神經細胞

顯微鏡之下睇到嘅幾粒神經細胞^{[註 1]}

一粒典型嘅神經細胞嘅結構圖解

神經細胞san4 ging1 sai3 baau1（英文：nerve cell），又有叫神經元san4 ging1 jyun4（neuron），係一種特殊嘅細胞：一粒神經細胞喺俾某啲特定嘅嘢刺激到嗰陣，會以電同化學方式射訊號，如果有一大柞神經細胞連埋一齊，形成一個神經網絡嘅話，就可以做到大規模噉同時處理大量資訊。舉個例說明，而家有個人感覺到佢隻手俾針拮到，跟住即刻縮手。喺呢個過程入面，佢個神經系統內部發生咗以下嘅事：佢隻手塊皮膚上面嗰啲對力敏感嘅神經細胞對支針施嘅力有反應，於是就射訊號，呢啲訊號會沿住條脊椎上腦，等個腦知「隻手俾針拮到」；個腦話咁快自動決定應該縮手嚟保護隻手，於是又射返個「縮手」嘅訊號出嚟；呢啲訊號沿脊椎落返去隻手臂嗰啲運動神經細胞嗰度，啲運動神經細胞即刻令隻手臂嗰幾嚿肌肉郁，做「縮手」嘅動作^{[註 2]}^[1]^[2]。

神經細胞係各種動物神經系統－包括腦同脊椎－最主要同基本嘅組件。據估計，一個男人嘅腦平均有成 860 億（8.6 × 10¹⁰）粒神經細胞^[3]^[4]，構成一個極之龐大嘅神經網絡，而資訊喺神經細胞之間嘅傳遞速度可以去到成 119 米每秒咁快^[5]。於是乎喺一個大人嘅腦入面，時時刻刻都有幾百億粒神經細胞喺度以極之快嘅速度互相傳遞資訊，令到嗰個人能夠接收同處理外界嘅資訊，並且按照收到嘅資訊嚟決定做啲乜^[6]。

一粒典型嘅神經細胞係由三個主要部份組成嘅^[6]^[7]：

細胞體（cell body，或者 soma）；細胞體係粒神經細胞嘅中心，有細胞核等嘅結構。
樹突（dendrites）；樹突負責接收訊號。
軸突（axon）；軸突負責傳訊號－軸突會好似樹枝噉由細胞體嗰度散開去，並且接駁落去第啲神經細胞嘅樹突嗰度。

細胞體、樹突同軸突嘅結構主宰咗神經細胞嘅功能：包住一粒神經細胞嗰浸細胞膜會有機制令粒神經細胞裏面同周圍永遠都會有個非 0 嘅電勢差^{[註 3]}；當一粒神經細胞受到電嘅刺激嗰陣，佢浸細胞膜內外嘅電勢差會有所改變，而如果呢個電勢差改變嘅數值夠勁嘅話，噉個神經細胞就會產生一個動作電位（action potential）－一個新嘅電勢差改變。呢個動作電位會沿住粒神經細胞條軸突傳開去。而如果個動作電位夠勁，佢可以令到下一粒神經細胞再做呢一個過程傳送一個新嘅動作電位－如是者，資訊就沿住個神經網絡傳落去^[8]^[9]。

基礎

點發現

同第啲細胞一樣，神經細胞喺用染料上咗色之後可以喺顯微鏡下清楚噉睇到：喺 19 世紀尾，有唔少科學家都認為細胞學說唔適用於神經組織，認為神經組織係生物身上唯一唔係由細胞組成嘅嘢；喺 1887 年，西班牙神經科學家桑地牙哥·拉蒙卡哈（Santiago Ramón y Cajal）用嗰時新興嘅染色法高基方法（Golgi's method）將各種神經組織樣本染色，然後攞嚟喺顯微鏡下睇，最後發現當時前所未知嘅細胞－神經細胞。打後經過進一步嘅研究，拉蒙卡哈發現神經細胞彼此之間係有空間嘅（詳情睇下面突觸），而唔係好似嗰時某啲科學家所諗嘅噉，係一個完全相連嘅網^[10]。

拉蒙卡哈嘅研究最後令學界確立咗神經細胞教義（The neuron doctrine）：根據呢條教義，神經系統係由互相之間分開嘅個體細胞－神經細胞－所組成嘅，而呢啲神經細胞之間會有連繫，組成一個大嘅網絡嚟控制隻動物嘅行為。呢條原則係現代神經科學嘅根基^[11]^[12]。

定義

及後廿世紀嘅神經科學研究確立咗神經細胞嘅定義^{[註 4]}。定義上，神經細胞係指有以下兩樣特徵嘅細胞^[6]^[13]：

當一粒神經細胞俾某啲特定嘅電同化學訊號刺激到嗰陣，佢會以電同化學方式傳達新嘅訊號（signal）。
神經細胞之間有突觸（synapse）－一種令相連神經細胞能夠交流嘅結構。

