Nerve impulses (A-level only) Flashcards
Neurones
Cells that transmit information from receptors to effectors
3 main types of neurones
Sensory
Motor
Relay
Sensory neurone
Sensory neurones carry nervous impulses from receptors (e.g. pressure receptors) into the Central Nervous Systems (CNS).
Motor neurone
Motor neurones carry impulses from the CNS to effector organs (e.g. muscles or glands).
Relay neurone
Relay neurones are intermediate neurones.
Relay neurones receive impulses from a sensory neurone and relay them to motor neurones.
Basic stucture of neurone
Neurones can be myelinated or non-myelinated but all neurones have the following basic structures:
Dendrites - carries nervous impulses towards a cell body.
Axons - carries nervous impulses away from the cell body.
Cell body - where the nucleus is normally located.
Myelinated motor neurone
Motor neurones in vertebrates are usually myelinated.
Schwann cells are wrapped around the axon of the neurone.
These cells form the myelin sheath.
Gaps between adjacent Schwann cells are called nodes of Ranvier.
Potential difference
At resting state there is a difference in charge across the neurone membrane: the inside of the neurone is more negatively charged than outside.
This is because there are more positive ions outside the cell than inside.
The difference in charge is called a potential difference.
Sodium-potassium pumps
The resting potential is maintained by sodium-potassium pumps in the neurone membrane.
Three Na+ ions are actively transported out of the neurone by the pumps for every two K+ ions that are transported in.
This leads to a build-up of positive ions outside the cell.
Potassium ion channels
There are potassium ion channels in the neurone membrane.
This means it is permeable to K+ ions.
When K+ ions are transported into neurones, they can diffuse back out.
The neurone membrane is also impermeable to Na+ ions so the ions cannot diffuse back into the cell after they have been transported out.
Resting potential
Together the action of sodium-potassium pumps and potassium ion channels leads to a potential difference across the neurone membrane.
This potential difference is called the resting potential.
The neurone is said to be polarised.
Resting potential is about −70mV.
Repolarisation
When a resting neurone is stimulated, its membrane experiences a change in potential difference.
This change is called repolarisation.
Stages of repolarisation
Stimulation
Depolarisation
All-or-nothing
Repolarisation
Hyperpolarisation
Resting potential
Stimulation
Na+ ion channels in the cell membrane open when a neurone is stimulated.
Na+ ions flood into the neurone.
The potential difference across the membrane changes to become more positive inside the neurone.
Depolarisation
If the potential difference increases above the threshold value (about −55mV) then the membrane will become depolarised.
More sodium channels open and there is a sharp increase in potential difference to about +30mV.
All-or-nothing
Depolarisation is an all-or-nothing response.
If the potential difference reaches the threshold, depolarisation will always take place and the change in potential difference will always be the same.
If the stimulus is stronger, action potentials will be produced more frequently but their size will not increase.
Repolarisation
After the neurone membrane has depolarised to +30mV, the sodium ion channels close and potassium ion channels open.
K+ ions are transported back out of the neurone and the potential difference becomes more negative.
This is called repolarisation.
Hyperpolarisation
There is a short period after repolarisation of a neurone where the potential difference becomes slightly more negative than the resting potential.
This is called hyperpolarisation.
Hyperpolarisation prevents the neurone from being restimulated instantly.
This is called the refractory period.
Resting potential
After the refractory period, the potassium ion channels close and the membrane returns to its resting potential.
The process where a neurone is depolarised and returns to resting potential is called an action potential.
4 stages of an action potential
Sodium ions
Sodium ion channels
Wave of depolarisation
Refractory period
Sodium ions
When an action potential is generated, there are more Na+ ions inside the neurone than outside.
Some of these Na+ ions diffuse sideways along the neurone axon.
Sodium ion channels
The presence of Na+ ions creates a change in potential difference further along the neurone membrane.
If this reaches the threshold value, sodium ion channels at this part of the membrane open.
Na+ ions diffuse into the neurone.
This part of the neurone now becomes depolarised.
Wave of depolarisation
Na+ diffuse all along the neurone in this way.
Depolarisation takes place along the neurone membrane.
This creates a wave of depolarisation.
Refractory period
The period of hyperpolarisation in an action potential is called the refractory period.
The ion channels are recovering during the refractory period.
This means an action potential cannot be stimulated again instantly.
This ensures that the wave of depolarisation travels in one direction.
Factors that speed up the transmission of nerve impulses include:
Myelination
Temperature
Axon diameter
Myelination
Schwann cells wrap around the axon of neurones to create a myelin sheath.
The myelin sheath acts as an electrical insulator because it is impermeable to ions (Na+ and K+).
Depolarisation and action potentials cannot occur at the myelinated parts of the axon and can only occur in the gaps between (the nodes of Ranvier).
The nervous impulse jumps from one node to the next.
This is called saltatory conduction.
Temperture
An increase in temperature increases kinetic energy.
Ions move across the membrane more rapidly when they have more kinetic energy.
Axon diameter
Giant axons are found in the giant squid and allow it to have a rapid escape response.
Greater axon diameter means there is a greater surface area for the movement of ions across the cell membrane.
