Some responses to stimulus happen automatically, they bypass the conscious part of the brain when a quick response is essential. It would not be a good thing if you had to pause for however short a time to think “have I been burnt?”. Your body just gets on with it and responds accordingly.


These are reflex responses, fast automatic responses to certain stimuli


  • If a bright light enters the eye, your pupils automatically contract


  • If you receive a shock or sudden surprise, adrenaline is released into the body to set you up for the “fight or flight” response


  • The “knee-jerk” helps to maintain posture and balance, doctors test this response by tapping with a small hammer on the receptors below the knee which cause the upper leg muscles (the effector) to contract, causing the lower leg to jerk upwards


Synapses are small gaps between neurones, where they connect. The electrical signals travel quickly along the neurones but are slowed down (but still very fast transmission) when they have to cross the gap. The electrical signal is transferred across the synaptic gap chemically, setting up a new electrical signal which runs along the next neurone.





Exercises:


Q1. You’re stung on the thumb by a bee. Using the diagram below, explain how the reflex arc is involved in your automatic response to this stimulus.



A1. 

  • The Receptors in the thumb detect the Stimulus (the bee sting)
  • Electrical Impulses travel along the Sensory Neurone
  • the Sensory Neurone passes the impulses via a synaptic connection along a Relay Neurone
  • Impulses travel along a Motor Neurone via another synaptic connection, to The Effector
  • When the impulses reach The Effector (in this case the muscle), it contracts
  • The muscular contraction causes the hand to move away from the Stimulus


Q2. How do nerve impulses travel between neurones?

A2. Chemicals diffuse across the gap (synapses) sending off a new electrical impulse in the new neurone


Q3. Reflex arcs travel through the conscious part of the brain, is this true or false?

A3. This is false, reflexes are automatic and bypass the conscious part of the brain


Q4. Name the neurone which comes after the sensory neurone in a reflex arc

A4. This is the relay neurone


Q5. If the target effector is a gland, what will happen when the messages are received?

A5. The gland will secrete a hormone in response.


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