Download The anatomy of a typical neuron Neurons vs. nerves

Ch. 7 – Neurons: The Matter of the Mind
• The nervous system:
– Controls, integrates and coordinates the body’s activities
– Provides rapid and brief responses to stimuli
– Has 2 major divisions:
• 1. The central nervous system (CNS) = the brain and spinal cord
• 2. The peripheral nervous system (PNS) = nerves
– Is made up of nervous tissue, which includes:
• Neurons (nerve cells) – are excitable; they can generate and transmit
electrochemical signals that carry information
• Neuroglial (or glial) cells – support and protect neurons
The anatomy of a typical neuron
• The red arrows (→) indicate the direction of information
(electrochemical signal or action potential or nerve impulse)
flow
Figure 7.2
Neurons vs. nerves
• Types of neurons:
– 1. Sensory (afferent)
neurons – carry info from
sensory receptors toward
the CNS
– 2. Motor (efferent) neurons –
carry info away from the CNS to
an effector (= a muscle or gland)
– 3. Interneurons
(association neurons) –
carry info within the CNS
(between sensory and
motor neurons)
• Nerves = bundles of neuron
fibers (axons and/or
dendrites) in the PNS
– Nerves may be sensory, motor,
or mixed (mixed nerves contain
both sensory and motor fibers)
CNS
Figure 7.1
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An example of glial cells in the PNS:
Schwann cells
•
Main function: form the myelin sheath (which electrically insulates and
protects the axon, and speeds up the conduction of the nerve impulse)
Figure 7.3
Ion flow across the neuron membrane
• The electrochemical signal (action potential or nerve impulse)
depends on the flow of ions across the neuron membrane
• Ions may passively diffuse down their concentration
gradients through open channels or be actively pumped
against their concentration gradients
Na+ + Na+
Na
K+
K+
Note: leak channels
(which are always
open) for Na+ and K+
are not shown here
K+ K+
Gated channels may open/close in
response to electrical or chemical stimuli
Na+
Na+
Na+
Na+-K+ pump
(requires ATP)
Figure 7.4
Membrane potential (in general)
• = a voltage difference across a cell membrane due
to a separation and unequal distribution of electrical
charges
– The main charged particles involved: Na+ and K+ ions
• It’s usually measured in millivolts (mV)
• A membrane potential is an electrochemical
gradient:
– The ions have the potential to (“want to”) diffuse across
the membrane due to concentration differences and
electrical forces, but most of these ions are not able to
diffuse across because of limited permeability (i.e., most
of the channel proteins for these ions are closed)
• The inside of the membrane of most cells is more
negative compared to the outside of the membrane
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The synapse
• = the junction/connection
between a neuron and
another cell (typically
another neuron, or a
muscle or gland cell)
• Axon endings terminate in
bulblike swellings called
synaptic knobs, which:
– Store chemicals
called
neurotransmitter
in synaptic
vesicles
– Release
neurotransmitter
in response to
an incoming AP
Figure 7.6a
An electron micrograph of a synapse
Figure 7.6b
Transmission at an excitatory synapse
(Slide 1 of 3)
Figure 7.7
3
Transmission at an excitatory synapse
(Slide 2 of 3)
Figure 7.7
Transmission at an excitatory synapse
•
•
Why is it excitatory? The receptors are gated Na+ channels, which (Slide 3 of 3)
when opened cause the membrane of the postsynaptic neuron to
depolarize toward threshold
If the receptors are gated K+ channels, opening them would cause the membrane of
the postsynaptic neuron to hyperpolarize away from threshold, and the synapse
would be inhibitory
Figure 7.7
Information processing by neurons
•
•
A neuron may have up to 10,000 synapses with other neurons from
different sources – some of these are excitatory synapses, and some are
inhibitory synapses
The first part of the axon of the postsynaptic neuron “sums up” the input:
– If the net effect = threshold is reached, then it generates an AP
– If the net effect = threshold is not reached, then it doesn’t generate an AP
Figure 7.8
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