Gated Ion Channels:- Voltage-Gated Ion Channels Produce Neuronal Action Potentials

Signaling in the nervous system is accomplished by networks of neurons, specialized cells that carry an electrical impulse (action potential) from one end of the cell (the cell body) through an elongated cytoplasmic extension (the axon). The electrical signal triggers release of neurotransmitter molecules at the synapse, carrying the signal to the next cell in the circuit. Three types of voltage-gated ion channels are essential to this signaling mechanism. Along the entire length of the axon are voltage-gated Na⁺ channels (Fig. 12–5; see also Fig. 11–50), which are closed when the membrane is at rest (Vm=-60mV) but open briefly when the membrane is depolarized locally in response to acetyl choline (or some other neurotransmitter). The depolarization induced by the opening of Na channels causes voltage-gated K⁺ channels to open, and the resulting efflux of K⁺ repolarizes the membrane locally.

A brief pulse of depolarization traverses the axon as local depolarization triggers the brief opening of neigh boring Na channels, then K channels. After each opening of a Na channel, a short refractory period follows during which that channel cannot open again, and thus a unidirectional wave of depolarization sweeps from the nerve cell body toward the end of the axon. The voltage sensitivity of ion channels is due to the presence at critical positions in the channel protein of charged amino acid side chains that interact with the electric field across the membrane. Changes in trans membrane potential produce subtle conformational changes in the channel protein. At the distal tip of the axon are voltage-gated Ca²⁺ channels. When the wave of depolarization reaches these channels, they open, and Ca²⁺ enters from the extracellular space. The rise in cytoplasmic [Ca²⁺] then triggers release of acetylcholine by exocytosis into the synaptic cleft (step 3 in Fig. 12–5). Acetylcholine diffuses to the postsynaptic cell (another neuron or a myocyte), where it binds to acetylcholine receptors and triggers depolarization. Thus, the message is passed to the next cell in the circuit. We see, then, that gated ion channels convey signals in either of two ways: by changing the cytosolic con centration of an ion (such as Ca²⁺), which then serves as an intracellular second messenger (the hormone or neurotransmitter is the first messenger), or by changing V_m and affecting other membrane proteins that are sensitive to V_m. The passage of an electrical signal through one neuron and on to the next illustrates both types of mechanism.

FIGURE 12–5 Role of voltage-gated and ligand-gated ion channels in neural transmission. Initially, the plasma membrane of the pre synaptic neuron is polarized (inside negative) through the action of the electrogenic Na+ K+ ATPase, which pumps 3 Na out for every 2 K⁺ pumped into the neuron (see Fig. 12–3). 1 A stimulus to this neuron causes an action potential to move along the axon (white arrow), away from the cell body. The opening of one voltage-gated Na⁺ channel allows Na⁺ entry, and the resulting local depolarization causes the adjacent Na⁺ channel to open, and so on. The directionality of movement of the action potential is ensured by the brief refractory period that follows the opening of each voltage-gated Na channel. 2 When the wave of depolarization reaches the axon tip, voltage gated Ca²⁺ channels open, allowing Ca²⁺ entry into the presynaptic neuron. 3 The resulting increase in internal [Ca²⁺] triggers exocytic release of the neurotransmitter acetylcholine into the synaptic cleft. 4 Acetylcholine binds to a receptor on the postsynaptic neuron, causing its ligand-gated ion channel to open. 5 Extracellular Na and Ca²⁺ enter through this channel, depolarizing the postsynaptic cell. The electrical signal has thus passed to the cell body of the post synaptic neuron and will move along its axon to a third neuron by this same sequence of events.

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بمشاركة طالبات دورة (ينابيع الرحمة).. شعبة السيدة فاطمة بنت أسد تحيي ذكرى زواج النورين (عليهما السلام)

قسم شؤون المعارف يوضح مراحل الإخراج الطباعي لإصدارات مركز تراث الجنوب

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قسم الشؤون الفكرية وجمعية العميد يدعوان المهتمّين لحضور فعّاليات مسابقة العميد البحثية الأولى

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طريقة مبتكرة للكشف المبكر عن أمراض القلب والشرايين

منظمة الصحة تحذر من خطر إدمان الشباب لأكياس النيكوتين

خبراء يحذرون من عادة شائعة تهدد صحة الملايين

خبير يكشف 5 حقائق طبية صادمة وغير معروفة

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الأولى في العالم.. الصين تجري تجربة رائدة في الفضاء باستخدام "جنين اصطناعي"

دراسة حديثة تفند ربط التستوستيرون بالمخاطرة لدى الرجال

ميتا تضيف وضع "المحادثة المخفية" إلى تطبيق "واتساب"

فوبيا المرتفعات ليست من الدماغ.. دراسة تكشف السبب

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مواضيع ذات صلة

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Receptor Enzymes:- Receptor Guanylyl Cyclases Generate the Second Messenger cGMP

Receptor Enzymes: -The Insulin Receptor Is a Tyrosine-Specific Protein Kinase

Gated Ion Channels:- Neurons Have Receptor Channels That Respond to Different Neurotransmitters

Gated Ion Channels:- Ion Channels Underlie Electrical Signaling in Excitable Cells

Solute Transport across Membranes:- Defective Ion Channels Can Have Adverse Physiological Consequences

Solute Transport across Membranes:- The Acetylcholine Receptor Is a Ligand-Gated Ion Channel

Solute Transport across Membranes:- The Neuronal Na+ Channel Is a Voltage-Gated Ion Channel

Solute Transport across Membranes: -The Structure of a K+ Channel Reveals the Basis for Its Specificity

Solute Transport across Membranes:- Ion-Channel Function Is Measured Electrically

Solute Transport across Membranes:- Ion-Selective Channels Allow Rapid Movement of Ions across Membranes