CHAPTER 3: BIOLOGICAL BASES OF BEHAVIOUR
The human brain is so vastly complex that at any given moment millions of signals are travelling between
the neurons and along various neuronal pathways within its structures.
Understanding the anatomical and physiological drivers of behaviour and cognition provides a base from
which we can obtain a more holistic view of human psychology including biological, psychological and
cultural domains.
UNIT 1: COMMUNICATION IN THE NERVOUS SYSTEM
Behaviour depends on rapid information processing. Information travels immediately from your eye to
your brain, from your brain to the muscles of your arm and hand, and from your fingers back to your brain.
In essence, your nervous system is a complex communication network in which signals are constantly
being transmitted, received, and integrated.
3.1.1 NERVOUS TISSUE: THE BASIC HARDWARE
Your nervous system is living tissue composed of cells. The cells in the nervous system fall into 2 major
categories: Neurons and Glia.
Neurons:
Neurons: are individual cells in the nervous system that receive, integrate, and transmit information. →
They are basic links that permit communication within the nervous system.
A fast majority of them communicate only with other neurons. However a small minority of them receive
signals from outside the nervous system or carry messages from the nervous system to the muscles that
move the body.
The soma contains the cell nucleus and much of the chemical machinery common to most cells. → The
rest of the neuron is devoted exclusively to handling information. → Neurons have several branched
structures called dendritic trees. Each individual branch is a dendrite.
Dendrites: are the parts of a neuron that are specialized to receive information. → From the many
dendrites, information flows into the cell body, then travels away from the soma along the axon.
The axon is a long, thin fibre that transmits signals away from the soma to other neurons, muscles, or
glands. → Axons vary in length and may communicate with a number of other cells. Axons are wrapped
in cells with a high concentration of a white, fatty substance called myelin.
24
,The myelin sheath acts as insulating material and aids in accelerating the transmission of signals that
move along axons. If an axon’s myelin sheath deteriorates, its ability to conduct signals is less effective.
The loss of muscle control that is seen in the disease multiple sclerosis is due to a degeneration of myelin
sheaths, leading to an interruption of the signal, which affects movement.
The axon ends in a cluster of terminal buttons, which are small knobs that secrete chemicals called
neurotransmitters. These chemicals serve as messengers that can activate nearby neurons. The points at
which neurons interconnect are called synapses.
Synapse: is a junction where information is transmitted from one neuron to another.
To summarize, information is received at the dendrites, passed through the soma and along the axon,
and transmitted to the dendrites of other cells at meeting points called synapses. The synapse is a
connection between the neurons.
Glia:
Glia: are cells found throughout the nervous system that provide various types of support for neurons.
Glia tends to be much smaller than neurons but are much more abundant within the human brain.
Glial cells serve many functions. They supply nourishment to neurons, help remove neurons’ waste
products and provide insulation around many axons. The myelin sheaths that encase some axons are
derived from special types of glial cells (Schwann Cell). Glial cells also play a complicated role in the
development of the nervous system in human embryos.
Although glia may contribute to information processing in the nervous system, the bulk of this crucial work
is handled by the neurons. Thus, we need to examine the process of neural activity in more detail.
3.1.2 THE NEURAL IMPULSE: USING ENERGY TO SEND INFORMATION
The neuron at rest: a tiny battery:
Hodgkin and Huxley (1952) learned that the neural impulse is a complex electrochemical reaction. Both
inside and outside the neuron are fluids containing electrically charged atoms and molecules called ions.
Positively charged sodium and potassium ions and negatively charged chloride ions flow back and forth
across the cell membrane, but they do not cross at the same rate. → The difference in flow rates leads to
a slightly higher concentration of negatively charged ions inside the cell. The resulting voltage means that
the neuron at rest is a tiny battery, a store of potential energy.
The resting potential of a neuron is its stable, negative charge when the cell is inactive. → Therefore,
when the neuron is inactive there is no neural impulse being transmitted.
The action potential:
As long as the voltage of a neuron remains constant, the cell is quiet, and no messages are being sent.
When the neuron is stimulated, channels in its cell membrane open, briefly allowing positively charged
sodium ions to rush in.
Action potential: is a very brief shift in a neuron’s electrical charge that travels along an axon. → The firing
of an action potential is reflected in the voltage spike.
After the firing of an action potential, the channels in the cell membrane up. Some time is needed before
they are ready to open again and until that time, the neuron cannot fire.
25
, Absolute refractory period: is the minimum length of time after an action potential before another action
potential can begin.