神經細胞嘅呢啲特徵令佢哋可以輕易噉用嚟喺神經系統入面傳達資訊：資訊定義上係指「不確定性嘅減少」（reduction of uncertainty；可以睇吓資訊理論）；舉個例說明，對眼嘅視網膜上面每一粒感光細胞喺受到外界嘅光刺激嗰陣，會向個腦塊視覺皮層（個腦處理視覺資訊嗰一忽）射神經訊號，而個訊號嘅物理性質（例如係頻率）會按照佢感覺到嘅光嘅屬性而有所不同－例： $f=\beta I$ ，當中 $f$ 係傳嘅訊號嘅頻率， $\beta$ 係一個固定嘅常數，而 $I$ 係受嘅光嘅亮度，呢啲訊號跟住會上腦，個腦就可以透過讀取個訊號嘅頻率（ $f$ ），得知對眼見到乜嘢亮度嘅光（ $I$ ）－個腦對於「眼前有啲乜」嘅不確定性減少咗^[14]。

有好多跡象顯示，神經細胞之間傳嗰啲訊號就係隻動物諗緊同感受緊嘅嘢：以人類做例子，如果一個人嘅神經系統入面啲神經細胞冇同某樣嘢相關嘅資訊，佢就唔會諗或者感知到嗰樣嘢－由行為上睇，佢唔會對嗰樣嘢有反應、事後唔會記得嗰樣嘢、唔會傾嗰樣嘢... 等等；而且神經細胞傳訊號嘅規律－好似係射訊號嘅頻率或者訊號之間嘅時間差呀噉－仲能夠預測嗰個人嘅行為以及佢所感知到嘅嘢（後者根據佢嘅口頭報告）。喺廿一世紀嘅心靈科學上一般認為，一個人類心靈嘅內容就喺佢體內嗰啲神經細胞所傳嘅訊號當中^[15]。

顯微鏡下睇到嘅神經細胞^{[註 5]}

結構

神經細胞往往喺結構上有高度嘅特化，喺神經系統入面做多樣功能：有啲神經細胞專門處理感官資訊（感覺神經細胞），有啲專門將有關郁動嘅訊號傳俾身體嘅肌肉（運動神經細胞），又有啲會專門負責控制唞氣同心跳呢啲生存必要嘅基本功能。因為噉，神經細胞喺形狀、大細同電化性質上都有相當大嘅內部差異，舉個例，一粒神經細胞嘅細胞體嘅直徑可以由 4 至 100 微米不等－即係話最大同最細嘅神經細胞嘅直徑差異可以係成廿倍咁多^[16]。雖然係噉，有一啲結構特徵係神經細胞普遍都有嘅^[6]：

三大主要結構：
- 細胞體（cell body / soma）：係粒神經細胞嘅核心－細胞體係粒神經細胞嘅細胞核（nucleus）嘅所在，粒細胞生存必需要嘅蛋白質合成過程大部份都係喺細胞核嗰度發生嘅，所以細胞體對一粒神經細胞嚟講不可或缺。神經細胞嘅細胞核直徑多數都係喺 3 至 18 微米之間^[17]。細胞體由一個好似網噉嘅結構性蛋白質－叫神經細系（neurofilament）－撐住^[18]。
- 樹突（dendrites）：由細胞體上面多個地點分枝出嚟嘅結構，呢種好似樹枝噉嘅形狀令佢哋俾人比喻式噉嗌做「樹」突。多數神經細胞接收訊號，都係靠連接住第啲神經細胞樹突嚟接收嘅，樹突會向四周圍嘅其他神經細胞嗰度分枝，令一粒神經細胞可以同時接受數以千計嘅輸入^[19]。
- 軸突（axon）：係細胞體嗰啲幼細啲、好似電線噉嘅延伸；軸突可以延伸到去等如粒細胞體個直徑嘅成千百倍咁長，例如長頸鹿嘅神經細胞軸突有啲會沿住嗰條好長嘅頸一路伸，伸到成超過 1 米咁長；軸突主要做嘅嘢係將資訊由細胞體帶走，而有陣時又會帶一啲資訊俾返細胞體。有好多神經細胞都係得一條軸突，但呢條軸突嘅尾通常會有好多條分枝，令粒神經細胞有得同好多粒神經細胞通訊^[20]^{[註 6]}^[21]。軸突由一浸髓磷脂（myelin）包住，呢浸脂幫手將條軸突絕緣，等神經訊號傳遞得快啲^[22]。
- 樹突同軸突嘅結構特性可以按「粒細胞屬神經系統邊部份」而有所不同：喺中央神經系統（central nervous system；喺脊椎動物入面係腦同脊椎）入面嘅神經細胞樹突同軸突偏向比較幼，通常都係得嗰 1 微米左右咁粗；而喺周邊神經系統（peripheral nervous system）入面嗰啲會粗好多，直徑可以去到成 20 微米，而且仲可以好長－人體內最長嘅運動神經細胞軸突可以去到超過 1 米，由腳趾一路伸到上去脊椎嘅基，而感覺神經細胞嘅軸突都有成超過 1.5 米長嘅，由腳趾一路上到去脊椎^[23]。
除咗呢三大主要結構之外，神經細胞仲俾一啲膠質細胞（glia / glial cell）圍住：膠質細胞唔會射電訊號，主要功能係負責支援神經細胞，例如係負責霸佔神經細胞之間嘅空間，等神經細胞彼此會維持分隔開、向神經細胞提供養分、以及攻擊對神經細胞造成威脅嘅病原體等等。雖然話膠質細胞本身唔曉射神經細胞會射嘅訊號，但佢哋依然能夠透過上述嘅功能左右神經訊號嘅傳遞^[24]^[25]。

