Chapter 3 – Neuroscience and Behaviour
It was an unusual night, even for the late shift in the hospital emergency room. Seventeen-year-
old David saw people who weren’t there and 75-year-old Betty saw, but didn’t recognize, her own
husband. They discovered he was suffering from hallucinations–a side effect of abusing
methamphetamine. David’s prolonged crystal meth habit had altered the normal functioning of some
chemicals in his brain, distorting his perception of reality and “fooling” his brain into perceiving things
that were not actually there. Doctors diagnosed Betty with a rare disorder called prosopagnosia, which
is an inability to recognize familiar faces –a result of the brain damage caused by her stroke.
The anticipation you have, the happiness you feel, and the speed of your feet are the result of
information processing in your brain. In a way, all of your thoughts, feelings, and behaviors spring
from cells in the brain that take in information and produce some kind of output trillions of times a day.
These cells are neurons, cells in the nervous system that communicate with one another to perform
Cajal was the first to see that each neuron was composed of a body with many threads
extending outward toward other neurons. Surprisingly, he also saw that the threads of each neuron did
not actually touch other neurons. Cajal discovered that neurons are complex structures composed of
three basic parts: the cell body, the dendrites, and the axon. Like cells in all organs of the body, neurons
have a cell body (also called the soma), the largest component of the neuron that coordinates the
information-processing tasks and keeps the cell alive. Functions such as protein synthesis, energy
production, and metabolism take place here. The cell body contains a nucleus; this structure houses
chromosomes that contain your DNA, or the genetic blueprint of who you are. The cell body is
surrounded by a porous cell membrane that allows molecules to flow into and out of the cell. Unlike
other cells in the body, neurons have two types of specialized extensions of the cell membrane that
allow them to communicate: dendrites and axons. Dendrites receive information from other neurons
and relay it to the cell body. The term dendrite comes from the Greek word for “tree”; indeed, most
neurons have many dendrites that look like tree branches. The axon transmits information to other
neurons, muscles, or glands. In many neurons, the axon is covered by a myelin sheath, an insulating
layer of fatty material. The myelin sheath is composed of glial cells, which are support cells found in
the nervous system.An axon insulated with myelin can more efficiently transmit signals to other
neurons, organs, or muscles. In fact, with demyelinating diseases, such as multiple sclerosis, the
myelin sheath deteriorates, slowing the transmission of information from one neuron to another.
There’s a small gap between the axon of one neuron and the dendrites or cell body of another.
This gap is part of the synapse: the junction or region between the axon of one neuron and the
dendrites or cell body of another.
There are three major types of neurons, each performing a distinct function: sensory neurons,
motor neurons, and interneurons. Sensory neurons receive information from the external world and
convey this information to the brain via the spinal cord. They have specialized endings on their
dendrites that receive signals for light, sound, touch, taste, and smell. In our eyes, sensory neurons’
endings are sensitive to light. Motor neurons carry signals from the spinal cord to the muscles to
produce movement. These neurons often have long axons that can stretch to muscles at our extremities.
However, most of the nervous system is composed of the third type of neuron, interneurons, which
connect sensory neurons, motor neurons, or other interneurons. Some interneurons carry information
from sensory neurons into the nervous system, others carry information from the nervous system to
motor neurons, and still others perform a variety of information-processing functions within the
Besides specialization for sensory, motor, or connective functions, neurons are also somewhat
specialized depending on their location. For example, Purkinje cells are a type of interneuron that carries information from the cerebellum to the rest of the brain and spinal cord. These neurons have
dense, elaborate dendrites that resemble bushes. Pyramidal cells, found in the cerebral cortex, have a
triangular cell body and a single, long dendrite among many smaller dendrites. Bipolar cells, a type of
sensory neuron found in the retinas of the eye, have a single axon and a single dendrite.
