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NROB60 Chapter 5 Introduction Imagine you stepped on a thumbtack and how it can be converted into a neural signal: First, specialized ion channels of the sensory nerve endings allow positive charge to enter the axon If this depolarization reaches threshold, then action potentials are generated o Because the axonal membrane is excitable and has voltage-gated sodium channels, action potentials can propagate without decrement up the long sensory nerves For this information to be processed by the rest of the nervous system, it is necessary that these neural signals be passed on to the other neurons that lead to a coordinated reflex response (e.g. lifting foot). By the end of the 19 century, it was recognized that this transfer of information from one neuron to another occurs at specialized sites of contact. In 1897, Charles Sherrington gave these sites their names: synapse. The process of information transfer at a synapse is called synaptic transmission The physical nature of synaptic transmission was argued over and two prominent hypotheses arose: One suggested it was electrical current flowing from one neuron to the next o The existence of such electrical synapses was finally proven in 1959 by Edwin Furshpan and David Potter. The other hypothesis suggested it was a chemical transfer of information o This was also proven to be correct with solid support for the concept of chemical synapses by Otto Loewi in 1921. Types of Synapses Electrical Synapses o Electrical synapses are relatively simple in structure and function, and the allow the direct transfer of ionic current from one cell to the next. o They occur at specialized sites called gap junctions The membrane of two cells is separated by only about 3nm and this narrow gap is spanned by clusters of special proteins called connexins. Six of these combine to form a channel called a connexon and two connexons combine to form a gap junction channel It allows ions to pass directly from the cytoplasm of one cell to the cytoplasm of the other. The pore is relatively large (1-2 nm) and is big enough for all the major cellular ions and many small organic molecules to pass through Most allow ionic current to pass equally well in both directions (unlike the majority of chemical synapses). Cells connected by gap junctions are electrically coupled o Transmission at the electrical synapses is very fast and, if the synapse is large, fail-safe. An action potential in the presynaptic neuron can produce almost instantaneously, and action potential in the postsynaptic neuron. o Also occur in the vertebrae brain and is common in every part of the mammalian CNS o When two neurons are electrically coupled, an action potential in the presynaptic neuron causes a small amount of ionic current to flow across the gap junction channels into the other neuron Causes a postsynaptic potential (PSP) in the second neuron o Since most electrical synapses are bidirectional, when the second neuron generates an action potential, it will in turn induce a PSP in the first neuron o The PSP generated by a single electrical synapses in the mammalian brain is small and may not by large enough to trigger an action potential in the postsynaptic cell One neuron usually makes electrical synapses with many other neurons so several PSPs occurring simultaneously may strongly excite a neuron (synaptic integration) o The precise roles of electrical synapses vary from one brain region to another. Chemical Synapses Most synaptic transmission in the mature human nervous system is chemical The presynaptic and postsynaptic membranes at chemical synapses are separated by a synaptic cleft that is 20 50 nm wide. o It is filled with a matrix of fibrous extracellular proteins. o One function is to make the pre- and post-synaptic membranes adhere to each other o The presynaptic side of the synapse is usually an axon terminal and typically contains dozens of small membrane-enclosed spheres called synaptic vesicles. These vesicles store neurotransmitter, the chemical used to communicate with the postsynaptic neuron. Many axon terminals also contain larger vesicles called secretory granules They contain soluble protein that appears dark in the electron microscope and sometimes called dense-core vesicles. o Dense accumulations of protein adjacent to and within the membranes on either side of the synaptic cleft are collectively called membrane differentiations. On the presynaptic side, proteins jutting into the cytoplasm of the terminal along the intracellular face of the membrane sometimes look like a field of tiny pyramids. These are the actual sites of neurotransmitter release called active zones Synaptic vesicles cluster adjacent to the active zones (Fig. 5.3) o The protein thickly accumulated in and just under the postsynaptic membrane is called the postsynaptic density. It contains the neurotransmitter receptors, which convert the intercellular chemical signal into an intracellular signal in the postsynaptic cell o CNS Synapses Different types of synapse may be distinguished by which part of the neuron is postsynaptic to the axon terminal If the postsynaptic membrane is on a dendrite, the synapse is said to be axodendritic If the postsynaptic membrane is on the cell body, the synapse is said to be axosomatic. If the postsynaptic membrane is on another axon, these synapses are called axoaxonic In certain specialized neurons, dendirtes actually form synapses with one another. These are called dendrodendritic synapses CNS synapses may be further classified into two general categories based on the appearance of their presynaptic and postsynaptic membrane differentiations Synapses in which the membrane differentiation on the postsynaptic side is thick than on the presynaptic side are called asymmetrical synapses or Grays type I synapses. Those in which the membrane differentiations are of similar thickness are called symmetrical synapses or Grays type II synapses These structural differences predict functional differences. o The Neuromuscular Junction Chemical synapses also occur between the axons of motor neurons of the spinal cord and skeletal muscle. This is called a neuromuscular junction Has many of the structural feathers of chemical synapses in the CNS It is fast and reliable o An action potential in the motor axon always causes an action potential in the muscle cell it innervates o Reliability is accounted for, in part, by structural specializations of the neuromuscular junction Its most important specialization is its size o One of the largest synapses in the body The presynaptic terminal also contains a large number of active zones The post-synaptic membrane, motor end plate, contains a series of shallow folds The presynaptic active zones are precisely aligned with these junctional folds, and the postsynaptic membrane of the folds Is packed with neurotransmitter receptors o It ensures that many neurotransmitter molecules are focally released onto a large surface of chemically sensitive membrane Principles of Chemical Synaptic Transmission Neurotransmitters o Most neurotransmitters fall into one of three chemical categories: Amino acids Amines Peptides o The amino acid and amine neurotransmitters are all small organic molecules containing at least one nitrogen atom, and they are stored in and released from synaptic vesicles. o Peptide neurotransmitters are large molecules stored in and released from secretory granules Secretory granules and synaptic vesicles are frequently observed in the same axon terminals o Different neurons in the brain release different neurotransmitters Fast synaptic transmission at most CNS synapses is mediated by the amino acids glutamate (Glu), gamma-aminobutyric acid (GABA) and glycine (Gly). The amine acetylcholine (Ach) mediates fast synaptic transimission at all neuromuscular junctions. Slower fo
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