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Module: BIOM - 1010

Lecturer: Dr Zhu

Date: 3/11/16

The Nervous System Part 3
o

An axon transmits an action potential from the axon initial segment, downwards towards the terminals
 Once initiated, the action potential is self-propagating
 In nonmyelinated axons, each successive segment of membrane is depolarised then
repolarised
 It differs in myelinated fibres (see later)
 The action potential can only travel down the axon as the Na+ channels behind it are still
inactive

o

All action potentials are alike and are independent of stimulus intensity
 Neurones can tell the difference between a weak stimulus and a strong one by the frequency of
impulses

o

The refractory period that occurs due to the inactivity of sodium channels, occurs in two stages (see right):
 Absolute refractory period
 This is the period of time from the opening of
the Na+ channels until the resetting of the
channels
 This ensures that an action potential is
either triggered or not, no in-between
 It also ensures that the action potential is
one way
 Relative refractory period
 This stage follows the absolute refractory
period
 Most Na+ channels have returned to
their resting state
 Some K+ channels are open
 Repolarisation is occurring
 The threshold for another action potential is increased and therefore, only very
strong stimuli can stimulate another action potential

o

The rate of an action potential traveling down an axon depends on two factors:
 The axon diameter
 Large diameter – faster conduction
 The degree of myelination in the axon
 Unmyelinated – continuous conduction
 This is relatively slow
 Myelinated axon – saltatory conduction
 This is fast

o

Myelinated fibres transmit an action potential faster due to the nodes of Ranvier
 The action potential is only generated in the nodes of Ranvier and so does not have to depolarise
the whole length of the axon

o

Multiple sclerosis is an autoimmune disease that affects young adults
 In multiple sclerosis, the myelin sheaths are destroyed when the immune system attacks the
myelin
 This turns the myelin into harden lesions called scleroses
 The impulses conduct slower and eventually stop
 Demyelinated axons gradually express Na+ channels causes cycles of relapse and remission



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