Neurons Part 1
Distinguish between resting potential, graded potential and action potential. Relate all of these concepts to a neuron.
Resting potential is the electrical potential of the membrane when not stimulated in any way. The resting potential of a neuron is -70mV. A graded potential is when ion channels open and the ions flow in or out. These can vary in magnitude and duration depending on the strength of the stimulus. The more neurotransmitter there is, the more ion channels on the dendrites open, the larger the magnitude. Na or Ca depolarize and K or Cl hyperpolerize. They occur at the dendrites and cell body and are caused by the opening and closing of ion channels. An action potential only occurs when the membrane potential at the axon hillock reaches threshold; it is all-or-nothing. It has three phases: depolarization, repolarization, and hyperpolarization. After, there is an absolute refractory period and a relative refractory period. They are always the same magnitude and the same duration. They can be transmitted long distances and occur in axons. They are caused by the opening of voltage-gated ion channels.
Describe the functional anatomy of a neuron
A neuron has four functional zones: signal reception, integration, conduction, and transmission. Signal reception takes place on the dendrites and the cell body (soma). The incoming signal is received on the dendrites and converted to a change in membrane potential. Signal integration takes place at the axon hillock. If the signal is strong enough, it is converted to an action potential. Signal conduction takes place on the axon, which has some parts wrapped in myelin. The AP travels down the axon. Signal transmission takes place at the axon terminals (synapse). This is when the neurotransmitters are released and the signal changes from electrical to chemical.
Distinguish between the absolute and relative refractory period of an action potential
Absolute refractory period is when the cell is incapable of generating a new action potential. This is the time when an action potential is already underway, from the threshold to the undershoot. The relative refractory period is when it is very difficult to generate an action potential, but it can be done. This is the time after an action potential has occurred, consisting of the undershoot period.
Describe how acetylcholine is regulated at the synapse
Acetyl CoA is made in the mitochondria. Choline acetyl transferease catalyzes the conversion of choline and acetyl CoA to acetylcholine. ACh is packed into vesicles and released into the synapse. ACh binds to a receptor on the post synaptic cell. Acetylcholineterase breaks down ACh into choline and acetate, terminating the signal in the postsynaptic cell. Choline is recycled and the acetate diffuses out of the synapse.
Understand why APs are unidirectional
Action potentials start at the axon hillock and travel towards the axon terminal. After an AP has gone through a section, that "up-stream" portion is in a absolute refractory period which prevents transmission and summation of APs in that area. No APs can happen during the absolute refractory period as the membrane is trying to level out and recover from the recent AP. After the absolute period, the area goes through a relative refractory period in which is takes a very strong stimulus to cause another AP, but it is possible to have another AP.
Discuss how cobra toxin, black widow toxin, and botulinin toxin affect activities at an ACH synapse
Cobra venom binds to acetylcholine receptors, which causes ACh not to be able to bind, which causes death. Black widow toxin, latrotoxin, causes pores in the the membrane and allows Ca to enter the synapse. With no Ca flow control, too many vesicles bind and release neurotransmitters which causes pain and death. Botulinin toxin blocks vesicle docks and the neurotransmitter cannot be released.
Describe decrement and how it is overcome to generate an action potential (spatial and temporal summation)
Decrement is when a signal fades. The magnitude of a graded potential decreases with increasing distance from the opened ion channel. It is caused by leakage of charged ions across the membrane, electrical resistance of the cytoplasm, and electrical properties of the membrane. These are overcome by spatial and temporal summation. Spatial summation is the summed graded potentials generated from different synapses that influence the net charge; how many and where the graded potentials are occurring. Temporal summation is when the frequency of graded potentials time can influence the net charge; how often and close together the graded potentials are occurring. Temporal and spatial summation can occur at the same time.
Define hyperpolarization, depolarization, and repolarization. Understand what they mean.
Hyperpolarization is when the membrane potential becomes more negative than the resting value (it becomes more negative than -70 mV; i.e. -80 or -90). Depolarization is when the membrane potential becomes less negative (it becomes more positive than -70 mV; i.e. -50 or +40). Repolarization is when the membrane potential returns to resting value (returns to -70 mV).
