Nervous System etc

Note: only combined science biology content - separate/triple science have additional things to learn which aren't here!


1. Synapses and the Reflex Arc

Synapses are the gaps that connect two different neurones, eg the sensory and the relay neurones.

In order to pass the messages (nerve impulses) on, at the end of one neurone, chemicals called neurotransmitters are released, which diffuse across the synapse (gap) to the other neurone. Receptors on the other neurone bind to these neurotransmitters, so a second electrical impulse is sent.

The Reflex Arc is for an action that is quick and we do not have to think about it

Receptor detects a stimulus, then the sensory neurone sends a message to the relay neurone (CNS) but does not go to the brain (only to the spinal chord). Finally, the motor neurone brings about the effector to do a response.

This is innate behaviour for survival (prevent damage to the body)

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2. Required Practical: Reflexes

Investigating reflexes and how they are impacted (requires two people)

1. use a ruler, person A holds it just above person B's thumb (they have their hand outstretched)

2. without warning, person A drops the ruler and records the distance (cm) it took for person B to catch the ruler. Repeat three times at least, and calculate a mean

3. Person B drinks a certain volume (eg 250ml) of coke. Repeat the above steps after five minutes (allow the caffeine to kick in)


  • same person (B) and same hand (eg right hand for all measurements)
  • same height that the ruler is dropped at (improves accuracy)
  • repeat the same amount of times
  • use the same ruler
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3. Hormones and Glands

Glands are examples of effectors (carry out responses as a result of the detection of a stimulus by receptors) and secrete hormones

Important glands - pancreas, thyroid, adrenal, pituitary, ovaries, testes

Hormones are secreted by glands, and are chemical messages which are carried in the blood

Hormones are different to nervous system responses - hormones are slow-acting and have long term impacts, whereas responses by effectors like muscles as instant and short-term

Negative Feedback = the reaction causes the opposite of the stimulus to occur (eg something is too hot, so cold will bring it back to normal temperature)

Homeostasis = maintaining a stable internal environment (including temperature, pH, etc). This is important as all the reactions in the body must be able to occur

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Controlling Blood Glucose

INSULIN = the hormone secreted by the pancreas when blood glucose levels are too high - it brings it down

GLUCAGON = the hormone secreted by the pancreas when blood glucose levels are too low - it brings the levels up

GLYCOGEN = the polymer that glucose is stored as in the liver when there's too muct glucose in the blood

ongoing high blood glucose levels can cause harm to nerves, vessels and organs

low blood glucose levels can cause weakness (fainting), confusion, blurred vision and unusual behaviour

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Adrenaline and Thyroxine

ADRENALINE = hormone secreted by the adrenal gland (above the kidneys)

Stimulus - fright or stress (it's the fight or flight response)

Effects - triggers fast heart rate and ventilation (breathe quicker etc to get more oxygen and glucose to the muscles)

THYROXINE = hormone secreted by the thyroid gland (in the throat, made using iodine and amino acids)

Effects - regulates basal mtabolic rate - important for processes like protein sythesis and for growth and development

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