Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson on Nerves as Information Pathways. Today, we will be discussing the structure of a nerve, and we will also talk about reflexes as the simplest nerve pathways. So we're going to start by discussing the structure of a nerve.
So nerves are basically just bundles of nerve fibers. And nerve fibers are just the long axons of either a sensory or a motor neuron. And each axon of these sensory or motor neurons is surrounded by something called a myelin sheath.
So a myelin sheath, basically what it does is it covers the axon of this nerve, allowing action potentials to propagate faster. So a myelin sheath is made of something called glial cells. So these cells allow these action potentials to happen much faster than they would otherwise.
And myelin sheaths are present around the axons of neurons in both the central and peripheral real nervous system, but their structure is slightly different. So in the central nervous system, glial cells, called oligodendrocytes form the myelin sheath. Whereas in the peripheral nervous system, glial cells called Schwann cells form the myelin sheath. So they're a little bit different, but basically their structure is similar, allowing these action potentials to propagate much more quickly.
So if we take a look right here, we have a nerve fiber. And we have the axon, the myelin sheath. So as I mentioned, the myelin sheath kind of wraps around the axon. You'll notice how it just kind of wraps around it in these rolls. So this is the myelin sheath.
And then we have an unsheathed node. So these unsheathed nodes basically separate each cell from the next. So action potentials will jump from one unsheathed node to another. And when that happens, it allows for new action potentials to happen.
So at each node it's going to cause sodium channels to open, sodium to flow in, action potential can occur, and then that action potential will propagate to the next unsheathed node. And just kind of a fun fact, action potentials can actually travel up to 400 feet per second thanks to these myelin sheaths made of glial cells. So it really does allow those action potentials to happen very, very, very quickly.
So next we're going to take a look at this diagram on reflexes, and we are going to discuss how a reflex works. So reflexes are the simplest nerve pathway. And reflexes are an automatic movement as a result of a stimulus, so it doesn't take conscious effort for a reflex to occur.
And reflexes can involve interneurons, or sometimes a motor neuron will synapse directly with a sensory neuron. So a very simple reflex just involves sensory neurons and motor neuron synapsing with each other. However, most interactions involve an interneuron when we're talking about reflex pathways.
So we're going to take a look at the patellar reflex arc as an example of a very simple reflex. So if you've ever been to the doctor before and they've tapped your knee with that little rubber mallet and it makes your foot kick upward. So we're going to discuss why that happens.
OK, so let's take a look at this diagram right here. So basically, when that mallet strikes your knee, it will strike the patellar tendon. And that causes muscle spindles in your quad to stretch.
So then a signal will travel towards your spinal cord via a sensory neuron. So it's going to travel, this causes this muscle spindle to stretch. That signal is sent towards your spinal cord via a sensory neuron. And then in your spinal cord, it's going to synapse directly with a motor neuron. And then that motor neuron is going to carry that information back to your quadriceps muscle, triggering a contraction and causing your leg to kick upward.
So again, this is a very simple reflex. It doesn't involve an interneuron. Sensory neuron, in this case, is synapsing directly on a motor neuron with no interaction with an interneuron. So it's a very simple reflex. So in this lesson has been an overview on reflexes and nerves.
An insulating layer that surrounds axons neurons in the CNS and PNS and allows action potentials to propagate more quickly.
A nerve is a bundle of axons of neurons.
An automatic movement of the body that occurs as a response to stimuli.
The glial cells that compose the myelin sheath in the peripheral nervous system.