1.2 - Nerves & Hormones

The nervous system and hormones enable us to respond to external changes. They also help us to control conditions inside our bodies. Hormones are used in some forms of contraception and in fertility treatments. Plants also produce hormones and respond to external stimuli.

Candidates should use their skills, knowledge and understanding to:

• evaluate the benefits of, and the problems that may arise from, the use of hormones to control fertility, including In Vitro Fertilisation (IVF)
• evaluate the use of plant hormones in horticulture as weedkillers and to encourage the rooting of plant cuttings.

B1.2.1 The nervous system

a) The nervous system enables humans to react to their surroundings and coordinate their behaviour.

b) Cells called receptors detect stimuli (changes in the environment).

Receptors and the stimuli they detect include:

• receptors in the eyes that are sensitive to light
• receptors in the ears that are sensitive to sound
• receptors in the ears that are sensitive to changes in position and enable us to keep our balance
• receptors on the tongue and in the nose that are sensitive to chemicals and enable us to taste and to smell
• receptors in the skin that are sensitive to touch, pressure, pain and temperature changes.

c) Light receptor cells, like most animal cells, have a nucleus, cytoplasm and cell membrane.

d) Information from receptors passes along cells (neurones) in nerves to the brain. The brain coordinates the response. Reflex actions are automatic and rapid. They often involve sensory, relay and motor neurones.

e) Candidates should understand the role of receptors, sensory neurones, motor neurones, relay neurones, synapses and effectors in simple reflex actions.

In a simple reflex action:

• impulses from a receptor pass along a sensory neurone to the central nervous system
• at a junction (synapse) between a sensory neurone and a relay neurone in the central nervous system, a chemical is released that causes an impulse to be sent along a relay neurone
• a chemical is then released at the synapse between a relay neurone and motor neurone in the central nervous system, causing impulses to be sent along a motor neurone to the organ (the effector) that brings about the response
• the effector is either a muscle or a gland, a muscle responds by contracting and a gland responds by releasing (secreting) chemical substances.

B1.2.2 Control in the human body

a) Internal conditions that are controlled include:

• the water content of the body – water leaves the body via the lungs when we breathe out and via the skin when we sweat to cool us down, and excess water is lost via the kidneys in the urine
• the ion content of the body – ions are lost via the skin when we sweat and excess ions are lost via the kidneys in the urine
• temperature – to maintain the temperature at which enzymes work best
• blood sugar levels – to provide the cells with a constant supply of energy.

b) Many processes within the body are coordinated by chemical substances called hormones. Hormones are secreted by glands and are usually transported to their target organs by the bloodstream.

c) Hormones regulate the functions of many organs and cells. For example, the monthly release of an egg from a woman’s ovaries and the changes in the thickness of the lining of her womb are controlled by hormones secreted by the pituitary gland and by the ovaries.

d) Several hormones are involved in the menstrual cycle of a woman. Hormones are involved in promoting the release of an egg:

• follicle stimulating hormone (FSH) is secreted by the pituitary gland and causes eggs to mature in the ovaries. It also stimulates the ovaries to produce hormones including oestrogen
• luteinising hormone (LH) stimulates the release of eggs from the ovary
• oestrogen is secreted by the ovaries and inhibits the further production of FSH.

e) The uses of hormones in controlling fertility include:

• giving oral contraceptives that contain hormones to inhibit FSH production so that no eggs mature
  • oral contraceptives may contain oestrogen and progesterone to inhibit egg maturation
  • the first birth-control pills contained large amounts of oestrogen. These resulted in women suffering significant side effects
  • birth-control pills now contain a much lower dose of oestrogen, or are progesterone only
  • progesterone-only pills lead to fewer side effects
• giving FSH and LH in a ‘fertility drug’ to a woman whose own level of FSH is too low to stimulate eggs to mature, for example in In Vitro Fertilisation (IVF) treatment
• IVF involves giving a mother FSH and LH to stimulate the maturation of several eggs. The eggs are collected from the mother and fertilised by sperm from the father. The fertilised eggs develop into embryos. At the stage when they are tiny balls of cells, one or two embryos are inserted into the mother’s uterus (womb).

B1.2.3 Control in plants

a) Plants are sensitive to light, moisture and gravity:

• their shoots grow towards light and against the force of gravity
• their roots grow towards moisture and in the direction of the force of gravity.

b) Plants produce hormones to coordinate and control growth. Auxin controls phototropism and gravitropism (geotropism).

c) The responses of plant roots and shoots to light, gravity and moisture are the result of unequal distribution of hormones, causing unequal growth rates.

d) Plant growth hormones are used in agriculture and horticulture as weed killers and as rooting hormones.

Suggested ideas for practical work to develop skills and understanding include the following:

• investigation into candidates’ reaction times – measuring reaction times using metre rules, stop clocks or ICT
• using forehead thermometers before and after exercise
• demonstrating the speed of transmission along nerves by candidates standing in a semi-circle and holding hands and squeezing with eyes closed
• design an investigation to measure the sensitivity of the skin demonstrating the knee jerk reaction
• investigation to measure the amount of sweat produced during exercise
• investigate:
  • the effect of light on the growth of seedlings
  • the effect of gravity on growth in germinating seedlings
  • the effect of water on the growth of seedlings
  • using a motion sensor to measure the growth of plants and seedlings
  • the effect of rooting compounds and weed killers on the growth of plants.