1. Ch. 15 Coordination Part 3

4. 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

5. How Your Taste Buds Work
Papillae  small bumps that cover tongue
Each contains many taste buds
Taste bud contains receptor cells sensitive to SPECIFIC chemicals (detecting 5 different tastes)
Sweet
Sour
Salt
Bitter
Savory

6. 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 Voltage- gated 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

7. 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

8. 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

9. 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

10. Factors that Affect Speed of Conduction of AP
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
Diameter of axon
Thick axons  faster transmission of action potential
Thin axons slower transmission of axon potential

11. 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

12. 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

13. 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

16. 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