proteins (with the help of ribosomes). What make the morphology of neurons
specific are the extensions that proceed from the cell body, including a number
of dendrites and a single axon.Dendrites(think of the Greek worddendron,
“tree”) are shorter, bushy extensions that collect information from other
neurons. The axon is a single“cord,”which is usually longer than the
dendrites and ends in several branches called presynaptic terminals. The
axon transmits information to other neurons across thesynapses, to which
we will come back in a moment.
As the distinctive contribution of neurons is to carry information across the
human brain and body, it is important to ask the question of how they
perform this task. Thought is often referred to as very swift such as in the
expression“at the speed of thought.”However, if we compare the speed at
which a single piece of information spreads along an axon with the speed of
electric signals in a computer or telephone wire (which can be close to the
speed of light), it is in fact rather slow, amounting to 1 to 120 meters per
second. We will see, in turn, that the human brain and body deals with
information with muchflexibility and efficiency. Information spreads within
and across neurons with the help of a combination of electric and chemical
processes (Byrne, 2013a; Kalat, 2016, pp. 26–34). A typical way for neurons to
conduct information is to summarize incoming impulses from other neurons
and send anaction potentialdown the axon (that is, to“fire”) when the inputs
added up pass a threshold (McCormick, 2014). This happens in an all-or-none
fashion: the action potential does not vary in strength, being somewhat
similar to the“zeroes”and“ones”representing information in a computer.
Therefore, more intense signaling in neurons can occur through the sending
of more frequent action potentials. The accumulation of impulses and
the forwarding of the action potential are based on the regulatedflow of
DendritesSomaAxonMyelin sheathNucleusPresynaptic terminalsFigure 3.1.The neuron.52 Cognitive Science and the New Testament