Biology, Eighth Edition (Raven)

Chapter 44: The Nervous System

Action Potential Propagation in Unmyelinated Neurons

What is the function of unmyelinated neurons? Action potentials in neurons that lack myelin sheaths travel much more slowly than action potentials in equivalent neurons sheathed in myelin. The speed of action potentials is also dependent on the diameter of the axon. Wider axons have lower resistance than narrow axons and signals can travel faster in large axons. Some invertebrates, such as squids, have giant unmyelinated axons that can transmit signals rapidly. In most cases, unmyelinated neurons are short so that the speed of the signal is not that important. The gray matter on the exterior of the brain is composed on unmyelinated tissue.

View the animation below, then complete the quiz to test your knowledge of the concept.


1.	An action potential...

		A)	causes the neuron cell membrane to become unable to alter its charge.
		B)	causes the outside of the neuron cell membrane to become positively charged in reference to the inside.
		C)	causes the inside of the neuron cell membrane to become positively charged in reference to the outside.
		D)	causes the inside of the neuron cell membrane to become negatively charged in reference to the outside.
		E)	causes the inside of the neuron cell membrane to become neutrally charged in reference to the outside.

2.	Depolarization of the neuron inner membrane is caused by...

		A)	the arrival of an inhibitory post synaptic potential (IPSP).
		B)	the arrival of an excitatory post synaptic potential (EPSP).
		C)	the opening of the sodium gates, and the diffusion of sodium ions into the neuron.
		D)	the opening of the potassium gates, and the diffusion of potassium ions out of the neuron.
		E)	the opening of the chloride gates, and the diffusion of chloride ions into the neuron.

3.	Repolarization of the neuron inner membrane is caused by...

		A)	the arrival of an inhibitory post synaptic potential (IPSP).
		B)	the arrival of an excitatory post synaptic potential (EPSP).
		C)	the opening of the sodium gates, and the diffusion of sodium ions into the neuron.
		D)	the opening of the potassium gates, and the diffusion of potassium ions out of the neuron.
		E)	the opening of the chloride gates, and the diffusion of chloride ions into the neuron.

4.	Signal conduction on an unmyelinated neuron is a chain reaction occurring along the neural membrane.

		A)	True
		B)	False

5.	An action potential cannot occur until the inner membrane reaches resting membrane potential, at which point an action potential will be propagated.

		A)	True
		B)	False