Nervous System: Neurones and Action Potentials PDF

Nervous system: Neurones and action potentials Paul Austin School of Medical Sciences https://www.theguesthouseocala.com/ The Neurone The basic structural unit of the nerve system is the neurone (nerve cell). According to one estimate we have 86 billion neurones Each neurone is an individual cell –...

Nervous system: Neurones and action potentials Paul Austin School of Medical Sciences https://www.theguesthouseocala.com/ The Neurone The basic structural unit of the nerve system is the neurone (nerve cell). According to one estimate we have 86 billion neurones Each neurone is an individual cell – they communicate at synapses. They have a single cell body with dendrites – that receive information axons – that transmit information. They branch to communicate with many cells. Information is coded in the form of action potentials The University of Sydney A basic neurone The University of Sydney User:Dhp1080, CC BY-SA 3.0 , via Wikimedia Commons Revising some basic concepts Body water occupies 2 “compartments” the intracellular fluid (ICF - 66% of total body water) the extracellular fluid (ECF - 33% of total body water) the ICF and ECF are separated by the cell membrane. Structure of the Cell Membrane Phospholipid bilayer Phosphate heads (hydrophilic) on the surfaces Lipid tails (hydrophobic) centrally. Protein molecules are inserted into the bilayer. The University of Sydney The cell membrane The University of Sydney OpenStax, CC BY 4.0 , via Wikimedia Commons Revising some basic ideas Substances with low lipid solubility cross the membranes by several methods including through highly specific transmembrane transport proteins that form leak channels; form gated channels for ions; or transport ions. Na-K ATPase Pumps three Na+ from the cell and takes two K+ into the cell for each molecule of ATP - sets up an electrical gradient. It is found in all parts of the body. The University of Sydney The composition of the ICF and ECF is different The University of Sydney There is an electrical potential across the cell membrane The membrane is most permeable to K+ because some K+ channels are leaky à K+ moves through them down the concentration gradient. The membrane is much less permeable to Na+. The inside of the cell becomes negative with respect to the outside Equilibrium is reached when the concentration gradient is balanced by the electrical gradient. The membrane potential is –50 to –90 mV in most cells. The University of Sydney Voltage gated ion channels The University of Sydney EredLuin [CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)] Action potential Depolarisation Repolarisation Threshold potential -55 mV Resting potential -65 mV Stimulus The University of Sydney Hyperpolarisation Action potential Resting potential Depolarisation Repolarisation Resting potential The University of Sydney Propagation of the action potential The University of Sydney https://cpb-us-w2.wpmucdn.com/sites.gatech.edu/dist/6/1810/files/2017/06/Figure_35_02_04.png All action potentials are the same All action potentials produce exactly the same electrical change. A stronger stimulus doesn't produce a larger action potential. If there is a stronger stimulus the frequency of action potentials will increase. The University of Sydney Myelinated axons https://upload.wikimedia.org/wikipedia/commons/6/64/Coronal_cross-section_of_human_brain.jpg The University of Sydney Doc. RNDr. Josef Reischig, CSc., CC BY-SA 3.0 , via Wikimedia Commons Myelin Myelin increases the speed of nerve conduction. Myelin is present around nerves in the CNS and PNS where it insulates axons. It is about 80% lipid. The University of Sydney Myelination The University of Sydney Neuron_with_oligodendrocyte_and_myelin_sheath.svg: *Complete_neuron_cell_diagram_en.svg: LadyofHatsderivative work: Andrew c, Public domain, via Wikimedia Commons Myelin and saltatory conduction Direction of movement à The University of Sydney Fibre types Diameter (µm) Conduction velocity (m/s) Function A-a 13 - 22 70 – 120 motor neurones A-b 8 - 13 40 - 70 touch, movement A-g 4-8 15 - 40 touch, pressure A-d 1-4 5 - 15 pain, pressure, temperature B 1-3 2 - 14 autonomic C 0.1 - 1 0.2 - 2 pain, touch, pressure, temperature Fibre type The University of Sydney Synapse Neurotransmitters are synthesised in the cell body, transported to the end of the axon and stored at the end of presynaptic nerve terminal in packages called ‘vesicles’. When an action potential reaches the end of the axon Ca2+ moves in allowing neurotransmitter release. Neurotransmitters diffuse across the synaptic cleft to bind with receptors on the post synaptic membrane. Neurotransmitters are Broken down by enzymes or Undergo reuptake into the presynaptic neurone The University of Sydney Neurotransmission The University of Sydney Young, KA., Wise, JA., DeSaix, P., Kruse, DH., Poe, B., Johnson, E., Johnson, JE., Korol, O., Betts, JG., & Womble, M., CC BY 4.0 , via Wikimedia Commons Neurotransmitters Glutamate major excitatory neurotransmitter opens Na+ channels in post-synaptic membrane – cause excitatory post synaptic potentials (EPSPs) GABA major inhibitory neurotransmitter opens Cl- channels in post-synaptic membrane – cause inhibitory post synaptic potentials (IPSPs) Acetylcholine can be excitatory or inhibitory in the brain excitatory in the muscle The University of Sydney Coding information One EPSP isn’t enough to produce an action potential – a bigger stimulus is needed. This is achieved by summation. Temporal summation The University of Sydney Spatial summation