Lesson 6 — The Nerve Impulse: Transporting the Message


The Neuron at Rest

Read pages 372 - 374


In 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. Various lobes of your brain process this information, and motor pathways communicate appropriate responses to your effectors (muscles or glands). Study the flow chart below to review the steps involved when a sensory pathway results in the stimulation of motor pathways after perception in the brain:


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 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 that surround the neural membrane.


ADLC.

How does a neuron transmit information? It communicates through nerve impulses. The nerve impulses are electrochemical signals called action potentials that travel along the axon. The action potential is the changes in the membrane of a neuron that translate into a signal.

An action potential or nerve impulse has three main stages: resting potential (polarized), depolarization, and repolarization.


Resting Potential: Na+ on the outside

© Jul 24, 2014 Robert Bear, David Rintoul.  Nervous System. OpenStax CNX. Creative Commons Attribution 4.0. Download for free.


A neuron at rest has all ion channels closed. No nerve impulses travel along the axon, and the axon is ready for an electrochemical signal, the action potential. The two charged particles of interest are sodium ions (Na+) and potassium ions (K+).

A neuron has a charge across its membrane due to ion difference. A polarized membrane stores energy that holds opposite charges apart. In a resting state, the sodium ions (purple) become more concentrated outside the neuron, and the potassium ions (yellow) as well as negatively charged ions (which are chlorine and negatively charged proteins) become more concentrated inside the neuron. The result is that the interior of the neuron (cytoplasm) 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, a difference in charge or a resting membrane potential of approximately -70 mV occurs. This may vary from cell to cell and in various situations. The minus sign indicates that the inside is negative compared to the outside. The potential of a membrane is the voltage difference. For example, the mV in the graph measures the membrane potential.


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The neuron membrane has many sodium-potassium exchange pumps that use ATP to transport actively Na+ out of a neuron and K+ into a neuron.

Active transport requires ATP and uses a protein channel to transport the ions. The sodium ions are moved by the sodium potassium exchange pump across the neuron membrane, which means from cytoplasm (inside the cell) to extracellular fluid (outside of the cell). The potassium ions moved from extracellular fluid to cytoplasm. The carrier protein of the ion exchange pump is located in the membrane of the cell. The binding of the ions and the binding of ATP cause shape changes in the carrier protein.

When the neuron is resting, a movement of ions along the concentration gradient still occurs. The sodium-potassium exchange pumps constantly work to maintain the balance of sodium and potassium ions even when no action potential occurs.


Watch and Listen

  1. To explore further the role of ions in establishing a resting or polarized state, watch this animation titled Sodium Potassium Exchange Pump.



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  2. The following segment of Biologix-02 may help you to understand the processes of message transmission:

     

 ©Alberta Education. Nerve Impulse Conduction: Dentists Calm Your Nerves (1:50 - 2:57); Series 02.  Learn Alberta.

 



Self-Check

  1. What is the definition of the resting state of a neuron? 

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

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

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

  5. Draw a diagram of a polarized neuron membrane. Show the sodium, potassium, and chloride ions as well as negatively-charged proteins.

Self-Check Answers

  1. The neuron's resting state is the period when no nerve impulse is being generated. 

  2. The charge difference across the resting neuron membrane 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 that has 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. 

  3. 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 produces an electric potential. 

    3. special transit proteins that allow potassium ions to diffuse from the neuron (Fewer sodium ions are allowed to diffuse into the neuron, resulting in more positive charges outside the neuron than inside the neuron.)

  4. In a resting neuron, more sodium ions are on the outside of the neuron membrane than inside it, and more potassium ions are on the inside than on the outside of the neuron membrane. More negatively-charged protein particles and chloride ions are on the inside than 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. 

  5. Your diagram may resemble the illustration below.
Biology 30 © 2008  Alberta Education & its Collaborative Partners ~ Updated by ADLC 2019