突觸

突觸（synapse）係神經細胞之間溝通嘅途徑。突觸位於一粒神經細胞嘅軸突末端，會差啲掂住下一粒神經細胞嘅樹突，有陣時亦會有兩粒神經細胞嘅軸突之間有突觸。當有一粒神經細胞所射嘅動作電位（指神經細胞射嘅電訊號）去到條軸突嘅末端嗰陣，個訊號會令到鈣離子通道打開，令到鈣離子（Ca²⁺）走入去個軸突尾嗰度。啲鈣離子跟手會令神經傳遞素呢種化合物湧出嚟，而呢啲神經傳遞素會移動去下一粒神經細胞嗰度，啟動嗰粒神經細胞嘅受容體^[26]。

人腦入面嘅突觸數量龐大：小腦入面某啲類型嘅神經細胞可以有成過百條樹突分枝，一粒噉嘅神經細胞可以同成幾萬粒其他神經細胞相連接；另一方面，又有某啲類嘅神經細胞得 1 至 2 條樹突分枝，但係每條會掂到成千個突觸。喺人腦嗰 100 億（有多）粒－即係 10¹¹ 粒－神經細胞入面，平均每粒神經細胞有 7,000 個突觸掂住第啲神經細胞。據估計，一個三歲嘅細路個腦有大約 10¹⁵ 個突觸，而突觸數量似乎會隨年紀下跌，變成大人之後就進入穩定。至於大人平均有幾多個突觸，現時估計由 10¹⁴ 個至 5 x 10¹⁴ 個不等^[27]。

神經傳遞素

當一粒神經細胞所傳嘅訊號行去到條軸突最尾嘅突觸嗰陣，突觸會釋放神經傳遞素（neurotransmitter）嚟影響下一粒神經細胞。呢啲神經傳遞素會黐落去下一粒神經細胞嘅受容體（receptor）嗰度，而下一粒神經細胞跟住落嚟會有點嘅行為－會唔會傳新訊號－就係由「邊種受容體受到啟動」話事嘅。受容體大致上可以按佢哋對下一粒神經細胞嘅影響，分做三大種^[28]：

刺激性（excitatory）會令到下一粒神經細胞射資訊嘅機會率提升、
壓抑性（inhibitory）會令到下一粒神經細胞射資訊嘅機會率下降、同埋
調制性（modulatory）會有第啲長遠效果。

當中穀氨酸（glutamate）同 γ-氨基丁酸（簡稱 GABA）係好重要嘅神經傳遞素。穀氨酸同 γ-氨基丁酸係個腦入面最常見嗰兩種神經傳遞素－腦入面超過 90% 嘅神經細胞都係靠呢兩種神經傳遞素嚟傳達資訊嘅：穀氨酸嘅作用好簡單直接－穀氨酸會對好幾種唔同嘅受容體有作用，並且對其中幾類嘅受容體有刺激性作用，又同對第啲類嘅受容體有調制性作用，例如所謂嘅「閂咗」（OFF）嘅孖極神經細胞同多數神經細胞一樣，俾穀氨酸射到會受刺激，但佢側面嗰啲「開咗」（ON）孖極神經細胞因為用第種受容體，俾穀氨酸射到反而會受壓抑^[29]^[30]；γ-氨基丁酸都會對好幾種受容體有作用，不過佢啲作用（至少喺成年動物當中）冚唪唥都係壓抑性嘅。因為 γ-氨基丁酸咁常見，神經科學家為咗貪方便，有陣時會索性就噉嗌曉釋放 γ-氨基丁酸嘅神經細胞做「壓抑性細胞」^[31]。

