CNS (SYNAPSE)

Synapse At the end of this lecture you should be able to ; Synapse and its types Properties of Synapses

The Synapse Junctional point between two neurons that transmits impulse from first to the second Neuron.

Classification Anatomical Classification 1. Axo-Dendritic (on the Basis of Communication ) 1. Axo-Dendritic 2. Axo-Somatic 3. Axo-Axonic 4. Dendro-dendritic (amacrine cells in Eye) PHYSIOLOGICAL Classification: (According to Nature) 1. Electrical 2. Chemical

Chemical synapse

Electrical synapse

Physiological Anatomy Components 1. Presynaptic terminal 3. Postsynaptic membrane 2. Synaptic Cleft Typical Anterior Motor Neuron

Mechanism of Action of Neuro-transmitter on the Post-synaptic Membrane Receptor Proteins have Two Components: Binding Component Ionophore Component Anionic channel a. Ion Channels { Cationic channel b. Second Messenger Activator

Second Messenger System in the Postsynaptic Neuron G-Proteins; is Attached to Receptor Protein on its inner surface. Three components of G-Protein. 1. Alpha - Activator 2. Beta & Gamma - attached to alpha component On activation, ά-Component separates from β & γ components - is Free to perform the function specific for that particular Neuron.

Second Messenger System in the Postsynaptic Neuron

Functions Which Can be Performed By α-Component Opening of specific Ion channel in the Postsynaptic Membrane. Activation of Cyclic AMP Activation of intracellular Enzymes. Activation of Gene Transcription

Rapidly Acting Transmitters (Small Molecules) Class-I. - Acetylcholine Class-II. (Amines) Adrenalin (Epinephrine) Nor-adrenaline (Nor-Epinephrine) Dopamin Serotinin Histamine CLASS-III. ( Amino Acids) Gamma Amino Butyric Acid (GABA) Glutamate Glycin Aspartate Class-IV- Nitrous Oxide Class-V- Neuropeptides. Substance-P, Enkephalin, Endorphins

Neurotransmitter Removal Monoamine Oxidase

Properties of Synapses or Synaptic Transmission 1. One Way Conduction 2. Synaptic Delay. minimum 0.5 m.sec 3. Excitation (EPSP) Presynaptic 4. Inhibition-{ Postsynaptic

Excitation & Inhibition Excitation: is carried out by: 1. Opening of Sodium Channels 2. Decreased conduction through K+ or Cl- Channels. 3. Various changes in the metabolism of Postsynaptic Neuron Inhibtion: Pre & Post Synaptic Inhibition 1. Opening of chloride Channels 2. Increase in K+ ion efflux by opening K+ Channels 3. activation of Enzymes which inhibit metabolism. > By producing Hyperpolarization

Negative feedback inhibition of Spinal Motor Neuron via Inhibitory interneuron (Renshaw Cell) in the spinal cord Renshaw Cell

Reciprocal Inhibition

Properties of Synapses 5. Synaptic fatigue : Partially due to exhaustion of stores of neurotransmitter & Probable two other factors include; 1. Progressive inactivation of postsynaptic receptors 2. Slow development of abnormal concentration of ions inside the postsynaptic neuronal cell. Significance: Stabilization of Nervous system

Properties of Synapses Spatial summation ( > one Neuron) 6. Summation { Temporal summation (Only One N repetitively)

Properties of Synapses 7. Facilitation

Properties of Synapses 8.Occlusion – Net Response < than the individual Presynaptic N & Subliminal Fringe Subliminal fringe is due to spatial summation. Occlusion is due to Overlapping of the fibers.

Occlusion The decrease in expected response due to pre synaptic fibers sharing post synaptic neurons is called occlusion

Properties of Synapses 9.Convergence e.g on ά-motor N CNS correlates ,summates and sort different types of Information. 10.Divergence Amplifing type Cortico spinal pathway

Divergence into multiple Tracts

Properties of Synapses 11.Recruitment. involving more & more neurons as the stimulus becomes stronger 11. After discharge.- Prolonged out put discharge after the incoming signal is over. Mechanism: a. Synaptic after discharge (parallel Collaterals) b. Reverberatory circuits

Properties of Synapses . 13.Posttetanic Potentiation; Production of enhanced postsynaptic potentials in response to stimulation. Due to accumulation of Ca++ in the Presynaptic Neuron. 14. Habituation; Gradual Disappearance of response to a repeated benign stimulus is called Habituation. > Decrease Release of Ach > Decrease Intracellular Ca++. .

Properties of Synapses Effect of Acidosis & Alkalosis: Acidosis: Greatly depresses neuronal activity Fall in pH from 7.4 to 7.0 > Comma ( Diabetic or Uremic ) Alkalosis: Greatly Increase Neuron Excitability Hypoxia: > unconsciousness Within 3-7 seconds. Drugs like Caffeine, theophyline & theobromine > Increase Excitability Decrease in ca++ > Increased Excitability & Vice versa Anesthetics > Increase in threshold for excitation