Lesson 1 Chapter 3 – Membrane Potential, Graded Potentials, Action Potentials

Lesson 1 Chapter 3 – Membrane Potential, Graded Potentials, Action Potentials 1 Unit 6 Study Guide Excluded from study: Chapter 3: Exclude pages 64 – 66 The Central Nervous System Membrane Potential The plasma membranes of all living cells have a membrane potential, or are polarized electrically. This potential allows for cellular communication in nervous tissue and muscle. The term membrane potential refers to a separation of charges across the membrane or to a difference in the relative number of cations (+) and anions (-) in the ICF (inside the cell) and ECF (outside the cell). - ICF (cytosol) is negatively charged. ECF is positively charged. A neuron in its resting state has a membrane potential of -70 mV. 3 Stages of Nerve Impulse include: 1) Resting Potential 2) Action Potential 3) Refractory Potential (aka Polarization) (aka Depolarization) (aka Repolarization) Two Kinds of Potential Change o Graded Potentials  Small signals  Short range o Action Potentials  Large signals  Travel long distances Lesson 1 Chapter 3 – Membrane Potential, Graded Potentials, Action Potentials 2 The Resting Potential - - - Outside the neuron’s membrane is high concentration of sodium ions causing it to be positively charged compared to the inside Inside the neuron is a high concentration of potassium ions, chloride ions (- ions) and – charged proteins. There are ion channels in the lipid bilayer membrane that allows certain ions to move out (because there are more open potassium ion channels), than sodium ions have the potential to move in (almost no sodium ions channels open) At resting potential, more potassium ions have the potential to move out (because there are more open potassium ion channels), than sodium ions have the potential to move in (almost no sodium ion channels open) Also, another membrane protein called sodium potassium pump moves 3 Na+ out while moving 2 K+ in (ATP must be used = active transport) The Action Potential - Only occurs at the Nodes of Ranvier in myelinated neurons A nerve impulse is an action potential. An action potential begins when the dendrites are stimulated with certain chemicals or an electrical jolt. When neurons receives a stimulus, the cell membrane becomes more permeable to Na+ than K+ (Na+ channels open, and the K+ channel closes.) The highly concentrated Na+ ions rush into the cell by diffusion and charge attraction. The rapid inflow of Na+ ions rush into the cell by diffusion and charge attraction. This charge reversal is referred to as depolarization. The Refractory/Repolarization Potential - Once the voltage inside the cell becomes positive, the Na+ channels slam closed, stopping the inflow of Na+ K+ channels then open and Potassium ions rush outside. The return to the resting state is called hyperpolarization (since Potassium pump is slower to close, thus excess K+) Refractory period is the period of time the nerve will not respond to a stimulus (needs to return to resting potential. All or None Response - For an impulse to be generated, the stimulus must be strong enough to “jolt” the membrane to allow ions to rush in. This level of intensity is called the “threshold level”. Different neurons have different threshold levels. If stimulus is strong enough, the impulse will be transmitted along the entire neuron. However, if the stimulus is stronger, the neuron will not fire any stronger. (Fires maximally) Lesson 1 Chapter 3 – Membrane Potential, Graded Potentials, Action Potentials 3 Anatomy of a Nerve Cell Two types of cells: o Neurons and Glial cells - - Neuron (nerve cell) – the fundamental structural unit of the nervous system that enables us to receive info, process it, and produce action o A “bundle” of neurons forms a nerve Glial cells – (“glue”) are non-conducting cells that help the neurons out, tying them together and providing support (nourishment and remove wastes) o Large concentration found in the brain. Axon - the long extension of a neuron that carries nerve impulses away from the body of the cell. Axon terminal – the hair-like ends of the axon Axon hillock – initial segment has the lowest threshold, and as a result, action potential tends to originate here Cell body – the cell body of the neuron; it contains the nucleus (also called the soma) Dendrites – the branching structure of a neuron that receives messages (attached to the cell body) Myelin Sheath – the fatty substance that surrounds and protects some nerve fibers Node of Ranvier – one of the many gaps in the myelin sheath, this is