NROB60 Chapter 5.docx

10 Pages
Unlock Document

University of Toronto St. George
Human Biology
Janelle Le Boutillier

NROB60 – Chapter 5 Types of Synapses Electrical Synapses - They allow the direct transfer of ionic current from one cell to the next o They occur at specialized sites called gap junctions - At a gap junction, the membranes of two cells are separated by only about 3 nm o This narrow gap is spanned by clusters of special proteins called connexins  6 connexins combine to form a channel called a connexon • 2 connexons combine to form a gap function channel allowing ions to pass directly from the cytoplasm of one cell to the cytoplasm of the other - The pore of most gap junction channels is relatively large o Its diameter is about 1-2 nm - Most gap junctions allow ionic current to pass equally well in both directions; therefore, unlike the vast majority of chemical synapses, electrical synapses are bidirectional o Because electrical current can pass through these channels, cells connected by gap junctions are said to be electrically coupled - In invertebrate species, such as the crayfish, electrical synapses are sometimes found between sensory and motor neurons in neural pathways mediating escape reflexes - Electrical synapses also occur in the vertebrate brain o When 2 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  This current causes a postsynaptic potential (PSP) in the second neuron - 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 is usually small (about 1 mV or less at its peak) and may not be large enough to trigger an action potential in the postsynaptic cell o One neuron usually makes electrical synapses with many other neurons, so several PSPs occurring simultaneously may strongly excite a neuron  This is an example of synaptic integration - The precise roles of electrical synapses vary from one brain region to another o Often found where normal function requires that the activity of neighboring neurons be highly synchronized - Gap junctions between 2 neurons are particularly common during early embryonic stages o During brain development, gap junctions allow neighboring cells to share both electrical and chemical signals that may help coordinate their growth and maturation - Gap junctions also interconnect many non-neural cells, including glia, epithelial cells, smooth and cardiac muscle cells, liver cells and some glandular cells Chemical Synapses - The presynaptic and postsynaptic membranes at chemical synapses are separated by a synaptic cleft that is 20-50 nm wide o 10 times the width of the separation at gap junctions - The cleft is filled with a matrix of fibrous extracellular protein o One function of this matrix is to make the pre- and postsynaptic membranes adhere to each other - The presynaptic side of the synapse, also called the presynaptic element, is usually an axon terminal o The terminal contains dozens of small membrane-enclosed spheres, each about 50 nm in diameter, called synaptic vesicles  These vesicles store neurotransmitter, the chemical used to communicate with the postsynaptic neuron - Many axon terminals also contain larger vesicles, 100 nm in diameter, called secretory granules o The granules contain soluble protein that appears dark in the electron microscope, so they are sometimes called large, dense-core vesicles - Dense accumulations of protein adjacent to and within the membranes on either side of the synaptic cleft are collectively called membrane differentiations o On the postsynaptic side, proteins jutting into the cytoplasm of the terminal along the intracellular face of the membrane sometimes look like a field of tiny pyramids.  The pyramids, and the membrane associated with them are the actual sites of neurotransmitter release, called active zones  Synaptic vesicles are clustered in the cytoplasm adjacent to the active zones o The protein thickly accumulated in and just under the postsynaptic membrane is called the postsynaptic density  The postsynaptic density contains the neurotransmitter receptors • Converts the intercellular chemical signal (neurotransmitter) into an intracellular signal in the postsynaptic cell (change in membrane potential) CNS Synapses: - In the CNS, different types of synapse may be distinguished by which part of the neuron is postsynaptic to the axon terminal o If the postsynaptic membrane is on a dendrite, the synapse is said to be axodendritic o If the postsynaptic membrane is on the cell body, the synapse is said to be axosomatic o In some cases, the postsynaptic membrane is on another axon, and these synapses are called axoaxonic o In certain specialized neurons, dendrites actually form synapses with one another; these are called dendrodendritic synapses - CNS synapses may be further classified into 2 general categories based on the appearance of their presynaptic and postsynaptic membrane differentiations, under magnification of the electron microscope o Synapses in which the membrane differentiation on the postsynaptic side is thicker than on the presynaptic side are called asymmetrical synapses or Gray’s type I synapses o Those in which the membrane differentiations are of similar thickness are called symmetrical synapses or Gray’s type II synapses The Neuromuscular Junction: - Chemical synapses also occur between the axons of motor neurons of the spinal cord and skeletal muscle o Such a synapse is called a neuromuscular junction and it has many of the structural features of chemical synapses in the CNS - An action potential in the motor axon always causes an action potential in the muscle cell it innervates o This reliability is accounted by structural specializations of the neuromuscular junction  Its most important specialization is size, one of the largest synapses in the body - The presynaptic terminal also contains a large number of active zones o The post-synaptic membrane, also called the motor end-plate, contains a series of shallow folds o The presynaptic active zones are precisely aligned with these junctional folds, and the postsynaptic membrane of the folds is packed with neurotransmitter receptors  This structure ensures that many neurotransmitter molecules are focally released onto a large surface of chemically sensitive membrane - Neuromuscular junctions are also of considerable clinical