Nervous systems vary in structure a

Nervous systems vary in structure and complexity. In (a) cnidarians, nerve cells form a decentralized nerve net. In (b) echinoderms, nerve cells are bundled into fibers called nerves. In animals exhibiting bilateral symmetry such as (c) planarians, neurons cluster into an anterior brain that processes information. In addition to a brain, (d) arthropods have clusters of nerve cell bodies, called peripheral ganglia, located along the ventral nerve cord. Mollusks such as squid and (e) octopi, which must hunt to survive, have complex brains containing millions of neurons. In (f) vertebrates, the brain and spinal cord comprise the central nervous system, while neurons extending into the rest of the body comprise the peripheral nervous system. (credit e: modification of work by Michael Vecchione, Clyde F.E. Roper, and Michael J. Sweeney, NOAA; credit f: modification of work by NIH)
Compared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. While there is great diversity among different vertebrate nervous systems, they all share a basic structure: a CNS that contains a brain and spinal cord and a PNS made up of peripheral sensory and motor nerves. One interesting difference between the nervous systems of invertebrates and vertebrates is that the nerve cords of many invertebrates are located ventrally whereas the vertebrate spinal cords are located dorsally. There is debate among evolutionary biologists as to whether these different nervous system plans evolved separately or whether the invertebrate body plan arrangement somehow “flipped” during the evolution of vertebrates.
The nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glia, cells that provide support functions for the neurons by playing an information processing role that is complementary to neurons. A neuron can be compared to an electrical wire—it transmits a signal from one place to another. Glia can be compared to the workers at the electric company who make sure wires go to the right places, maintain the wires, and take down wires that are broken. Although glia have been compared to workers, recent evidence suggests that also usurp some of the signaling functions of neurons.

There is great diversity in the types of neurons and glia that are present in different parts of the nervous system. There are four major types of neurons, and they share several important cellular components.

NEURONS

The nervous system of the common laboratory fly, Drosophila melanogaster, contains around 100,000 neurons, the same number as a lobster. This number compares to 75 million in the mouse and 300 million in the octopus. A human brain contains around 86 billion neurons. Despite these very different numbers, the nervous systems of these animals control many of the same behaviors—from basic reflexes to more complicated behaviors like finding food and courting mates. The ability of neurons to communicate with each other as well as with other types of cells underlies all of these behaviors.

Most neurons share the same cellular components. But neurons are also highly specialized—different types of neurons have different sizes and shapes that relate to their functional roles.

PARTS OF A NEURON

Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. Neurons also contain unique structures, illustrated in Figure 35.3 for receiving and sending the electrical signals that make neuronal communication possible. Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. Although some neurons do not have any dendrites, some types of neurons have multiple dendrites. Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible synaptic connections.

Once a signal is received by the dendrite, it then travels passively to the cell body. The cell body contains a specialized structure, the axon hillock that integrates signals from multiple synapses and serves as a junction between the cell body and an axon. An axon is a tube-like structure that propagates the integrated signal to specialized endings called axon terminals. These terminals in turn synapse on other neurons, muscle, or target organs. Chemicals released at axon terminals allow signals to be communicated to these other cells. Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons. Some axons are covered with myelin, which acts as an insulator to minimize dissipation of the electrical signal as it travels down the axon, greatly increasing the speed on conduction. This insulation is important as the axon from a human motor neuron can be as long as a meter—from the base of the spine to the toes. The myelin sheath is not actually part of the neuron. Myelin is produced by glial cells. Along the axon there are periodic gaps in the myelin sheath. These gaps are called nodes of Ranvier and are sites where the signal is “recharged” as it travels along the axon.

It is important to note that a single neuron does not act alone—neuronal communication depends on the connections that neurons make with one another (as well as with other cells, like muscle cells). Dendrites from a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as 200,000 other neurons.

ผลลัพธ์ (ไทย) 3:

ระบบประสาทแตกต่างกันในโครงสร้างและความซับซ้อน ใน ( ) ไนดาเรีย ประสาทเซลล์รูปแบบเครือข่ายประสาทแบบกระจายอำนาจ . ( ข ) เซเว่นอัพ ประสาทเซลล์จะรวมลงในเส้นใยเรียกว่าเส้นประสาท ในสัตว์แสดงทวิภาคีสมมาตร เช่น planarians ( C ) , กลุ่มเซลล์ประสาทในด้านหน้าสมองที่ประมวลผลข้อมูล นอกจากสมอง ( D ) แมลงมีกลุ่มของร่างกายของเซลล์ประสาทเรียกว่าปมประสาทไขสันหลังเส้นประสาทต่อพ่วงอยู่ด้วย ) . เช่น ปลาหมึก หอย และปลาหมึก ( E ) ซึ่งต้องล่าเพื่อความอยู่รอด มีมันสมองที่ซับซ้อนที่มีนับล้านของเซลล์ประสาท ( ฉ ) สัตว์มีกระดูกสันหลังสมองและไขสันหลังประกอบด้วยระบบประสาทส่วนกลาง ในขณะที่เซลล์ประสาทมีการขยายเข้าไปในส่วนต่างๆของร่างกายประกอบด้วยระบบประสาทนอกส่วนกลาง ( เครดิต E :การปรับเปลี่ยนการทำงานโดยไมเคิล vecchione ไคลด์ Roper f.e . และไมเคิลเจสวีนี่ โนอา สินเชื่อ F : ปรับเปลี่ยนการทำงานโดย NIH )
เมื่อเทียบกับสัตว์ไม่มีกระดูกสันหลัง ระบบประสาทของสัตว์มีกระดูกสันหลังที่มีความซับซ้อนมากขึ้น จากส่วนกลาง และผู้เชี่ยวชาญ ในขณะที่มีความหลากหลายแตกต่างกันมากในหมู่สัตว์มีกระดูกสันหลังระบบประสาทพวกเขาทั้งหมดใช้โครงสร้างพื้นฐาน