Download “You`ve Got Mail”: How Neurons Send Messages The firing of a

“You’ve Got Mail”: How Neurons Send Messages
The firing of a neuron is a biochemical process that uses electrical impulses to relay information
in the brain. Much like the postal service that brings you your mail every day, neurons sort,
package, and deliver information to other neurons. This document describes how multipolar
neurons send electrical messages to each other using that sort, package, and deliver process.
Other types of neurons as exist well, but they are outside the scope of this text.
Parts of a Neuron
A multipolar neuron is named for the numerous appendages extending from the central hub of
the cell. This type of neuron is designed much like a tree. It is branched at both ends and has a
“trunk” in the middle. Beginning at the “top” of the tree, the parts of a neuron are:
 The dendrites, a branched structure that reaches to the ends of other neurons.
 The soma, or the body of the neuron. The soma holds the nucleus of the cell, and is
responsible for keeping the cell alive.
 The axon, a long appendage similar to the trunk of a tree. The neuron receives most of
its length from the axon. This length allows the messages to be carried great distances.
 The axon terminal (the roots of a tree), which connects to the dendrites of the next
neuron.
Figure 1: The Neuron [animatlab.com]
“Sorting the Mail”: Dendrites React to a Message Being Received
When dendrites receive a message from a preceding neuron, changes begin to occur in the cell. A
cell that is not firing has a resting voltage of -70 mV. However, as messages begin to be
received, the cell registers the communication by becoming more positive.
This change in voltage is achieved by transport channels embedded in the lipid bilayer of the cell
wall (the barrier that separates the inside of the cell from the outside). Usually these transport
channels keep ions and unnecessary materials from entering the neuron and changing its biology.
Figure 2: Transport Channel [www.sonefe.org]
However, a neuron receiving a message will allow the transport channels to open- but only allow
sodium (Na+) in and potassium (K+) out. Like a bouncer at the clubs downtown, the transport
channel is selective about which ions are allowed into the cell, and they must show ID before
they’re allowed in. They prove themselves by being the correct shape, and fitting together with
the side of the transport channel much like a puzzle piece. ATP supplies the energy for this
exchange, and it has its own shape and puzzle, as shown in Figure 2.
For every 2 K+ that leave the neuron, 3 Na+ enter, making the cell more positive. The
positive ions in turn raise the voltage of the cell to 30 mV, which is the signal the soma needs to
begin preparing an action potential.
“The Letter”: Action Potentials Continue the Message Movement
Imagine a neuron as a see-saw. Some messages try to get one side to go down, some try to make
the other. More and more incoming messages add weight to a certain side. In order to begin
again though, one side must touch the ground. This is how the all-or-nothing principle works
with neurons. A threshold must be reached for an action potential to be generated. If the
threshold is not reached, then no action potential is generated and the “letter” is not sent to the
next neuron.
An action potential is the “message” or “letter” that gets carried down to the next cell. In order
for an action potential to be generated, a certain threshold must be reached. This can be difficult
because some neurons try to hinder an action potential from being created. These neurons are
called inhibitory neurons (as opposed to excitatory) and the messages they send create a sort of
balance with other messages.
Once an action potential is created, the cell needs some time to go back to -70 mV before it can
try to fire again. In this refractory period, the transport channels reverse the previous flow, and
K+ travels back into the cell as NA+ is pushed out.
The now-existing action potential is ready to be delivered. It travels from the soma down the
axon and to the synaptic terminals. Like a wave travelling across a smooth body of water, the
action potential is an area of positive charge created from the NA+ ions, which travels down the
axon. After the action potential passes, the axon “calms” and the negative charge is
reestablished.
The axon is covered in insulation known as the myelin sheath, which helps keep the electrical
impulse from weakening in its travels. It’s similar to putting bubble-wrap in a fragile package; it
keeps the message safe.
“Into the Mailbox”: Action Potentials Release Neurotransmitters
At the axon terminal, the action potential reaches the end of its journey. The action potential
letter is “opened” with the physical opening of vesicles, which are voids in the terminal knobs
that hold neurotransmitters. The vesicles are told by the action potential to move toward the edge
of the terminal. As it reaches the edge, these neurotransmitter bubbles “pop” and allow the
neurotransmitters to exit the neuron.
Neurons do not physically touch each other, so when neurotransmitters are released they occupy
the space between the sending neuron’s synaptic terminal and the receiving neuron’s dendrites.
This in-between is known as a synapse.
Figure 3: The Synapse [www.txtwriter.com]
Neurotransmitters that are released into the synapse do one of 3 things:
A) Attach to a receptor on the receiving dendrite and work to create the next action potential
B) Get destroyed by enzymes in the extracellular fluid, serving no role in the next action
potential
C) Re-enter the synaptic terminal, and get re-packaged into a vesicle to be ejected later on
The number and type of neurotransmitters emitted influences the next action potential. The more
neurotransmitters that reach dendritic receptors, the quicker the “see-saw” can hit the ground and
cause an action potential.
The Big Picture
This whole process is repeated on an endless loop (like your favorite song). Action potentials are
generated, travel down the axon, and release neurotransmitters in different combinations to
create nerve impulses, and in a larger scope, thoughts.
Figure 4 (below) illustrates the action potential through its delivery.
Figure 4: A Firing Neuron [wwhfblog.com]