Transmission of Impulses: Synapse
தூண்டல் கடத்தல்: சினாப்ஸ்
Electrical synapses are faster than chemical ones — not slower. And neurotransmitter receptors sit on the post-synaptic membrane, never on the pre-synaptic side.
The idea
A nerve impulse is transmitted from one neuron to another through junctions called synapses. A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called the synaptic cleft. There are two types of synapses — electrical and chemical. At electrical synapses the membranes of pre- and post-synaptic neurons are in very close proximity, and electrical current can flow directly from one neuron into the other across these synapses; transmission of an impulse across an electrical synapse is very similar to impulse conduction along a single axon, and is always faster than that across a chemical synapse. Electrical synapses are rare in our system. At a chemical synapse the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called the synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses. The axon terminals contain vesicles filled with these neurotransmitters. When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane, where they fuse with the plasma membrane and release their neurotransmitters into the synaptic cleft. The released neurotransmitters bind to their specific receptors present on the post-synaptic membrane. This binding opens ion channels, allowing the entry of ions which can generate a new potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory.
Why it matters
Chemical synapses let one neuron excite or inhibit another and let signals be adjusted, combined or blocked — the basis of all decision-making in the nervous system.
NEET lens
Statement questions: chemical synapses use neurotransmitters; electrical synapses — membranes very close, current flows directly, faster, rare in our system; chemical — synaptic cleft (fluid-filled), vesicles fuse and release neurotransmitter, receptors on post-synaptic membrane, new potential may be excitatory or inhibitory. 'Transmission across a chemical synapse is always faster than across an electrical synapse' is the incorrect statement.
Clinical significance
Many medicines and drugs act at chemical synapses — for example, some antidepressants keep neurotransmitters longer in the synaptic cleft. In myasthenia gravis, antibodies block acetylcholine receptors on the post-synaptic muscle membrane.
Introduction
Impulses pass from one neuron to the next at synapses, which are either electrical or chemical.
Core concept
Synapse = pre-synaptic membrane + post-synaptic membrane (± synaptic cleft). Electrical synapse: membranes very close; current flows directly; similar to conduction along an axon; always faster than chemical; rare in humans. Chemical synapse: fluid-filled synaptic cleft; axon terminal vesicles contain neurotransmitters → action potential arrives → vesicles move to and fuse with membrane → neurotransmitter released into cleft → binds specific receptors on post-synaptic membrane → ion channels open → ions enter → new potential (excitatory or inhibitory) in post-synaptic neuron.
Go deeper
Because receptors exist only on the post-synaptic membrane, impulses at a chemical synapse travel in one direction only.
From Class 10
Class 10 described the synapse as a gap where chemicals carry the signal. NEET adds electrical synapses and where the receptors are.