Anatomy and Physiology I Electrical Signals in Neurons Action Potentials The Synapse Instructor: Mary Holman

Electrical Signals in Neurons Two basic features of cell membranes of neurons and other excitable cells Membrane potential Ion channels Allow neurons to communicate using electrical signals Action potentials

Fig c + + – – + + – – + + – + – – + – + – + – + – + – + – + – + – + – –70 mV Low Na + Low K + High K + High Na + Na + K+K+ Pump The Membrane Potential of the Resting Neuron -70 millivolts 1 mV= 0.001Volts Impermeant anions Trigger Zone Membrane Potential = unequal distribution of + and - ions on either side of a cell membrane

Resting Membrane Potential of the Neuron From: Principles of Anatomy & Physiology by Tortora & Grabowski

Ion Channels Gated channels that open and close in response to a stimulus 1.Voltage-gated ion channels as in action potentials 2.Ligand-gated ion channels as in neurotransmitters 3.Mechanically-gated ion channel

Fig Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gate-like Mechanism of Ion Channels Protein (b) Channel open (a) Channel closed Cell membrane Fatty acid tail Phosphate head Stimulus -action potential -chemical -mechanical ions Membrane potential changes due to move- ment of ions when channels are open

A cell that has a membrane potential is said to be polarized When the ion flow due to the opening of an ion channel produces a more negative membrane potential it is called hyperpolarized If the membrane potential becomes more positive, it is called depolarized Membrane Polarization

Action Potential If the membrane is depolarized to ~ -55mV it is said to have reached threshold potential When a threshold potential is reached, a nerve impulse or action potential is generated

Fig a Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. –62 mV Na + Neurotransmitter Ligand-gated Na + channel Presynaptic neuron Sub-threshold Depolarization Does not Result in an Action Potential Trigger Zone

Fig b Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. –55 mV Na + Trigger zone Voltage-gated Na + channel An Action Potential is Generated When Depolarization Reaches ~55mV Ligand-gated Na + channel

Fig (a) Region of depolarization (b) Region of repolarization –70 –0 –70 –0 –70 –0 K+K+ Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Threshold stimulus Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Na + channels open K + channels closed K + channels open Na + channels closed Action Potential & Nerve Membrane

An Action Potential = Nerve Impulse All-or-none response all impulses are the same strength more stimuli leads to more impulses Refractory Period Absolute vs Relative period of time between impulses when membrane is unresponsive to stimuli ~1--30 milliseconds

Fig Milliseconds Membrane potential (millivolts) Action potential Hyperpolarization –40 –20 –60 –80 Resting potential Resting potential reestablished Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Oscilloscope Recording of an Action Potential

Fig Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Direction of nerve impulse ––––––––– ––––––––– ––––––––– ––––––––– ––––––––– ––––––––– Region of action potential A Nerve Impulse = moving Zone of Depolarization

Myelinated nerves conduct action potentials from node to node This mode of transmission is called saltatory conduction Impulse Conduction

Fig Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Electric current Nodes Axon Schwann cells (a) (b) –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– Action potential Action potential Action potential A Nerve Impulse Moving along a Myelinated Axon

The Synapse Nerve impulses pass from neuron to neuron at synapses, moving from a pre-synaptic neuron to a post-synaptic neuron. Neurotransmitters are released when the impulse reaches a synaptic knob

Fig Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites Impulse Synaptic cleft Axon of presynaptic neuron Cell body of postsynaptic neuron Axon of postsynaptic neuron Axon of presynaptic neuron Synapses of Three Neurons

Events at the Synaptic Cleft Action potential races along axon to the terminal knobs or varicosities. The change in polarization of the membrane of the knob opens voltage sensitive Ca ++ gates & Ca ++ floods in. The increased [Ca ++ ] stimulates vesicles to move to the cell membrane and release neurotransmitters via exocytosis. Neurotransmitter molecules bind to receptors on post- synaptic neuron membrane which opens ligand-gated ion channels. Depending on which ion channels are opened, membrane is hyperpolarized or depolarized.

Fig a Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mitochondrion Synaptic knob Synaptic cleft Neurotransmitter Axon Ca +2 Presynaptic neuron Direction of nerve impulse Synaptic vesicles Cell body or dendrite of postsynaptic neuron Synaptic vesicle Vesicle releasing neurotransmitter Axon membrane Polarized membrane Depolarized membrane Ca +2 Activity at a Synaptic Cleft

Fig b Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mitochondrion Synaptic vesicle Synaptic cleft Postsynaptic membrane © Don W. Fawcett/Photo Researchers, Inc. A Synaptic Cleft TEM 37,500x

Neurotransmitters Released from vesicles in terminal knobs of presynaptic axons React with receptors on membrane of postsynaptic neuron Many different molecules act as neuro- transmitters Many serious diseases result from imbalances of neurotransmitters

Can cause excitatory (EPSP) or inhibitory (IPSP) post-synaptic membrane potentials by opening or closing various ion channels Usually the net effect the EPSPs and IPSPs is determined in the region of the trigger zone of the neuron Actions of Neurotransmitters

Fig Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nucleus Neuron cell body Presynaptic knob Presynaptic axon Neurons Receive Multiple Signals Simultaneously

Impulse Processing Neuron pools in CNS interpret impulses from many neurons Convergence Multiple axons delivering impulses to the same neuron Divergence An axon that branches and delivers impulses to several neurons

Fig a Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Impulse Convergence

Fig b Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Impulse Divergence

Fig. 10A Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ion channel Nicotine Outside nerve cell Membrane lipid bilayer Inside nerve cell Addictions Some addicting compounds bind to sites that bind intrinsic pain re- lievers (endorphins) Others alter binding of neurotransmitters acting either as an agonist or antagonist