How does the Facial Nerve actually work.

[BMac]

Excuse me for being a complete novice to Neurology but I have a question regarding how the Facial Nerve actually controls different muscles.

 

For example, the Facial Nerve controls the obicularis oculi, procures, corrugator supercilii muscles but how does this work?

 

When we open/close the eyelid are we sending certain frequencies of electrical signals down the nerve and each muscle works on different frequencies? Like tuning your radio?

 

If so is it possible to use an electrode to stimulate the obicularis oculi, procures, corrugator supercilii muscles only with a certain frequency of electrical current or am i totally wrong here?

 

Thanks

Barry

[andrewcellini]

i'm not sure how different the facial nerves work in stimulating muscle contraction as other skeletal muscles, so someone more versed in the subject should definitely chime in.

 

when an action potential is propagated to an innervated muscle, acetylcholine is diffused across the membrane of the presynaptic terminal of the axon. when acetylcholine binds to receptors on the muscles, the membrane potential of the muscle cells change (due to the influx of Na+ ions). when the potential in the muscle cells is raised around -50 mV, the sarcoplasmic reticulum releases Ca2+ ions which is key to the function of the contractile proteins (which i frankly don't remember completely enough to talk about specifics).

 

different responses of the muscle depends on which cells are contracting and which aren't.

Edited November 3, 2015 by andrewcellini

[CharonY]

Excuse me for being a complete novice to Neurology but I have a question regarding how the Facial Nerve actually controls different muscles.

 

 

When we open/close the eyelid are we sending certain frequencies of electrical signals down the nerve and each muscle works on different frequencies? Like tuning your radio?

 

 

No, the target muscles are activated by individual neuronal cells. What you need to know is that a nerve is a bundle of neuronal fibers that transmit signals to and from the brain. The facial nerve specifically originates from the brainstem and then branches out, and connects with the various muscles. Which muscles are activated at any given time is dependent on which branches carry action potentials. The frequency of action potentials typically only encodes the strength of a signal (as the amplitude of APs do not change).

 

So if you close your eyelid a signel travels through the facial nerve, goes to the zygomatic branches and end with the orbicularis oculi, which, upon activation, close the eyelids. The opening of the eyelids, however, is controlled by the levator palpebrae superioris which, IIRC, is innervated by a different pathway.

Edited November 3, 2015 by CharonY

[BMac]

So in a way the Facial Nerve is like a wire only inside there are say 100 wires that branch out to different parts of the face. So does that mean that those 100 wires at the brain are branched out too in that area of the brain? And thus the activation of certain muscles is dependent on which of the 100 quadrants of the brain send a signal down the main wire (Facial Nerve)?

 

I have another question regarding electrical stimulation of nerves. In theory would it be possible to attach an electrode to the facial nerve (Lets leave out where and how to attach one) and be able to control different parts of the facial muscles using different frequencies/impulses of electrical stimulation? So in effect would it be possible to stimulate 1 of those 100 wires using certain frequencies/impulses or would you just end up stimulating the whole face?

[Endy0816]

You can do that for some muscles, I don't know about the face though.

 

 

but you could fake it

 

[CharonY]

Yes the facial nerve is basically a bundle of neurons. In principle you could try to activate parts of the bundle (or more easily, the target muscles directly) and trigger movement. However, to re-iterate, the frequency typically controls the intensity of the signal, not the targeting or fine control per se.

You have to know (or remember) that the signal is transmitted in the form of action potentials (AP). If you read up on the properties of APs you will realize why that is the case.

[BMac]

So it is still possible to activate parts of the bundle to contract certain muscle groups of the face?

 

How in theory can you stimulate those bundles?

 

I noticed the video above moving your arm is possible but they are using the signal from the nerve that is at that specific muscle group. To do like wise on the face then obviously he would need to place the same electrodes on say the nerves at the eyebrow and be capable of doing the same thing, cause another persons eyebrows to move.

 

Again would it be possible to have a small electrode implant on the main cervical nerve before it branches out to the arms to do the same thing? ie pick up the signal being created at that point and then repeat it. Once repeated would it stimulate the arm the same way?

I'm thinking that there would be a lot of signals being picked up and in turn a lot of muscles will be stimulated again, not just the forearm. But in a case like that could the signals be separated?

Are they different from one another or like the video above, its just a case of on/off on the nerve in that muscle group and the same on/off on the cervical nerve will stimulate all of the muscles associated with it with no rhythm, ie they all just tense up that's it.

 

But say we did pick up that signal say on C5 where the patient does a few movements, we record that signal, and then we repeat it directly on the C5 nerve. Would it recreate the same movements of all those muscles?

 

What equipment is that guy actually using on that first video?

[CharonY]

Even before nerves branch out they are a collection of neuronal cells (or more precisely, projections of axons of neural cells). If you stimulate the whole nerve, you will most likely have cramp-like convulsions.

You could, using microelectrodes, stimulate individual cells and their respective targets. While possible, it is also very very tricky and if you wanted to stimulate a muscle, it is far more easier to insert electrodes there.

[CPG]

Just thought I'd add that individual cells (including neurons and muscle cells) are sensitive to the frequency of their inputs.

 

The way I think of this is to first consider the passive properties of cell membranes, namely its resistance and capacitance. Any given cell will have a resting permeability (or conductance), and the membrane behaves like a capacitor so the surface area of a cell membrane largely determines its capacitance. These two are in parallel with one another, so as in any basic RC circuit low frequencies exhibit more influence on the membrane potential, and higher frequencies have less influence.

 

With this knowledge you can generate a curve that describes impedance as a function of the frequency of current input (e.g. synaptic currents).

 

Now let's add in the active properties of excitable cells: ion channels that are gated dynamically by factors like membrane voltage and intracellular calcium concentration. Each of these ion channel types will have its own specific voltages at which it opens and closes (activation, deactivation, inactivation, deinactivation). This opening and closing is due to conformational changes in the channel proteins, and these conformational changes have their own time constants.

 

When all this is said and done, that curve you'd expect based on the passive properties will be re-shaped by a cell's active properties and the cell of interest will display a unique impedance profile for how effective particular input frequencies affect the membrane voltage.

 

If that sounds really complex, you're right. But it's also what give these types of cells an incredible diversity in their ability to integrate signals over time and ultimately determine cell-type specific activity in response to a given signal.

Edited December 4, 2015 by CPG

[Xalatan]

The facial nerve controls the muscles of facial expression through action potential, basically electrical signal coupled to transduced chemical acetylcholine signal at the neuromuscular synapse.

 

Different neurons in the facial motor nucleus (pons/medulla) each controls a corresponding motor unit, which consists of a neuron and the muscle fibres it synapses with. Depending on which muscle of the facial nerve is to be activated, different neurons in the facial nucleus will fire off action potentials to contract that muscle (and specify strength of contraction), for example orbicularis oculi.

 

Nervous control doesn't work on different frequencies per se because action potential is an all-or-none response. It is either on, or off. Rather it works by specifying which motor unit is to be activated, and the number of action potential being fired off per unit time (barring refractory period, called temporal summation) determines the strength of contraction.

Edited December 24, 2015 by Xalatan