Mod3-Obj9: Depolarisation/hyper-polarisation, graded and action potentials Flashcards
Resting membrane potential
A charge difference (voltage) exits across the plasma membrane
- External surface is positively charged
- Internal surface is negatively charged
Resting membrane potential: Oppositely charged attract each other
- Energy is required to keep them apart
- Energy is released as they move towards each other
An electrical current is…
The movement of charged particles
-Has the potential to do work
Separating opposite charges across the plasma membrane
Generates a potential energy, measured in volts, known as the resting membrane potential
Every cell in the body has a resting membrane potential
- ) Differential distribution of ions
- ) Selective membrane permeability
- ) Na+K+ pump uses ATP energy to maintain differential distribution
- Leakage channels, from an area of high concentration to low concentration, down a concentration gradient
- In ECF lots of Na+ ion few K+ ions, more positive ions exit the cell than enter, more positive that inside
- In ICF lots of K+ ions few Na+ ions, fewer positive charges (more negative) than outside=RELATIVE CHARGE DIFFERENCE
In neurons the RMP is approximately…
-70mV
The minus sign indicates that the inside of a cell is negative relative to the outside
Neurons are excitable because they can…
Change their membrane potential
-Generate electrical signals that facilitate neural communication
Less negative, more positive
Depolarisation
More negative, less positive
Hyperpolarisation
Changes in RMP of neurons occur when…
A stimulus alters the permeability of the plasma membrane, which
- Opens membrane ion channels, which allows
- Na+ or K+ ions to flow across the membrane; down their chemical concentration gradient, along an electrical gradient (to an area of opposite charge)
Ion flow of changes in RMP alters the…
- Na+ and K+ concentration in the ECF and ICF
- Overall charge on either side of the membrane
- Membrane potential (voltage across the membrane)
There are two types of membrane channels
Leakage channels : always open
Gated channels: Open and closed in response to specific stimuli: chemicals, mechanical stimulation, voltage channels
Chemically gated channels
Open in response to a chemical stimulus e.g.) binding of neurotransmitters
-Located in the plasma membrane of the dendrites and cell body
Mechanically-gated channels
Open in response to mechanical stimulation e.g.) touch, vibration and pressure
Located in the plasma membrane of the dendrites and cell body
Voltage-gated channels
Open and close in response to changes in the membrane potential (voltage) across the membrane
-Located in the plasma membrane of the axon and axon terminals (not present in the plasma membrane of the dendrites or cell body)
Changes in membrane potential
The electrical signals used by neurons to relay sensory and motor information throughout the body are generated when Na+ or K+ ions flow across the plasma membrane and change the membrane potential (voltage)
Changes in membrane potential are…
Relative to the RMP
- Occur when a gated channel opens and allows the movement of ions
- Are described by the terms depolarisation and hyperpolarisation
Polarity
Is a state in which two properties are completely opposite
Depolarisation
To decrease the “oppositeness”
-A reduction in membrane potential (decreasing the charge polarity across the membrane)
Repolarise
Restore the “oppositeness”
Hyperpolarise
To increase the “oppositeness”
-An increase in membrane potential (increasing the charge polarity across the membrane)
Depolarisation: If gated channels that allow the passage of Na+ ions open:
- Influx of Na+ ions into ICF
- INTERIOR BECOMES LESS NEGATIVE
- Membrane potential moves towards or above zero e.g.) -70mV to -60mV
Hyperpolarisation: If gated channels that allows the passage of K+ ions open:
- Efflux of K+ ions out of the ICF
- INTERIOR BECOMES MORE NEGATIVE (i.e. less positive)
- Membrane potential moves further from zero e.g.) -70mV to -80mV
The nervous system generates two types of electrical signals:
- ) Graded potentials
2. ) Action potentials (nerve impulse)
Graded potentials
Short distance, short lived changes in membrane potential (localised)
- Either depolarisation or hyperpolarisation
- Usually generated in the dendrites or cell body of a neuron, when a stimulus causes chemically gated or mechanically gated channels to open
Graded potentials are…
Proportional to the strength of the stimulus
-The stronger the stimulus=the greater the voltage charge the further the signal travels
If a stimulus strong enough, a graded potential can:
- Travel from the site of stimulation (dendrites or cell body) to the initial segment of an axon
- Depolarise the initial segment to threshold (-55mV)
- Trigger the opening of voltage-gated Na+ channels
- generate an action potential
Action potentials…
- Are long distance electrical signals
- Arise at the initial segment of an axon when voltage gated channels are stimulated to open
- Channels in membrane potential are independent of stimulus strength
Action potentials don’t…
Decrease in size/strength with distance along axon
- Are all or none response
- Are self-propagating; once initiated, a chain reaction of action potentials will travel down the entire axon
