Action Potentials

Question 1

List the sequence of events that generate an action potential in a mechanoreceptor, and the sequence of events that propagate the signal through interneurons and motor neurons, focusing on EPSPs, IPSPs, and integration.

Answer 1

1. A force applies pressure on a mechanoreceptor, and depolarization of a graded potential occurs proportionally to the force of the pressure
2. Graded potentials are temporally or spatially summed up until they hit the threshold of -55mV, triggering an all-or-none action potential
3. Graded potentials are generated at the dendrites and if they sum enough to become an action potential, the signal transduction will travel down the axon to an interneuron in the central nervous system
4. At the interneuron, EPSP depolarizes the dendritic membrane with graded potentials(excite) and IPSP hyperpolarize(inhibit)
5. The interneuron of the CNS temporally and spatially sums EPSP and IPSP from other sensory receptors, and if the threshold is reached, generates an AP that travels down the axon to syna[pse with a motor neuron in the CNS
6. The motor neuron of the CNS temporally and spatially sums EPSP and IPSP and if the threshold is reached, generator an Ap that travels down the axon to synapse with an effector (muscle or gland) in the PNS
7. The effector in the PNS accomplishes a motor task in response to the original stimulus

Question 2

Contrast graded potentials with action potentials in terms of size and proportionality to the size of the stimulus

Answer 2

• The size of any one graded potential in a sensory receptor is proportional to the strength/size of the stimulus
• Graded potentials may depolarize or hyperpolarize and action potentials are huge depolarizations if a threshold is reached (reaching -55mV)
• Graded potentials determine whether or not an action potential is generated. So, if there is a large enough stimulus, a graded potential may reach a threshold, and an AP is generated

Question 3

Describe the steps of an action potential, referencing the channels involved in each step.

Answer 3

Resting State (-70): all voltage-gated channels are closed. The membrane is at the resting potential, but experiencing (EPSP) and hyperpolarizing (IPSP) graded potentials below the threshold. The inactivation gate of the voltage-gated Na+ channel is open, but the activation gate is closed (so overall closed)

Depolarization phase(-55): Threshold depolarization is reached. The activation gate of voltage-gated Na+ channels opens and Na+ pours into the neuron. Positive charge builds up on the inside surface of the membrane and the membrane depolarizes significantly (the beginning of the AP)

Repolarization phase begins: Inactivation gates of voltage-gated Na+ channels close, shutting off Na+ flows into the cell. Voltage-gated K+ channels open and K+ leaves the cell to begin repolarizing the membrane by making the inner surface of the membrane less positive and more negative.

Repolarization phase continues: Outflow of K+ continues, as more negative charge builds up along the inner surface of the neuronal membrane. Membrane potential falls toward the resting potential. With the inactivation gates of Na+ channels close, the activation gates re-open to prepare for the next AP. K+ channels are mostly closed.

After-hyperpolarization phase(-90): short dip of the membrane potential below the resting potential as the last K+ channels close and K+ outflow stops entirely