1. Chapter 12b Neurophysiology

2.

Processing of sensory information and communication

• Messages are conveyed as action potentials

• Communication depends on membrane potentials, graded potentials and action potentials

3.

Resting potential :

• transmembrane potential(TMP) of resting cell

• Results from uneven distribution of ions across membrane

• Usually -70mV for average neuron

5 Neural Membrane Processes 4.

Graded potential :

• temporary, localized change in TMP

• caused by stimulus

• Generated in soma or dendrite

Action potential :

• electrical impulse

• produced by graded potential

• moves along surface of axon to synapse

5.

Synaptic activity :

• releases neurotransmitters at presynaptic membrane

• produces graded potentials in postsynaptic membrane

Information processing :

• response (integration of stimuli) of postsynaptic cell

6. Figure 127 (Navigator) 7. How is resting potentialcreated and maintained? 8.

Concentration gradient of ions (Na + , K + )

• ECF has high concentration of Na+ & Cl-

• Cytosol has high concentration of K+

Selectively permeable through channels

Maintains charge difference across membrane ( - 70 mV)

9. Figure 128 (Navigator) Product of both passive and active forces 10. PassiveForcesAcross the Membrane

Chemical gradients :

• concentration gradients of ions (Na + , K + )

Electrical gradients :

• potential difference across membrane

• Slightly negative on inner surface

• Slightly positive charge on outer surface

Electrochemical gradient:

• Sum of chemical and electrical forces

11. Electrical Currentsand Resistance

Electrical current :

• movement of charges to eliminate potential difference

Resistance :

• the amount of current a membrane resists

• May be altered by opening/closing channels creating a current

12. Electrochemical Gradients Figure 129a, b 13. Electrochemical Gradients Figure 129c, d 14. ActiveForcesAcross the Membrane

Sodiumpotassium ATPase(exchange pump):

• are powered by ATP

• carries 3 Na +out and 2 K +in

• balances passive forces of diffusion

• maintainsresting potential(70 mV)

15. Changes inTransmembrane Potential

Transmembrane potential rises or falls:

• in response to temporary changes in membrane permeability

• resulting from opening or closing specific membrane channels

16. Sodium and Potassium Channels

Membrane permeability to Na +and K +determines transmembrane potential

Sodium and potassium channels are eitherpassiveoractive

17. Passive Channels

Also calledleak channels

Are always open

Permeability changes with conditions

18. Active Channels

Also calledgated channels

Open and close in response to stimuli

At resting potential, most gated channels are closed

19. Gated Channels Figure 1210 20. 3Classesof Gated Channels

Chemically regulated channels :

• open in presence of specific chemicals at a binding site

• found on neuron cell body and dendrites

21.

Voltage-regulated channels :

• respond to changes in transmembrane potential

• characteristic ofexcitable membrane

• found in neural axons, skeletal muscle sarcolemma, cardiac muscle

22.

Mechanically regulated channels :

• respond to membrane distortion

• found in sensory receptors (touch, pressure, vibration)

23. Graded Potentials

Any stimulus that opens a gated channel:

• produces a graded potential

• Also calledlocal potentials

Changes in transmembrane potential:

• cant spread far from site of stimulation

24.

Opening sodium channel produces graded potential

Figure 1211 (Navigator) 25. Figure 1211 (Step 1) Graded Potentials: Step 1 26. Figure 1211 (Step 2) Graded Potentials: Step 2 27.

Repolarization

• stimulus is removed, transmembrane potential returns to normal

Hyperpolarization

• Increasing the negativity of the resting potential

• Result of opening a potassium channel

28. Figure 1212 29. Effects of Graded Potentials

At cell dendrites or cell bodies:

• trigger specific cell functions

At motor end plate:

• releases ACh into synaptic cleft

30. What events are involvedin the generationand propagation ofan action potential? 31. Action Potentials

Propagated changes in transmembrane potential

Affect an entire excitable membrane

Link graded potentials at cell body with motor end plate actions

32. Initiating Action Potential

Initial stimulus:

• a graded depolarization to change resting potential tothresholdlevel (60 to 55 mV)

All or none principle

• stimulus exceeds threshold amount and action potential is triggered or it wont

33. Generating the Action Potential Figure 1213 (Navigator) 34. Steps of A P Generation

Depolarization to threshold

Activation of Na +channels and rapid depolarization

Inactivation of Na +channels, activation of K +channels

Return to normal permeability

35. The Refractory Period

time period:

• from beginning of action potential

• to return to resting state

• during which membrane will not respond normally to additional stimuli

• Absolutevs.Relative

36. Propagation of Action Potentials

moves along entire length of axon

• series of repeated actions, not passive flow

Continuous propagation :

• unmyelinated axons

Saltatory propagation :

• myelinated axons

37. Saltatory Propagation

Faster and uses less energy than continuous propagation

Myelin insulates axon, prevents continuous propagation

Local current jumps from node to node

Depolarization occurs only at nodes

38. Comparison of graded andaction potentials 39. Graded Potential

Depolarizes or hyperpolarizes

No threshold value

Dependent of intensity of stimuli

Effect decreases with distance

No refractory period

Occurs in most cell types

40. Action Potential

Depolarizes only

Distinct threshold value

All or none phenomenon

No decrease in strength along axon

Refractory period occurs

Occurs only in excitable cells

41. What factors affectthe propagation speedof action potentials? 42. Axon Diameterand Propagation Speed

Ion movement is related to cytoplasm concentration

Axon diameter affects action potential speed

The larger diameter, the lower the resistance

43. 3 Groups of Axons

Classified by:

• diameter

• myelination

• speed of action potentials

Type A ,Type B , andType C fibers

44.

Information travels within the nervous system as propagated electrical signals (action potentials)

The most important information (vision, balance, motor co