The all – or – none law:
The neural impulse is an all-or-none proposition, like firing a gun. Either the neuron fires or it doesn’t, and
its action potentials are all the same size. The intensity of a stimulus is usually conveyed by the rate at
which neurons are fired, as well as the number of neurons or set of neurons that are fired.
3.1.3 THE SYNAPSE: WHERE NEURONS MEET
In the nervous system, the neural impulse functions as a signal. For that signal to have any meaning for
the system, it must be transmitted from the neuron to other cells. This transmission takes place at special
junctions called synapses, which depend on chemical messengers.
Sending the synapse: where neurons meet:
The two neurons don’t touch. They are separated by the
synaptic cleft, a microscopic gap between the terminal
button of one neuron and the cell membrane of another
neuron. → Signals must cross this gap to permit neurons to
communicate. In this situation, the neuron that sends a
signal across the gap is called the presynaptic neuron, and
the neuron that receives the signal is called
the postsynaptic neuron.
The arrival of an action potential at an axon’s terminal
buttons triggers the release of neurotransmitters.
Neurotransmitters are chemicals that transmit information
from one neuron to another. They are stored in sacs called
synaptic vesicles.
Neurotransmitters are released when a vesicle fuses with membrane of the presynaptic (sending neuron)
cell, and its content spill into the synaptic cleft. → They bind with receptor sites on post-synaptic neuron:
sites respond to specific neurotransmitters.
Receiving signals: postsynaptic potentials:
When a neurotransmitter and a receptor molecule combine, reactions in the cell membrane cause a
postsynaptic potential (PSP), a voltage change at a receptor site on a postsynaptic cell membrane.
Postsynaptic potentials do not follow the all-or-none law like action potentials do. Instead, postsynaptic
potentials vary in size (the amount of voltage change). Moreover, they increase or decrease the
probability of a neural impulse in the receiving cell in proportion to the amount of voltage change.
Two types of messages can be sent from cell to cell: excitatory and inhibitory.
An excitatory PSP is a positive voltage shift that increases the likelihood that the postsynaptic neuron will
fire action potentials. An inhibitory PSP is a negative voltage shift that decreases the likelihood that the
postsynaptic neuron will fire action potentials.
The direction of the voltage shift, and thus the nature of the PSP (excitatory or inhibitory), depends on
which receptor sites are activated in the postsynaptic neuron and depends on the types of
neurotransmitters that are released.
26
The human brain is so vastly complex that at any given moment millions of signals are travelling between
the neurons and along various neuronal pathways within its structures.
Understanding the anatomical and physiological drivers of behaviour and cognition provides a base from
which we can obtain a more holistic view of human psychology including biological, psychological and
cultural domains.
UNIT 1: COMMUNICATION IN THE NERVOUS SYSTEM
Behaviour depends on rapid information processing. Information travels immediately from your eye to
your brain, from your brain to the muscles of your arm and hand, and from your fingers back to your brain.
In essence, your nervous system is a complex communication network in which signals are constantly
being transmitted, received, and integrated.
3.1.1 NERVOUS TISSUE: THE BASIC HARDWARE
Your nervous system is living tissue composed of cells. The cells in the nervous system fall into 2 major
categories: Neurons and Glia.
Neurons:
Neurons: are individual cells in the nervous system that receive, integrate, and transmit information. →
They are basic links that permit communication within the nervous system.
A fast majority of them communicate only with other neurons. However a small minority of them receive
signals from outside the nervous system or carry messages from the nervous system to the muscles that
move the body.
The soma contains the cell nucleus and much of the chemical machinery common to most cells. → The
rest of the neuron is devoted exclusively to handling information. → Neurons have several branched
structures called dendritic trees. Each individual branch is a dendrite.
Dendrites: are the parts of a neuron that are specialized to receive information. → From the many
dendrites, information flows into the cell body, then travels away from the soma along the axon.
The axon is a long, thin fibre that transmits signals away from the soma to other neurons, muscles, or
glands. → Axons vary in length and may communicate with a number of other cells. Axons are wrapped
in cells with a high concentration of a white, fatty substance called myelin.
24
,The myelin sheath acts as insulating material and aids in accelerating the transmission of signals that
move along axons. If an axon’s myelin sheath deteriorates, its ability to conduct signals is less effective.
The loss of muscle control that is seen in the disease multiple sclerosis is due to a degeneration of myelin
sheaths, leading to an interruption of the signal, which affects movement.
The axon ends in a cluster of terminal buttons, which are small knobs that secrete chemicals called
neurotransmitters. These chemicals serve as messengers that can activate nearby neurons. The points at
which neurons interconnect are called synapses.
