10 6 Cell Membrane Potential A cell membrane

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10. 6: Cell Membrane Potential • A cell membrane is usually electrically charged, or

10. 6: Cell Membrane Potential • A cell membrane is usually electrically charged, or polarized, so that the inside of the membrane is negatively charged with respect to the outside of the membrane (which is then positively charged). • This is as a result of unequal distribution of ions on the inside and the outside of the membrane. 1

Distribution of Ions • Potassium (K+) ions are the major intracellular positive ions (cations).

Distribution of Ions • Potassium (K+) ions are the major intracellular positive ions (cations). • Sodium (Na+) ions are the major extracellular positive ions (cations). • This distribution is largely created by the Sodium/Potassium Pump (Na+/K+ pump). • This pump actively transports 3 sodium ions out of the cell and 2 potassium ions into the cell. 2

Resting Potential • Resting Membrane Potential (RMP): • 70 m. V difference from inside

Resting Potential • Resting Membrane Potential (RMP): • 70 m. V difference from inside to outside of cell • It is a polarized membrane • Inside of cell is negative relative to the outside of the cell • RMP = -70 m. V • Due to distribution of ions inside vs. outside • Na+/K+ pump restores Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. High Na+ Low K+ Low Na+ Impermeant anions High K+ Cell body Axon terminal + – (a) + – +– – – + + + – – – + + – 70 m. V + – – + (b) + + – Na – Low + Na+ Pump – – K+ + – Low K++ – High K+– + + High Na+ + – – – + + – 70 m. V + – – + (c) 3 + – – +

Local Potential Changes • Caused by various stimuli: • Temperature changes Gate-like mechanism •

Local Potential Changes • Caused by various stimuli: • Temperature changes Gate-like mechanism • Light • Pressure Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Protein Cell membrane (a) Channel closed Fatty acid tail Phosphate head (b) Channel open • Environmental changes affect the membrane potential by opening a gated ion channel • Channels are 1) chemically gated, 2) voltage gated, or 3) mechanically gated 4

Local Potential Changes • If membrane potential becomes more negative, it has hyperpolarized •

Local Potential Changes • If membrane potential becomes more negative, it has hyperpolarized • If membrane potential becomes less negative, it has depolarized • Graded (or proportional) to intensity of stimulation reaching threshold potential • Reaching threshold potential results in a nerve impulse, starting an action potential 5

Local Potential Changes Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction

Local Potential Changes Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Na+ – 62 m. V Neurotransmitter (a) Chemically-gated Na+ channel Presynaptic neuron Voltage-gated Na+ channel Trigger zone (axon hillock) Na+ Na+ Na+ – 55 m. V (b) 6

Action Potentials • At rest, the membrane is polarized (RMP = -70) • Threshold

Action Potentials • At rest, the membrane is polarized (RMP = -70) • Threshold stimulus reached (-55) • Sodium channels open and membrane depolarizes (toward 0) • Potassium leaves cytoplasm and membrane repolarizes (+30) • Brief period of hyperpolarization (-90) Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Na+ Na+ Na+ K+ K+ K+ K+ Na+ Na+ – 0 – 70 Na+ Na+ Na+ Na+ (a) K+ Na+ K+ K+ K+ – 0 K+ Threshold stimulus K+ K+ Na+ channels open K+ channels closed K+ Na+ Na+ Na+ Na+ – 70 Region of depolarization (b) K+ K+ Na+ Na+ Na+ K+ K+ K+ K+ Na+ Region of repolarization (c) Na+ Na+ Na+ – 0 K+ channels open Na+ channels closed – 70 Na+ 7

Action Potentials Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or

Action Potentials Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. +40 Membrane potential (millivolts) Action potential +20 0 – 20 Resting potential reestablished – 40 Resting potential – 60 – 80 Hyperpolarization 0 1 2 3 4 5 Milliseconds 6 7 8 8

Action Potentials Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or

Action Potentials Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Region of action potential + + + – – – – – + + + + (a) + + + – – – – – + + + + Direction of nerve impulse + + + (b) – – – – + + – – + (c) + + + + 9

All-or-None Response • If a neuron axon responds at all, it responds completely –

All-or-None Response • If a neuron axon responds at all, it responds completely – with an action potential (nerve impulse) • A nerve impulse is conducted whenever a stimulus of threshold intensity or above is applied to an axon • All impulses carried on an axon are the same strength 10

Refractory Period • Absolute Refractory Period • Time when threshold stimulus does not start

Refractory Period • Absolute Refractory Period • Time when threshold stimulus does not start another action potential • Relative Refractory Period • Time when stronger threshold stimulus can start another action potential 11

Impulse Conduction 12

Impulse Conduction 12

10. 7: Synaptic Transmission • This is where released neurotransmitters cross the synaptic cleft

10. 7: Synaptic Transmission • This is where released neurotransmitters cross the synaptic cleft and react with specific molecules called receptors in the postsynaptic neuron membrane. • Effects of neurotransmitters vary. • Some neurotransmitters may open ion channels and others may close ion channels. 13

Synaptic Potentials • EPSP • Excitatory postsynaptic potential • Graded • Depolarizes membrane of

Synaptic Potentials • EPSP • Excitatory postsynaptic potential • Graded • Depolarizes membrane of postsynaptic neuron • Action potential of postsynaptic neuron becomes more likely • IPSP • Inhibitory postsynaptic potential • Graded • Hyperpolarizes membrane of postsynaptic neuron • Action potential of postsynaptic neuron becomes less likely 14

Summation of EPSPs and IPSPs Copyright © The Mc. Graw-Hill Companies, Inc. Permission required

Summation of EPSPs and IPSPs Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. • EPSPs and IPSPs are added together in a process called summation • More EPSPs lead to greater probability of an action potential Neuron cell body Nucleus Presynaptic knob Presynaptic axon 15

Neurotransmitters 16

Neurotransmitters 16

Neurotransmitters 17

Neurotransmitters 17

Neuropeptides • Neurons in the brain or spinal cord synthesize neuropeptides. • These neuropeptides

Neuropeptides • Neurons in the brain or spinal cord synthesize neuropeptides. • These neuropeptides act as neurotransmitters. • Examples include: • Enkephalins • Beta endorphin • Substance P 18

10. 8: Impulse Processing • Way the nervous system processes nerve impulses and acts

10. 8: Impulse Processing • Way the nervous system processes nerve impulses and acts upon them • Neuronal Pools • Interneurons • Work together to perform a common function • May excite or inhibit • Convergence • Various sensory receptors • Can allow for summation of impulses • Divergence • Branching axon • Stimulation of many neurons ultimately 19

Neuronal Pools • Groups of interneurons that make synaptic connections with each other •

Neuronal Pools • Groups of interneurons that make synaptic connections with each other • Interneurons work together to perform a common function • Each pool receives input from other neurons • Each pool generates output to other neurons 20

Convergence Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

Convergence Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. • Neuron receives input from several neurons • Incoming impulses represent information from different types of sensory receptors • Allows nervous system to collect, process, and respond to information 1 2 • Makes it possible for a neuron to sum impulses from different sources 3 (a) 21

Divergence • One neuron sends impulses to several neurons Copyright © The Mc. Graw-Hill

Divergence • One neuron sends impulses to several neurons Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. • Can amplify an impulse • Impulse from a single neuron in CNS may be amplified to activate enough motor units needed for muscle contraction 4 6 5 (b) 22