Biomedical Engineering Reference
In-Depth Information
Graded Response and Action Potentials
A neuron can change the membrane potential of another neuron to which it is connected
by releasing its neurotransmitter. The neurotransmitter crosses the synaptic cleft, interacts
with receptor molecules in the postsynaptic membrane of the dendrite or cell body of the
adjacent neuron, and changes the membrane potential of the receptor neuron (Figure 12.3).
The change in membrane potential at the postsynaptic membrane is due to a transforma-
tion from neurotransmitter chemical energy to electrical energy. The change in membrane
potential depends on how much neurotransmitter is received and can be depolarizing or
hyperpolarizing. This type of change in potential is typically called a
, since
it varies with the amount of neurotransmitter received. Another way of envisaging the
activity at the synapse is that the neurotransmitter received is integrated or summed, which
results in a graded response in the membrane potential. Note that while a signal from a
neuron is either inhibitory or excitatory, specific synapses may be excitatory and others
inhibitory, providing the nervous system with the ability to perform complex tasks.
The net result of activation of the nerve cell is the action potential. The action potential is
a large depolarizing signal of up to 100 mV that travels along the axon and lasts approxi-
mately 1-5 ms. Figure 12.4 illustrates a typical action potential. The action potential is an
all or none signal that propagates actively along the axon without decreasing in amplitude.
When the signal reaches the end of the axon at the presynaptic terminal, the change in
potential causes the release of a packet of neurotransmitter. This is a very effective method
of signaling over large distances. Additional details about the action potential are described
throughout the remainder of this chapter after some tools for better understanding this
phenomenon are introduced.
graded response
Dendrites
Converging Axons
Axon
Axon
Cell Body
FIGURE 12.3
A typical neuron with presynaptic terminals of adjacent neurons in the vicinity of its dendrites.
