سيال عصبي

Figure 1. A. view of an idealized action potential shows its various phases as the action potential passes a point on a cell membrane. B. Recordings of action potentials are often distorted compared to the schematic view because of variations in electrophysiological techniques used to make the recording.

السيال العصبي أو النبض العصبي (Nerve Impulse) ، هو عملية نقل المعلومات أو النبضات العصبية داخل الأعصاب. وتتم عملية النقل إما بواسطة كهربائية (أنظر كمون الفعل) أو عن طريق التفاعلات الكيماوية بين الأعصاب. وإن سرعة السيال العصبي في الأعصاب تقدر بـ 120 متراً بالثانية أي ما يعادل 432 كم في الساعة.

يتكون الجهاز العصبي من خلايا منفردة، تتعاون معاً لإنجاز وظائف معقدة، وتدعى هذه الخلايا العصبونات. ان السيال العصبي هو اللغة الوحيدة التي تتفاهم بها العصبونات والشكل الذي تترجم إليه أنواع المؤثرات جميعها التي تؤثر في الجسم. ينتقل السيال من خلية عصبية لاخرى من خلال الروابط الفسيحة (gab junction) والتي هي عبارة عن قنوات دقيقة تسمح بسريان التيار من خلالها مباشرة وتكون هذه الطريقة أسرع مقارنة بالانتقال عبر التشابك الكيمياوي.

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السياق الطبيعي الحيوي والخلوي

الأيونات والقوى التي تقود حركتهم

الأيونات (الدوائر الوردية) ستنساب عبر غشاء من التركيز العالي إلى التركيز المنخفض، مسببة تياراً. However, this creates a voltage across the membrane that opposes the ions' motion. When this voltage reaches the equilibrium value, the two balance and the flow of ions stops.^[1]

غشاء الخلية الكاره للماء يمنع الجزيئات المشحونة من الانتشار بسهولة خلاله، مما يتيح فرق potential لأن يتواجد عبر الغشاء.

غشاء الخلية

Despite the small differences in their radii,^[2] ions rarely go through the "wrong" channel. For example, sodium or calcium ions rarely pass through a potassium channel.

Membrane potential

قنوات الأيونات

All-atom figure of the open potassium channel, with the potassium ion shown in purple in the middle. When the channel is closed, the passage is blocked.

مضخات الأيونات

Resting potential

E_{eq,K^{+}}={\frac {RT}{zF}}\ln {\frac {[K^{+}]_{o}}{[K^{+}]_{i}}},

حيث

E_eq,K⁺ is the equilibrium potential for potassium, measured in volts
R is the universal gas constant, equal to 8.314 joules·K^-1·mol^-1
T is the absolute temperature, measured in kelvins (= K = degrees Celsius + 273.15)
z is the number of elementary charges of the ion in question involved in the reaction
F is the Faraday constant, equal to 96,485 coulombs·mol^-1 or J·V^-1·mol^-1
[K⁺]_o is the extracellular concentration of potassium, measured in mol·m^-3 or mmol·l^-1
[K⁺]_i is the intracellular concentration of potassium

E_{m}={\frac {RT}{F}}\ln {\left({\frac {P_{\mathrm {K} }[\mathrm {K} ^{+}]_{\mathrm {out} }+P_{\mathrm {Na} }[\mathrm {Na} ^{+}]_{\mathrm {out} }+P_{\mathrm {Cl} }[\mathrm {Cl} ^{-}]_{\mathrm {in} }}{P_{\mathrm {K} }[\mathrm {K} ^{+}]_{\mathrm {in} }+P_{\mathrm {Na} }[\mathrm {Na} ^{+}]_{\mathrm {in} }+P_{\mathrm {Cl} }[\mathrm {Cl} ^{-}]_{\mathrm {out} }}}\right)}

Action potentials arriving at the synapses of the upper right neuron stimulate currents in its dendrites; these currents depolarize the membrane at its axon hillock, provoking an action potential that propagates down the axon to its synaptic knobs, releasing neurotransmitter and stimulating the post-synaptic neuron (lower left).

تشريح العصب

Initiation

Neurotransmission

الخلايا العصبية الحسية

Pacemaker potentials

In pacemaker potentials, the cell spontaneously depolarizes (straight line with upward slope) until it fires an action potential.

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الأطوار

Stimulation and rising phase

طور القمة والسقوط

Hyperpolarization ("undershoot")

Refractory period

الانتشار

In saltatory conduction, an action potential at one node of Ranvier causes inwards currents that depolarize the membrane at the next node, provoking a new action potential there; the action potential appears to "hop" from node to node.

Myelin and saltatory conduction

Comparison of the conduction velocities of myelinated and unmyelinated axons in the cat.^[3] The conduction velocity v of myelinated neurons varies roughly linearly with axon diameter d (that is, v ∝ d),^[4] whereas the speed of unmyelinated neurons varies roughly as the square root (v ∝√ d).^[5] The red and blue curves are fits of experimental data, whereas the dotted lines are their theoretical extrapolations.

