Chapter 3: The Biological Bases
Communication in the Nervous System
Nervous Tissue: The Basic Hardware
The cells in the nervous system fall into two major categories.
Neurons are individual cells in the nervous system that receive, integrate and transmit
information. Most of them only communicate with other neurons, but some receive
signals from outside the nervous system.
The soma or cell body, contains the cell nucleus and the chemical machinery common to
cells. The rest is devoted to handling information.
Dendrites are specialiazed to receive information. It then travels from the soma along
The axon is a long thin fibre that transmits signals away from the soma to other neurons
or muscles or glands. In humans, they are usually wrapped in a white, fatty substance
called myelin. This speeds up the transmission of signals.
The axon ends in a cluster of terminal buttons, small knobs that secrete
neurotransmitters. These may activate neighbouring neurons.
Where two neurons connect are called the synapses.
Glia are cells found throughout the nervous system that support the neurons. They are
smaller and more abundant than neurons.
They supply nourishment to neurons, help remove their waster products, and some
They also play a role in orchestrating the development of the nervous system in the
The Neural Impulse: Using Energy to Send Information
The Neuron at Rest: A Tiny Battery
The neural impulse is an electorchemical reaction. Fluids containing ions are both inside
and outside of the neuron.
The difference in flow rates of different chemicals going into and out of the cell create a
slightly higher negative charge inside the neuron, resulting in voltage. The resting potential of a neuron is it’s stable, negative charge when inactive.
When a neuron is stimulated, certain channels open, allowing positive ions to flow in,
causing the neuron to be less negative, or even positive, resulting in an action potential.
An action potential is a very brief shift in a neurons electrical charge that travels along
The passageways close up again, and some time is needed for they can open again, and
until then, the neuron cannot fire. The absolute refractory period is the min. length of
time after an action potential in which another can begin. The relative refractory period
follows, which is when the neuron can fire, but it has a higher threshold for sending
The All-or-None Law
Weaker stimuli do not produce smaller action potentials. The neuron either fires, or it
doesn’t. All action potentials are the same for it.
However, neurons can convey the strength of a signal by varying the rate at which they
transmit them. Thicker axons transmit more rapidly than thinner ones.
The Synapse: Where Neurons Meet
Sending Signals: Chemicals as Couriers
The neuron that sends the signal is the presynaptic neuron, and the one that receives it is
the postsynaptic neuron.
The arrival of an action potential triggers the release of neurotransmitters, most stored in
At the postsynaptic neuron, the neurotransmitters may bind with special molecules at the
Receiving Signals: Postsynaptic Potentials
When a neurotransmitter and a receptor meet, the reactions create a postsynaptic
potential (PSP), a voltage change at a receptor site.
These do not follow the allornone law, but vary in size, and vary the probability of a
postsynaptic neuron sending a signal or not in proportion to the amount of voltage
An excitatory PSP is positive and increases the probability, and an inhibitory PSP
decreases the probability. They type all depends on which receptor it bonds to.
These effects last for only a fraction of a second, and then the neurotransmitters are
inactivated by enzymes or drift away from the receptors.
Most are reabsorbed back into the presynaptic neuron by reuptake. Integrating Signals: Neural Networks
Elimination of old synapses plays a more crucial role in sculpting the brain than creating
new ones does. The nervous system creates more than needed, and then eliminates the
lessactive ones. This is called synaptic pruning.
One neuron stimulating another repeatedly causes a change in the synapses, and learning
has take place.
Neurotransmitter and Behaviour
The only transmitter between motor neurons and voluntary muscles.
Also contributes to attention, arousal and memory.
An agonist is a chemical that mimics the action of a neurotransmitter.
An antagonist is a chemical that opposes the action of a neurotransmitter.
Include 3 neurotransmitters: dopamine, norepinephrine and serotonin.
Dopamine is used by neurons that control voluntary movements.
Serotoninreleasing neurons play an important role in the regulation of sleep and
wakefulness, and eating behavior. Also modulates aggression in animals, and perhaps
The dopamine hypothesis asserts that abnormality in the activity of dopamine synapses
play a role in the development of schizophrenia.
GABA & Glutamate
Consists of amino acids.
GABA and Glycine seem to produce only inhibitory PSPs. GABA is present at nearly
40% of all synapses.
GABA is involved in the regulation of anxiety in humans.
Glutamate always has excitatory PSPs. It is best known for its contribution to learning
and memory. Endorphins
Endorphins are internally produced chemicals that resemble opiates in effect and
structure. They contribute to the modulation of pain, eating behavior, and the body’s
response to stress, as well as feelings of pleasure.
Organization of the Nervous System
The Peripheral Nervous System
The Peripheral Nervous System is made up of all of the nerves outside of the brain.
Nerves are bundles of neuron fibres (axons).
The Somatic Nervous System
Made up of nerves that connect to voluntary muscles and sensory receptors.
They carry info from the receptors in the skin, joints and muscles to the CNS and from
the CNS to the muscles.
Afferent nerve fibres are axons that carry information inwards to the CNS from the
periphery of the body.
Efferent nerve fibres carry info outward.
So somatic nerves are “twoway streets”.
The SNS lets you move around in and feel the world around you.
The Autonomic Nervous System
Made up of nerves that connect to the heart, blood vessels, smooth muscles and glands.
It controls automatic, involuntary, visceral functions that people don’t think about such as
digestion, heart rate, etc.
It mediates much of the physiological arousal that occurs when people experience
The sympathetic nervous system mobilizes the body’s resources for emergencies.
It creates the fightorflight response.
In contrast, the parasympathetic nervous system generally conserves bodily resources.
It activates the processes that allow the body to save and store energy.
The Central Nervous System
The Central Nervous System consists of the brain and the spinal cord. It is protected by enclosing sheaths called meninges. It is also “bathed” in a substance
called cerebrospinal fluid that nourishes the brain and provides cushion. This is in the
hollow spaces called ventricles.
Looking Inside the Brain: Research Methods
The electroencephalograph (EEG) is a device that monitors the electrical activity of the
brain over time by means of recording electrodes attached to the scalp.
The recordings are translated into line tracings, known at brain