The communication of information within and between neurons proceeds in two stages—
conduction and transmission. Together, these stages are what scientists generally refer to as the
electrochemical action of neurons.As you’ll recall, the neuron’s cell membrane is porous: It allows
small electrically charged molecules, called ions, to flow in and out of the cell. Neurons have a natural
electric charge called the resting potential, which is the difference in electric charge between the
inside and outside of a neuron’s cell membrane. The resting potential is similar to the difference
between the “+” and “−” poles of a battery, and just like a battery, resting potential creates the
environment for a possible electrical impulse.
This electric impulse is called an action potential, which is an electric signal that is conducted
along the length of a neuron’s axon to the synapse. The action potential occurs only when the electric
shock reaches a certain level, or threshold. When the shock was below this threshold, the researchers
recorded only tiny signals, which dissipated rapidly. When the shock reached the threshold, a much
larger signal, the action potential, was observed. The action potential is all or none: Electric stimulation
below the threshold fails to produce an action potential, whereas electric stimulation at or above the
threshold always produces the action potential.After the action potential reaches its maximum, the
membrane channels return to their original state, and K flows out until the axon returns to its resting
potential. This leaves a lot of extra Na ions inside the axon and a lot of extra K ions outside the axon.
During this period where the ions are imbalanced, the neuron cannot initiate another action potential, so
it is said to be in a refractory period, the time following an action potential during which a new action
potential cannot be initiated.
Myelin doesn’t cover the entire axon; rather, it clumps around the axon with little break points between
clumps, looking kind of like sausage links. These breakpoints are called the nodes of Ranvier, after
French pathologist Louis-Antoine Ranvier, who discovered them. When an electric current passes
down the length of a myelinated axon, the charge seems to “jump” from node to node rather than
having to traverse the entire axon. This process is called saltatory conduction, and it helps speed the
flow of information down the axon.
Axons usually end in terminal buttons, which are knoblike structures that branch out from an
axon.Aterminal button is filled with tiny vesicles, or “bags,” that contain neurotransmitters,
chemicals that transmit information across the synapse to a receiving neuron’s dendrites. The dendrites
of the receiving neuron contain receptors, parts of the cell membrane that receive neurotransmitters
and either initiate or prevent a new electric signal. The action potential travels down the length of the
axon to the terminal buttons, where it stimulates the release of neurotransmitters from vesicles into the
synapse. These neurotransmitters float across the synapse and bind to receptor sites on a nearby
dendrite of the receiving neuron, or postsynaptic neuron.Anew electric potential is initiated in that
neuron, and the process continues down that neuron’s axon to the next synapse and the next neuron.
This electrochemical action, called synaptic transmission, allows neurons to communicate with one
another and ultimately underlies your thoughts, emotions, and behavior.
Neurotransmitters leave the synapse through three processes. First, re-uptake occurs when
neurotransmitters are reabsorbed by the terminal buttons of the presynaptic neuron’s axon. Second,
neurotransmitters can be destroyed by enzymes in the synapse in a process called enzyme deactivation;
specific enzymes break down specific neurotransmitters. Finally, neurotransmitters can bind to the
receptor sites called autoreceptors on the presynaptic neurons.Autoreceptors detect how much of a
neurotransmitter has been released into a synapse and signal the neuron to stop releasing the
neurotransmitter when an excess is present. - Acetylcholine (ACh), a neurotransmitter involved in a number of functions, including voluntary
motor control, was one of the first neurotransmitters discovered.Acetylcholine is found in neurons of
the brain and in the synapses where axons connect to muscles and body organs, such as the heart.
Acetylcholine activates muscles to initiate motor behavior, but it also contributes to the regulation of
attention, learning, sleeping, dreaming, and memory
- Dopamine is a neurotransmitter that regulates motor behavior, motivation, pleasure, and emotional
arousal. Because of its role in basic motivated behaviors, such as seeking pleasure or associating
actions with rewards, dopamine plays a role in drug addiction
- Glutamate is a major excitatory neurotransmitter involved in information transmission throughout
the brain. This means that glutamate enhances the transmission of information. Too much glutamate
can overstimulate the brain, causing seizures. GABA(gamma-aminobutyric acid), in contrast, is the
primary inhibitory neurotransmitter in the brain. Inhibitory neurotransmitters stop the firing of
neurons, an activity that also contributes to the function of the organism. Too little GABA, just like too
much glutamate, can cause neurons to become overactive.
