The nervous system

The nervous system

1. Responses to a stimulus

   • A stimulus is a change in the internal or external environment of an organism.
   • Tactic/taxis is a directional movement in response to a stimulus
     • Negative taxis/kinesis/tropism means the response is away from the stimulus and positive is the opposite.
     • An example of this is the movement of humans towards a nice smelling food. It increases the chances of survival, most of the time.
       • This is an example of odor-taxis. There are other types of taxis as well, such as photo-, chemo-, baro- etc.
     • Barotaxis is a response to a change in blood pressure.
     • Phototaxis is a response to a change in light intenisity.
     • Chemotaxis is a response to change in chemicals
   • Kinetic/Kinesis is the nondirectional movement in response to a stimulus.
     • There are also different types of kinesis, which are basically the same as taxis.
     • An example of this is the random movement of people when it rains. They tend to walk away towards some cover and they move faster.
   • Tropism is the directional growth in response to a stimulus.
     • An example of this is a plant shoot growing towards the light (positive phototropism), and plant root growth away from light (negative phototropism).
     • Gravitropism also occurs in plants and it is basically a response towards gravity. Negative is growth upwards and positive is downwards
     • Shoot and root growth is determined by the auxin, IAA (indoleacetic acid). This diffuses backwards at the tips of growing parts and stimulates the growth of cells below the tip.
       • These hormones move around the plant by diffusion or active transport via the phloem. They tend diffuse back towards the side in the shade, so that shoot grows towards light.

2. The reflex arc

   • 1. Receptor cells detect a stimulus e.g. a sharp pin.
     • 2. AN electrical impulse is initiated.
       • 3. The sensory neuron carries this impulse from the receptor to the CNS.
         • 4. The CNS (brain or spinal cord) receive info and coordinates a response, which flows through the relay neuron.
           • 5. The motor neuron carries this impulse to the effectors e.g. muscles which contracts (or glands).

3. Simple reflexes help the chances of survival

   • Moving towards food by the smell of it, using odor receptors in your nose.
   • Running away from predators using your photoreceptors to detect this stimulus
   • Blinking to prevent your eye from getting damaged if they are open for too long.
   • The heart beats continuously keeping you allow by pumping your blood around the body.