fes devices take advantage of this property to electrically activate nerve cells, which then may go on to activate muscles or other nerves. however, special care must be taken in designing safe fes devices, as electric current through tissue can lead to adverse effects such as decrease in excitability or cell death. this may be due to thermal damage, electroporation of the cell membrane, toxic products from electrochemical reactions at the electrode surface, or over-excitation of the targeted neurons or muscles. typically fes is concerned with stimulation of neurons and nerves. in some applications, fes can be used to directly stimulate muscles, if their peripheral nerves have been severed or damaged (i.e., denervated muscles). however, the majority of the fes systems used today stimulate the nerves or the points where the junction occurs between the nerve and the muscle. the stimulated nerve bundle includes motor nerves (efferent nerves—descending nerves from the central nervous system to muscles) and sensory nerves (afferent nerves—ascending nerves from sensory organs to the central nervous system).