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Different parts of the body need to communicate with each other for things to work well, this is called co-ordination. Animals evolved 2 mechanisms of co-ordination, mainly for survival. These are:

  • The Nervous System- which communicates through electrical messages called Impulses.
  • The Hormonal (endocrine system)- communicates through chemical messangers called Hormones.

The Central Nervous system

Our nervous system is made up of Nerve Cells, also called Neurons. These are cells that are specialised to carry messages over long distances, so they are long, more like a wired/landline phone (we say elongated in proper Biology speak).


All over the body, we have receptors which detect incoming messages (stimuli). These are groups of sensitive cells which are located in sense organs:

  • Nose- sensitive to chemicals (chemoreceptors)
  • Skin- touch, pressure, temperature, pain (baroreceptors and thermoreceptors)
  • Tongue- chemicals (chemoreceptors)
  • Ears- sound and balance
  • Eyes- Light

Structure of main types of nerve cells

Nerve cells are just like any other animal cell except that they have a few adaptations that enable them to detect and carry messages around the body.

There are 3 main types of neurons:

  • Sensory neuron- carries impulses from the sense organ to the Brain or spinal cord
  • Relay neurons- are found in the spinal cord and brain
  • Motor neuron- carry impulses from the spinal cord or brain to the effector.

The effector is part of the body that brings about the required change. It can be a muscle or a gland.

Glands are a tissue which is specialised to make and secrete a particular chemical or hormone.


Neurons are not joined up together. There are small gaps between one neuron and the next, more like switches.

The gaps between neurons are called Synapses. When an impulse reaches the synapse, a conducting chemical called a neurotransmitter is released. This allows the impulse to pass through the gap. After the impulse has passes, the neurotransmitter is broken down and reabsorbed.

Unfortunately, because the neurotransmitter has to diffuse across the gap/synapse, it results in a small delay of about 0.2 milliseconds. Not much, is it? Since there are many more synapses along the route of the impulse, the delay accumulates and becomes substantial.

The Reflex pathway

A Reflex is a quick, automatic response to a stimulus. It does not need you to think, you just do it. Ever said 'that was a knee-jerk reaction'?

  • Yes a knee jerk is a reflex, so is:
  • Blinking
  • Removing your hand from a hot Bunsen

Why do we need reflexes?

  • For survival
  • To prevent organ damage

If we had to think about the response, it would take too long to react and damage our affected body part or even get us killed!!! This is because of many synapses that the impulse will need to cross causing a significant delay.

Reflex arc/pathway- is the order of neurons that the impulse follows during a reflex action. In simple terms, this is:

Receptor- Sensory Neuron- Relay Neuron- Motor Neuron- Effector

Just memorise this and you will be ok for exams....


Homeostatic control

Co-ordination is also about keeping body conditions the same and confortable to support life processes. This is called Homeostasis. Our bodies need to keep these constant to sustain a healthy life:

  • Salts- Sodium ions, Potassium ions, Chloride ions etc
  • Sugar (Glucose)- A reactant in respiration
  • Temperature- for optimal enzyme activity
  • Water
  • pH- for optimal enzyme activity.

The Menstrual cycle

The menstrual cycle is the series of changes that take place in the female reproductive cycle over 28 days. There are at least 4 different hormones that are responsible, these and the major events are summarised below.

Changes in the uterus lining

Days Changes
1-5 The period- the uterus lining breaks down. Dead cells and blood from raptured blood vessels released.
5-14 Uterus lining begins to grow, reaches maximum thickness around Day 14
11-14 Ovulation- The ovary releases an egg
14-28 Uterus lining thickness maintained, If there is no pregnancy, back to Day 1

Hormones and their roles

Hormone Where it is produced Role (job)
Oestogen Ovary Causes uterus lining to thicken- Day 5-14, Inhibits FSH production.
Progesterone Ovary Maintains the uterus lining- Day 14-28
Luetinizing Hormone (LH) Ovary Stimulates ovulation -around Day 14
Follicle Stimulating Hormone (FSH) Pituitary gland (brain) Causes an egg to mature in the ovary


Controlling fertility- The pill, fertility drugs and IVF

1. The pill

The contraceptive pill was developed by Gregory Pincus in the 1950s. Pincus carried out a controverisal test of the pill on Puerto Rican women in the ealy 1950s. It is widely held responsible of the 1960s 'swing' where women felt sexually liberated to have sex without fear of getting pregnant.

The most common type of pill is the Combined pill containing both Oestrogen and Progesterone. Oestrogen inhibits FSH production which means no eggs mature = no pregnancy. Progesterone thickens mucus at the cervix so that sperm may not penetrate through to the uterus.

2. IVF

IVF stands for In Vitro Fertilisation. (vitreous- glass)- Literally meaning fertilisation in a glass container (petri dish).

IVF is used to help women who cannot conceive naturally. The steps are:

  • Injection with FSH to stimulate egg production
  • A number of eggs mature and are harvested
  • The eggs are put in a petri dish where sperm are added to fertilise them.
  • The woman is injected with Oestrogen to prepare the uterus lining to receive the embryo
  • Viable (living/potential) embryos are identified and a few are implanted into the uterus.
  • Pregnancy is monitored

IVF pros and Cons

Pros (good things)
Cons (bad things)
Allows couples who would never have children to have them Very expensive
Allows screening for genetic defects Ethical objections- Are we playing God
  Fate of spare embryos- Killed or used in research
  Multiple pregnancies


Plant Hormones- Tropisms

Plants also show various responses. They respond to water, sunlight and gravity. These growth responses are called Tropisms and are controlled by a growth hormone called Auxin.

Auxin is produced at the tips of shoots and roots.

In shoots

Auxin diffuses to the shaded part of the shoot and stimulates cell growth. So the region in darkness grows longer than the one receiving light and the shoot bends towards light. This is called phototropism.

In roots

Auxin is attracted down by gravity where it slows down cell growth. The part above therefore grows more cells, causing the root to bend downwards. This is called geotropism or gravitropism.

Shoots are negatively geotropic (grow away from gravity) and positively phototropic (grow towards light).

Roots are positively geotropic and negatively phototropic.