The vocabulary of nerve cells and the senses. the transduction and coding of stimuli (resting and action potentials)

Transcription

1 The vocabulary of nerve cells and the senses the transduction and coding of stimuli (resting and action potentials)

2 The Bottom Line For any external or internal signal to be recognized by the brain it must first be transduced into an electrical (neural) signal. Hence, whatever properties of neurons that influence this transduction also influence the ability to perceive the signal and its properties. Once transduced the signal must be sent to the central nervous system and processed. Hence, any properties of neurons influencing the transmission and processing of signals also influence the ability to perceive the signal and its properties.

3 Site of the action: the nerve membrane The nerve cell membrane is much the same as the cell membrane of any other cell (phospholipids and proteins). It differs essentially only in the specialized proteins present in it, which determine the function of the cell. One such function is the formation and control of an electrical potential (voltage) across the membrane.

4 Membrane Potential: I 8, 8 5, 5 At rest, the membrane is impermeable to charged particles (ions). Both sides are electrically neutral (equal numbers of anions [ ] and cations [here, ] on both sides). However there is more on the left side.

5 Membrane Potential: II Next we reveal the presence of protein pores (channels) in the membrane. These pores are selective and will only allow through them ( is blocked). At the outset the pores are closed.

6 Membrane Potential: III In response to a stimulus, some pores open.

7 Membrane Potential: IV 6, 8 8, Potassium flows down its concentration gradient. Because the anions ( ) cannot follow, a potential forms across the membrane. This potential opposes and eventually stops further flow of. The more channels that open, the greater the amplitude of the potential needed to stop the flow (i.e., come to equilibrium). Thus, an external stimulus has been turned into a membrane potential, and the potential is proportional to the strength of the stimulus.

8 the bottom line Since all external signals must be transduced into voltage in order for the brain to perceive them, and Since all changes in electrical signals in the nervous system are the result of changes in membrane proteins, then For any signal (stimulus) to be perceived by a cell there must be one or more membrane proteins that can be influenced by that signal.

9 Control of permeability: ion channels Neuron membranes have many thousands of channels. Channels differ in the specificity of what they will transport and what they will not: Na +,, Cl -, Ca ++,etc. Channels also differ in what causes them to open and/or close: Chemicals outside cell Chemicals inside cell Mechanical deformation of the membrane Etc. This allows neurons to respond selectively to stimuli and to encode the stimulus strength as the amplitude of an electrical potential.

10 For a signal to get from a peripheral receptor to the brain, we need a reliable method to transport the signal over the distance. This is the job of nerve axons. Axons transmit their information using all-ornone pulses in the membrane called action potentials.

11 The Action Potential: long-distance carrier of information The action potential (AP) uses the same mechanisms for producing a trans-membrane voltage as we just saw. However, the AP uses sodium (Na + ) as well as ), and the pores are responsive to the voltage across the membrane. In the resting neuron there are more Na ions on the outside and more K ones on the inside. Thus opening Na gates makes the membrane more positive on the inside, while opening more K gates makes it more negative.

12 The Action Potential The action potential depends on a stimulus opening up voltage-gated Na + channels which depolarizes the membrane which opens up voltage-gated Na + channels which Eventually the channels spontaneously close and become inactive for a time before returning to their active state. Also, slower channels open, repolarizing the membrane. An action potential at any point on the axon membrane causes the adjacent membrane to undergo an action potential. Thus the signal is transmitted along the axon.

13 All Na channels closed and inactive. More and more Na channels return to their active state. Refractory period and stimulus coding The refractory period is determined by The time that Na + channels are in their inactive state The time that channels are open, opposing renewed depolarization. Time to the next action potential during this period is determined by stimulus strength. Thus action potential frequency is proportional to stimulus strength.

14 the bottom line Since action potentials are all-or-none (i.e., positive feedback) phenomena, their amplitude is constant and thus conveys no information. All the information then is coded as the frequency of action potentials, not their amplitude. The range of intensity of the stimulus must thus be coded into the possible range of frequencies of a neuron. The minimum detectable change in frequency depends on the constancy of firing of the signaling neuron (most neurons fire constantly). The absolute refractory period governs the maximum frequency. The range in frequency between these two factors governs the range of intensity of stimulus which can be reported. (Since an action potential lasts about one millisecond you might expect the maximum frequency of action potentials to be about 1000 per second. Given the absolute refractory period and other considerations, the practical limit is more like per second.)

15 But there s no free lunch! The both the action and receptor potentials involve the movement of ions across the cell membrane. If this were all there were to it, after a while the ion gradients would run down and no further potentials could be developed. Instead, cells use an energy-driven pump (Na-K-ATPase) to pump Na out of the cell in exchange for bringing K in. As the name implies, the pump burns ATP to do this. This constitutes a large portion of your resting energy needs.

16 And that s enough neurobiology for today!!