Describe the role of ligand gated channels and/or voltage gated channels in graded potentials and action potentials
Ligand gated ion channels help to change the membrane potential by letting the ions diffuse through them and changing the resting potential. This causes a graded potential to spread through the cell and if it is strong enough when it gets to the axon hillock, an action potential occurs. Voltage gated channels are used throughout an AP by allowing K and Na to diffuse across the membrane and move the AP down the axon.
Describe the role of myelin in vertebrate neurons of the peripheral nervous system (PNS)
Myelin in the PNS helps to increase speed and decrease time of an AP. Myelin is wrapped around the axon of the neurons and helps to conduct the APs down the axon without them fading as they continue down the axon.
Explain why sodium rushes in at the beginning of an action potential and why potassium rushes out at the apex
Na rushes in because they gates open and ions want to go down their electrochemical gradient to reach equilibrium. When the gates open, the electrical gradient is positive on the outside compared to the inside, so positive Na wants to make the inside positive by entering. There is also less Na inside than out, so it is flowing down both is electrical and chemical (electrochemical) gradient. The reverse happens for K. Once the gates for K open, the inside of the cell is more positive than outside, so to equal this out, positive K leaves. There is also more K on the inside of the cell than the outside. This movement allows for K to flow down its electrical and chemical (electrochemical) gradient.
Understand chain of events at synapse - from opening of calcium channels to receptor- ligand binding on post-synaptic membrane
Once the AP arrives at the synapse, the voltage-gated Ca2+ ion channels open in response. Ca enters the cell and signals to vesicles to move to the memebrane and dock with docking proteins. Once the vesicles dock, they exocytose their contents (neurotransmitters) into the synaptic cleft. The neurotranmitters diffuse across the cleft and bind to the receptors on the target cell. This signals a tranduction pathway inside of the target cell.
Describe how SSRIs affect serotonin in the synapse
SSRIs (Selective Serotonin Reuptake Inhibitors) uptake and degrade serotonin in the synapse.
Describe saltatory conduction
Saltatory conduction is when action potentials "leap" from one node of Ranvier to another. Action potentials occur at the nodes, and an electrotonic current spreads though the internodes. This causes conduction to be very rapid. APs don't actually jump from node to node, but due to myelination, they move very quickly though the neuron to the next node.
Describe the role of the sodium-potassium pump (AKA Na+/K+ ATPase) in the neuron
The Na/K pump restores concentration gradients following repeated action potentials.
Describe the role of the axon hillock
The axon hillock integrates the signal and if it reaches threshold, converts the signal into an action potential.
Know main ions involved in hyperpolarization and depolarization (and directional flow)
The ions involved in hyperpolarization are K and Cl. K flows out but Cl (which is negative) flows in. The ions involved in depolarization are Na and Ca, which both flow outwards.
Goldman equation - Understand that this equation shows you the mathematical relationship between the membrane potential and the concentration of three ions inside and outside of the cell. Know which ions are largely responsible for the membrane potential at any given point in time and be able to explain why selective permeability is so important.
The three ions responsible for the membrane potential are Cl, K, and Na. Selective permeability is important because neurons depolarize and hyperpolarize by selectively altering permeability. If the membrane was not selectivity permeable, the ions would diffuse across at any time and there would be no membrane potential and no electrochemical gradient. An action potential could not occur.
Understand all stages of an action potential
There are five stages to an action potential. The resting state, threshold, depolarization phase, repolarization, and the undershoot. The resting state is when all the channels are closed and the membrane potential is -70 mV. Threshold is when Na gates open and the membrane potential begins to get more positive. Once it reaches a certain number, an action potential happens. In the depolarization phase, Na rushes into the cell, raising the membrane potential, and K gates remain closed. In the repolarization phase, the Na gates finally close and the K gates open, allowing K to rush out and lower the membrane potential back to resting state. In the undershoot, K gates are still open and Na gates are closed. This causes the membrane potential to lower beyond the resting potential. Once the gates finally close, the Na/K pump returns the membrane potential to resting potential.
