B1.2 Nerves and Hormones

B1.2 Nerves and 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.


The nervous system

  • The nervous system enables humans to react to their surroundings and coordinate their behaviour.
  • Central nervous system = brain plus spinal cord.
  • Stimuli = changes in the environment.
  • Receptors = cells that detect stimuli
  • Nerve impulse = electrical message that passes along a neurone.
  • Neurones = nerve cells.


  • Neurones are highly specialised cells:
    • Very long so nerve impulses can travel quickly to different parts of the body.
    • Branched ends to form connections with many other neurones.
    • Insulating sheath to maintain the nerve impulse.
  • Nerve = a bundle of neurones connected to brain or spinal cord.
  • Sensory neurone = nerve cell that transmits nerve impulse from a receptor to the central nervous system.
  • Relay neurone = neurone in the central nervous system.
  • Motor neurone = nerve cell that transmits nerve impulse from the central nervous system to an effector.
  • Effector = a structure that the nervous system causes to respond – a muscle or gland.



  • Synapses = junctions between nerve cells.
  • When a nerve impulse arrives at the end of a neurone, chemicals are released.
  • These diffuse across the synapse, and cause a new nerve impulse in the next neurone.




  • 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 to temperature changes.
  • Light receptor cells, like most animal cells, have a nucleus, cytoplasm and cell membrane.


  • Information from receptors passes along neurones in nerves to the spinal cord and the brain.
  • The brain coordinates the response.

Reflex actions

  • Reflex actions are automatic and rapid.
  • They are simple responses to stimuli that often protect the body from harm.
  • They often involve sensory, relay and motor neurones.
  • The pathway starting with a stimulus and resulting in a response does not require conscious control by the brain.
  • In a simple reflex action:
    • Impulses from a receptor pass long a sensory neurone to the central nervous system
    • There is a synapse between a sensory neurone and a relay neurone in the central nervous system
    • A chemical is released at the synapse between the sensory neurone and a relay neurone.
    • This causes an impulse to be sent along the relay neurone
    • A chemical is then released at the synapse between a relay neurone and motor neurone in the central nervous system
    • This causes impulses to be sent along a motor neurone to the effector
    • This is either a muscle or a gland
    • A muscle responds by contracting
    • A gland responds by releasing (secreting) chemical substances eg salivary gland releases saliva.

Control in the human body

  • Internal conditions that are controlled include:
    • The water content of the body:
      • Water leaves the body:
        • via the lungs when we breathe out
        • via the skin when we sweat to cool us down.
      • 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
      • Excess ions are lost via the kidneys in the urine
    • Temperature:
      • To maintain the temperature at which enzymes work best.
      • Enzymes are protein molecules that control reactions inside and outside cells.
      • They are sensitive to changes in temperature and work best at body temperature – 37o
    • Blood sugar levels:
      • To provide the cells with a constant supply of energy.
      • We take in sugars as carbohydrate in our food.



  • Many processes within the body are coordinated by chemical substances called hormones.
  • Hormones are secreted by glands.
  • They are transported to their target organs by the bloodstream.
  • Hormones regulate the functions of many organs and cells.


Menstrual cycle

  • The monthly release of an egg from a woman’s ovaries
  • The changes in the thickness of the lining of her womb
  • These are controlled by hormones secreted by the pituitary gland and by the ovaries.
  • They are involved in promoting the release of an egg.
  • Follicle stimulating hormone (FSH):
    • Secreted by the pituitary gland
    • Causes eggs to mature in the ovaries.
    • It also stimulates the ovaries to produce hormones including oestrogen.
  • Oestrogen:
    • Secreted by the ovaries.
    • Inhibits the further production of FSH.
    • Brings about the release of LH.
  • Luteinising hormone (LH):
    • Stimulates the release of eggs from the ovary

The use of artificial fertility controlling hormones

  • Hormones can be synthesised artificially.
  • These are very similar to human hormones, and can be used to affect the way the body works.
  • Some people are concerned about the use of hormones that control fertility.


Oral contraceptives:

  • Oral contraceptives 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
  • Progesterone-only pills lead to fewer side effects.
  • Birth-control pills now contain a much lower dose of oestrogen, or are progesterone only.
  • Some religions do not encourage the use of hormones that prevent conception.


Fertility drugs

  • Fertility drugs can be given to women whose own level of FSH is too low to stimulate eggs to mature.
  • They contain FSH and LH.
  • This stimulates eggs to mature.
  • This increases the chances of getting pregnant.
  • These drugs are also used 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).
  • Some people believe that the human population is growing too quickly anyway.
  • Fertility drugs can result in multiple pregnancies, which can be dangerous to the mother.
  • Excess embryos may be used for embryo research, and many people disagree with this, because embryos have the potential to become a living human.

Control in plants

  • Plants are sensitive to light, moisture and gravity:
  • Their shoots grow:
    • towards light
    • against the force of gravity
  • Their roots grow:
    • towards moisture
    • in the direction of the force of gravity.
  • Plants produce hormones to coordinate and control growth.
  • Auxin controls phototropism and gravitropism (also called geotropism).
  • The responses of plant roots and shoots to light, gravity and moisture are the result of unequal distribution of hormones, causing unequal growth rates.


  • The auxin diffuses away from the stimulus.
  • It affects the growth of cells in different ways.
  • In the shoots:
    • It causes increased cell growth
    • This causes the shoot to curve towards the stimulus.




  • In the roots:
    • It inhibits cell growth.
    • This causes the root to curve away from the stimulus.


The use of artificial plant hormones

  • Plant growth hormones are used in agriculture and horticulture.
  • Agriculture = large scale business involving cultivating of soil, to produce crops, and raise livestock.
  • Horticulture = small scale cultivation of fruits, vegetables, flowers, or ornamental plants typically in a garden.
  • Chemicals are synthesised that are similar to plant hormones.
  • Some people are concerned about these chemicals entering the food chain and causing toxic effects.


Weed killers

  • Chemicals that are used that are specific to the weeds eg dandelions.
  • They cause the weed to grow very quickly.
  • The weed cannot sustain this rate of growth and dies.
  • This also kills other wild plant species that are not weeds.


Rooting hormones

  • Cuttings are taken from a plant.
  • The cutting is dipped into rooting powders.
  • The hormone causes cells in the cutting to develop into roots.