What does the motor unit consist of?
- - Consists of an anterior horn cell, the alpha motor neuron coming from it and all the individual muslce fibres it supplies
- - all the mm fibers of a particular motor unit are of the same type
Voluntary contractions of mm- how does this occur?
- - asynchronous firing of motor units resulting in smooth mm contraction
How is the force of a mm contraction grades?
- - the number of motor units recruited
- - the frequency of nerve impulses
How does eletrical stimulation of innervated muscles differ from a volunatry contract?
- - there is a synchronous firing of all motor units stimulated
- - electrical stimulation will not stimulate motor units in the same recruitment order as volunatry contraction because larger diameter fibres are more easily stimulated and sensory nerves are inevitably stimulated, and the frequency of firing is fixed
Where voluntary active exercise is restricted, electrical stimulation may be substituted
- - usually applied by surging a series of short pulses at a frequency between 30- 100Hz
- - speed, length and strength of contraction can be varied by altering surge time, rate and fall rates intensity
- - may be used for a variety of therapeutic purposes
How can mm stimulation increase the strength of a normal mm?
- - electrical stimulation can increase mm strength but not as much as equivalent voluntary exercirse
- - Hn Soi Lun et al 88 and Pfiefer et al 97
- - initial gain in strength are due to neural mechanisms
How can mm stimulation increase the strength of a weakened mm?
- - value of electrical stimulation is clearer
- - gibson et al 88 obalulwa 91
- - evidence points to the need for a maximum tolerable contraction for strengthening
How can mm stimulation increase the strength by usuing facilitation of mm control
- - used extensively to initate and facilitate voluntary contraction of mm
- - proposed uses have- inhibition due to pain (eg post trauma or surgery)
- - in situations where the mm action is not readily under voluutary control without practice
- - in circumstances where the new mm action has to be learn (post motor nerve transplant)
- - in the latter stages of a recovering peripheral nerve lesion to encourage voluntary mm contraction
What effects does mm stimulation have on the metabolism and blood flow?
- - electrical stimulation will havethe same effects as normal vountary mm contraction in causing a temporary increase in mm metabolism
- - increase oxygen uptake
- - carbon di-oxide, lactic acid and other metabolite production
- - raised local temp
- - increase blood flow
Stimulation of a denervated mm
- different from innervated mm in many respects
- can only be caused to contract by direct stinulation of the mm fibre
- the differences between stimulation mm directly and via its nerve are
- - mm tissue is less excitable than nerve so that a greater electric charge is required
- - slow worm like contraction resuls because of theslow spread of contraction through the mm and diminished rate of contraction compared with innervated mm
- - slow rising electrical pulses can stimulate because it has less ability to accomodate than nerve
- Current is usually most effective applied long axis of the mm fibres ie with the stimulation electrodes at each end of the mm belly
- Rationale for treatment ahs been the maintenance of the mm is as health state as possible by electrically induced artifical exercise while awaiting reinnervation
- - the best results seem to have been achieved with vigorous isometric contraction to fatigue 2/3 times a day
- - problems in practice