BIOB30 Ch8 (258-266).docx

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Biological Sciences
Joanne Nash

BIOB30 Chapter 8 (258-266) 1 Neurons: Cellular and Network Properties Graded Potentials Reflect Stimulus Strength  Graded potentials – depolaraizations or hyperpolarizations that occur in the dendrites and cell body (or sometimes in the axon terminals) o The changes in membrane potential have an amplitude directly proportional to the strength of the triggering event  What causes graded potential o In the CNS and the efferent division, chemical signals from neurons open chemically gated ion channels  Allows ions to enter or leave the neuron o In some sensory neurons, a mechanical stimuli (e.g. stretch) opens or closes ion channels  Closing ion gates can cause depolarization too o E.g. closing the K+ gate and retaining K+  Figure 8.7 show graded potential where: o A stimulus opens a monovalent cation channel allowing sodium influx o Positive charge sends a wave of depolarization through the cytoplasm  Local current – the wave of depolarization that moves through the cell  Convention: current is the net movement of positive electric charge o Initial amplitude (or depolarization strength) in a graded potential is determined by how much charge enters the cell  Stronger initial means it will spread farther without dying  Why graded potentials lose strength as they move through the cytoplasm 1. Current Leak – positive ions leak out as the depolarization wave moves (membrane has open leak channels so it is a bad insulator) 2. Cytoplasmic resistance – the cytoplasm provides resistance to the flow of electricity  Trigger zone – the integrating center of the neuron which contains a high concentration of voltage-gated sodium channels in its membrane o In efferent neurons and interneurons: is at the axon hillock  At the initial segment (the very first part of the axon) o In sensory neurons, it is immediately adjacent to the receptor (where the dendrite joins the axon)  If the trigger zone is depolarized to threshold voltage, the voltage-gated Na+ channels open and an action potential is initiated  Excitatory – depolarizing graded potentials  Inhibitory – hyperpolarizing graded potentials  Subthreshold graded potential – one that is below threshold by the time it reaches the trigger zone Action Potentials Travel Long Distances  Action potentials or spikes do not diminish in strength through the neuron o Amplitude is around 100mVBIOB30 Chapter 8 (258-266) 2 Neurons: Cellular and Network Properties o All-or-none – they occur as a maximal depolarization or don’t occur at all  Excitability – the ability of a neuron to respond rapidly to a stimulus and fire an action potential  A suprathreshold (above threshold) stimulus causes an action potential that requires only 2 types of gated ion channels o Voltage gated sodium and potassium channels o Also need a leak channel for resting membrane potential  Signaling in the CNS can be much more complex o Some neurons fire without an external stimulus  Tonically active – neurons that fire regular trains of action potentials (beating pacemakers)  Bursting – bursts of action potentials rhythmically alternating with intervals of quiet (rhythmic pacemakers) o The different firing patterns are created by ion channel variants that differ in their activation and inactivation voltages, opening and closing speeds, and sensitivity to neuromodulators Na+ and K+ Move Across the Membrane During Action Potentials  Figure 8.9 shows voltage and ion permeability changes during an action potential o There are 3 phases: the rising phase, the falling phase, and the after- hyperpolarization phase Rising Phase of the Action Potential  Due to a sudden temporary increase in permeability to Na+  Steps: o An action potential begins (graded potential reaches the trigger zone) o Membrane is depolarized to threshold (-55mV) o Voltage-gated sodium channels open o Na+ moves down the electrochemical gradient (into the cell) o Further depolarizes the membrane o Overshoot – the portion of the action potential above 0mV o When membrane potential becomes positive, electrical gradient for sodium disappears  Concentration gradient remains so influx continues  Membrane potential moves toward +60mV (E ) as long as it is Na permeable to Na+ o Before E ,Naodium channels in the axon close a permeability decreases a lot o The action potential peaks at +30mV Falling Phase of the Action Potential  Due to an increase in K+ permeabilityBIOB30 Chapter 8 (258-266) 3 Neurons: Cellular and Network Properties  Steps: o Voltage-gated K+ channels open in response to depolarization  These are very slow to open so K+ per
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