tailieunhanh - Ebook Texbook of medical physiology: Part 2

Part 2 book “Texbook of medical physiology” has contents: The nervous system, gastrointestinal physiology, metabolism and temperature regulation, endocrinology and reproduction, sports physiology. | U N I The Nervous System: A. General Principles and Sensory Physiology 45. Organization of the Nervous System, Basic Functions of Synapses, “Transmitter Substances” 46. Sensory Receptors, Neuronal Circuits for Processing Information 47. Somatic Sensations: I. General Organization, the Tactile and Position Senses 48. Somatic Sensations: II. Pain, Headache, and Thermal Sensations T IX C H A P T E R 4 5 Organization of the Nervous System, Basic Functions of Synapses, “Transmitter Substances” The nervous system is unique in the vast complexity of thought processes and control actions it can perform. It receives each minute literally millions of bits of information from the different sensory nerves and sensory organs and then integrates all these to determine responses to be made by the body. However, before beginning this discussion of the nervous system, the reader should review Chapters 5 and 7, which present the principles of membrane potentials and transmission of signals in nerves and through neuromuscular junctions. General Design of the Nervous System Central Nervous System Neuron: The Basic Functional Unit The central nervous system contains more than 100 billion neurons. Figure 45–1 shows a typical neuron of a type found in the brain motor cortex. Incoming signals enter this neuron through synapses located mostly on the neuronal dendrites, but also on the cell body. For different types of neurons, there may be only a few hundred or as many as 200,000 such synaptic connections from input fibers. Conversely, the output signal travels by way of a single axon leaving the neuron. Then, this axon has many separate branches to other parts of the nervous system or peripheral body. A special feature of most synapses is that the signal normally passes only in the forward direction (from the axon of a preceding neuron to dendrites on cell membranes of subsequent neurons). This forces the signal to travel in required directions for performing specific .