Synapse part 1.ppt

 OBJECTIVES Definition Physiological anatomy Types of synapse Mechanism of synaptic transmission Synaptic potentials Properties of synapse Functions of synapse  Synapse  Sir Charles Sherrington coined the word synapse from the Greek word synap...

 OBJECTIVES Definition Physiological anatomy Types of synapse Mechanism of synaptic transmission Synaptic potentials Properties of synapse Functions of synapse  Synapse  Sir Charles Sherrington coined the word synapse from the Greek word synaptein  Syn - together , haptein – to clasp DEFINITION  Synapse is a junction between two neurons  Junction between a neuron and muscle cell / gland cell is also referred to as synapse  Functional anatomy of synapse - The neuron from which information passes through the synapse is the presynaptic neuron - The neuron which receives the information is the postsynaptic neuron - The part of the presynaptic terminal forming the synapse is the presynaptic membrane - The part of the postsynaptic terminal forming the synapse is the postsynaptic membrane - The space between the presynaptic and postsynaptic membrane is the synaptic cleft STRUCTURE OF SYNAPSE 1. SYNAPTIC KNOB OR BUTTON The end of pre synaptic Nerve terminal is enlarged to form synaptic knob /terminal button It contains: a.Synaptic vesicles b. Mitochondria containing ATP c. Microtubules d. Presynaptic membrane  a. Synaptic vesicles  Types of vesicles 1.Small clear-contain Neurotransmitters [NT] - ACh, glycine, GABA, or glutamate 2. Small dense core – catecholamines [epinephrine, nor-epinephrine, dopamine] 3.Large dense core – neuropeptides [enkephalin, endorphin, substance P, etc]  Synthesis Vesicles synthesized in cell body & rapidly transported along axon to synaptic knob b. Mitochondria containing ATP c. Microtubules d. Presynaptic membrane - The pre synaptic membrane is thickened to form active zone – contains many proteins and voltage gated calcium channels  [ADD CALCIUM CHANNEL TO THE DIAGRAM] 2. SYNAPTIC CLEFT - The space between the presynaptic and postsynaptic membrane is the synaptic cleft - 20-30nm wide - This space is filled with ECF - Neurotransmitter molecules released from the presynaptic terminal diffuse across the cleft to reach the postsynaptic membrane 3. POSTSYNAPTIC MEMBRANE Contains receptor proteins for binding with neurotransmitters The area of postsynaptic membrane modified for synaptic transmission is called postsynaptic density It is a cluster of receptors for transmitter embedded within the postsynaptic membrane SYNAPSE Synapse Synapse  Types of synapse A. Structural classification 1. Axodendritic 2. Axosomatic 3. Axoaxonic 4. Dendrodendritic B. Classification based on type of transmission across the synapse 1. Chemical 2. Electrical 3. conjoint  A. Structural classification of synapse The pre synaptic neuron can terminate on different parts of post synaptic neuron 1. Axo dendritic - most common type 98% of cerebral cortical & 80% of spinal cord synapses Synaptic knob of pre synaptic neuron makes contact with the dendrite of post synaptic neuron 2. Axo somatic Synaptic knob of pre synaptic neuron terminate on the soma of post synaptic neuron 20% 0f spinal & 2% of cerebral cortical synapses, Basket cell of cerebellum 3. Axo axonal Synaptic knob of some pre synaptic neuron terminate on the axon of post synaptic neuron - spinal cord 4. Dendro dendritic – dendrite of presynaptic neuron connects to the dendrite of post synaptic neuron Eg: mitral & granule cells of olfactory bulb Types of synapse  B. Classification based on type of transmission across the synapse 1. Chemical - most common  The transmission of impulse occurs through release of neurotransmitters  There is a gap of 20-40 nm between the 2 neurons – synaptic cleft  The first neuron- pre synaptic neuron secretes Neurotransmitter (NT) that acts on the receptor of second neuron - post synaptic neuron to excite / inhibit it 2. Electrical – only few - some neurons in the lateral vestibular nucleus  The transmission of impulse occurs through gap junctions  The pre & post synaptic cells come close together and form gap junctions - low resistance bridges - ions can pass freely from one cell to the other - AP is transmitted like in cardiac & smooth muscle 3. Conjoint – a few  Both electrical &chemical transmission occurs Chemical synapse Electrical synapse Most common Very few Synaptic cleft of 20-40 The cells are close nm present between together & form gap cells junctions Chemical transmission Electrical transmission of impulse via NT from via passage of ions pre syn acting on post through gap junction syn cell Transmission in either Transmission is only in direction one direction ie pre to No synaptic delay post syn Synaptic delay present  SYNAPTIC TRANSMISSION Process by which the information from presynaptic neuron passes to the post synaptic neuron through the synapse 1. Action potential[AP] reaches synaptic knob 2. Opening of voltage gated calcium channels 3.release of neurotransmitter [NT] from synaptic vesicles 4. Development of EPSP & IPSP 5. removal of NT from synaptic cleft 6. development of AP  STEPS OF SYNAPTIC TRANSMISSION 1.Action potential[AP] reaches synaptic knob 2.opening of voltage gated calcium channels 3.release of neurotransmitter [NT] from syn.vesicles Action Potential that arrives at presynaptic axon terminal depolarizes the presynaptic membrane opening of voltage gated Ca2+ channels calcium enters the axon terminal through active zone Ca++ triggers the fusion, exocytosis of vesicle & discharge of NT into synaptic cleft  Synaptic vesicles containing neurotransmitter molecules that are concentrated at active zone of the presynaptic axon undergo docking and priming Docking is the process by which vesicles attach with the membrane Priming is the process by which the vesicles become ready to discharge their content in response to a stimulus  Synaptic vesicle - docking Synaptic vesicle dock on the active zone by means of docking proteins synaptobrevin and syntaxin Synaptic vesicle -v snare protein – synaptobrevin Pre synaptic membrane – t snare protein – syntaxin Other proteins also involved[symptosomal nerve associated protein[SNAP 25]] The synaptobrevin attaches and interacts with syntaxin for docking and priming of vesicles The amount of NT released is directly related to the no of Ca++ that enter SYNAPTIC PROTEINS: Synaptobrevin, syntaxin, symptosomal nerve associated protein[SNAP 25] Applied aspect Tetanus & botulinum toxins block NT release by inactivating docking & fusion proteins Tetanus toxin blocks presynaptic NT release in CNS & cause spastic paralysis Botulinum toxin blocks NT release in NMJ & cause flaccid paralysis  Neurotransmitters Excitatory transmitters Acetyl choline , noradrenaline, adrenaline, histamine, serotonin , glutamate Inhibitory transmitters GABA , glycine NT binds with receptors in the post synaptic membrane & cause effects Excitatory transmitters - depolarization Inhibitory transmitters - hyperpolarization  Various steps in release of NT & recycling of NT The vesicles and the proteins contained in their walls are synthesized in the neuronal cell body and transported along the axon to the endings by fast axoplasmic transport. The neuropeptides in the large dense-core vesicles must also be produced by the protein-synthesizing machinery in the cell body. However, the small clear vesicles and the small dense-core vesicles recycle in the ending i.e. terminal button  They are loaded with transmitter in the terminal button, fuse with the cell membrane, and discharge the transmitter by exocytosis, then are retrieved by endocytosis. In some instances, they enter endosomes and are budded off the endosome and refilled, starting the cycle over again. More commonly, however, the synaptic vesicle discharges its contents through a small hole in the cell membrane, then the opening reseals rapidly and the main vesicle stays inside the cell ("kiss-and-run" discharge).  Small synaptic vesicle cycle in presynaptic nerve terminals Vesicles bud off the early endosome and then fill with neurotransmitter (NT; top left). They then move to the plasma membrane, dock, and become primed. Upon arrival of an action potential at the ending, Ca2+ influx triggers fusion and exocytosis of the granule contents to the synaptic cleft. The vesicle wall is then coated with clathrin and taken up by endocytosis. In the cytoplasm, it fuses with the early endosome, and the cycle is ready to repeat.  Synaptic delay When an impulse reaches a pre synaptic terminal an interval of at least 0.5 ms occurs before a response is obtained in post synaptic neuron Due to the time taken for the NT to be released from pre synaptic terminal & then act on the post synaptic neuron  IMPORTANT QUESTIONS Define synapse. Types of synapse. Explain the properties of synapse Synaptic transmission Term synapse coined by ? Docking proteins?