5 I. Nerve Signals and Their Transmission A. Resting Potential – maintained by the Sodium Potassium Pump1. Cells pump 3 Sodium ions (3Na+) out Potassium ions (2K+) ina. Cell is more negative inside than outb. This makes the cell like a batteryB. How do the ions get “pumped” in/out?1. An integral protein (enzyme) uses ATPa. its name is ATPaseb. it “countertransports” – pumping two things in opposite directionsc. it is also referred to as an electrogenic pump because it generates an electrical charge
6 C. Now that the electrical gradient is established, how is it used? 1. Cell’s “resting membrane potential” is -70mV2. Depolarizing a cell:1. enables us to use the charge in our cell membrane2. There are sodium channels in our membrane. They open when we need to depolarize the membrane.a. this allows Na+ to rush in which makes the inside of the cells less negative.b. cells depolarize to +30mV (when Na+ is at equilibrium with itself, there is still an excess of K+)
7 D. How do we “repolarize” our cell membranes? 1. NOT through the Sodium Potassium Pump…would take too longa. it keeps membrane charged continually2. Potassium channels open brieflyResting Na+ channel open Na+ closed, K+ open Resting “polarized” “depolarized” “repolarization” “polarized”
8 E. This “depolarization” (opening Sodium gates) and E. This “depolarization” (opening Sodium gates) and “repolarization (opening Potassium gates) is called an action potential.1. What signals a cell to depolarize?a. a receptor (lock) on a cell membrane binds to its ligand (key) causing the receptor to change shape-- change involves the opening of the sodium channelb. the receptor is called a neurotransmitter receptor and the ligand is called the neurotransmitter-- Acetylcholine is the neurotransmitter in skeletal muscle cells.
9 c. A chemically gated sodium channel opens. Na+ flow in 2. How does an Action Potential work?a. Begins in a location where chemically gated sodium channels are present. These locations are called “postsynaptic membranes” in neurons and “motor endplates” in muscle cells.b. Neurotransmitter (ligand) binds to a neurotransmitter receptor. The receptor changes shape.c. A chemically gated sodium channel opens. Na+ flow ind. Once one Na+ channel opens, the others follow due to the change in electrical conditions. This point is called “threshold”e. K+ channels open slowly so that when the Na channels close, the K+ channels are fully open (repolarization). System resets when K+ channels close.f. this causes a moving wave of depolarization and repolarization within the cell
11 Post-synaptic membranes or Motor Endplates Neurotransmitter binds to Neurotransmitter receptor Neurotransmitter receptor changes shape Sodium Channel opens Sodium flows in Other Sodium Channels open = “Threshold” Potassium Channels open slowly Sodium Channels close Potassium Channels close.Resting Na+ channel open Na+ closed, K+ open Resting “polarized” “depolarized” “repolarization” “polarized”
12 F. Action potential is how information is carried along a F. Action potential is how information is carried along a neuron, and how an entire muscle contraction is stimulated through one muscle cell.1. Neuron to neuron communication occurs between structures called synapses.2. Neurons communicate with other cells (effector cells) and muscle cells in structures called neuroeffector junctionsa. the term synapse is used for both even though it is incorrect
13 G. How this communication takes place: 1. Action potential moves down the neuron through the axon, then through the axon terminals (“telodendria”), and finally through the synaptic end bulbs.a. Synaptic end bulbs contain vesicles of neurotransmitter2. Voltage-gated calcium channels in the membrane of the end bulb open.3. Calcium diffuses in.4. Calcium’s presence causes the synaptic vesicles to fuse with the cell membrane to release neurotransmitter (exocytosis) into the synaptic cleft.5. Neurotransmitters diffuse across the synaptic cleft and bind to neurotransmitter receptors on the membrane on the opposite side of the cleft.6. Neurotransmitters are broken down by enzymes in the synaptic cleft.
14 A Chemical Synapse Action Potential axon axon terminals Synaptic end bulbs Calcium Channels open Calcium diffuses in Synaptic vesicles leave via exocytosis Neurotransmitters diffuse across synaptic cleft Neurotransmitters bind to neurotransmitter receptors Action potential triggered in next cell.
15 Toilet Flushing H. The All-or-None Principle 1. If stimulus is too small….no action potential2. If stimulus is “threshold”…action potential3. If stimulus is larger than threshold…same sized action potential as aboveI. The Absolute Refractory Period1. A brief period of time after a stimulus in which an additional stimulus will not produce an additional action potentialJ. The Relative Refractory Period1. A certain time period after a stimulus during which a second stimulus will produce a second action potential, but only if it is a larger stimulus than usual.
17 Neurotransmitters Acetylcholine transmit signal to skeletal muscleEpinephrine (adrenaline) & norepinephrinefight-or-flight responseDopaminewidespread in brainaffects sleep, mood, attention & learninglack of dopamine in brain associated with Parkinson’s diseaseexcessive dopamine linked to schizophreniaSerotoninNerves communicate with one another and with muscle cells by using neurotransmitters. These are small molecules that are released from the nerve cell and rapidly diffuse to neighboring cells, stimulating a response once they arrive. Many different neurotransmitters are used for different jobs:glutamate excites nerves into action;GABA inhibits the passing of information;dopamine and serotonin are involved in the subtle messages of thought and cognition.The main job of the neurotransmitter acetylcholine is to carry the signal from nerve cells to muscle cells. When a motor nerve cell gets the proper signal from the nervous system, it releases acetylcholine into its synapses with muscle cells. There, acetylcholine opens receptors on the muscle cells, triggering the process of contraction. Of course, once the message is passed, the neurotransmitter must be destroyed, otherwise later signals would get mixed up in a jumble of obsolete neurotransmitter molecules. The cleanup of old acetylcholine is the job of the enzyme acetylcholinesterase.
18 Neurotransmitters Weak point of nervous system any substance that affects neurotransmitters or mimics them affects nerve functiongases: nitrous oxide, carbon monoxidemood altering drugs:stimulantsamphetamines, caffeine, nicotinedepressantsquaaludes, barbiturateshallucinogenic drugs: LSD, peyoteSSRIs: Prozac, Zoloft, PaxilpoisonsSelective serotonin reuptake inhibitor