Diffusion
• Temperature-dependent random movement of ions and molecules tends to distribute the ions evenly throughout the solution so that there is a net movement of ions from regions of high concentration to regions of low concentration.
• Generating a steady electrical potential difference across a membrane requires
– An ionic concentration gradient – Selective ionic permeability
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Before moving on to neurons, four important points should be made:
Each of the different polypeptides contributing to a protein with quaternary structure is called a subunit (亚基).
1.4 Channel Proteins
• Channel protein is suspended in a phospholipid bilayer, with its
• The protein channels can be highly selective for specific ions. (离子通道对离子具有高度的选择性)
• The movement of any ion through channel depends on the concentration gradient and the difference in electrical potential across the membrane. (离子的跨膜运动依赖于膜两侧的浓度梯度和电位差)
Electrical current flow across a membrane
• Driving an ion across the membrane electrically requires
– The membrane possesses channels permeable to the ions – There is a electrical potential difference across the membrane
Electricity
• Two important factors determine how much current (I) will flow:
– Electrical potential (V, 电势) – Electrical conductance (g, 电导)
• Electrical conductance Electrical resistance (电阻, R=1/g) Ohm’s law: I = gV
• Ion selectivity (离子选择性) – The diameter of the pore – The nature of the R groups lining it
• Gating (门控特性) – Channels with this property can be opened and closed-gated by changes in the local microenvironment of the membrane:
– hydrophobic (疏水的) portion inside the membrane
– hydrophilic (亲水的) ends exposed to the watery environments on either side
A Membrane Ion Channel
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Two Properties of Ion Channels
Vm = - 80 mV
Before moving on to situation in real neurons, four important points should be made:
② The net difference in electrical charge occurs at the inside and outside surfaces of the membrane (膜内外两侧电荷的不同 仅仅分布于膜的内外侧 面，而不是分布于整个 细胞的内外液)
Ion channel and Neuronal Electric Activities
王志举 Dept. of Physiology School of basic medical sciences
1. Neuronal membrane
1.1 Cytosol and Extracellular Fluid
1.6 THE MOVEMENT OF IONS
• A channel across a membrane is like a bridge across a river.
• An open channel A net movement of ions across the membrane.
– The membrane possesses channels permeable to the ions
– There is a concentration gradient across the membrane
Electricity
• Another way to induce a net movement of ions in a solution is to use an electrical field (电场), because ions are electrically charged particles.
① Large changes in membrane potential are caused by minuscule changes in ionic concentrations (仅需要微小的离子浓 度改变就可以引起膜 电位大幅度的变化)
100 mM Vm = 0 ement requires that external forces be applied to drive ions across.
• Two factors influence ion movement through channels:
– Diffusion (扩散) – Electricity (电势差)
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1.3 Protein
• These proteins provide routes for ions to cross the neuronal membrane. •The resting and action potentials depend on special proteins that span the phospholipid bilayer.
2. THE IONIC BASIS OF THE RESTING MEMBRANE POTENTIAL (静息电位)
• 2.1 The membrane potential (膜电位) is the voltage across the neuronal membrane at any moment, represented by the symbol mV or Vm (membrane potential ).
1.2 The Phospholipid Membrane (磷脂膜)
The lipids of the neuronal membrane forming:
a barrier to water-soluble ions a barrier to water
头端-极性磷酸盐-亲水
尾端-非极性碳氢化合物 -疏水
• Microelectrode (微电极) and mV measurement
2.2 Establishing Equilibrium Potential
20 :1
No potential difference Vm = 0 mV
The diffusional force = The electrical force
This movement is called diffusion (扩散).
• A difference in concentration is called a concentration gradient (浓度梯度).
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Diffusion
• Driving ions across the membrane by diffusion happens when
• Opposite charges attract and like charges repel.
The movement of ions influenced by an electrical field
Opposite charges attract and like charges repel
Protein – Amino Acids
The Peptide Bond (肽键) and a Polypeptide (多肽)
Protein Structure
The tertiary structure
The primary structure The secondary structure The quaternary structure
• Chemically-gated channel • Voltage-gated channel • Mechanical-gated channel