Alkenes

Fractions that are produced by the distillation of crude oil can go through a process called cracking, a chemical reaction which produces smaller hydrocarbons, including alkanes and alkenes. Ethene and other alkenes are unsaturated hydrocarbons and can be used to make polymers.
Cracking
-Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient: they do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand. This is where cracking comes in.
Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are heated to vaporise them.

The products of cracking include alkenes (for example ethene and propene). The alkenes are a family of hydrocarbons that share the same general formula:
CnH2n
The general formula means that the number of hydrogen atoms in an alkene is double the number of carbon atoms. For example, ethene is C2H4 and propene is C3H6.
Alkene molecules can be represented by displayed formulas in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line.

The nervous system allows the body to respond to changes in the environment in a process usually coordinated by the brain. Reflex actions are extra-rapid responses to stimuli; this process also involves the nervous system but bypasses the brain.
Receptors

Receptors are groups of specialised cells that can detect changes in the environment called stimuli. Receptors are often located in the sense organs, such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus.
Most animal cells have a nucleus, cytoplasm and cell membrane. Light receptors have these cell components too.
The human central nervous system (CNS) consists of the brain and spinal cord. When a receptor is stimulated it sends a signal along the nerve cells, also called neurones, to the brain. The brain then coordinates the response.

Neurones

Neurones are nerve cells that carry information as tiny electrical signals. There are three different types of neurones, each with a slightly different function:
sensory neurones carry signals from receptors to the spinal cord and brain
relay neurones carry messages from one part of the CNS to another
motor neurones carry signals from the CNS to effectors.
This diagram shows a typical neurone (a motor neurone), which has tiny branches at each end and a long fibre that carries the signals.

A motor neurone
Synapses
Where two neurones meet there is a tiny gap called a synapse. Signals cross this gap using chemicals. One neurone releases the chemical into the gap. The chemical diffuses across the gap and makes the next neurone transmit an electrical signal.

Neurones

Neurones are nerve cells that carry information as tiny electrical signals. There are three different types of neurones, each with a slightly different function:
sensory neurones carry signals from receptors to the spinal cord and brain
relay neurones carry messages from one part of the CNS to another
motor neurones carry signals from the CNS to effectors.
This diagram shows a typical neurone (a motor neurone), which has tiny branches at each end and a long fibre that carries the signals.

A motor neurone
Synapses
Where two neurones meet there is a tiny gap called a synapse. Signals cross this gap using chemicals. One neurone releases the chemical into the gap. The chemical diffuses across the gap and makes the next neurone transmit an electrical signal.

Reflex actions
-When a receptor is stimulated it sends a signal to the central nervous system, where the brain coordinates the response, but sometimes a very quick response is needed, one that does not involve the brain: this is a reflex action.
Sequence
In a simple reflex action:
stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector
An effector is any part of the body that produces the response. Here are some examples of effectors: -a muscle contracting
a gland releasing (secreting) a hormone or other chemical.
Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. This animation allows you to step through each stage of the reflex arc.
You have an old or no version of flash - you need to upgrade to view this funky content!
Go to the WebWise Flash install guide
This is what happens:
receptor detects a stimulus - a change in the environment
sensory neurone sends impulses to relay neurone
motor neurone sends impulses to effector
effector produces a response.