Neurophysiology

What has electrical charge

protons and electrons

Ions

atoms or molecules that bear a net charge because they have unequal numbers of protons and electrons

Electric Current

• movement or flow of charges make up it

• similar to flow of water through pipes

• what neurons use to transmit a signal or message another cell

Voltage or Potential Difference

when we separate positive and negative electrical charges

• can do work when charges are allowed to flow as a current

Transmembrane Potential

• in our bodies, the positive and negative charges are separated across cell membranes

• inside is more negative than outside

  • resting neuron (resting membrane potential) is -70 mV

Resting Neuron Potential

• aka resting neuron or resting membrane potential

• neuron is at rest, not transmitting signals

What is responsible for the difference?

• Ions are distributed unequally

  • Extracellular cation – Na+

  • Intracellular cation – K+

  • Extracellular anion – Cl-

• inside the cell we also have negatively charged proteins

Semipermeable

• cell membranes are _______

  • must be protein channels available for Na+ to move through the K+ and Cl-, proteins will not move

    • too big to move through channels

Chemical Gradients

• Based on concentration of the ion, moves from high to low

  • Na+ and Cl- goes into cell membrane

  • K+ goes out of the cell membrane

  • causes the ions to flow in or out of the cell if open

Electrical Gradients

• Movement based on charge, opposites attract

  • Na+ and K+ goes into cell membrane

  • Cl- goes out of cell membrane

  • causes the ions to flow in or out of the cell if open

Electrochemical Gradients

• Movement based on chemical and electrical gradients

  • Na+ - moves into cell membrane due to chemical and electrical gradients

  • K- - moves out of the cell membrane because chemical gradient has a higher force than electrical gradient, having it go out

    • Chemical gradient wants to go out, electrical gradient wants to go in, chemical gradient overpowers it

  • causes the ions to flow in or out of the cell if open

Maintaining Resting Membrane Potential and return cells to RMP

• Sodium-potassium (Na⁺-K⁺) ATPase pump

  • Move against concentration gradient

  • 3 Na+ out for every 2 K+ in

• after a change in membrane potential

Passive Channels / Leak Chanels

• Always open

• K+ leak channels

• Na+ leak channels

• allow ion movement

Chemically Regulated (Ligand Regulated) Channels

• Open or close in response to a specific chemical

• allow ion movement

Mechanically Regulated Channels

• Open or close in response to membrane distortion

  • Touch, pressure, vibration

• allow ion movement

Voltage Regulated Channels

• Open or close in response to change in transmembrane potential

• allow ion movement

ATP

• 3 Na+ - moves out

• 2 K+ - moves in

Dendrites + Soma

• Leak channels

• Chemically gated

• Mechanically gated

• Has Na+ - K+ ATPase pump

Both Axon and Terminals

• Leak channels

• Voltage gated

• Has Na+ - K+ ATPase pump

Local Potentials or Graded Potentials

When a neuron is stimulated by a signal from another neuron, a ligand binding to a chemical channel or a shape change in a mechanically regulated channel it causes small local disturbances in the membrane potential. These channels allow Na⁺ to flow into the cell.

• Happens in dendrites and soma

• Gets name because it is in a very small area of the membrane

• Overall goal: axon hillock to reach threshold -> i.e. -60mV

• To speed up/bring it to threshold, it can open more channels, open channels closer to the axon hillock, or open channels for longer

Depolarization

• Incoming Na+ ions diffuse short distances from the initial site producing a current along the dendrite and cell body toward the axon hillock or trigger zone

• local potential – short distance

Graded

• Strength varies in magnitude dependent on stimulus

• Open more channels or open channels longer

• part of local potential

Decremental

• signal weakens the further it travels

• part of local potential

Reversible

• Remove stimulus = stop signal

• Restores resting membrane potential

• part of local potential

Excitatory

• Open Na+ channels

• Depolarize

• part of local potential

Inhibitory

• Open K+ channels

• Hyperpolarize (make more negative)

• part of local potential

Repolarization

• Na⁺-K⁺ ATPase pumps return cell to resting membrane potential

• part of local potential

Action Potential

• Neurons can generate an electrical signal

• The ion channels that produce action potentials are voltage-gated channels

  • opening depends on the membrane potential

Action Potential Step 1

• Local potential at axon hillock increases until it rises to threshold

  • Depolarization event

Action Potential Step 2

• Neuron produces an action potential; voltage-regulated Na⁺ channels open; more and more Na⁺ gates open as Na⁺ enters the cell; K⁺ gates open more slowly when threshold is reached

  • rapid depolarization

Action Potential Step 3

• When 0mV is reached/passed, Na⁺ gates are; voltage peaks at approx. +35mV (0mV in some, +50mV in others)

Action Potential Step 4

• K⁺ gates now fully open; K⁺ leaves the cell repolarizing the membrane; causing shift back to negative inside and positive outside

Action Potential Step 5

• K⁺ channels remain open a little longer than the Na⁺ channels and more K⁺ leaves than Na⁺ came in causing a 1 or 2 mV overshot or hyperpolarization

All or None Rule

• If threshold reached – Action Potential will occur

• If not, it won’t

No Signal Degradation

• Action Potential remain same strength all the way down length of axon

Irreversible

• Removing signal will not stop it from occurring once Action Potential starts

Refractory Period

• During an action potential and a few msec after, it is difficult or impossible to stimulate to produce another action potential

• impossible to make another A.P. on a membrane segment

• Two phases

  • Absolute

  • Relative

Absolute Refractory Period

• no matter what, cannot generate another action potential

• threshold to +35 mV all Na+ channels are open

• +35mV to -50mV the inactivation gate is closed and will not open

Relative Refractory Period

• Can generate another A.P. but requires very strong stimulus

• Difficult but not impossible

Unmyelinated Fibers

• continuous propagation

Myelinated Fibers

• saltatory propagation

  • saltatory meaning skipping

• only happens at nodes of ranvier

Axon Diameter and Propagation Speed

• Myelinated = faster

• Larger diameter = faster

• Type A = largest + myelinated

  • Sensory - balance, touch, pressure

• Motor – skeletal muscles

• Type B = medium + myelinated

• Type C = smallest + unmyelinated

  • Both Type B and C

    • Sensory – temp, pain, touch, pressure

    • Motor – smooth + cardiac muscle, adipose tissues, glands