Ganglion Cell bodies Myelin sheath Artery Vein Axon 100 µm Nerve-consists of hundreds (thousands) of axons wrapped together in connective tissue. bullfrog
A mass of nerve
Initiation of action potential all or nothing must reach a threshold -Membrane potential is the voltage difference across a cell’s membrane (cytoplasm is more negative than outside the cell – resting potential is -70mV) - Nerve impulses are detected as a wave of electrical activity. (electrochemical change) - With an all or nothing response, how is intensity detected?
Resting Potential p889
Resting Membrane Potential-Polarized K + leak Na + /K + ATP channel pump K + leak channels maintain negative voltage inside the cell. There are few Na + leak channels.
Taste buds – a collection of chemosensitive epithelial cells associated with a sensory neuron. Circumvallate papilla
Each taste bud is “onion” shaped structures of between 50-100 taste cells. Molecules in the food dissolve in saliva and contact the taste receptors through the taste pores.
Smith &Margolskee Scientific American March 2001 Salts Na + ions enter Na + channels & depolarize the cell. The cell repolarizes by opening K + gates.
Smith &Margolskee Scientific American March 2001 Sour - Acids 1. H + directly enter channels. 2. H + bind to Na + channels causing them to open. 3. H + bind to K + channels and close these channels (no K + leaves). Taste Cell
Smith &Margolskee Scientific American March 2001 Sugar molecules bind to a receptor. This activates a G-protein & and the secondary messenger cAMP causing K + leak channels to close. The Na + leak channels allow Na + in a & the neuron depolarizes.
G protein GTP K + channel open Adenylyl cyclase Protein kinase A Sugar molecule Receptor K + channel closes activates 1 2 3 4 5 6 Gustducin cAMP closes K + leak channels, but Na + leak channels stay open.
The Tongue Taste Map Smith &Margolskee Scientific American March 2001 1. Although each neuron responds more strongly to one type of tastant, it can also generate a stimulus to other dissimilar molecules. 2. Specific tastes might be distinguished by the brain due to a pattern of activity across neural networks. 3. Smith & Margolskee claim that taste discrimination depends on the relative activity of different neuron types, each of which must contribute to the overall pattern of activity in order to distinguish between different stimuli.