PHYSIOLOGY OF NERVE AND MUSCLE CELLS lecture 6

THE RESULT OF SUMMATION OF PSPs
1. INHIBITORY STATE
This occurs when the inhibitory input is greater than the excitatory input so IPSPs are produced and the cell membrane becomes hyperpolarized and its excitability decreases.
2. EXCITATORY STATE
This occurs when the excitatory input is greater than the inhibitory input so more EPSPs are produced and the cell membrane becomes depolarized. When depolarization reaches a critical threshold level (the firing level) a propagated action potential (nerve impulse) is produced. The propagated action potential starts at the initial segment of the neuronal axon not at the soma of the neuron because:
•i. The density of the voltage-gated Na+ channels at the initial segment is seven times as much as those at the membrane of the soma. This allows greater and faster Na+ influx which creates more rapid depolarization up to the firing level.
•ii. The depolarization required to open the voltage gated Na+ channels at "the initial segment is only +15 mV, whilst the required depolarization at the soma is +30 mV.
•This explains why the axoaxonic synapses are the most effective in exciting the postsynaptic neuron. This is because they are the nearest to the initial segment. The least effective synapses are the axodendretic synapses.