Synaptic Transmission

Synapses The “synapse” is the physical gap between pre- and post-synaptic membranes (~20-30 nMeters) Presynaptic membrane is typically an axon The axon terminal contains Mitochondria that provide energy for axon functions Vesicles (round objects) that contain neurotransmitter Cisternae that are a part of the Golgi apparatus: recycle vesicles Postsynaptic membrane can be A dendrite (axodendritic synapse) A cell body (axosomatic synapse) Another axon (axoaxonic synapse) Postsynaptic thickening lies under the axon terminal and contains receptors for transmitters

Synapses Junction is the connection between a nerve cell and another (muscle fiber or gland). Synapse is the connection between 2 nerve cells.

Synapses

CLASSIFICATION OF SYNAPSES

Synapses could be classified according to 1- Either their location between the pre and postsynaptic neurons (histological classification) 2- The mechanism of transmission of impulses across them (physiological classification)

histological classification synapses are classified into three types:- 1. Axodendritic synapses: These are synapses between the axon terminals of the presynaptic neuron and the dendrites of the postsynaptic neuron. 2. Axosomatic synapses: These are synapses between the axon terminals of the presynaptic neuron and the soma of the postsynaptic neuron 3. Axoaxonic synapses: These are synapses between the axon terminals of the presynaptic neuron and the axon of the postsynaptic neuron.

histological classification

PHYSIOLOGICAL CLASSIFICATION synapses are classified into three types: 1. CHEMICAL SYNAPSES: 2. ELECTRICAL SYNAPSES: 3. CONJOINT SYNAPSES: (ELECTROCHEMICAL)

CHEMICAL SYNAPSES In these synapses, transmission of signals occurs by releasing a ''chemical transmitter" from the presynaptic terminal into the synaptic cleft. The transmitter then acts on specific receptors on the postsynaptic membrane to generate postsynaptic potential. There are more than 40 different synaptic transmitters in the CNS which are either small molecule rapidly acting (acetylcholine) or large molecule slowly acting (substance P). Chemical synapses are the only type of synapses found in the human nervous system.

ELECTRICAL SYNAPSES In these synapses, there are gap junctions between the pre and postsynaptic membranes which allow the transmission of the depolarization wave directly from the pre to the postsynaptic membrane.

CONJOINT SYNAPSES In these synapses, transmission of impulses occurs by both mechanisms electrical and chemical. They are found in some fish and invertebrates.

SYNAPSES Action potential  synaptic knob  opening of Ca+ channels neurotransmitter vesicles fuse with membrane release of neurotransmitter into synaptic cleft binding of neurotransmitter to protein receptor molecules on receiving neuron membrane opening of ion channels triggering of new action potential Neurotransmitter is broken down by enzymes & ion channels close.

SYNAPTIC

Cell-to-Cell Communication Between Neurons Takes Place At Synapses A synapse is a region at which a neuron communicates with its target cell. The synapse is composed of 3 parts: (a) the axon terminal; (b) the synaptic cleft, the space between the cells and (c) the membrane of the postsynaptic cell.

Acetylcholine (ACh): Synthesis and Breakdown Synthesis requires the enzyme choline acetyltransferase (ChAT) Made from choline (Ch) and acetyl coenzyme (acetyl CoA) in the axon terminal then filled into synaptic vesicles. Once released into the cleft, its rapidly broken down by the enzyme acetylcholinesterase (AChE). Ch is transported back into the axon terminal and reused to make ACh.