where the action potential occurs during saltatory conduction along the axon. Nucleus – the organelle in the cell body of the neuron that occupies the genetic material of the cell Schwann’s cell – cells that produce myelin, they are located within the myelin sheath. Lesson 1 Chapter 3 – Membrane Potential, Graded Potentials, Action Potentials Conduction via a Nerve Fibre • Two types of propagation – – • Contiguous conduction • Conduction in unmyelinated fibres • Action potential spreads along every portion of the membrane Saltatory conduction • Rapid conduction in myelinated fibres • Impulse jumps over sections of the fibre covered with insulating myelin Two types of synpases – – Electrical synapses • Neurons linked by gap junctions • Second messenger molecules move through • Found in retina, some parts of cerebral cortex Chemical synapses • Majority 4 Lesson 1 Chapter 3 – Membrane Potential, Graded Potentials, Action Potentials 5 Common neurotransmitters - Acetylcholine Dopamine Norepinephrine Epinephrine Serotonin - Histamine - Glycine - Aspartate - Gamma-amino butyric acid (GABA) EPSP vs IPSP An excitatory postsynaptic potential (EPSP) is a temporary depolarization of postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell. A postsynaptic potential is defined as excitatory if it makes it easier for the neuron to fire an action potential. [Excitatory potentials are depolarizations] They are the opposite of inhibitory postsynaptic potentials (IPSPs), which usually result from the flow of negative ions into the cell. An Inhibitory Postsynaptic Potential is the change in membrane voltage of a postsynaptic neuron which results from synaptic activation of inhibitory neurotransmitter receptors. [An IPSP produces a hyperpolarization] A postsynaptic potential is considered inhibitory when the resulting change in membrane voltage makes it more difficult for the cell to fire an action potential, lowering the firing rate of the neuron. Lesson 1 Chapter 3 – Membrane Potential, Graded Potentials, Action Potentials - The axon hillock of the post synaptic neuron acts as an integrator Adds up all the signals called “summation” Temporal and Spatial summation: o Spatial summation occurs when excitatory potentials from many different presynaptic neurons cause the postsynaptic neuron to reach its threshold and fire. o Temporal summation occurs when a single presynaptic neuron fires many times in succession, causing the postsynaptic neuron to reach its threshold and fire. Excitatory neurotransmitters - Acetylcholine Glutamate Inhibitory neurotransmitters - Gamma-amino butyric acid (GABA) Glycine What are the differences between Action potentials and Graded Potentials? Graded Potential Action Potential Graded potential change: magnitude varies with magnitude of triggering event. All-or-none response; magnitude of triggering even coded in frequency rather than amplitude of action potentials Duration varies with duration of triggering events Constant duration Decremental conduction; magnitude diminishes with distance from initial site Propagated throughout membrane in undiminishing fashion 6 Lesson 1 Chapter 3 – Membrane Potential, Graded Potentials, Action Potentials Passive spread to neighbouring inactive areas of membrane Self-regeneration in neighbouring inactive areas of membrane No refractory period Refractory period Can be summed Summation impossible Can be depolarized or hyperpolarization. Triggered by stimulus, by combination of neurotransmitters with receptor, or by spontaneous shifts in leak-pump cycle Always depolarized and reversal of charges. Triggered by depolarization to threshold, usually through spread of graded potential. Occurs in specialized regions of membrane designed to respond to triggering event Occurs in regions of membrane with abundance of voltage-gated Na+ channels Intercellular communication refers to communication between cells. It can take place either directly or indirectly: - - Direct intercellular communication involves physical contact between interacting cells (Without ever touching ECF) o Gap junction o Transient direct linkup of surface makers Indirect intercellular communication involves chemical messengers or signal molecules with four types: (Releasing into ECF) o Paracrine o Neurotransmitters o Hormones o Neurohormones Signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another, most often involving ordered sequences of biochemical reactions inside the cell. It is carried out by enzymes and linked through second messengers resulting in what is thought of as a “second messenger pathway”. 7