how does the pre-synaptic neuron transmit an impulse across the synaptic cleft to the post synaptic neuron ?
Synaptic cleft is the gap between the terminal end of axon of pre synaptic neuron and the dendrites of post synaptic neuron. When an impulse reaches at terminal end of axon of pre synaptic neuron, it stimulates the movement of synaptic vesicles towards the membrane. Here, they fuse with the plasma membrane and release their neurotransmitters at the synaptic cleft where they bind with their specific receptors present on the post synaptic neuron. This binding opens the ion channels , allowing entry of ions which can generate a new impulse in post synaptic neuron. This way transmission of impulse occurs across the synaptic cleft form presynaptic neuron to the post synaptic neuron.
Hope this information will clear your doubts about the transmission of impulse.
Neurotransmission at a chemical synapse begins with the arrival of an action potential at the presynaptic axon terminal. When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated Na+ channels. Na+ ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated Ca2+ channels to open. Calcium ions entering the cell initiate a signaling cascade. A calcium sensing protein binds calcium and interacts with SNARE proteins. These SNARE proteins are involved in the membrane fusion. The synaptic vesicles fuse with the presynaptic axon terminal membrane and empty their contents by exocytosis into the synaptic cleft. Calcium is quickly removed from the terminal.Fusion of a vesicle with the presynaptic membrane causes neurotransmitters to be released into the synaptic cleft. The neurotransmitter diffuses across the synaptic cleft, binding to receptor proteins on the postsynaptic membrane.The binding of a specific neurotransmitter causes particular ion channels, in this case ligand-gated channels, on the postsynaptic membrane to open. The binding of a neurotransmitter to its receptor is reversible. As long as it is bound to a post synaptic receptor, a neurotransmitter continues to affect membrane potential. The effects of the neurotransmitter generally lasts few milliseconds before being terminated. The neurotransmitter termination can occur in three ways. First, reuptake by astrocytes or presynaptic terminal where the neurotransmitter is stored or destroyed by enzymes. Second, degradation by enzymes in the synaptic cleft such as acetylcholinesterase. Third, diffusion of the neurotransmitter as it moves away from the synapse..