9.5

• Resting potential is the potential difference (voltage) across a neuron membrane when not stimulated, usually about -70 mV in humans.
• Resting potential is established by making the membrane more permeable to K+ than Na+, and by the active transport of 3Na+ out of the cell and 2K+ into the cell, which establishes an electrochemical gradient.
• The stages in generating an action potential are depolarisation, repolarisation, hyperpolarisation, and return to resting potential.
• During depolarisation, a stimulus causes the facilitated diffusion of Na+ into the cell down an electrochemical gradient, the p.d across the membrane becomes more positive, and if the membrane reaches threshold potential (-50mV), voltage-gated Na+ channels open, causing a significant influx of Na+ ions that reverses the p.d to +40mV.
• During repolarisation, voltage-gated Na+ channels close and voltage-gated K+ channels open, facilitated diffusion of K+ ions out of the cell down their electrochemical gradient, and the p.d across the membrane becomes more negative.
• During hyperpolarisation, there is an 'overshoot' when K+ ions diffuse out, causing the p.d to become more negative than the resting potential, and a refractory period ensues, during which no stimulus is large enough to raise the membrane potential to threshold.
• The refractory period ensures that no action potential can be generated in hyperpolarised sections of membrane, ensuring unidirectional impulse and discrete impulses.
• Myelinated axons conduct impulses faster than unmyelinated axons due to saltatory conduction, where the impulse ‘jumps’ from one node of Ranvier to another.
• The function of synapses includes preventing the electrical impulse from crossing the junction, sending impulses between neurons or from neurons to effectors for excitatory or inhibitory response, summation of sub-threshold impulses, and initiation of new impulses in several different neurons for multiple simultaneous responses.
• Action potential is propagated along an unmyelinated neuron by stimulus leading to influx of Na+ ions, depolarisation of the first section of membrane, opening of sodium voltage-gated channels further along the membrane, and a sequential wave of depolarisation.
• The structure of a synapse includes a presynaptic neuron that ends in a synaptic knob containing lots of mitochondria, endoplasmic reticulum, and vesicles of neurotransmitter, a synaptic cleft which is a 20-30 nm gap between neurons, and a postsynaptic neuron that has complementary receptors to the neurotransmitter, which in this case are ligand-gated Na+ channels.
• The additional features of a myelinated motor neuron include Schwann cells that wrap around the axon many times, a myelin sheath made from myelin-rich membranes of Schwann cells, and very short gaps between neighbouring Schwann cells where there is no myelin sheath, known as Nodes of Ranvier.
• The role of acetylcholine is to transmit impulses across synapses.
• The structure of a motor neuron includes a cell body containing organelles and a high proportion of RER, dendrons that branch into dendrites which carry impulses towards the cell body, and an axon which is a long, unbranched fibre that carries nerve impulses away from the cell body.
• ATP is used to reform acetylcholine for storage in vesicles.
• p.d. becomes more negative, resulting in hyperpolarisation, so no action potential is generated in the postsynaptic neuron.
• Noradrenaline increases the force of skeletal muscle contraction and increases the rate and force of heart contraction.
• Acetyl and choline diffuse back into presynaptic membrane.
• Vesicles move towards and fuse with the presynaptic membrane.
• Acetylcholine causes muscle contraction at motor end plate and causes excitation at preganglionic neurons.
• Neurotransmitters cross the synaptic cleft via simple diffusion.
• In the postsynaptic neuron, the neurotransmitter binds to a specific receptor on the postsynaptic membrane.
• In an inhibitory synapse, the neurotransmitter binds to and opens Cl- channels on the postsynaptic membrane, triggering K+ channels to open.
• Acetylcholine causes inhibition at parasympathetic postganglionic neurons such as heart or breathing rate.
• Acetylcholine is hydrolysed into acetyl and choline by acetylcholinesterase (AChE).
• Exocytosis of neurotransmitter into the synaptic cleft occurs.
• In the presynaptic neuron, a wave of depolarisation travels down the neuron, causing voltage-gated Ca2+ channels to open.
• Cl- moves in and K+ moves out via facilitated diffusion in the postsynaptic neuron.
• If the influx of Na+ ions raises the membrane to threshold potential, an action potential is generated in the postsynaptic neuron.
• Ligand-gated Na+ channels open in the postsynaptic neuron.
• Noradrenaline, also known as norepinephrine, is primarily released from sympathetic neurons.