The Human Nervous System
In IGCSE biology curriculum, your central nervous system - the brain and spinal cord - give you rapid responses to changes in your environment. it uses information from your sense organs to co-ordinate reflexes and other actions. Your peripheral nervous system runs all over your body and is made up of spinal nerves and cranial nerves. These nerves carry information from the sensory organs to your central nervous system and also from your CNS to effector organs (muscles and glands).
stimulus ---> receptor ---> co-ordination ---> effector ---> response
An individual nerve cell is called a neuron and it responds to stimuli. It can conduct electrical impulses.
Wherever one neuron ends, another one begins however there is a tiny gap which is called a synapse. The electrical nerve impulse cannot cross this gap so chemical transmitters are released. They diffuse across the gap. When they reach the next neuron they bind to receptors on the membrane and a new impulse starts.
Reflex actions are very fast and protect you from danger. They also run everyday functions such as breathing. After a reflex has taken place impulses travel to your brain which makes you aware of what has happened. In IGCSE biology, spinal reflexes involve the spinal cord. Cranial reflexes involve the brain.
Sensory receptors are usually found in sense organs. They give information about changes in the environment around you and inside your body. Sensory receptors transform energy from one form to another. The main sense organs are the eye, ear, tongue, nose, skin and muscles.
You focus your eyes by changing the shape of your lens. This called accommodation. When you look at distant objects, the ciliary muscles relax and the suspensory ligaments pull tight. The lens goes thin as the light needs to be bend to focus on the retina. When you look at close objects the ciliary muscles contract and the suspensory ligaments go slack. The lens is more spherical and fatter as the light needs to bend much more to be focused on the retina.
In IGCSE biology, it is importatn that the amount of light that enters your eye is controlled by the iris through the pupil reflex. The pupil enlarges in dim light to let as much light as possible into the eye. It gets smaller in bright light to protect the retina from damage.
Stimuli (light) ---> receptor (retina) ---> sensory neurons in optic nerve ---> unconscious part of brain ---> motor neurons in nerve to iris ---> iris muscles (effector) ---> response (change in pupil size)
There are four main components of the brain in IGCSE biology curriculum, they are:
- The cerebrum controls conscious thoughts
- Cerebellum is involved in co-ordination and balance
- Medulla controls basic body functions
- Hypothalamus and pituitary gland are involved in chemical control of the body
The end of part III ! (remember to read part IV)
Drafted by Gina (Biology)