The nervous system enables humans to react to their surroundings.
Cells called receptors detect stimuli e.g. receptors in the eye detect light.
Neurons are nerve cells. They carry information as electrical signals. There are three types:
- Sensory - e.g pain receptors to the spinal cord and brain
- Relay - carry messages from one part of the spinal cord/brain to another
- Motor - carry signals to effectors (muscles)
Tiny branches are called dendrons and the fibre that carries the signal is called the axon. It is surrouned by the myelin sheath which helps impulses travel faster.
In a simple reflex action:
- Impulses from a receptor pass along neurons to the central nervous system where the brain cooridnates a response.
- A sensory neurone sends a signal to the spinal chord.
- At a synapse*, between the sensory and the relay neuron, a chemcial is reelased to stimulate the impulse.
- This impulse is passed to the relay neuron in the spinal chord.
- The impulse is passed on to the motor neurone.
- This carries the impulse to the effector, the muscle.
A muscle contracts and a gland secretes.
* A synapse is a gap in between two neurones. The impulse stimulates vesicles to release a neurotransmitter (chemical)
Many processes are controlled by chemical substances called hormones. They are secreted by glands and are tansported in the bloodstream.
Hormones regulate the functions of many organs.
For example in the menstural cycle. The hormones are secreted by the pituitary gland and by the ovaries. The hormones control the monthly release of an egg and changes the thickness of the lining of the uterus.
The hormones responsible are:
- FSH - Secreted by the pituitary gland and causes an egg to mature in the ovaries. Stimulates the production of oestrogen.
- LH - Stimulates the release of eggs in the ovaries.
- Oestrogen - Secreted by the ovaries and inhibits the further production of FSH.
Hormones in fertility
Female contraceptives prevent pregnancy. They work by preventing FSH production. This stops the maturation of the egg.
- The pill. The first birth control pill contained high doses of oestrogen. These resulted in side effects.
- Now pills contain a mixture of progesterone and oestrogen, and this leads to fewer side effects.
- FSH and LH can be given to a woman who does not produce enough to release an egg.
- The hormones cause the maturation of several eggs.
- They are collected and fertilised with sperm.
- The fertilised egg cell divides.
- At this stage, the embryo is implanted into the woman's uterus.
Plants are sensitive to light, moisture and gravity. Known as phototropism, hydrotropism, and geotropism.
Their shoots grow towards the light and against gravity. They will grow in the direction of these three factors. This is called tropism.
Their roots grow towards moisture and in the direction of the force of gravity.
Plant hormones which coordinate and control growth are called auxins.
The respnses of plant roots and shoots are the result of unequal distribution of hormones causing unequal growth rates.