Receptor And Synaptic Potentials

Question 1

What does the Axon hillock do?

Answer 1

Can convert graded EPSPs to (all or none) action potential

Question 2

What does the Axon terminals do?

Answer 2

Axon terminals converts electrical signals (e.g., action potentials ) to chemical signals

Question 3

What do Axons do?

Answer 3

Axon conducts action potentials to axonal terminals. Propagation of action potentials hastened with myelination and increased axonal diameter

Question 4

What do dendrites do?

Answer 4

Dendrites convert chemical signals into graded summating postsynaptic potentials (i.e., EPSPs, IPSPs)

Question 5

What is a graded potential?

Answer 5

• Amplitude graded- dependent on stimulus intensity
• Decay over time and distance
• Ligand gated ion channels
• Example ligsnds - neurotransmitters (e.g., acetylcholine )

Question 6

What dies graded potential vary in amplitude?

Answer 6

• stimulus due to Ligand binding to receptor at synapse (e.g. acetylcholine)
• Occur at dendrites, soma, terminal, neuromuscular junction
• Amplitude relates to stimulus intensity
• Signal amplitude decays with from point of initiation

Question 7

What is excitatory postsynaptic potentials (EPSPs)?

Answer 7

Input is excitatory —> depolarization of membrane potential (Vm)

Question 8

What is inhibitory postsynaptic potentials (IPSPs)?

Answer 8

• Input is inhibitory —> no depolarization as membrane potential (Vm) pushed further from threshold
• Hyperpolarization: inhibitory neurotransmitters (e.g., GABA) can cause influx of negative ions (e.g., Cl-)

Question 9

How are inputs integrated on the some/cell body?

Answer 9

• Summation

- EPSP/IPSP

Question 10

What is summation?

Answer 10

When multiple signals arriving at the trigger zone are superimposed (summed)

• temporal
• spatial

Question 11

What is temporal summation?

Answer 11

Change in frequency of stimulation (time)

Question 12

Explain a temporal summation

Answer 12

• Single presynaptic neuron fires many times in quick succession
• Increased frequency- graded potentials sum together
• If summed graded potentials reach threshold —> generates action potential.
• Depends on passive properties (time constant) of cell membrane

Question 13

What is spatial summation?

Answer 13

Multiple inputs at different locations on neuron (space)

• PSPs arriving at the trigger zone (axon hillock) at the same time can summate.
• Sufficient spatial summation of EPSPs can trigger action potentials

Question 14

What are the series of events caused by an EPSPs and spatial summation ?

Answer 14

1. Three excitatory neurons fire. Their graded potentials separately are all below threshold.
2. Graded potentials arrive at trigger zone together and sum to create a suprathreshold signal.
3. An action potential is generated

Question 15

What is an action potential?

Answer 15

• All-or-none (I.e., present-or-absent), amplitude often less subject to modulation (I.e., largely independent of stimulus size)
• Voltage gated ion channels

Question 16

List sequentially major synaptic events that move an action potential from the pre- to post-synaptic potentials

Answer 16

Graded potentials > threshold > action potentials

Question 17

What are the Action potential (AP) features?

Answer 17

1. All-or-none: action potentials ONLY occurs if stimulus > threshold, if not then no AP
2. Stimulus magnitude: has greater impact on frequency of firing of action potentials
3. Amplitude: generally no loss in action potential (AP) amplitude during conduction over the length 9f the axon (but, amplitude can be modulated at the terminal)

Question 18

In the conversion of graded to action potentials, voltage of summated EPSPs must be above threshold, why?

Answer 18

Voltage of summated EPSPs must be above threshold at which a certain # of voltage gated Na+ channels open and initiate an action potential

Question 19

What is propagation of the action potential?

Answer 19

AP initiated at axon hillock
(Summation/EPSP/IPSP > action potential)

• but how is that signal then sent down an axon?
  1. Depolarization in one area
  2. Followed by repolarization in that area with depolarization moving dish (wave of action potential )
  3. That area then has a period of resting and repolar8zation cint8nues to move down

Question 20

What role does the Axon hillock/threshold have in propagation ?

Answer 20

Axon hillock/threshold

• Voltage-gated Na+ channels largely absent on soma and dendrites. Hillock (junction of soma and axon) is heavily populated with voltage-gated Na+ channels
• if graded potentials depolarization > threshold, signal converted to action potential

Question 21

What events occur during propagation of action potential

Answer 21

• membrane depolarization rapid influx of Na+
• depolarization of one axonal region —> positive charge flows to adjacent regions and propagate depolarization
• Voltage gated Na+ channels open in adjacent region and AP moves down axon
• Action potential propagates without losing amplitude

Question 22

What happens after an action potential is propagated?

