Neuromuscular Junti

8/16/2019 Neuromuscular Junti

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Skeletal muscles are told what to do by the nervous system. They onlycontract when told to do so.

You already learned that the electrically active, signalling cells of thenervoussystem aretheneurons.Neurons are

cells withmanyprocessessticking offof them,

and one of these processes provides the output of the electrical information forthe neuron. This output process is the thinnest process, and there is only oneof them; it is called the axon. Do you see the aon in the diagram of thisneuron to the left!

You do not need to learn all the names of the parts of the neuron yet. That

will come soon enough. "ll # want you to see for now is that the neuron sendsits information down its aon to its presynaptic terminals $%pre% for before,because the information comes from this end&. #t's presynaptic terminals inthis diagram are all at the far right(hand edge of the drawing.

" neuron that communicates directly with a muscle is called a motor neuron$%motor% for movement&. So you may also see the presynaptic terminals calledthe motor neuronal terminals. )ith what does the motor neuroncommunicate! *ertainly not with the entire muscle.

Think about it this way... if one motor neuron controlled all the muscle

fibers in an entire muscle, every time that motor neuron was active, the entiremuscle would contract. +ur muscular contractions would then be all(or(none."nd we would move about this world in a erky fashion. Since that is not thecase, can you figure out what the motor neuron connects to! #t connects tomuscle fibers. Somewhere between a few muscle fibers and many(( but that isthe topic for the net web page on motor units.

You should epect that if the muscle fiber receives information from a motor

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neuron, it should have an anatomical speciali-ation at the point of motorneuronal contact. The speciali-ed region on the muscle fiber is called themotor end plate $that's ust a fancy way of saying that it is the muscle'spostsynaptic terminal, %post% for after, because the information is received at

this end&.

The neuromuscular junction is the term for the connection between thenervous system and the muscle fiber. #t is a type of synapse(( a connectionfrom a neuron to either another neuron or a muscle or another effector organ. egardless, it is a connection from that sends electrical information toanother place. This electrical information has to travel from one cell toanother. The problem is that each cell is an independent unit, and there isspace between the two cells that must communicate electrically.

You probably know from eperience that if you cut the electrical cord from a

lamp right in half and then plug in the half that can get plugged in, even if youput the two halves of the cord close together, if the two halves don't touch,your light won't turn on. $don't try this, though(( big fire ha-ard/& *ells thatcommunicate electrically have to have a gap between them in order to bedifferent cells. So how do they bridge this gap! They transfer the electricalinformation into chemical information to cross the gap, then the chemicalinformation is transformed back into electrical information.

0ollow along through the net description on 0igure 1.2 in your tetbook...

The electrical information $called an Action Potential, or AP& reaches the

tip of the motor neuronal terminal. There, the "3 causes the motor neuronalterminal to secrete a chemical. The specific chemical is called acetylcholine $or ACh&. The "*h, once secreted, simply diffuses across the gap between themotor neuron and the muscle fiber. This gap is called the synaptic cleft,literally meaning the space within the synapse. )hen the "*h reaches themotor end plate, the membrane has specific receptors to recogni-e "*h, calledacetylcholine receptors $or AChRs&. )hen "*h comes in contact with the"*hs on the motor end plate, the "*hs are able to trigger the generation ofan "3 on the sarcolemma. Thus, an electrical signal is restoredpostsynaptically.

To summari-e what happens across the neuromuscular unction,

electrical signal (((((4 chemical signal (((((4 electricalsignal "3 "*h "3on motor neuron in synaptic cleft on sarcolemma

You will get a lot more detail on synapses when we cover the nervous system.


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5ntil then, this is enough terminology.

6eanwhile, you should now be able to see that only those muscle fibers thatreceive information from a particular motor neuron will be ecited to contract

when that particular motor neuron is electrically active. To understand thisfurther, go on to the motor unit page.

� 7899 ST** 0oundation 3resswritten by Dawn ". Tamarkin, 3h.D.

Definition: a motor unit is a singlemotor neuron plus all the musclefibers to which it connects.

Some motor neurons only connect to a few muscle fibers. +ther motorneurons connect to hundreds of muscle fibers. )hich of these two options doyou think allows for greater control of your movement!

:ave you guessed yet!

:ow about now!

+. #f a motor neuron connects to only a few muscle fibers you will havegreater control of movement, because you can very gradually recruit more and

more motor neurons.

# think you'll need more information.

http://faculty.stcc.edu/AandP/AP/AP1pages/Units5to9/Unit7/motor.htmhttp://faculty.stcc.edu/AandP/AP/AP1pages/Units5to9/Unit7/unit7.htmhttp://faculty.stcc.edu/AandP/http://faculty.stcc.edu/AandP/AP/AP1pages/Units5to9/Unit7/motor.htm


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:ere's an image that # stolefrom that other professor'spowerpoint presentation, slide


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too erky if we have large motor units. "nd we will get a lot of force out ofthem.

"nother image from that 3ower3oint

presentation is shown here. The only reason #added it is so that you can picture a muscle withmore motor units. #n this figure we are up tohaving > motor units. "nd ust that little leapfrom 7 to > makes it look so much morecomplicated. #magine now having hundreds...with each motor unit containing hundreds of muscle fibers/ That's the situation in yourCuadriceps muscles/

Now think about your eye muscles. They are

small. They have many fewer muscle fibers. They have to be able to contractin tiny amounts so that your eyes can make tiny movements rather thanumping all over as they look around. # hope you can guess that these musclesneed to have small motor units.

7899 ST** 0oundation 3resswritten by Dawn ". Tamarkin, 3h.D.

HE ROLE OF THE NEUROMUSCULAR JUNCTION IN MUSCLE

CONTRACTION

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As the axon of a motor

neuron approaches a

muscle that t nner!ates"t #!#es nto multple

$ranches" each of %hch

ma&es a s'napse calle# aneuromuscular (uncton

%th an n#!#ual muscle

f$re) It thus follo%s thatan' one muscle f$re s

nner!ate# $' onl' one

motor neuron)

The set of muscle f$resthat contract %hen an

acton potental

propa*ates #o%n the $ranches of one partcular

axon s &no%n as a

+motor unt+) A motor unt

s thus the smallestcontractle element that

the ner!ous s'stem can

act!ate)

,hen an acton potental

reaches a neuromuscular

(uncton" t causesacet'lcholne to $e

release# nto ths s'napse)

The acet'lcholne $n#s tothe ncotnc receptors

concentrate# on the motor

en# plate" a specal-e#area of the muscle f$re.s

post/s'naptc mem$rane)

Ths $n#n* causes thencotnc receptor

channels to open an# let

so#um ons enter themuscle f$re)

If enou*h of these so#um ons enter the muscle f$re to rase t from ts restn* potental

of /01 m2 to a$out /13 m2" the' tr**er a muscular acton potental that sprea#sthrou*hout the f$re) Ths potental tra!els frst alon* the surface of the sarcolemma" the

excta$le mem$rane surroun#n* the !arous contractle c'ln#rcal structures &no%n as

m'of$rls) To reach the m'of$rls" some of %hch are locate# #eep n the muscle f$re"

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Mem$rane excta$lt' s a trat that s shar e# $' neurons an# muscle f$res an# that

can pro#uce acton potentals) 4ut ths trat s not exclus!e to neurons an# muscle

f$res) For example" t s also foun# n *lan#ular cells" fertl-e# o!ules" an# certan

plant cells)
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Neuromuscular Junti

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