Understanding Nerve Cells: Structure and Function

1. 38 The Nervous System 0

2. Chapter 38 At a Glance • 38.1 What Are the Structures and Functions of Nerve Cells? • 38.2 How Do Neurons Produce and Transmit 发送 Information? • 38.3 How Does the Nervous System Process 处理Information and Control Behavior? • 38.4 How Are Nervous Systems Organized? • 38.5 What Are the Structures and Functions of the Human Nervous System?

3. 38.1 What Are the Structures and Functions of Nerve Cells? • The nervous system has two principal 主要的 cell types 1. Neurons, often called nerve cells, which receive, process, and transmit information 2. Glia, which assist neuronal function by • Providing nutrients 养分 • Regulating the composition 组成 of the extracellular fluid 细胞外液 in the brain and spinal cord • Modulating 调整 communication between neurons • Speeding up the movement of electrical signals 电讯号within neurons

4. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell • Typical neurons have four distinct parts that carry out these four functions 1. Dendrites 树突 2. A cell body 3. An axon 轴突 4. Synaptic terminals 突触末端

5. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell (continued) • A neuron must perform four functions 1. Receive information from the environment 2. Process the information and produce electrical signals 3. Conduct 传导 electrical signals over distances to a junction where it meets another cell 4. Transmit information to other neurons, muscles, or glands 腺体

6. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell (continued) • Dendrites respond to stimuli 刺激 • Dendrites are branched 有枝的 tendrils 卷须状物protruding 突出 from the cell body that perform the “receive information” function • Their branches provide a large surface area for receiving signals, either from the environment or from other neurons

7. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell (continued) • Dendrites respond to stimuli (continued) • Dendrites of sensory neurons 感觉神经元 respond to specific stimuli, such as pressure, odor气味, light, body temperature, blood pH, or the position of a joint 关节 • Dendrites of neurons in the brain and spinal cord usually respond to chemicals, called neurotransmitters 神经递质, that are released by other neurons

8. Table 38-1 Some Important Neurotransmitters

9. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell (continued) • The cell body processes signals from the dendrites • Electrical signals travel down the dendrites and converge 聚集 on the neuron’s cell body, which integrates 整合 incoming information, performing the “process information” function • If incoming signals are positive enough, a large, rapid electrical signal called an action potential 动作电位 is produced

10. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell (continued) • The cell body processes signals from the dendrites (continued) • The cell body also contains other organelles 细胞器such as the nucleus 细胞核, endoplasmic reticulum 内质网, and Golgi apparatus 高尔基体 that are typical of other cells, synthesizing complex molecules and coordinating 协调 the cell’s metabolism 代谢作用

11. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell (continued) • The axon conducts action potentials long distances • In a typical neuron, a long, thin strand called an axon extends outward from the cell body and conducts action potentials from the cell body to synaptic terminals at the axon’s end

12. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell (continued) • The axon conducts action potentials long distances (continued) • Single axons may stretch 延伸 from our spinal cord to our toes, a distance of about 3 feet • Axons are typically bundled 捆 together into nerves, much like wires are bundled within an electrical cable 电缆

13. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell (continued) • At synapses, signals are transmitted from one cell to another • The site where a neuron communicates with another cell, or innervates 支配, is called a synapse

14. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell (continued) • At synapses, signals are transmitted from one cell to another (continued) • A typical synapse consists of 1. The synaptic terminal , which is a swelling at the end of an axon of the “sending” neuron 2. A dendrite or cell body of a “receiving” neuron, muscle, or gland cell 3. A small gap separating the two cells

15. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell (continued) • At synapses, signals are transmitted from one cell to another (continued) • Most synaptic terminals contain neurotransmitters that are released in response to an action potential reaching the terminal

16. 38.1 What Are the Structures and Functions of Nerve Cells? • The functions of a neuron are localized in separate parts of the cell (continued) • At synapses, signals are transmitted from one cell to another (continued) • The plasma membrane 细胞质膜 of the receiving neuron bears receptors that bind the neurotransmitters and stimulate a response in this cell • Therefore, at a synapse, the output 输出 of the first cell becomes the input 输入to the second cell

