Nerve Impulses

1. Nerve Impulses

2. Membrane Potentials • All living cells maintain a difference in the concentration of ions across their membranes. • There is a slight excess of positives on the outside and a slight excess of negatives on the inside. • This results in a difference in electrical charge across the plasma membrane called membrane potential.

3. The Voltmeter

4. Resting Membrane Potentials • When a neuron is not conducting electrical signals, it is said to be “resting.” • At rest, a neuron’s membrane potential is typically maintained at about -70 mV.

5. Sodium-Potassium Pump • Active transport mechanisms in the plasma membrane that transports sodium ions (Na+) and potassium ions (K+) in opposite directions at different rates. • Three Na+ out for every two K+ in • Maintains an imbalance in the distribution of positive ions, thus maintaining a difference in electrical charge—the inside becomes slightly less positive (slightly negative).

7. The Sodium-Potassium Pump at Work

8. Some K+ channels are open when at rest K+ diffuses down its concentration gradient Adds to the positive on the outside of the cell Na+ channels are closed Role of Channels in Membrane

9. Local Potentials • In neurons, membrane potentials can fluctuate above or below the resting membrane potential in response to certain stimuli. • A slight shift away from the RMP in a specific region of the plasma membrane is often called a local potential.

10. Excitation of a neuron occurs when a stimulus triggers the opening of stimulus-gated channels allows Na+ to enter the cell • Depolarization – movement of the membrane potential towards zero • Inhibition occurs when a stimulus triggers the opening of stimulus-gated K+ channels. As K+ diffuses out of the cell the positive ions outside the cell increases • Hyperpolarization – movement of the membrane potential away form zero (thus below the usual RMP)

12. Action Potential • An action potential is a nerve impulse in which an electrical fluctuation travels along the surface of a neuron’s plasma membrane. • Voltage Gated Channels (-59 mV= threshold potential) • All or nothing response

13. Steps of the Mechanism that Produces an Action Potential

14. A stimulus triggers stimulus gated Na+ channels to open and allow inward Na+ diffusion. This causes the membrane to depolarize. Step 1

15. As the threshold potential is reached, voltage gated Na+ channels open. Step 2

16. As more Na+ enters the cell through voltage gated Na+ channels, the membrane depolarizes even further. Step 3

17. The magnitude of the action potential peaks (at + 30 mV) when voltage gated Na+ channels close. Step 4

18. Repolarization begins when voltage gated K+ channels open, allowing outward diffusion of K+. Step 5

19. After a brief period of hyperpolarization, the resting potential is restored by the sodium-potassium pump and the return of ion channels to their resting state. Step 6

20. Action Potential

21. Absolute – can not respond to any stimulus no matter how strong Relative -- the membrane is repolarizing and can only respond to very strong stimuli Refractory Period

23. Conduction of the Action Potential • The action potential causes voltage gated channels to open in adjacent areas of the axon membrane causing the action potential to move down the length of the axon. • In myelinated fibers, electrical changes in the membrane can only occur at gaps in the sheath (nodes of Ranvier). This is called saltatory conduction.

25. Action Potential Travels Along the Axon

27. Saltatory Conduction

28. Conduction Speed • The speed of conduction of a nerve fiber is proportional to its diameter. The larger the diameter, the faster it conducts impulses. • Myelinated fibers conduct impulses more rapidly than unmyelinated fibers.

29. Synaptic Transmission

30. Synapses • A synapse is where signals are transmitted between neurons. • Presynaptic and postsynaptic neurons. • Electrical synapse – occur when two cells are joined end to end by gap junctions. The action potential just continues on between cells (cardiac cells).

31. Chemical Synapse • Three structures make up a chemical synapse: • A synaptic knob – contains vesicles with neurotransmitters • A synaptic cleft – space in between (20-30 nm) • The plasma membrane of a postsynaptic neruon.

32. Electrical and Chemical Synapses

33. Action potential can not cross synaptic cleft • Neurotransmitters are released from the synaptic knob where they travel across the cleft to the receptors on the plasma membrane of the postsynaptic neuron. • Neurotransmitters cause either depolarization (excitatory) or hyperpolarization (inhibitory) of the postsynaptic membrane.

34. The Chemical Synapse • Calcium influx • Neruotransmitter vesicles move to membrane and open • Neurotransmitter diffuses across synaptic cleft and bind to receptor molecules.