電訊號

神經編碼（neural coding）係神經科學一個子領域，研究神經細胞射嘅訊號（signal）點攜帶資訊：定義上，一個訊號係一個物理量，呢個物理量嘅數值 ${\text{s}}$ 同收訊者想知嘅物理量 $x$ 成某啲特定關係， ${\text{s}}=f(x)$ ，所以假如收訊者經已知道 $f$ 呢個函數，佢就可以靠讀取收到嘅訊號嚟得知 $x$ 嘅數值；舉個假想嘅例子說明，一個人工視網膜喺每一個時間點 $t$ 會受到一定強度嘅光（ $I$ ；想知嘅物理量），而塊膜上嘅人工感光細胞嘅物理化學特性令佢哋喺受光刺激嗰陣會產生一個動作電位，個電位嘅電壓值 $V_{t}$ 同光嘅強度成一個固定嘅關係， $V_{t}=\beta I$ ，而 $\beta$ 係一個已知嘅固定數值（訊號特性同物理量定固定關係），個電位跟住會傳上一個人工腦，個腦就可以靠讀取 $V_{t}$ 嘅數值，得知 $I$ 係乜嘢數值（用個已知函數解讀想知嘅物理量嘅數值）^[32]。神經編碼想解答嘅問題如下：由動物嘅外顯行為睇得出^{[註 7]}，神經細胞能夠傳達資訊；噉到底神經訊號嘅邊啲物理量－頻率、電壓值同時間差等－係傳達緊邊啲資訊^[33]^[34]？

神經編碼研究其中一個研究重點係嘗試搵出 ${\text{s}}=f(x)$ 當中嘅 $f$ ，靠噉嚟「解讀」一個腦諗緊乜：舉個假想嘅例子說明，如果而家知道咗人類每逢睇到紅色物件（ $x$ 係物件嘅色水）嗰陣，佢哋嘅視覺皮層（大腦皮層主管視覺資訊嗰忽）啲神經細胞會射頻率係 50 赫茲嘅電訊號（ ${\text{s}}$ ），而且淨係喺個人見到紅色物件嗰陣先會有啲噉嘅訊號（知道咗 $f$ ）；噉下次見到有個人，佢塊視覺皮層出現頻率係 50 赫茲嘅電訊號嗰陣時，就能夠唔使睇第啲嘢都知「呢個人睇到紅色嘅物件」－理論上，科學家將會能夠齋靠睇神經訊號嘅物理特性，就知一隻動物諗緊乜同感受緊乜^[33]^[35]。

事實係，神經編碼研究經已有俾人應用落去腦機介面（brain-computer interface）等嘅技術嗰度。例如科學家可以運用神經科學上嘅量度方法，監察住一個人嘅腦活動變數 $s_{1}$ ，而由打前嘅研究已知 $s_{1}$ 反映嗰個人望緊視野嘅邊一忽（ $s_{1}=f(x)$ ，當中 $x$ 係對眼望緊嘅位置， $f$ 係已知，所以可以由 $s_{1}$ 讀取 $x$ ），再將呢啲量度架生駁落去一部電腦嗰度，教部電腦按 $s_{1}$ 嘅數值決定行乜嘢程式－嗰個人就可以靠控制自己望邊，嚟控制一部電腦嘅運作^[36]^[37]。

膜電勢

一粒神經細胞喺靜止嗰陣都會有個膜電勢（membrane potential）：一粒神經細胞內外有好多唔同種嘅離子，包括鈉離子（Na⁺）、鉀離子（K⁺）、氯離子（Cl^-）同鈣離子（Ca²⁺）呀噉^[38]－離子係一種帶正或者負電荷嘅粒子，所以呢啲離子嘅存在同濃度會影響一個空間入面嘅電壓；各種唔同嘅刺激－包括咗壓力、拉伸、神經傳遞素同浸細胞膜內外嘅電壓改變等等－都會令到神經細胞上嗰啲離子通道打開，然後離子會流過神經細胞浸細胞膜，改變粒神經細胞內外嘅電壓。即係話是但搵粒神經細胞上嘅一點，嗰點嘅電勢 $V_{0}$ 會取決於一啲固定嘅物理量， $V_{0}=f({\text{inputs}})$ ，令神經細胞適合用嚟傳資訊^[39]。

神經細胞浸細胞膜可以大致上噉想像成上圖嘅電路：細胞外嘅空間係 extracellular，而細胞內嘅空間係 intracellular，電容 $C$ 會令電荷積聚，每款離子都有佢專屬嘅離子通道 $g$ ，每個 $g$ 都可以當係一個可變電阻，一旦 $g$ 打開，嗰款離子會流過嗰條通道，而離子量（離子帶有電荷）嘅改變會造成兩個空間嘅電荷比例唔同咗^[40]。

動作電位

動作電位（action potential），又有叫神經脈衝（nerve impulse）或者「突升」（spike），係指一粒神經細胞上某一個位置嘅膜電勢突然猛升猛跌：當粒神經細胞表面一個位置所受嘅電或者化學刺激超過一個臨界點，嗰個位置嘅離子通道開合會造成嗰度嘅電壓突然猛升或者猛跌；當軸突嘅一個位置產生動作電位，會刺激到佢旁邊嘅地方（去極化），進而又再產生動作電位^[41]；就噉樣一忽傳一忽，個動作電位就會好似波浪噉沿住條軸突傳落去。神經細胞就係噉將原本喺細胞體嘅資訊經軸突傳到離開好遠嘅突觸。令個訊號沿住粒細胞傳落去^[42]^[43]。

