Q.Compare the following:

(a) Central neural system (CNS) vs Peripheral neural system (PNS):

• The CNS consists of the brain and the spinal cord; the PNS consists of all the nerves of the body that are connected to the CNS.
• The CNS is the command centre where information is processed and control is exercised; the PNS carries information between the CNS and the rest of the body.
• The PNS has afferent (sensory) fibres that bring impulses towards the CNS and efferent (motor) fibres that carry commands away from it; the PNS is grouped into somatic and autonomic systems.

(b) Resting potential vs Action potential:

• Resting potential is the charge difference across the membrane of a neuron that is at rest and not conducting an impulse; action potential is the charge difference produced at a site that has been stimulated.
• In the resting state the membrane is polarised — positive outside, negative inside; during the action potential the polarity reverses — negative outside, positive inside (depolarisation).
• Resting potential is maintained by high internal potassium and the sodium–potassium pump; the action potential arises from a sudden influx of sodium ions, and it is what we call the nerve impulse.

(a) Central neural system (CNS) and Peripheral neural system (PNS)

The human neural system is organised into two major divisions that differ in what they are made of and what they do.

• Make-up. The CNS consists of the brain and the spinal cord. The PNS is made up of all the nerves of the body that are connected to the CNS — that is, to the brain and spinal cord.
• Role. The CNS is the command centre: it is the site where incoming information is processed and from which control over the body's activities is exercised. The PNS is the connecting network that carries information between the CNS and the tissues and organs.
• Direction of traffic. The nerve fibres of the PNS are of two kinds — afferent (sensory) fibres carry impulses from the tissues towards the CNS, and efferent (motor) fibres carry regulatory impulses from the CNS out to the organs. The PNS is further grouped into the somatic system (to skeletal muscles) and the autonomic system (to involuntary organs), whereas the CNS has no such peripheral subdivisions.

(b) Resting potential and action potential

Both are electrical potential differences across the neuron's membrane, but they describe opposite conditions.

• When they occur. The resting potential exists when the neuron is at rest and not conducting any impulse. The action potential appears at a site where a stimulus has been applied.
• State of the membrane. At the resting potential the membrane is polarised — the outer surface is positive and the inner surface is negative. During the action potential this polarity is reversed: the outer surface becomes negative and the inner surface positive, a change called depolarisation.
• How they arise. The resting potential is set up by the uneven distribution of ions — high potassium and negatively charged proteins inside, high sodium outside — and is actively maintained by the sodium–potassium pump, which pumps three sodium ions out for every two potassium ions in. The action potential arises when the stimulated membrane suddenly becomes freely permeable to sodium ions, which rush inward and reverse the charge.