Neural Control And Coordination

Synapses are junctions across which impulses pass from one neuron to the next, and they exist as electrical or chemical types. NCERT Class 11, Chapter 21 (Neural Control and Coordination) explains that at a chemical synapse the pre- and postsynaptic membranes are separated by a synaptic cleft, and the arriving impulse causes synaptic vesicles to release chemical neurotransmitters that diffuse across the cleft and bind receptors on the postsynaptic membrane, generating a new potential. Direct flow of ions through gap junctions describes electrical synapses, where membranes are in very close proximity; this is not the predominant mode at chemical synapses, so that option is wrong here. Movement of the entire axon does not occur; the axon stays in place while signalling proceeds chemically, making this physically incorrect. Diffusion of carbon dioxide plays no role in synaptic transmission and confuses respiratory gas exchange with neural signalling. Hence release of neurotransmitters is correct. A mechanistic plausibility check supports it: a fluid-filled cleft physically separates the two neurons at a chemical synapse, so the signal must be carried by a diffusible chemical messenger rather than direct electrical continuity.