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Neurons & synapses

Topic 6
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5

Biology SL

Nerve impulse
➢ The signal being sent through the neuron’s axon is called nerve impulse
➢ An action potential is an electric signal
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But not by the movement of electrons which is regular electricity, but due to
movement of ions
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Actual potential = nerve impulse

➢ Some animals, such as humans, have highly developed neurons where axons are
covered by a myelin sheath
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This greatly increases the travelling rate of the nerve impulse

➢ The nerve impulse can be divided into 3 steps
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Resting potential
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Depolarization
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Not sending signal
Sending a signal

Repolarization
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Returning to resting potential

Resting potential
➢ The resting potential is the time period during which an area of a neuron is ready to
send an action potential, but it isn’t actually sending one
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This area of the neuron is said to be polarized
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The axon has a positive and a negative side

➢ The polarization occurs across the cell membrane and it’s due to the active
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transport of Na and K in 2 different directions
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➢ The majority of the Na are actively transported out of the axon cell into the
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intercellular fluid and the majority of K are transported into the cytoplasm
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This active transport mechanism is called sodium-potassium pump
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For every 3Na that are pumped out of the axon, 2K are pumped in

➢ There are negatively charged organic ions permanently located in the cytoplasm of
the axon
➢ The net result of the position of the charged ions leads to a net positive charge
outside of the axon membrane and a net negative charge inside the axon
membrane
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➢ Creation of a concentration gradient of Na and K across the membrane for
depolarization

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Neurons & synapses

Topic 6
...
5

Biology SL
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➢ Repolarization is a series of events beginning with K diffusing out of the localized
area of the membrane
➢ Action potentials can be measured by measuring the voltage changes across the cell
membrane
Myelin sheath
➢ The neurons of higher organisms have often myelinated axons (=myelin sheath)
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These axons are referred to as myelinated fibers

➢ The myelin sheath is a series of cells called Schwann cells, that have wrapped
themselves around the axon many times creating multiple layers of the same cell
membrane
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These cells are spaced evenly along the axon with small gaps in between,
called nodes of Ranvier

➢ Saltatory conduction is the phenomenon whereby an action potential of myelinated
axons skips from one node of Ranvier to the next, as the impulse progresses along
the axon towards the synaptic terminals
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The action potential doesn’t undergo ion movements underneath the
myelin material
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The myelin sheath acts as an insulator

➢ The cytoplasm of the axon is electrically conductive
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Allows the electrical potential to skip from one node to the next

➢ Benefits:
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The impulse travels faster, as the in/out ion movement takes time and
saltatory conduction allows areas to be skipped

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Less ATP (=energy) is needed, as the only locations where the Na/K pump
needs to re-establish resting potential is at the nodes of Ranver

Synapses
➢ The communication between neurons is chemical and occurs where two or more
neurons adjoin each other in an area called synapse
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The axon’s synaptic terminals of a neuron adjoin the dendrites of the next

➢ The neurotransmitter (=chemical) is always released from the synaptic terminal
buttons of the first neuron
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Typically it results in a continuation of the impulse when it’s received by the
dendrites of the second neuron

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The neuron that releases the neurotransmitter is called presynaptic neuron
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Neurons & synapses
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Topic 6

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