Quick questions on Neurons, action potentials, synapses and reflex arcs (QCE Biology Unit 2)

A neuron is a specialised cell built for long-distance signalling.

At rest, the inside of the neuron is around 70 mV more negative than the outside. - The sodium-potassium pump exports 3 Na+ for every 2 K+ imported, using ATP. - The membrane is more permeable to K+ at rest, and K+ leaks out down its gradient, leaving the inside negative. - Large negatively charged proteins inside the cell also contribute.

If a stimulus depolarises the membrane past the threshold (around minus 55 mV), an action potential is triggered. The response is all-or-nothing: once threshold is crossed, the action potential goes ahead at full size; below threshold, nothing happens.

A synapse is the junction between two neurons (or between a neuron and an effector). Most synapses in mammals are chemical.

A reflex arc is the neural pathway responsible for a reflex: a rapid, involuntary response to a specific stimulus. The classic example is the patellar (knee-jerk) reflex.

1. Receptor. Detects the stimulus (muscle spindle in the quadriceps for the knee-jerk). 2.

They are all-or-nothing once threshold is crossed.

Calcium entry is the trigger for neurotransmitter release; sodium and potassium drive the action potential, but calcium handles the synapse.

Most vertebrate synapses are chemical; electrical synapses exist but are uncommon and rarely assessed at QCE level.

Mark schemes require all five components.