Neuronal Communication: From Electricity to Neurotransmitters

How do our neurons communicate with one another? - To put it simply, neurons communicate by using electricity to create bursts of energy - These bursts of energy trigger the release of neurotransmitters, which then pass the message along to the next neuron in line - This chain reaction of electricity and chemicals is what allows our brain to function - What makes all of this possible? The neural membrane - The bilayered cell membrane of a neuron is: ● Thin ● Made up of lipid (fat) and protein ❖ Lipid molecules: have a hydrophilic head, hydrophobic tails ❖ Forms a double membrane - Holds cell together - Controls environment inside and outside of cell through protein channels - Selective permeability: the property of the neuron’s cell membrane that allows it to regulate the passage of ions and molecules across the membrane - The cell membrane’s selective permeability makes neurons’ polarization possible ● Polarization: a state in which there is a difference in electrical charge between the inside and outside of the neuron ❖ A difference in electrical charge between two points is also called voltage - Resting potential: difference in charge between inside and outside of membrane of a neuron at rest ● Negative ● Varies from -40 to -80 millivolts ● Here, the neuron is not actively sending a signal (resting) - The resting potential of the neuron is negative because of an unequal amount of ions on either side of the cell membrane ● Ions: atoms that have lost or gained electrons ❖ Positive: sodium Na, potassium K ❖ Negative: chloride Cl, certain organic anions A ❖ Outside neuron: more Na, Cl ions ❖ Inside neuron: more K, anions - Molecules like to move from high concentration to low concentration ● Force of diffusion: tendency of ions to move through membrane to less concentrated side ● Electrostatic pressure: force in which ions are repelled from side similarly charged and attracted to oppositely charged side ● These forms are trying to create balance, or equilibrium How does the neuron stay polarized? - anions= too big to pass through the membranes and keep chloride ions from entering - Potassium’s force of diffusion is stronger than its electrostatic pressure so it is highly permeable to the cell membrane - Sodium-potassium pump: large protein molecules that actively move sodium ions through the cell membrane ot the outside and potassium ions back inside ● Pumps move three sodium ions out for every two potassiums they let in ● This keeps the inside of the membrane negative - Ion channels: pores in the membrane which allow for the passive flow of ions between extra- and intracellular fluids ● Chemically gated ion channels: opened by ligands (neurotransmitters or hormones) ● Electrically gated ion channels: opened by change in membrane’s electrical potential - Local potentials: small partial depolarizations that can communicate messages using a decremental signal which dies out over distance ● Partially stimulates the neuron; may set off other partial depolarizations near it ● If local potentials exceed the threshold to activate electrically gated ion channels, it will inflate an action potential ● Graded potential: a local potential which varies in magnitude with the strength of the stimulus that produced it What happens when the neuron gets a signal? - Action potential: abrupt depolarization of area of membrane that allows neuron to communicate ● Sodium ion channels open ● This results in a quick depolarization (balance) - Then, the potassium ion channels open which returns the voltage back to its resting potential - Depolarization triggers nearby sodium channels to open, sending new action potentials through the axon until the terminal - At terminal signal goes to next neuron that will repeat the process - Action potential: abrupt depolarization of area of membrane that allows neuron to communicate over long distances ● Graded potential: a local potential which varies in magnitude with the strength of the stimulus that produced it ● All-or-none law: an ungraded action potential follows this principle, means that it occurs at full strength or it does not occur at all ● Nondecremental: a quality of action potentials in which they travel down the axon without any decrease in size, propagated news and at full strength at each successive point along the way - Refractory periods: the segment of an axon that has just produced an action potential has reduced or zero responsiveness to subsequent graded potentials ● Absolute refractory period: during the action potential and initial recovery, the sodium ion channels are open and unresponsive to further stimulation, no matter how intense. ❖ Resetting ❖ Duration of the absolute refractory period limits the rate of generating new action potentials ❖ Ion channels cannot transmit ions at this point- prevents impulses from moving backwards towards the cell body, making it unidirectional, and preventing self-propagation in both directions- which would result in an impulse bouncing back and forth in the cell and block the ability to receive new signals ● Relative refractory period: ❖ Period in which another action potential can be generated but only by a stronger-than-threshold stimulus ❖ Sodium channels close, but potassium channels remain open in this period to allow potassium ions to exit the cell ❖ Overall polarity of the cell membrane is slightly more negative than the resting potential during this period, so a stimulus must be stronger than the threshold to fire an action potential ● Rate law: axon encodes stimulus intensity not in the size of its action potential but in its firing rate - Glial cells: non-neural cells that provide a number of supporting functions to neurons, including myelination - Myelination and conduction speed ● Survival of the signal depends in part on how rapidly messages can move through the nervous system ● Can increase conduction