Different species of animal have neurones with different characteristics.
(a) Fig. 1.1 is a diagram of a motor neurone of a rat and a motor neurone of a snail.
(i) Name the structures labelled A, B and C on Fig. 1.1.
(ii) The rat motor neurone has an impulse transmission speed of 50 m s-1. The snail motor neurone has an impulse transmission speed of 8 m s-1.
Explain why the rat motor neurone has a faster impulse transmission speed than the snail motor neurone.
(b) Fig. 1.2 shows an action potential in a rat neurone and Fig. 1.3 shows an action potential in a snail neurone.
Contrast the two action potentials shown in Fig. 1.2 and Fig. 1.3.
▶️ Answer/Explanation
(a)(i)
A = dendrite(s)
B = nucleus/nuclei
C = synaptic knob(s)
Explanation: The structures are identified based on their location and function in the neurone. Dendrites receive signals, the nucleus contains genetic material, and synaptic knobs are the terminal ends that release neurotransmitters.
(a)(ii)
The rat motor neurone has a myelin sheath/Schwann cell/myelinated axon which allows for saltatory conduction where the impulse jumps from one node of Ranvier to another, significantly increasing transmission speed.
Detailed Explanation: The key difference lies in myelination. Rat neurones are myelinated, meaning they have a fatty sheath that insulates the axon. This allows the action potential to “jump” between nodes of Ranvier (gaps in the myelin sheath), a process called saltatory conduction. In contrast, snail neurones are unmyelinated, so the action potential must travel continuously along the entire length of the axon, which is much slower. Additionally, the rat neurone has a larger axon diameter (40 μm vs 7 μm), which also contributes to faster conduction.
(b)
1. The snail action potential shows greater depolarization (reaches a more positive membrane potential) compared to the rat.
2. The snail action potential is slower (takes longer to complete) than the rat’s.
3. The snail has a longer refractory period (time before another action potential can be generated).
4. The snail shows longer hyperpolarization (extended period where membrane potential is more negative than resting potential).
Detailed Explanation: The action potentials differ in several key aspects. The snail’s action potential peaks at a higher membrane potential (more positive) indicating greater depolarization. The entire cycle from resting potential through depolarization, repolarization, and hyperpolarization takes significantly longer in the snail neurone. The refractory period (the time when the neurone cannot fire another action potential) is also longer in the snail. These differences reflect variations in ion channel density, types, and kinetics between the species. The rat’s faster, more efficient action potential is better suited for rapid nervous system responses needed by a more active organism.