Propagating the action potential

===Lecture Details===
Helena Parkington; Week 8 MED1011; Physiology
===Lecture Content===
Ca channels have low AP spikes as there is a small number of ions involved. They also have a reduced amplitude.
Voltage gated channels are membrane spanning proteins, have 4 subunits with 6 membrane spanning domains. S4 senses membrane voltage and S2 forms the conducting pore, opening and closing is determined by membrane potential. Closed at large negative values of membrane potential. Selective for relevant ion. Cation channels also exist and have ion channel properties. They are not regenerative and have spontaneous transmitter release; eg pacemakers. 
Action propagation proceeds down an axon at 2-5m/s. Very fast but it decays, and is not regenerative. The wider the fibre the less resistance/further the action potential can travel (Ia afferents compared to C efferents). Active AP maintains full size but is slow, passive is fast but decays. Nodes of Ranvier allow active at nodes and passive between nodes. Occurs on myelinated axons (decreases severity of membrane potential difference). Demyelination occurs in Guillain-Barre syndrome, dementias and MX. Branching can fail due to insufficient current in parent branch.
Sodium channels are most abundant at the axon hillock, node of Ranvier and soma, least under myelin and at dendrites. K channels are absent in node of Ranvier. Ca channels are abundant at terminals, dendrites, soma.
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