3.1.3 The Reflex Arc

• The reflex arc is an involuntary response so it does not involve the conscious part of the brain as the coordinator of the reaction
• Awareness of a response having happened occurs after the response has been carried out
• Responses are therefore automatic and rapid - this helps to protect the body from harm

The neurones of the reflex arc

• There are three main types of neurone in a reflex arc: sensory, relay and motor
  • Sensory neurones carry impulses from sense organs to the CNS (brain or spinal cord)
  • Relay neurones are found inside the CNS and connect sensory and motor neurones
  • Motor neurones carry impulses from the CNS to effectors (muscles or glands)

• Sensory neurones are long and have a cell body branching off the middle of the axon
• Relay neurones are short and have a small cell body at one end with many dendrites branching off it
• Motor neurones are long and have a large cell body at one end with long dendrites branching off it

An example of a reflex arc

1. A stimulus (e.g. a pin) is detected by a  receptor (e.g. pain/pressure/touch receptors in the skin)
2. A sensory neurone sends electrical impulses to the spinal cord (the coordinator)
3. An electrical impulse is passed to a relay neurone in the spinal cord
4. A relay neurone synapses with a motor neurone
5. A motor neurone carries an impulse to a muscle in the effector (e.g. the leg)
6. The effector will initiate a response when stimulated by the motor neurone e.g. the muscle will contract and pull the foot up and away from the sharp object

Aim: to plan and carry out an investigation into the effect of a factor on human reaction time

• Reaction time is the time taken to respond to a stimulus; reactions are usually very quick responses typically taking less than a second to occur
  • This can be affected by factors such as age, stress or use of drugs (such as caffeine)
• Reaction time could investigated by measuring the effect of background noise or consumption of caffeine on how quickly a person catches a ruler
  • The further down the length a ruler is caught, the slower an individual's reaction time
• The experiment can be repeated several times to calculate a mean average distance caught and this can be equated to a reaction time
• The process is then repeated after a condition is changed (the person drinks a can of caffeinated drink or has to repeat the experiment when loud background music is playing)

Variables

• The independent variable is the variable that will be changed
  • In this investigation, this could be the consumption of caffeine or the effect of background noise
  • Remember to only change one factor (independent variable) in the investigation
• The dependent variable is the variable that is being measured
  • In this investigation, it would be the reaction time of the person tested in each value of the independent variable
  • The reaction time is calculated using the measurement on the ruler when the individual caught it, this measurement can be converted to a reaction time using a pre-populated conversion chart
• There are many control variables that should be considered in this experiment such as:
  • The person catching the ruler using their dominant hand each time
  • Dropping the same ruler from the same height each time, with the ruler orientated in the same direction (0 cm facing down)
  • If the effect of caffeine is being investigated then none should have been consumed before the start of the experiment, or the room is silent for the first run if background noise is being altered
• It is possible to carry out a reaction time investigation using a computer; this is obviously more accurate as it removes human error from measurements
• Reaction time can also be recorded in milliseconds which is more precise than seconds

Interpreting data on reaction times

• In the exam, you must be able to extract and interpret data from graphs, charts and tables about the functioning of the nervous system

• You could be asked to calculate the mean reaction time for each student, or be given the means and asked to identify if any had been calculated incorrectly if any anomalous results were included
  • There is an anomaly recorded for student C on their third attempt – 573 ms is much higher than the other results recorded so should not be used to calculate a mean

• The lowest reaction time equates to the person with the fastest reaction time – in this example, student A has the fastest reaction time (255 ms) and student C the slowest (267 ms)