I. Nerve Signals and Their Transmission A. Resting Potential – maintained by the Sodium Potassium Pump 1. Cells pump 3 Sodium ions (3Na+) out 2 Potassium ions (2K+) in a. Cell is more negative inside than out b. This makes the cell like a battery B. How do the ions get “pumped” in/out? 1. An integral protein (enzyme) uses ATP a. its name is ATPase b. it “countertransports” – pumping two things in opposite directions c. it is also referred to as an electrogenic pump because it generates an electrical charge
C. Now that the electrical gradient is established, how is it used? 1. Cell’s “resting membrane potential” is -70mV 2. Depolarizing a cell: 1. enables us to use the charge in our cell membrane 2. 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+)
D. How do we “repolarize” our cell membranes? 1. NOT through the Sodium Potassium Pump…would take too long a. it keeps membrane charged continually 2. Potassium channels open briefly Resting Na+ channel open Na+ closed, K+ open Resting “polarized” “depolarized” “repolarization” “polarized”
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 channel b. the receptor is called a neurotransmitter receptor and the ligand is called the neurotransmitter -- Acetylcholine is the neurotransmitter in skeletal muscle cells.
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 in d. 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
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”
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 junctions a. the term synapse is used for both even though it is incorrect
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 neurotransmitter 2. 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.
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.
H. The All-or-None Principle 1. If stimulus is too small….no action potential 2. If stimulus is “threshold”…action potential 3. If stimulus is larger than threshold…same sized action potential as above I. The Absolute Refractory Period 1. A brief period of time after a stimulus in which an additional stimulus will not produce an additional action potential J. The Relative Refractory Period 1. 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.