significance; disease, drugs, and poisons that interfere with this chemical synapse have direct effects on vital bodily functions Principles of Chemical Synaptic Transmission Neurotransmitters - Most neurotransmitters fall into one of 3 chemical categories: o 1) amino acids o 2) amines o 3) peptides - The amino acid and amine neurotransmitters are all small organic molecules containing at least one nitrogen atom and stored in and released from synaptic vesicles - Peptide neurotransmitters are large molecules stored in and released from secretory granules o They often exist in the same axon terminals that contain amine or amino acid neurotransmitters - Different neurons in the brain release different neurotransmitters o Fast synaptic transmission at most CNS synapses is mediated by the amino acids glutamate (Glu), gamma-aminobutyric acid (GABA) and glycine (Gly) o The amine acetylcholine (Ach) mediates fast synaptic transmission at all neuromuscular junctions o Slow forms of synaptic transmission in the CNS and in the periphery are mediated by transmitters from all 3 chemical categories Neurotransmitter Synthesis and Storage - Different neurotransmitters are synthesized in different ways o Ex. glutamate and glycine are among the 20 amino acids that are the building blocks of protein o In contrast, GABA and the amines are made only by the neurons that release them  These neurons contain specific enzymes that synthesize the neurotransmitters from various metabolic precursors - The synthesizing enzymes for both amino acid and amine neurotransmitters are transported to the axon terminal, where they locally and rapidly direct transmitter synthesis o Once synthesized in the cytosol of the axon terminal, the amino acid and amine neurotransmitters must be taken up by the synaptic vesicles o Concentrating these neurotransmitters inside the vesicle is the job of transporters, special proteins embedded in the vesicle membrane - Peptides are formed when amino acids are strung together by the ribosomes of the cell body o In the case of peptide neurotransmitters, this occurs in the rough ER o A long peptide synthesized in the rough ER is split in the Golgi apparatus, one of the smaller peptide fragments is the active neurotransmitter o Secretory granules containing the peptide neurotransmitter bud off from the Golgi apparatus and are carried to the axon terminal by axoplasmic transport Neurotransmitter Release - Neurotransmitter release is triggered by the arrival of an action potential in the axon terminal - The depolarization of the terminal membrane causes voltage-gated calcium channels in the active zones to open o These membrane channels are permeable to Ca2+ ions; there is a large inward driving force on Ca2+ o The internal calcium ion concentration at rest is very low, Ca2+ will flood the cytoplasm of the axon terminal as long as the calcium channels are open o The resulting elevation in calcium ion concentration is the signal that causes neurotransmitter to be released from synaptic vesicles - The vesicles release their contents by a process called exocytosis o The membrane of the synaptic vesicles fuses to the presynaptic membrane at the active zone, allowing the contents of the vesicle to spill out into the synaptic cleft o Calcium can achieve very high concentrations, greater than 0.1 mM - The speed of neurotransmitter release suggests that the vesicles involved art those already docked at the active zones o Docking is believed to involve interactions between proteins in the synaptic vesicle membrane and the active zone - In the presence of high [Ca2+], these proteins alter their conformation so that the lipid bilayers of the vesicle and presyanptic membranes fuse o Which forms a pore that allows the neurotransmitter to escape into the cleft - The vesicle membrane is recovered by the process of endocytosis, and the recycled vesicle is refilled with neurotransmitter o During periods of prolonged stimulation, vesicles are mobilized from a reserve pool that is bound to the cytoskeleton of the axon terminal - Secretory granules also release peptide neurotransmitters by exocytosis, in a calcium- dependent fashion but not at the active zones o The release of peptides generally requires high-frequency trains of action potentials, so that the [Ca2+] throughout the terminal can build to the level required to trigger release away from the active zones o The release of peptides takes 50 msec or more Neurotransmitter Receptors and Effectors - There are over 100 different neurotransmitter receptors but can be classified into 2 types: o Transmitter-gated ion channels and G-protein-coupled receptors Transmitter-Gated Ion Channels: - These are membrane-spanning proteins consisting of 4 or 5 subunits that come together to form a pore between them o In the absence of neurotransmitter, the pore is usually closed o When neurotransmitter binds to specific sites on the extracellular region of the channel, it induces a conformational change which within microseconds causes the pore to open - If the open channels are permeable to Na+, the net effect will be to depolarize the postsynaptic cell from the resting membrane potential o Because it tends to bring the membrane potential toward threshold for generating action potentials, this effect is said to be excitatory o A transient postsynaptic membrane depolarization caused by the presynaptic release of neurotransmitter is called an excitatory postsynaptic potential (EPSP)  Synaptic activation of ACh-gated and glutamate-gated ion channels causes EPSPs - If the transmitter-gated channels are permeable to Cl-, the net effect will be to hyperpolarize the postsynaptic cell from the resting membrane potential o Because it tends to bring the membrane potential away from threshold for generating action potentials, this effect is said to be inhibitory o A transient hyperpolarization of the postsynaptic membrane potential caused by the presynaptic release of neurotransmitter is called an inhibitory postsynaptic potential (IPSP)  Synaptic activation of glycine-gated or GABA-gated ion channels cause an IPSP G-Protein-Coupled Receptors: - Fast chemical synaptic transmission is mediated by amino acid and amine neurotransmitters acting on tra
More Less

Related notes for HMB200H1

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.