Chapter 3: Biological Foundations of Behaviour
- The Neural Bases of Behaviour:
o Neurons: the basic building blocks of the nervous system. They are nerve cells linked
together in circuit and vary in shape and size. These cells are composed of 3 main parts:
the cell body, dendrites and axon. The cell body (soma) contains the biochemical
structures needed to keep the neuron alive, and its nucleus carries the genetic
information that determines how the neuron functions and develops. The dendrites are
branchlike fibres extending from the cell body, which collect messages from
neighbouring neurons and send them on to the cell body. At the cell body the incoming
information is combined and processed. Extending from one side of the cell body is the
axon, which conducts electrical impulses away from the cell body to other neurons,
muscles or glands. The axon branches out into many axon terminals. Neurons do two
important things: they generate electricity that creates nerve impulses, and release
chemicals that allow them to communicate with other neurons, muscles and glands.
o Glial cells: surround neurons and hold them in place. Also manufacture nutrient
chemicals neurons need, form the myelin sheath around some axons, and absorb
toxins/waste materials that can damage neurons.
o Blood-brain barrier: prevents many substances from entering the brain. It is formed by
the walls of blood vessels and a special type of glial cell.
- The Electrical Activity of Neurons:
o Action potential: a sudden reversal in the neurons membrane voltage
Resting potential: when the interior charge of an axon is -70mV. Caused by the
uneven distribution of positively charged sodium ions outside the cell, and
presence of negatively charged proteins ions within the cell. It is maintained by
the sodium potassium pumps, which constantly pump out three sodium ions for
every two potassium ions pumped into the cell.
If a neurons axon is stimulated and the interior voltage differentiation shifts
from the resting potential of -70mV to the action potential threshold of about
-55mV, then an action potential will be triggered. Once an action potential is
trigged, sodium channels open for an instant, and sodium ions flow into the
axon. Quickly, the interior voltage differentiation shifts to +40mV and the
sodium channels close, then the potassium channels open. During this absolute
refractory period, potassium ions flow out of the axon and restore the negative
interior charge. Eventually the axon returns to the resting potential of -70mV.
If the stimulation does not reach a negative resting potential of -55mV, then it is
called a graded potential. This is also demonstrated by the all-or-none law,
which states: action potentials occur at a uniform and maximum intensity, or
they do not occur at all.
o The Myelin Sheath: a fatty, whitish insulation layer around the axon derived from glial
cells during development. It is interrupted at regular intervals by the nodes of Ranvier,where the myelin is either thin or absent. Electrical conduction can skip from node to
node, which travels much faster than electrical conduction without a myelin sheath.
Damage to the myelin sheath can be seen in people who suffer from multiple sclerosis
because the persons immune system attacks the myelin sheath, and this disrupts the
timing of nerve impulses.
- How Neurons Communicate Synaptic Transmission:
o Synapse: a functional connection between a neuron and its target.
o Synaptic cleft: a tiny gap between the axon terminal of one neuron and the dendrite of
the next neuron.
o Neurotransmitters: [discovered by Otto Loewi] chemical substances produced by
neurons that carry messages across the synapse to either excite other neurons, or
This process involves five steps:
1. Synthesis: the chemical molecules are formed inside a neuron.
2. Storage: the molecules are stored in chambers called synaptic vesicles
within the axon terminals.
3. Release: when an action potential comes down the axon, vesicles move to
the surface of the axon terminal and the molecules are released into the
fluid-filled space between the axon of the sending (presynaptic) neuron, and
the membrane of the receiving (postsynaptic) neuron.
4. Bind: the molecules cross the synaptic space and bind to receptor sites.
5. Two possible cases: