University of Cambridge - Human Physiology - Neuronal action potential propagation to target
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Course
Medicine (A100)
Institution
Cambridge University (CAM)
Neuronal action potentials are propagated using local circuit currents which depend on the length and time constant to determine their speed. Also chemical synapses use neurotransmitters to bind to post-synaptic membrane ionotropic and metabotropic receptors to produce EPSPs and/or IPSPS that summa...
Action potentials are propagated in myelinated and non-myelinated neurones using local
circuit currents. They are regenerated along the length of the axon either at the Nodes of
Ranvier for myelinated neurones or along any point in non-myelinated neurones. For the
action potential to be regenerated, the local current must have sufficient charge to
depolarise the adjacent length of the membrane to the threshold. These action potentials
produced are identical in amplitude. However, there are also non-regenerative potentials
such as synaptic potentials generated by opening of activated ion channels in the post-
synaptic membrane that don’t have an all-or-nothing nature. They are graded responses,
subthreshold potentials that spread along short distances of the cell and dissipate within a
few millimeters. Whereas action potentials can propagate up to a metre. With a graded
response, the greater the neurotransmitter concentration, the greater the voltage change.
Local circuit currents
When an action potential is generated at one point in the neurone by the influx of Na+ ions,
the cytoplasm of this point is slightly more positively charged that adjacent regions that
have a slightly negative charge. Therefore, this charge imbalance causes ions to diffuse from
the depolarised area to the adjacent areas. The current carried by the ions moves along the
path of least resistance so it spreads longitudinally along the axon from the positive region
to the negative region. When the Na+ ions move into adjacent areas they cause a
depolarisation to threshold which then allows that area to generate its own action potential
and as more Na+ ions move into the neurone at that point the local circuit current can
continue to move along the axon. The refractory period caused by the inactivation of
voltage gated Na+ channels only allows the propagation in one direction so the signal is
transmitted from one end to the opposite.
Variables affecting speed of conduction
The internal resistance of the axon cytoplasm is inversely related to the internal cross
section. Therefore, to improve the conducting speed of action potentials nerve fibres
increase their axon diameter to decrease the internal resistance caused by the cytoplasm to
the movement of ions in the local current. For example, the giant squid axon can be up to
1mm in diameter which can propagate action potential at velocities approximately 25m/s.
However, in the human body there is limited space so axons can’t have as large diameter,
therefore an adaptation to increase conduction speed is myelinating the fibers to increase
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