6.5 Neurons and synapses Flashcards
What are the two ways of internal communication?
Via the endocrine system and the nervous system. The endocrine system consists of glands that release hormones. The nervous system consists of neurons.
What is a neuron?
Neurons have a cell body with cytoplasm and a nucleus but they also have narrow outgrowths called nerve fibres along which nerve impulses travel.
- Dendrites are short branched nerve fibres, for example those used to transmit impulses between neurons in one part of the brain or spinal chord.
- Axons are very elongated nerve fibres, for example those that transmit impulses from the trips of the toes or the fingers to the spinal chord.
What is myelination?
Some nerve fibres are coated along most of their length by a material called myelin. It consists of many layers of phospholipid bi-layer. Special cells called schwann cells deposit the myelin by growing around and round the nerve fibre, each time they grow around the nerve fibre a double layer of phospholipid bilayer is deposited.
There is a gap between the myelin deposited by adjacent Schwann cells. The gap is called a node of Ranvier. In myelinated nerve fibres the impulse can jump from one node of Ranvier to the next. This is called saltatory conduction. It is much quicker than continuous transmission along a nerve fibre so myelinated nerve fibres transmit nerve impulses much more rapidly than unmyelinated nerve fibres.
What is a Swann cell?
A cell that grows around a nerve and as it does deposits myelin.
What is a node of Ranvier?
A gap in myelinated nerve cells between the Swann cells.
How is the resting potential of a neuron maintained?
A neuron has a resting potential difference of -70 inside the membrane.
A Na+ /K+ pump, pumps Na+ out of the cell and brings K+ in, the pump takes 3 Na+ out and only brings 2 K+ back in. However the membrane is about 50 times more permeable to K+ ions than it is to Na+ ions and therefore K+ ions leak back across the membrane, so we are losing +1 charge overall through the pump and then more because the K+ we do have are leaking back through.
In addition to this there are proteins inside the nerve fibre that are negatively charged, which increases the charge imbalance.
So it is -70.
How is an action potential triggered?
- Sodium channels open in the membrane, positive Na+ flood into the membrane down a concentration gradient. This raises the charge from -70 to +30. Then after this re-polarisation happens:
- The sodium channels close and the potassium channels open, the positive K+ leave and the channels stay open until the inside of the membrane is negative again. It then takes a few more milliseconds to restore the Na+ and K+ balance.
How do nerve impulses travel?
Nerve impulses are action potentials propagated along the axons of neurons.
A nerve impulse is an action potential that starts at one end of a neuron and is then propagated along the axon to the other end of a neuron.
The propagation of the action potential happens because the ion movements that depolarise one part of the neuron trigger depolarisation in the neighbouring part of the neuron.
Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential. In the axon when the sodium channels open in one part of the axon the ions will diffuse down inside, and this makes the next bit of the neuron more positive, the Na+ ions outside will be moving in the opposite direction and overall this will increase the potential to -50 rather than -70, this is the threshold potential that will the open sodium channels in the next part of the axon because these channels are voltage gated. The movement of Na+ ions that moves the impulse along is called a local current.
Why do axon potentials always go in the same direction?
Because an impulse can only be initiated at one terminal of a neuron and can only be passed on to other neurons or different cell types at the other terminal. Also, there is a refractive period after a depolarisation that prevents propagation of an action potential backwards along an axon.
What are local currents?
Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential. In the axon when the sodium channels open in one part of the axon the ions will diffuse down inside, and this makes the next bit of the neuron more positive, the Na+ ions outside will be moving in the opposite direction and overall this will increase the potential to -50 rather than -70, this is the threshold potential that will the open sodium channels in the next part of the axon because these channels are voltage gated. The movement of Na+ ions that moves the impulse along is called a local current.
What are synapses?
Synapses are junctions between neurones and between neurons and receptor or effector cells. Chemicals called neurotransmitters are used to send signals across synapses. This system is used at all synapses where the pre-synaptic and post-synaptic cells are separated by a fluid filled gap, so electrical impulses cannot pass across. This gap is called the synaptic cleft and is only about 20 nm wide.
How does synaptic transmission work?
- A nerve impulse is propagated along the presynaptic neuron until it reaches the end of the neuron and the presynaptic membrane.
- Depolarisation of the presynaptic membrane causes calcium ions Ca2+ to diffuse through channels in the membrane into the neurone
- Influx of calcium causes vesicles containing neurotransmitter to move to the presynaptic membrane and fuse with it.
- Neurotransmitter is released into the synaptic cleft by exocytosis.
- The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post-synaptic membrane,
- The binding of the neurotransmitter to the receptors causes adjacent sodium ion channels to open.
- Sodium ions diffuse down their concentration gradient into the post synaptic neuron causing the post synaptic membrane to reach the threshold potential.
- An action potential is triggered in the post synaptic membrane and is propagated on along the neuron.
- The neurotransmitter is rapidly broken down and removed from the synaptic cleft.
What is the space between the two synaptic membranes called?
Synaptic cleft
What does acetylcholine do?
Acetylcholine is used as the neurotransmitter in many synapses. It is produced by combining choline absorbed from the diet with an acetyl group produced during aerobic respiration.
It only remains in the receptors for long enough for one action potential to be fired as the enzyme acetylcholinesterase rapidly breaks it down. It is reabsorbed re made up and ready to use again.
What are neonicotinoids?
Neonicotinoids are synthetic compounds similar to nicotine, they bind to the acetylcholine receptors in the central nervous system of insects. Acetylcholinesterase does not break down neonicotinoids and so the binding is irreversible. The receptors are blocked and so acetylcholine is unable to bind and synaptic transmission is prevented, the insect is paralysed.
They can therefore be used as pesticides.