- Norepinephrine, a neurotransmitter that influences mood and arousal, is particularly involved in
states of vigilance, or a heightened awareness of dangers in the environment. Similarly, serotonin is
involved in the regulation of sleep and wakefulness, eating, and aggressive behavior. Because both
neurotransmitters affect mood and arousal, low levels of each have been implicated in mood disorders.
- Endorphins are chemicals that act within the pain pathways and emotion centers of the brain. The
term endorphin is a contraction of endogenous morphine, and that’s a pretty apt description. Morphine
is a synthetic drug that has a calming and pleasurable effect; an endorphin is an internally produced
substance that has similar properties, such as dulling the experience of pain and elevating moods.
The drug LSD, for example, is structurally very similar to serotonin, so it binds very easily with
serotonin receptors in the brain, producing similar effects on thoughts, feelings, or behavior.
Many drugs that affect the nervous system operate by increasing, interfering with, or mimicking
the manufacture or function of neurotransmitters. Agonists are drugs that increase the action of a
neurotransmitter. Antagonists are drugs that block the function of a neurotransmitter.
For example, a drug called L-dopa has been developed to treat Parkinson’s disease, a movement
disorder characterized by tremors and difficulty initiating movement and caused by the loss of neurons
that use the neurotransmitter dopamine. Dopamine is created in neurons by a modification of a
common molecule called L-dopa. Ingesting L-dopa will elevate the amount of L-dopa in the brain and
spur the surviving neurons to produce more dopamine. In other words, L-dopa acts as an agonist for
Many other drugs, including some street drugs, alter the actions of neurotransmitters. Let’s look at a
few more examples.
Methamphetamine affects pathways for dopamine, serotonin, and norepinephrine at the neuron’s
synapses, making it difficult to interpret exactly how it works. But the combination of its agonist and
antagonist effects alters the functions of neurotransmitters that help us perceive and interpret visual
Amphetamine is a popular drug that stimulates the release of norepinephrine and dopamine. In
addition, both amphetamine and cocaine prevent the reuptake of nor-epinephrine and dopamine. The
combination of increased release of norepinephrine and dopamine and prevention of their reuptake
floods the synapse with those neurotransmitters, resulting in increased activation of their receptors.
Nor-epinephrine and dopamine play a critical role in mood control, such that increases in either
neurotransmitter result in euphoria, wakefulness, and a burst of energy. However, norepinephrine also
increases heart rate. An overdose of amphetamine or cocaine can cause the heart to contract so rapidly
that heartbeats do not last long enough to pump blood effectively, leading to fainting and sometimes to
Prozac, a drug commonly used to treat depression, is another example of a neurotransmitter agonist. Prozac blocks the reuptake of the neurotransmitter serotonin, making it part of a category of drugs
called selective serotonin reuptake inhibitors, or SSRIs. Patients suffering from clinical depression
typically have reduced levels of serotonin in their brains. By blocking reuptake, more of the
neurotransmitter remains in the synapse longer and produces greater activation of serotonin receptors.
Serotonin elevates mood, which can help relieve depression
Neurons are the building blocks that form nerves, or bundles of axons and the glial cells that
support them. The nervous system is an interacting network of neurons that conveys electrochemical
information throughout the body.
There are two major divisions of the nervous system: the central nervous system and the
peripheral nervous system. The central nervous system (CNS) is composed of the brain and spinal
cord. The central nervous system receives sensory information from the external world, processes and
coordinates this information, and sends commands to the skeletal and muscular systems for action. The
peripheral nervous system (PNS) connects the central nervous system to the body’s organs and
muscles. The peripheral nervous system is itself composed of t