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Ch. 15 Coordination
Part 3
What begins an Action Potential?
• Variety of stimuli
received by the receptor
cell
• Stimuli: light, pressure,
temperature
• Receptor Cell:
• End of sensory neuron
• Specialized cell in sense
organ
• Detect specific type of
stimulus and influence
electrical activity of
neurons
• Ex. Light receptors in
eyes or
chemoreceptors in
taste buds
How Your Taste Buds Work
• Papillae  small
bumps that cover
tongue
• Each contains many
taste buds
• Taste bud contains 50100 receptor cells
sensitive to SPECIFIC
chemicals (detecting 5
different tastes)
•
•
•
•
•
Sweet
Sour
Salt
Bitter
Savory
Salt Detection
• Chemoreceptors in taste buds detect
Na+ ions from salt
• Ions diffuse through highly selective
channel proteins in cell surface
membrane of microvilli of receptor
cells
• Leads to DEPOLARIZATION
• Receptor potential generated an
increase in positive charge inside of a
cell
• When receptor potential is large
enough, it stimulates Voltagegated CALCIUM ION channels
opening
• Calcium ions (Ca2++) enter cytoplasm
• Lead to EXOCYTOSIS of vesicles
containing neurotransmitter from
basal membrane
• Neurotransmitter stimulates an action
potential in sensory neuron
• Impulse is transmitted to taste center
in cerebral cortex of brain
Sweetness Detection
• Contain protein
receptors that
stimulate a G
protein
• G protein
activates enzyme
to produce cAMP
• cAMP is second
messenger that
activates signal
cascade that
leads to closure of
K+ ion channels
• Closing K+
channels 
depolarizes cell
produces action
potential
More About Action Potential
• Do NOT change in size as they travel
• Do NOT change in size according to
intensity of stimulus
• Do NOT speed up with intensity of
stimulus
• Speed of action potential transmission
is ALWAYS the same
• Frequency of action potentials change
depending on strength of stimulus
• Peak of Action Potential always +30 mV
• Frequencies of action potentials vary
• Strong stimulus produce rapid succession
of action potentials
• More neurons are stimulated for that ONE
stimulus
• Weak stimulus fewer action potentials
per second
• Stimulates fewer neurons for that one
stimulus
Brain Interpretation of Signals
• Brain interprets:
• FREQUENCY of action potentials arriving along the axon of a
sensory neuron
• NUMBER of neurons carrying action potentials
• Both of these give brain info on STRENGTH of stimulus
• NATURE of stimulus determined from position of
sensory neuron bringing in the info
• If neuron is from the retina, brain interprets signal as light
• If different stimulus stimulates receptor in retina (like
pressure), brain can still interpret as light sensation
Factors that Affect Speed of
Conduction of AP
1.
Myelin
• Unmyelinated neuron slow speed
of conduction (0.5 m/s)
• Myelinated neuron faster
conduction (100 m/s)
• Myelin speeds up rate at which action
potential travels by insulating axon
• Na+ and K+ cannot flow through parts
of the cell membrane that has myelin
• Action Potentials only occur at Nodes
of Ranvier
• All membrane proteins are located in
unmyelinated portions of cell
membrane (NoR)
• Action potentials “jump” past portion
of axon that is myelinated (1-3mm)
• Called SALTATORY CONDUCTION
• Increases speed of transmission of
action potential by 50 x than in an
unmyelinated axon
2.
Diameter of axon
• Thick axons  faster transmission of
action potential
• Thin axons slower transmission of
axon potential
All or Nothing Law
• Neurons either transmit impulses from one end to the
other or they do not send an impulse at all
• Action potential initiated when threshold value is
reached
Maintenance
of
Resting
Potential
• Axon phospholipid bilayer is impermeable to K+/Na+ ions
• Na-K pump (a globular, transmembrane protein) maintains resting membrane potential
• ATP used to pump 3 Na+ ions out and 2 K+ ions in
• Na-K pump has binding site for ATP
• Cell membrane has more K+ ion channels open than Na+ ion channels
• Membrane is more permeable to K+ ions
• K+ ions diffuse OUT of cell
• Inside of cell becomes more negative (less positive) than the outside of the cell (due to all
those K+ ions leaving)
• Membrane potential INSIDE is -65 mV less than the potential outside the cell
• Leaky K+ channels play a big role in maintaining resting membrane potential
• Na-K pump creates an electrochemical gradient
• As long as resting potential is maintained, then all voltage-gated channels are closed
How Action Potentials Are Generated:
• Sensory neurons respond to stimuli
• Stimulus causes Na+ channels to
open in cell membrane
• Na+ ions enter the sell
• This causes depolarization
• Receptor/generator potential is
created (local circuit)
• If receptor potential (local circuit) is
greater than the threshold (-60mv
to -50mV), then the action potential
is generated
• This is the “all-or-nothing response”
• Increased stimulus leads to
increased FREQUENCY of action
potentials
Transmission of Action Potentials
• Action potential stimulates
neighboring area of membrane
• Na+ ions move sideways/attracted
to areas of resting potential
• This creates a local circuit
• This depolarization causes Na+ ion
channels to open up (more
depolarization)
• Transmission in one direction is
due to
hyperpolarization/refractory
periods
• Myelin sheath enables a faster
transmission of action potential
because the action potentials
must jump from one Node of
Ranvier to the next
• Saltatory Conduction