Welcome to this lesson on nerves as information pathways. Today, you will be learning about the structure of a nerve, as well as reflexes as the simplest nerve pathways. Specifically, you will learn about:
Nerves are basically just bundles of nerve fibers and nerve fibers are just the long axons of either a sensory or a motor neuron. Each axon of these sensory or motor neurons is surrounded by something called a myelin sheath.
A myelin sheath covers the axon of this nerve, allowing action potentials to propagate faster. A myelin sheath is made of something called glial cells. These cells allow these action potentials to happen much faster than they would otherwise.
Myelin sheaths are present around the axons of neurons in both the central and peripheral real nervous system, but their structure is slightly different. 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.
Take a look at the diagram of a nerve fiber below.
In the diagram, you can see the axon and myelin sheath of the nerve fiber. The myelin sheath wraps around the axon in what appears to be rolls; you can see above that it's rolled around the axon.
You may also noticed the unsheathed node. This unsheathed node separates each cell from the next. Action potentials will jump from one unsheathed node to another, allowing new action potentials to happen.
At each node sodium channels open, allowing sodium to flow in, and action potential can occur, and then that action potential will propagate to the next unsheathed node.
Action potentials can actually travel up to 400 feet per second thanks to these myelin sheaths made of glial cells.
Take a look at the diagram on reflexes below to see how they work.
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.
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.
Think about the patellar reflex arc as an example of a very simple reflex. 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.
When that mallet strikes your knee, it will strike the patellar tendon and that causes muscle spindles in your quad to stretch. Then a signal will travel towards your spinal cord via a sensory neuron, and then in your spinal cord it's going to synapse directly with a motor neuron. 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.
Sensory neuron, in this case, doesn’t involve an interneuron. It 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 of the nerve structure and how reflexes cause nerves to work as information pathways. You also learned how action potentials will jump from one unsheathed node to another, allowing new action potentials to happen and how the myelin sheath helps speed up the action potential process.
Keep up the learning and have a great day!
Source: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND
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.