Nerve Impulse

Resting Potential

• voltage of a nerve is always a measure of charge inside the cell relative to the outside of the cell

• Have a relative resting charge of -70mV

• Charge is due to a ratio of 2K+ inside the cell and 3Na+ outside the cell

Na+/K+ Pump

1. Three sodium ions enter the enzyme from within the cell

2. ATP phosphorylates the enzyme, causing it to pump3 Na+ out of the cell

3. Two potassium ions (K+) enter the enzyme from outside the cell

4. The now unphosphorylated enzyme pumps the 2 K+ into the cell

Action Potential

• An ‘excited’nerve transferring an impulse along it’s length

• The Na+ gates open → Na+ rushes in

• The voltage jumps to +40mV

• Lasts milliseconds

• Movement is called depolarization

Moving Impulse

• a nerve impulse begins when a neuron is stimulated by another neuron or by its environment

• Impulse travels along the axon, away from the cell body, toward the axon terminal

Repolarization

• After a maximum voltage has been achieved the Na+ gates close

• Prior to the maximum action potential, K+ gates open and K+ rushes out

• The voltage drops to -90mV

• Process inhibits an action potential

Hyperpolarization

• range of -70mV to -90mV

• Refractory Period: no action potential is possible

Resting Potential

Na/K pumps being and reestablished a resting potential of -70mV

Graph of Nerve Impulse

1. Resting Potential -70mV

2. Initiation of nerve impulse -70mV to -55mV

3. Threshold -55mV, less than -55mV → failed initiation or no action potential

4. Depolarization -55mV to 30mV

5. Action Potential +30mV (Na gates closes)

6. Repolarization +30 to -70 (K gate open)

7. Hyperpolarization -70 to -90

8. Refractory Period -80 to -70

9. Resting Potential -70 (3Na+ out/ 2K+ in)

Key Diploma Points

1. Na+ is an excitatory ion

2. K is an inhibitory ion

3. A drug that increases Na+ or decreases K+ will increase nerve transmission

4. A drug that decrease Na+ or increases K+ will decrease nerve transmission

Threshold Level: All-or-one

• minimum level of stimulus required to produce a response (>55mV)

• all or none - nerves or muscles respond completely or not at all

• a charge greater than minimum required will not produce an increased response

• to increase a nerve response, more nerves need to be recruited

Message Priority

1. greater frequency of impulses the greater the intensity of response

2. some neurons have higher threshold levels that require a greater stimulus

3. brain gates and decides which impulse to interpret

Summation

• Summation acts as a filter, preventing the neuron from firing in response to weak, random signals. 

• inhibitory and excitatory neurotransmitters can be released

• summative effect on a post-synaptic neuron will determine if an impulse is generated

Synapse

• an area located between nerve cells

• protein is transferred across this space

• this protein is necessary to transfer a nerve impulse from one neuron to the next

Neurotransmitters

• proteins that have either an excitatory or inhibitory effect on the postsynaptic membrane

• excitatory → receptor proteins will trigger ion channels to open to allow Na to flow into post synpatic nueron

• inhibitory → will trigger K channels to open, allowing K ion to flow out = hyperpolarization, less potential for a neural impulse

Acetylcholine

• excitatory neurotransmitter, generates a speeding up of the nervous system

• cholinesterase is the enzyme that breaks down the neuro transmitter acetylcholine

• can be reabsorbed by the presynapse

Key Diploma Points - Summation vs Strenght of Signal

• combination of weak depolarization’s can add up to a threshold impulse

• strength of signal directly affects reaching threshold. a stronger signal does not equal a stronger impulse

• an interference with Na flow = no nerve impulse

• an interference with K flow = nerve impulses