1/1/2016 The Nervous Impulse • Dependent upon a resting potential across the cell membrane The Nervous System – Magnitude of potential is determined by • Leakage channels for sodium and potassium • Active transport carriers (Sodium/Potassium pump) Neural Signaling – The neuron is polarized Chapter 11 • Impulse results from depolarization The concentrations of Na + and K + on each side of the membrane are different. Factors that contribute to resting membrane potential Outside cell + The Na concentration Na + is higher outside the K+ (5 m M ) (140 m M ) cell. The K + concentration + K Na + is higher inside the (140 m M ) (15 m M ) + + cell. Inside cell Na -K ATPases (pumps) maintain the concentration gradients of Na + and K + across the membrane. The permeabilities of Na + and K + across the membrane are different. + + Suppose a cell has only K channels... K leakage channels + K+ K+ K loss through abundant leakage channels establishes a negative membrane potential. + + Cell interior K K –90 mV Now, let’s add some Na + channels to our cell... K+ K+ Na + Na + entry through leakage channels reduces the negative membrane potential slightly. K K+ Na + Cell interior –70 mV Na+-K+ pump Finally, let’s add a pump to compensate K+ K+ Na + for leaking ions. Na +-K+ ATPases (pumps) maintain the concentration gradients, resulting in the resting membrane potential. Check out the A&P Flix on Mastering “Resting Membrane Potential” K+ K+ Na + Cell interior –70 mV Figure 11.8 Voltmeter The Nervous Impulse • Polarization Plasma Ground electrode – membrane outside cell Voltage across the plasma membrane – Inside of the cell is more negative than the outside Microelectrode inside cell • Resting potential – Axon Polarization leads to attraction between opposite charges across the membrane – When a neuron is at rest, average potential is -70mV • Neurons use changes in membrane potential as Neuron signals to receive, integrate, and send information Figure 11.7 1 1/1/2016 The Nervous Impulse Depolarizing stimulus • Types of changes Inside – Depolarization positive • Decrease in membrane potential (interior becomes less Inside negative) negative Depolarization – Hyperpolarization • Increase in membrane potential (inside becomes more Resting negative) potential Time (ms) (a) Depolarization: The membrane potential moves toward 0 mV, the inside becoming less negative (more positive). This increases the probability of nerve impulse production. Figure 11.9a Hyperpolarizing stimulus The Nervous Impulse • Changes in polarization are produced by… – Anything that changes ion concentration across the membrane – Anything that changes membrane permeability to Resting an ion (most important) potential • Largely due to changes in the number of open ion channels Hyper- polarization • Membrane channels – Chemically gated (ligand gated) Time (ms) (b) Hyperpolarization: The membrane – potential increases, the inside becoming Voltage gated more negative. This decreases the probability of nerve impulse production. Figure 11.9b The Nervous Impulse • There also are mechanically gated membrane Receptor Neurotransmitter chemical attached to receptor Na + + channels Na Na + Na + – Open in response to physical deformation (touch, Chemical Membrane binds voltage pressure, sound waves) changes – K+ Found in sensory receptors K+ Closed Open Closed Open (a) Chemically (ligand) gated ion channels open when the (b) Voltage-gated ion channels open and close in response appropriate neurotransmitter binds to the receptor, to changes in membrane voltage. allowing (in this case) simultaneous movement of Na + and K +. Figure 11.6 2 1/1/2016 The Graded Potential • Short lived, localized changes in membrane potential Stimulus Depolarized region – Due to incoming signals, usually energy or neurotransmitters • Channels open → ions flow • Short distance signals Plasma membrane • May be depolarization or hyperpolarization events (a) Depolarization: A small patch of the membrane (red area) has become depolarized. Figure 11.10a Active area (site of initial depolarization) –70 Membranepotential (mV) Resting potential (b) Spread of depolarization: The local currents (black arrows) that are created depolarize Distance (a few mm) (c) Decay of membrane potential with distance: Because current adjacent membrane areas and allow the wave of is lost through the “leaky” plasma membrane, the voltage declines depolarization to spread. with distance from the stimulus (the voltage is decremental ). Consequently, graded potentials are short-distance signals. Figure 11.10b Figure 11.10c The Action Potential Stimuli that Initiate Action Potentials Long distance signal Initiated by sufficient depolarization at site of Sensory Neurons Motor & Association Neurons • graded potential Light • Chemical