PSYCH CH. 3.doc

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10 Apr 2012

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Chapter 3: The Biological Bases of Behaviour
Focus of the chapter: study of the brain and its relations to behaviour, and to discover which part
of the brain is associated with which functions/behaviours
Communication in the Nervous System
- Nervous system:
oHandles information (it is a complex communication network in which signals are
transmitted, received, and integrated)
Nervous Tissue
- Nervous system is living tissue composed of cells (2 major categories):
Cells found throughout the nervous system that provide various types of
support for neurons
Tend to be smaller than neurons but outnumbers neurons (10 to 1)
Glia cells—50% of the brain’s volume
Supply nourishment to neurons, help remove neuron’s waste products, and
provide insulation around many axons (myelin sheath)
New research suggest that glial cells may also send and receive chemical
May play an important role in memory formation
Gradual deterioration of glial tissue might contribute to the emergence of
Some types of glia can detect neural impulses and send signals to other glial
Individual cells in the nervous system that receive, integrate, and transmit
Basic links that allow communication to occur within the nervous system
Most communicate with other neurons
Small minority receive signals from outside the nervous system (sensory
organs) or carry messages from the nervous system to the muscles that
move the body
Common features:
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Soma (cell body)
oContains the cell nucleus and much of the chemical machinery
common to most cells
oParts of a neuron that are specialized to receive information
oLong, thin fibre that transmit signals away from the soma to other
neurons or to muscles or glands
oIn humans, many axons are wrapped by myelin
Myelin sheath
oHigh concentration of a white, fatty substance
oInsulating material derived from glial cells
oSpeeds up the transmission of signals that move along axons
If axon’s myelin sheath deteriorates, its signals may not be
transmitted effectively: loss of muscle control in multiple
sclerosis is due to degeneration of myelin sheaths
Terminal buttons
oSmall knobs where the axon ends in a cluster
oSecrete chemicals called neurotransmitters: serve as messengers
that may activate neighbouring neurons
oThe points at which neurons interconnect and information is
transmitted from one neuron to another
oInfo is received at the dendrites, passed through the soma and along the axon, and is
transmitted to the dendrites of other cells at meeting points called synapses
Neural Impulse: Using Energy to Send Information
Neuron at Rest
- Neural impulse is a complex electrochemical reaction
- Inside and outside the neuron are fluids containing electrically charged molecules and atoms
- Cell membrane: semipermeable: allows movement of some ions
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- Positively charged Na and K ions and negatively charged Chloride ions flow back and forth
across the cell membrane
oBut not at the same rate
oDifference in flow rates leads to a slightly higher concentration of negatively charged
ion inside the cell
Resulting voltage: neuron at rest is a tiny battery, a store of potential energy
Resting potential of a neuron = stable, negative charge when the cell is
Charge is about -70 millivolts
Action Potential
- As long as the voltage of a neuron remains constant, the cell is quiet and no messages are
being sent
- When neuron is stimulated:
oCell membrane opens
Allows positively charged Na ions to rush in
The neuron’s charge thus becomes less negative, or even positive
oThis creates an action potential
- Action potential: a very brief shift in a neuron’s electrical charge that travels along an axon
- After the firing of an action potential, the channels in the cell membrane that opened up for
the Na ions close up
oSome time is needed before they are ready to open again
oUntil that time, the neuron cannot fire
Absolute refractory period: the minimum length of time after an action
potential during which another action potential cannot begin
Referred to as the “downtime”
oUsually for 1 or 2 milliseconds
Relative refractory period: follows absolute refractory period
The neuron can fire, but its threshold for firing is elevated (i.e. more
intense stimulation is required to initiate an action potential)
All or None Law
- Neural impulse is an all-or-none proposition: either it fires or it doesn’t
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