Chapter 4 Neuronal Physiology V edit. Pg. 99-131 VI edit. Pg. 85-113 VII edit. Pg. 87-113
Input Zone Dendrites and Cell body Nucleus Trigger Zone Axon hillock Conducting Zone Axon (may be from 1mm to more than 1m long) Arrows indicate the direction in which nerve signals are conveyed. Output Zone Axon terminals
Cell body: contains nucleus, cytoplasm, mitochondria, Golgi apparatus, Nissl bodies (ER containing chromatophilic substance), ribosomes Dendrites: small cellular processes that receive input signals in a neuron Axon: long cellular process designed for carrying electrical signals to other cells. Bundles of axons form nerves
Morphology of various type of neurons in the central nervous system
OUT IN
Electrical signals are produced by changes in ion movement across the plasma membrane: Action potentials & synaptic (or graded) potentials
Ion Channels 1) Leak channels resting membrane potential 2) Voltage-gated ion channels action potential 3) Ligand-gated ion channels graded potential
Action potentials are brief changes in membrane potential that occur when the inside of the cell becomes more positive. Action potentials are self-generating signals that can propagate over long distances overshoot Action potential=spike
Voltage-gated Sodium Channels Membrane depolarization Closed Open Inactivated
Voltage-gated Sodium Channels Membrane depolarization Closed Open Inactivated X Tetrodotoxin X Local anesthetics
Voltage-gated Potassium Channels Closed Membrane depolarization Open
Action Potential Are triggered once membrane potential reaches threshold All-or-none, with constant amplitude and duration Depolarizing
At rest: P K :P Na :P Cl = 50/1/10 At the peak of the AP: P K :P Na :P Cl = 1/20/0.45
At rest, all voltage-gated channels are closed Threshold Rest
The triggering event causes the opening of some voltage-gated Na + channels, which brings the RMP to threshold Threshold
The triggering event causes the opening of some voltage-gated Na + channels, which brings the RMP to threshold Threshold
Once the membrane potential reaches threshold, all voltagegated Na + channels open Threshold
The influx of Na + causes depolarization of the membrane, which generates the rising phase of the action potential Why do Na + ions rush in? Threshold
Voltage-gated Na + channels inactivate, Na + ions cease to enter the cell Threshold
Simultaneously, voltagegated K + channels open, K + ions leave the cell Threshold
Voltage-gated K + channels open, K + ions leave the cell causing membrane repolarization Why do K + ions rush out? Threshold
The exit of K + brings the membrane potential close to rest Threshold
Membrane potential returns to rest, both voltage-gated Na + and K + channels return to the closed state Threshold Rest http://www.blackwellpublishing.com/matthews/channel.html
Permeability changes during an action potential
Generation of an Action Potential http://www.blackwellpublishing.com/matthews/channel.html http://www.sumanasinc.com/webcontent/anisamples/neurobiology/signalin g.html
Permeability changes during an action potential
General considerations of ion changes at rest and during an action potential (for a neuron with a RMP=-70 mv and a diameter of 25 mm) At rest During an action potential (to 30 mv) Number of displaced charges across the membrane 9 000 000 12 000 000 Equivalent number of moles 1.4 x 10-17 (0.014 fm) 2 x 10-17 (0.02 fm)
How can we restore the concentration gradient disrupted by the action potential?
The Na + /K + pump restore the concentration gradient disrupted by the action potential OUT IN
Action potentials are generated in the axon hillock and they propagate along the axon in order to reach the axon terminal
What happen to the action potential once is generated in the axon hillock?
