第二章 细胞的基本功能
2011-06-08 18:12:43   来源：   作者：  评论：0 点击：

Summary
This chapter describes the basic information of all kinds of tissue cells, which are the important foundation for understanding physiology. It's content includes: 1.the structure of cell membrane and transportation of substances through cell membrane; 2.electrical phenomenon of the cells; 3.signal transduction; 4.muscular contraction.
The mammalian cell membrane is composed of two layers of lipid in which protein molecules are embedded. Lipid composition of the cell membrane acts as a barrier, by which cell membrane limits transmembrane movement for most of molecules inside and outside of the cell. Some of the proteins in the cell membrane, however, form structures that permit transmembrane movement for some of water-soluble molecules. The cell membrane is therefore, named semipermeable, through which different kinds of substances pass across in different ways. Lipid-soluble molecules moving freely across the cell membrane down its concentration gradient called simple diffusion. Most of molecules inside and outside of cells, however, can not cross membrane without assistance. Two kinds of proteins in the cell membrane called channels and carriers provide permeability for those water-soluble substances, through which ions and glucose/amino acid pass across membrane. Those two kinds of transmembrane movement called facilitated diffusion. In some situations, the molecules pass though the membrane against the concentration gradient called active transport. The energy derived from ATP is necessary for this process, and the protein involved in active transport named pump. If the molecules are bigger, they can not cross the membrane through the channel or carrier, and those substances get into or out of the cell through even more complicated mechanisms called exocytosis/endocytosis.
Signal transduction refers to the processes by which intercellular signals, such as neurotransmitters, hormones, neurotrophic factors, and cytokines are converted into biochemical signals within cells and in turn modify cellular function in different ways. Four general patterns of signal transductions occur in almost all mammalian cells. One pattern is involved a special kind of membrane receptor that coupled with guanine nucleotide-binding proteins or G proteins, binding of ligands to these receptors initiates receptor-G protein interactions that produce a range of biological effects on target cells. The main effect is to trigger the complex cascade of intracellular messengers that lead to the generation of second messengers and the regulation of protein phosphorylation, and ultimately to produce physiologic response adapted to extracellular stimuli. Protein phosphorylation seems to be the final common pathway in the regulation of cellular function. A second pattern is characterized by direct activation of a class of protein kinase called tyrosine protein kinases. Binding of ligand to these receptors triggers cascade of further phosphorylation and lead to activate MAPK, which is involved in regulation of the process of gene expression. A third pattern of signal transduction is mediated by ligand-gated ion channels or receptor ionophores. In response to the binding of transmitters, the receptor undergoes a conformational change, opening the gate and allowing ions to diffuse along their concentration gradient and lead to the change of membrane potential on the target cell. A fourth pattern, which characterized by activation receptors that locate inside of the cells. After bounding to hormones, those receptors translocated to the nucleus, where they binding DNA and function as transcription factors and regulate the gene expression.
The plasma membrane of all excitable cells exhibit a small difference in electrical charges between inside and outside of the cell called the membrane potential, including resting potential and action potential. In resting state and without stimulation, cells maintain a negative electrical potential inside in relative to outside. Two characteristics of cells contribute to their ability to maintain this electrical potential. First, the cell membrane is differently permeable to ions, in resting state all cells are highly permeable to K+, and relatively impermeable to other ions. Second, different types of ions are unequally distributed across the cell membrane. Generally, there are higher concentration of K+,P－and lower concentration Na+, Cl－ inside of the cell than they are in outside. Taken together, K+ would flow down its concentration gradient from inside to outside of the cell, the positive charges in this way accumulated outside of the cell membrane because the P－ cannot cross the membrane in company with the K+, and the electrical membrane potential developed in this process. The net movement of K+ between inside and outside of the cell membrane stops until the electrical force repelling K+ of further flowing equals to the force of the concentration gradient, at this point K+ has reached its equilibrium potential (Ek), which can be estimated with the Nernst equation. Action potential is a rapid reversal change of the membrane potential which can be propagated over the surface of the cell. At the peak of action potential, the membrane potential become zero or even positive quite close to the equilibrium of ENa. Before the cell generation of the action potential, membrane potential must first decrease (depolarization) to reach a special value called threshold potential, in which the permeability of Na+ increase rapidly and in turn triggers the action potential. However, the increase of the Na+ conductance maintain quite a short time (1～2ms), and the K+ channel opened again with the membrane depolarization. Both of the factors contribute to the process of the returning potential to its resting value. All action potentials in a given cell are the same size regardless of their amplitude of stimulus, this phenomenon is called all-or-none rule. During the course of a spike, the cell become completely inexcitable, that means the cell will not fire again no matter how strong the stimulus is.
Muscles can be divided into two groups, striated and smooth, based on their appearance under light microscope. Striated muscle is characterized by the regular striated seen under the microscope including skeletal muscle that response for the body movement and cardiac muscle that response for the pumping action of the heart. Muscle cell consists of bundless of still smaller fibers called myofibrils. Under electrical microscope, myofibrils can be seen to consist of two kinds of longitudinally oriented filaments called thick and thin filaments. The thick filaments are aggregated of the protein called myosin, and the myosin molecule containing an ATP splitting enzyme (ATPase) swings out from the thick filament and this extension is called cross-bridge. The thin filaments are largely made up of the protein actin. The basic unit of contraction of muscle is sarcomere, and it is a special structure between two Z lines. The excitation of muscle cell is resulted from the excitatory transmission through nerve-muscle junction and sequent generation of action potential of the muscle cell. This action potential initiate contraction of the cell by the process of excitation-contraction coupling, in which the elevation of Ca2+ is the critical factor to trigger muscle contraction. In the process of contraction, neither the thick or thin filament change in their length, rather, shorting occurs because the thick filament pull the thin filament pass them, on the other hand, the thin filament slide between thick filaments towards to middle line of the sacomere. Force developed during the contraction is due to the interaction of thick and thin filaments and can be affected by different factors, including initial length which is the length before muscle contraction. The maximum force can be produced if the muscle reaches a special length called optimal initial length before contraction. The mechanism underlying this phenomenon is the maximum overlap between thick and thin filaments occur and almost all cross-bridge are involved in the interaction of thick and thin filaments, ultimately lead to creating of maximum force.

参考文献
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复习思考题
1． 试述细胞膜中脂质和蛋白质各自的功用。
2． 物质被动跨膜转运的方式有哪几种？各有何特点？
3． 比较物质被动转运和主动转运的异同。
4． 跨膜信号转导包括哪几种方式？
5． G蛋白耦联受体介导的信号转导包括几种方式？G蛋白在跨膜信号转导过程中发挥何种作用？
6． 试述静息电位的形成原理，列举实验证据说明静息电位相当于K+的平衡电位。
7． 试述动作电位的形成机制，列举实验证据说明锋电位相当于Na+的平衡电位。
8． 动作电位产生的条件是什么？为什么刺激必须使细胞去极化达到阈电位才能产生动作电位？
9． 局部兴奋有何特征？
10． 刺激运动神经会引起骨骼肌的收缩，列举每一个环节并论述其机制。
11． 前负荷和初长度如何影响骨骼肌收缩？