Chapter 9 & 10

• Resting potential: the sodium potassium pump, the leakage of positive potassium ions out of the cell and the presence of negative proteins and nucleic acids inside the cell results in the buildup of charge differences across the cell membrane (resting potential of a neuron is -70 mV)
• Disruptions to the resting potential: Graded potential, gated channels and action potential
• Graded potentials: continues changes to the membrane potential (depolarization-excitatory and hyperpolarization-inhibitory)
• Depolarization-excitatory: membrane potential is less negative and closer to achieving the threshold
• Hyperpolarization-inhibitory: membrane potential is more negative and farther away from achieving the threshold
• Gated ion channels: normally close, but open in response to a stimulus. When open they allow positive or negative ions to enter or leave the cell, causing changes in membrane potential.
• Two types of gated channels: chemically gated channels and voltage gated channels
• Chemically gated channels/ligand gated channels: open in response to a specific chemical stimulus (E.g.: neurotransmitter, such as acetylcholine, or a hormone). The binding of the neurotransmitter will cause the channel to open and allows sodium to diffuse into the cell. Since sodium is a cation (+), this will make the membrane less negative or depolarize it.
• Voltage gated channels: open in response to changes in membrane potential and result in action potentials.
• Action potentials are short-lived disruptions in the membrane potentials and are the actual signals that move along the axon, and they start in the axon hillock
• When a level of depolarization is reached, which is about -55 mV, on nerve impulse or action potential may be generated in the axon.
• Threshold potential: the level of depolarization needed to produce an action potential. Is accompanied by the opening and closing of voltage-gated ion channels
• Voltage-gated sodium channels and Voltage-gated potassium channels are used to create an action potential in neurons
• Action potential steps: 1. Resting phase, 2. Rising phase, 3. Top of curve, 4. Falling phase
• Action potential

  
• 1. Resting Phase: The interior of the nerve cell is more negatively charged than the exterior of the cell. There is a greater concentration of potassium ions on the inside of the cell, and there is a greater concentration of sodium ions on the outside of the cell. Equilibrium is maintained to keep the membrane potentials around -70mV at rest.
• 2. Rising Phase: The membrane potential becomes less negative (more positive) due to depolarization. This is a result of an influx of positively charged sodium ions causes the membrane potential to have a less negative (more positive) voltage.
• 3. Top of the curve: the sodium inactivation gate closes, stopping the influx of sodium. Abut this time the potassium gate opens. Potassium moves out of the cell.
• 4. Falling Phase: As the positively charged potassium ions leave the cell, repolarization of the membrane occurs. The potassium gates are slower to close than the sodium gate. Excess potassium moves out before the gate is closed, causing a temporary undershoot or refractory period (when the membrane potential is lower than resting potential). Sodium potassium pumps restore ion concentration to their equilibrium states. Sodium channel activation gate closes and inactivation gate reopens.
• Important points about action potentials: action potentials are all or non events. An action potential occurs if the threshold voltage is reached, but not while the membrane potential remains below the threshold
• Myelinated axons and increase in the diameter of an axon would INCREASE the velocity of action potentials along a neuron
• Neurotransmitters can have different effects, as they can either be excitatory or inhibitory.
• Excitatory post-synaptic potential (EPSP): Excitatory neurotransmitters bind to a gated ion channel which allows for the influx of positively charged sodium ion. This depolarizes the membrane, resulting in a less negative voltage.
• Inhibitory post-synaptic potential (IPSP): Inhibitory neurotransmitters bind to a gated ion channel, which allows for the influx of negatively charged chloride ions. This hyperpolarizes the membrane, resulting in a more negative voltage
• Action potentials are initiated at the base of the axon hillock
• Propagation: the movement of an action potential down the axon
• Myelination of the axon is one mechanism that speeds up propagation.
• In myelinated axons, action potential is only generated in the nodes of Ranvier because these are the only areas with the membrane proteins open to the exterior environment
• Neurotransmitters are the chemical ligands that carry the message across the synapse.
• Neurons communicate with cells to initiate responses: When an action potential reaches the end of the pre-synaptic neuron, there is an influx of positively charges calcium ions. This influx in the exocytosis of the synaptic vesicles with neurotransmitters into the synaptic cleft. The neurotransmitters that are released into the synaptic cleft then bind to receptors on the post-synaptic neuron
• Transmission of a neurotransmitter from the signaling pre-synaptic neuron to the receiving post-synaptic neuron

  
• Action potential

  
• Three categories of vertebrate sensory receptors: Mechanoreceptors, Chemoreceptors and Electromagnetic receptors
• Mechanoreceptors: receptors re stimulated by mechanical forces; touch, temperature, nociceptors, proprioceptors, baroceptors, hearing and balance
• Chemoreceptors: receptors detect changes in chemistry; taste, smell, and pH
• Electromagnetic Receptors: detect changes in heat and light energy; photoreceptors of the eyes and thermal receptors that are found in some reptiles
• 4 steps involved in conveying sensory information to the CNS: stimulus, transduction, transmission and interpretation
• 1. stimulus: A physical stimulus acts on a receptor
• 2. Transduction: The energy from the stimulus is converted into graded potentials (electrical energy)
• 3. Transmission: Graded potentials are converted into action potentials, which are conducted to the central nervous system.