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 50- 100 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 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

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 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

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