stimuli • Must reach threshold – usually a change of ~100mV Heat from other Opening of specific voltage gated channels • Chemicals neurons Does not decrease in strength with distance • Mechanical energy “All or none” A.K.A. nerve impulse Threshold stimulus always required 3 1/1/2016 Dendrites Cell body Neuron cell body The Action Potential (receptive (biosynthetic center regions) and receptive region) The players: Sodium Na + channel Potassium channel Nucleolus Activation gates K+ Inactivation gate Axon (a) (impulse Dendritic Potassium Channel spine Nucleus conducting Impulse Sodium Channel region) direction Slow to open Nissl bodies Node of Ranvier • Opens instantly Axon terminals • Can’t sustain (self-inhibits) Slow to close Axon hillock Schwann cell (secretory (impulse Neurilemma (one inter- region) (b) generating node) Terminal region) branches The events Sodium Na + The big picture channel Potassium channel 1 Resting state2 Depolarization 3 Repolarization Activation gates K+ Inactivation gate Na + Na + 1 Resting state 3 4 Hyperpolarization 2 Action K+ potential K+ 4 Hyperpolarization Na + 2 Depolarization Membrane potential (mV) potential Membrane Threshold 1 1 4 K+ Time (ms) 3 Repolarization Figure 11.11 (1 of 5) The Action Potential The Action Potential • Resting potential is quickly restored • Once initiated, AP is self-propagating – Thousands of Na +/K + pumps redistribute ions – Once Na + channels in one region are inactivated, no – May seem like a huge task new AP is generated there – Only a small number of ions actually cross the • Continues along axon in one direction at membrane constant velocity • Change in 0.012% of intracellular Na + concentration • Factors affecting conduction velocity: – Axon diameter – larger diameter = faster conduction – Degree of myelination 4 1/1/2016 The Action Potential The Action Potential • Myelinated Neurons • Refractory period – Nodes of Ranvier – only place where current can – Neuron cannot respond to a second stimulus pass through the membrane • During repolarization – Saltatory conduction – 30X faster than continuous – Limits number of impulses per second conduction Absolute refractory Relative refractory period period The Action Potential Depolarization (Na + enters) • Coding for stimulus intensity – All APs are independent of stimulus strength – CNS must discern strong from weak signals to initiate appropriate response Repolarization – (K + leaves) Stimulus intensity is coded for by frequency of action potentials After-hyperpolarization Stimulus Time (ms) Figure 11.14 Dendrites Cell body Neuron cell body The Synapse (receptive (biosynthetic center regions) and receptive region) • Nervous system operates through chains of neurons connected by synapses • Syn = “to clasp or join” Nucleolus • Junction between… – Adjacent neurons Axon – Neuron and an effector cell (a) (impulse Dendritic spine Nucleus conducting Impulse • region) direction Mediates information transfer Nissl bodies Node of Ranvier – Electrical Axon terminals ? Axon hillock Schwann cell (secretory (impulse – Neurilemma (one inter- region) Chemical (b) generating node) Terminal region) branches 5 1/1/2016 The Synapse • Presynaptic neuron – Axodendritic Conducts impulses toward the synapse synapses • Postsynaptic neuron Dendrites – Axosomatic Transmits impulses away from the synapse synapses • Most neurons are both Cell body Axoaxonic synapses (a) Axon Axon of presynaptic The Synapse neuron • Electrical Synapses – Direct transmission of electrical signals from one cell to another Axosomatic – Less common than chemical synapses synapses • Neurons are electrically coupled (joined by gap junctions) • Communication is very rapid – May be unidirectional or bidirectional • Important in – Embryonic tissue Cell body (soma) – Some brain regions of postsynaptic – Synchronizing groups of neurons (example: jerky eye movements) (b) neuron Electrical Synapses The Synapse • Chemical Synapses – Indirect communication between cells – Electrical signal of AP is changed to a chemical signal (neurotransmitter) in the presynaptic neuron – Neurotransmitter is released into the synaptic space and diffuses toward the postsynaptic neuron – Postsynaptic neuron changes chemical signal back to electrical signal for conduction along its own axon – Unidirectional – Here’s how it works… 6 1/1/2016 Chemical Synapses Chemical Synapses 1. Action potential 2. Voltage-gated arrives at the Ca 2+ channels axon terminal in open, and Ca 2+ the presynaptic enters the axon neuron terminal Chemical Synapses Chemical Synapses 3. Ca 2+ entry causes 4. Neurotransmitter synaptic vesicles diffuses across the to release synaptic cleft and neurotransmitter binds to specific by exocytosis receptors on the postsynaptic neuron’s membrane Chemical Synapses