Monosynaptic reflexes do not require a neuron between the pre-synaptic and post-synaptic neuron, and do not require input from the brain. These reflexes can be triggered even in brain-dead individuals. The knee-jerk reflex is an example of a monosynaptic reflex.

The accommodation reflex is used to adjust the focus of the eye. The acoustic reflex reduces sound intensity by adjusting the bones of the middle ear. The suckling reflex is the complicated reflex of an infant mammal being able to breast feed. Somatic reflexes are a broad category simply involving muscle reflexes. Some of these reflexes involve input from the brain, while others (like the knee-jerk reflex) do not.

The sympathetic nervous system is activated during times of stress, and is responsible for initiating the "fight or flight" response. Part of this response in an increase in heart rate, allowing better conduction of blood and delivery of oxygen throughout the body.

The other answer options are effects of parasympathetic stimulation, which allows for rest. During this time, the body stores energy from glucose into glycogen, and allows for digestion and reproduction.

By pumping two positively-charged potassium molecules in for every three positively-charged sodium molecules that are pumped out of the cell, the sodium-potassium pump maintains a resting potential of –70mV relative to outside of the cell. This function is important for creating an electrochemical gradient along the neuron.

Remember that sodium flows down its gradient to enter the cell during depolarization, while potassium flows down its gradient to exit a cell after an action potential, causing hyperpolarization during the refractory period.

After the sodium channel is opened, sodium rushes into the cell down its concentration gradient (as previously created by the sodium-potassium pump). This causes depolarization of the membrane as its potential reaches a value of +35mV, which is eventually lowered by the opening of the potassium channels. This leads to hyperpolarization, which prevents the signal from traveling backwards.

The refractory period, a phase in which action potentials cannot be fired, is the result of hyperpolarization, during which the membrane potential drops below –70mV. The membrane potential is at this –70mV level while the threshold, which needs to be reached to fire action potential, is slightly higher at –50mV. During the period of extreme hyperpolarization, an action potential will not form.

Threshold potential is defined as the potential that must be reached in order for an action potential to be initiated by a neuron. Threshold potential is around -55mV in humans, which is slightly higher than the resting potential of -70mV. Once this threshold is reached, the electrical signal will propagate as the membrane depolarizes to a positive potential.

Sub-threshold stimuli, such as stimulus causing depolarization to -65mV, will not trigger action potentials. Muscle contractions can result from action potentials or provide sensory feedback, but the contractions themselves do not play a role in initiating action potentials.

The refractory period is the span of time during which the neuron "recovers" and generally does not respond to a second stimulus as strongly as it did to the first. As the name implies, the absolute refractory period refers to the time when no stimulus, no matter how strong, can provoke a second action potential. This occurs because sodium channels, the opening of which causes depolarization, are sealed by a gating mechanism.

The relative refractory period follows the absolute refractory period. During the relative refractory period, the cell will not respond to normal stimuli, but can generate an action potential if an exceptionally large stimulus occurs. During this period the sodium channels are closed, but not sealed by the gating mechanism; they are essentially normal. The relative refractory period is caused by hyperpolarization as potassium rushes out of the cell after the action potential. Because the potential is lower than normal, only a very large stimulus can overcome the threshold.

Though threshold, saltatory conduction, and depolarization do relate to nervous system potentials, they do not refer to this specific period. Apoptosis is a completely unrelated process referring to a type of cell death.

At the end of an action potential, the voltage-gated potassium channels are slow to close. This allows both the normal "leaky" potassium channels and the voltage-gated potassium channels to be open simultaneously. Large amounts of potassium are able to flow down their concentration gradient, exiting the cell. The exiting of these positively charged ions results in the negative dip in cell membrane potential, known as hyperpolarization.

During hyperpolarization, voltage-gated potassium channels close and the sodium-potassium pump is activated to return the cell to the resting potential by moving potassium back into the cell, and sodium out of the cell.

Temporal summation refers to the phenomenon that an individual neuron will fire with such a high frequency that previous changes in potential have not yet normalized before a new one begins. This summative effect can cause the generation of an action potential, once the threshold potential is surpassed.

Spatial summation refers to the simultaneous activation of several unique neurons to affect another. Numerous individual inputs sum together on the target neuron to stimulate an action potential.

The resting membrane potential is approximately –70mV, while the threshold potential is roughly –55mV. When a neuron receives a stimulus, the binding of neurotransmitters elicits small, localized influxes of sodium known as postsynaptic potentials. These small potentials must sum together in order to raise the local region of the neuron to –55mV. Once this threshold potential is reached, an action potential is generated and the neuron perpetuates the signal.