3: Graded / action potentials

• Axosomatic synapses interact directly with the cell body, while axodendritic synapses interact with the dendrites, with both affecting the initial segment and thus action potential frequency.

  On the other hand, axoaxonic synapses interact with postsynaptic axons, affecting the axon terminal and thus neurotransmitter release.
• Interneurons are located completely in the central nervous system.
• Stimuli can cause graded potentials, while graded potentials alter action potential frequency. They can either increase (excite) or decrease (inhibit) this frequency.
• Graded potentials can cause either a depolarizing or hyperpolarizing change in membrane potential.
• The equilibrium potential for Na+ is 60 mV. Thus, a stimulus that opens channels leading to influx of this ion is excitatory.
• The equilibrium potential for K+ is -90 mV. Thus, a stimulus that opens channels leading to efflux of this ion is inhibitory.
• The equilibrium potential for Ca2+ is 134 mV. Thus, a stimulus that opens channels leading to influx of this ion is excitatory.
• The equilibrium potential for Cl- is -80 mV. Thus, a stimulus that opens channels leading to influx of this ion is inhibitory.
• Stimulus intensity is encoded by action potential frequency. Maximum frequency is determined by the length of the absolute refractory period.