17. Figure 38-1 A neuron, showing its specialized parts and their functions Dendrites:Receive signalsfrom other neurons Synaptic terminals:Transmit signals fromother neurons Cell body:Integrates signals;coordinates theneuron’s metabolicactivities An actionpotential starts here neurotransmitters synapticterminal dendrite Axon: Conducts the action potential receptors Synaptic terminals:Transmit signals toother neurons synapse Dendrites:(of other neurons):Receive signals

18. 38.2 How Do Neurons Produce and Transmit Information? • Information is carried within a neuron by electrical signals and is transmitted between neurons by neurotransmitters released from one neuron and received by a second neuron

19. 38.2 How Do Neurons Produce and Transmit Information? • Information within a single neuron is carried by electrical signals • An unstimulated, inactive neuron maintains a constant electrical voltage difference 电压差, or potential, across its plasma membrane, called a resting potential静息电位. The voltage inside the cell is always negative and ranges from about 40 to 90 millivolts (mV) • If the membrane potential becomes less negative, it reaches a level called threshold 阈值 and triggers an action potential

20. 38.2 How Do Neurons Produce and Transmit Information? • Information within a single neuron is carried by electrical signals (continued) • During an action potential, the membrane potential rises rapidly to 50 mV inside the cell, then returns to resting potential • The action potential signal flows down the axon to the synaptic terminals with no change in voltage from the cell body to the synaptic terminals

21. Figure 38-2 Electrical events during an action potential 80 action potential 40 0 potential(millivolts) threshold restingpotential 40 morenegative lessnegative 80 time (milliseconds)

22. 38.2 How Do Neurons Produce and Transmit Information? • Information within a single neuron is carried by electrical signals (continued) • Myelin 髓鞘 speeds up the conduction of action potentials • The thicker an axon, the faster the action potential moves • A much more effective way to speed up conduction is to cover the axon with a fatty insulation 绝缘 called myelin • Myelin is formed by glial cells that wrap themselves around the axon, leaving naked nodes 结节in between • In myelinated neurons, action potentials “jump” rapidly

23. Figure 38-3 A myelinated axon An action potential jumpsfrom node to node, greatlyspeeding up conductiondown the axon myelin-producingglial cell node myelin myelinsheath axon axon

24. 38.2 How Do Neurons Produce and Transmit Information? • Neurons use chemicals to communicate with one another at synapses • At electrical synapses 电突触 , electrical activity can pass directly from neuron to neuron through gap junctions connecting the insides of the cells • Although electrical synapses occur in many places in the mammalian nervous system, neurons use chemicals far more frequently to communicate

25. 38.2 How Do Neurons Produce and Transmit Information? • Neurons use chemicals to communicate with one another at synapses (continued) • Two neurons do not actually touch at a synapse • A tiny gap, the synaptic cleft 突触间隙, separates the first, or presynaptic neuron突触前神经元, from the second, or postsynaptic neuron突触后神经元 • The presynaptic neuron sends neurotransmitter molecules across the gap to the postsynaptic neuron

26. 38.2 How Do Neurons Produce and Transmit Information? • Neurons use chemicals to communicate with one another at synapses (continued) • Communication between neurons begins with an action potential in a presynaptic neuron, usually beginning near the cell body and traveling down the axon until it reaches a synaptic terminal • The synaptic terminal contains scores of 大量的vesicles小囊泡, each full of neurotransmitter molecules

27. 38.2 How Do Neurons Produce and Transmit Information? • Neurons use chemicals to communicate with one another at synapses (continued) • When the action potential invades the synaptic terminal, the inside of the terminal becomes positively charged 带正电荷的, triggering a cascade of 级联changes that causes some of the vesicles to release neurotransmitters into the synaptic cleft • The neurotransmitter molecules diffuse 弥散 across the cleft and bind to receptor proteins on the outer surface of the plasma membrane of the postsynaptic neuron

28. Figure 38-4 The structure and function of the synapse presynaptic neuron An action potentialis initiated The action potentialreaches the synapticterminal of thepresynaptic neuron synapse synapticvesicle The positive charge ofthe action potential causesthe synaptic vesicles torelease neurotransmitters Neurotransmittersbind to receptors on the postsynaptic neuron synaptic terminalof presynapticneuron dendrite ofpostsynapticneuron neurotransmitters synaptic cleft Neurotransmittersare taken back into the synaptic terminal,are degraded, ordiffuse out of thesynaptic cleft neurotransmitter Neurotransmitterbinding causes ionchannels to open, andions flow in or out ions postsynaptic neuron receptor

