Nerve Impulses: Getting From A to B Quiz

Answer: The phospholipid bilayer

Ions, being polar, cannot freely diffuse through the non-polar phospholipid bilayer. The resting potential (around -70 millivolts) is established by the Na+/K+ ATPase pump, which moves three sodium ions out of the cell and two potassium ions into the cell.

Answer: Amount of solvent present

A similar equation, the Nernst equation, calculates the equilibrium potential for a single ion (e.g. Na+, K+, Ca++ or Cl-). The equilibrium potential (Eion) is the potential difference required to maintain and ion concentration gradient across the plasma membrane (i.e. with no net flow of ions).
The Goldman equation is similar, but takes multiple ions into consideration, as well as taking into account the different permeabilities of the membrane to the different ions.

Answer: Dendrites

Santiago Cajal established the Principle of Dynamic Polarization, which describes how neurones (as cells) are highly polarized - a body (soma) with two processes: the dendrite (which transmits information to the soma) and the axon (which transmits information from the soma).

Answer: Temporal summation

Dentrites are poor insulators and the signal decreases exponentially along their length. Another reason for this attenuation is the dendrite's inability to generate its own action potential, due to a low density of voltage-gated sodium channels along its length.

Answer: Axon hillock

Excitatory inputs cause a small depolarization (makes the inside of the cell less negative), whereas inhibitory inputs cause a small hyperpolarization (makes the inside of the cell more negative). If the inside of the cell becomes depolarized to a point (the threshold potential), a full action potential is fired.

Answer: The frequency of the action potentials

This system means that all action potentials will be the same strength, and that action potentials will not be sent if the stimulus is small (sub-threshold). A strong stimulus will send more action potentials from the axon hillock than a weaker stimulus.

Answer: Saltatory conduction

Myelin sheath is provided by Schwann cells. Many axons are myelinated, but dendrites never are. Nodes of Ranvier (unmyelinated points) are found regularly along the length of myelinated axons, allowing the signal to "jump" between nodes, transmitting the signal more rapidly.

Answer: They are more common than chemical synapses in the human body

Electrical synapses are formed when two adjacent neurones are joined by a gap juntion, made of proteins called connexins (or innexins in invertebrates). These channels allow the neurones to exchange ions.

Answer: It ensures impulse propagation occurs in one direction only

Full depolarization involves a reversal of potential difference across a neurone, usually from around -70 mV to +40 mV. The inhibition of voltage-gated sodium channels (preventing cation influx) and the opening of potassium channels (encouraging cation efflux) is what causes hyperpolarization. During the absolute refractory period, sodium channels are inactivated and so it is impossible for that section to undergo another depolarization.

This prevents retrograde impulse conduction.

Answer: Effectors

The general sequence of events is as follows: stimulus > receptor > CNS > effector > response.
This sequence is typical when the stimulus is an unwelcome change to your internal environment, e.g. a change in temperature or carbon dioxide levels. With both of these examples, the response (either decreasing body temperature or excreting carbon dioxide) is a reflex and can be called a homeostatic response.