speeds by insulating nervous cells with myelination ● Myelin: fatty tissue that wraps around axon to insulate it - Two types of glial cells secrete myelin: ● Oligodendrocytes: glial cells which produce myelin in the brain and spinal cord ● Schwann cells: glial cells which produce myelin in the rest of the nervous system Glial cells - Schwann cells: glial cells which produce myelin in the rest of the nervous system ● Action potentials cannot occur across the membrane if it is surrounded by the myelin sheath as there are few sodium channels beneath it - Nodes of ranvier: gaps in the myelin sheath ● At these nodes, there are plenty of sodium channels which can trigger an action potential - Saltatory conduction: a form of transmission in which action potentials appear to jump from node to node ● Insulating effect of myelin reduces capacitance (how well something stores energy), which slows the movement of ions down the axon, this gives the graded potential a speed boost ● Breaks in myelination mean signal is regenerated by an action potential at every node of ranvier - Myelinated neurons use much less energy with reduced usage of sodium-potassium pumps - Myelin-destroying disorders increase capacitance, the few voltage-sensitive sodium channels under the myelin prevents action potential from generating; this causes conduction to slow or stop in affected neurons Other types of glial cells - Radial glia: fetal development, form “scaffolds”, that guide new neurons to their destinations - Microglia: provide energy to neurons and respond to injury and disease by removing cellular debris - Astrocytes: trigger the formation of many connections in neurons; their absence may result in the loss of synapses. Also appear to play a key role in learning How neurons communicate with each other - Synapse: the connection between two neurons - Synaptic cleft: a small gap that separates neurons so they are not in direct physical contact at the synapse ● Presynaptic: a neuron that is transmitting to another neuron ● Postsynaptic: a neuron that is receiving a transmission from a presynaptic neuron - Neurotransmitters: chemicals that the neuron releases to communicate with a muscle, an organ, or the next neuron in a chain - Vesicles: membrane-enclosed bubbles at the axon terminals which store neurotransmitters - Vesicles that are fused with the cell membrane release their neurotransmitters into synaptic cleft - Neurotransmitters dock into specific receptors, which then open ion channels 1. Ionotropic receptors: receptors that form the ion channel and open quickly to produce the immediate reactions required for muscle activity and sensory processing 2. Metabotropic receptors: receptors that open channels indirectly through a second messenger; they act slowly and produce longer-lasting effects - Opening channels sets off graded potentials that initiates the action potential - Opening channels can have 1 of the following effects: 1. Partial depolarization: depolarization that is excitatory and facilitates the occurrence of an action potential 2. Hyperpolarization: increased polarization that is inhibitory and makes an action potential less likely to occur - An exciting or inhibiting response? ● Determined by which neurotransmitter is being released AND what type of receptors are on the postsynaptic neuron - We USED to think that a neuron could only release a single neurotransmitter (Dale’s Principle) 1. Corelease: transmitters are packaged in the same vesicles, but not necessarily at the same rate or in the same numbers 2. Contransmission: transmitters are packaged in separate vesicles, which allows release of just one (at low rates of impulse) or both (at higher rates of impulse) 3. Release of different transmitters from various terminals to produce different effects at separate destinations - The output of a single neuron is insufficient to cause the postsynaptic neuron to fire or prevent it from firing ● Typical neuron receives inputs from approximately 1,000 other neurons and each neuron has numerous terminals - So the postsynaptic neuron combines potentials from multiple neurons to fire ● Ensures that neurons will not be fired by spontaneous activity of a single presynaptic neuron and provides a more complex message with a variety of inputs ● Spatial summation: combines potentials occurring simultaneously at different locations on the dendrites and cell body (spatial space) ● Temporal summation: combines potentials arriving a short time apart from either the same or separate inputs (temporal time) - Neurons do not just conduct messages; they integrate the info from those messages ● To fire or not to fire? - After that decision is made, transmitters must be inactivated to prevent continued stimulation to postsynaptic receptors or leakage into other synapses ● Reuptake: transmitters are taken back into the terminals by membrane proteins called transporters, where they are repackaged into vesicles for reuse. ● At some synapses, it is absorbed by nearby astrocytes; others, it is inactivated by an enzyme - Controlling the reuptake of neurotransmitters is one way to affect behavior; used in psychopharmacology ● Antidepressant medications may block the reuptake of serotonin, norepinephrine, or both Neural codes and neural networks - Neural codes: neurons don’t produce a train of equally spaced impulses ● Bursts of different length separated by different intervals ● The firing patterns an be correlated with sensory input on one end and behavior on the others ● Also coded by which nerves detected the stimulation (nerves with specific inputs are sometimes called “Labeled lines”) - Neural Networks: groups of neurons that function together to carry out a process; where the most complex neural processing happens ● In single, small areas or larger regions of brain….more on this later TERMS TO MEMORIZE - Acetylcholine - Serotonin - Dopamine - Norepinephrine - Epeneophorin - Appa gaba