29. 38.2 How Do Neurons Produce and Transmit Information? • Neurons use chemicals to communicate with one another at synapses (continued) • Synapses produce inhibitory 抑制的 or excitatory 兴奋的 postsynaptic potentials • At most synapses, the binding of neurotransmitter molecules to receptors on a postsynaptic neuron opens ion channels 离子通道 in the neuron’s plasma membrane • Depending on which channels are associated with the receptors, ions such as Na, K, Ca2, or Cl may move through these channels, causing a small, brief 短暂的change in voltage, called a postsynaptic potential or PSP

30. 38.2 How Do Neurons Produce and Transmit Information? • Neurons use chemicals to communicate with one another at synapses (continued) • Synapses produce inhibitory or excitatory postsynaptic potentials (continued) • If the postsynaptic neuron becomes more negative, its resting potential moves farther away from threshold, reducing the likelihood of firing an action potential • This change in voltage is called an inhibitory postsynaptic potential (IPSP)抑制性突触后电位

31. 38.2 How Do Neurons Produce and Transmit Information? • Neurons use chemicals to communicate with one another at synapses (continued) • Synapses produce inhibitory or excitatory postsynaptic potentials (continued) • If the postsynaptic neuron becomes less negative, then its resting potential will move closer to threshold, and it will be more likely to fire an action potential • This voltage change is called an excitatory postsynaptic potential (EPSP)兴奋性突触后电位

32. 38.2 How Do Neurons Produce and Transmit Information? • Neurons use chemicals to communicate with one another at synapses (continued) • Neurotransmitter action is usually brief • Some neurotransmitters—like acetylcholine 乙酰胆碱, the transmitter that stimulates skeletal 骨骼muscle cells—are rapidly broken down 分解 by enzymes in the synaptic cleft • Many other neurotransmitters are transported back into the presynaptic neuron

33. 38.2 How Do Neurons Produce and Transmit Information? • Neurons use chemicals to communicate with one another at synapses (continued) • Integration of postsynaptic potentials determines the activity of a neuron • The dendrites and cell body of a single neuron often receive EPSPs and IPSPs from the synaptic terminals of thousands of presynaptic neurons • The voltages of all the PSPs that reach the postsynaptic cell body at about the same time are added up, a process called integration整合

34. 38.2 How Do Neurons Produce and Transmit Information? • Neurons use chemicals to communicate with one another at synapses (continued) • Integration of postsynaptic potentials determines the activity of a neuron (continued) • If the excitatory and inhibitory postsynaptic potentials, when added together, raise the electrical potential inside the neuron above threshold, the postsynaptic cell will produce an action potential

35. Animation: The Synapse

36. 38.3 How Does the Nervous System Process Information and Control Behavior? • The nervous system performs marvelous 非凡的feats of computations 计算, stores prodigious 庞大的amounts of information, and directs complex behaviors • These accomplishments arise from 起因于three interacting properties 1. Specialization 分工 of individual neurons 2. Networks of connections between neurons 3. Outputs from the nervous system to specific muscles and glands that actually perform the behaviors dictated 指示 by the nervous system

37. 38.3 How Does the Nervous System Process Information and Control Behavior? • Most behaviors are controlled by pathways composed of four elements 1. Sensory neurons 感觉神经元 respond to an internal or external stimulus 2. Interneurons 中间神经元 receive signals from sensory neurons, hormones, and neurons that store memories 3. Motor neurons 运动神经元 receive instructions from sensory neurons or interneurons and activate muscles 4. Effectors 效应器, usually muscles or glands, perform the response directed by the nervous system

38. 38.3 How Does the Nervous System Process Information and Control Behavior? • These four elements, when properly connected, carry out the basic operations required of any nervous system 1. Determine the type of stimulus 2. Determine and signal the intensity 强度 of a stimulus 3. Integrate information from many sources 4. Initiate and direct appropriate responses

39. 38.3 How Does the Nervous System Process Information and Control Behavior? • The nature 性质 of a stimulus is encoded by specialization of sensory neurons and their connections to specific parts of the brain • The senses inform the brain about the properties of its environment, both outside the body (such as images, sounds, or odors) and inside the body (such as the body temperature or concentration of salts and glucose 葡萄糖 in the blood)