Propagation of the action potential: always forward
http://www.blackwellpublishing.com/matthews/actionp.html http://harveyproject.science.wayne.edu/development/nervous_system/cell_neuro/action_potential/a.html
Why is the action potential unable to travel backward? Absolute and relative refractory period
Factors that affect Nerve Conduction: Axon Myalination Fiber diameter
PNS Schwann cells One Schwann cell forms the myelin sheath for one axon CNS Oligodendrocytes One oligodendrocyte forms the myelin sheath for several axon
Propagation of the action potential in myelinated axons: Saltatory Conduction
Saltatory Conduction
Conduction of the Action Potential http://www.blackwellpublishing.com/matthews/actionp.html Notice: action potentials do not travel along the axon! Rather, each membrane segment generates its own action potential
Factors that affect Nerve Conduction: Axon Myalination Fiber diameter: V=f(d)
Why saltatory conduction is the best way to transmit an action potential? 1) Diameter constrains 2) Faster transmission 3) Prevent large scale ionic disruption
Action Potential Are triggered once membrane potential reaches threshold All-or-none, with constant amplitude and duration Depolarizing Occurs only in the axon hillock or areas with high density of voltage-activated Na + channels Self-generating spread, propagates continuously or by saltatory conduction Travel in one direction, refractory period
Once the action potential reaches the axon terminal, it sends a chemical signal to other neurons or effector organs: synaptic transmission
In chemical synapses, information is transmitted in only one direction: from the presynaptic neuron to the postsynaptic neuron Synapses are places where two neurons meet There are chemical and electrical synapses The main components of a chemical synapse are: presynaptic membrane, synaptic cleft, postsynaptic membrane
Synaptic knob
Synaptic transmission
Events Leading to Neurotransmitter Release http://www.bishopstopford.com/faculties/science/arthur/synapse.swf http://www.lifesci.ucsb.edu/~mcdougal/neurobehavior/modules_homework/lect3.dcr http://www.wisc-online.com/objects/index_tj.asp?objid=ap1201
Ion Channels 1) Leak channels resting membrane potential 2) Voltage-gated ion channels action potential 3) Ligand-gated ion channels graded potential
Chemically-gated Ion Channels What are ligandgated ion channels?
Excitatory and Inhibitory Synapses Excitatory neurotransmitters: acetylcholine, noradrenaline Inhibitory neurotransmitters: glycine, GABA +60 E Na 0-70 -90 E K Brooks/Cole - Thomson Learning
Neurotransmitters Small chemical molecules, synthesized and packaged in axon terminal Involve in fast synaptic transmission Can be excitatory or inhibitory Are fast removed from synaptic cleft Examples: Glutamate, Acetylcholine, Dopamine, GABA, Glycine Neuropeptides Polypeptide molecules, synthesized in cell body Involve in slow synaptic transmission (or neuromodulation) Only regulate a preexisting function Slowly diffuse out of synaptic cleft Examples: Substance P, endorphins, enkephalins
Termination of Neurotransmitter Action http://www.bishopstopford.com/faculties/science/arthur/synapse.swf
Synaptic transmission and generation of local potentials (or synaptic potentials, or graded potential)
Graded Potentials (also know as local or synaptic potentials)
Generation of graded potentials in dendrites
Local potentials spread by passive current flow
Propagation of local potentials Influenced by: 1) voltage difference 2) resistance to current flow
Local potentials only propagate over short distances WHY?
Local potentials only propagate over short distances
Excitatory Postsynaptic Potentials What ions are involved in the generation of an EPSP?
Inhibitory Postsynaptic Potentials What ions are involved in the generation of an IPSP?
Action Potential Are triggered once membrane potential reach threshold All-or-none, with constant amplitude and duration Depolarizing Occurs only in the axon hillock or areas with high density of voltage-activated Na + channels Self-generating spread, saltatory conduction to inactive membrane areas Travel in one direction, refractory period Graded Potential Linear relationship between amplitude of stimulation and amplitude of local potential Local response, with variable amplitude and duration Depolarizing or hyperpolarizing Occurs in any area of the membrane (except in axons) Passive spread to inactive membrane areas Bi-directional
Integration of synaptic potentials
Presynaptic inhibition & facilitation
Convergence and Divergence
Conclusions Local potentials Action potential generation Action potential propagation Synaptic release
Central nervous system (spinal cord) Peripheral nervous system Axon terminals Cell body Afferent neuron Receptor Central axon Peripheral axon (afferent fiber) Interneuron Cell body Efferent neuron* Axon (efferent fiber) Axon terminals Effector organ (muscle or gland)