6.5 ~ Neurons and Synapses

Endocrine and nervous system

• Internal communication within the body happens via either the endocrine or the nervous system

• Endocrine system: glands release hormones into circulatory stems —> longer time to initiate effect but potentially longer lasting

• Nervous system: nerves transmit electrical impulses —> very quick effect is transient

Parts of the neuron

1. Cell body

   1. Contain cytoplasm and nucleus

2. Dendrites

   1. Short, branched nerve fibres

   2. Usually transmit impulses between neurons in the brain and/or bring impulses towards the cell body

3. Axons

   1. Longer nerve fibres that take impulses away from the cell body

4. Myelin sheath

   1. Insulating fatty layer that speeds transmission

5. Schwann cell

   1. Make the myelin

6. Axon terminal

   1. Transmitter

Active transport via neurons pump

• Cells have a membrane potential

  • The difference in charge on the inside of the cell compared with the outside

  • Is essentially a voltage or potential difference

• Arises due to an imbalance of charged ions on the inside and outside of the cell

  • This membrane potential is maintained using sodium-potassium pumps

• Na+ ions are actively pumped out of the cell and K+ ions are pumped in

  • However, this is unequal; three Na+ ions is therefore, much steeper than the K+ gradient

• There are also many proteins inside the nerve fibre that are negatively charged

• All of these give the neurons a resting membrane potential of -70mV (the outside being more positive than the inside)

Nerve impulses

• An action potential that starts at one end of the neuron and is propagated along the axons to the other end

• This happens because ion changes caused by depolarisation is one part of the neuron trigger the depolarisation in the neighbouring part of the neuron

• Nerve impulses can only travel in one direction along the neuron in humans and other vertebrates: an action potential can only be triggered at one terminal of the neuron and can only be passed on at the other terminal

Propagation of nerve impulses

• Due to the movement of Na ions

• During depolarisation, Na+ ions rapidly diffuse into the cell down their concentration gradient

  • Now crates a concentration gradient in lateral regions of the nerve fibre

  • I.e. regions that have not yet depolarised

• As such, Na+ ions also diffuse laterally within the cell

• This generates local currents where Na+ ions are moving between polarised and depolarised regions of the neuron

• This diffusion of Na+ ions into adjacent region of the nerve fibre that is still polarised causes the membrane potential in that region to increase slightly to -50mV

  • Threshold potential

  • Na channels in the membrane are voltage-gated: they are controlled by the membrane potential and therefore open when a certain membrane potential is reached

• Therefore, these local currents cause the adjacent part of the neuron to reach its threshold potential which opens the sodium channels, therefore depolarisation the region

End of the propagation

• Once an area of the neuron has been fully depolarised, the change in potential causes voltage-gated K+ channels to open

  • Trigger to open the protein channels is a membrane of +30mV

• As a result of channels opening, K+ ions that are at higher concentration inside the neuron diffuse out and result in a decrease in membrane potential

  • Called depolarisation

• K+ channels remain open until the membrane potential becomes at least as negative as the resting potential

• In many cases, the membrane potential becomes even more negative than the resting potential for a brief period )approx. 2ms)

  • Hyperpolarisation

• The reason for this is that not all K+ channels close immediately after the resting potential has been reached

• Right after hyperpolarisation, that part of the neuron enters a refractory period and cannot be depolarised (to generate an action potential) as its Na+ channels are inactivated

Nerve fibres

• Can either be myelinated or unmyelinated

• Myelination involves the deposition of a material called myelin along the nerve fibre

  • Cells called Schwann cells deposit multiple layers of phospholipid bilayer around the nerve fibre

• There are gaps between adjacent Schwann cells where the nerve is not myelinated

  • These gaps are called nodes of Ranvier

Myelination

• In myelinated nerve fibres, the electrical impulse ‘jumps’ from one node of Raniver to the next in a process called saltatory conduction

