muscle physiology intro

8/18/2019 Muscle Physiology Intro

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Intro to motor systems


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What are lower motor neuro(LMN)?

• Located in:

- Anterior grey column

- Anterior nerve roots (spinal LMN) – controls voluntmuscle movement

-Cranial nerve nuclei of brainstem

- Cranial nerves with motor function (cranial nerv


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Types of LMN

There are 3 types of LMN:

1. Alpha (α-MNs) involved in muscle contraction !shortens musinnervates extrafusal muscle !res

2. Beta (β-MNs) involved in endurance activities$ innervates i!slow twitch# muscle !res !these %bres warm up slowly and are morema"ing them good for long'distance e*ercise#

3. Gamma (γ-MNs) innervate intrafusal %bres which) togethesensory a+erents) contracts muscle spindles$ used for propr


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"rrangement of lower motorneurons•#roximal , medial

• $istal , lateral

•

%lexors , posterior) but in front of e*tensors

• Extensors , posterior) dorsal to -e*ors


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&ome denitionsMotor unit , single alpha'MN and the muscle !res it innerva

Motor neuron pool , collection of alpha'MN and the muscle itinnervates

NB a sin!le m"scle is ma#e "p $% man& m"scle 'res

Motor recruitment , the process of producing graded musclecontractions depending on the action re.uired e.!. !rippin! a pethr$*in! a #isc"s

This depends on 3 things:

- /iring rate (%aster + m$re %re,"ent 'rin! rate str$n!er c$ntra

- No0 of motor units (m$re m$t$r "nits recr"ite# str$n!er)

- 1i2e of motor units (same as a$ve)


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What is the myotatic re'ex?

Myotatic re-e* , stretch re'ex

• If muscle lengthens) muscle spindle stretches

• Muscle spindle stretching results in increased alpha M

• Muscle %bres will then contract to counteract stretchin

• /unctions to maintain constant muscle length


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Myotatic re'ex explained(hopefully)

##*+E#T*N• Muscle spindles monitor muscle length

- roup 4a sensory neurons can sense stretch on

alpha motor neurons

- Considered monosynaptic i.e. $nl& $ne s&napse

Bet*een 2 #ierent t&pes $% ne"r$ns

WE-E, After e*trafusal %bres have contracted) the mspindle is slac" hence) during this period) it technicallysense or signal muscle length (ta"t *ire)


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.amma co/acti0ation1ear in mind2alpha MN shortens e*trafusal %breonce shortened) cannot detect change in muscle le

&o2

In order to compensate) gamma MN are activated

• amma MN shortens muscle spindles i0e0 if botshortened) muscle spindle would no longer be slaonce again signal muscle length


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What actually signals muscltension?

Muscle tension is signalled by a particular body) "now.olgi tendon organ (31 axons)

This activates the E-E&E M4T"T*+ E%LE5 (mam"scle tensi$n *ithin $ptimal ran!e)

40 Increases muscle tension

60 Inhibits alpha MN

30 7educes rate of muscle contraction

All serves to return muscle to normal tension8length


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eciprocal inhi!ition 6'exor7crossed/extensor re'e

•

roup 4a sensory a*ons send message to inhibitory interneuronto rela* the antagonist muscle !within the same action#

/.!. i% &$" *ant t$ 0e &$" arm (iceps) &$"r triceps (etens$r)

This prevents the myotatic re-e* from resisting intentional move

i.e. on the same side/action

%lexor 6 crossed/extensor re'exes

If one side -e*es) the other side will e*tend to compensate (thinas $ne %$$t steps the $ther le! *ill tense t$ s"pp$rt *ei!ht + maalance)

i.e. during an action, refex will happen on the other side


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$i8erent types of LMNconditions

39 Ner0e damage

reduced conduction velocitcarpal t"nnel

:9 $ener0ation %brillations (increase# sensiti A5h)

;9 $egeneration fasciculations (t*itch c$ntra#"e t$ ran#$m 'rin! $% ne"r$ns)


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=pper motor neurons

• 9pper motor neurons , sensory neurons , a+erents

They are li"e the command centres5 which send signals telling muscles what

• 1ynapse in the anterior !ventral# horn of the grey

matter of the spinal cord along with the LMN

Located in the lateral corticospinal tract

$E&+EN$*N. #"TW"4&

Basal ganglia , movement selection ; initiation

Cerebellum , everything else


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muscle physiology intro

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