Topic 8 - Grey Matter

• Neurones are amongst the largest cells in the body, especially in terms on length.
• Neurones are highly specialised (differentiated) cells, adapted to rapidly carrying electrochemical changes called nerve impulses.
• An action potential is propagated along an axon.
• The propagation of action potentials is different in myelinated and unmyelinated neurons.
• Neurones are a means of conveying information over large distances within the body.
• The resting potential is the state where the concentration of sodium ions out of the axon membrane is very high.
• The cell body of a neurone contains the nucleus and a large amount of rough endoplasmic reticulum to produce large amounts of protein, particularly neurotransmitters.
• Dendrons are extensions of the cell body, the 'thickest' parts of the branches, subdivided into dendrites, which are small 'fingers' furthest away from the cell body, collecting nerve impulses and carrying them towards the cell body.
• The axon is the single, long fibre that runs the length of the neurone, carrying the nerve impulse away from the cell body, propagating it from one place to another.
• Schwann Cells wrap themselves around the axon many times, providing protection, electrical insulation, and aiding in the regeneration of damaged axons.
• The myelin sheath is formed by the many wrappings of the Schwann Cell, making it rich in a lipid known as myelin, which allows nerve impulses to be carried much faster than unmyelinated ones.
• Nodes of Ranvier are gaps between adjacent Schwann Cells, areas with no myelin sheath, around 2-3µm long, and occurring every 1-3mm in humans.
• Sensory Neurons transmit nerve impulses from a receptor to the central nervous system or an intermediary neurone, usually with a long axon and many short dendrites.
• The action potential is propagated once again, this time in a different area of the axon.
• The area of the second action potential is removing potassium ions in just the same way, this area has been hyperpolarised.
• The area where the first action potential occurred is returning to its resting potential, this area has been repolarised.
• A stimulus causes a sudden influx of sodium ions and hence a reversal of charge on the axon membrane, this is the action potential.
• Once again, everything has just shifted to the right by ‘one section’, notice how the original ‘section’ is now pumping out sodium ions, ensuring that the resting potential of around -65mV is established.
• The concentration of potassium ions inside the axon is lower, which means that the outside of the axon is positive in relation to the inside.
• The part of the axon that has just repolarised is now ready to receive a new stimulus and start the whole process off again.
• The axon membrane is polarised with Na+, Na+, K+ ions.
• The myelin sheath is an electrical insulator, which prevents action potentials from forming in area of ‘myelination’.
• Passage through a myelinated neurone involves the structure of myelinated neurones, with the many ‘wrappings’ of the Schwann cells around the axon leading to layers of a lipid called myelin.
• Action potentials can only occur at Nodes of Ranvier.
• As a result of this node-hopping, the propagation of action potentials through myelinated neurones is much faster than in unmyelinated ones, this method of action potential propagation is known as Saltatory Conduction.
• Motor Neurons transmit nerve impulses from the CNS or an intermediary neuron to an effector, usually having a long axon and many short dendrites.
• Intermediary Neurons transmit impulses between other neurones, for example, between sensory and motor neurones, with lots of dendrites at both sides, to propagate an impulse between two neurones.
• The refractory period is the time after an action potential when no new action potential can occur.
• The refractory period separates one impulse from the next.
• The all-or-nothing principle states that an action potential is either completely successful or completely unsuccessful.
• Once an action potential has been initiated at the start of an axon, the subsequent action potentials do not decrease in size.
• The all-or-nothing principle is a fundamental principle of neuronal function, stating that an impulse is either completely blocked or completely conducted.
• The number of nerve impulses increases with a stronger stimulus, leading to more impulses.
• The refractory period is a crucial aspect of neuronal function, separating one impulse from the next.
• Different neurons have different threshold values, contributing to the speed of conductance of an action potential.
• The action potential at the end of an axon will have the same size (or 'electrical energy') as the one initiated at the beginning.
• The speed of action potentials can vary, depending on the properties of the axon.
• The speed of action potentials can range from just 0.5ms-1 to 120ms-1.
• All action potentials are the same size; threshold value for action potential to occur
• Several (sub-threshold) impulses add to produce an action potential