Transcript The Nerve Impulse and Reflex Arc

Pages 234-239
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http://highered.mheducation.com/sites/007
2495855/student_view0/chapter14/animatio
n__the_nerve_impulse.html
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Neurons have two functional properties
◦ Irritatability – respond to a stimulus and convert it
into a nerve impulse
◦ Conductivity – ability to transmit the impulse to
other neurons, muscles, or glands
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When neurons are in a “resting” state:
◦ They are polarized
 inside the plasma membrane is more negative than
outside
 more potassium (K) inside the cell
 More sodium (Na) outside the cell
 a gradient is created
[Na+]
[K+]
 the cell stays at rest
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A stimulus depolarizes the neuron’s
membrane
The membrane is now permeable to sodium
 sodium channels open
 sodium (Na) diffuses into the membrane
 the inside becomes more positive
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This is how an action potential can be initiated
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A nerve impulse (action potential) is generated:
 If the stimulus is strong enough
 If sodium influx great enough
Na+
Na+
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If enough sodium enters the cell, the action potential
(nerve impulse) starts and is propagated over the entire
axon
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the nerve impulse is propagated or it is not
 There are no partial impulses
K+
K+
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Repolarization:
 the membrane is restored to resting potential
 Negative inside, positive outside
 The sodium-potassium pump restores resting potential
using energy supplied by ATP
 Another impulse cannot take place until the entire
membrane has been repolarized
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When the action potential reaches the axon
terminal:
1. calcium channels open
2. vesicles containing neurotransmitters fuse with
the axonal membrane
3. neurotransmitters diffuse across the synapse and
bind to receptors on the membrane of the next
neuron
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The amount of neurotransmitter released will
determine if the action potential will continue
◦ This is known as graded potential
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Slide 2
Axon of
transmitting
neuron
Receiving
neuron
1 Action
Dendrite
Axon terminal
potential
arrives.
Vesicles
Synaptic
cleft
Slide 3
2 Vesicle Transmitting neuron
fuses with
plasma
membrane.
Synaptic
cleft
Ion
channels
Receiving neuron
Neurotransmitter
molecules
Slide 4
2 Vesicle Transmitting neuron
fuses with
plasma
membrane.
3 Neurotrans-
mitter is
released into
synaptic cleft.
Synaptic
cleft
Ion
channels
Receiving neuron
Neurotransmitter
molecules
Slide 5
2 Vesicle Transmitting neuron
4 Neurotransfuses with
3 Neurotrans- mitter binds
plasma
membrane.
mitter is
released into
synaptic cleft.
Synaptic
cleft
Ion
channels
Receiving neuron
to receptor
on receiving
neuron’s
membrane.
Neurotransmitter
molecules
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The transmission of a nerve impulse down
neuron is electrical
The transmission of a nerve impulse to next
neuron is chemical
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Somatic reflexes
◦ Reflexes that stimulate the skeletal muscles
◦ Example: pulling hand away from a hot object
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Autonomic reflexes
◦ Regulates smooth muscles, heart, and glands
◦ Example: regulation of blood pressure, glands,
digestive system
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Reflexes are rapid, predictable, involuntary
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A reflex arc can include:
responses to a stimulus
1. Sensory receptor—reacts to a stimulus
2. Sensory neuron—carries message to integration
center
3. Integration center (CNS) –
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processes information
directs motor output
4. Motor neuron —carries message to an effector
5. Effector organ —muscle or gland to be stimulated
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Stimulus at distal
end of neuron
1
Spinal cord
(in cross section)
Skin
2
Receptor
5
Effector
Sensory
neuron
4
Motor neuron
3
Integration
center
Interneuron