Neurons, Action Potentials and the Synapse

Question 1

Give the three states a sodium (Na+) channel may be in.

Answer 1

Open, closed or inactive.

Question 2

Give the states a potassium (K+) channel may be in.

Answer 2

Open or closed.

Question 3

What is the All-or-Nothing Principle?

Answer 3

Unless the threshold is reached, no action potential will be fired no matter what.

Question 4

How are we able to distinguish between one pinch and several? What happens to the action potential?

Answer 4

All action potentials are the same, it is their FREQUENCY that changes. Increase in number of action potentials for greater pain.

Question 5

What happens to the membrane potential when Na+ ions move inwards?

Answer 5

It becomes less negative (increases).

Question 6

Outline the first stage of an action potential (depolarization to threshold).

Answer 6

Resting potential is -70mv. Na+ channels open, Na+ ions move inwards, membrane potential becomes less negative (-60mV). Stimulus exceeds theshold (all or nothing), action potential is fired - at this point voltage-gated Na+ channels open, leading to a rapid depolarization of the membrane.

Question 7

Describe what happens during the rapid depolarization phase of an action potential.

Answer 7

Positive feedback loop created - membrane is less negative inside and so more voltage gated Na+ channels open, increasing influx of Na+ ions, which makes membrane even more positive. Na+ voltage gated channels close at +30mV.

Question 8

Describe what happens during repolarisation.

Answer 8

At +30mV, voltage-gated Na+ channels are inactivated. Voltage-gated K+ channels slowly open, K+ ions move out, causes membrane to repolarise (membrane becomes more negative again).

Question 9

Describe what happens during hyperpolarisation.

Answer 9

Hyperpolarisation is an overshoot of the resting potential. Na+ voltage gated channels return to closed (but activatable) state after 0.4 sec. Slow K+ voltage gated channels begin to close at -70mV, causing membrane to become more negative than when at resting potential. Eventually K+ voltage gated channels close. Na+/K+ pump then restores ion concentrations, resting potential restored.

Question 10

When is the ABSOLUTE refractory period, and what is its function?

Answer 10

Begins just after threshold has been exceeded - keeps action potentials separate and distinguishable. From moment Na+ voltage channels open until Na+ channel inactivation ends, membrane cannot react to another stimulus.

Question 11

When is the RELATIVE refractory period, and what is its function?

Answer 11

Occurs immediately after the absolute refractory period. The neuron can generate an action potential again, but ONLY if membrane is depolarised to a value more POSITIVE than threshold.

Question 12

Which three factors effect the speed at which an AP is conducted at?

Answer 12

Diameter, temperature and myelination of a neuron.

Question 13

What are the two types of synapse?

Answer 13

Electrical and chemical.

Question 14

Describe an electrical synapse.

Answer 14

Requires direct communication (gap junctions) between cells which is VERY RARE. Located in the CNS and PNS.

Question 15

Describe a chemical synapse.

Answer 15

Cells not directly coupled. Involves release of NEUROTRANSMITTERS, very common.

Question 16

Describe the transmission of a nerve impulse across a synapse.

Answer 16

In the synaptic knob, Na+ influx triggers voltage gated Ca2+ (calcium) channels to open. Influx of Ca2+ is detected by SNARE proteins, which move vesicles containing Ach closer to the membrane for exocytosis. Ach diffuses across synaptic cleft down concentration gradient. Binds to specific receptors on post-synaptic membrane, which increases membrane permeability to Na+. Membrane thus depolarises to threshold, stimulates Na+ voltage gated channels to open and AP continues on.

Question 17

What does a SNARE protein do?

Answer 17

Links a vesicle to the plasma membrane so that exocytosis of its contents can occur. Membrane and vesicle ‘zipper’ together when triggered which pulls the vesicle closer to the membrane.

Question 18

How is Ach removed from receptors on the post-synaptic membrane?

Answer 18

Ach is broken down by Achesterase.

Question 19

Give the two categories that neurotransmitters (e.g. Ach) fall into.

Answer 19

Inhibitory and excitatory.

Question 20

Give a brief explanation of how inhibitory neurotransmitters work.

Answer 20

They suppress the generation of an action potential by causing HYPERPOLARISATION of the post-synaptic neuron. (K+ leaves)

Question 21

Give a brief explanation of how excitatory neurotransmitters work.

Answer 21

Cause the generation of an action potential in the post-synaptic neuron by depolarising the post-synaptic membrane. (Na+ influx)

Question 22

Name the two types of receptor that Ach may join with.

Answer 22

Muscarinic (slow) and Nicotinic (fast)

Question 23

Where are nicotinic receptors found, and what is their role within the body?

Answer 23

Found in neuromuscular junctions. Nicotinic receptors are EXCITATORY, and thus are direct and fast way to open the ion channels on the post-synaptic neuron.

Question 24

Where are muscarinic receptors found, and what is their role within the body?

Answer 24

May be INHIBITORY or EXCITATORY. Inhibitory found in synapses in ANS/PNS/CNS, excitatory found in synapses in the heart. Both types are INDIRECT AND SLOW TO OPEN THE ION CHANNELS.

Question 25

What do ADRENERGIC SYNAPSES release?

Answer 25

Norepinephrine (NE), otherwise known as noradrenaline.

Question 26

Where are alpha1 adrenergic receptors found?

Answer 26

Blood vessels to skin/mucosa/abdominal viscera.

Question 27

Where are beta1 adrenergic receptors found?

Answer 27

Located in the heart (used to increase heart rate).

Question 28

Where are beta2 adrenergic receptors found?

Answer 28

Respiratory airways, blood vessels to skeletal muscles and heart.

Question 29

Dopamine receptors may be excitatory or inhibitory - what do inhibitory dopamine receptors allow?

Answer 29

Precise control of movement.

Question 30

Damage to dopamine-releasing neurons may cause which disease?

Answer 30

The symptoms of rigidity and stiffness found in Parkinson’s.

Question 31

How does Cocaine effect dopamine receptors?

Answer 31

Cocaine inhibits the removal of dopamine from synapses, resulting in a feeling of intense pleasure.

Question 32

What is an IPSP?

Answer 32

Inhibitory post-synaptic potential - makes a post-synaptic neuron less likely to generate an action potential.

Question 33

What is an EPSP?

Answer 33

Excitatory post-synaptic potential - makes a post-synaptic neuron more likely to generate an action potential.

Question 34

Define TEMPORAL summation.

Answer 34

Temporal summation occurs when a single presynaptic neuron fires many times in succession, causing the postsynaptic neuron to reach its threshold and fire.

Question 35

Define SPATIAL summation.

Answer 35

Spatial summation occurs when excitatory potentials from many different presynaptic neurons cause the postsynaptic neuron to reach its threshold and fire.