topic 6 : human health and physiology

Resting potential

Resting potential Graded potential Active potential

• Membrane potential of a resting neuron.• State of neuron at rest which is not sending

signals.• Volts : -60 - -80 mV• -ve sign means that inside of a neuron has –ve

charge to the outside.

Formation of resting potential

• Involve changing of the charge of plasma membrane of nerve cell.

• Resting potential is driven by changing of chemical potential energy to electrical potential energy.

• Importance of substance involve in this process are :* potassium ion* sodium ion

• Each of the ion has concentration gradient within the plasma membrane of the cell.

• Concentration gradient of potassium and sodium within the plasma membrane represent the chemical form of potential energy.

• During formation of resting potential, chemical energy is convert into electrical energy.

• This conversion involves sodium - potassium pumps to be used.

• Sodium – potassium pumps generates and maintain the ionic gradient of Na+ and K+ into the cell.

• ATP also is used to transport Na+ and K+ in opposite direction.

Step by step

protein pore is formed by clusters of specialized protein that span the membrane. it is also carried out selective permeability (specific ion can bind to).

Involves ion channel (potassium channel + sodium channel).

1

• Ions diffuse through ion channel. • Each ion carry the units of electrical charge.• Diffusion of ions will result net movement of

positive or negative charge.• Produce voltage across membrane.

2

• During resting potential, a lot of potassium channels open and few sodium channel open.

• Diffusion of K+ occurred• From inside to outside of the cell.• [K+ inside] higher than [K+ outside].

3

4

• [Cl-] will present inside of the cell.• Cl- cannot diffuse out of the cell because there

are very few Cl- channels open.• Leaving high negative charge inside the cell

thus build up -ve charge inside membrane.

5

• The separation of charge cause voltage to be produced.

• This in turn cause the electrical gradient to counterbalance the [K+].

6

Limitation of diffusion of [K+]

1. The building up of negative charge inside the cell indefinitely is prevented by excess of negative charge.

2. Excess negative charge exerts attractive force that oppose the flow of additional positively charge K+ out of the cell.

Graded potential

• Intermediate potential before action potential take place.

• Involves ion channels which are potassium and sodium gated channels.

• Gated channels can open and closed in response to one of three kinds of stimuli.

Gated channe

l

Stretch – gated ion channels

Voltage – gated ion channels

Ligand – gated ion channels

• Sense stretch cells • Open when membrane is

mechanically deformed.

• Found at synapses.• Open and closed

when a specific chemical (neurotransmitter) binds to the channel.

• Found in axon.• Open or closed when

the membrane potential changes.

• Sodium and potassium gated channels are therefore voltage – gated ion channel.

Process occurs during graded potential

• Increase the magnitude of membrane potential.

• Inside of the membrane becomes more negative.

• More K+ ions channels open.• Movement of more K+ to the outside of cell.• Inside becomes more negative thus more

negative value of potential is produced.

hyperpolarisation

• Reduction in the magnitude of membrane potential.

• Inside of the membrane becomes less negative.

• More Na+ ions channels open.• More movement of Na+ into the cell. Inside of

the cell becomes less negative.• Less negative value of potential is produced.

depolarisation

Action Potential

Nerve impulseNerve impulse of signals that carry information along the axon.

The transport of information can occur at great distances, for example, from toes to spinal cord.

Action potential is very brief.• 1-2 millisecond in duration• Enable a neuron to produce the voltage at

high frequency• This feature is important as neuron encode

information in their action potential frequency. For example, knee-jerk reflex

2

Action potential frequency• Related to magnitude and suddenness of

stretch in the quadriceps muscle

3

Production of active potential

Resting potential

• On Na+ ion channel, activation gate is closed. Meanwhile, inactivation gate is open in most channel.

• On K+ ion channel, activation gate is closed.

1

Depolarization

• Occur when stimulus is present and depolarizes membrane.

• On Na+ ion channel, both gates open. Na+ diffuse into the cell. Then Na+ influx causes further depolarization. More Na+ diffuse into the cell, so on.

• Inside membrane become less negative.• On K+ ion channel, activation gate remain close.

2

Rising phase ofthe action potential

• Threshold crossed, membrane potential close to ENa (62 mV) .

• On Na+ ion channel, activation gate is open and inactivation gate open too.

• On K+ ion channel, activation gate is closed.

3

Falling phase of action potential

• On Na+ ion channel, activation gate is open. Inactivation gate is closed. Which block the Na+ influx.

• On K+ ion channel, activation gate is open. Which permit K+ reflux. This caused the inside of the membrane becomes negative and membrane potential close to EK, -72 mV.

4

Undershoot

• Membrane’s permeability of K+ is higher than at rest. Membrane potential close to EK.

• On Na+ ion channel, both activation and inactivation gates are closed.

• On K+ ion channel, the activation gate is open initially, soon it will closed and increase the membrane potential to rest potential.

• Second depolarising stimulus occurs during this period, but it will not able to trigger an action potential, refractory period.

5

Conduction of action potential

• Action potential is along distance signal without diminishing from the cell body to the synaptic terminals.

• Long distance signals does regenerating.

1

• At the site where action potential is initiated, the Na+ influx during rising phase creates electric current that depolarised axon the neighbouring membrane.

• Depolarisation in neighbouring region is large enough to reach threshold. This is the point where action potential is reinitiated.

• This process is repeated many times as action potential travels the length of axon.

2

• Behind travelling zone of depolarization due to Na+ influx, this region is known as zone of repolarization due to K+ efflux.

• At this zone, inactivation gate Na+ channel closed. • Inward current that depolarises the axon membrane

ahead of the action potential cannot produce action potential behind it.

• This prevents action potential from travelling back toward the cell body.

• Action potential moves in only one direction.

3

• Depolarisation – repolarisation process is repeated in the next region of the membrane.

• Local current of ions across the membrane caused action potential to be propagated along the length of the axon.