Conduction of the impulse and the synapse · Biology… | StemPy Lab
Course 17.1·Lesson 3 of 3
Conduction of the impulse and the synapse
An impulse moves along an axon because a site that has fired makes a current that fires the site ahead. In a myelinated fibre, the impulse jumps from node to node and is much faster. At the end of the axon the signal crosses a synapse. At an electrical synapse the current passes directly. At a chemical synapse neurotransmitters cross the cleft and may excite or inhibit the next neuron.
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How the impulse moves on
A fibre does not carry the impulse like a wire carries a current. The impulse is made again at every site along the way.
At site A the polarity has reversed. The outside of A is now negative and the inside is positive.
At site B just ahead, the membrane is still at rest: the outside is positive and the inside is negative.
A current flows between A and B to complete the circuit. Inside the axon it runs from A to B. Outside it runs from B to A.
This current reverses the polarity at B. An action potential is generated at B.
The sequence repeats, and the impulse travels the length of the axon.
Three stages of an impulse in an axon, one under the other. In the top stage the whole axon is at rest, with positive charges on the outside and negative charges on the inside. In the middle stage, site A has reversed its polarity, with negative charges outside and positive charges inside, while site B ahead is still at rest. Arrows show current flowing inside the axon from A to B and outside from B to A. In the bottom stage, site B has reversed and site A has returned to the resting polarity, so the impulse has moved forward to B.
The impulse goes forward and not back because the site just left behind is recovering and cannot fire again at once. NCERT does not say this, and it is not in the NEET syllabus, but A-Level and IB ask it.
Myelin makes the impulse faster
In a myelinated fibre the myelin sheath insulates the axon, so ions cross the membrane only at the nodes of Ranvier. The impulse jumps from node to node. This is saltatory conduction, from the Latin word for jumping. The word is not used by NCERT, and it is not in the NEET syllabus, but A-Level and IB ask it.
Two axons, one above the other. In the non-myelinated axon, a small red patch of depolarised membrane moves along in many small steps. In the myelinated axon, the myelin sheath covers the axon and the depolarised membrane appears only at the nodes of Ranvier, with arrows showing the impulse jumping from one node to the next. A note says that the myelinated fibre conducts much faster.
Typical speeds are about 1 m/s in a thin non-myelinated fibre and up to about 100 m/s in a thick myelinated fibre. These are standard values from reference texts, not from NCERT. A thicker fibre also conducts faster.
You should find that the myelinated fibre takes a hundredth of the time at 100 m/s. A message from the spinal cord to the foot would be slow if all fibres were non-myelinated.
The synapse
A synapse is the junction between a presynaptic neuron and a postsynaptic neuron. The membranes may or may not be separated by a gap called the synaptic cleft. There are two types.
Electrical synapse
The membranes of the two neurons are very close.
Electrical current flows directly from one neuron into the other.
The transmission is similar to conduction along one axon, and it is always faster than at a chemical synapse.
Electrical synapses are rare in our system.
Chemical synapse
A fluid filled synaptic cleft separates the two membranes.
Neurotransmitters carry the signal across.
The axon terminal holds vesicles filled with neurotransmitter.
Two panels. On the left, a chemical synapse: the axon terminal of the presynaptic neuron holds vesicles filled with neurotransmitter. An arriving action potential makes the vesicles fuse with the membrane and release the neurotransmitter into the synaptic cleft. The neurotransmitter binds to receptors on the postsynaptic membrane and opens ion channels. On the right, an electrical synapse: the membranes of the two neurons are very close and current flows directly from one neuron into the other. A note says that it is faster but rare.
What happens at a chemical synapse
An action potential arrives at the axon terminal.
The synaptic vesicles move towards the membrane, fuse with it and release the neurotransmitter into the cleft.
The neurotransmitter binds to specific receptors on the postsynaptic membrane.
The binding opens ion channels, and ions enter the postsynaptic neuron.
A new potential may develop. It may be excitatory (it makes firing more likely) or inhibitory (it makes firing less likely).
NCERT does not name a neurotransmitter in this chapter. It names acetylcholine in the chapter on muscle, where the neural signal releases it at the neuromuscular junction. The names dopamine, serotonin, GABA and glutamate come from A-Level and IB. The signal crosses a chemical synapse in one direction only, because the vesicles are on the presynaptic side and the receptors are on the postsynaptic side. Crossing takes a short synaptic delay, about 0.5 ms in the usual textbook value.
Try it in Python
The program adds up the time a message takes along a path with fibres and synapses.
Practise with new numbers
Common mistakes
Saying that the impulse flows like current in a wire. The impulse is made again at each site, and the local current only reverses the polarity of the next site.
Saying that the myelin sheath makes the impulse by itself. The sheath only insulates the axon. The impulse still depends on Na+ and K+ moving at the nodes.
Saying that transmission is always faster at a chemical synapse. An electrical synapse is always faster.
Saying that electrical synapses are the usual type. They are rare in our system. Most synapses are chemical.
Saying that every neurotransmitter excites the next neuron. The new potential may be excitatory or inhibitory.
Remember these
Current from A to B inside the axon, from B to A outside. The impulse moves on.
Electrical synapse: close membranes, direct current, faster, rare. Chemical synapse: cleft, vesicles, neurotransmitter, receptors.
Steps at a chemical synapse: arrival, vesicles fuse, release, binding, channels open, excitatory or inhibitory potential.
A myelinated fibre conducts much faster than a non-myelinated one.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
In which direction does the current flow inside the axon between a firing site A and the resting site B ahead of it?
Why is the nerve impulse faster in a myelinated fibre?
Give two differences between an electrical and a chemical synapse.
Put in order: neurotransmitter binds to receptors, vesicles fuse with the membrane, action potential arrives, neurotransmitter released into the cleft.
A signal takes 10 ms on a pathway of one fibre and one synapse. Another pathway has the same fibre but three synapses. Which is slower, and why?
References
Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 35, The Nervous System, section 35.2, How Neurons Communicate.
Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 12, The Nervous System and Nervous Tissue, section 12.5.
NCERT (2023). Biology, Textbook for Class XI, chapter 18.
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