Physiology Lecture l16 PDF

Physiology Synapses LECTURE (16) DR. El-Sawy 1  Physiology Synapses Definition: Areas of contact between neurons....

Physiology Synapses LECTURE (16) DR. El-Sawy 1  Physiology Synapses Definition: Areas of contact between neurons. Types of Synapses : Electrical Chemical  Extreme rare  Common. Site  Present in hippocampus &  Represent almost all retina. synapses in CNS.  Gap junctions () presynaptic &  Consist at least of 2 neurons: postsynaptic neurons. Presynaptic neuron:  Composed of proteins called release chemical transmitter (connexons) → highly Postsynaptic neuron: Formed of Receiving the signal permeable.  So allow transmission of potential changes from one neuron to the next. Fatigue Resist fatigue Show fatigue Conduction Both directions One direction Speed Faster Slower DR. El-Sawy 2  Physiology Synapses  Transmits impulse towards the synapse.  Are dilated forming certain swelling called presynaptic knobs that contain the neurotransmitter vesicles.  These presynaptic terminals (Knobs) may synapse on: Pre-synaptic 1. Dendrites of postsynaptic neuron (Axo-dendritic synapse) → most common type. 2. Soma (Axo-somatic synapse) or 3. Axon (Axo-axonic Synapse).  10-30 nm. Synaptic cleft  Is full of interstitial fluid.  Separates nerve ending from next neuron or effector organ. Post-synaptic  Transmits impulse away from the synapse. DR. El-Sawy 3  Physiology Synapses A. Release of neurotransmitter into synaptic cleft:  When presynaptic neuron is stimulated and action potential reaches synaptic knob → opening of voltage sensitive Ca2+ channels → Ca2+ entry into knob → Ca2+ will bind transmitter vesicles with knob membrane → then vesicles open into the synaptic cleft and release the transmitter by exocytosis. B. Action of neurotransmitter on postsynaptic membrane:  Produces its effects through binding the post-synaptic membrane.  Types of post-synaptic potentials: Excitatory postsynaptic potential Inhibitory postsynaptic potential (EPSP) (IPSP)  On stimulation: presynaptic  On stimulation of presynaptic terminals release excitatory knob → release inhibitory neurotransmitter which open cation neurotransmitter which either: channels → depolarization. a. Open anion channel → Cl influx→ hyperpolarization. b. ↑ K+ efflux → hyperpolarization. C. Termination of synaptic transmission: a. Active reuptake of the transmitter into the presynaptic terminal. b. Enzymatic degeneration. c. Diffusion. DR. El-Sawy 4  Physiology Synapses DR. El-Sawy 5  Physiology Synapses  At synapses, impulses are conducted only in one direction One-way conduction from the presynaptic neuron to the post-synaptic neuron. Synaptic delay  The minimum time for synaptic delay is about 0.5 msec.  Importance: Protective mechanism against excess neuronal activity It is the most important means by which the excess excitability of an epileptic circuit is cut off and stopped. This leads to spontaneous ending of epileptic fit.  Causes: a) Exhaustion: Of the chemical transmitter in presynaptic terminals. b) Inactivation of some postsynaptic receptors: Due to accumulation of metabolites. c) Marked ↑ of intracellular Ca2+ in postsynaptic neuron: Synaptic High Ca2+ level → opens K+ channels → K+ efflux fatigue and hyperpolarization of postsynaptic membrane →↓ excitability of postsynaptic neuron. DR. El-Sawy 6  Physiology Synapses Effects of  Marked hypoxia for short period (just few seconds) → loss of hypoxia excitability of neurons & stoppage of synaptic transmission.  Alkalosis: ↑ excitability. Effects of pH  Acidosis: ↓ excitability.  Caffeine & theophylline (in coffee - tea): Increase neuronal excitability.  Strychnine: Effects of Causes hyper-excitability of neurons. drugs  Anesthetics and hypnotics: Decrease neuronal activity Decrease synaptic transmission. DR. El-Sawy 7