Answer 22

Refractory period of action potential

-in refractory region, voltage gated Na+ channel inactivation gate closed

Question 23

Summarize the steps of the propagation of the action potential

Answer 23

1. A graded potential above threshold reaches the trigger zone
2. Voltage-gated Na+ channels open and Na+ enters the axon.
3. Positive charge flows into adjacent sections of the axon by local current flow
4. Local current flow from the active region causes new sections of the membrane to depolarize p
5. The refractory period prevents backward conduction. Loss of K+ from the cytoplasm repolarizes the membrane

Question 24

How does conduction of action potential along an axon occur?

Answer 24

• passive

- saltatory in myelinated axons

Question 25

Describe the distribution of voltage-gated channel

Answer 25

Unmyelinated axon Dense distribution at impulse initiation zone Myelinated axon Dense distribution at impulse initiation zone Dense distribution between nodes of Ranvier At Nodal-paranodal-juxtaparanidal region(node of ramvjer before terminal) Na+ = nodal +terminal K+ = paranodal + juxtaparanodal Ca +2= terminal

Question 26

How does a myelin sheath effect conduction of action potential?

Answer 26

Rapid conduction in myelinated axons

Question 27

How is velocity affected by axonal diameter?

Answer 27

High intracellular resistance = short length constant This causes lower velocity in small diameter neurons Low intracellular resistance = long length constant Tsmall very similar to Tlarge

Question 28

Contrast the amplitude of action and graded potentials

Answer 28

Graded potentials- amplitude varies with intensity of stimulus, I.e., the response is graded Action potential- Once the threshold is reached, the amplitude of an action potential is not dependent on the initial stimulus, I.e., it is an all or none phenomenon

Question 29

Contrast the threshold of graded and action potentials

Answer 29

Graded potentials- there is no threshold Action potential- there is a threshold

Question 30

Contrast the refractory period of graded and action potentials

Answer 30

Graded potentials- there is no refractory period Action potential- there is a refractory period

Question 31

Contrast the duration of graded and action potentials

Answer 31

Graded potential- Duration is dependent on the initial stimulus Action potential- duration is constant

Question 32

Contrast conduction of graded and action potential

Answer 32

Graded potential- conduction decreases with decreases with distances (decremental conduction) Action potential- conduction is not decremental

Question 33

Constrast the depolarization occurrences of graded and action potentials

Answer 33

Graded potential- can be depolarizing or hyperpolarizing Action potential- are always initiated by depolarization

Question 34

Contrast the summation of graded potentials and action potentials

Answer 34

Graded potential- summation can occur Action potential- no summation occurs

Question 35

Contrast the mediation of graded potentials and action potentials

Answer 35

Graded potentials- are mediated by a receptor (ligand gated) Action potential- Are mediated by voltage-gated ion channels

Question 36

What are electrical events at a synapse ?

Answer 36

- Action potential in presynapse result in calcium ion influx at axonal terminal/presynapse - Neurotransmitter stored within vesicles released into synaptic cleft as free neurotransmitter (T) - Neurotransmitter (T) binds to ligand-gated ion channels - provokes graded potential - Conversion of signals electrical (presynaptic) —> chemical —> electrical (postsynaptic)

Question 37

What is the neuromuscular junction ?

Answer 37

Functional interface between a motor neuron and a muscle cell

Question 38

What happens at the neuromuscular junction?

Answer 38

- Nerve impulse converted to a muscle action potential | - Where disease, inhibition, by drugs/poisons occurs (botulism, curare,etc.), yielding weakness or spasm

Question 39

List the steps involved in in neuromuscular transmission in skeletal muscle and point out location of each step

Answer 39

- Acetylcholine released from synaptic vesicles into synaptic cleft - ACh diffuses across cleft - ACh binds to nicotinic receptors in the sacrolemma - Na+ influx through nicotinic receptors (ligand-gated ion channels) - Channels induced graded endplate potential (EPP) - The EPP migratesacross motor endplate surface (membrane)

Question 40

What is the graded response to the neuromuscular junction ?

Answer 40

Graded response | -Under normal conditions ACh released results in suprathreshold EPP

Question 41

Describe the action potential of the neuromuscular junction

Answer 41

EPP migrated along muscle cell surface to activate voltage-gated Na+ channels present -Depolarization of muscle cell surface due to opening of voltage-gated channels, triggering action potentials

Question 42

What is Myasthenia Gravis?

Answer 42

Severe muscle weakness - Autoimmune disease: Auto-antibodies bind to and decrease # nicotinic acetylcholine (ACh) receptors at skeletal muscle neuromuscular junction (NMJ) - Muscular weakness is elicited due to a decrease in the response of the muscle fiber to acetylcholine

Question 43

What are the symptoms of Myasthenia Gravis?

Answer 43

- Weakness of the eyelids, eye muscles, oropharyngeal muscles, and limb muscles. Increased during exercise and reduced by rest - Diplopia (“double vision”), ptosis(“drooping”) of Both eyelids after repeated blinking exercises

Question 44

What are the treatments for Myasthenia Gravis?

Answer 44

Anticholinesterase drugs (e.g., neostigmine). Inhibition of acetylcholinesterase allows released ACh to remain at the NMJ (unhydrolyzed) for a longer time and increases the chances of its interaction with the nAChRs