- a cell body containing the usual organelles including a large amount of RER which is associated with the production of proteins and neurotransmitters
- dendrons which are extensions of the cell body which subdivide into dendrites that carry nerve impulses towards the cell body
- an axon which is a single long fibre that carries nerve impulses away from the cell body
- Schwann cells which surround the axon protecting it providing electrical insulation. They also carry out phagocytosis
- A myelin sheath which forms a covering to the axon and is made up of the membranes of the Schwann cells
- nodes of Ranvier are constrictions between adjacent Schwann cells where there is no myelin sheath
Resting potential
The inside of an axon is negatively charged relative to the outside. This is a resting potential and in this condition the axon is said to be polarised and this can be established due to the following events:
- sodium ions are actively transported out of the axon by the sodium-potassium pump
- potassium ions are actively transported into the axon by the sodium-potassium pump
- the active transport of sodium ions is greater than the active transport of potassium ions so an electrochemical gradient is established
- the sodium ions begin to diffuse back in while the potassium ions begin to diffuse back out
- most of the gates in the channels that allow potassium through are open whilst most of the gates in the channels that allow sodium ions to move through are closed
When a stimulus is detected by a receptor it causes a temporary reversal of the charges either side of the axon. If the stimulus is large enough the inside of the membrane becomes a positive charge. This is known as an action potential and the axon membrane is now said to be depolarised. This occurs because the voltage-gated channels change shape opening/closing depending on the voltage across the membrane:
- at resting potential some potassium voltage-gated channels are open (the permanently open ones) but the sodium voltage-gated channels are closed
- the energy of the stimulus causes some sodium voltage-gates channels to open causing sodium ions to diffuse into the axon through these channels along an electrochemical gradient. This triggers a reversal in the potential difference across the membrane as they are positively charged
- as sodium move in more sodium channels open causing a greater influx (this is positive feedback)
- once an action potential of ~40mV is established the voltage gates on the sodium channels close to prevent further influx of sodium ions and the voltage gates in potassium channels begin to open
- lots of potassium ions diffuse out causing more potassium ions to diffuse out starting repolarisation of the axon
- the outward diffusion of the potassium ions causes a temporary overshoot of electrical gradient with the axon inside being more negative relative to the outside than usual (hyperpolarisation). The potassium ion channels close and the sodium potassium pump causes sodium to move out and potassium to move in reestablishing the resting potential. The axon is said to be repolarised.
Once created the action potential moves rapidly along the axon. As one region of the axon produces an action potential and depolarises it acts as a stimulus for the depolarisation of the next region of the axon. The action potential is a wave of depolarisation and the previous region of the membrane returns to its resting potential by undergoing repolarisation.
How an action potential moves along an unmyelinated axon:
- at resting potential the axon membrane is polarised and negatively charged on the inside relative to the outside
- a stimulus causes an influx of sodium ions and hence a reversal of charge on the axon membrane. This is the action potential and the membrane is depolarised
- the localised electric currents established by the influx of sodium ions cause the opening of sodium voltage-gated channels a little further along resulting in depolarisation in that region. Behind this region sodium channels close and potassium channels open beginning repolarisation
- the action potential is propagated in the same way further along the axon
- repolarisation allows sodium ions to be actively transported out and potassium ions to be actively transported in returning the axon to its resting potential
How an action potential moves along a myelinated sheath: In myelinated axons the sheath of myelin acts as an electrical insulator preventing action potentials from forming. At intervals there are breaks in the insulation (nodes of Ranvier). Action potentials can occur at these points and so the action potential effectively jumps from node to node in a process known as saltatory conduction. An action potential travels along a myelinated neurone faster than along the axon of an unmyelinated one of the same diameter.
The transmission of an action potential along the axon of a neurone is the nerve impulse. A number of factors affect the speed at which an action potential passes along an axon:
- the myelin sheath
- the diameter of an axon - the larger the diameter the faster the speed
- temperature - this affects the rate of diffusion of ions so a higher temperature means a faster nerve impulse
Once an action potential has been created in any region of an axon there is a period after where inward movement of sodium ions is prevented as the sodium voltage-gated channels are closed. During this time it is not possible for a further action potential to be generated. This is known as the refractory period. It has 3 purposes:
- ensures action potentials are propagated in one direction only as action potentials cannot be propagated in a region of refractory
- produces discrete impulses as a new action potential cannot be formed immediately behind the first one ensuring action potentials are separated from one another
- it limits the number of action potentials as they are separated from one another
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