Neuromuscular junction

1. How is acetylcholine stored in the presynaptic region? How many ACh molecules are in a quanta?
2. What is the synaptic cleft and what enzyme can be found in this region?
3. What is a miniature endplate potential (MEPP)?
4. How is an endplate potential (EPP) formed?
5. What is the safety factor?

Answers:
1. ACh is stored in three storage compartments (main --> mobilized --> immediate), where it is contained in packets called quanta. Each quanta has 5,000-10,000 ACh molecules. The immediate store contains 1,000 quanta.
2. This is the space between the presynaptic and postsynaptic region. It contains the enzyme ACh-esterase.
3. A MEPP is formed during a period of inactivation and it occurs when there is a spontaneous release of a quanta, which occurs every 5 seconds.
4. An EPP is formed when depolarization of the nerve opens up voltage-gated Ca channels, which floods the nerve terminal. This leads to the release of multiple quanta.
5. The safety factor refers to the initial excess amplitude of the EPP (4x needed for an AP), which allows time for ACh to move from the main and mobilizing storage to the immediate storage.

Impulse propagation

1. How do Na ions enter the neuron during depolarization?
2. What is the absolute refractory period and how does it differ from the relative refractory period?
3. Why does temperature change the AP? And what effect does it have on latency, amplitude, duration, and conduction velocity?
4. What are the areas between the myelin sheath covering called and what is the process of propagating a current from one node to another?
5. How are the depolarized membranes brought back to resting state?

Answers:
1. Na enters the cell through Na voltage gated channels with activation and inactivation gates. When the cell is activated, the activation gate is open. When the cell is inactivated, the inactivation gate is closed.
2. The absolute refractory period is the time after closure of the inactivation gates, when no action potential can be formed, no matter how strong the stimulus. The relative refractory period occurs after the absolute refractory period and a more intense stimulation is required to form an AP.
3. Decreased temperature affects the protein configuration of the Na channels and causes a delay in opening and closing of the gates. Decreased temperature results in prolonged latency, increased duration, and decreased C.V. Amplitude may be increased, but may also decrease due to increased temporal dispersion phase cancellation.
4. Nodes of Ranvier are the areas between the myelin. Saltatory conduction is propagating a current from one node to another.
5. K voltage-gated channels open from depolarization after a slight delay. K moves out of the cell to establish a charge equilibrium.

Action Potentials

1. What is the difference between the endoneurium, perineurium, and epineurium?
2. What are leak channels?
3. How is the negative potential maintained inside the neuron? What is this normal resting membrane potential?
4. What is an action potential and how is it formed?
5. What does it mean to have an all-or-none response?


Answers:
1. The endoneurium surrounds the individual axon and its myelin sheath. The perineurium is the strong covering that surrounds bundles of nerve fibers. The epineurium surrounds the entire nerve and holds the fascicles together.
2. Leak channels are openings that allow Na and K to move in and out of the neuron membrane.
3. The negative potential is maintained via Na-K ATP-dependent pumps that export three ions of Na and import two ions of K. The normal restinging membrane potential is -70 to -90 mV.
4. The action potential is a voltage change occuring from an excited cell. It results when the cell membrane becomes increasingly permeable to Na, which rushes into the cell through open voltage-gated channels.
5. A stimulus must be strong enough to reach a threshold of activation. If below threshold, no activation will occur. A high enough stimulus will produce an action potential, but higher stimuli will not produce a larger potential.

Basic neuron anatomy

1. What is an alpha motor neuron and where is it located?
2. What is an axon and what is used for myelination of an axon?
3. What are the terminal nerve branches and what is the significance of the innervation ratio?
4. What is the difference between Type I and Type II motor neurons? Which is tested in electrodiagnosis?
5. What is the Henneman Size Principle?


Answers:
1. An alpha motor neuron is the cell body of the motor nerve and is located in the anterior horn of the spinal cord.
2. An axon is the branch of the cell body that propagates current. It is myelinated by Schwann cells. (Or may be unmyelinated.)
3. Terminal nerve branches extend from the distal axon and innervate individual muscle fibers. The innervation ratio (IR) refers to the number of fibers innervated by one axon. A higher IR implies more force is generated while a lower IR is associated with muscles for fine movement.
4. Type I has smaller cell body, thinner axons, lower IR, and are slow twitch. Type II has larger cell body, thicker axons, higher IR, and are fast twitch. Only Type I fibers are tested in electrodiagnosis.
5. The Henneman Size Principle says that smaller alpha motor neurons (Type I) are recruited first.