Rod cells and the detection of stimuli (Edexcel Int. A-level Biology)

This lesson describes how the nervous system detects stimuli, focusing on the detection of light by the rods in the the retina of mammals. The PowerPoint has been designed to cover the content of specification point 8.8 of the Edexcel International A-level Biology specification and includes descriptions of the roles of rhodopsin, opsin, retinal, sodium ions, cation channels and hyperpolarisation in the formation of action potentials in the optic neurones.

The lesson begins by using a quiz to get the students to recognise the range of stimuli which can be detected by receptors. This leads into a task where the students have to form 4 sentences to detail the stimuli which are detected by certain receptors and the energy conversion that happen as a result. Students will be introduced to the idea of a transducer and learn that receptors always convert to electrical energy which is the generator potential. It is likely that students will be aware that the human retina contains rod and cone cells, so the next part of the lesson builds on that knowledge and adds the detail needed at this level. Students will discover that the optical pigment in rod cells is rhodopsin and that the bleaching of this into retinal and opsin results in a cascade of events that allows an action potential to be initiated along the optic nerve. Time is taken to go through the events that occur in the dark and then the students are challenged to use this as a guide when explaining how the events differ in the light. Key terms like depolarisation and hyperpolarisation, that were met earlier in topic 8, are used to explain the changes in membrane potential and the resulting effect on the connection with the bipolar and ganglion cells is then described.
The remainder of the lesson focuses on the Pacinian corpuscle and describes how this responds to pressure on the skin, resulting in the opening of the sodium channels and the flow of sodium ions into the neurone to cause depolarisation