Synaptic transmission

Synthesis and transport of neurotransmitter

Neurotransmitters 1.Small clear vesicles: Acetylcholine glycine Glutamate GABA (gamma- amino butyric A.) 2. Small dense core vesicles: Catecholamines 3. Large dense core vesicles: neuropeptides: hypothalamic hormones, opioid polypeptides

Synaptic Transmission of Impulse Neural signals are transmitted over long distances through a chain of neurons. A synapse is the junction between two neurons. Typically, the axon terminal divides into several branches and each branch terminates as a knob (synaptic knob) on the soma or a dendrite of another neuron. Ventral horn cells of the spinal cord have thousands of synaptic knobs of different neurons

Types of synapse Axo -somatic Axo - dendritic Axo - axonic

Structure of a Synapse

Chemical Events at a Synapse

Electrical Events at a Synapse

EPSP and IPSP

Ionic Basis of EPSP and IPSP The neurotransmitters that produce EPSP do so by combining with specific receptors on the post synaptic membrane and opening ligand -gated sodium channels. Since a small part of the postsynaptic membrane is involved, the overall decrease in membrane potential of the post synaptic cell is small. Mechanism of IPSP: Opening of ligand -gated chloride channels

5. EPSP No threshold. Decreases resting membrane potential. Closer to threshold. Graded in magnitude. Have no refractory period. Can summate.

6. Synaptic Summation EPSPs can summate, producing AP. Spatial summation: Numerous boutons converge on a single postsynaptic neuron (distance). Temporal summation: Successive waves of neurotransmitter release (time).

a. EPSP b.Spatial summation c.Temporal summation within 1-15msec

Generation of Action Potential If due to spatial or temporal summation, the EPSP is strong enough, it generates an action potential in the axon hillock region of the post-synaptic neuron by current sink action. Axon hillock region has the highest conc. Of Na+ channels, and hence has the lowest threshold of excitation. Axon potential travels down the axon as well as spreads in the soma to clear it of all previous influences

Fate of Neurotransmitters Partly diffuses out of synaptic cleft Partly degraded / hydrolyzed Reuptake: more important : in case of NE Dopamine GABA Glycine Glutamate Reuptake mechanism saves need for re-synthesis Certain drugs can prevent reuptake of certain neurotransmitters. Given to prolong or increase its effect

Synaptic Inhibition Post-synaptic inhibition Pre-synaptic inhibition

6. Presynaptic Inhibition Post-synaptic inhibition Presynaptic inhibition Motor Neuron

Functional significance of Synaptic transmission Synaptic transmission of impulse allows modulation of neural signals like:- 1.Inhibition. 2.Amplification. 3. Divergence. 4.Convergence 5.Interaction of multiple neural inputs