Resting potential, the imbalance of electrical charge that exists between the interior of electrically excitable neurons (nerve cells) and their surroundings. If the inside of the cell becomes less negative (i.e., the potential decreases below the resting potential), the process is called depolarization.

axon hillock

• an action potential arrives at the synaptic terminal.
• calcium channels open, and calcium ions enter the synaptic terminal.
• vesicles containing neurotransmitter fuse with the plasma membrane of the sending neuron.
• neurotransmitter molecules diffuse across the synaptic cleft.

What event triggers the generation of an action potential? The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV. This is the minimum value required to open enough voltage-gated Na+ channels so that depolarization is irreversible.

This is important because the increased flow of positively charged potassium ions out of the cell (relative to the rate of Na+ movement into the cell) results in a net negative charge inside the cell; the negative sign in the resting membrane potential represents the negative environment inside the cell relative to the …

Graded potentials are brought about by external stimuli (in sensory neurons) or by neurotransmitters released in synapses, where they cause graded potentials in the post-synaptic cell. Action potentials are triggered by membrane depolarization to threshold.

Graded potentials can be of two sorts, either they are depolarizing or hyperpolarizing (Figure 1).

Graded potentials

• are proportional in amplitude to the size of the input stimulus.
• may be depolarizing or hyperpolarizing.
• they can be integrated both temporally and spatially (see the discussion of synaptic integration)
• travel passively, uniformly in all directions. don’t require voltage-gated channels.

a response that increases with the amount of energy supplied as opposed to the reaction brought about by the ALL-OR-NONE LAW.