• Saltire conduction is much faster than continuous transmission along the nerve

  • Speed of transmission in unmyelinated neurons is around 1m/s: whereas in myelinated neurons can be up to 100 m/s

Synapses

• Are junctions between cells in the nervous systems

• This can be between

  • Sensory receptors cells and neurons

  • Neurons and other neurons

  • Neurons and effector cells (muscles or glands)

• Neurotransmitters are chemicals that are used to send signals across synapses

  • Pre synaptic and postsynaptic neurons are separated by a fluid-filled gap called the synaptic cleft

  • Electrical impulses cannot pass across directly

Process of synapse transmission

1. A nerve impulse is propagated along a presynaptic neuron until it reaches the presynaptic membrane

2. Depolarisation of the presynaptic membrane opens calcium channels causing Ca2+ ions to diffused into the neuron

3. Influx of Ca2+ ions causes vesicles containing the neurotransmitter to be moved to the membrane and fuse with it

4. Neurotransmitter is released by exocytosis into the synaptic cleft

5. Neurotransmitter diffuses across the synaptic cleft and binds to the receptors on the postsynaptic membrane

6. Binding of the neurotransmitter to the receptors opens sodium channels on the postsynaptic membrane

7. Na+ ions diffuse into the postsynaptic neuron, causing it to reach its threshold potential and depolarisation

8. The impulse is propagated along the postsynaptic neuron

   1. The neurotransmitter is rapidly removed from the postsynaptic cleft

Nerve impulse and the threshold potential

• Nerve impulses follow and all-or-nothing

  • An action potential is only generated if the threshold potential is reached

  • This is because the voltage-gated sodium channels only open at a specific membrane potential

• As the membrane potential continues to increase, more Na channels open —> this is a positive feedback response

  • If the threshold potential is reached then full depolarisation will occur

• It is possible that insufficient neurotransmitter is released at the synapse to cause depolarisation of the postsynaptic neuron are pumped out by the Na-K pump and the membrane reverts back to its resting potential

• A postsynaptic neuron may receive inputs from a number of presynaptic neurons

  • It may require neurotransmitter release from a number of presynaptic neurons to depolarise one postsynaptic

  • In brain, this mean information can be gather from a variety of sensory inputs in order to initiate a response

Secretion and re-absorption of acetylcholine by neurons

• Is the neurotransmitter used at a number of synapses, including those at the nueromuscular joint

  • Synpases that use Ach are called cholinergic synapses

• Ach is produced from choline, which is absorbed in the diet, and an acetyl group, which is produced during aerobic respiration

• Ach is synthesised in the presynaptic neuron and packaged into vesicles

  • Is then released into the synaptic cleft during synaptic transmission

• The postsynaptic memrbane will have receptors for Ach

  • Ach remains bound to the receptor for a short time where one action potential is generated

• Ach is broken down by an enzyme present in the synaptic cleft acetylcholinersterase (AChE)

  • This enzyme breaks down Ach into acetate and choline

• Choline is reabsorbed by the presynaptic membrane and reused to make more Ach

• ACh breakdown and Re absorption needs to be efficient to control the intensity and duration of the signal is controlled

Blocking of synaptic transmission

• Neonicotinoids are synthetic compounds similar to nicotine

  • These are powerful insecticides that cause paralysis and death of insects by blocking the Ach receptors

    • E.g. imidacloprid

• These compounds bind to the Ach receptor on the postsynaptic membrane in cholinergic synapses in the CNS of insects

  • Ach receptors are therefore blocked and so synaptic transmission is prevented

• Using neonicotinoids as insecticides is advantageous because they do not cause much harm to humans and other mammals

  • A smaller proportion of synapses in the CNS of mammals are cholinergic

  • Neonicotinoids bind less strongly to Ach receptors in mammals

• However, these insecticides will also kill insects that might be benificial

  • E.g. honey bees

• Widespread Neonicotinoids use has been attributed to the collapse of honeybee colonies in a number of ecosystems

  • Therefore, widespread use on crops can have negative impacts on an ecosystem