PRINCIPLES OF TOXICOLOGY

ions moving out of the cell brings the membrane state back to its original resting potential (Figure
7.1c).
Neurotransmitter Activity
7.1 MECHANISMS OF NEURONAL TRANSMISSION 147
Figure 7.1 Propagation of an action potential along a nerve fiber. (a) The resting electrochemical potential of a
nerve. (b) Stimulation alters the sodium permeability of the nerve. (c) As sodium ions rush in, the adjacent gradient
begins to depolarize, which increases sodium permeability and allows sodium to enter this part of the nerve as well.
This action propagates down the nerve as a small, local current. (d) Repolarization begins in the same place the
impulse started. The high positive charge inside the cell increases the potassium permeability. Potassium ions flow
out of the cell and reestablish the resting potential. (e) Repolarization travels down the nerve until it is complete.
Communication between neurons and other cells occurs by both electrical and chemical signals.
Electrical signals, the fastest means of communication, are transmitted between tightly packed neurons
through membrane pores called gap junctions. The slightly slower chemical signals consist of
neurotransmitters released at the synapse, which then bind to receptors on the postsynaptic cell,