C 2.2 SL Neural Signaling Student Notes PDF

Organisms – C 2.2 SL Neural Signaling Guiding Questions “How are electrical signals generated and moved within neurons?” “How can neurons interact with other cells?”...

Organisms – C 2.2 SL Neural Signaling Guiding Questions “How are electrical signals generated and moved within neurons?” “How can neurons interact with other cells?” Syllabus Overview & Objectives Students should understand that cytoplasm and a nucleus form the cell Neurons as cells within the body of a neuron, with elongated nerve fibres of varying length projecting C2.2.1 nervous system that carry from it. An axon is a long single fibre. Dendrites are multiple shorter electrical impulses fibres. Electrical impulses are conducted along these fibres. Students should understand how energy from ATP drives the pumping of Generation of the resting potential sodium and potassium ions in opposite directions across the plasma by pumping to establish and C2.2.2 membrane of neurons. They should understand the concept of a maintain concentration gradients membrane polarization and a membrane potential and also reasons that of sodium and potassium ions the resting potential is negative. Nerve impulses as action Students should appreciate that a nerve impulse is electrical because it C2.2.3 potentials that are propagated involves movement of positively charged ions. along nerve fibres Compare the speed of transmission in giant axons of squid and smaller Variation in the speed of nerve C2.2.4 non-myelinated nerve fibres. Also compare the speed in myelinated and impulses non-myelinated fibres. Synapses as junctions between Limit to chemical synapses, not electrical, and these can simply be C2.2.5 neurons and between neurons and referred to as synapses. Students should understand that a signal can effector cells only pass in one direction across a typical synapse. Release of neurotransmitters from Include uptake of calcium in response to depolarization of a presynaptic C2.2.6 a presynaptic membrane membrane and its action as a signalling chemical inside a neuron. Include diffusion of neurotransmitters across the synaptic cleft and Generation of an excitatory binding to transmembrane receptors. Use acetylcholine as an example. C2.2.7 postsynaptic potential Students should appreciate that this neurotransmitter exists in many types of synapse including neuromuscular junctions. 1|Page Neurons as cells within the nervous system that carry electrical impulses The nervous system consts of nerve cells called neurons. The neurons help with internal communication by transmitting nerve impulses. Neurons transmit information along nerve fibers in the form of electrical impulses. The electrical impulse is not like an electrical current that flows along wires. An impulse is a momentary reversal in electrical potential difference in the membrane – a change in the position of charged ions between the inside and outside of the membrane of the nerve fibres. Can you annotate the structures of the neuron? 2|Page Generation of the resting potential: A nerve impulse is a result of a change in concentration of sodium (Na+) and potassium (K+) ions along the cell membrane. Depending on the membrane potential (voltage), we can distinguish between The resting potential and The action potential. What is the resting potential? What is the action potential? All cells have a measurable membrane potential (voltage), which is established by the imbalance of ions inside and outside of a cell. The membrane potential in neurons is – 70mV. Which three factors contribute to the establishment of a negative membrane potential? 1. Sodium potassium pump: Sodium-potassium pumps in the membrane transfer Na+ out of the neuron and K+ into the neuron. This is active transport and requires ATP. The number of ions pumped is inequal – three Na+ go out, two K+ ions go in. This causes an imbalance of ions and concentration gradients for both. 3|Page 2. Leakage of Sodium and Potassium ions: Leakage of potassium and sodium ions also contributes to the imbalance of ions across the membrane. The axon membrane has a higher permeability of K+, which leaks out of the cell through its membrane channels faster than Na+ leaks in through its Na+ protein channels. 3. negatively charged proteins inside the cell: Due to this unequal distribution of ions, and the abundance of negatively charged proteins inside the axon fibre, the membrane potential is found to be at ca. -70mV – meaning that the inside of the neuron is more negatively charged compared to the outside. 