the next neuron in the chain.
• The neurotransmitter either excites or inhibits the
postsynaptic receptors (dendrites) of another neuron.
• Thus an individual neuron receives information via its
dendrites from the terminal buttons of axons from other
neurons, the terminal buttons of its axon send information
to other neurons.
Presynaptic Terminals.
Axons from other
nuerons influence
neuron A
Neuron A
Message flows down axon of neuron
A to influence neuron B
Neuron B
2. Support Cells.
• Neurons have a high metabolic rate and must be constantly
supplied with oxygen and glucose or they will die.
• The various support cells are thus very important.
• Glial cells hold neurons in place, control their supply of
chemicals, insulate them, and remove neurons that have
died. There are several forms:
• i) Astrocytes (astroglia): Provide physical support to
neurons and clear up debris (called phagocytosis).
• ii) Oligodendrocytes: Produce myelin in the CNS. In the PNS
the same function is provided by Schwann Cells. These
digest dying cells and then guide the axons to re-grow to a
limited extent.
• This does not happen in the CNS so that nerve damage (e.g.
in spinal neurons) is to be permanent.
Electrical Activity Within a Neuron.
• A microelectrode is placed
in the axon of a giant
squid.
• An electrode is placed in
the surrounding medium.
• Both are connected to a
voltmeter.
• The inside of an inactive
axon is negatively charged
with respect to the outside.
• This resting potential is
• -70mV.
voltmeter
microelectrode
electrode
The Action Potential.
• A positives charge applied to the inside of the axon makes it
more positive (depolarisation).
• If a sufficiently strong charge is applied then the threshold of
excitation is reached, and the neuron produces an action
potential.
• Here the
membrane potential is rapidly reversed and
becomes strongly positive (up to +40mV) with respect to the
exterior.
• The membrane potential quickly returns to normal, but first it
briefly overshoots its resting potential and drops to around
-75mV (hyperpolarisation).
• This entire process takes about 2msec.
The Action Potential.
The Membrane Potential.
• The electrical charge within the axon results from the
balance between two opposing forces:
• i) Diffusion: Molecules distribute themselves
evenly
throughout the medium in which they reside.
• ii). Electrostatic pressure: Particles with the same electrical
charge repel one another while particles with the opposite
charge attract one another.
• The environment inside the axon and in the fluid
surrounding it contain different ions.
• Organic ions (A-) only found inside the axon.
• Potassium (K+) found predominantly inside the axon.
• Chloride ions (Cl-) found predominantly outside the axon.
• Sodium ions (Na+) found predominantly outside the axon.
Resting Potential.
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•
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The axonal membrane is
selectively permeable.
At rest, ion channels permit
potassium and sodium to pass
through slowly.
Most of the sodium channels
remain closed.
 The sodium-potassium
pump expels sodium ions
and draws in potassium ions
in the ratio of 3 sodium out
to 2 potassium in.
 During an action potential
the sodium channels open
and allow sodium ions to
flood into the axon.
Events During the The Action Potential.
After the Action Potential.
• Neurons may have different thresholds of excitation but all
obey the rule that once the threshold is reached, an action
potential is triggered – this is called the ‘all-or-none rule’.
• Following the action potential, the sodium gates remain
closed for around 1ms and so further action potentials
cannot be triggered regardless of the stimulation.
• This is called the absolute refractory period.
• The sodium gates then open but the potassium gates
remain open for a further 2-4ms ensuring the no action
potentials can be generated.
• This is called the relative refractory period.
• The axon cannot cope with repeated excitation as the
sodium-potassium pump cannot keep up, as a result
sodium accumulates within the axon and no more action
potentials are possible. Scorpion venom keeps open the
sodium channels and causes paralysis.
Conduction of the Action Potential in
Unmyelinated Axons.
•
Each point along the axon membrane generates the action
potential. The next area of membrane is depolarised, reaches its
threshold and generates another action potential. In this manner
the action potential passes down the axon like a wave.
Conduction of the Action Potential in
Myelinated Axons.
• These axons are covered with an insulating layer of myelin,
separated by small unmyelinated gaps (nodes of Ranvier).
• Action potentials travel down the axon reducing in strength
until they reach the next node where another action
potential is triggered.
Saltatory Conduction.
• The jumping of action potentials from one node to another
in myelinated axons is referred to as saltatory conduction.
• There are two advantages to this
• 1. Energy is saved as sodium-potassium pumps are only
required at specific points along the axon.
• 2. Conduction of an action potential is much faster within a
myelinated axon (around 120 m/sec as opposed to around
35 m/sec) in unmyelinated ones.