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Transcript
Interneuronal connections Functional classification or Types of comunication A.Chemical synapse Almost all synapses used for signal transmission in the CNS of human being are chemical synapses. First neuron secretes a chemical substance called neurotransmitter at the synapse to act on receptor on the next neuron to excite it, inhibit or modify its sensitivity. The chemical synapse is a specialized junction that transfers nerve impulse information from a presynaptic membrane to a postsynaptic membrane using neurotransmitters. Axo-dendritic synapse Axo-somatic synanpse Axo-axonic synapse The Chemical Synapse R.E.B, 4MedStudents.com, 2003 Neurotransmitters Properties of neurotransmitters: 1) synthesized in the presynaptic neuron 2) Localized to vesicles in the presynaptic neuron 3) Released from the presynaptic neuron under physiological conditions 4) Rabidly removed from the synaptic cleft by uptake or degradation 5) Presence of receptor on the post-synaptic neuron. 6) Binding to the receptor elicits a biological response 6 Neurotransmitters found in the nervous system EXCITATORY Acetylcholine Dopamine Histamine Nonepinephrine Epinephrine Glutamate Serotonin INHIBITORY GABA Glycine 7 The Synapse Structure of a synapse Excitatory postsynaptic potential Single stimuli applied to the sensory nerves in the experimental situation described above characteristically do not lead to the formation of a propagated action potential in the postsynaptic neuron. Instead, the stimulation produces either a transient, partial depolarization or a transient hyperpolarization. If positive ion gates open (which allow more Na+ and Ca2+ to enter than K+ to exit), the membrane becomes depolarized, which results in an excitatory postsynaptic potential (EPSP). If the threshold potential is exceeded, an action potential is generated. If K+ or chlorine ion (Cl−) gates open (allowing K+ to exit or Cl− to enter), the membrane becomes more depolarized (hyperpolarized), which results in an inhibitory postsynaptic potential (IPSP). As a result, it becomes more difficult to generate an action potential on this membrane. Summary of Synaptic Transmission Chemical Synapse Events at a chemical synapse 1. Arrival of action potential on presynaptic neuron opens volage-gated Ca++ channels. 2. Ca++ influx into presynaptic term. 3. Ca++ acts as intracellular messenger stimulating synaptic vesicles to fuse with membrane and release NT via exocytosis. 4. Ca++ removed from synaptic knob by mitochondria or calcium-pumps. 5. NT diffuses across synaptic cleft and binds to receptor on postsynaptic membran 6. Receptor changes shape of ion channel opening it and changing membrane potential 7. NT is quickly destroyed by enzymes or taken back up by astrocytes or presynaptic membrane. Note: For each nerve impulse reaching the presynaptic terminal, about 300 vesicles are emptied into the cleft. Each vesicle contains about 3000 molecules. Postsynaptic Potentials NT affects the postsynaptic membrane potential Effect depends on: – The amount of neurotransmitter released – The amount of time the neurotransmitter is bound to receptors The two types of postsynaptic potentials are: – EPSP – excitatory postsynaptic potentials – IPSP – inhibitory postsynaptic potentials Inhibitory Synapses Neurotransmitter binding to a receptor at inhibitory synapses: – Causes the membrane to become more permeable to potassium and chloride ions – Leaves the charge on the inner surface more negative (flow of K+ out of the cytosol makes the interior more negative relative to the exterior of the membrane – Reduces the postsynaptic neuron’s ability to produce an action potential Electrical Synapses Pre- and postsynaptic neurons joined by gap junctions – allow local current to flow between adjacent cells. Connexons: protein tubes in cell membrane. Rare in CNS or PNS Found in cardiac muscle and many types of smooth muscle. Action potential of one cell causes action potential in next cell, almost as if the tissue were one cell. Important where contractile activity among a group of cells important. The Synapse The junction between two neurons is termed a synapse (synapsis = point of contact) The narrow gap between the two neurons at the synapse is the synaptic cleft; the cleft is filled with extracellular fluid and spans an area of = synapse approximately 20 nm A neuron that conducts impulses toward a synapse is called