Receptors respond to a stimulus and send impulses along sensory neurons to the CNS. The CNS coordinates the information and sends impulses along motor neurons to the effectors, which bring about a response. The sequence is as follows:
- Sensory neuron
- Central nervous system
- Motor neuron
Sensory neurons - carry messages from sense neurones to the CNS
Motor neurons - carry messages out of the CNS to effector organs
Some receptors are found in the skin. Other receptors can form part of complex organs, such as:
- light receptor cells in the retina of the eye
- hormone-secreting cells in hormone glands
- muscle cells
- position receptors in the inner ear
- sound receptors in the ear
- touch, pressure, temperature and pain receptors in skin
- chemical receptors in the nose and tongue
B6 PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system
The peripheral nervous system (PNS) consists of motor and sensory neurons that carry information from the receptors to the CNS, as well as instructions from the CNS to the effectors.
Neurons carry electrical signals, and are connected by synapses.
Neurons carry signals from one place to another, around the many parts of the nervous system. They connect sense receptors to the central nervous system and also connect one part of the nervous system to another, for example in the brain and spine. They also carry signals from the nervous system to effector organs, such as muscles and glands.
The cytoplasm forms a long fibre that is surrounded by a cell membrane. This is called an axon
B6 REFLEX ARC
Reflex reactions in humans are controlled by the reflex arc.
When the safety of an organism demands a very quick response, the signals may be passed directly from a sensory neuron, via a relay neurone, to a motor neurone for instant, unthinking action. This is a reflex action.
A reflex arc is the nerve pathway which makes such a fast, automatic response possible. It does not matter how brainy you are - you will always pull your hand away from a flame without thinking about it. It is in-built, or innate, behaviour, and we all behave in the same way. The animation allows you to go through the stages of the reflex arc one by one:
Conditioned reflexes are useful because they increase an animal’s chances of survival.
The human brain consists of billions of neurons. These neurons are connected together to form even more billions of different pathways. Whenever we have a new experience, a new pathway in the brain is used. Each new experience changes our behaviour - this is called learning.
The cerebral cortex is the part of the brain responsible for intelligence, language, memory and consciousness.
Scientists have used different methods to find out which parts of the cerebral cortex do different jobs. These include:
MRI brain scans
B6 DRUGS AND SYNAPSES
Ecstasy (also called MDMA) is a drug that blocks the serotonin receptor sites in the synapses in the brain. This prevents the serotonin from being absorbed by the receptor molecules. As a result, the level of serotonin in the synapse increases. This produces a feeling of wellbeing.
However, there is evidence to suggest that the use of Ecstasy reduces memory. Ecstasy can also cause severe dehydration which can result in death.
C6 SYNTHESISED CHEMICALS
Synthesised chemical group Example of chemical
Examples of use
food additives aspartame sweetening food fertilisers ammonium nitrate improving crop yields dyestuffs procion colouring clothing paints emulsion paint decorating homes pigments titanium oxide making white paint pharmaceuticals aspirin relieving pain
acid + metal → salt + hydrogen
acid + metal oxide → salt + water
acid + metal hydroxide → salt + water
acid + carbonate → salt + water + carbon dioxide
Neutralisation - Higher
When an acidic compound dissolves in water it produces hydrogen ions, H+. These ions are responsible for the acidity of the solution.
When an alkaline compound dissolves in water it produces hydroxide ions, OH−. These ions are responsible for the alkalinity of the solution.
Acids react with alkalis to form salts. These are called neutralisation reactions. In each reaction, water is also formed:
acid + alkali → salt + water
Many reactions are carried out using reactants that are in solution. This allows the ions of molecules of reactants to collide with each other and react. The most common solvent is water, but some reactions involve the use of other solvents.
When a product is made as a solution, one way to separate it from the solvent is to make crystals. This involves evaporating the solution to a much smaller volume and then leaving it to cool. As the solution cools, crystals form, and can be obtained by filtration.
When a solution is heated the solvent evaporates. This can be used prior to crystallisation, or continued until all of the solvent has evaporated, to leave the product as a dry powder.
A solid product that has been separated from a liquid reaction mixture by filtration or crystallisation will still contain some of the solvent. It can be dried by placing it in an oven or into a desiccator.
This technique is used to separate a solid from a liquid. It can be used to obtain product that is free from unreacted chemicals, by-products or solvent
C6 PERCENTAGE YIELD
The yield from a chemical reaction is the mass of product made.
A balanced symbol equation shows the number of atoms and molecules in the reactants and products. The reactants are on the left of the arrow and the products are on the right of the arrow.
A balanced equation can tell us the theoretical yield for a reaction, by using the relative formula masses of reactants and products involved.
C6 ACTUAL YIELD/THEORETICAL YIELD
Theoretical vs actual yield
The difference between the theoretical yield and the actual yield for any reaction may have one or more of a number of causes:
- Some of the reactants may remain unreacted when the reaction is complete.
- Some of the product may be lost when liquids or solids are transferred from one container to another.
- Some of the reactants may form other products.
If you have both the theoretical yield and actual yield, you can work out the percentage yield for the synthesis:
percentage yield = (actual yield ÷ theoretical yield) × 100
What are waves?
Waves are vibrations that transfer energy from place to place without matter (solid, liquid or gas) being transferred
Light and other types of electromagnetic radiation are transverse waves. Water waves and S waves (a type of seismic wave) are also transverse waves. In transverse waves, the vibrations are at right angles to the direction of travel.
Sound waves and waves in a stretched spring are longitudinal waves. P waves (a type of seismic wave) are also longitudinal waves. In longitudinal waves, the vibrations are along the same direction as the direction of travel.
It is the distance from the middle to the top.
The wavelength of a wave is the distance between a point on one wave and the same point on the next wave.
The frequency of a wave is the number of waves produced by a source each second.
wave speed = frequency x wavelength
P6 OPTICAL FIBRES
An optical fibre is a thin rod of high-quality glass. Very little light is absorbed by the glass. Light getting in at one end undergoes repeated total internal reflection, even when the fibre is bent, and emerges at the other end.
When waves meet a gap in a barrier, they carry on through the gap. However, the waves spread out to some extent into the area beyond the gap. This is diffraction.
Where two waves meet, their effects are added together. This is called interference.
Sound waves and light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction.
When they arrive out of step, they cancel out.
This is called destructive interference.
P6 ELECTROMAGNETIC SPECTRUM
FrequencyType of electromagnetic radiationTypical useWavelength highest gamma radiation killing cancer cells shortest X-rays medical images of bones ultraviolet radiation sunbeds visible light seeing infrared radiation optical fibre communication microwaves cooking lowest radio waves television signals longest
P6 ANALOGUE AND DIGITAL
Analogue and digital
Before a sound or piece of information is transmitted, it is encoded in the transmitter in one of the ways described below - analogue or digital. The receiver must then decode the signal to produce a copy of the original information or sound.
Analogue signals vary continuously in amplitude, frequency or both. Digital signals are a series of pulses with two states - on or off. Digital signals carry more information per second than analogue signals and they maintain their quality better over long distances.
P6 ANALOGUE AND DIGITAL