TONE IS THE RESISTANCE OF MUSCLE TO STRETCH. TONE TESTS IN TWO WAY: PHASIC AND POSTURAL. PHASIC TONE IS TESTED WITH DTR RESPONSE. POSTURAL TONE IS THE PROLONGED CONTRACTION OF ANTIGRAVITY MUSCLES WHEN POSTURAL TONE IS DEPRESSED, THE TRUNK AND LIMBS CANNOT MAINTAIN THEMSELVES AGAINST GRAVITY, AND THE INFANT APPEARS HYPOTONIC.THE FIRST STEP IN DIAGNOSIS IS TO DETERMINE WHETHER THE DISEASE LOCATION IS IN THE BRAIN, SPINE, OR MOTOR UNIT., OF COURSE, MORE THAN ONE SITE MAY BE INVOLVED.FOR EXAMPLE, THE BRAIN AND THE PERIPHERAL NERVES ARE INVOLVED IN SOME LYSOSOMAL AND MITOCHONDRIAL DISORDERS...