臨床基礎班生理課件2-nervous system新

1、Organization of the nervous system; basic functions of synapses and transmitter substancesDemao Song, Ph.D.Department of Physiology & PathophysiologyORGANIZATION OF THE NERVOUS SYSTEMThe Center Nervous System Neuron The Basic Functional UnitThe central nervous system contains more than 100 billion n

2、eurons.The ing signal enter the neuron through synapses mainly on the neuronal dendrite, but also on the cell body.A special feature of most synapses is that the signal normally passes only in the forward direction (from axon to dendrites).For different types of neurons, there may be only a few hund

3、red or as many as 200,000 such synaptic connections from the input fibers.Sensory Division of the Nervous System Sensory ReceptorsMost activities of the nervous system are initiated by sensory experience emanating from sensory receptors, whether visual receptors in the eyes, auditory receptors in th

4、e ears, tactile receptor on the surface of the body, or other kinds of receptors.This sensory experience can cause an immediate reaction from the brain, or memory of the experience can be stored in the brain for minutes, weeks, or years and determine bodily reactions at some future date.Motor Divisi

5、on The EffectorsThe most important eventual role of the nervous system is to control the various bodily activities. This is achieved by controlling:Contraction of appropriate skeletal muscles throughout the bodyContraction of smooth muscle in the internal organsSecretion by both exocrine and endocri

6、ne glands in many parts of the bodyProcessing of information “Integrative” function of Nervous systemOne of the most important functions of the nervous system is to process ing information in such a way that appropriate mental and motor responses will occur. More than 99% of all sensory information

7、is discarded by the brain as irrelevant and unimportant.When important sensory information excites the mind, it is immediately channeled into proper integrative and motor regions of the brain to cause desired responses.Role of synapses in Processing informationThe synapse is the junction point from

8、one neuron to the next, and, therefore, it is an advantageous site for control of signal transmission.The synapses determine the directions that the nervous signals will spread in the nervous system.CENTRAL NERVOUS SYSTEM SYNAPSESTypes of synapsesChemical synapses: The first neuron secretes a chemic

9、al substance called a neurotransmitter at the synapse, and this transmitter in turn acts on receptor proteins in the membrane of the next neuron to excite the neuron, inhibit it, or modify its sensitivity in some other way.Electrical synapses, in contrast, are characterized by direct open fluid chan

10、nels that conduct electricity from one cell to the next. Most of these consist of small protein tubular structures called gap junctions that allow free movement of ions from the interior of one cell to the interior of the next. “One way” conduction a chemical synapsesThey always transmit the signals

11、 in one direction that is, from the neuron that secretes the transmitter substance, called the presynaptic neuron, to the neuron on which the transmitter acts, called the postsynaptic neuron.This is the principle of one-way conduction at chemical synapses, and it is quite different from conduction t

12、hrough electrical synapses, which usually can transmit signals in either direction.Physiologic anatomy of the synapsePresynaptic terminals: also be called terminal knobs, buttons, end-feet, synaptic knobs.Mechanism by which action potentials cause transmitter release from the presynaptic terminals r

13、ole of calcium ionsThe quantity of transmitter substance that is then released into the synaptic cleft is directly related to the number of calcium ions that enter the terminal.When the calcium ions enter the presynaptic terminal, it is believed that they bind with special protein molecules on the i

14、nside surface of the presynaptic membrane, called release sites. This binding in turn causes transmitter vesicles in the terminal to fuse with the release sites and to open through the membrane to the exterior by the process called exocytosis.Action of the transmitter substance on the postsynaptic n

15、euron function of “receptor proteins”.At the synapse, the membrane of the postsynaptic neuron contains large numbers of receptor proteins. The molecules of these receptors have two important components: (1) a binding component that protrudes outward from the membrane into the synaptic cleft here it

16、binds with the neurotransmitter from the presynaptic terminal and (2) an ionophore component that passes all the way through the membrane to the interior of the postsynaptic neuron.Action of the transmitter substance on the postsynaptic neuron function of “receptor proteins”.The ionophore in turn is

17、 one of two types: (1) an ion channel that allows passage of specified types of ions through the membrane of (2) a “second messenger” activator that is not an ion channel but instead is a molecule that protrude into the cell cytoplasm and activates one or more substances inside the postsynaptic neur

18、on.Ion channelsThe ion channels in the postsynaptic neuronal membrane are usually of two types:Cation channels: that most often allow sodium ions to pass when opened but sometimes allow potassium and/or calcium ions as well.Anion channels: that allow mainly chloride ions to pass but also minute quan

19、tities of other anions.“Second Messenger” system in the postsynaptic neuronInside the cytoplasm, the separated alpha component performs one or more of multiple functions, depending on the specific characteristic of each type of neuron.Opening specific ion channels through the postsynaptic cell membr

20、ane.Activation of cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP) in the neuronal cell.Activation of one or more intracellular enzymes.Activation of gene transcription.Excitatory Receptors in the Postsynaptic MembraneExcitationOpening of sodium channels to allow large

21、numbers of positive electrical charges to flow to the interior of postsynaptic cells.Depressed conduction through chloride or potassium channels, or both.Various changes in the internal metabolism of the cell to excite activity.Inhibitory Receptors in the Postsynaptic MembraneInhibitionOpening of ch

22、loride ion channels through the receptor molecule.Increase in the conductance of potassium ions through the receptor.Activation of receptor enzymes that inhibit cellular metabolic functions or that increase the number of inhibitory synaptic receptors or decrease the number of excitatory receptors.Ch

23、emical substances that function as synaptic transmittersMore than 50 chemical substances have been proved or postulated to function as synaptic transmitters. One group comprises small-molecule, rapidly acting transmitters. The other is made up of a large number of neuropeptides of much larger molecular size that are usually much more slowly acting.Small-Molecule, Rapidly Acting TransmittersClass IAcetylcholine Class II: The AminesNorepinephrineEpinephri