B1

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.

• Receptor detects a stimulus - change in the environment
• Sensory neurone sends signal to relay neurone
• Motor neurone sends signal to effector
• Effector produces a response.

• stimulants cause more neurotransmitter molecules to diffuse across the synapse
• depressants stop the next neurone sending nerve impulses – they bind to the receptor molecules it needs to respond to the neurotransmitter molecules

Cigarette smoke contains many harmful chemicals. The cells lining the trachea (windpipe), bronchi and bronchioles (the branches inside the lungs) are damaged by cigarette smoke. These epithelial cells have tiny hair-like cilia on their surface. Normally these cilia move to push mucus out of the lungs. Damaged cells cannot do this, leading to a build-up of mucus and a smokers’ cough.

The liver removes alcohol from the bloodstream. It has enzymes that break down alcohol but the products of the reactions involved are toxic. They damage the liver and over time this leads to cirrhosis.