AP 2 1.4: Action Potentials Flashcards
Resting Potential
Neurons are specialized to conduct electrical impulses called action potentials. The nerve impulse is an electrochemical charge moving along an axon created by the movement of unequally distributed ions on either side of an axon’s plasma membrane.
polarized
At rest, the plasma membrane is said to be polarized, meaning that one side has a different charge than the other side.
resting potential
When the axon is not conducting an impulse, this difference in electrical charge is called resting potential, or the resting state of a neuron, and is equal to about -70mV (millivolts). The charge is negative because the charge on the inside of the axon’s cell membrane is 70 millivolts less than the outside of the membrane
sodium-potassium pump
The resting potential is maintained by a sodium-potassium pump, which uses active transport to carry ions across the plasma membrane. The pump works by using an integral carrier protein that, for every three sodium (Na+) ions pumped out, two potassium (K+) ions are pumped in. The pump must keep in constant operation because the Na+ and K+ ions will naturally diffuse back to where they originated. Because the plasma membrane is more permeable to K+ diffusing outward and because more Na+ ions are being pumped outward than K+ pumped inward, a relative positive charge develops and is maintained on the outside of the membrane.
action potential
If the axon is stimulated to conduct a nerve impulse, there is a rapid change in the polarity. This change in polarity is called the action potential. The resting potential becomes an action potential if the membrane becomes depolarized. Once an action potential occurs, it continues through the entire length of the axon.
depolarization
Electrical Propagation
First, the membrane potential becomes more positive (called depolarization), indicating that the inside of the membrane is now more positive than the outside.
re-polarization
Electrical Propagation
Then the potential returns to normal (called re-polarization), indicating that the inside of the axon is negative again
special protein-lined channels
Electrical Propagation
The action potential is due to special protein-lined channels in the membrane, which can open to allow either sodium or potassium ions to pass through.
sodium gates
special protein-lined channels
The action potential is due to special protein-lined channels in the membrane, which can open to allow either sodium or potassium ions to pass through. These channels have gates, called sodium gates and potassium gates
potassium gates
special protein-lined channels
The action potential is due to special protein-lined channels in the membrane, which can open to allow either sodium or potassium ions to pass through. These channels have gates, called sodium gates and potassium gates. These channels and their gates are voltage activated, as proteins respond to changes in voltage with changes in shape.
Phase 1: Resting Potential
phases of an action potential step by step.
Phase 1: Resting Potential: During the resting phase, both sodium and potassium gates are closed.
Phase 2: Depolarization
phases of an action potential step by step.
Phase 2: Depolarization: The sodium gates open, and sodium rushes into the axon during the depolarization phase of the action potential. Voltage travels to zero and then on up to +40 mV.
Phase 3: Repolarization
phases of an action potential step by step.
Phase 3: Repolarization: The sodium gates close, and potassium gates open allowing potassium to rush out of the axon. This returns a negative voltage to the inside of the axon
Phase 4: Afterpolarization
phases of an action potential step by step.
Phase 4: Afterpolarization, also called hyperpolarization. Potassium gates are slow to close, and there is an undershoot of the potential. The voltage drops below -70mV and then returns to -70mV as the resting state begins.
self-propagating
phases of an action potential step by step.
The action potential travels along the length of an axon like a wave. It is self-propagating because the ion channels are prompted to open whenever the membrane potential decreases (depolarizes) in an adjacent area.
action potential is an all-or-nothing
phases of an action potential step by step.
An action potential is an all-or-nothing response, either occurring or not. Since no variation exists in the strength of a single impulse, intensity of a sensation (minor or major pain) is distinguished by the number of neurons stimulated and the frequency with which the neurons are stimulated.
Chemical Transmission of an Action Potential:
An impulse passing from one nerve cell to another always moves in only one direction. There is a very short delay in transmission of the nerve impulse from one neuron to another because neurons do not touch.
synapse
There is a minute fluid-filled space, called a synapse, between the axon terminal of the sending (presynaptic) neuron and the dendrite of the receiving (postsynaptic) neuron
When a nerve impulse reaches the end of an axon
When a nerve impulse reaches the end of an axon, voltage-gated calcium channels open. As calcium ions (Ca2+) rushes in, it causes vesicles containing the neurotransmitters to fuse with the plasma membrane and release the neurotransmitter into the synapse. When the neurotransmitter released binds with a receptor on the next neuron, sodium ion (Na+) channels in the receiving dendrites open.
nerve impulses is electrochemical
The transmission of nerve impulses is electrochemical in nature as chemicals called neurotransmitters allow the signal to jump the synaptic gap. The signal moves from electrical (through the neuron) to chemical (in the synapse) to electrical again once the signal reaches the next neuron.
Depolarization
When a nerve impulse reaches the end of an axon
Depolarization occurs in the next neuron, and the impulse is propagated forward to another neuron or to a target organ, always in one direction.
Once a neurotransmitter has been released into a synapse
When a nerve impulse reaches the end of an axon
Once a neurotransmitter has been released into a synapse, it has only a short time to act. Some synapses contain enzymes that rapidly inactivate the neurotransmitter.
inhibition
When a nerve impulse reaches the end of an axon
The short existence of neurotransmitters in the synapse prevents continuous stimulation of postsynaptic membranes. Prevention of continuous stimulation is called inhibition
acetylcholinesterase
When a nerve impulse reaches the end of an axon
For example, the enzyme acetylcholinesterase, or simply cholinesterase, breaks down the neurotransmitter acetylcholine. In other synapses, the synaptic ending rapidly reabsorbs the neurotransmitter. Some neurons repackage the neurotransmitters in synaptic vesicles while others chemically breakdown the neurotransmitters. The short existence of neurotransmitters in the synapse prevents continuous stimulation of postsynaptic membranes.
Types of Neurotransmitters
Norepinephrine and epinephrine are neurotransmitters produced by the adrenal glands. Dopamine is a specialized brain neurotransmitter to help regulate emotional responses and muscle tone.
Acetylcholine
Types of Neurotransmitters
Acetylcholine is a neurotransmitter found at neuromuscular junctions (NMJ) in the peripheral nervous system. The NMJ is located where a motor neuron ends on a muscle instead of another neuron. For a muscle to contract, the nervous system must work together with the muscular system
The Neuromuscular Junction
The nervous system interacts with the muscular system at neuromuscular junctions to enable muscular contraction. First, a nerve impulse must be sent to the muscle by the presynaptic motor neuron. A neuromuscular junction is a special type of synapse formed between a motor neuron and muscle tissue. Once the nerve impulse reaches the muscle fiber (at the neuromuscular junction), acetylcholine is released into the synapse (see Figure 1.20). Acetylcholine binds to receptors on the muscle fiber that cause sodium channels to open. Sodium rushes into the muscle cell, triggering an action potential, which reaches the sarcoplasmic reticulum.
sarcoplasmic reticulum
The Neuromuscular Junction
The sarcoplasmic reticulum is a specialized type of smoother ER found within striated muscle tissue. Calcium ions are released from the sarcoplasmic reticulum of the muscle cell, causing the muscle to contract.
