Module 1

1. Module 1

1.46. Page 2

Lesson 7

Module 1—The Nervous System

Explore

The Neuron at Rest

Two photographs are shown. The photograph on the left is of an exploding firework in the night sky. The photograph on the right is of a person using his or her hands to cover his or her ears. The person is grimacing in response to a loud sound.

left: © Laurin Rinder/shutterstock; right: © Bairachnyi Dmitry/shutterstock

In the previous lessons you examined how communication pathways, called sensory pathways, send information about taste, smell, touch, sight, and sound to the brain. Different lobes of your brain process this information, and motor pathways communicate appropriate responses to your effectors. In your mind, review how this works by adding information that you recall to the summary flow chart and relating it to the diagram 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, which 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 characteristics of the neuron's membrane and explore the role of sodium and potassium ions as well as negatively charged particles, such as proteins and chloride ions, that surround the neural membrane.

Read

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: the 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 averaging approximately – 70 mV

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

To understand the characteristics of a neuron at rest and a nerve impulse, read "The Nerve Impulse" on pages 372 to 374 in the textbook. Study “Figure 11.12” on page 374 of your textbook, which describes how the sodium-potassium ion pump works. Summary notes and a chart similar to the one on page 374 will prove to be invaluable in mastering these concepts. Make sure you include the concepts of membrane potential, voltage, polarized membrane, polarization, depolarization, and repolarization in your work.

Watch and Listen

To further explore the role of ions in establishing a resting or polarized state, go to the McGraw-Hill Ryerson online learning centre at www.albertabiology.ca. Navigate to Student Resources, choose “Chapter 11” from the menu on the left, and then select “Animations.” Choose the animation called “Sodium-Potassium Exchange Pump” from the list. This is a challenging and important concept. Be sure that you are able answer the following questions that are based on the animation:

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 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, or to have a polarized membrane. 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 animated illustration of the polarized or resting neuron to review these concepts.

Where is the greatest concentration of the red circles, which represent the sodium ions? If you guessed outside the membrane, you were right. Where is the greatest concentration of the blue circles, which represent the potassium ions? If you guessed inside the membrane, you were correct. Besides movement of potassium ions by the ion exchange pump, what other ways can you see in the diagram that facilitate 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.

You may wish to review the concepts of diffusion, carrier molecules, permeability, and impermeability by using the Internet or reviewing your Science 10 materials.

Watch and Listen

The following segments of “Nerve Impulse Conduction: Dentists Calm Your Nerves” may help you to understand the processes of message transmission:

“Bio Review: Neuron Structure and Function”
“Ion Movement”
“Ion Distribution”
“Bio Simulation: Establishment of Membrane Potential”

Self-Check

In your own words, answer the following questions in order to check your understanding of the concepts to this point. When you have finished, check your answers, make any corrections, and file your work in the course folder before you move on in the lesson.

SC 1. Define the term resting state as it is used to describe a neuron.

SC 2. Explain what the resting membrane potential is and why it is significant to the functioning of the neuron.

SC 3. Identify and explain the three factors that contribute to maintaining the resting membrane potential.

SC 4. Describe the distribution of sodium ions, potassium ions, and negatively charged particles in a resting neuron.

SC 5. What is meant by a “resting potential of – 70 mV”?

Check your work.

Self-Check Answers

SC 1. The neuron’s resting state is defined as the period when no nerve impulse is being generated.

SC 2. 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, 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.

SC 3. Neurons generate a resting membrane potential (polarized state) by

the selectively permeable membrane of the neuron being impermeable to the negatively charged particles, namely chloride ions and negatively charged proteins

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.

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.

SC 4. 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.

SC 5. A “resting potential of – 70 mV” means that the difference between the net positive charge on the outside and the net negative charge on the inside is – 70 mv.