Neurophysiology

Neurophysiology

Electrical communication between neurons

What is used in the chemical communication of neurotransmitters?

Neurotransmitters

3 Types of Nerve Impulses

• Resting Potential

• Action Potential

• Graded Potential

Polarity

The difference in charge between the inside and outside of a cell

What happens when salts go into a solution?

They break apart into ions

Sodium is more concentrated (inside/outside) a cell at resting potential.

Outside

Potassium is more concentrated (inside/outside) a cell at resting potential.

Inside

What is the charge of the inside of cell compared to outside at resting potential?

-70 millivolts

How do ions move in and out of cells?

Through protein channels, which are specific to the type of ion

What two forces drive ion movement?

• Electrical Gradient

• Chemical (concentration) Gradient

Chemical Gradient (4 processes)

• Like ions repel each other

• Ions diffuse from areas of higher concentration to areas of lower concentration

• Forces act to drive Na+ from outside to inside

• Forces act to drive K+ from inside to outside

Electrical Gradient (3 processes)

• Similar charges repel each other (opposites attract)

• + ions driven towards - (and - to +)

• Na+ outside (which is more +) is driven towards the inside (more -)

What are the chemical gradient and electrical gradient together called?

Electrochemical gradient

Where does the action potential begin?

Axon hillock

Action Potential

Information is sent through the axon from the cell body to the axon terminal

Depolarization

The charge inside the cell moves closer to 0

Hyperpolarization

The polarity inside the cell moves away from resting potential, i.e., becomes more negative

Threshold of Excitation for an Action Potential

-55 millivolts

What happens when the threshold of excitation is reached?

Voltage gated Na+ open, Na+ enters the cell which depolarizes the inside of the cell

9 Steps of an Action Potential

1. Threshold of Excitation is reached

2. Voltage gated Na+ gates open

3. Na+ enters the cell through ion channels

4. The inside of the cell becomes more positive - depolarization

5. The inside of the cell becomes positive

6. Voltage gated Na+ channels close

7. Voltage gated K+ channels open

8. K+ exits the cell and the inside of the cell becomes more negative - hyperpolarization

9. After-Hyperpolarization - K+ channels are slow to close and potassium leaks out

Na+/K+ Pump

Works to restore balance of Ca+ and K+ ions by moving 3 Na+ outside and 2 K+ inside; requires ATP

Absolute Refractory Period

• Impossible to generate another action potential

• When the inside of a cell is above the threshold of excitation

Relative Refractory Period

• Can generate another action potential, but it’s more difficult

• During the after-hyperpolarization

• Need much more positive current to reach the threshold of excitation (-55 mv)

3 Phases of the Action Potential

• Depolarization

• Repolarization

• Hyperpolarization

In which phase of polarization does the relative refractory period occur?

Hyperpolarization

In which phases of polarization does the absolute refractory period occur?

Depolarization and repolarization

Toxins

Noxious or poisonous substances; many have impacts on the action potential

Tetrodotoxin (TTX)

Found in the flesh of pufferfish. Blocks voltage gated Na+ gates so they can’t open, so the diaphragm can’t contract, leading to death by asphyxiation.

Saxitoxin (STX)

Produced by algae during the red tide. Blocks voltage gated Na+ channels from opening.

How do localized anesthetics block the action potential?

Blocks voltage gated Na+ channels in a localized area.

How do venoms block the action potential?

They keep voltage gated Na+ gates from opening and block voltage gated K+ channels from opening

Propagation of the Action Potential

Action potential is regenerated along the axon, beginning at the axon hillock and ending at the axon terminal.

Nodes of Ranvier

Spaces between sections of myelin where there is a high concentration of ion channels

EPSP

Excitatory Postsynaptic Potential; graded depolarization due to Na+ or Ca2+ influx

IPSP

Inhibitory Postsynaptic Potential; usually results from Cl- influx, a graded hyper polarization.

3 Overall Steps to Chemical Transmission

• Receive (EPSPs and IPSPs)

• Integrate (Summation at the axon hillock)

• Send (action potential and the release of chemicals)

What are graded potentials proportional to?

The size of stimulus that generates them; they decay over time

Saltatory Conduction

Propagation in myelinated axons; axon potential skips along axon (no ion channels beneath myelin)

Multiple Schlerosis

Autoimmune disease where a loss of myelin sheath causes ions to leak out. This leads to muscle slowness, weakness.

All-or-None

A neuron either generates an action potential or doesn’t, no in between

Non-decremental

Action potential is re-generated along the axon, and size and shape is the same every time

3 Steps to Neuronal Communication

• Receive (graded potentials)

• Integrate (what happens at axon hillock - does the membrane reach the threshold of excitation?)

• Send (action potential and the release of chemicals)

Graded Membrane

Small membrane potentials; can be depolarizations (excitatory) or hyperpolarizations (inhibitory)

Depolarizations are (excitatory/inhibitory).

Excitatory

Hyperpolarizations are (excitatory/inhibitory).

Inhibitory

Excitatory

Brings membrane closer to threshold of excitation

Inhibitory

Brings membrane further away from threshold of excitation

Where do EPSPs and IPSPs summate together?

Axon hillock

At what charge do Na+ gated ion channels close?

+50 mV

When to K+ gated channels open?

+50 mV - when the Na+ gated ion channels close