THAN KINH NGOAI VI
1. Dien the mang neuron, dan truyen xung dongtrong sOi true trong sOi true
a. Dien the nghi
Don vi cau true - chile nang cua c0 the don bao cung nhii da bao la te bao. Moi te bao la don vi song, co kha nang trao doi chat vdi dich ngoai bao, san xuat cac san pham cua minh va phan chia sinh te bao con.
Khi neuron 6 trang thai nghi, dien the 6 mat trong mang te bao am hdn mat ngoai. Sii chenh lech dien the nay la do sii chenh lech nong do ion am va ion dutdng gitia trong va ngoai mang te bao. Sii chenh lech nong do ion nay la do:
- Trong te bao, nong do protein (mang dien tich am) rat cao va khong the khuyech tan qua mang te bao de ra ngoai diidc nen nong do ion am trong mang cao hOn, ngoai mang te bao.
- Sii khuyech tan cua ion Na+ va K+: binh thtfdng hai loai ion nay tham qua mang te bao vdi so' liidng rat it. Do nong do ion K+ trong mang cao hdn ngoai mang te bao nen kha nang khuyech tan ion K+ tti trong ra ngoai mang 16n hdn nhieu (100 lan) so vdi ion Na+ di tti ngoai vao trong nen nong do ion am trong mang te bao von da cao lai cang cao hdn so vdi ngoai mang
- Bom Na+ - K+: day la ly do chi'nh de tao ra stf chenh ech nong do ion gitfa trong va ngoai mang te bao. Bdm nay loat dong lien tuc (trti khi te bao chet). Cd m6i lan bdm loat dong co 3 ion Na+ di ra ngoai va chi co 2 ion K+ dtfpc )dm tii ngoai vao trong. Noi cach khac, sau moi lan bdm, 6 igoai mang tang them 1 ion dtfOng nen stf chenh lech nong lo ion am gitfa trong va ngoai mang cang tang len.
NhU vay, khi neuron khong hoat dong, dien the mat ,rong am hdn mat ngoai mang neuron (hien ttfdng phan ;tfc). Dien the d mang sdi true la -90mV va d mang than leuron la -65mV.
3. Dien the hoat dong
Dien the hoat dong cua mang b it dau bang stf bien doi dot ngot tuf dien the" am, luc nghi sang dien the difdng roi lai rat nhanh trO ve dien the am:
- Giai doan nghi (giai doan phan cue):
+ Vai tro cua kenh natri: la loai kenh co canh cong dien the, cong phia ngoai la cong hoat hoa, cong phi'a trong la :ong khu hoat. Khi dien the mang la -90mV, cong hoat hoa dong (Na+ khong vao dtfdc), cong khtf hoat md
+ Vai tro cua kenh kali: cung la kenh co canh cong dien the nhtfng chi co mot cong hoat hoa nam phia trong mang te bao. O trang thai nghi cong kenh dong, K* khong ra ngoai dtfdc.
Giai doan khtf ctfc:
Hoat hoa kenh natri: dien the mang tang d in tii - 90mV len -50mV gay bien doi cong hoat hoa lam no md ra, Na+ ua qua kenh vao trong te bao. Tinh tham cua mang vdi Na+ luc nay tang gap 500 den 5000 lan. Trang thai nay chi keo dai vai phan van giay.
- Giai doan tai cilc:
Ngay sau khi md vai phan van giay kenh natri bat dau dong, kenh kali md lam ion K+ khuyech tan nhanh ra ngoai, tai tao lai trang thai phan ctfc luc nghi ve tri so
-90mV.
c. Dan truyen xung dong tren sdi true.
Stf lan truyen dien the hoat dong tao nen mot mach dien tai cho gitfa vung dang khtf ctfc va phan mang 6 vung tiep giap. Lan song lan truyen nhtf vay goi la xung dong than kinh. Khi dien the hoat dong dtfpc tao ra 6 mot diem tren mang neuron, qua trinh khtf ctfc se lan toa toan bo mang theo quy luat "tat ca hoac khong". Dien the hoat dong xuat hien tai mot diem cua spi true se lan toa theo hai phia. Dong dien trong te bao chay ttf vung hoat dong (noi ti'ch dien dtfOng) den vung khong hoat dong ben canh (noi ti'ch dien am). Phia ngoai te bao xuat hien mot dong dien chay ngtfpc chieu ttf vung khong hoat dong den vung hoat dong. Hien ttfpng nay lam nhtfng vung khong hoat dong 0 hai phia cua vung hoat dong bi khtf ctfc tao nen dong dien chay theo hai htfdng. Tuy nhien ve mat sinh ly, xung dong thtfc te chi dan truyen theo chieu thuan.
Doi vdi cac spi than kinh khong co myelin, cac xung dong d in truyen theo tting diem mot tren suot chieu dai spi true nen toe do d in truyen khong nhanh.
Cac spi th in kinh co myelin, cac xung dong d in truyen theo kieu "nhay coc" qua cac eo Ranvier (do myelin la mot loai chat cach dien, mat khac mang tai eo Ranvier co tmh tham rat cao vdi cac ion (gap 500 lan cac spi khong co myelin) nen toe do dan truyen cua cac spi
than kinh co bao myelin cao hon rat nhieu so vdi cac sdi khong co bao myelin.
b
Hin 2.1: Dan truyen nhay cua cac sdi than kinh co myelin, a: dong dien cuc bo, b: kich thich dien gay khCr ctfc d cac eo Ranvier ke can
va dong dien chay theo hai htfdng.