Module 1 Lesson 6 - 6
Lesson 6 — The Nerve Impulse: Transporting the Message
Lesson Summary
In previous lessons, you learned about the various parts of the nervous system and nerve transmission pathways. You learned about sensory receptors that allowed you to hear the mosquito ringtone of a phone. You learned that various lobes of the brain are involved in processing this information, and motor pathways lead to responses.
This lesson focused on the neuron, the type of cell that makes up the communication pathways of the nervous system. You discovered the special parts and functions of a neuron, and you considered how these structures facilitated action potentials and could accelerate the rate of signal transmission. This leads to an appreciation of the significance of myelination and an understanding of how symptoms of diseases such as MS indicate interruptions in signal transmission.
In this lesson, you investigated the following focusing questions:
- How does the structure of a neuron facilitate reception and transmission of a nerve impulse to the synaptic gap?
- How are signals impeded by diseases such as multiple sclerosis?
To answer these questions, you explored the three major parts of nerve impulse transmission through a neuron:
- the resting or polarized state
- the action potential involving depolarization
- the reestablishment of the resting state or repolarization
You investigated the importance of specific ions in signal transmission. 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 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 produces 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 +30 mV and the permeability to Na+ is lost (sodium gates close). Then, the K+ ions 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 forces 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, then, is ready to be stimulated again. The nerve impulse is passed along the neuron in a wave of depolarization.
In this lesson, you learned that myelinated neurons conduct impulses much more quickly than unmyelinated neurons do because depolarization can occur only at the nodes of Ranvier. Action potentials at the node generate 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
and/or 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.