1. Unit 12:Regulation and Reproduction
Lesson 3: Reflex Actions and Action Potentials

2. Starter Activity
List some reflexes:

4. Reflexes: Pupil dilation Moving hand away from something hot. Yawning
Flinching when being tickled
Blinking in response to bright light / dust
Baby hand grasp
Mammalian diving response
(lots of other baby ‘primitive’ ones!!)
Blushing
Gag reflex
Knee jerk reflex
Plus many, many more!!!!

6. Lesson Objectives
To know how a reflex action happens and be able to draw and interpret a reflex arc.
To know how an action potential causes a nerve impulse.

7. Label the neurones

8. Label the neurones Sensory Neuron Motor Neuron Effector Muscle
Sensory Receptor
Relay Neuron
Sensory
Neuron
Motor Neuron
Effector Muscle

9. The Reflex Action How does a reflex action come about?

10. Class Work Task:
Draw a reflex arc and annotate

11. Action Potentials
Action potential is just the scientific name for a nerve impulse.
Nerve cells at rest are said to have a ‘Resting Potential’
It isn’t technically at rest as the neuron is always actively transporting ions across its cell membrane.
This maintains a negative potential on the inside of the cell (compared with outside)
The potential difference (voltage) is -70mV. (This is the resting potential)

12. Resting Potential
In a resting Neurone, How many Na ions are pumped out of the cell?
And how many K ions are pumped in?
(2 K+ in, 3 Na+ out)

13. Voltage gated ion channels
As well as sodium / potassium pumps, the nerve cell membrane contains:
Voltage gated ion channels
These remain closed at rest (i.e. during a resting potential)
Sodium and Potassium can only use Na/K pumps and voltage gated ion channels to move in/out of cells.
This is because they are not fat soluble (lipid soluble)
(The channels / pumps are made of protein so are sometimes called protein channels)

14. The Action Potential
Once a sensory receptor has been activated (by a stimulus) a ‘Generator Potential’ is produced.
This happens through depolarisation of the cell membrane.
If this depolarisation is large enough to reach a ‘threshold potential’ it will open gated ion channels. (Threshold Potential = -50mV)
When ion channels open, many sodium ions flood in.
This causes the inside of the cell to become more positive (Na+)
The potential difference rises to +40mV
This is an action potential.

15. Hyperpolarisation / Refractory Period
Once an action potential has been reached, sodium channels close, K+ channels open.
Potassium diffuses out making the inside negative (compared to outside) again.
This is repolarisation
During this stage, the recovery ‘overshoots’ a little meaning the potential difference becomes even more negative.
During the refractory period, it is impossible to stimulate another impulse.

16. The Action Potential Graph

17. Saltatory Conduction

18. Saltatory Conduction Speeds up nerve transmission
Happens in myelinated neurons
Speed of myelinated saltatory conduction can be as fast as 120 m/s!!!!

19. What happens to the AP when it arrives at a synapse?

20. Action potentials review

21. Video clip on the nervous system
(Body Stories)

22. Class work / Homework tasks
Complete the worksheet/ hand-out booklets you were given before half term. (Nerve Cells and Synapses)
Rewrite today’s notes in the form of a poster. This could include a graph (action potential) as the main feature with annotations to explain exactly what is happening at each stage.