نظرية الكابل

مقالة مفصلة: نظرية الكابل

Figure.1: Cable theory's simplified view of a neuronal fiber. The connected RC circuits correspond to adjacent segments of a passive neurite. The extracellular resistances r_e (the counterparts of the intracellular resistances r_i) are not shown, since they are usually negligibly small; the extracellular medium may be assumed to have the same voltage everywhere.

\tau {\frac {\partial V}{\partial t}}=\lambda ^{2}{\frac {\partial ^{2}V}{\partial x^{2}}}-V

where V(x, t) is the voltage across the membrane at a time t and a position x along the length of the neuron, and where λ and τ are the characteristic length and time scales on which those voltages decay in response to a stimulus. Referring to the circuit diagram above, these scales can be determined from the resistances and capacitances per unit length^[6]

\tau =\ r_{m}c_{m}

\lambda ={\sqrt {\frac {r_{m}}{r_{l}}}}

Termination

Chemical synapses

Electrical synapses between excitable cells allow ions to pass directly from one cell to another, and are much faster than chemical synapses.

Electrical synapses

Neuromuscular junctions

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أنواع أخرى من الخلايا

Cardiac action potentials

Phases of a cardiac action potential. The sharp rise in voltage ("0") corresponds to the influx of sodium ions, whereas the two decays ("1" and "3", respectively) correspond to the sodium-channel inactivation and the repolarizing eflux of potassium ions. The characteristic plateau ("2") results from the opening of voltage-sensitive calcium channels.

Muscular action potentials

Plant action potentials

التوزيع التصنيفي والمزايا التطورية

Comparison of action potentials (APs) from a representative cross-section of animals^[7]
Animal	Cell type	Resting potential (mV)	AP increase (mV)	AP duration (ms)	Conduction speed (m/s)
Squid (Loligo)	Giant axon	−60	120	0.75	35
Earthworm (Lumbricus)	Median giant fiber	−70	100	1.0	30
Cockroach (Periplaneta)	Giant fiber	−70	80–104	0.4	10
Frog (Rana)	Sciatic nerve axon	−60 to −80	110–130	1.0	7–30
Cat (Felis)	Spinal motor neuron	−55 to −80	80–110	1–1.5	30–120

طرق تجريبية

The giant axons of the European squid (Loligo vulgaris) were crucial for scientists to understand the action potential.

As revealed by a patch clamp electrode, an ion channel has two states: open (high conductance) and closed (low conductance).

الذيفانات العصبية

Tetrodotoxin is a lethal toxin from the pufferfish that inhibits the voltage-sensitive sodium channel, halting action potentials.

التاريخ

صورة لإثنان من Purkinje cells (معنونين A) رسمها سانتياگو رامون إ كخال. Large trees of dendrites feed into the soma, from which a single axon emerges and moves generally downwards with a few branch points. The smaller cells labeled B are granule cells.

Ribbon diagram of the sodium–potassium pump in its E2-Pi state. The estimated boundaries of the lipid bilayer are shown as blue (intracellular) and red (extracellular) planes.

نماذج كمية

Equivalent electrical circuit for the Hodgkin–Huxley model of the action potential. I_m and V_m represent the current through, and the voltage across, a small patch of membrane, respectively. The C_m represents the capacitance of the membrane patch, whereas the four g's represent the conductances of four types of ions. The two conductances on the left, for potassium (K) and sodium (Na), are shown with arrows to indicate that they can vary with the applied voltage, corresponding to the voltage-sensitive ion channels. The two conductances on the right help determine the resting membrane potential.

انظر أيضا

قالب:Neuroscience portal

ملاحظات

المصادر

^ Campbell Biology, 6th edition
^ CRC Handbook of Chemistry and Physics, 83rd edition, ISBN 0-8493-0483-0, pp. 12–14 to 12–16.
^ Schmidt-Nielsen, Figure 12.13.
^ خطأ استشهاد: وسم <ref> غير صحيح؛ لا نص تم توفيره للمراجع المسماة hursh_1939
^ Rushton WAH (1951). "A theory of the effects of fibre size in the medullated nerve". Journal of Physiology. 115: 101–22.
^ Purves et al., pp. 52–53.
^ Bullock TH, Horridge GA (1965). Structure and Function in the Nervous Systems of Invertebrates. San Francisco: W. H. Freeman.

ببليوگرافيا

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Deutsch S, Micheli-Tzanakou E (1987). Neuroelectric Systems. New York: New York University Press. ISBN 0-8147-1782-9.
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Stevens CF (1966). Neurophysiology: A Primer. New York: John Wiley and Sons.LCCN 66-15872.