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1. Nervous Tissue: The Basic Hardware
2. The Neural Impulse: Using Energy to Send Information
3. The Synapse: Where Neurons Meet
4. Neurotransmitters and Behaviour: Multi-talented Chemical Messengers
→ Behaviour depends on rapid information processing.
→ Nervous system = complex communication network in which signals are constantly being
transmitted, received, and integrated
1. Nervous Tissue: The Basic Hardware
→ Nervous system = living tissue composed of cells.
→ 2 major categories:
a. Neurons
b. Glia
a. Neurons:
→ Individual cells in the nervous system that receive, integrate, and Soma
transmit information.
→ Basic chains of communication within the nervous system.
→ Most neurons only communicate with other neurons.
→ Some neurons:
○ Receive signals from outside nervous system (form sensory
organs)
○ Carry messages from the nervous system to the muscles that
move the body.
Structure:
○ Soma / cell body:
Contains nucleus and most of the other structures
common to most cells
○ Dendrites: (individual branches of dendritic trees)
Parts specialised to receive information.
Information flows into the cell body and then travels away
from the soma along the axon
○ Axon (from Greek “Axle”):
Long thin fibre that transmits signals away from the soma
to other neurons, muscles, or glands
Vary in length
May communicate with several other cells.
Generally wrapped in myelin.
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b. Glia
→ Cells found throughout the nervous system – provide various types of support for neurons
→ Glia = literally means “glue”
→ Smaller than neurons and more abundant in the brain (account for 50% of the brain’s volume)
→ Serves multiple support functions;
○ Provision of certain nutrients for neurons
○ Insulation
○ Removal of waste products
→ Myelin sheaths that encase some axons = derived from special types of glial cells
→ Also plays complicated role in development of nervous system in the human embryo.
→ Recent research:
○ Glial cells may also play a role in sending and receiving chemical signals.
○ Some glia can detect neural impulses and send signals to other glial cells.
○ Glia cells may play an important role in memory formation
○ Gradual deterioration of glial tissue might contribute to Alzheimer’s disease
○ Glia cells play crucial role in the experience of chronic pain.
○ Impaired neural glial communication may contribute to psychological disorders, such as
schizophrenia.
2. The Neural Impulse: Using Energy to Send Information
→ The electrochemical properties of the neuron allow it to transmit signals.
→ The electric charge of a neuron can be measured with a pair of electrodes connected to an
oscilloscope, as Hodgkin and Huxley showed with a squid axon.
○ Because of its exceptionally thick axons, the squid has frequently been used by scientists
studying the neural impulse.
→ SEE PAGE 31 FOR MORE
A. At rest, the neuron’s voltage hovers around –70
millivolts.
B. When a neuron is stimulated, a brief jump occurs in its
voltage, resulting in a spike on the oscilloscope
recording of the neuron’s electrical activity. This change
in voltage, called an action potential, travels along the
axon like a spark travelling along a trail of gunpowder.
Neurons at Rest: A Tiny Battery
→ Neural impulse = complex electrochemical reaction.
→ Inside and outside neurons are fluids containing electrically charged atoms and molecules called ions.
→ Positively charged sodium and potassium ions + negatively charged chloride ions = flow back and
forth across cell membrane
○ Do not cross at the same rate.
→ Difference in flow rates leads to slightly higher concentration of negatively charged ions inside the cell
○ Creates negative charge within the neuron
→ Neuron at rest = a tiny battery – a store of potential energy
→ Resting potential of a neuron is its stable, negative charge when the cell is inactive.
○ Charge is about -70 millivolts (roughly one twentieth of the voltage of a torch)
The Action Potential: How Neurons Fire
→ If the voltage of a neuron remains constant = the cell is quiet + no messages are being sent.
→ When neuron is stimulated, channels in its cell membrane open = briefly allowing positively charged
sodium ions to rush in.
○ Neuron’s charge is less negative / or even positive
○ Creates action potential: a shift in a neuron’s electrical charge that travels along an axon.
After firing of action potential = channels in the cell membrane close (cannot fire until they
open again)
→ Absolute refractory period: minimum length of time after an action potential before another action
potential can begin.
○ Not very long -> about 1 or 2 milliseconds
○ Followed by a brief relative refractory period
Neuron can fire but threshold for firing is elevated
More intense stimulation is required to start an action potential.
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