4|Page Nerve impulses as action potentials An action potential starts when Na+ ions suddenly rush into the neuron upon stimulation of voltage gated membrane channels and reverse the membrane potential making it more positive. An action potential is composed of two subsequent stages: 1. Depolarization: The cell membrane's charge becomes positive to generate an action potential. This is usually caused by positive sodium ions going into the cell. 2. Repolarization: The cell membrane's charge returns to negative after depolarization. This is caused by positive potassium ions moving out of the cell. Read through the information on the right and answer the questions to check your understanding: State the 2 phases of an action potential: What causes depolarization? What causes repolarization? The action potential then progresses along the whole length of the axon fiber. 5|Page Variation in the speed of nerve impulses: Signal transmission in humans is at about 1m per second. This speed is largely due to the small diameter (1µm). Some animals have nerve fibres with larger diameters. A bigger diameter reduces resistance and therefore increases conduction speed. Griant axons in squids have diameters of up to 500 µm. How does this help the squid as an adaptation to life? Read the article and answer the questions on your sheet. https://www.nature.com/scitable/topicpage/myelin-a-specialized- membrane-for-cell-communication-14367205/ What exactly is myelin? What type of cells make myelin in the Peripheral Nervous System? Are all axons myelinated? Which ones are, which ones aren’t? How does myelin insulation speed up signal transmission in nerve cells? What type of diseases cause a damage to the myelin sheath, and what are the consequences? 6|Page 7|Page Mylination is another factor which affects the speed at which nerve impulses are transmitted. The axon fibres of neurons can be mylinated or unmylinated. This refers to a lipid-rich substance surrounding the nerve fibre. This coating is provided by a series of Schwann cells with gaps between called nodes of Ranvier. In mylinated nerve fibers the transmission can be up to 100m/s. The presence of a myelin sheath surrounding the axon fibre increases the speed of transmission of the action potential. Only at the junctions in the sheath (nodes of Ranvier) the axon membrane exposed. Elsewhere along the fibre, the electrical resistance of the myelin sheath prevents depolarization of the nodes. The action potentials actually ‘jump’ from node to node (this is called saltatory conduction, meaning ‘to leap’). This greatly speeds up the rate of transmission. Use the website below to investigate the effect of axon diameter and myelination on velocity of signal transmission. https://ilearn.med.monash.edu.au/physiology/action-potentials/axon-diameter#simulation 1. Carry out the simulation with unmyelinated fibres. 2. Record the results in the table blow. 3. Then repeat the experiment with myelinated fibers. Record the results. 4. Export the spreadsheet and produce a graph with a line of best fit and R2 value. 5. What is the effect of axon diameter on speed? 6. What is the effect of myelination on speed? 7. Suggest why, for very small axon diameters, there is little benefit of myelinated velocity. 8|Page Synapses as junctions between neurons: What is a synapse? What parts is a synapse composed of? There are 3 main types of synapses: Synapses between neurons, in both the brain and spinal cord. Synapses between neurons and muscle fibres (effectors). Synapses between neurons and glands (effectors). Release of neurotransmitters from a presynaptic membrane: An action potential cannot cross the synaptic cleft between neurons, and instead the nerve impulse is carried by chemicals called neurotransmitters. What is a neurotransmitter? Give an example of a neurotransmitter: Where is a neurotransmitter made and where is it stored? 9|Page Generation of an excitatory postsynaptic potential: The postsynaptic membrane contains specialized receptors. These neuroreceptors are protein channels embedded in the cell membrane of the postsynaptic synapse. The receptors are chemical gated, allowing Na+ to rush into the neuron upon binding of the neurotransmitter to the binding site on the neuroreceptor-. Annotate the diagram to explain how neurotransmitters result in the generation of a new excitatory action potential: 10 | P a g e Neuroreceptors are protein channels embedded in the cell membrane of the postsynaptic synapse. Upon binding of the neurotransmitter to the binding side of the receptor protein Na+ rushes into the neuron. Acetylcholine is one of the most common neurotransmitters and is used at many synapses, including neuromuscular junctions. Acetylcholine - Secretion and Reabsorption : Acetylcholine is an organic molecule composed of acetyl CoA and choline. It acts as a neurotransmitter for both, PNS and CNS. In the PNS, it is the main neurotransmitter in the autonomic nervous system where it is responsible for activating muscles. To avoid overstimulation of the postsynaptic neuro-receptor, Acetylcholine needs to be broken down immediately. What is the name of the enzyme responsible for this action (Look at the diagram to the left)? As a consequence, the ion channel of the receptor protein closes, and the resting potential in the post-synaptic neuron is re-established. 11 | P a g e 12 | P a g e