C2.2 Neural Signaling

• neurons are specially adapted cells that can carry electrical impulses (nerve impulses)
• nerve impulses are electrical signals that can be passed between cells
• there are two groups to nerve impulses - transmission along neurons and between neurons
• membrane potential is the voltage created by an imbalance of charges (ions) along or on either side of the membrane
• at rest the inside of neurons must be relatively negative at -70mV (resting membrane potential)
• Sodium potassium protein pumps are embedded in the membrane, and they establish a resting membrane potential by actively transporting sodium to the outside of the membrane and potassium inside
• the active transport of sodium potassium pumps means they require ATP and they generate a concentration gradient
• at rest for transmission alongside the membrane, what you'll notice is the sodium ions are on the outside of the membrane and the potassium ions are on the inside and the relative voltage of the cell is -70mV (this is all established by the active transport by the sodium potassium pump)
• during nerve transmission sodium ions will move into the cell (previously on the outside) and because sodium ions are positive hence their movement into the cell will cause that cell to become relatively positive in its' voltage (depolarization)
• depolarization is when the membrane potential (in voltage) goes from negative to positive (due to the movement of sodium ions inside the cell)
• in depolarization, the positive voltage will cause changes in some of the channel proteins in the membrane and will result in the potassium ions leaving the cell
• repolarization is when the membrane potential (in voltage) goes from positive back to negative (due to the movement of potassium ions leaving the cell)
• repolarization allows the membrane to go back down to resting potential
• after depolarization and repolarization the sodium potassium pumps re-pump the sodium ions outside and potassium ions inside the cell in order to restore the resting potential
• The steps of nerve impulses in action potentials are as follows;

  1. Voltage-gated sodium ion channels open
  2. Sodium ions diffuse into the cell (facilitated diffusion)
  3. Depolarization occurs (membrane potential going from negative to positive)
  4. Voltage-gated sodium ion channels close, and voltage-gated potassium ion channels open
  5. Potassium ions diffuse out of the cell (facilitated diffusion)
  6. Repolarization (membrane potential going from positive to negative)
  7. sodium-potassium pump re-establishes resting potential by actively pumping sodium ions out and potassium ions in the cell
• Voltage-gated ion channels mean they open and close based on voltage, hence when there's a stimulus strong enough it'll open the channels
• the nerve impulses along neurons are self-propagating meaning that once they're initiated they introduce a sequence of events that continue to move along the axon
• nerve transmissions only occur in one direction, they start at the dendrites and move along the axon and end at the axon terminal
• once that initial impulse occurs that opens up the sodium ion channels, the voltage becomes positive and that positive voltage in one part will open the ion channels in the next part of the axon allowing ions to come in becoming positive and so on (a wave of positive voltage)
• self-propagating is when depolarization in one part of the axon triggers depolarization in next part of the axon due to the opening of voltage gated channels (one-way direction)
• the average speed for a nerve impulse is around 1m/s
• there are several adaptations that organisms have to speed up nerve impulses;

  1. increase the diameter (ohms law) - which decreases the resistance to flow allowing electrical impulses to travel faster along the neuron
  2. myelination - results in a structure called a myelin sheath made up of several cells (each cell called a schwan cell) and in between each cell there's a space known as the node.
• myelin sheaths being separated by nodes are a great set-up for saltatory conduction (this only applies to myelinated neurons)
• The schwan cells in the myelin sheaths insulate the electrical charge (instead of it escaping and being put back in by gated pumps) the impulses just jump from node to node decreasing the energy requirement of ATP but it greatly increases the speed (as pumps are only needed at the nodes and not continuously along the neuron)
• the increase in speed due to myelination (only myelinated neurons) increases the speed from 1m/s to 100m/s (implications on response time, messaging and evolutionary advantage)
• transmission between neurons is often known as synaptic transmission
• the synapse is a gap between the cells which signals are passed through by neurotransmitters (signaling molecule)
• transmission between neurons can be between; neuron to neuron, or neuron to effector, or sensory organ to neuron
• the synapse is approximately 20nm
• the end of a presynaptic neuron and the beginning of a postsynaptic neuron is where signaling or transmission between cells occur
• 1.0 - steps of transmission between cells (presynaptic);

  1. an action potential (wave of positive voltage) will reach the terminal end of the presynaptic neuron
  2. that wave of voltage will cause voltage-gated calcium ion channels to open (high conc of Ca ions outside the cell)
  3. Ca ions enter the presynaptic neuron through facilitated diffusion
  4. the Ca ions force vesicles with neurotransmitters to fuse with the membrane
  5. neurotransmitters are released into the synapse (exocytosis) once that fusion happens
• 2.0 - steps of transmission between cells (postsynaptic);

  1. once the neurotransmitters diffuse across the synapse, they bind to the receptors on the postsynaptic membrane
  2. this will cause ion channels to open and the receptor may also act as an ion channel
  3. sodium ions (or any other ions) can enter the cell and start a new action potential in the postsynaptic cell
  4. the neurotransmitter is then removed from the synapse to eliminate the prospect of a continuing message (could be repumped into the presynaptic neuron or an enzyme will destroy it)
• Acetylcholine is an example of a neurotransmitter responsible for carrying messages between motor neurons and muscles and undergoes transmission between presynaptic and postsynaptic membranes
• when the neurotransmitter acetylcholine binds to the postsynaptic neuron it'll give messages to contract and when these messages aren't needed, the enzyme acetylcholinesterase breaks down the acetylcholine into acetyl and choline in the synapse (the choline will be reabsorbed back into the presynaptic neuron to generate more acetylcholine and acetyl will be used in cell respiration)
• acetylcholinesterase is the enzyme responsible for breaking down or destroying acetylcholine when its' signals aren't needed or when a new action potential is generated
• for an action potential to be propagated the potential difference must be above the potential needed for channels to open
• threshold potential in the membrane is the potential that must be reached in order for the voltage-gated ion channels to open
• an action potential in one neuron signifies the change to a positive voltage (depolarization)
• the change to a positive voltage allows the sodium channels to open in one neuron but also nearby neurons also open allowing ions to flood into the cell (causing a wave of depolarization and repolarization which is called a self-propagating wave)
• the movement of Na ions into the cell and K ions out of the cell will cause depolarization and repolarization in that particular spot called a local current