Q.Differentiate between:

1. Myelinated vs non-myelinated axons: A myelinated axon is enveloped by Schwann cells that form a myelin sheath around it, with gaps between adjacent sheath segments called nodes of Ranvier; such fibres occur in the spinal and cranial nerves. A non-myelinated axon is enclosed by a Schwann cell that does not form a myelin sheath, and has no nodes of Ranvier; such fibres are common in the autonomic and somatic neural systems.
2. Dendrites vs axons: Dendrites are short, repeatedly branching fibres projecting from the cell body that carry impulses towards the cell body; a neuron has many. An axon is a single long fibre, branched only at its far end into synaptic knobs, that carries impulses away from the cell body towards a synapse or a neuro-muscular junction.
3. Thalamus vs hypothalamus: The thalamus is a major coordinating centre that relays and integrates sensory and motor signals passing to and from the higher brain centres. The hypothalamus lies at the base of the thalamus and, though small, controls body temperature and the urges to eat and drink, and contains neurosecretory cells that secrete hypothalamic hormones. …

(a) Myelinated and non-myelinated axons

Axons come in two kinds depending on whether an insulating wrapping is present.

• A myelinated axon is enveloped by Schwann cells that lay down a myelin sheath around the fibre. Between two adjacent segments of this sheath there are small gaps called the nodes of Ranvier. Myelinated fibres are found in the spinal and cranial nerves.
• A non-myelinated axon is enclosed by a Schwann cell that does not form a myelin sheath around it, so there are no nodes of Ranvier. Such fibres are commonly found in the autonomic and somatic neural systems.

(b) Dendrites and axons

Both are fibres projecting from the neuron's cell body, but they differ in shape, number and the direction in which they carry impulses.

• Dendrites are short fibres that branch repeatedly and project out from the cell body; they contain Nissl's granules and carry impulses towards the cell body. A neuron usually has several dendrites.
• An axon is a single long fibre whose far end is branched, each branch ending in a bulb-like synaptic knob that holds synaptic vesicles filled with neurotransmitters. The axon carries impulses away from the cell body towards a synapse or a neuro-muscular junction.

(c) Thalamus and hypothalamus

Both belong to the forebrain and lie close together, but they do very different jobs.

• The thalamus is wrapped by the cerebrum and works as a major coordinating centre for sensory and motor signalling, relaying and integrating these signals as they pass to and from the higher centres of the brain.
• The hypothalamus lies at the base of the thalamus. Although small, it contains centres that regulate body temperature and the urges for eating and drinking, and it holds neurosecretory cells that secrete hypothalamic hormones, linking the nervous system to hormonal control.

(d) Cerebrum and cerebellum …