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.

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.

Resting Potential Resting Membrane Potential (RMP): 70 mV 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 mV Due to distribution of ions inside vs. outside Na+/K+ pump restores Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. High Na+ Low Na+ Impermeant anions High K+ Low K+ Cell body Axon Axon terminal + + + + – – – (a) – + + – + + + – – – – – – – – – + + – + – – – + + + + + + –70 mV (b) + + High Na+ + + – – – Low Na+ – Na+ + + Pump – – + + – – K+ – – – – + – – Low K+ High K+ – + + + + + + – + –70 mV (c)

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

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

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

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

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

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

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

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

Impulse Conduction

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.

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

Summation of EPSPs and IPSPs Copyright © The McGraw-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

Neurotransmitters

Neurotransmitters

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

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

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

Convergence Neuron receives input from several neurons Copyright © The McGraw-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 Makes it possible for a neuron to sum impulses from different sources 2 1 3 (a)

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