動作電位係一種「一係有一係冇」（all-or-none）嘅訊號。噉講意思即係話，如果有一粒神經細胞要產生動作電位，佢就一定要完成成個動作電位，唔會話中途停咗得半個，又或者一個得一半大細嘅動作電位－喺呢啲情況下，一個強啲嘅刺激唔會造成一個強啲嘅訊號，而係會令到粒神經細胞射訊號嘅頻率提升（可以睇埋數碼訊號）^[44]^[45]。

一般認為，動作電位嘅時間性特質，例如係頻率同到達時間，係個腦所處理緊嘅資訊嘅所在：

感官細胞會對外界刺激起好多種反應，有啲感官細胞會對穩定嘅刺激起反應，並且以穩定嘅率嚟射訊號；呢啲噉嘅感官細胞喺受到強度更高嘅刺激嗰陣，例如係強度高啲、掁眼啲嘅光，會提升佢哋嘅發射頻率－發射頻率以每秒嘅脈衝數量計嘅話好多時會同個外界刺激形成一個冪函數（power function）^[46]；
另一方面，又有啲感官細胞係所謂嘅間歇性（phasic）感官細胞－佢哋嘅發射頻率喺穩定刺激之下會下降或者停，皮膚就係一個例子－當皮膚俾一嚿物件掂到嗰陣，啲觸覺感官細胞會射訊號，但係如果嚿物件同塊皮膚維持住一股穩定嘅壓力嘅話，噉啲感官細胞會停止唔再射訊號住^[44]。
相比之下，動作電位嘅大細同形狀係由粒神經細胞嘅軸突四圍嘅環境話嗮事嘅－動作電位嘅大細同形狀同粒神經細胞傳緊嘅資訊嘅內容唔啦更^[44]。

分類

神經細胞喺形狀同大細等各方面嗰度都有唔細嘅內部差異，神經科學家可以根據呢啲特徵嚟將佢哋分類。

按輸入輸出

神經細胞可以輸入同輸出係乜分做三大類^[6]^[47]：

感覺神經細胞（sensory neuron），又有叫傳入神經細胞（afferent neuron），專門負責喺啲感覺器官（即係眼耳口鼻等等）嗰度對外界嘅刺激起反應，並且將呢啲刺激帶嘅資訊以電流同化學訊號嘅方式傳達；好多種動物嘅眼入面有感光細胞就係一種感覺神經細胞，會對光有反應，會按受到嘅光嘅特徵決定傳乜嘢款嘅訊號，話俾個腦知佢哋感受到點樣嘅光^[14]^[48]。
運動神經細胞（motor neuron），又有叫傳出神經細胞（efferent neuron），負責由個腦同脊椎接收描述「想要做乜嘢」嘅訊號，並且將資訊傳去身體嘅好多嚿肌肉同腺等嘅部位嗰度，等個腦有得指揮個身體做佢想個身體做嘅嘢；例：腦處理完眼嘅訊號之後，知道環境入面有件障礙物，於是就傳個表達「係噉咦向側面移動，避開障礙物」嘅訊號落肌肉嗰度，令身體作出適當嘅動作^[49]。運動神經細胞有病變往往會搞到身體郁動出問題^[50]。
聯合神經細胞（interneuron）就喺個腦或者脊椎嘅同一個區域入面將啲神經細胞連埋一齊，等佢哋有得一齊做嘢；例如喺「感光細胞傳訊號話俾個腦知感受到乜嘢光」同「運動神經細胞傳訊號指揮肌肉」之間有唔少嘅資訊處理過程，好似係「判斷啲光反映眼前有乜物件」呀噉，呢啲過程往往係由聯合神經細胞負責嘅^[51]。


	輸入	輸出
感覺神經細胞（sensory neuron）	外界嘅物理刺激	描述外界刺激嘅訊號
運動神經細胞（motor neuron）	描述目標行動嘅訊號	傳俾肌肉等組織嘅指令
聯合神經細胞（interneuron）	由第啲神經細胞嚟嘅訊號	傳去第啲神經細胞嗰度嘅訊號

按訊號規律

粒粒神經細胞都有佢自己嘅電性質，好似係「浸細胞膜內外嘅電壓變化嘅規律」等嘅變數可以隨細胞而有異^[52]。所以神經細胞又有得按佢哋嘅電生理特徵嚟分類：

恆久式（tonic / regular spiking）：有啲神經細胞就算唔受乜嘢刺激，都會持續噉以恆定頻率射資訊嘅。
間歇式（phasic / bursting）：有啲神經細胞喺受到某啲特定刺激嗰陣，先會突然猛咁射資訊。
快升式（fast spiking）：有啲神經細胞射起資訊上嚟發射頻率好高，例子有大腦皮層（cerebral cortex；大腦最外面嗰浸）嘅某啲壓抑性聯合神經細胞^[53]。

按結構

按極嘅數量

結構上，大多數嘅神經細胞都屬於^[54]：

單極（unipolar）：得一條渠道嚟收發訊號，係冇脊椎動物體內最常見嘅神經細胞。
孖極（bipolar）：一條軸突同一條樹突，喺眼同鼻等嘅感官入面零舍多。
多極（multipolar）：一條軸突同兩條或者以上嘅樹突。
無軸突（anaxonic）：分唔到邊條係樹突邊條係軸突。
疑似單極（pseudounipolar）：得一條渠道收發訊號，但係嗰條嘢同時係軸突又係樹突。

