(a) What are hormones? (b) In a tabular form, state four differences between nervous coordination and hormonal coordination in humans. (c) State three effec...

Answer Details

(a) Hormones

Hormones are chemical substances (organic compounds) produced in very small quantities by the endocrine (ductless) glands of the body, and carried by the bloodstream to specific target organs where they regulate and coordinate the body's activities, growth and development.

(b) Four differences between nervous and hormonal coordination

(c)(i) Three effects of over-secretion of thyroxin

• Raised metabolic rate leading to loss of body weight despite increased appetite.
• Rapid or irregular heartbeat, restlessness, nervousness and excitability.
• Protrusion of the eyeballs and enlargement of the thyroid gland (exophthalmic goitre / Grave's disease).

(c)(ii) Three effects of under-secretion of thyroxin

• Lowered metabolic rate leading to gain in body weight and sluggishness.
• In children, retarded physical and mental growth (cretinism).
• In adults, swelling of the tissues, dry skin and mental dullness (myxoedema); enlargement of the thyroid (simple goitre) where iodine is lacking.

(d) Diagram of the reflex arc

A labelled diagram, 10 - 12 cm long, should show a spinal-cord reflex arc with these labelled parts arranged in order: receptor in the skin, sensory (afferent) neurone with its cell body in the dorsal root ganglion, dorsal root, the grey matter and white matter of the spinal cord with a relay (intermediate) neurone, synapses, motor (efferent) neurone leaving by the ventral root, and the effector (muscle). Arrows should show the direction of the impulse from receptor to effector.