The Neuron and the Nerve Impulse: Quick Revision

A brief note on The Neuron and the Nerve Impulse. Read it before a test. Try each question at the end before you open its answer.

The nervous system

• CNS: brain and spinal cord. PNS: the nerves.
• Afferent: towards the CNS. Efferent: away from the CNS.
• Somatic: to skeletal muscles. Autonomic: to involuntary organs and smooth muscles, with a sympathetic and a parasympathetic part.
• Visceral nervous system: nerves, fibres, ganglia and plexuses between the CNS and the internal organs.

The neuron

• Cell body (cyton): nucleus and Nissl granules. Dendrites: bring impulses in. Axon: carries impulses away. Synaptic knobs: vesicles with neurotransmitter.
• Multipolar: one axon, two or more dendrites (cerebral cortex). Bipolar: one axon, one dendrite (retina). Unipolar: one axon only (embryonic stage).
• Myelinated: Schwann cells form the sheath, with nodes of Ranvier (spinal and cranial nerves). Non-myelinated: no sheath (autonomic and somatic systems).

The impulse

• At rest: more permeable to K+, almost impermeable to Na+. Outside positive, inside negative. The pump moves 3 Na+ out and 2 K+ in.
• Action potential: the stimulus makes the membrane freely permeable to Na+. The polarity reverses. Then the permeability to K+ rises, K+ leaves and the resting potential returns.
• Conduction: current flows inside from the active site to the next site, and outside the other way.
• Model values: rest -70 mV, threshold -55 mV, peak +30 mV. NCERT gives no numbers.

The synapse

• Electrical: membranes very close, current passes directly, faster, rare.
• Chemical: cleft, vesicles, neurotransmitter, receptors, channels open, excitatory or inhibitory potential.

A worked example

A myelinated fibre conducts at 60 m/s. How long does an impulse take to travel 1.2 m?

1. Time in seconds: $1.2 \div 60 = 0.02$ s.
2. In milliseconds: $0.02 \times 1000 = 20$ ms.

Practice with new numbers

A program to try

The program works out the time of a message in two kinds of fibre.

Mistakes to avoid

• Mixing up afferent and efferent. Afferent fibres arrive at the CNS.
• Putting the Nissl granules in the axon. They are in the cell body.
• Saying that the resting membrane is more permeable to Na+. It is more permeable to K+.
• Saying that the pump moves 2 Na+ out and 3 K+ in. It moves 3 Na+ out and 2 K+ in.
• Saying that electrical synapses are the common kind. They are rare.

Check yourself

1. What does an afferent fibre do?

2. Which part of a neuron carries the impulse away from the cell body?

3. Which ion rushes in when a stimulus acts on a site of the axon?

4. Name one difference between electrical and chemical synapses.

Sources

• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 35, The Nervous System.
• NCERT (2023). Biology, Textbook for Class XI, chapter 18.