Transmission of Nerve Impulses (see Fig , p. 403)

Transcription

1 How a nerve impulse works Transmission of Nerve Impulses (see Fig , p. 403) 1. At Rest (Polarization) outside of neuron is positively charged compared to inside (sodium ions outside, chloride and potassium ions inside). At rest, the cell membrane is not permeable to sodium ions but very permeable to potassium ions so they diffuse out of the cells. However, a mechanism called the sodium/potassium pump (active transporter) pulls 3 sodium ions to the outside of the cell while pulling 2 potassium ions inside the cell resting potential: the difference in charge from the inside to the outside of a cell at rest. It is approximately 70 mv. 1

2 Transmission of Nerve Impulses the two sides of a nerve cell membrane have unequal charges meaning the neuron is polarized Polarity is caused by unequal distribution of sodium (Na + ) & potassium (K + ) ions Resting/membrane Potential Na+ ions are actively transported out of the cell and K+ ions into the cell (often called the sodiumpotassium pump). special passageways for these two ions that are commonly referred to as GATES or CHANNELS IN A RESTING NERVE CELL MEMBRANE, all the sodium gates are closed and some of the potassium gates are open 2

3 sodium cannot diffuse through the membrane & largely remains outside the membrane. As a result, the number of positive ions outside the cell increases 3

4 4

5 Transmission of nerve impulses 2. Depolarization when a neuron is stimulated enough, gates for potassium ions channels (transporters) close and gates for sodium ion channels open. Na + ions move in while K + ions move out. More positive charges move in than out, and this neutralizes the negative charge inside the cell (actually makes the inside slightly positive). The resulting difference in charge during depolarization is called the action potential. Action potentials occur at cell bodies and dendrites as well. depolarization at one point of the axon causes the neighboring Na + channels to open, and the depolarization continues down the length of the axon 5

6 Action Potential rapid change in membrane potential that occurs when a nerve cell membrane is stimulated. the membrane potential goes from the resting potential (typically -70 mv) to some positive value (typically about +30 mv) in a few milliseconds A stimulus changes the permeability of the axon membrane making it permeable to Na ions so they diffuse quickly into the axon. This causes a change in the distribution of ions and a reversal of polarity. This reversal of polarity and flow of ions caused by a stimulus results in a current and is called the action potential. 6

7 7

8 Transmission of nerve impulses 3. Repolarization after the Na + channels open, K + channels reopen to cause K + to move out. At the same time, Na + channels close. the sodium/potassium pump restores the original concentrations of Na + and K + by pumping Na + out and K + in. This entire process of depolarization and repolarization occurs very quickly. An axon can send many impulses along its length every second if it is sufficiently stimulated. 8

9 A series of action potentials sweeping down an axon is a nerve impulse. A refractory period is needed to restore the resting potential where sodium ions move out and potassium ions move into the axon Refractory period: the brief time between the triggering of an impulse along an axon and the axon readiness for the next impulse. During that brief time, the axon cannot transmit an impulse. For many neurons, the refractory period is about s. 9

10 A stimulus must have a certain intensity to change polarity in a neuron and start an impulse. This level of intensity is called the threshold. The impulse is either transmitted or not transmitted. 10

11 Speed of Nervous Transmission As mentioned earlier, myelinated neurons transmit impulses much quicker than unmyelinated ones. This is due to the fact that depolarization only occurs at the nodes of Ranvier. In a sense, the impulse jumps from node to node until it reaches the end of the neuron. Speeds of impulses on myelinated neurons can reach 120 m/s. 11

12 12