Motor Control

Allan Bretag

Muscle Tone: What is it?

• Tension in a muscle acting against a load

– The load may be due to gravitational force or another external force

– The load may be due to the force exerted by the antagonist muscle

Muscle tone: What use is it?

• It takes up the slack in muscles - some crossbridges already pulling

• Tendons and elastic elements in muscle are already under strain

• This means that any new motor output will produce an immediate response in the form of movement

Antagonistic pairs

http://www.mrothery.co.uk/muscles/MusclesNotes.htm

Scan down page to find good notes on muscle and eventually a figure showing antagonistic pairs. The electron microscope pictures are not so good.

Muscle Tone

•Alpha tone

•Gamma tone

•Alpha-Gamma co-activation

Alpha tone
(via pyramidal tracts)

http://www.radiology.wisc.edu/Med_Students/neuroradiology/NeuroRad/Systems/cs%20tract.gif

Gamma tone

• Gamma motor neurones are activated by action potentials passing down extrapyramidal pathways from the brainstem

• These stimulate intrafusal muscle fibres inducing a stretch reflex

• If this is done in antagonistic pairs, background tone can be established without using valuable “bandwidth” on the pyramidal - alpha motor neurone line

Origins of extrapyramidal pathways

•Tecto-spinal tracts

-from superior colliculus

• Vestibulo-spinal tracts

-from vestibular nuclei

• Reticulo-spinal tracts

-from reticular formation

Alpha-Gamma Co-activation

• Alpha motor neurones activate extrafusal muscle fires directly (produce alpha tone)

• Gamma motor neurones activate intrafusal muscle fibres (this is done in such a way that it would produce exactly the same amount of tone as the alpha tone – if the alpha tone did not exist)

• Therefore, if the alpha tone in a particular muscle has the required effect, the gamma tone will do nothing

• But if the alpha tone falls short of producing sufficient muscle shortening (against a load), then gamma tone via the stretch reflex will reinforce it to achieve the desired response

• This is a mechanism designed to be able to cope with moment to moment load changes (running over uneven ground), unexpected load (an object is heavier or lighter than expected) or muscle fatigue

Alpha Gamma Co-activation

http://www.sante.cc/stress/articles/stressmusc/stress02max.jpg

The Cerebellum

http://www.on-with-life.org/WebFrames/Cerebellum.htm

Cerebellum function

• Receives most forms of sensory input

• Adjusts rate, range, force and direction of movements to achieve most efficient movement

• Is involved with the co-ordination of voluntary movements on the background of necessary balance, equilibrium and muscle tone

• Seat of motor learning

• Compares result of motor command with expectation

• Powerful timing and sequencing function

• Important in “putting the brakes on” a movement at the appropriate time

Spinocerebellar pathways

Muscle spindle pathways to the cerebellum – collaterals from the spindle afferents on one side of the body synapse in the grey matter of the spinal cord from where second order neurones pass messages either ipsilaterally along the cord to the ipsilateral cerebellar hemisphere or contralaterally along the cord to the same (ipsilateral) hemisphere). The symmetrical arrangement is true of spindle afferents from the opposite side of the body.

Cerebellar cortex structure

** Two kinds of input to the cerebellar cortex via mossy and climbing fibres (possibly for comparison). Only one kind of output – inhibitory utilising GABA via Purkinje cells to deep cerebellar and brainstem nuclei.

http://bbf-www.uia.ac.be/models/models.shtml

Cerebellar disease

• Ataxia

• Dysdiadochokinesia

• Dysmetria

• Dysynergia

• Scanning speech

Basal ganglia (Basal nuclei)

http://www.dbios.com/basal-ganglia-big.htm

Basal nuclei

http://pharyngula.org/~pzmyers/neuro/chap9/basalganglia.jpg

Basal nuclei and their connect-ions

http://pharyngula.org/~pzmyers/neuro/chap9/9-10.jpg

Diseases of the basal nuclei

• Parkinson’s disease

• Huntington’s disease

• Ballism

Motor cortex

http://www.fmrib.ox.ac.uk/~stuart/thesis/chapter_7/section7_1.html

http://www.pbs.org/wgbh/nova/mind/prob_map.html

Diseases of pyramidal tracts