In human beings, certain chemicals which cross the synapse between nerve cells are released from

Correct — C, nerve endings of nerve cell. A synapse is the narrow gap between the ending of one neuron and the next cell. When an electrical impulse arrives at the axon terminal, it triggers vesicles stored there to fuse with the membrane and release neurotransmitter molecules into the gap. Those molecules diffuse across and bind receptors on the next cell, starting a fresh impulse. The release happens at the terminal endings, not in the cell body, the nucleus, or along the length of the axon.

• (a)cell body of nerve cell — The cell body receives incoming signals and houses the machinery that manufactures the transmitter, but the release site is far away at the far end of the axon.
• (b)nucleus of nerve cell — The nucleus holds the DNA that codes for the enzymes making the transmitter. It is a long way from the synaptic gap and releases nothing into it.
• (d)axon of nerve cell — The axon conducts the impulse and is the road, not the destination. The vesicles that release the transmitter are concentrated at the swollen terminals in which the axon ends.

A nerve impulse travels electrically within a neuron and chemically between neurons. Along the axon it is a wave of membrane depolarisation; at the synapse it converts into a puff of neurotransmitter that crosses a gap of about twenty nanometres. That conversion imposes a short delay and, importantly, makes transmission one-way, because only the sending side has vesicles and only the receiving side has receptors.

The wording of the stem is the clue — it says the chemicals cross the synapse, so the release point has to be the structure that borders the synapse. That is the terminal ending, sometimes called the synaptic knob. Acetylcholine, dopamine, serotonin and noradrenaline are among the transmitters used, and a great many drugs and toxins act precisely here, by blocking a receptor, by preventing reuptake, or by stopping the enzyme that clears the transmitter away.

• The synapse is the junction between the terminal ending of one neuron and the next cell, with a gap of about twenty nanometres.
• Neurotransmitters are stored in vesicles at the axon terminal and released when the impulse arrives.
• Because vesicles sit only on the sending side and receptors only on the receiving side, transmission across a synapse is one-way.
• Acetylcholine is the transmitter at the neuromuscular junction; dopamine, serotonin and noradrenaline act within the brain.
• A neuron carries the impulse from dendrites to cell body to axon to terminal endings.

• Placing the release in the cell body because the transmitter is manufactured there.
• Assuming a synapse is a physical contact; it is a gap, and the signal crosses chemically.
• Treating synaptic transmission as two-way when the structure makes it one-way.

A precise-site item. The options walk through the parts of a neuron, so the answer depends on knowing which part borders the synapse.

• practice — not a real PYQ

  Transmission of a nerve impulse across a synapse is achieved by

  • (a)direct electrical contact between the two cells
  • (b)chemical messengers released into the synaptic gap
  • (c)the flow of blood between the neurons
  • (d)movement of the whole nerve cell

  Answer(b) chemical messengers released into the synaptic gap — the electrical signal is converted into a chemical one and back again.

• practice — not a real PYQ

  The neurotransmitter released at the junction between a motor neuron and a skeletal muscle fibre is

  • (a)adrenaline
  • (b)acetylcholine
  • (c)insulin
  • (d)thyroxine

  Answer(b) acetylcholine — it crosses the neuromuscular junction and triggers contraction of the muscle fibre.