Chng 7

CC KHI NIM C BN V MNG N RON NHN TO 7.1. Cu trc v m hnh ca mng n ron

Mng n ron l s ti to bng k thut nhng chc nng ca h thn kinh con ngi. Trong qu trnh ti to khng phi tt c cc chc nng ca b no con ngi u c ti to, m ch c nhng chc nng cn thit. Bn cnh cn c nhng chc nng mi c to ra nhm gii quyt mt bi ton nh trc.

Mng n ron bao gm v s cc n ron c lin kt truyn thng vi nhau trong mng, hnh 7.1 l mt phn ca mng n ron bao gm hai n ron.

N ron cn c th lin kt vi cc n ron khc qua cc r. Chnh v cch lin kt a dng nh vy nn mng n ron c lin kt rt cao.

Cc r ca n ron c chia lm hai loi: loi nhn thng tin t n ron khc qua axon, ta gi l r u vo v loi a thng tin qua axon ti n ron khc gi l r u ra. Mt n ron c th c nhiu r u vo, nhng ch c mt r u ra nh vy c th xem n ron nh mt m hnh nhiu u vo mt u ra, (hnh 7.2).

Hnh 7.1. Mt mng n ron n gin gm hai n ron.

Chiu thng tin

R u ra

axon

Axon c ni vi r u vo ca n ron 2

R u ra ca n ron 1 c ni vi axon Nhn

Nhn

Hnh 7.2. M hnh ca mt n ron

y

x1 x2 . . . . xn

N ron

Mt tnh cht rt c bn ca mng n ron sinh hc l cc p ng theo kch thch c kh nng thay i theo thi gian. Cc p ng c th tng ln, gim i hoc hon ton bin mt. Qua cc nhnh axon lin kt t bo n ron ny vi cc n ron khc, s thay i trng thi ca mt n ron cng ko theo s thay i trng thi ca nhng n ron khc v do lm thay i ton b mng n ron. Vic thay i trng thi ca mng n ron c th thc hin qua mt qu trnh dy hoc do kh nng hc t nhin.

S thay th nhng tnh cht ny bng mt m hnh ton hc tng ng c gi l mng n ron nhn to. Mng n ron nhn to c th c ch to bng nhiu cch khc nhau v vy trong thc t tn ti rt nhiu kiu mng n ron nhn to.

M hnh n ron c m u vo x1, x2, ...xm v mt u ra y (hnh 7.3), m hnh ny gm c ba thnh phn c bn:

+ Cc kch thch u vo ca t bo n ron c th nng tc ng vo mng membran khc nhau c biu din qua trng lng wi, i = 1, .., m tng ng vi cng kch thch ca tng u vo. Cc kch thch u vo c thc hin qua mt b tng, l gi tr o kch thch u vo tc ng vo t bo n ron.

+ u ra ca b tng c a n b p ng a(.), b ny khng ch c chc nng to ra p ng tc thi m cn c kh nng lu gi cc p ng theo thi gian. Thnh phn ny hot ng theo nguyn l nh ng.

+ N ron b kch thch trong thi gian th nng ca mng membran vt qu ngng. Quan h ny c thc hin nh hm a(.), n c chc nng ca b to tn hiu ngng, xc nh ph thuc ca tn hiu ra y vo cc kch thch u vo.

Cch thnh lp n ron nhn to nh vy to ra mt t do trong thit k. Vic la chn php cng tn hiu u vo v p ng a(.) s cho ra cc kiu mng n ron nhn to khc nhau v tng ng l cc m hnh mng khc nhau.

Theo hnh 7.3 th tn hiu u ra yi l:

( ) ( )

=

=+m

j itjxijwatiy 1

1 (7.1)

Hnh 7.3. Cu trc ca mt n ron.

yi

wim

wi2

wi1

xm

x2

x1

u vo

u ra

Cc trng s

f(.) a(.)

i

trong hm kch hot a(f) dng hm bc nhy:

( )