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Slide 1-

11 Fundamentals of the Nervous System and Nervous Tissue Part B
Slide 2-

Potential energy generated by separated charges is called voltage. Reflects the flow of ions rather than electrons There is a potential on either side of membranes when the number of ions is different across the membrane Electrical Current and the Body
Slide 3-

Types of plasma membrane ion channels: Passive, or leakage, channels – always open Chemically gated channels – open with binding of a specific neurotransmitter Voltage-gated channels – open and close in response to membrane potential (change in charge) Mechanically gated channels – open and close in response to physical deformation of receptors Role of Ion Channels InterActive Physiology®: Nervous System I: Ion Channels PLAY
Slide 4-

Operation of chemical Gated Channel Figure 11.6a
Slide 5-

Operation of a Voltage-Gated Channel Figure 11.6b
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When gated channels are open: Ions move along chemical gradients, diffusion from high concentration to low concentration. Ions move along electrical gradients, towards the opposite charge. Together they are called the Electrochemical Gradient An electrical current and Voltage changes are created across the membrane Gated Channels
Slide 7-

Electrochemical Gradient The EG is the foundation of all electrical phenomena in neurons. It is also what starts the Action Potential.
Slide 8-

The potential difference (–70 mV) across the membrane of a resting neuron It is generated by different concentrations of Na+, K+, Cl, and protein anions (A) The cytoplam inside a cell is negative and the outside of the cell is positive. (Polarized) Resting Membrane Potential (Vr)
Slide 9-

Used to integrate, send, and receive information Membrane potential changes are produced by: Changes in membrane permeability to ions Alterations of ion concentrations across the membrane Types of signals – graded potentials and action potentials Membrane Potentials: Signals
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Changes are caused by three events Depolarization – the inside of the membrane becomes less negative Repolarization – the membrane returns to its resting membrane potential Hyperpolarization – the inside of the membrane becomes more negative than the resting potential Changes in Membrane Potential
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Changes in Membrane Potential Figure 11.9
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Short-lived, local changes in membrane potential Decrease in intensity with distance Their magnitude varies directly with the strength of the stimulus Sufficiently strong graded potentials can initiate action potentials Graded Potentials
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Graded Potentials A stimuli from sensory input causes the gated ion channels to open for a short period of time. Positive Cations flow into the cell and move towards negative locations around the stimuli. Alternately the now negative area on the outside of the cell will flow towards the positive areas. However, this spread of depolarization is short lived because the lipid membrane is not a good conductor and is very leaky, so charges quickly balance out.
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Graded Potentials Figure 11.10
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Graded Potentials Figure 11.11
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A brief change in membrane potential from -70mV(resting) to +30mV (hyperpolarization) Action potentials are only generated by muscle cells and neurons They do not decrease in strength over distance An action potential in the axon of a neuron is a nerve impulse Action Potentials (APs)
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Na+ and K+ channels are closed Leakage accounts for small movements of Na+ and K+ Each Na+ channel has two voltage-regulated gates Activation gates – closed in the resting state Inactivation gates – open in the resting state Action Potential: Step 1 Resting State Figure 11.12.1
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The local depolarization current flips open the sodium gate and Na+ rushes in. Threshold: when enough Na+ is inside to reach a critical level of depolarization (-55 to -50 mV) threshold, depolarization becomes self-generating. Action Potential: Step 2 Depolarization Phase Figure 11.12.2
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Action Potential: Step 2 Cont. Na + will continue to rush in making the inside less and less negative and actually overshoots the 0mV (balanced) mark to about +30mV.
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After 1 ms enough Na+ has entered that positive charges resist entering the cell. Sodium inactivation gates close and membrane permeability to Na+ declines to resting levels As sodium gates close, voltage-sensitive K+ gates open K+ exits the cell and internal negativity of the resting neuron is restored Action Potential: Step 3 Repolarization Phase Figure 11.12.3
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Action Potential: Step 4 Hyperpolarization Potassium gates are slow and remain open, causing an excessive efflux of K+ This efflux causes hyperpolarization of the membrane (undershoot). The neuron is insensitive to stimulus and depolarization during this time Figure 11.12.4
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Repolarization Restores the resting electrical conditions of the neuron Does not restore the resting ionic conditions Ionic redistribution back to resting conditions is restored by the sodium-potassium pump Action Potential: Role of the Sodium-Potassium Pump
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Phases of the Action Potential 1 – resting state 2 – depolarization phase 3 – repolarization phase 4 – hyperpolarization
Slide 24-

When one area of the cell membrane has begun to return to resting the positivity has opened the Na+ gates of the next area of the neuron and the whole process starts over. A current is created that depolarizes the adjacent membrane in a forward direction The impulse propagates away from its point of origin Propagation of an Action Potential
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Propagation of an Action Potential (Time = 0ms) Figure 11.13a
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Propagation of an Action Potential (Time = 1ms) Figure 11.13b
Slide 27-

Propagation of an Action Potential (Time = 2ms) Figure 11.13c
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All action potentials are alike and are independent of stimulus intensity Strong stimuli can generate an action potential more often than weaker stimuli The CNS determines stimulus intensity by the frequency of impulse transmission Coding for Stimulus Intensity
Slide 29-

Conduction velocities vary widely among neurons Rate of impulse propagation is determined by: Axon diameter – the larger the diameter, the faster the impulse Presence of a myelin sheath – myelination dramatically increases impulse speed Conduction Velocities of Axons InterActive Physiology®: Nervous System I: Action Potential PLAY
Slide 30-

Current passes through a myelinated axon only at the nodes of Ranvier Voltage-gated Na+ channels are concentrated at these nodes and Action potentials jump from one node to the next Much faster than conduction along unmyelinated axons where the entire axon has continuous conduction. Saltatory Conduction
Slide 31-

Saltatory Conduction Figure 11.16