Question

Is it right to say that the giant squid have thick nerve fibres for the quicker conduction of nerve impulses? If not, why?

Answer 1

Neurons with myelin (or myelinated neurons) conduct impulses much faster than those without myelin.
Schwann cells (or oligodendrocytes) are located at regular intervals along the process (axons and, for some neurons, dendrites).Between areas of myelin are non-myelinated areas called the nodes of Ranvier.
Because fat (myelin) acts as an insulator, membrane coated with myelin will not conduct an impulse.
So, in a myelinated neuron, action potentials only occur along the nodes and, therefore, impulses 'jump' over the areas of myelin - going from node to node in a process called saltatory conduction (with the word saltatory meaning 'jumping').
Because the impulse 'jumps' over areas of myelin, an impulse travels much faster along a myelinated neuron than along a non-myelinated neuron.

Velocity of nerve impulses not only depends on myelination but also on the thickness of the nerve fibres. The impulse travels faster in thicker nerve fibres than in thinner nerve fibres. In invertebrates like squids they have nonmyelinated very thick nerve fibres. These help in conducting impulses rapidly to distant parts like the long arms of squids. Vertebrates howeever have evolved a different mechanism for rapid conduction of nerve impulses. They have thin myelinated nerve fibres which carry nerve impulses much faster than the non-myelinated nerve fibres. This has removed the necessity of having inconveniently thick fibres in animals with long limbs.