General Physics. Nerve Conduction. Newton s laws of Motion Work, Energy and Power. Fluids. Direct Current (DC)

Transcription

1 Newton s laws of Motion Work, Energy and Power Fluids Direct Current (DC) Nerve Conduction Wave properties of light Ionizing Radiation General Physics Prepared by: Sujood Alazzam 2017/2018

2 CHAPTER OUTLINE Nerve Conduction 18.1 Structure of nerve Cell The Resistance and Capacitance of an Axon 18.3 Ionic concentration and the resting potential 18.4 Response to weak stimuli The action potential.

3 Lecture 14: 18.3 Ionic concentration and the resting potential

4 Objectives Identify the types of ions found in neurons Explain Ionic concentration in nerve cell. Describe the resting potential.

5 REVISION Diffusion Permeability

6 Diffusion: The process by which molecules spread from areas of high concentration, to areas of low concentration. When the molecules are even throughout a space - it is called EQUILIBRIUM.

7 Permeability refers to the ability of a particular molecule to cross the plasma membrane of a cell by diffusion. If a molecule can cross the membrane, the membrane is said to be permeable to that molecule. If a molecule cannot cross the membrane, the membrane is not permeable (is impermeable) to that molecule.

8 18.3 Ionic concentration and The resting membrane potential Neurons can respond to stimuli and conduct impulses because a membrane potential voltage- is established across the cell membrane. Electric Potential -Voltage- means the same thing in a neuron as it does in an electrical circuit.

9 18.3 Ionic concentration and The resting membrane potential However, current in wires is carried by electrons. In contrast, in neurons and other cells, current is carried through the movement of ions. These may include both positively charged ions (cations) and negatively charged ions (anions).

10 Imagine taking two electrodes and placing one on the outside and the other on the inside of the plasma membrane of a living cell. If you did this, you would measure an electrical potential difference, or voltage, between the electrodes.

11 Inside neuron, is slightly negative relative to the outside. This difference is referred to as the Resting Membrane Potential, or in other words, Membranes are polarized.

12 It is called a RESTING potential because it occurs when a membrane is not being stimulated or conducting impulses (in other words, it's resting).

13 In most resting neurons, the potential difference across the membrane is about 30 to 90 mv (1 mv = 1/1000 volt), with the inside of the cell more negative than the outside. That is, neurons have a resting membrane potential -30 to -90 mv. The electric charges are distributed asymmetrically across the cell membrane -redistribution of electrons across the anode/cathode of a battery-.

14 The resting potential is determined by the uneven distribution of ions (charged particles) between the inside and the outside the cell, and by the different permeability of the membrane to different types of ions.

15 As in all cells, the cell membrane of a neuron is polarized (resting potential) (This means that there is an electrical difference across the cell membrane). If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarizations. If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized.

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17 Types of ions found in neurons: In neurons and their surrounding fluid, the most abundant ions are: 1. Positively charged (cations): Sodium Na+, and potassium K+. 2. Negatively charged (anions): Chloride Cl-, and organic anions

18 In a resting neuron (polarized), the membrane is much more permeable to K + than to Na+.

19 In most neurons K+, and organic anions (such as those found in proteins and amino acids) are present at higher concentrations inside the cell than outside. In contrast Na+ and Cl- are usually present at higher concentrations outside the cell. This means there are stable concentration gradients across the membrane for all of the most abundant ion types.

20 An unequal distribution of these ions occurs on the two sides of a nerve cell membrane because carriers actively transport these two ions: sodium from the inside to the outside and potassium from the outside to the inside.

21 The Na+ - K+ pump maintains Na+ and K+ gradients As a result of this active transport mechanism (commonly referred to as the Sodium - Potassium Pump), there is a higher concentration of sodium on the outside than the inside and a higher concentration of potassium on the inside than the outside

22 The Na+ - K+ pump maintains Na+ and K+ gradients In sodium-potassium pump, 3Na+ ions are moved from the inside to the outside of the cell, and 2K+ ions are moved from the outside to the inside.

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24 Because 3Na+ are exported for every 2K+ brought into the cell, the pump makes a small direct contribution to the resting membrane potential (polarized) (making it slightly more negative than it would otherwise be).

25 Ions can't pass directly through the lipid regions of the membrane. Instead, they have to use specialized channel proteins that provide tunnel across the membrane. Some channels, are open in resting neurons. Others are closed in resting neurons and only open in response to a signal.

26 Ion channels that mainly allow K+, to pass are called potassium channels, and ion channels that mainly allow Na+ to pass are called sodium channels. In neurons, the resting membrane potential (polarized ) depends mainly on movement of K+ through potassium channels.

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28 In a resting nerve cell membrane (polarized), all the sodium gates are closed and some of the potassium gates are open. AS A RESULT, sodium cannot diffuse through the membrane & largely remains outside the membrane. HOWEVER, some potassium ions are able to diffuse out.

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30 This means that there are more positive charges on the outside than on the inside. In other words, there is an unequal distribution of ions or a resting membrane potential (polarized ). This potential will be maintained until the membrane is disturbed or stimulated. Then, if it's a sufficiently strong stimulus, an action potential will occur.

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