Receptors, neurones and the Venus fly trap

Last time

Before we start: last time, in Resting potential, action potentials and myelin, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Touch a hot pan

You pull your hand away from a hot pan before you even feel the pain. A chain of three neurones did it, running through your spinal cord and not your brain. That chain is a reflex arc. We start with the cell that notices the stimulus, and finish with a plant that has its own fast signalling system.

Sensory receptors

A sensory receptor is a cell or nerve ending that detects a stimulus. Each type responds to one kind of energy, such as light, pressure or chemicals. Receptors act as transducers. They convert the energy of the stimulus into an electrical signal, called a generator potential. If the generator potential reaches the threshold, an action potential is sent along the sensory neurone.

A chemoreceptor: the taste cell

Sodium ions in food contact microvilli on a taste cell. They enter through channels in the cell surface membrane and depolarise the cell. The depolarisation opens voltage gated calcium channels. Calcium ions flow into the cell. Calcium causes vesicles of neurotransmitter to fuse with the membrane and release their contents across the synapse onto the sensory neurone dendrite. The neurone then fires.

A past-paper example

February March 2025, Paper 42, question 10 part a. Describe how the contact of sodium ions with the microvilli of a chemoreceptor cell can lead to release of a neurotransmitter. Four marks. Sodium ions enter through channels and depolarise the cell. Voltage gated calcium channels open, and calcium ions enter. Calcium causes vesicles to fuse with the membrane and release neurotransmitter by exocytosis.