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Physiology of the Nervous System. 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. Ionic basis of E m.
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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
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
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.
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.
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.
Changes in Em • When the V-G Na+ channels open, there is a rush of Na+ into the cell, making the inside positive. • The Na+ channels close at the same time the V-G K+ channels open. • When this happens, there is a rush of K+out of the cell, making the inside more negative.
Myelination I • In the central nervous system, myelin is formed by the oligodendrocytes. • One oligodendrocyte can contribute to the myelin sheath of several axons.
Myelination II • In the peripheral nervous system, myelin is formed by Schwann cells. • Each Schwann cell associates with only one axon, when forming a myelinated internode.
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.
Structure of the spinal cord I • The CNS is made up not only of the brain, but also the spinal cord. • The spinal cord is a thick, hollow tube of nerves that runs down the back, through the spine.
