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Chapter 3

PSYC 1010 Chapter Notes - Chapter 3: Excitatory Postsynaptic Potential, Amyotrophic Lateral Sclerosis, Inhibitory Postsynaptic Potential

Course Code
PSYC 1010
Rebecca Jubis

of 16
Readings- Chapter 3 The Biological Bases of Behaviour
Nervous Tissue: The Basic Hardware
Nervous system is living tissue composed of cells
Cell are either glia or neurons
Neurons: individual cells in the nervous system that receive, integrate & transmit
Every neuron looks different
Soma or cell body: contains the cell nucleus & much of the chemical machinery common to
most cells
Neurons have a bunch of branched, tree-like structures called dendrites
Dendrites: the parts of a neuron that are specialized to receive information
Axon: a long, thin fibre that transmits signals away from the soma to other neurons or to
muscles or glands; they can be as long as a metre
Axons are wrapped by white, fatty substance called myelin sheath
Myelin sheath: insulating material, derived from glial cells, that encases some axons
Myelin sheath speeds up the transmission of signals that move along axons
If an axons myelin sheath deteriorates, signals may not be effectively transmitted 
sometimes lead to multiple sclerosis
Axon ends in a clutter of terminal buttons
Terminal buttons: small knobs that secrete chemicals called neurotransmitters; these
chemicals are like messengers
Synapses: a junction where information is transmitted from one neuron to another
Overall, information is received at the dendrites, passed through the soma, along the
axon, & is transmitted to the dendrites of another cell at meeting points called
Glia: cells found throughout the nervous system that provide various types of support
for neurons
They tend to be smaller than neurons but they outnumber neurons (10-1)
Glial cells take up about 50% of the brains volume
Glial cells supply nourishment to neurons, help remove neurons waste products, &
provide insulation around many axons
Glial also help the development of the nervous system in the human embryo
Glia may also send & receive chemical signals
Can be implicated in many diseases; amyotrophic lateral sclerosis (ALS), &
Parkinsons disease
Glial cells also play an important role in memory formation, & can lead to Alzheimers
Impaired neural-glial communication might also contribute to schizophrenia & mood
The Neuron at Best: A Tiny Battery
Hodgkin & Huxley learned that the neural impulse is a complex electrochemical
Inside & outside the neuron are fluids containing electrically charged atoms &
molecules called ions
+ charged sodium & potassium ions & - charged chloride ions flow back & forth
across the cell membrane. But dont cross at the same rate
Usually a slightly higher concentration of charged ions inside the cell
Resting potential of a neuron: its stable, - charge when the cell is inactive
At rest, its resting potential is at -70 millivolts
The Action Potential
When the neuron is stimulated, channels in its cell membrane open, allowing +
charged sodium ions to rush in which causes an action potential
Action potential: a very brief shift in a neurons electrical charge that travels along an
Absolute refractory period: the minimum length of time after an action potential
during which another action potential cant begin
This downtime usually lasts 1-2 milliseconds; followed by a brief relative refractory
period; the neuron can fire, but its threshold for firing is elevated; more intense
stimulation is required to initiate an action potential
The All-or-None Law
Neural impulse is all or nothing; not halfway
Neurons have control over how strong the stimulus is
A stronger stimulus will cause a cell to fire a more rapid volley of neural impulses
than a weaker stimulus
Thicker axons transmit neural impulses quicker than thinner ones
Can move up to 100 metres per second
The Synapse: Chemicals as Couriers
During the synapse part, the two neurons dont actually touch, they are divided by a
synaptic cleft
Synaptic cleft: a microscopic gap between the terminal button of one neuron & the
cell membrane of another neuron
The neuron that sends a signal across the gap is called the presynaptic neuron; the
neuron that receives the signal is called the postsynaptic neuron
Neurotransmitters: chemicals that transmit information from one neuron to another
Most of these chemicals are stored in synaptic vesicles
The neurotransmitters are released when a vesicle fuses with the membrane of the
presynaptic cell; its contents spill into the synaptic cleft
After release, neurotransmitters diffuse across the synaptic cleft to the membrane of
the receiving cell
There, they mix in the postsynaptic cell membrane
Receiving Signals: Postsynaptic Potentials
When a neurotransmitter & a receptor molecule combine, reactions cause a
postsynaptic potential (PSP)
Postsynaptic potential: a voltage change at a receptor site on a postsynaptic cell
They dont follow the all or nothing law; they are graded
They vary in size & they increase or decrease the probability of a neural impulse in
the receiving cell to the amount of voltage change
Two types of messages can be sent from cell to cell excitatory or inhibitory
Excitatory postsynaptic potential: a positive voltage shift that increases the likelihood
that the postsynaptic neuron will fire action potentials.