Module 1

Module 1—The Nervous System

Lesson Summary

 

In this challenging lesson you investigated the following focusing question:

• How does the structure of a neuron facilitate reception and transmission of a nerve impulse to the synaptic gap?

To answer this question, you explored the three major parts of nerve impulse transmission through a neuron:

• the resting or polarized state
• the action potential involving depolarization
• the re-establishment of the resting state, or repolarization

You investigated the importance of specific ions in communication. The distribution of the ions in the resting state produces a polarized membrane. In the resting state, the membrane of the neuron is impermeable to Na+ ions. The sodium-potassium ion exchange pump uses ATP to pump Na+ ions out of the neuron and K+ ions into the neuron. K+ ions diffuse through the selectively permeable membrane, but the negative ions attract them to keep most of the K+ ions inside the neuron. Thus, the positive charges dominate outside the neuron and the negative charges dominate the inside of the neuron. This creates the polarized state, with a voltage difference of about – 70 mV. Stimulation of a neuron causes depolarization, a shift in ion concentrations and electrical charges. You examined the mechanism of this shift.

 

When the neuron membrane becomes permeable to Na+ and the Na+ ions flood the inside of the neuron, the voltage difference becomes approximately + 40 mV and the permeability to Na+ is lost (sodium gates close). The K+ ions then leave the neuron and the resting potential of – 70 mV is restored. The sodium-potassium ion exchange pump works to bring the K+ back inside and kicks the Na+ out. This process takes only a millisecond or two and is called the refractory period. The neuron repolarizes in the refractory period and is then ready to be stimulated again. The nerve impulse is passed along the neuron in a wave of depolarization.

 

In this lesson you also learned that myelinated neurons conduct impulses much more quickly than nonmyelinated neurons because depolarization can only occur at the nodes of Ranvier. Action potentials at the node create enough current flow to activate the sodium gates in the next node, so the impulse “jumps” from node to node. Electrical current flow between nodes is instantaneous. Multiple sclerosis is an autoimmune disorder that causes the destruction of the myelin sheath, which ultimately slows down and stops the transmission of nerve impulses. This can result in blurred vision, loss of balance, and an inability of muscles to respond to commands from the brain, among other possible symptoms.

 

Lesson Glossary

 

Consult the glossary in the textbook for other definitions that you may need to complete your work.

 

action potential: the change in charge that occurs when the gates of the potassium ion channels close and the gates of the sodium ion channels open; a large depolarization event that is conducted along the membrane of a nerve cell or a muscle cell

 

all-or-none response: action that occurs either completely or not at all, such as the generation of an action potential by a neuron

 

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

 

hyperpolarization: the process of generating a membrane potential that is more negative than the normal resting membrane potential

 

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
  
myelinated neuron: a neuron whose axon is wrapped by Schwann cells, which produces a myelin sheath

 

Myelinated neurons make up the white matter of the brain and the spinal cord and transmit nerve impulses very quickly.

 

overshoot: the situation that results when more potassium ions leak out of the neuron than should because the potassium gates are slow to close; results in hyperpolarization

 

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

 

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

 

refractory period: the short time immediately after an action potential in which the neuron cannot respond to another stimulus; period of time it takes to re-establish the net positive charge on the outside of the neuron and the net negative charge on the inside of the neuron, where there are more sodium ions on the outside and more potassium ions on the inside of the neuron

 

repolarization: restoring the resting membrane potential (– 70 mV) from the depolarized state

 

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

 

saltatory conduction: rapid transmission of a nerve impulse along an axon resulting from the action potential jumping from one node of Ranvier to another, skipping the myelinated regions of the membrane

 

threshold potential: the smallest change in the membrane potential of a cell membrane that is needed to initiate an action potential; approximately – 55 mV

 

threshold stimulus: weakest possible stimulus that is needed to initiate a nerve impulse

 

unmyelinated neuron: a neuron that does not have Schwann cells and, therefore, lacks a myelin sheath

 

Unmyelinated neurons make up the grey matter of the brain and spinal cord and transmit nerve impulses much more slowly than myelinated neurons.

 

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

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