a pre-synaptic neuron A neuron that conducts impulses away from a synapse is called a post-synaptic neuron The Synapse axon terminal synaptic knob A single neuron may have many thousands of synaptic junctions on its dendrites and cell body mitochondrion synaptic vesicle (contains neurotransmitter) pre-synaptic synaptic post-synaptic membrane (with receptors for membrane cleft neurotransmitter) Events at the synapse Voltage-gated calcium ion channels open in the pre-synaptic membrane An action potential travels down the axon of the neuron to the synaptic knob and depolarises the pre-synaptic membrane Calcium ions diffuse into the synaptic knob post-synaptic membrane calcium ions in the extra-cellular fluid Events at the synapse Voltage-gated calcium channels open in the pre-synaptic membrane Calcium ions diffuse into the synaptic knob post-synaptic membrane An action potential travels down the axon of the neuron to the synaptic knob and depolarises the pre-synaptic membrane The uptake of calcium ions triggers the fusion of the synaptic vesicles with the pre-synaptic membrane Events at the synapse Voltage-gated calcium channels open in the pre-synaptic membrane Calcium ions diffuse into the synaptic knob The uptake of calcium ions triggers the fusion of the synaptic vesicles with the pre-synaptic membrane Neurotransmitter is released into the synaptic cleft by EXOCYTOSIS Neurotransmitter diffuses across the cleft and binds to specific protein receptors embedded in the post-synaptic membrane post-synaptic membrane receptors in the post-synaptic membrane Events at the synapse Binding of neurotransmitter opens Na+ gates in the membrane and there is an influx of Na+ into the post-synaptic neuron post-synaptic membrane Neurotransmitter diffuses across the cleft and binds to specific protein receptors embedded in the post-synaptic membrane An excitatory post-synaptic potential (EPSP) builds up across the membrane and if this reaches threshold, an action potential is triggered in the post-synaptic neuron sodium ions Depolarisation of the post-synaptic membrane Events at the synapse Following activation of the post-synaptic membrane, neurotransmitter is removed from the synaptic cleft to enable further stimulation to occur post-synaptic membrane The neurotransmitter, acetylcholine, is hydrolysed by the enzyme acetylcholinesterase, which is located at the surface of the post-synaptic membrane The neurotransmitter, noradrenaline, is actively transported back into the axon terminals sodium ions Depolarisation of the post-synaptic membrane Unidirectionality Unidirectionality describes the one-way transmission of nerve impulses between neurons Neurotransmitter is stored and released only on the pre-synaptic side of the synaptic cleft Receptors for neurotransmitter are only located on the post-synaptic membrane synaptic vesicle (contains neurotransmitter) This arrangement allows for the transmission of impulses between neurons in one direction only post-synaptic membrane (with receptors for neurotransmitter) The nature of the neurotransmitter determines the response of the post-synaptic membrane During hyperpolarisation, the post-synaptic membrane potential becomes more negative than its resting potential and results from either the efflux of positive charge or the influx of negative charge Inhibition occurs at synapses where transmitter release results in the hyperpolarisation of the post-synaptic membrane Occlusion On account of divergence one neuron may pass excitive signals on the other neurons. Another neuron may excite several neurons. But if from both neurons which is divergented excitement will be simultaneously the total quantity of excited neurons will be decrease. Opposite inhibition Lateral inhibition If in a neurons' chain, which secure opposite inhibition collaterals of axons of inhibition neurons form synaptic connection with neighboring excitive cells in these cells develop lateral inhibition Spatial summation The adding together of EPSPs generated simultaneously at many different synapses on a dendrite. Two or more presynaptic inputs are active at the same time A space (spatial) dependent process. Occurs in a Convergent Synapse Temporal summation The adding together of EPSPs generated at the same synapse if they occur in rapid succession, within 1-15 msec of one another. The same presynaptic fiber fires AP in quick succession A Time (Temporal) dependent process Occurs in a Divergent Synapse