Synapse: is a junction where information is transmitted from one neuron to another.
To summarize, information is received at the dendrites, passed through the soma and along the axon,
and transmitted to the dendrites of other cells at meeting points called synapses. The synapse is a
connection between the neurons.
Glia:
Glia: are cells found throughout the nervous system that provide various types of support for neurons.
Glia tends to be much smaller than neurons but are much more abundant within the human brain.
Glial cells serve many functions. They supply nourishment to neurons, help remove neurons’ waste
products and provide insulation around many axons. The myelin sheaths that encase some axons are
derived from special types of glial cells (Schwann Cell). Glial cells also play a complicated role in the
development of the nervous system in human embryos.
Although glia may contribute to information processing in the nervous system, the bulk of this crucial work
is handled by the neurons. Thus, we need to examine the process of neural activity in more detail.
3.1.2 THE NEURAL IMPULSE: USING ENERGY TO SEND INFORMATION
The neuron at rest: a tiny battery:
Hodgkin and Huxley (1952) learned that the neural impulse is a complex electrochemical reaction. Both
inside and outside the neuron are fluids containing electrically charged atoms and molecules called ions.
Positively charged sodium and potassium ions and negatively charged chloride ions flow back and forth
across the cell membrane, but they do not cross at the same rate. → The difference in flow rates leads to
a slightly higher concentration of negatively charged ions inside the cell. The resulting voltage means that
the neuron at rest is a tiny battery, a store of potential energy.
The resting potential of a neuron is its stable, negative charge when the cell is inactive. → Therefore,
when the neuron is inactive there is no neural impulse being transmitted.
The action potential:
As long as the voltage of a neuron remains constant, the cell is quiet, and no messages are being sent.
When the neuron is stimulated, channels in its cell membrane open, briefly allowing positively charged
sodium ions to rush in.
Action potential: is a very brief shift in a neuron’s electrical charge that travels along an axon. → The firing
of an action potential is reflected in the voltage spike.
After the firing of an action potential, the channels in the cell membrane up. Some time is needed before
they are ready to open again and until that time, the neuron cannot fire.
25
, Absolute refractory period: is the minimum length of time after an action potential before another action
potential can begin.
The all – or – none law:
The neural impulse is an all-or-none proposition, like firing a gun. Either the neuron fires or it doesn’t, and
its action potentials are all the same size. The intensity of a stimulus is usually conveyed by the rate at
which neurons are fired, as well as the number of neurons or set of neurons that are fired.
3.1.3 THE SYNAPSE: WHERE NEURONS MEET
In the nervous system, the neural impulse functions as a signal. For that signal to have any meaning for
the system, it must be transmitted from the neuron to other cells. This transmission takes place at special
junctions called synapses, which depend on chemical messengers.
Sending the synapse: where neurons meet:
The two neurons don’t touch. They are separated by the
synaptic cleft, a microscopic gap between the terminal
button of one neuron and the cell membrane of another
neuron. → Signals must cross this gap to permit neurons to
communicate. In this situation, the neuron that sends a
signal across the gap is called the presynaptic neuron, and
the neuron that receives the signal is called
the postsynaptic neuron.
The arrival of an action potential at an axon’s terminal
buttons triggers the release of neurotransmitters.
Neurotransmitters are chemicals that transmit information
from one neuron to another. They are stored in sacs called
synaptic vesicles.
Neurotransmitters are released when a vesicle fuses with membrane of the presynaptic (sending neuron)
cell, and its content spill into the synaptic cleft. → They bind with receptor sites on post-synaptic neuron:
sites respond to specific neurotransmitters.
Receiving signals: postsynaptic potentials:
When a neurotransmitter and a receptor molecule combine, reactions in the cell membrane cause a
postsynaptic potential (PSP), a voltage change at a receptor site on a postsynaptic cell membrane.
Postsynaptic potentials do not follow the all-or-none law like action potentials do. Instead, postsynaptic
potentials vary in size (the amount of voltage change). Moreover, they increase or decrease the
probability of a neural impulse in the receiving cell in proportion to the amount of voltage change.
Two types of messages can be sent from cell to cell: excitatory and inhibitory.
An excitatory PSP is a positive voltage shift that increases the likelihood that the postsynaptic neuron will
fire action potentials. An inhibitory PSP is a negative voltage shift that decreases the likelihood that the
postsynaptic neuron will fire action potentials.
The direction of the voltage shift, and thus the nature of the PSP (excitatory or inhibitory), depends on
which receptor sites are activated in the postsynaptic neuron and depends on the types of
neurotransmitters that are released.
26