按軸突長度

生物學家卡米路·高基（Camillo Golgi；高基方法嘅發明者）提倡按軸突嘅長度，將神經細胞分做兩大類^[55]：

軸突長、能夠長距離傳訊號嘅高基-I（Golgi-I），以及係
軸突短－短到一睇可能以為係樹突－嘅高基-II（Golgi-II）。

醫療研究

神經修復

就算到咗廿一世紀初，神經細胞相關疾病依然係一啲難醫嘅病：喺多數情況之下，神經細胞嘅生成係源自喺細路個腦嘅發育－喺呢啲過程當中，某啲特殊嘅幹細胞（stem cell）會變成神經細胞；大人嘅腦好少可會再做細胞分裂，除咗係喺腦嘅某幾個特定部份之外，大人嘅腦唔多會產生新神經細胞，所以大人腦嘅神經細胞一旦萎縮就好難補救。腦退化同柏金遜症等嘅病到咗 2020 年都仲係不治之症^[63]^[64]。

一般嚟講，如果想要修復神經細胞疾病造成嘅破壞，可以由兩大思路去諗：

神經生成（neurogenesis）係指新神經細胞由幹細胞嗰度生出嚟嘅過程。呢種喺成年嘅脊椎動物嘅腦入面好少可會自然噉發生，就算發生都只係限於腦入面嘅某啲部份，而研究顯示，喺新皮層嗰浸神經細胞喺個人出世之前經已形成咗，而且研究亦都表示咗，一粒神經細胞喺功能上冇得由第粒同類細胞取代－呢個諗法就係所謂嘅神經細胞冇得互換定律（law of no inter-changeability of neurons）^[63]。雖然係噉，廿一世紀初有科學家嘅研究發現，有人工方法可以將人體第啲部份嘅幹細胞－好似係皮膚嗰度嘅幹細胞噉－變做新嘅神經細胞。理論上，如果有辦法由一個病人嘅幹細胞嗰度產生神經細胞，再用呢啲神經取代受損嗰啲神經細胞嘅話，就有可能局部醫治神經細胞疾病^[65]。
神經重生（neuroregeneration）指受咗損嘅神經細胞再生同修復。根據現時嘅神經科學知識，呢樣嘢的確係有可能嘅：證據顯示咗，周邊神經系統嘅神經細胞啲軸突如果俾啲嘢砍斷咗，係可以再生嘅^[66]；不過「要點樣先可以令一粒神經細胞自我修復」係一個大問題，所以對於神經重生呢個諗頭，廿一世紀初嘅神經科學界都仲係有啲保留^[67]^[68]。

比較研究

有唔少神經科學家都致力研究唔同動物物種嘅神經細胞之間有乜差異，以及呢啲差異又點樣引致唔同物種喺認知能力等方面嘅差異：絕大多數嘅現存動物物種都有神經細胞，淨係得好似海綿等構造極之簡單嘅動物會冇神經細胞^[69]；喺有神經系統嘅動物之間，唔同動物嘅神經系統喺規模同結構上可以好唔同，而神經系統嘅規模以及結構同嗰種動物嘅行為以及智能有關，例如喺哺乳類動物當中，腦嘅大細就同智能有正相關^[70]^[71]。


物種	全身神經細胞數量	描述
秀麗隱桿綫蟲（C. elegans）	302	一種行為極之簡單嘅動物；每一條蟲有嘅神經細胞數量同神經細胞之間嘅聯繫都一模一樣，所以神經科學家好興攞佢哋嚟做啲簡單嘅研究^[72]。
白蚱（Cnidaria）	≈ 5,600 ^[73]	冇腦，能夠對外界刺激俾反應，但冇乜智能可言^[74]。
黑腹果蠅（Drosophila melanogaster）	≈ 250,000	經已有相當複雜嘅行為；除咗基本嘅維生能力之外，黑腹果蠅公出咗名喺向啲乸求偶嗰陣會進行快速嘅學習，按求偶對象嘅反應調整自己嘅行為^[75]^[76]。
蜜蜂（Apis）	≈ 960,000	具有高度嘅社會性，曉用語言向同伴傳達相當複雜嘅資訊，採蜜嗰時能夠將花嘅位置記住超過一個禮拜，仲識得建築大過自己身體好多嘅結構^[77]。
八爪魚（Octopoda）	≈ 500,000,000 ^[17]	係非人類嘅動物當中首屈一指咁聰明嘅；八爪魚具有長期記憶能力、識得學習、曉運用工具^[78]，喺水族館入面出咗名成日會用各種方法逃走，搞到水族館工作人員好頭痛^[79]。
貓（Felis silvestris catus）	≈ 760,000,000 ^[80]	有複雜嘅行為，有長期記憶能力、識得學習，仲能夠學做各種嘅指令^[81]。
人類（Homo Sapien）	≈ 8.6 × 10¹⁰ ^[3]	能夠建立文明。

