Synaptic Transmission: Electrical and Chemical Synapses

NCERT grounding

Section 18.3.2 of NCERT Class 11 Biology, titled Transmission of Impulses, opens with a definition the examiner returns to year after year: 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. NCERT then names exactly two kinds — electrical synapses and chemical synapses — and devotes the rest of the section to contrasting how they conduct the impulse. The NIOS Senior Secondary Biology lesson 17 echoes the same partition under the heading Conduction of Nerve Impulse along the Neuron and Over the Synapse, stating plainly that conduction through the nerve fibre is electrical while conduction through the synapse is chemical.

"Impulse transmission across an electrical synapse is always faster than that across a chemical synapse. Electrical synapses are rare in our system."

NCERT Class 11 · Chapter 18 · §18.3.2

That single sentence has been examined in NEET 2022 as a direct true/false trap (with the options inverted to catch quick readers) and supports the receptor-site question of NEET 2017. The rest of the section anchors a second tested fact: the receptor sites for neurotransmitters lie on the post-synaptic membrane, never on the pre-synaptic side or on the vesicle wall. Master these two NCERT lines and you have already locked the conceptual core of the subtopic.

How a synapse transmits an impulse

The synapse is where the all-or-none action potential — the topic of the previous sibling page — leaves the axon and decides whether the next cell fires. That decision is not a wire connection in most human synapses. It is a brief chemical event lasting under a millisecond, but every NEET-relevant step happens inside the synaptic knob, the bulb-like swelling at the tip of the axon terminal. The knob holds two NCERT-named structures you must be able to label on a diagram: synaptic vesicles filled with neurotransmitter, and the pre-synaptic membrane that faces the next cell.

The two NCERT-named synapse types

NCERT recognises only two architectural categories of synapse, and the way they conduct an impulse is radically different. A chemical synapse converts the electrical signal into a chemical messenger and back into an electrical signal; an electrical synapse skips the chemistry entirely and lets ionic current flow continuously through cytoplasmic bridges.

Anatomy of the chemical synapse

Walk from the axon hillock outward and the axon eventually narrows to a non-myelinated tip. That tip is the synaptic knob. Inside it you will find dozens to hundreds of synaptic vesicles, each a small membrane-bound sac stuffed with one species of neurotransmitter molecule — acetylcholine, glutamate, GABA, glycine, or one of the monoamines, depending on the neuron. Vesicles cluster near the inner face of the pre-synaptic membrane at specialised release zones, ready to fuse when calcium signals them to do so.

Across the synaptic cleft sits the post-synaptic membrane, which belongs either to a dendrite or cell body of the next neuron or, at a neuromuscular junction, to a muscle fibre. Embedded in this membrane are ligand-gated ion channels — the receptors. They differ from the voltage-gated channels of the axon in one crucial respect: they open in response to a chemical (the neurotransmitter), not to a change in voltage. The cleft itself is filled with extracellular fluid and is so narrow that a released neurotransmitter molecule diffuses across it in microseconds.

The six-step chemical-synapse mechanism

NCERT compresses the chemical-synapse mechanism into a short paragraph; NEET expands it into six examinable steps. Memorise them in order — every plausible distractor in this subtopic is built from swapping or omitting one of these stages.

Two NCERT phrases sit inside this flow and reward exact wording. Step 3 — NCERT says vesicles fuse with the plasma membrane of the axon terminal. Step 5 — NCERT says the released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane. Step 6 — NCERT closes the section with: "the new potential developed may be either excitatory or inhibitory." That single sentence is the textbook source for both EPSP and IPSP, even though NCERT itself does not use those acronyms.

Calcium is the trigger — not sodium

The most commonly tested mechanistic detail at this synapse is the identity of the trigger ion. Along the axon, sodium drives depolarisation; at the synaptic knob, the action potential is already there — what the knob needs is a signal to release neurotransmitter. That signal is Ca²⁺. Voltage-gated Ca²⁺ channels in the pre-synaptic membrane open in response to the arriving depolarisation, Ca²⁺ floods into the knob, and the rise in intracellular Ca²⁺ is what unbolts the vesicles for fusion. Block these channels (as some neurotoxins do) and the axon still fires action potentials, but the synapse goes silent — vesicles never fuse, no neurotransmitter is released, the post-synaptic cell hears nothing.

