Download Action Potentials

Chapter 4
Neural Conduction and
Synaptic Transmission
A Case Study
P. 77 The Lizard, a Case of
Parkinson’s Disease
Neuron’s Resting Membrane Potential
Membrane Potential
• Difference in electrical
charge between the inside and
outside of a cell
Resting Potential
• Steady membrane potential
of a neuron at rest
-70 mV (millivolts)
Ions
• Positively or negatively
charged particles
Illustration
“Like all salts in a solution, the
salts in neural tissues separate into
positively and negatively charge
particles called ions.” –Pinel, p. 78
Postsynaptic
Potentials
Excitatory Postsynaptic
Potentials (EPSP)
• Neurotransmitters from a
neural message cause a
deplolarization in the next cell
-70 mV up to -67 mV
• Make it more likely the next
cell will fire
Postsynaptic
Potentials
Inhibitory Postsynaptic
Potentials (IPSP)
• Neurotransmitters… cause a
hyperpolarization in the next
cell
-70 mV up to -72 mV
• Less likely the next cell will
fire
Postsynaptic
Potentials
Factors: Determining EPSP
and IPSP
• Type of neurotransmitter
• Type of receptor
Graded Responses
• EPSP and IPSP can vary in
strength
Video: Lecture 4 Neural
Communication_Excititory and
Inhibitory
Action Potentials
“The postsynaptic potentials created at a single synapse typically
have little effect on the firing of the postsynaptic neuron (Bruno &
Sakmann, 2006). The receptive areas of most neurons are covered
with thousands of synapses, and whether or not a neuron fires is
determined by the net effect of their activity.” –Pinel, p. 81
Action Potentials
Integration
• EPSP and IPSP travel to the
base of the axon hillock where
they are summed
Action Potential
Integration
• EPSP and IPSP travel to the
base of the axon hillock where
they are summed
• Two EPSPs in rapid
succession at one synapse are
additive
• Same for IPSPs
Action Potentials
“Each neuron continuously integrates signals over both time and
space as it is continually bombarded with stimuli through the
thousands of synapses covering its dendrites and cell body.
Remember that, although schematic diagrams of neural circuitry
rarely show neurons with more than a few representative synaptic
contacts, most neurons receive thousands of such contacts.”
-Pinel, p. 82
Action Potentials
Threshold of Excitation
-65 mV (typically)
Action Potentials
Action Potentials
• Reversal of the
membrane potential,
signal travels down
the axon, contains
neural message
(-70 to +50)
• Lasts 1 millisecond
• All-or-nothing
Action Potentials: How are they
Produced?
Sodium Ion Channels
• When threshold of
excitation is reach Sodium Ion
Channels open wide
• Na+ rushes in
Potassium Channels
• Influx of Na+ triggers
opening of Potassium channels
• K+ pumped out
• After cell has been
repolarized, they close slowly
Action Potential
Refractory Period
• Hyperpolarization
• 1-2 milliseconds
• Keeps action potential
moving in one direction
Absolute Refractory Period
• Impossible to fire
Relative Refractory Period
• Higher than normal amount
of stimulation necessary to
fire
Structure of Synapses
Structure of Synapses
• Synapses can occur between
axon and
* Dendrite (most common)
* Soma
* Axons
Directed Synapses
• Site of release and site of
reception are in close
proximity
Structure of Synapses
Nondirected Synapses
• Site of release is at some
distance from the site of
reception
Figure 4.9, p. 87
Neurotransmitter Molecules
• Over 100 neurotransmitters
molecules
Two Types
• Small neurotransmitters
• Neuropeptides
Small Neurotransmitters
• Synthesized in terminal
button
• Packaged in vesicles by the
button’s Golgi Complex
• Vesilces are smaller, typically
near the membrane
Neurotransmitter Molecules
Neuropeptides
• Chains of amino acids
(essentially short proteins)
• Synthesized, packaged in
vesicles in cell body
Neurotransmitter Molecules
• Many neurons only produce
one neurotransmitter
Coexistence
• When one neuron makes a
small neurotransmitter and a
neuropeptide
Release of Neurotransmitters Molecules
Vocab
• Presynaptic neuron
• Postsynaptic neuron
Exocytosis
• Release of neurotransmitter
molecules
• Action potential opens
voltage activated calcium
channels in the button
Exocytosis
• Calcium ions (+) cause vesicles
to fuse to membrane
• Contents emptied into
synaptic cleft
Activation of Receptors
Receptor
• Protein that contains binding
sites for neurotransmitters
“Most neurotransmitters bind to
several different types of
receptors.” –p. 89
Reuptake and Degradation
Reuptake
• Almost immediately,
neurotransmitters that have
been released are drawn back
into the presynaptic neuron by
transporter mechanisms
• Recycling
Enzyme Degradation
• Enzymes break apart
neurotransmitters
Glial Cells
Astrocytes
• Release chemicals
• Contain receptors for
neurotransmitters
• Conduct signals
• Participate in neurotransmitter
reuptake
Gap Junctions
Gap Junctions
• Narrow spaces between neurons
that are bridged by fine tubular
channels
• Allow electrical signals and
molecules to flow from one neuron
to the next
Neurotransmitters
Small Molecules
• GABA
• Dopamine
• Epinephrine
• Norepinephrine
• Serotonin
• Acetylocholine
Neuropeptides
• Over 100 discovered
• Some act as hormones and
neurotransmitters