人工神經細胞

人工神經細胞（artificial neuron）係廿世紀開始出現嘅一種技術，大致上就係人造嘅人工細胞：一粒人工神經細胞係一個人造嘅物體，會好似動物神經細胞噉接收訊號、並且按收到嘅訊號改變自己內在嘅狀態，跟住再按照自己嘅內在狀態，決定係咪要射新訊號以及要射點樣嘅新訊號，原理上同動物嘅神經細胞極之相似（但唔完全一樣^[82]）。實用嘅研究證實咗，當一柞人工神經細胞結合埋一齊，組成一個網絡嗰陣，個網絡嘅輸出值同輸入值會成某啲特定嘅關係，而呢點表示人工神經網絡可以攞嚟做一啲有用嘅運算^[83]^[84]。

最抽象噉講，一粒簡單嘅人工神經細胞嘅數學模型如下^[83]：

$a_{1}$ 至 $a_{n}$ 係各個輸入嘅數值，啲輸入可以係嚟自外界嘅，但又可以嚟自同一個網絡入面嘅第啲人工神經細胞都得。
$w_{1}$ 至 $w_{n}$ 係每個輸入嘅權重，代表咗嗰一個輸入對呢粒人工神經細胞嘅啟動程度有幾大影響力。
$b$ 指偏向（bias），即係嗰粒神經細胞本身喺啟動上嘅傾向，例如如果有某一粒人工神經細胞嘅 $b$ 係正數而且數值大，佢就會傾向無論輸入係幾多都有強烈嘅啟動^[85]^[86]。
${\text{SUM}}$ ${\text{SUM}}$ （以下簡稱「 $S$ $S$ 」）指粒人工神經細胞嘅啟動程度（activation level），呢個數值由上面嗰柞因素話事，個函數如下：
- $S(T+1)=f(S(T),a(T),\theta _{s})$ ； $[1]$
- $S(T+1)$ 指粒細胞喺時間點 $T+1$ 嘅啟動程度， $S(T)$ 指粒細胞喺時間點 $T$ 嘅啟動程度，而 $a(T)$ 指喺時間點 $T$ 嗰柞輸入嘅數值。呢條式表示， $S(T+1)$ 嘅值由 $S(T)$ 、 $a(T)$ 、同埋 $\theta _{s}$ 呢三個數值話事。
- 好多人工神經細胞嘅啟動函數會包含 $\theta _{s}$ 呢個數值。呢個係所謂嘅門檻值（threshold）：一粒典型嘅動物神經細胞係一定要所受到嘅刺激有返咁上下勁，先會射訊號嘅；如果佢受嘅刺激數值唔超過門檻值嘅話，佢就乜嘢訊號都唔會射；為咗要更加接近動物神經細胞，好多研究人員喺設計神經網絡嗰時會特登每粒人工神經細胞俾個門檻值佢，令個網絡嘅各粒人工神經細胞好似動物神經細胞噉，收到嘅輸入唔超過門檻值就完全唔射訊號。
$t$ 係粒人工神經細胞嘅輸出數值。
$f$ 係 $S$ 同埋 $t$ 之間嗰個函數，可以有多個款，但 $f$ 好多時會係一個恆等函數（identity function）－即係話 $t$ 數值好多時同 $S$ 一樣^[83]。

好似上述嗰個噉嘅模型係抽象模型：理論上，只要有件物體會做出個模型所描述嘅行為－無論件物體係用血肉造定用矽等嘅物料造嘅都好－呢件物體都符合「神經細胞」嘅定義；於是喺廿一世紀初，有科學家開始嘗試用人工物體模擬神經細胞嘅行為，整一啲曉好似生物神經細胞噉收同射電訊號嘅人造細胞，而且呢啲人工細胞嘅輸入同輸出之間嘅函數同生物神經細胞嘅一樣，打後少少仲有研究將呢啲噉樣嘅人工神經細胞駁落去生物神經細胞嗰度，並且成功令到啲人工細胞同啲生物神經細胞一齊組成一個行得到嘅簡單網絡（睇埋人工神經網絡）^[87]。

有別於生物神經細胞模型

不過，人工神經細胞有別於生物神經細胞模型（biological neuron model）：生物神經細胞模型指嘅係一啲描述神經細胞嘅數學模型，力求嘅係準確噉描述神經細胞嘅行為，例子有正話提到，將神經細胞表面想像成電路嘅模型－呢啲模型係純科學嘅產物，以準確描述現實為目標；相比之下，人工神經細胞係受生物神經細胞啟發嘅一種模型，並唔追求準確噉描述生物神經細胞嘅行為，例如生物神經細胞好少可會好似人工神經細胞噉，清楚噉分做一層層而且每層都淨係受打前嗰層嘅細胞影響^[88]^[89]－人工神經細胞係工程學嘅產物，整咗個數學模型出嚟，唔要求個模型同現實世界嘅某啲事物一樣，而如果一個唔真實嘅模型比較能夠有效率噉做到設計者想佢做嘅工作，設計者會選用唔真實嗰個模型^[90]。

