A nerve impulse is a self propagating wave of electrical disturbance that travels along the surface of the axon membrane of a neurone.
It is the temporary reversal of the electrical potential difference across the axon membrane.
The reversal is between the resting potential and the action potential.
THE AXON MEMBRANE
- The axon membrane is a phospholipid bilayer
- it prevents sodium and potassium from being able to diffuse across it
- intrinsic proteins span the bilayer, there are three types.
- sodium ion channels (some are voltage gated, some are always open)
- potassium ion channels (some are voltage gated, some are always open)
- Sodium- potassium pump (actively transports potassium ions into the axon and sodium ions out.)
- POTASSIUM IN (KI) SODIUM OUT (SO) REMEMBER KISO PUMP
- inside axon are potassium ions, a few sodium ions and negatively charged organic molecules that are too large to pass through channels.
- outside the axon are sodium ions, some potassium ions and chloride ions.
- inside of axon is negatively charged relative to the outside
- resting potential, 50-90mV (usually 65mV)
- in this state it is said to be polarised
HOW IS RESTING POTENTIAL ESTABLISHED?
- sodium is transported out, potassium is transported in by sodium-potassium pump. (KISO)
- more sodium ions move out than potassium ions in (two potassium to every three sodium)
- the outward movement of sodium ions is greater than the inward movement of potassium ions
- chemical gradient is established
- sodium ions begin to diffuse back naturally into the axon, potassium diffuse out (REMEMBER SIKO NOW)
- however, most of the sodium gates are now closed and the membrane is 100% more permeable to potassium ions than to sodium ions.
- potassium ions diffuse out fast, further increasing potential difference between positive outside and negative inside of axon.
- asisde from the chemical gradient, there is also an electrical gradient.
- as more potassium ions diffuse out of the axon, the outside becomes more positively charged. the positive outside begins to repel the positively charged potassium ions, which are attracted to the negatively charged inside of the axon. This prevents them from moving out more.
- equilibrium between chemical and electrical gradient is established and there is no net movement of ions.
- action potential is the temporary reversal of charges on the axon membrane, or depolarisation.
- -65mV to +40mV
- action potential is caused when a stimulus exceeds threshold value
- depolarisation occurs because the channels in the membrane change shape, depending on the voltage across the membrane
PROCESS OF ACTION POTENTIAL
- at resting potential, most sodium voltage-gated channels are closed
- energy from stimulus opens them and sodium ions diffuse into the axon, along the electrochemical gradient.
- they are positively charged and trigger a reversal in the potential difference across the membrane
- as more sodium ions diffuse into the axon, more sodium channels open, causing influx of sodium ions
- once action potential of +40 has been established. the voltage gates on sodium ion channels close and the voltage gates for potassium ions begin to open.
- Potassium ions diffuse out and more voltage gated potassium ion channels open. outwards influx of potassium ions
- the outward diffusion of these potassium ions causes a temporary overshoot of the electrical gradient, with the inside of the axon being more negative than usual, this is called hyperpolarisation.
- The gates on the potassium ion channels now close and the sodium-potassium pump re-establish resting potential of -65mV. The axon is repolarised.
ACTION POTENTIAL IN UNMYELINATED AXON
- action potential (impulse) is passed rapidly along the axon
- the size of the action potential remains the same from one end to the other
- as one region of the axon produces an action potential and becomes depolarised, it acts as a stimulus for depolarisation of the region of the axon next to it
- action potentials are regenerated at each part of the axon
- in the meantime, the previous region of the membrane returns to resting potential, becoming repolarised, as it has passed the impulse on.
- the action potential passes like a ripple down the neurone.
ACTION POTENTIAL IN MYELINATED NEURONE
- axon is insulated by schwann cells (myelin sheath)
- action potentials cannot form on the myelinated parts of the membrane
- instead, action potentials occur at the nodes of ranvier (the breaks in the myelin sheath)
- The action potential 'jumps' from node to node in a process called Saltatory conduction.
- as a result, the action potential passes faster in a myelinated axon than in an unmyelinated.
FACTORS AFFECTING ACTION POTENTIAL SPEED
- myelin sheath - saltatory conduction in myelinated neurones, faster than unmyelinated
- diameter of the axon - greater the diameter, faster the speed of conductance (less leaks)
- temperature - rate of diffusion is faster at higher temperatures.
- however, sodium potassium pump is fuelled by atp from respiration. respiration requires enzymes. enzymes function fastest at optimum temperature but denature at high temperatures.
Refractory period is when inward movement of sodium ions is prevented because the sodium voltage gated channels are closed. During this time, no action potential can be generated. This occurs for three reasons:
- makes the neurone impulse unidirectional - action potential can only pass from an active region to a resting region, prevents the action potential from spreading out in other directions.
- produces discrete impulses - creates a break between impulses, action potentials are separated from one another.
- It limits the number of action potentials - action potentials are separated from one another and therefore number of action potentials that can pass along an axon at a given time is limited.
All or nothing principle.
If stimulus intensity does not exceed threshold value, it will not generate any action potential. Therefore no impulse is created.
any stimulus that exceeds threshold value will suceed in creating an action potential and therefore generating an impulse. No matter the intensity, it will only generate one impulse.
The larger the stimulus, the more impulses are generated in a given time. Different neurones with different threshold values serve to tell the brain how large the stimulus is.