Module 1 The Nervous System

Lesson 1.1.7

1.1.7 page 2

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The Neuron at Rest

In the previous lessons you examined how communication pathways throughout the Nervous System called sensory pathways sent information about taste, smell, touch, sight, and sound to the brain. Different lobes of your brain processed this information, and motor pathways communicated appropriate responses to your effectors. You can review how this works in the flow chart below:

Sensory receptors → sensory pathway → lobes of brain → motor pathways → effectors

Now you will examine some characteristics of the special type of cell, the neuron, that makes up the nerves in these pathways. As part of this exploration you will need to understand how messages are communicated through the neuron from dendrite to terminal end. To do this, you must investigate the neuron membrane and explore the role of sodium and potassium ions as well as negatively charged particles such as proteins and chloride ions which surround the neural membrane.

To understand the characteristics of a neuron at rest and a nerve impulse, read pages 372 – 374 of the textbook. Be sure to make summary notes for your course folder. Study Figure 11.12 on page 374 of your textbook, which describes how the sodium potassium ion pump works. Summary notes and a similar chart in your course folder will prove to be invaluable in mastering these concepts.

membrane potential: a form of potential energy resulting from the separation of charges between the inside and the outside of a cell membrane; voltage across the cell membrane

voltage: electrical potential difference across a membrane as measured by a voltmeter

polarized membrane: state of the cell membrane in an unstimulated neuron, in which the inside of the neuron is negatively charged in comparison to the outside of the neuron; the resting state of a membrane averages approximately – 70 millivolts

polarization: the process of generating a resting membrane potential averaging approximately – 70 millivolts

depolarization: the loss or reduction of the negative resting membrane potential

repolarization: restoring the resting membrane potential (- 70 millivolts) from the depolarized state

Action potential Crash Course

Sodium Potassium Exchange Pump

Identify two characteristics of active transport.
In which direction are sodium ions moved across the neuron membrane by the sodium potassium exchange pump?
In which direction are the potassium ions moved?
Where is the carrier protein of the ion exchange pump located?
What causes the shape changes in the carrier protein?
Where is ATP used?

resting membrane potential: the voltage that exists across a cell membrane during the resting state of an excitable cell such as the neuron; averages around -70 mV, but may range from – 50 to – 200 millivolts (mV) depending on the cell

You should now know that sodium ions become more concentrated outside the neuron and potassium ions become more concentrated inside the neuron. The net result is that the interior of the neuron (intracellular fluid) becomes negatively charged compared to the exterior (extracellular fluid) which becomes positively charged. When this occurs, the neuron is said to be polarized. This can be verified by inserting a tiny electrode into the axon of the neuron and touching another to its surface. Usually there is a difference in charge or a resting membrane potential of approximately 70 mV. This may vary from cell to cell and in different situations. Use the illustration of the polarized or resting neuron below to review these concepts.

Where is the greatest concentration of the red circles, representing the sodium ions? If you guessed outside the membrane, you were right. Where is the greatest concentration of the blue circles, representing the potassium ions? If you guessed inside the membrane you were correct. Besides movement of potassium ions by the ion exchange pump, what other way can you see in the diagram that facilitates movement of potassium ions towards the outside of the cell? If you guessed diffusion and carrier molecules you were correct. What do the black circles represent? If you guessed chloride ions you were correct. Why are there no black circles on the exterior of the neuron? If you guessed that the membrane is impermeable to chloride ions you were correct. What is the net charge on the exterior of the axon membrane? If you guessed positive you were correct. What is the net charge on the interior of the axon? If you guessed negative you were correct.

Watch and Listen

The following video segments may help you to understand the processes of message transmission:

Neuron Structure and Function

Sodium Potassium Pump

Action Potentials

Self-Check

In your own words, answer the following questions in order to determine whether or not you understand the concepts to this point. When you have finished, autocheck your answers, make any corrections, and file your work in your Biology 30 Course Folder for later review.

What is the term used to describe the resting state of a neuron?
Explain what the resting membrane potential is and why it is significant to the functioning of the neuron.
Identify and explain the three factors that contribute to maintaining the resting membrane potential.
Describe the distribution of sodium ions, potassium ions, and negatively charged particles in a resting neuron.
What is meant by a “resting potential of -70mV”?

Check your work.

Self-Check Answers

The neuron’s resting state is defined as the period when no nerve impulse is being generated.
The charge difference across the neuron membrane in a non-stimulated, polarized or resting neuron is called the resting membrane potential. The resting membrane potential is approximately -70 mV (millivolts), with the outside of the membrane having a positive net charge relative to the inside which would have a negative net charge. The resting membrane potential is significant because it provides energy for the generation of a nerve impulse in response to an appropriate stimulus.
Neurons generate a resting membrane potential (polarized state) by
1. the selectively permeable membrane of the neuron being impermeable to the negatively charged particles, namely chloride ions and negatively charged proteins.
2. the sodium potassium ion exchange pump, which uses energy to pump three sodium ions out of the neuron and two potassium ions into the neuron, resulting in an uneven distribution of positive charge inside and outside the membrane. This buildup of positive charge on the outside creates an electric potential.
3. special transit proteins that allow potassium ions to diffuse out of the neuron. Fewer sodium ions are allowed to diffuse into the neuron, resulting in more positive charges outside the neuron than inside the neuron.
In a resting neuron, there are more sodium ions on the outside of the neuron membrane than inside, and there are more potassium ions on the inside than there are on the outside of the neuron membrane. There are more negatively charged protein particles and chloride ions on the inside than there are on the outside of the neuron membrane. Therefore, the resting neuron has a net positive charge on the outside of the membrane, and a net negative charge on the inside of the neuron.
A “resting potential of -70mV” means that the difference between the net positive charge on the outside and the net negative charge on the inside is -70mv.