Electrophysiology (Action Potential and Synapses)

Question 1

Neuronal membranes consist of a lipid bilayer with proteins.

What do you call these proteins and what are some of the functions of the proteins that float in or penetrate the cell membrane?

Answer 1

These proteins are called transmembrane proteins.

They can function as ion channels or receptors for neurotransmitters or peptide hormones.

Question 2

What is the “resting potential” a comparison of?

In neurons, what is the resting potential?

Answer 2

It is a comparison of the voltage of the inside of the cell at rest to the outside of the cell.

The resting potential of neurons is ~70 to 90 mV.

Question 3

At rest, compare the concentrations of Na+ and K+ of inside and outside a cell.

Question 4

At rest, Na+ will passively flow out of the cell and K+ will passively flow inside the cell. (Also, at rest, K+ has an easier time flowing inside than Na+.)

At rest, how do cells maintain and restore the concentration gradient?

Answer 4

Cells use ion pumps that require metabolic energy (ATP) to maintain gradients by pumping Na+ OUT and K+ INTO the cell.

Question 5

Local changes in membrane potential occur near synapses.

• What is an EPSP?
• What is an IPSP?

Answer 5

EPSP - Excitatory post-synaptic potential
- A small depolarization (from -70 to -60 mV)

IPSP - Inhibitory post-synaptic potential
- A small hyperpolarization (from -70 to -80 mv)

Question 6

Graded potential changes decay over time and distance and cannot travel long distances along the neuron membrane.

However, graded potentials can add together. What are the two ways that they can summate?

Answer 6

1. Temporal summation: Multiple and rapid graded potentials at a single synapse can add together
2. Spatial summation: Multiple graded potentials at different synapses can add together

Question 7

Once an EPSP is large enough and reaches the cell’s threshold potential, an all-or-nothing action potential results.

What is a neuron cell’s threshold potential?

Answer 7

Around -55 mV

Question 8

Where do graded potentials typically occur?

Answer 8

Graded potentials usually occur around unmyelinated membrane synapses.

(Ex: Soma’s, dendrites, and the receptive end of sensory neurons)

Question 9

Why are nerve and muscle cells called excitable cells?

How can these cells be excited?

Answer 9

Nerve and muscle cells are called excitable cells because they can undergo transient, rapid changes in membrane potential.

These cells can be excited via synaptic input or electrical stimulation.

Question 10

What occurs in a cell when the membrane potential depolarizes to -55 mV?

• What occurs at Na+ channels?
• What propagates down the axon?

Answer 10

At -55 mV, the threshold potential is reached and an action potential is formed.

This causes voltage gated Na+ channels to open and for an action potential to propagate down the axon.

Question 11

Does Na+ membrane permeability increase or decrease during an action potential? How does this affect the membrane potential?

Answer 11

Na+ membrane permeability drastically increases.

This causes the membrane potential to DEPOLARIZE and go from -70 mV to (overshooting 0 mV) ~20 mV.

Question 12

Why does the cell membrane need to be rapidly depolarized in order to develop an action potential?

Answer 12

Rapid depolarization is critical, because Na+ channels are time and voltage sensitive.

They will only open at certain voltages and for a certain period of time.

Question 13

After depolarization of the membrane by Na+, what causes repolarization of the membrane potential?

Answer 13

Repolarization is caused by K+ efflux from the neuron.

Question 14

Repolarization by a K+ efflux causes a period of hyperpolarization to occur.

What channels mediate this hyperpolarization and what is this period called?

Answer 14

K+ channels mediate the hyperpolarization.

This period is known as the refractory period (which limits the frequency of action potentials).

Question 15

After an action potential, in order to return the cell to a resting state the ionic gradients must be restored to normal.

What ion pump accomplishes this?
What fuels this ion pump?

Answer 15

The sodium potassium ion pump accomplishes this, via ATP fuel.

This ion pump pumps Na+ OUT of the cell and K+ BACK into the cell.

Question 16

In the CNS after an action potential, astrocytes assist in returning the cell to a resting state.

How do they assist?

Answer 16

Astrocytes help the cell return to a resting state by removing excess K+ from the extracellular environment and preventing K+ accumulation.

Question 17

In neurons, what is a plateau potential?

• What ion are they mediated by?
• How are they possibly connected to spinal cord injuries and strokes?

Answer 17

Plateau potentials are relatively prolonged depolarizations mediated by calcium entry into the neuron.

They enable the neuron to produce sustained discharge of action potentials without continued input.

After SC injury and stroke, plateau potentials may underlie spasticity (or rigidness) and discharge of motor neurons with uncontrolled muscle contraction.

Question 18

What characterizes the condition of myotonia congenita?

Answer 18

Myotonia congenita is characterized by severe muscle cramps and is thought to be contributed to by Plateau potentials.

Question 19

What region of the neuron cell body has the lowest threshold for excitation?

Answer 19

The first portion of the axon and the region of the cell body from which the cell body leaves has the lowest excitement threshold.

Together, they are termed the axon hillock.

Question 20

The axon hillock region has the highest density of what channels?

Answer 20

The axon hillock region has the highest density of voltage-sensitive Na+ channels.

Once the threshold potential is met, an action potential is generated in this region with Na+ channels opening and Na+ entering the cell.

Question 21

Are action potentials for sensory neurons triggered in the PNS or CNS?

Answer 21

Sensory neurons : periphery initiation of action potentials

Question 22

Are action potentials for motor neurons triggered in the PNS or CNS?

Answer 22

Motor neurons : central initiation of action potentials

Question 23

Once an AP is generated, what are the two means of conduction flow?

