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Nerve physiology 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. Types of neurons Divisions of the nervous system Non-excitable cells of the nervous system Structure of gray matter Signal transmission in neurons Resting potential 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 patchclamp technique, we can measure the current through individual channels Gated channels: ligand-gated Gated channels: voltage-gated Gated channels: mechanically-gated Graded potential A change in potential that decreases with distance Localized depolarization or hyperpolarization Graded Potentials Graded Potentials 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 Graded potentials vs Action Potential Characteristics of action potentials Generation of action potential follows all-ornone 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 The Generation of an Action Potential Induction of an action potential I Induction of an action potential II 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 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. + K channels Cycling of V-G channels 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 voltagesensitive state. Until then, they won’t respond to a second depolarization. Propagation of an Action Potential along an Unmyelinated Axon Saltatory Propagation along a Myelinated Axon Saltatory Propagation along a Myelinated 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. Synaptic transmission 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. Post-synaptic integration Signal integration Signal integration cont. Presynaptic inhibition Presynaptic facillitation Neural circuits I Neural circuits II 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. White and gray matter in the nervous system 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. Structure of the spinal cord II Structure of the spinal cord III Structure of the spinal cord IV