Chapter 3.2 Nerve Impulses

Nerve impulse

Electrochemical change that travels along an nerve fibre.

A nerve impulse is electrochemical because?

There is a change in electrical voltage that is brough about by changes in chemicals, specifically, the concentration of ions inside and outside the cell membrane.

When a positive and negative charge come together…

energy is released

If a group of positive and negative charges are separated…

They have the potential to come together and release energy.

Voltage

the electrical potential difference between two places

Ions

electrically charges particles

Membrane Potential

Differences in the concentration of ions mean that there is a potential between the inside and the outside of the cell membrane.

resting membrane potential

-70 mV

Ions are unable to…

diffuse through the phospholipid bilayer of the cell membrane directly

Instead of moving through the phospholipid bilayer directly, IONS…

Move through protein channels

Leakage channels

Protein channels that are open all the time

Voltage-gated channels

Protein channels that are only open when the nerve is stimulated

The resting membrane potential of neurons is due mainly to…

differences in the distribution of potassium ions and sodium ions on either side of the cell membrane, making the extracellular fluid more positively charged than the intracellular fluid.

Concentration of sodium ions is outside the neuron is…

about 10 times higher than the inside of the cell.

the concentration of potassium ions inside the cell is…

30 times greater than inside the neuron than outside.

How permeable is the cell membrane to potassium and why?

The cell is highly permeable to potassium due to the larger number of potassium leakage channels.

How permeable is the cell membrane to sodium ions and why?

The cell is membrane is only slightly permeable to sodium ions due to the lower number of sodium leakage channels.

the concentration of chloride ions is…

Higher outside the cell than inside

How permeable is the cell membrane to chloride ions? and why?

The cell membrane is highly permeable to chloride ions allowing their diffusion through protein channels.

The concentration of large, negatively charged organic ions is…

Higher inside the neuron then outside.

How permeable is the cell membrane to large organic negatively charged ions?

It is impermeable to these ions and therefore they stay inside the cell.

Sodium-Potassium pump

In addition to protein channels the sodium and potassium ions move across the cell membrane through a carrier protein called the sodium potassium pump.

Function of the sodium-potassium pump

The pump moves 2 potassium ions into the cell for every 3 sodium ions that are removed.

Net reduction of positive ions inside the cell #1

S+P pump binds 3 sodium ions and a molecule of ATP

Net reduction of positive ions inside the cell #2

The splitting of ATP provides energy to change the shape of the channel. The sodium ions are driven through the channel.

Net reduction of positive ions inside the cell #3

Sodium ions are released to the outside of the membrane and the new shape of the channel allows 2 potassium ions to bind.

Net reduction of positive ions inside the cell #4

Release of the phosphate allows the channel to revert to its original form releasing the potassium ions on the inside of the membrane.

Why is the resting membrane potential negative #1

The combination of location of the ions, the permeability of the cell membrane and the S+P pump means that there is a net flow of positive ions outside the cell than there are sodium ions diffusing into the cell.

Why is the resting membrane potential negative #2

In addition to the negative organic ions inside the cell, results in the inside of the cell being more negative than the outside.

Action Potential

If the stimulus to a neuron is sufficient, the signal will pass along the neuron. This happens due to the opening and closing of voltage-gated channels, which causes the rapid, depolarisation or repolarisation of the of the membrane. Lasts approximately 1 millisecond.

Depolarisation

Is the sudden increase in membrane potential. This occurs of the level of stimulation, exceeds about 15 mV.

How does depolarisation occur?

When a neuron is stimulated by a neurotransmitter on a sensory receptor, some sodium channels are opened. These are called ligand-gated channels. Once they are opened more sodium ions move into the cell. This makes the intracellular fluid less negative, increasing the potential difference.

Repolarisation

After a short period, their is a sudden decrease in membrane potential.

How does repolarisation occur?

Sodium channels close, stopping the influx of sodium ions into the cell. At the same time, the voltage-gated potassium channels open, increasing the flow of potassium ions outside of the cell. This decreases the membrane potential, and the membrane is repolarised.

Hyperpolarisation

The potassium channels remain open for a little longer than necessary. This results the membrane potential dropping lower than the resting membrane potential, and the membrane is hyperpolarised.

Refractory Period

Once the sodium channels have opened, the quickly become inactivated, meaning that they will not respond to stimulus. For a brief period it will not go under another action potential. This lasts from when the membrane reaches the threshold of -55 mV till it reaches resting membrane potential again.

Transmission of a Nerve Impulse

Action potential occurs in one section of a membrane, however, it triggers an action potential in the adjacent membrane. This process occurs along the length of the neuron and is called a nerve impulse.

Conduction along unmyelinated nerve fibres

Depolarisation of one area of the membrane causes the movement of sodium ions into the adjacent area. This movement stimulates the opening of voltage gated sodium channels in the next part of the membrane which initiates action potential in that part of the membrane. This repeats along the whole membrane so that action potential moves away from the point of stimulation.

Transmission along myelinated fibres

The myelin sheath insulates the nerve fibre from extra cellular fluid so. This does not occur at the nodes of ranvier. therefore where the nerve fibre is surrounded by myelin ions cannot flow between the inside and outside the cell, this means that no action potential can be formed.

Saltatory conduction

In myelinated nerve fibres because action potential cannot be formed where there is myelin, instead action potential jumps from one node of ranvier to the next. This allows the impulse to travel much faster up to about 140 m/s rather than the unmyelinated 2m/s.

Transmission across a synapse #1

When a nerve impulse reaches an axon terminal, it activates voltage gated calcium ion channels.

Transmission across a synapse #2

As there is a higher concentration of calcium ions in the extracellular fluid, they flow into the cell at the presynaptic axon terminal.

Transmission across a synapse #3

This causes synaptic vesicles to fuse with the membrane releasing special chemicals called neurotransmitters by exocytosis.

Transmission across a synapse #4

The neurotransmitters diffuse across the gap and attach to receptors on the membrane of the next neuron.

Transmission across the synapse #5

This stimulates ligand gated protein channels to open, which allows the influx of sodium ions and initiates an action potential in the post synaptic membrane.