Neurotransmitters: ACh, GABA, and the excitatory/inhibitory split

NCERT names acetylcholine (ACh) as the classic neurotransmitter at chemical synapses and at the neuromuscular junction. Released from cholinergic neurons, ACh binds nicotinic receptors on skeletal muscle, opens cation channels and produces a depolarising end-plate potential — that is, an EPSP large enough to trigger a muscle action potential and a contraction. ACh is rapidly hydrolysed in the cleft by acetylcholinesterase, which ends the signal and prevents continuous firing; this is also why anticholinesterase agents such as organophosphate insecticides are dangerous.

The principal inhibitory neurotransmitter of the mammalian brain is GABA (gamma-aminobutyric acid). It opens Cl⁻ channels on the post-synaptic membrane; Cl⁻ flowing in (or, at some synapses, K⁺ flowing out) drives the membrane away from threshold, producing a hyperpolarising IPSP. Glycine plays the same inhibitory role in the spinal cord. Whether a chemical synapse is excitatory or inhibitory depends not on the neurotransmitter alone but on the ion channel the receptor opens — and that is determined by the post-synaptic side, not the pre-synaptic one.

EPSP, IPSP and summation: how the post-synaptic neuron decides

A single chemical synapse rarely fires the next neuron on its own. The post-synaptic potential it produces — whether EPSP or IPSP — is graded, meaning its amplitude depends on how much neurotransmitter was released. Graded potentials are also local: they decay with distance as they spread passively across the dendritic membrane toward the axon hillock. To trigger an action potential, many EPSPs must arrive close together in time (temporal summation) or at neighbouring synapses (spatial summation) and overwhelm any IPSPs reaching the cell. Only when the net membrane potential at the axon hillock crosses threshold does the post-synaptic neuron itself fire an all-or-none action potential, which then propagates down its axon to the next synapse.

Summation explains why the brain can do arithmetic with synapses. Each neuron is bombarded by thousands of pre-synaptic inputs, both excitatory and inhibitory, and its output reflects the running sum. A small change in GABAergic tone can shift the balance from firing to silent without altering any individual EPSP.

Why chemical transmission is unidirectional

Anatomy enforces direction. Vesicles loaded with neurotransmitter exist only on the pre-synaptic side; receptors exist only on the post-synaptic side. An impulse arriving backwards in the post-synaptic axon has no vesicles to release and, even if neurotransmitter leaked the other way, would find no receptors on the pre-synaptic membrane. Unidirectionality is therefore a structural property, not a regulatory one. Compare this with an electrical synapse, where gap junctions are symmetrical channels open in both directions — those synapses can pass current both ways, which is why NCERT-style answers describe them as bidirectional in addition to faster.

Electrical synapse — the rarer cousin

Although NCERT mentions them in a single paragraph, electrical synapses are worth understanding because they are tested directly. At an electrical synapse, the two membranes are bridged by gap junctions — paired protein channels (connexons) that span both cells and create a continuous cytoplasmic tunnel. Ions and small molecules pass through this tunnel, so an action potential in one cell instantly depolarises the next. There is no cleft to cross, no vesicle to fuse, no receptor to bind — and therefore no synaptic delay worth measuring. Electrical synapses are also typically bidirectional because the gap-junction channels are symmetrical.

In humans, electrical synapses are rare. They are found mainly where speed and synchrony matter more than computation — for example, between certain interneurons in the brainstem that need to fire together, in some cardiac muscle and smooth muscle (gap junctions in the heart are how the wave of contraction spreads), and in a few specialised sensory circuits. NCERT's exact phrasing is that electrical synapses are "rare in our system," which lets you eliminate any option claiming they dominate human signalling.

Worked examples

Common confusion & NEET traps