40. 38.3 How Does the Nervous System Process Information and Control Behavior? • The nature of a stimulus is encoded by specialization of sensory neurons and their connections to specific parts of the brain (continued) • Sensory neurons are specialized to respond to specific stimuli • For example, some sensory neurons respond to touch, but not temperature, light, or chemicals • Others respond to specific chemicals but not to touch or light, or even to other chemicals

41. 38.3 How Does the Nervous System Process Information and Control Behavior? • The nature of a stimulus is encoded by specialization of sensory neurons and their connections to specific parts of the brain (continued) • The nervous system encodes the nature of a stimulus—touch or temperature, for example—by which sensory neurons respond to the stimulus and to which parts of the brain the axons of those neurons connect • The brain interprets 解释action potentials that occur in the axons of the optic nerves as the sensation of light光觉

42. 38.3 How Does the Nervous System Process Information and Control Behavior? • The intensity of a stimulus is encoded by the frequency 频率 of action potentials • Because all action potentials are the same size and duration持续的时间, no information about the strength or intensity of a stimulus can be encoded in a single action potential

43. 38.3 How Does the Nervous System Process Information and Control Behavior? • The intensity of a stimulus is encoded by the frequency of action potentials (continued) • Intensity is coded in two ways 1. First, the intensity can be signaled by the frequency of action potentials in a single neuron • The more intense the stimulus, the faster the neuron fires action potentials

44. 38.3 How Does the Nervous System Process Information and Control Behavior? • The intensity of a stimulus is encoded by the frequency of action potentials (continued) • Intensity is coded in two ways (continued) 2. Second, many neurons may respond to the same stimulus • Stronger stimuli excite 刺激 more of these neurons, whereas weaker stimuli excite fewer neurons that fire at the same time

45. 38.3 How Does the Nervous System Process Information and Control Behavior? • The intensity of a stimulus is encoded by the frequency of action potentials (continued) • Intensity is coded in two ways (continued) 2. Second, many neurons may respond to the same stimulus (continued) • A gentle touch may cause a single touch receptor in the skin to fire action potentials very slowly • A hard poke may cause several touch receptors to fire, some very rapidly

46. Figure 38-5 Signaling stimulus intensity 40 70 sensory neuron 1 Sensory neuron 1fires slowly;sensory neuron 2is silent 40 sensoryneuron 2 sensoryneuron 1 70 sensory neuron 2 Gentle touch 40 70 sensory neuron 1 Sensory neurons1 and 2 both fire 40 sensoryneuron 2 sensoryneuron 1 70 sensory neuron 2 Hard poke time

47. 38.3 How Does the Nervous System Process Information and Control Behavior? • The nervous system processes information from many sources • The brain is continually bombarded by sensory stimuli from both inside and outside the body • The brain must evaluate 评价 these inputs, determine which ones are important, and decide how to respond • A large number of neurons may funnel 汇集 their signals to fewer neurons • For example, many sensory neurons may converge onto a small number of brain cells

48. 38.3 How Does the Nervous System Process Information and Control Behavior? • The nervous system processes information from many sources (continued) • Some of the brain cells act as “decision-making” cells, adding up the postsynaptic potentials that result from the synaptic activity of the sensory neurons • Depending on their relative strength (and other internal factors, such as hormones or metabolic activity), they produce appropriate behaviors

49. 38.3 How Does the Nervous System Process Information and Control Behavior? • The nervous system produces outputs to muscles and glands • Motor neurons in the brain, the spinal cord, or in the sympathetic 交感 and parasympathetic 副交感神经nervous systems stimulate activity in effectors—the muscles or glands that actually produce behaviors • The same principles of connectivity 连通性 and intensity coding for sensory inputs are used for the brain’s outputs to effectors

50. 38.3 How Does the Nervous System Process Information and Control Behavior? • Behaviors are controlled by networks of neurons in the nervous system • Simple behaviors, such as reflexes 反射作用, may be controlled by activity in as few as two or three neurons—a sensory neuron, motor neuron, and perhaps an interneuron in between—ultimately stimulating a single muscle • In humans, simple reflexes, such as the familiar knee-jerk 膝反射 or pain withdrawal reflexes, are produced by neurons in the spinal cord