Revision notes of muscle physiology – 31/10/13
Tendons in ankle / wrist – have tendon sheaths (synovial = 2 layered) to reduce
friction. Paratenonitis – e.g. Achilles paratenonitis – ‘in severe cases, tendon
can appear sausage-like, because it is so severely swollen’ – e.g. marathon
runners with acute presentation, or chronically – Sx present at beginning of run
(but push through the discomfort) – Sx agg by activity, relieved by rest. DD with
tendinosis (degeneration), insertional tendinitis / traction epiphysitis,
retrocalcaneal bursitis
Muscle tissue properties:
1)
2)
3)
4)
Excitability
Contractility
Extensibility
Elasticity
Elastic elements:
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titin molecules (extend from Z-disc attachments of actin, to M-line
attachments of myosin. Very elastic, can stretch to 4 times length without
harm
connective tissue of muscle fibre (endomysium, perimysium, epimysium)
tendons
As muscle starts to shorten, connective tissue coverings and tendons stretch first
– become taut – pull on bone (movement)
Electromyography (EMG):
Measures electrical activity (muscle action potentials) in resting and contracting
muscles.
Electromyograph equipment – used to produce an electromyogram.
Intramuscular: needle and fine wire. Provides valuable information about the
state of the muscle and its innervating nerve.
Wikipedia: Normal muscles at rest make certain, normal electrical signals when
the needle is inserted into them. Then the electrical activity when the muscle is
at rest is studied. Abnormal spontaneous activity might indicate some nerve
and/or muscle damage. Then the patient is asked to contract the muscle
smoothly. The shape, size, and frequency of the resulting electrical signals are
judged. Then the electrode is retracted a few millimetres, and again the activity is
analyzed until at least 10–20 motor units have been collected. Each electrode
track gives only a very local picture of the activity of the whole muscle. Because
skeletal muscles differ in the inner structure, the electrode has to be placed at
various locations to obtain an accurate study.
Normal results:
Muscle tissue at rest is normally electrically inactive. After the electrical activity
caused by the irritation of needle insertion subsides, the electromyograph should
detect no abnormal spontaneous activity (i.e., a muscle at rest should be
electrically silent, with the exception of the area of the neuromuscular junction,
which is, under normal circumstances, very spontaneously active). When the
muscle is voluntarily contracted, action potentials begin to appear. As the
strength of the muscle contraction is increased, more and more muscle fibres
produce action potentials. When the muscle is fully contracted, there should
appear a disorderly group of action potentials of varying rates and amplitudes (a
complete recruitment and interference pattern).
Abnormal results:
EMG is used to detect:
1) Neuropathies
2) Neuro-muscular junction disease
3) Myopathies
Neuropathic disease EMG characteristics:
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An action potential amplitude that is twice normal due to the increased
number of fibres per motor unit because of reinnervation of denervated
fibres
An increase in duration of the action potential
A decrease in the number of motor units in the muscle (as found using
motor unit number estimation techniques)
Myopathic disease EMG characteristics:
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A decrease in duration of the action potential
A reduction in the area to amplitude ratio of the action potential
A decrease in the number of motor units in the muscle (in extremely
severe cases only)
Surface EMG less invasive, electrodes monitor general muscle activity. Can be
used for biofeedback.
Nerve conduction testing – Usually at same time as EMG
Not invasive, but can be painful (electric shocks). Currents used are very small
so not dangerous to anybody; pacemakers etc. can still be done but adjustments
made.
Nerve conduction velocity (NCV) an important measurement during this test.
Motor: electrical stimulation of peripheral nerve; then recording from muscle.
Amount of time, distance between electrodes and amplitude of response
measured.
Sensory: electrical stimulation of peripheral nerve; then recording from purely
sensory part of the nerve, e.g. from a finger. Measurements based on time taken
and distance between electrodes.
Proprioception
Difference between joint position sense and ‘proprioception proper!’
unconscious proprioception via dorsal / ventral spinocerebellar tracts; joint
position sense via dorsal column/medial lemniscus pathway
Muscle spindles:
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sensory nerve endings wrapped around 3-10 specialised intrafusal
muscle fibres, enclosed in connective tissue capsule.
In parallel
More prolific in muscles with finely controlled movements (e.g. fingers,
eyes)
Measure MUSCLE LENGTH – stimulated by sudden and prolonged stretch
Signals to somato-sensory cortex, and cerebellum – for control of posture
/ movement
Gamma motor neurons supply muscle spindles – adjust tension in spindle
to variations in length of muscle. Maintains sensitivity of spindle
When stretched – activation of muscle spindles = spinal cord = synapse
with LMN in anterior horn (or in brain stem for head) = alpha motor
neuron to extra-fusal fibres in rest of muscle = contraction (to relieve
stretch on muscle)
‘Deep tendon reflex’ is then a complete misnomer!!! AKA myotatic stretch
reflex (more accurate)
Reflex – involves simple arc in spinal cord. But acted on by descending
inhibition from CNS. Therefore in UMN lesion, this descending control is
lost and the reflex is increased.
Golgi tendon organs:
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At myotendinous junction
In series
Function is to protect tendon/muscle from damage due to excessive
tension
Tendon reflexes = ‘inverse myotatic reflex’ = inhibition of alpha-motor
neurons to agonist muscle; also activation of antagonist muscle