15- الدرس الخامس - الظواهر الكهربائية || علوم بكالوريا بكسل

Summary

The lesson explains the resting potential of a nerve fiber as part of a broader unit on electrical phenomena in nerve cells. The teacher emphasizes understanding how ions move across the cell membrane rather than memorizing definitions. The next lesson will cover action potential.

Cell Membranes and Membrane Potential

• A cell membrane separates the inside of a cell from its surroundings and is made mainly of lipids.
• Ions cannot move freely through the lipid layer, so they pass through protein channels in the membrane.
• Because ions are distributed differently on either side of a membrane, an electrical potential difference exists across it. This is called the membrane potential.
• Membrane potential is found in all living cells, not just nerve cells:
  • In non-excitable cells, such as glial cells, it is relatively constant.
  • In excitable cells, ion movement can change it. Examples include neurons, sensory cells, muscle cells, gland cells, and the secondary oocyte.
• In this lesson, stimulation refers to ion movement across the membrane that changes the membrane potential.

Resting Potential

A nerve cell is at rest when it is not being stimulated. The resting potential is the potential difference between the outer and inner surfaces of the nerve-fiber membrane while the cell is at rest.

The lesson gives its value as approximately −70 mV. The negative sign indicates that the inside of the fiber is negative relative to the outside. The teacher emphasizes precise wording: the difference is between the surfaces of the fiber membrane, not simply “outside and inside the fiber.”

Ion Distribution at Rest

The lesson discusses sodium, chloride, potassium, and large negatively charged organic molecules, which are rendered in the subtitles as “precipitates” and are best understood as intracellular organic anions.

• Sodium and chloride ions are more concentrated outside the nerve fiber than inside.
• Potassium ions are more concentrated inside than outside.
• The large negatively charged organic molecules are inside the fiber and cannot cross the membrane.

These are differences in concentration; the ions are not necessarily found exclusively on one side.

Leak Channels and Ion Movement

Protein leak channels are located in the fiber membrane and are described as remaining open. Ions move through them down their concentration gradients, from higher to lower concentration:

• Sodium tends to move into the fiber because its concentration is higher outside.
• Potassium tends to move out of the fiber because its concentration is higher inside.

The membrane is more permeable to potassium than to sodium. The lesson attributes this to there being more potassium leak channels.

The Sodium–Potassium Pump

The sodium–potassium pump is located in the membrane and maintains the ion gradients. In each cycle, it moves three sodium ions out and two potassium ions in.

This movement goes against the ions’ concentration gradients, so it requires energy from ATP and is an example of active transport. By moving more positive charge outward than inward, the pump contributes to the outside being relatively positive.

Why the Membrane Is Polarized at Rest

The lesson identifies three factors that make the outer membrane surface relatively positive and the inner surface relatively negative:

1. Greater permeability to potassium than to sodium: More potassium leaves through leak channels than sodium enters, contributing to a positive charge outside.
2. Trapped negatively charged organic molecules: These remain inside the fiber and contribute to the negative charge inside.
3. The sodium–potassium pump: It exports three positive sodium ions for every two positive potassium ions it brings in, adding to the relative positivity outside.

Together, these factors create the membrane’s two-sided electrical polarity and the resting potential.

Key Distinction

A polarized membrane has a relatively positive outer surface and a negative inner surface. This differs from the neuron’s functional polarity, introduced in an earlier lesson, which concerns the direction in which a nerve impulse is transmitted.

Speakers and Sources Featured

• Main speaker: The teacher or instructor, who explains the material and refers to exam-style questions.
• Textbook and lesson diagrams: Used to present definitions, ion distributions, channels, the pump, and review questions.
• Illustrative hypothetical characters: A girl asking about an ECG and a doctor interpreting it. They are presented as an introductory example, not as independently identified speakers in the lesson.