註釋

↑ 粒細胞俾研究者用染料上咗色。
↑ 成個過程發生得好快，快到個人唔使有意識噉去諗呢樣嘢。
↑ 有關呢啲同電相關嘅概念嘅詳情，可以睇吓古典電磁學。
↑ 例如「神經細胞會用電傳訊號」呢點可以用微細嘅電極做量度得知。
↑ 喺呢幾幅相當中，研究者所用嘅染色方法唔同，所以啲細胞望落色水唔同。
↑ 順帶一提，一般軸突都要用顯微鏡先會睇到，不過魷魚嘅神經細胞軸突（睇魷魚巨型軸突）直徑可以有成 1.5 毫米咁粗。
↑ 例如動物曉對光等嘅刺激俾反應，而如果神經細胞受破壞，動物做呢啲行為嘅能力會下降。

睇埋

文獻

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[1] 粒細胞俾研究者用染料上咗色。

[2] 成個過程發生得好快，快到個人唔使有意識噉去諗呢樣嘢。

[10] 有關呢啲同電相關嘅概念嘅詳情，可以睇吓古典電磁學。

[16] 例如「神經細胞會用電傳訊號」呢點可以用微細嘅電極做量度得知。

[20] 喺呢幾幅相當中，研究者所用嘅染色方法唔同，所以啲細胞望落色水唔同。

[26] 順帶一提，一般軸突都要用顯微鏡先會睇到，不過魷魚嘅神經細胞軸突（睇魷魚巨型軸突）直徑可以有成 1.5 毫米咁粗。

[39] 例如動物曉對光等嘅刺激俾反應，而如果神經細胞受破壞，動物做呢啲行為嘅能力會下降。

[3] Rutecki, PA (1992). "Neuronal excitability: voltage-dependent currents and synaptic transmission". Journal of Clinical Neurophysiology. 9 (2): 195–211.

[4] The Nervous System.

[humanbrainnumber-5] 3.0 ^3.1 How many neurons make a human brain? Billions fewer than we thought. The Guardian.

[6] Azevedo, Frederico A.C.; Carvalho, Ludmila R.B.; Grinberg, Lea T.; Farfel, José Marcelo; Ferretti, Renata E.L.; Leite, Renata E.P.; Filho, Wilson Jacob; Lent, Roberto; Herculano-Houzel, Suzana (2009). "Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain". The Journal of Comparative Neurology. 513 (5): 532–541.

[7] Speed of Nerve Impulses. The Physics Factbook.

[:0-8] 6.0 ^6.1 ^6.2 ^6.3 ^6.4 Kandel E.R., Schwartz, J.H., Jessell, T.M. 2000. Principles of Neural Science, 4th ed., McGraw-Hill, New York.

[9] Parton, R. G., Simons, K., & Dotti, C. G. (1992). Axonal and dendritic endocytic pathways in cultured neurons. The Journal of cell biology, 119(1), 123-137.

[11] Herz, A. V., Gollisch, T., Machens, C. K., & Jaeger, D. (2006). Modeling single-neuron dynamics and computations: a balance of detail and abstraction. Science, 314(5796), 80-85.

[12] Kole, M. H., Ilschner, S. U., Kampa, B. M., Williams, S. R., Ruben, P. C., & Stuart, G. J. (2008). Action potential generation requires a high sodium channel density in the axon initial segment. Nature neuroscience, 11(2), 178-186.

[13] Know Your Neurons: The Discovery and Naming of the Neuron. Scientific American.

[14] Costandi, M. (2006). The discovery of the neuron. Neurophilosopy.

[15] Shepherd, G. M. (2015). Foundations of the neuron doctrine. Oxford University Press.

[17] What is a neuron?. Queensland Brain Institute.

[chen2004-18] 14.0 ^14.1 Chen, A. H., Zhou, Y., Gong, H. Q., & Liang, P. J. (2004). Firing rates and dynamic correlated activities of ganglion cells both contribute to retinal information processing. Brain research, 1017(1-2), 13-20.

[19] Brazier, MAB (1961). A History of the Electrical Activity of the Brain. London: Pitman.

[21] Davies, Melissa (2002-04-09). "The Neuron: size comparison". Neuroscience: A journey through the brain.

[octo-22] 17.0 ^17.1 Chudler, Eric H. "Brain Facts and Figures". Neuroscience for Kids.

[23] Löhrke S, Brandstätter JH, Boycott BB, Peichl L (April 1995). "Expression of neurofilament proteins by horizontal cells in the rabbit retina varies with retinal location". Journal of Neurocytology. 24 (4): 283–300.

[24] Hawkins, J., & Ahmad, S. (2016). Why neurons have thousands of synapses, a theory of sequence memory in neocortex. Frontiers in neural circuits, 10, 23.

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