How Neurons Communicate Quiz

PDF page 1127: Resting Membrane Potential A neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than the outside (−70 mV, note that this number varies by neuron type and by species). This voltage is called the resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell. If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the system would reach equilibrium. Because ions cannot simply cross the membrane at will, there are different concentrations of several ions inside and outside the cell, as shown in

PDF page 1125: depolarization, the inside of the membrane becomes more positive than the outside as Na flows to the inside.

PDF page 1125: Repolarization occurs when K flows across the membrane to the outside. In myelinated neurons, action potentials “jump” between gaps of unmyelinated axons (nodes of Ranvier), a phenomenon called saltatory conduction. Transmission of a nerve impulse from one neuron to another or to another type of cell such as a muscle cell occurs across a junction called a synapse. Synaptic vesicles at the axon terminal of the presynaptic neuron release chemical messengers called neurotransmitters into the junction; neurotransmitters then bind to receptors embedded in the membrane of the postsynaptic neuron. Neurotransmitters may be either excitatory (such as acetylcholine or epinephrine) or inhibitory (such as serotonin or GABA) as they either increase or decrease the change of an action potential in the postsynaptic neuron. Many drugs, including both pharmaceuticals and drugs of abuse, can induce changes in synaptic transmission; for example, tetrahydrocannabinol (more commonly known as THC) in marijuana binds to a naturally occurring neurotransmitter

PDF page 1125: Repolarization occurs when K flows across the membrane to the outside. In myelinated neurons, action potentials “jump” between gaps of unmyelinated axons (nodes of Ranvier), a phenomenon called saltatory conduction. Transmission of a nerve impulse from one neuron to another or to another type of cell such as a muscle cell occurs across a junction called a synapse. Synaptic vesicles at the axon terminal of the presynaptic neuron release chemical messengers called neurotransmitters into the junction; neurotransmitters then bind to receptors embedded in the membrane of the postsynaptic neuron. Neurotransmitters may be either excitatory (such as acetylcholine or epinephrine) or inhibitory (such as serotonin or GABA) as they either increase or decrease the change of an action potential in the postsynaptic neuron. Many drugs, including both pharmaceuticals and drugs of abuse, can induce changes in synaptic transmission; for example, tetrahydrocannabinol (more commonly known as THC) in marijuana binds to a naturally occurring neurotransmitter

PDF page 1131: particularly the Na channels, regenerates the action potential over and over again along the axon. This ‘jumping’ of the action potential from one node to the next is called saltatory conduction. If nodes of Ranvier were not present along an

PDF page 1127: Table 26.1. The difference in the number of positively charged potassium ions (K ) inside and outside the cell dominates the

PDF page 1127: resting membrane potential (Figure 26.10). When the membrane is at rest, K ions accumulate inside the cell due to a net movement with the concentration gradient. The negative resting membrane potential is created and maintained by increasing the concentration of cations outside the cell (in the extracellular fluid) relative to inside the cell (in the cytoplasm). The negative charge within the cell is created by the cell membrane being more permeable to potassium ion movement than sodium ion movement. In neurons, potassium ions are maintained at high concentrations within the cell while sodium ions are maintained at high concentrations outside of the cell. The cell possesses potassium and sodium leakage channels that allow the two cations to diffuse down their concentration gradient. However, the neurons have far more potassium leakage channels than sodium leakage channels. Therefore, potassium diffuses out of the cell at a much faster rate than sodium leaks in. Because more cations are leaving the cell than are entering, this causes the interior of the cell to be negatively charged relative to the outside of the cell. The actions of the sodium potassium pump help to maintain the resting potential,

PDF page 1127: once established. Recall that sodium potassium pumps brings two K ions into the cell while removing three Na ions per ATP consumed. As more cations are expelled from the cell than taken in, the inside of the cell remains negatively charged

PDF page 1130: cell or another neuron depolarizes the target neuron to its threshold potential (-55 mV). Na channels in the axon hillock open, allowing positive ions to enter the cell (Figure 26.10 and Figure 26.11). Once the sodium channels open, the neuron completely depolarizes to a membrane potential of about +40 mV. Action potentials are considered an "all-or nothing" event, in that, once the threshold potential is reached, the neuron always completely depolarizes. Once depolarization is

PDF page 1130: cell or another neuron depolarizes the target neuron to its threshold potential (-55 mV). Na channels in the axon hillock open, allowing positive ions to enter the cell (Figure 26.10 and Figure 26.11). Once the sodium channels open, the neuron completely depolarizes to a membrane potential of about +40 mV. Action potentials are considered an "all-or nothing" event, in that, once the threshold potential is reached, the neuron always completely depolarizes. Once depolarization is

PDF page 1125: At rest, the Na /K pump, powered by ATP, maintains this gradient, known as resting membrane potential. In response to a stimulus, such as an odorant molecule, membrane potential changes, and an action potential is generated along

PDF page 1131: Myelin and the Propagation of the Action Potential For an action potential to communicate information to another neuron, it must travel along the axon and reach the axon terminals where it can initiate neurotransmitter release. The speed of conduction of an action potential along an axon is influenced by both the diameter of the axon and the axon’s resistance to current leak. Myelin acts as an insulator that prevents current from leaving the axon; this increases the speed of action potential conduction. In demyelinating diseases like multiple sclerosis, action potential conduction slows because current leaks from previously insulated axon areas. The nodes of Ranvier, illustrated in Figure 26.13 are gaps in the myelin sheath along the axon. These unmyelinated spaces

PDF page 1132: Fusion of a vesicle with the presynaptic membrane causes neurotransmitter to be released into the synaptic cleft, the extracellular space between the presynaptic and postsynaptic membranes, as illustrated in Figure 26.15. The neurotransmitter diffuses across the synaptic cleft and binds to receptor proteins on the postsynaptic membrane.

PDF page 1132: Na ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated Ca channels to open. Calcium ions entering the cell initiate a signaling cascade that causes small membrane-bound vesicles, called synaptic vesicles, containing neurotransmitter molecules to fuse with the presynaptic membrane. Synaptic vesicles are shown in Figure 26.14, which is an image from a scanning electron microscope.