Which type of flow is faster?

Answer 23

Conduction flow via…

1. Local current flow (in unmyelinated fibers)
2. Saltatory conduction (in myelinated fibers)

Saltatory conduction is much faster due to myelination (reaching 120 m/sec, which is 50x faster).

Question 24

In an unmyelinated axon, Na+ and K+ channels exist down the entire axon. Once an AP is generated, these channels allows for conduction to travel down the length of the axon through what repeating cycle?

Answer 24

A self-perpetuating cycle of depolarization and repolarization allows for propagation of the action potential down the length of the axon.

Question 25

What two cells produce myelin for axons?

Answer 25

1. Schwann cells (in PNS) | 2. Oligodendrocytes (in CNS)

Question 26

There are ~1 mm spaces between Schwann cell (PNS) myelinated sections of an axon. What are these spaces called? What channels exist at these spaces?

Answer 26

They are called nodes of Ranvier. Voltage gated Na+ channels are concentrated at these spaces.

Question 27

How do the nodes of Ranvier allow for propagation of the action potential?

Answer 27

Action potential occurs at each node of Ranvier and allows the AP to jump from myelin sheath to myelin sheath. The myelin sheath prevents decay of the AP as it travels down the axon.

Question 28

How does the diameter of an axon affect conduction velocity?

Answer 28

The larger the diameter, the lower resistance to electrical charge, which correlates to more rapid conduction of AP.

Question 29

What is the difference between orthodromic conduction and antidromic conduction?

Answer 29

Orthodromic conduction - conduction in the same normal direction of AP Antidromic conduction - conduction in the opposite direction. Antidromic conduction can be elicited via mechanical or electrical stimulation of a neuron.

Question 30

Synaptic transmission is passing a signal from one neuron/motor cell to another. In simple (chemical/electric) terms, describe how a signal gets passed from one cell to another.

Answer 30

An electrical signal from the pre-synaptic neuron triggers a chemical signal to be released at the synapse. Those chemical signals cause the post-synaptic neuron to generate an electric signal.

Question 31

What are the three types of axon synapses?

Answer 31

1. Axo-somatic 2. Axo-dendritic 3. Axo-axonic

Question 32

Location wise, which synapses have a typically more powerful effect on the post-synaptic cell? - Those closer to the axon hillock - Those further, such as a distal dendrite

Answer 32

Synapses closer to the axon hillock (trigger zone) will have a more powerful effect on a post-synaptic cell. (Think about the chemical signal having to reach the post-synaptic membrane.)

Question 33

Synapses will tend to be inhibitory or excitatory based on where they occur. - Synapses on soma's tend to be...?

Answer 33

Soma synapses tend to be inhibitory (IPSP)

Question 34

Synapses will tend to be inhibitory or excitatory based on where they occur. - Synapses on dendrites tend to be...?

Answer 34

Dendritic synapses tend to be excitatory (EPSP)

Question 35

Synaptic vesicles are located where in an axon and contain what kind of chemicals?

Answer 35

Synaptic vesicles are located at pre-synaptic axon terminals and contain neurotransmitters.

Question 36

As an AP travels down an axon, it depolarizes an axon terminal and triggers the opening of what type of channels?

Answer 36

An AP will trigger the opening of voltage gated Ca channels at axon terminals.

Question 37

Once an AP reaches an axon terminal, it causes the opening of Ca channels. Will the opening of these channels cause Ca ions to go inside the axon terminal or rush out of the axon terminal?

Answer 37

At rest, Ca is higher outside than inside, so AP triggering the opening the Ca channels will cause Ca to rush inside the axon terminal.

Question 38

What does Ca entering the axon terminal cause synaptic vesicles to do?

Answer 38

An influx of Ca into an axon terminal will cause synaptic vesicles to: 1. Move to the pre-synaptic terminal 2. Fuse their membranes with the pre-synaptic terminal 3. Release their neurotransmitters into the synaptic cleft

Question 39

What do neurotransmitters do once released from their synaptic vesicles into the synaptic cleft?

Answer 39

Neurotransmitters will diffuse across the synaptic cleft and bind to neurotransmitter receptor proteins, which are embedded in the post-synaptic membrane.

Question 40

Once a NT binds to its post-synaptic receptor, different events can occur. 1. Negative ion channels can open. 2. Positive ion channels can open What happens with either event? What's an example of an ion that would be involved with either channel?

Answer 40

1. Negative ion channel opens - If negatively charged ions enter the post-synaptic cell (such as Cl-), the cell can hyperpolarize. - Hyperpolarization can trigger an IPSP 2. Positive ion channel opens - If positively charged ions enter the post-synaptic cell (such as Na+), the cell can depolarize - Depolarization can trigger an EPSP - If the cell reaches threshold potential, it can trigger an action potential to propagate.

Question 41

Once a synapse occurs, it must be terminated to prevent over-stimulation. What are the three mechanisms for terminating a synaptic transmission?

Answer 41

1. NT may diffuse from the synapse (norepinenephrine) 2. NT may be inactivated by enzymes (acetylcholinesterase on ACh) 3. NT may be re-uptaked by the pre-synaptic axon terminal (serotonin)

Question 42

IPSP's never occur at a motor neuron and a skeletal muscle cell. There is either no activation or an EPSP. What do you call the EPSP at the neuromuscular junction?

Answer 42

The EPSP at a neuromuscular junction is called the endplate potential (EPP). It is always above threshold, which allows for a 1:1 relationship. Basically, when an AP travels down an alpha motor neuron, there will be an AP generated by the muscle cell.