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Delve into the intricacies of nerve physiology, exploring neuron structure, signal transmission, and action potential generation. Learn about graded potentials, ion channels, and synaptic transmission, and understand how the nervous system regulates fast responses. Discover the roles of various channels and gradients in nerve function.
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Physiology of Nerves • There are two major regulatory systems in the body, the nervous system and the endocrine system. • The endocrine system regulates relatively slow, long-lived responses • The nervous system regulates fast, short-term responses
Neuron structure • Neurons all have same basic structure, a cell body with a number of dendrites and one long axon.
Ionic basis of Em • NaK-ATPase pumps 3Na+ out for 2 K+ pumped in. • Some of the K+ leaks back out, making the interior of the cell negative
Electrochemical Gradients Figure 12.12
Ion channels • Remember Ohm’s Law: I=E/R • When a channel opens, it has a fixed resistance. • Thus, each channel has a fixed current. • Using the patch-clamp technique, we can measure the current through individual channels
Graded potential • A change in potential that decreases with distance • Localized depolarization or hyperpolarization
Action Potential • Appears when region of excitable membrane depolarizes to threshold • Steps involved • Membrane depolarization and sodium channel activation • Sodium channel inactivation • Potassium channel activation • Return to normal permeability
The Generation of an Action Potential Figure 2.16.1
Characteristics of action potentials • Generation of action potential follows all-or-none principle • Refractory period lasts from time action potential begins until normal resting potential returns • Continuous propagation • spread of action potential across entire membrane in series of small steps • salutatory propagation • action potential spreads from node to node, skipping internodal membrane
Voltage-gated Na+ channels • These channels have two voltage sensitive gates. • At resting Em, one gate is closed and the other is open. • When the membrane becomes depolarized enough, the second gate will open. • After a short time, the second gate will then shut.
Voltage-gated K+ channels • Voltage-gated K+ channels have only one gate. • This gate is also activated by depolarization. • However, this gate is much slower to respond to the depolarization.
Action potential propagation • When the V-G Na+ channels open, they cause a depolarization of the neighboring membrane. • This causes the Na+ and K+ channels in that piece of membrane to be activated
AP propagation cont. • The V_G chanels in the neighboring membrane then open, causing that membrane to depolarize. • That depolarizes the next piece of membrane, etc. • It takes a while for the Na+ channels to return to their voltage-sensitive state. Until then, they won’t respond to a second depolarization.
Propagation of an Action Potential along an Unmyelinated Axon
Schwann cells cont. • In unmyelinated nerves, each Schwann cell can associate with several axons. • These axons become embedded in the Schwann cell, which provides structural support and nutrients.
g Aminobutyric Acid • Also know as GABA • Two know receptors for GABA • Both initiate hyperpolarization in the post-synaptic membrane • GABAA receptor allows an influx of Cl- ions • GABAB receptors allow an efflux of K+ ions
Transmitter effects on Em • Most chemical stimuli result in an influx of cations • This causes a depolarization of the membrane potential • At least one transmitter opens an anion influx • This results in a hyperpolarization.
EPSPs and IPSPs • If the transmitter opens a cation influx, the resulting depolarization is called an Excitatory Post Synaptic Potential (EPSP). • These individual potentials are sub-threshold. • If the transmitter opens an anion influx, the resulting hyperpolarization is called an Inhibitory Post Synaptic Potential (IPSP • All these potentials are additive.
