IGCSE Biology Paper-4: Specimen Questions with Answers 198 - 198 of 279

Glide to success with Doorsteptutor material for Bank-PO : get questions, notes, tests, video lectures and more- for all subjects of Bank-PO.

Question 198


Describe in Detail


Impulses travel between neuron across a synapse.

Describe how impulses travel across the synapse from one neuron to another.


Mechanism of Transmission of Nerve Impulse at a Synapse

The process of chemical transmission across synapses was discovered by Henry Dale (1936) . The physiological importance of synapse for the transmission of nerve impulses was established by McLennan in 1963. A brief description of the mechanism of synaptic transmission is given below:

  • When an impulse arrives at a presynaptic knob, calcium ions from the synaptic cleft enter the cytoplasm of the presynaptic knob.
  • The calcium ions cause the movement of the synaptic vesicles to the surface of the knob. The synaptic vesicles are fused with the presynaptic membrane and get ruptured (exocytosis) to discharge their contents (neurotransmitter) into the synaptic cleft.
  • The synaptic vesicles then return to the cytoplasm of the synaptic knob where they are refilled with neurotransmitter.
  • The neurotransmitter of the synaptic cleft binds with protein receptor molecules on the post synaptic membrane. This binding action changes the membrane potential of the postsynaptic membrane, opening channels in the membrane and allowing sodium ions to enter the cell. This causes the depolarization and generation of action potential in the post-synaptic membrane. Thus, the impulse is transferred to the next neuron.
  • Having produced a change in the permeability of the postsynaptic membrane the neurotransmitter is immediately lost from the synaptic cleft. In the case of cholinergic synapses, acetylcholine (ACh) is hydrolyzed by an enzyme acetylcholinesterase (AChE) which is present in high concentration at the synapse.
  • The products of the hydrolysis are acetate and choline which are reabsorbed into the synaptic knob where they are resynthesized into acetylcholine, using energy from ATP.

Neuromuscular Junction

Impulses are conducted from a neuron to a muscle cell across an area of contact called neuromuscular junction. When a nerve fiber ends on a muscle fiber, it forms motor end plate. The motor end plates have vesicles and mitochondria. The vesicles secrete neurotransmitter. When the motor impulse from the nerve is received on the motor end plates, a local depolarization occurs there resulting in the excitation of the muscle fiber.

Synaptic Vesicle

Developed by: