Biology the Nervous system

• When a neurone is not firing, there is a difference in charge between the inside and the outside of the membrane, referred to as the resting potential, usually around -70 mV.
• Polarisation of neuronal cell membranes at rest occurs due to the action of sodium-potassium ion pumps, found within the cell membrane and actively transporting sodium and potassium ions into and out of the neurone.
• Some trigger or inhibit the release of neurotransmitter from the presynaptic neuron.
• For every three sodium ions that the proteins pump out of the cell, they pump two potassium ions into the cell, ensuring that there are always more positive ions out of the cell compared to inside the cell and making sure there is a charge difference across the membrane.
• When a neurone is stimulated, the charge difference between the inside and outside of the cell membrane is lost and the membrane is depolarised.
• The presynaptic neuron has a swelling at the end which is called the synaptic knob.
• Depolarisation to +30 mV occurs regardless of the size of the stimulus, regardless of whether the threshold is reached, and regardless of whether an action potential is fired.
• Myelination is a process where some neurons are insulated with a fatty layer along the axon, known as a myelin sheath, which acts as an electrical insulator, preventing ions from moving into or out of the myelinated portions of the neuron.
• The speed at which an action potential moves along a neuron is known as the conduction velocity, with action potentials along myelinated neurons having a higher conduction velocity compared to those travelling along non-myelinated neurons.
• Conduction velocity slows due to the denaturation of channel and carrier proteins in the neuronal membrane at temperatures above 40 degrees.
• The vesicles fuse with the presynaptic membrane and their contents is released by exocytosis.
• The frequency of action potentials firing is related to the size of the stimulus, with larger stimuli resulting in more frequent action potentials.
• Action potentials occur only at the nodes of Ranvier, which are gaps in the myelin sheath where sodium ion channels and potassium ion channels are concentrated.
• The neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane.
• Action potentials travel quicker in neurons with bigger axons because there is less resistance to the flow of ions than in a neuron with a thinner axon.
• This triggers the opening of sodium ion channels in the postsynaptic membrane.
• A synapse is a gap found between neurons (or between a motor neuron and an effector), where electrical impulses cannot pass through, so neurons release neurotransmitters from one neuron to the next to stimulate an action potential in the next neuron.
• Factors that affect the speed of an action potential include myelination, axon diameter, temperature, and synapses.
• Saltatory conduction is a type of nervous transmission where the action potential jumps from node to node, much faster than transmission along non-myelinated neurons.
• Calcium ions move into the synaptic knob by facilitated diffusion and trigger the movement of vesicles containing neurotransmitters (such as acetylcholine or dopamine) towards the presynaptic membrane.
• As temperature increases, the speed of an action potential increases because the ions have more kinetic energy so diffuse faster.
• Synaptic transmission takes place in the following stages:
• An action potential arrives at the end of the presynaptic neuron (at the synaptic knob) and triggers the opening of voltage-gated calcium ion channels.
• If enough charge is lost and depolarisation exceeds -55 mV, an action potential will occur in that neurone, this is known as the threshold potential.
• Depolarisation during an action potential occurs because sodium ion channels open up in the membrane, facilitated by the sodium-potassium ion pump which has been actively transporting sodium ions out of the neurone, creating a sodium ion concentration gradient.
• Sodium ion channels close and potassium ion channels open, causing potassium ions to move out of the neurone down their concentration gradient, this is called repolarisation.
• The action of the sodium-potassium ion pump restores the balance between sodium and potassium ions on either side of the membrane and returns the neurone to its resting potential of -70 mV.
• Acetylcholine always acts as an excitatory neurotransmitter in NMJs.
• Some neurotransmitters are both excitatory and inhibitory, with their effect determined by where in the body they are acting.
• Acetylcholine is broken down by the enzyme acetylcholinesterase and the products are reabsorbed into the presynaptic neuron to resynthesize the neurotransmitter.
• The neurotransmitter is removed from the synaptic cleft, preventing the continuous stimulation of an action potential in the postsynaptic neurone.
• Drugs can have a range of effects on our body by interfering with neuronal signalling at the synapse.
• For example, acetylcholine is an excitatory neurotransmitter at cholinergic synapses in the CNS and neuromuscular junction, but in cholinergic synapses in the heart, acetylcholine acts as an inhibitory neurotransmitter by triggering the opening of potassium ion channels and making the membrane even more polarised (hyperpolarisation).
• Receptors which are complementary to neurotransmitters, such as acetylcholine, are only found on the postsynaptic membrane - there are none within the postsynaptic membrane.
• Neurotransmitters can be classed as excitatory if they trigger an action potential in the postsynaptic neuron or inhibitory if they prevent an action potential from happening.
• Some inhibit enzymes that break down the neurotransmitter, meaning that the neurotransmitter remains in the synaptic cleft and can continue stimulating the postsynaptic neuron.
• The action potential always travels in one direction only (unidirectional) and prevents the nerve impulse from travelling backwards.
• Spatial summation occurs when lots of presynaptic neurons converge on a single postsynaptic neuron.
• The neurotransmitter is either reabsorbed by the presynaptic neurone (and recycled) or broken down by enzymes in the synaptic cleft (and the products are reabsorbed).
• The postsynaptic membrane in NMJs contains a higher number of receptors.