Essential Cell Biology 5th edition

416 CHAPTER 12 Transport Across Cell Membranes
Once an action potential has passed, Na + pumps in the axon plasma
membrane labor to restore the Na + and K + ion gradients to their levels
in the resting cell. The human brain consumes 20% of the total energy
generated from the metabolism of food, mostly to power these pumps.
Voltage-gated Ca 2+ Channels in Nerve Terminals
Convert an Electrical Signal into a Chemical Signal
When an action potential reaches the nerve terminals at the end of an
axon, the signal must somehow be relayed to the target cells that the
terminals contact—usually neurons or muscle cells. The signal is transmitted
to the target cells at specialized junctions known as synapses.
At most synapses, the plasma membranes of the cells transmitting and
receiving the message—the presynaptic and the postsynaptic cells, respectively—are
separated from each other by a narrow synaptic cleft (typically
20 nm across), which the electrical signal cannot cross. To transmit the
message across this gap, the electrical signal is converted into a chemical
signal, in the form of a small, secreted signal molecule called a neurotransmitter.
Neurotransmitters are stored in the nerve terminals within
membrane-enclosed synaptic vesicles (Figure 12–39).
When an action potential reaches the nerve terminal, some of the
synaptic vesicles fuse with the plasma membrane, releasing their neurotransmitter
into the synaptic cleft. This link between the arrival of an
action potential and the secretion of neurotransmitter involves the activation
of yet another type of voltage-gated cation channel: voltage-gated
Ca 2+ channels located in the plasma membrane of the presynaptic nerve
terminal. Because the Ca 2+ concentration outside the nerve terminal is
more than 1000 times greater than the free Ca 2+ concentration in its cytosol
(see Table 12–1), Ca 2+ rushes into the nerve terminal through the open
channels. The resulting increase in Ca 2+ concentration in the cytosol of
the terminal immediately triggers the fusion of synaptic vesicles with the
plasma membrane, which releases the neurotransmitter into the synaptic
cleft. Thanks to these voltage-gated Ca 2+ channels, the electrical signal
has now been converted into a chemical signal (Figure 12–40).
presynaptic
nerve terminal
dendrite of
postsynaptic
nerve cell
postsynaptic
membrane
synaptic
cleft
presynaptic
membrane
synaptic vesicles
(A)
2 µm
(B)
Figure 12–39 Neurons connect to their target cells at synapses. (A) An electron micrograph and (B) a drawing of a cross section
of two nerve terminals (yellow) forming synapses on a single nerve cell dendrite (blue) in the mammalian brain. Neurotransmitters
carry the signal across the synaptic cleft that separates the presynaptic and postsynaptic cells. The neurotransmitter in the presynaptic
terminal is contained within synaptic vesicles, which release neurotransmitter into the synaptic cleft. Note that both the presynaptic and
postsynaptic membranes are thickened and highly specialized at the synapse. (A, courtesy of Cedric Raine.)