HUBS1416 Neurons and Action Potentials.docx

**[HUBS1416 Neurons and Action Potentials]** **Neuron Anatomy:** A purple and blue neuron Description automatically generated **Important parts of the Neuron** -------------------------------------- --------------------------------------- -------------------------------------------------------...

**[HUBS1416 Neurons and Action Potentials]** **Neuron Anatomy:** A purple and blue neuron Description automatically generated **Important parts of the Neuron** -------------------------------------- --------------------------------------- -------------------------------------------------------------------------------------------------------- Projecting out from the soma To receive neurochemical signals (neurotransmitters) from sensors and other neurons. On the dendrites Soma Nearest to the dendrites Axon Lipid based insulation that intermittently covers the axon. Is produced by glial cells for the neuron. Sodium (Na^+^) Channels Potassium (K^+^) Channels Sodium/Potassium (Na/K) ATPase pumps Calcium (Ca^2+^) Channels Vesicles Synapse Neurotransmitters Inside vesicles in the axon terminals -------------------------------------- --------------------------------------- -------------------------------------------------------------------------------------------------------- **Important Electrolytes (Ions) for the firing of the Neuron** ---------------- --------- ------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- **Element** **Ion** **Charge** **Purpose / Function** Sodium Na^+^ +1 (cation) Main extracellular (outside the cell) cation. Enters the neuron via voltage activated channels when the cell depolarises. Is removed from the cell using the Na/K ATPase pump. Potassium K^+^ +1 (cation) Main intracellular (inside the cell) cation. Potassium channels are mostly open, causing some leakage of K^+^ down its concentration gradients into or out of the cell, depending where the concentration is highest. Is also pumped into the cell by the Na/K ATPase pump. Calcium Ca^2+^ +2 (cation) Extracellular cation. Enters the cell via voltage activated channels near the axon terminal during depolarisation. This triggers the release of neurotransmitters from the vessicles in axon terminal into the synaptic cleft. Organic Anions P^-^ -1 (anion) Main intracellular anions. Molecules which have a net negative charge, these remain inside the cell at all times and establish its resting membrane potential of -70mVE. Changes in membrane potential are therefore produced by changing the number of balancing cations (K^+^, Na^+^) inside the cell, ie. more anions than cations in the cell = negative membrane potential. More cations than anions in the cell= positive membrane potential. Chlorine Cl^-^ -1 (anion) Main extracellular anion. Can be let into the neuron to cause it to hyperpolarise (become extra negative), therefore harder to fire. The chloride channels on the neuron are activated by the neurotransmitter GABA (gamma aminobutyric acid)- a neurotransmitter that is released from the axon terminals of other nerve cells. The action of GABA can be increased by certain chemicals- nitrous oxide, alcohol, benzodiazapines and bartiurates, for example- all of which cause numbness and drowsiness/numbness due to the inhibitory effect of the increased GABA activity that they stimulate. ---------------- --------- ------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- **Action Potentials in Neurons: Key Events** 1. **Excitatory and inhibitory signals are received at the dendrites**. Excitatory signals raise the membrane potential, whilst inhibitory signals keep it down. 2. If there are enough excitatory signals to raise the membrane potential from its resting point of -70mEV to around -55mEv, **sodium ion (Na^+^) channels in the cell membrane near the dendrites will open,** allowing sodium ions to flow down their concentraion gradient into the cell. This causes a rapid change in cell membrane potential called depolarisation. 3. **Depolarization moves as a wave down the length of of the axon**- each sodium channel opening raises the membrane potential in its immediate area and triggers the channel next to it to open. On myelinated cells the sodium channels are grouped together in the Nodes of Ranvier, forcing the depolarization to travel further through the cell to depolarize the next group of channels. This has the effect of increasing the speed of the depolarization since it effectivley jumps the myelinated sections of the axon. 4. **When the depolarization wave reaches the axon terminals it triggers calcium ion (Ca^2+^) channels** in the cell membrane near the terminals to open. Calcium enters the cell and causes neurotransmitter filled vesicles in the axon terminal to fuse with the cell membrane and release their contents into the synapse, send a chemical signal to the target tissue (muscle/ gland/ nerve). 5. **When the cell membrane potential reaches around +30mEV the sodium ion channels close** and are temporarily inactivated, stopping depolarisation. Potassium ion (K^+^) channels open to allow potassium to flow into and out of the cell. 6. **Sodium/ Potassium pumps remove 3 sodium ions from the inside of the cell for every 2 potassium ions that they put in**, resulting in the interior of the cell becoming more negative. This is called **repolarization** 7. **The cell becomes hyperpolarised** due to the action of the pumps and as the sodium channels are still inactivated during this period, the cell is unable to depolarize regardless of the level of excitatory signals it receives**. This is callled the refractory phase.** 8. **Open potassium channels allow the membrane potential to rise to its stable resting point (-70mEV)and the sodium channels become reactivated**. The cell is now ready to perform another action potential. 9. **Neurotransmitters in the synaptic cleft are reuptaken into vesicles in the axon terminal or broken down by enzymes** in the cleft (MAO -- Monoamine oxidase and COMT- Catechol-O-methyltransferase) stopping the chemical signal being sent. ------------------------------------------------------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- **Glial cells of Central Nervous System (Brain and Spinal Cord)** **Cell Type** **Function** Astrocytes Provide phyical protection and structural support for the neurons and form the blood-brain barrier between the capillaries and the neurons. Helps regulate the environment outside of the neurons. Oligodendicytes Produce myelin for the axons of the neurons. Can myelinate several neurons at once. Microglia Macrophages that provide protection against pathogens (bacteria, viruses and parasites) and remove cellular debris. Epindymal Cells Produce cerebrospinal fluid. **Glial cells of the Perpheral Nervous System** **Satellite Cell** Similar to astrocytes in that they provide physical protection and structural support for the cell bodies of peripheral neurons. They differ in appearance to astrocytes, as they are found on the outside of neuron cell bodies rather than between them. **Schwaan Cells** Similar to oligodendricytes in that they produce myelin for the axons of cells, however, they differ in that they wrap around the axons of cells, with each producing one patch of myelin. Therefore one myelinated axon will need multiple Schwaan cells to maintain it. ------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------  **Blausen.com staff (2014). \"Medical gallery of Blausen Medical 2014\". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=28761843**