Neutral & Action Potentials

Volt

Unit of measure for an electrical potential between 2 points

q =

a charge

V

Voltage, electrical potential energy. Resting membrane potential in a resting cell

Average cell resting membrane potential

-70 mV

In physiology the charged particles (q) are

dissolved ions

E

Ability to do work (Energy)

I

Current, movement of charges. Measured in amperes (amps)

As wire d increases

the current increases

As wire d decreases

the current decreases

The size of a wire effects

resistance

Water is a

good conductor

Lipids are

bad conductors; good insulators

Chemical Concentration gradient

Diffusion, chemical will diffuse from area of high to low

Na+ (sodium) tends to

diffuse into cell

K+ (potassium) tends to

diffuse out

Cl- (Chloride ion) tends to

diffuse in

K+ diffuses across membrane much faster than Na+. What does this mean?

There is a slight negative charge on inside, relative to outside

Na+/K+ pumps

Have a 3:2 ratio, always pumping to keep the membrane resting at -70 mV. Pumps in 2 potassium's, 3 sodiums out

Leak channels

Always open (important in establishing resting potential) allow potassium into the outside, and some sodiums in when there is too many. a SLOW leak

Chemically regulated channels

Open or closed in response to chemical signals (neurotransmitters) reaching them.

• Important in receiving messages from other cells

• Found mainly on dendrites and soma

Voltage-regulated channels

Open or close in response to changes in voltage potential across the plasma membrane

Found on axon

Responsible for propagating an action potential (electric signal along axon)

2 Gates: activation gate & inactivation gate

Mechanically regulated channels

Open or close in response to physical distortion

Important in sensory receptors

Found on sensory receptors

Graded potentials

Changes in transmembrane voltage potential which don’t spread far & do so in a graded manner

Occur on dendrites & soma

Temporal Summations

Multiple signals at one synapse

Spatial summation

Mutiple signals at multiple locations

Depolarization

Shift of membrane voltage potential away from resting levels towards 0 mV ( -60 to +30 in action potentials)

Hyperpolarization

Shift of membrane voltage potential away from 0 mV to -80 mV or more

Action Potentials

Carry signals along axon very rapidly, involve voltage regulated channels.

ALL or NOTHING ( threshold has to be at atleast -60 mV for action potential to occur)

Steps of Action potential

1. Depolarization (-60 mV)

2. Sodium channels open (voltage regulated)

   1. Sodium ions rush in, rapid depolarization (-60 mV to +30 mV)

3. Sodium channels close, potassium channels open at +30 mV

   1. Potassium ions rush out of cell

4. Potassium channels close when voltage reaches -70 mV. Membrane hyperpolarizes to -90mV

5. Refractory period. Sodium/Potassium pumps pump out sodium and in potassium to restore cell to normal

Speed of Propagation along unmyelinated acxon

1 meter per second, 2 mph

Speed of Propagation along myelinated axon

40 - 300 mph, 140 meters per second ( No voltage regulated channels on myelin)

Saltatory Propagation

Involves myelinated axons, no challenges along myelinated portions of axon. Current must jump from node to node.

Type A fibers

Largest axons, myelinated, (300 mph) Include sensory and motor of fine control

Type B fibers

Smaller axons, myelinated (40 mph) Less urgent, temp, pain, smooth muscles

Type C fibers

Smallest, unmyelinated (2 mph) Less urgent, temp, pain, smooth muscles

Electrical Synapse

Rare, presynaptic & postsynaptic membranes are locked together by proteins called connexons

Change in one membrane potential causes change in other by local currents. In graded potentials

Chemical Synpases

Most abundant synapse, most neuron/neuron synapses, all neuron/other cell type synapses

Communication through neurotransmitter

Excitatory Neurotransmitters

Causes depolarization of postsynaptic membrane

Inhibatory neurotransmitters

Cause hyperpolarization of postsynaptic membrane

Acetylcholine

Most understood and common, depolarizes membrane

Norepinephrine (NE) or Noradrenaline

Important in brain and ANS, depolarizes membrane

Dopamine

Found in CNS, may act excitatory or inhibtatory. Parkinson’s disease = inadequate dopamine production

Cocaine “high” is caused by inhibition of dopamine removal from synapses

Serotonin

Found in CNS, inadequate production can lead to depression and lack of attention

GABA ( Gamma aminobutyric acid)

Inhibiitory effect, reduces anxiety

Cholinergic synapse

Chemical synapse that released acetylcholine, most common & well known synapse

Steps of Cholinergic Synapse

1. Action potential arrives at synaptic knob & depolarizes membrane

2. Voltage regulated Calcium channels open & calcium rushes into synaptic knob

3. Presence of Calcium triggers the exocytosis of Acetylcholihne

4. Ach. diffuses across synapse, binds to postsynaptic membrane (chemically regulated channels)

5. Binding of Ach. to channels causes them to open Sodium channels, sodium rushes into postsynaptic cell, causing depolarization ( Graded potential)

6. AchE in synapse breaks down Ach into acetate and choline. These are then uptaken by cells & recycled

Neuromodulators

Chemicals that influence either release of neurotransmitter or response of postsynaptic cell to the neurotransmitter

Endorphins

Neuromodulator, produced in brain & spinal cord for pain relief