ions (electrically charged atoms and molecules) across the cell membrane,
a wall that bounds the whole cell and contains gates that can selectively let
ions through.
The spread of information through the neuron goes as follows. (1) <cf. e.g.
p. 53> At rest, the neuron constantly pumps some ions across the membrane
to maintain a slight imbalance; as ions are electrically charged, there is a small
negative voltage at the inside of the cell called theresting potential(around– 70
millivolts). (2) <cf. e.g. p. 53> Incoming information at the dendrites (as well
as at terminals on the cell body) lets ionsflow in and out through the cell
membrane; as ions carry electric charge, their migration changes the voltage
between the interior and exterior of the cell. (3) <cf. e.g. p. 53> When a certain
threshold is reached (between–40 and–55 millivolt for most cells), gates for
sodium ions on the membrane go open. (4) <cf. e.g. p. 53> Sodium ionsflow
into the cell and bring in positive electric charge, causing a sudden surge in
positive voltage. (5) <cf. e.g. p. 53> The impulse called the action potential
moves forward along the axon toward the presynaptic terminals, as changes in
voltage and theflow of ions through opening gates interact. (6) <cf. e.g. p. 53>
Finally, the cell returns to its original balance of ions and voltage.
What happens at thesynapses, which connect neurons with each other? The
(pre-synaptic) terminals of an axon are connected to the (post-synaptic)
terminals of another neuron at the synapses (Kalat, 2016, pp. 39–59). This
connection is, however, different from the connection between two wires that
are welded together. At the synapses, the cells are divided by thesynaptic cleft,
which is crossed by chemical messengers calledneurotransmitters. The pre-
synaptic neuron sends out neurotransmitters (Zucker et al., 2014) that are
picked up by the receptors of the post-synaptic neuron (Waxham, 2014). The
receptors can be thought of as docking stations for neurotransmitters, differ-
ent types of receptors specializing in the reception of different neurotransmit-
ters. The use of neurotransmitters adds a powerful tool to the communication
system of the brain. The shortage or abundance of neurotransmitters in the
brain or in some part of the brain can have large-scale effects on the spread of
information. It is important to note that neurotransmitters do not have a
specific“meaning”as such: the arrival of a neurotransmitter at a particular
receptor can result in different effects depending on the type of neuron and its
role in the nervous system. For example, the abundance of the neurotrans-
mitter dopamine has an inhibitory effect on most neurons (that is, it lessens
the chance of themfiring), whereas via a complex process it leads to the
increased activity of others. The net effect of the abundance of dopamine in
the brain is the generation of psychological“reward,”causing addiction to
some drugs, for example. Endorphins are neurotransmitters that in abundance
relieve pain and block the formation of memories; the drug morphine acts on
the same neural receptors that accept endorphins. These and other neuro-
transmitters regulate thoughts, emotions, and behavior effectively. Yet their
The Human Brain: A Guided Tour 53