Synapses, properties & Transmission BY DR. MUDASSAR ALI ROOMI (MBBS, M. PHIL)

SYNAPSES Definition: it is the site of functional contact b/w two neurons at which an electric impulse is transmitted from one neuron to another.

Types of synapses- on the basis of site of contact Axodendritic synapses (most common type) Axosomatic synapses Dendrodenritic synapses axosaxonic synapses

Types of synapses- on the basis of method of signal transmission Chemical synapses: Most common type Signal transmission is delayed for about 0.5 msec in these synapses. Electrical synapses (nexus): Less common Flow of ions from one neuron to another via gap junctions. Signal transmission is nearly instantaneous.

Anatomy of a typical synapse (synaptic morphology) Axon terminals Pre-synaptic membrane Post-synaptic membrane Synaptic cleft (20-30nm wide) Synaptic vesicles.

Events occurring at a chemical synapse during signal transmission (Synaptic Transmission Mechanism)

EPSP and IPSP Depending on type of neurotransmitter & type of change in permeability of post-synaptic membrane, post-synaptic neuron is either excited or inhibited. Neuro-transmitter binds with receptor on post-synaptic membrane  opening of ion channels  localized change in membrane potential  post-synaptic membrane potential (PSP)  2 types  Excitatory (EPSP), Inhibitory (IPSP).

EPSP Resembles EPP (end plate potential). There is localized hypo-polarization due to Na+ influx. Resting potential of cell body of neuron is -65mV. When EPSP is produced  hypo-polarization  potential becomes less negative  reach threshold of excitation (-45mV)  ACTION POTENTIAL in cell body.

Purpose of EPSP: To bring potential of membrane to threshold (-45mV) It is graded like EPP (directly proportional to amount of neuro-transmitter released).

IPSP: Produced when post-synaptic neuron is inhibited. Neuro-transmitter is of inhibitory type (GABA. Glycine) It binds with receptors on post-synaptic membrane  change in permeability of membrane for K+ or Cl- (there is opening of K+ or Cl- channels  efflux of K+  cell becomes more negative  hyper-polarization / IPSP. Opening of Cl- channels  extra-cellular Cl- moves into the cell  more negative  hyper-polarization / IPSP.

Effect of IPSP: Because of IPSP, resting potential which is -65mV, becomes -70 to -75mV  Post-synaptic neuron is inhibited  POST-SYNAPTIC INHIBITION. PRE-STNAPTIC INHIBITION: Synaptic knob has additional synapse with other nerve terminals  release of inhibitory neuro-transmitter from additional synapse synaptic knob is inhibited  no further transmission from synapse now to post-synaptic neuron.

EPSP Vs ACTION POTENTIAL: Property EPSP or IPSP or Graded potential Action Potential Magnitude Low High Propagation & Duration Nil; it remains localized ( up to 20 msec) Self propagating ( up to 2 msec) Refractory period absent present All or none law Not obeyed. It is graded. obeyed Summation Present Decrement (decline of size with distance) Absent. Size is constant Increased permeability to ions To Na+ & K+ at one time but Na+ influx > K+ efflux Na+ Influx , then K+ efflux

Properties of Synaptic Transmission DALE’S LAW: At a given synapse, only 1 type of neurotransmitter is released, it may be excitatory or inhibitory. Later on it was found that in certain cases  release of additional substances at a given synapse e.g., in noradrenergic synapses: along with nor-epinephrine, some dopamine, neuropeptide Y & prostaglandins are also released.

LAW OF FORWARD CONDUCTION: Through synapses, impulses are conducted always from pre-synaptic to post synaptic neuron, never in backward direction. (NO REVERSE GEAR!!)

SYNAPTIC DELAY At a synapse, there is delay due to time taken in events during synaptic transmission. Through each synapse, there is delay of 0.5 milli seconds.

FATIGUE OF SYNAPTIC TRANSMISSION If impulses are conducted through a synapse repeatedly  fatigue due to exhaustion of stores or progressive inactivation of receptors on post-synaptic membrane. Significance of fatigue?? Fatigue of synaptic transmission is protective in nature  termination of epileptic fit.

IN UNITY RESTS STRENGTH! SUMMATION: Adding up of effects of stimuli particularly if stimuli are subthreshold. On a single motor neuron, thousands of synaptic knobs terminate to form synapses. About 80% of these synapses are on dendrites, remaining on cell body & few on axons. So, single impulse coming to motor neuron through a synapse, can’t excite a motor neuron & there must be summation of effects of stimuli.

TEMPORAL Impulses transmit through 1 or few synaptic knobs repeatedly  effects on post-synaptic neurons are added  stimulation. Second stimulus must fall when effect of 1st one is still there.

SPATIAL Impulses are conducted along a number of synapses simultaneously  effects on postsynaptic neuron are added  excitation.

POST-TETANIC FACILITATION OR POTENTIATION (Rest is best for test!) If impulses are conducted through a synapse rapidly  then rest is given to synapse  then again impulses are conducted  response of post-synaptic neuron is increased. Mechanism: Calcium ions enter in synaptic knob in each transmission, before fatigue occurs  increase no. of calcium accumulate in knob  more neurotransmitter released  more EPSP.

Increase excitability Decrease excitability ALKALOSIS INCREASE EXCITABILITY OF SYNAPSES, ACIDOSIS DEPRESSES SYNAPTIC TRANSMISSION Increase excitability Caffeine (cerebral stimulant) Theophylline Strychnine / Kuchla Decreased calcium (tetany) Decrease excitability Anesthetics Hypoxia Increased calcium (stabilize the membrane)