Class Notes (835,166)
Canada (508,984)
Physiology (1,062)
Anita Woods (147)

4 Nerve cells.docx

13 Pages
Unlock Document

Physiology 2130
Anita Woods

Module 4: Nerve Cells  Virtually all cells in the body have membrane potentials  Inside electrically negative  Outside electrically positive  Electrochemical gradients of the major intercellular and extracellular ions establish and maintain membrane potential  Excitable cells (nerve and muscle cells), use resting membrane potential to generate electrochemical impulse called an action potential - i.e. nerve cell (neuron) - action potential is the ―language‖ of nervous system because this is how cells communicate with one another - action potentials needed for muscles to contract Structure of a Nerve Cell  Picture of multipolar neuron  Several type of neurons Structure Function Dendrites - Thin branching processes of cell body that receive incoming signals - ↑ overall SA of neuron so it can communicate with other neurons - # dendrites vary depending on where nerve cell is located in the nervous system Cell Body (Soma) - Control centre of nerve cell - Contains nucleus and all other organelles needed to direct cellular activity Axon - Projection of the cell body that carries the outgoing signal to the target cell in the form of an action potential - May or may not be myelinated Myelin sheath - Layered phospholipid membrane sheath wrapped tightly around the axon - Insulated with a fatty acid material called myelin, produced by special cells: Schwann cells - Schwann cells: in PNS and oligodendrocytes in CNS - Effect of this myelin is to insulate the axon so few ions can leak out through the membrane - Insulator for the axon forcing the ionic changes that comprise the action potential to take place in only small exposed regions of the axon called the Nodes of Ranvier - This jumping of action potential from node to node results in significant increase in transmission down the length of the axon Node of Ranvier - Small exposed regions of the axon - Jumping of action potential from node to node increases the speed of transmission down the length of the axon Collaterals - Branching of the axon near its terminal end - Serve to ↑ # of target cells with which the neuron can interact Terminal Bouton - Swelling at the end of an axon collateral (axon terminal) - Swelling contains mitochondria & membrane bound vesicles contain various neurocrin molecules - Chemical in this terminal facilitate the transmission of the signal across the synapse to the target cell Action Potential  Action potential: it is a rapid reversal of the resting membrane 1. Membrane potential rapidly increases from resting (-70mV) to +35mV - called depolarization 2. Membrane potential rapidly returns to -70mv - called repolarization 3. membrane briefly becomes more negative, -90mV - called hyperpolarization 4. Membrane returns to resting membrane potential, -70mV Q: What causes these rapid changes in membrane potential? A: the movement of ions across the membrane—principally sodium ions + + (Na ) and potassium ions (K ). These ions are allowed to move across the membrane through special channels Voltage-Gated Channels  Types of channels 1. Voltage gated sodium channel In neurons, these channels are found on the axon 2. Voltage gated potassium channel  Essential for generation of action potential  Channels are sensitive to changes membrane potential - They open when inside of cell becomes more + - i.e. -70mV to -60mV, depolarization Voltage-Gated Sodium Channel  Only allows sodium through, when there is a depolarization of the membrane (inside +), both gates are on the intracellular site + Na Steps: 1. Depolarization of the membrane occurs (membrane potential becomes more positive/less negative) 2. Activation gate opens immediately + 3. Na flow into the cell, down the concentration gradient 4. Inactivation gate closes (10 of a millisecond later) and Na can no longer flow into the cell; the channel cannot open 5. Channel returns to resting configuration (inactivation gate open and activation gate closed) 6. Channel is now ready to open again. Inactivation of Na Voltage-Gated Channel – The Absolute Refractory Period  Absolute refractory period: time period when inactivation gate is closed, channel will not open (channel has become inactivated), regardless of the strength of stimulation Voltage-Gated Potassium Channel  Only 1 gate, which opens when the membrane depolarizes  Unlike Na+ channel, they begin to open when the sodium voltage-gated channels starts to become inactivated - Do not open immediately - This is essential to the generation of the action potential - Unlike the Na voltage-gated channel, these channels do not have an inactivation period K Steps: 1. Depolarization of membrane occurs (membrane potential becomes more +/less -) 2. After a brief pause, K+ voltage –gated channels open located at the intracellular side (unlike Na+ channels that open immediately) + 3. K flow out of the cell, down their electrical and chemical gradients 4. Gate closes and channel returns to resting configuration 5. Channel is now ready to open again  These channels are essential to create action potential  Na voltage-gated channels open first and then become inactivated, producing the absolute refractory period +  K voltage-gated channels then begin opening as the Na channels begin entering the inactivated period The Action Potential  Membrane reverses from -70mV to +35mV  Begins at axon hillock: a region of neuron, most electrically sensitive area of the nerve - This region contains largest # of voltage-gated channels Steps: 1. Strong depolarization at the axon hillock triggers opening of most Na voltage-gated channels 2. Na rushes into the neuron, down its electrochemical gradient 3. Membrane depolarizes rapidly to roughly +35 mV 4. Na channels become inactivated while K channels begin opening. + 5. K rushes out of the cell, down its electrochemical gradient slowly 6. Membrane begins repolarizing back to normal (+35 mV back to –70 mV) 7. K continues to rush out of the cell and the membrane hyperpolarizes (reaches –90 mV). + + 8. K channels begin to close and K no longer leaves the cell 9. Membrane potential slowly returns to resting value of –70 mV → → → → → → Refractory Periods  Inactivation of Na channels cause absolute refrectory period, peiod when Na+ gates won`t open to fire another action potential  Relative refractory period is the period during action potential when membrane is hyperpolarized (less than -70mV) - Caused by the K voltage-gated channels, which are not only slow to open but are also slow to close - This allows K to continue to leave the cell even after it has repolarized to –70 mV - It is possible to fire another action potential, but it would require a stronger stimulus to reach threshold Threshold for Starting an Action Potential +  Action potentials require a strong depolarization at axon hillock → to open many Na voltage-gated channels + If small # of Na ions enter cell ↓ Small depolarization, small + charge build up ↓ Cell will attempt to maintain resting membrane potential (-70mV) ↓ + – + Charge build up affects other ions inside and outside the cell i.e. K and chloride (Cl ) ↓ + – K has a positive charge, it will leave the inside; at the same time, Cl (which is negative) will be attracted into the cell through leak channels which are ALWAYS open ↓ Movement of both of these ions (K out and Cl in) will repolarize the membrane potential back to normal +  In order to fire an action potention: depolarizing force from Na moving in must exceed the natural repolarizing forces from K moving out and Cl coming in +  If amount of positive charge (fr
More Less

Related notes for Physiology 2130

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.