Transcript Chap 28 – Nervous System Part 2 – Synaptic Transmission

Chapter 28
Nervous Systems
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition
Reece, Taylor, Simon, and Dickey
© 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Communication Between
Neurons:
Synaptic Transmission
© 2012 Pearson Education, Inc.
What happens when an action potential reaches
end of neuron?
 Signal must be passed to another cell
 The synapse:
– Relay point between neuron and an effector cell
– Another neuron
– Muscle cell
– Endocrine cell
© 2012 Pearson Education, Inc.
28.6 Neurons communicate at synapses
 Electrical synapses:
– Electrical signals directly passes from neuron to receiving
cell
– Ex: heart, smooth muscle of digestive tract
 Chemical synapses:
– Action potential of neuron converted to a chemical signal
(neurotransmitter)
– Neurotransmitter triggers response in effector cell
Animation: Synapse
© 2012 Pearson Education, Inc.
Figure 28.6
SENDING CELL
Axon of
sending
cell
Synaptic
vesicles
Synaptic
terminal
of sending
cell
(Pre-synaptic cell)
1
Action
potential
arrives
Synaptic
terminal
2
3
Vesicle fuses
with plasma
membrane
Dendrite
of receiving
cell
Neurotransmitter
is released into
synaptic cleft
SYNAPTIC
CLEFT
4
Neurotransmitter
binds to receptor
RECEIVING
CELL
(Post-synaptic cell)
Ion channels
Neurotransmitter
molecules
Neurotransmitter
Receptor
Neurotransmitter broken
down and released
Ions
5
Ion channel opens
6
Ion channel closes
Figure 28.6_1
AP reaching
end of presynaptic cell
triggers
increase in
intracellular
Ca2+, which
triggers
release of NT
1
Sending cell
Axon of
sending
cell
Synaptic
terminal
of sending
cell
Action
potential
arrives
Synaptic
vesicles
Synaptic
terminal
2
Vesicle fuses
with plasma
membrane
Dendrite
of receiving
cell
3
Neurotransmitter
is released into
synaptic cleft
Synaptic
cleft
4
Receiving
cell
Neurotransmitter
binds to receptor
Ion channels
Neurotransmitter
molecules
Figure 28.6_2
Neurotransmitter either broken down by enzymes
Or
Recycled back to sending cell
Neurotransmitter
Receptor
Neurotransmitter broken
down and released
Ions
5
Ion channel opens
6
Ion channel closes
Neurotransmitters May be Excitatory or
Inhibitory
 Excitatory
– Trigger Na-gated channels to open and receiving cell
starts to depolarize toward threshold
– Stimulate action potential in receiving cell
 Inhibitory
– Trigger K-gated channels to open and receiving cell
hyperpolarizes
– Inhibits action potential from occurring in receiving cell
© 2012 Pearson Education, Inc.
Figure 28.7
Synaptic terminals
Dendrites
Inhibitory
Excitatory
Myelin
sheath
Receiving
cell body
Axon
Synaptic
terminals
Figure 28.7_1
Synaptic
terminals
Examples of Neurotransmitters
 Acetylcholine
– in the brain and
– at synapses between motor neurons and muscle cells.
– Causes skeletal muscle contraction
 Serotonin and dopamine
– are important neurotransmitters in the CNS
– affect sleep, mood, and attention
 Epinephrine
– Involved in involuntary nervous system
– Flight or fright response
© 2012 Pearson Education, Inc.
Many drugs and toxins act at chemical synapses
 Many psychoactive drugs
– act at synapses and
– affect neurotransmitter action.
 Caffeine counters the effect of inhibitory
neurotransmitters.
 Nicotine acts as a stimulant by binding to
acetylcholine receptors.
 Alcohol is a depressant.
© 2012 Pearson Education, Inc.
Figure 28.9
Many drugs and toxins act at chemical synapses
 Botox
– Produced by many bacteria such as Clostridium botulinum
– Inhibits release of acetylcholine from synaptic vesicles
– Prevents skeletal muscle contraction
 Snake Venoms
– Inhibit acetylcholinesterase - the enzyme that breaks down
acetylcholine
– Can’t turn off induction of skeletal muscle contraction
– Results in uncontrolled muscle contractions (tetany)