▁one ▁functional ▁copy ▁of ▁the ▁SM N 2 ▁gene ▁( with ▁most ▁having ▁ 2 – 4 ▁of ▁them ) ▁which ▁still ▁codes ▁small ▁amounts ▁of ▁SM N ▁protein ▁– ▁around ▁ 1 0 – 2 0 % ▁of ▁the ▁normal ▁level ▁– ▁allowing ▁some ▁neur ons ▁to ▁surv ive . ▁In ▁the ▁long ▁run , ▁however , ▁reduced ▁avail ability ▁of ▁the ▁SM N ▁protein ▁results ▁in ▁grad ual ▁death ▁of ▁motor ▁neur on ▁cells ▁in ▁the ▁anterior ▁horn ▁of ▁sp inal ▁cord ▁and ▁the ▁brain . ▁Mus cles ▁that ▁depend ▁on ▁these ▁motor ▁neur ons ▁for ▁neural ▁input ▁now ▁have ▁decre ased ▁inner v ation ▁( also ▁called ▁d ener v ation ), ▁and ▁therefore ▁have ▁decre ased ▁input ▁from ▁the ▁central ▁nerv ous ▁system ▁( C NS ). ▁Dec re ased ▁imp ulse ▁transmission ▁through ▁the ▁motor ▁neur ons ▁leads ▁to ▁decre ased ▁contract ile ▁activity ▁of ▁the ▁d ener v ated ▁mus cle . ▁Con sequently , ▁d ener v ated ▁mus cles ▁under go ▁progress ive ▁at ro phy ▁( w aste ▁away ). ▁ ▁Mus cles ▁of ▁lower ▁extrem ities ▁are ▁usually ▁affected ▁first , ▁followed ▁by ▁mus cles ▁of ▁upper ▁extrem ities , ▁sp ine ▁and ▁neck ▁and , ▁in ▁more ▁severe ▁cases , ▁pul mon ary ▁and ▁m astic ation ▁mus cles . ▁Pro xim al ▁mus cles ▁are ▁always ▁affected ▁earlier ▁and ▁to ▁a ▁greater ▁degree ▁than ▁dist al . ▁ ▁The ▁sever ity ▁of ▁S MA ▁sympt oms ▁is ▁broad ly ▁related ▁to