nervous system B (neurons) Flashcards
(31 cards)
membrane potential changes
Membrane potential changes when:
Concentrations of ions across membrane change
Membrane permeability to ions changes
Voltage changes across the membrane can be classified into 2 basic types of electrical signals:
Action Potentials – Long-distance signals of axons that travel very rapidly
Graded Potentials – Incoming signals traveling over short distances
graded pontetials
travel from dendrites to axon hillock
action potentials
Action potentials are electrical signals that travel over long distances (from a neuron’s axon hillock to the end of its axon (axon terminals).
The signals are of uniform strength as they travel (do not lose strength). The action potential at the end of the axon is identical to the action potential that started at the axon hillock.
The high speed movement of an action potential along the axon is called conduction of the action potential.
In action potentials, voltage-gated ion channels in the axon membrane open sequentially as electrical current passes down the axon. They represent movement of Na+ and K+ across the membrane.
action potentials are all or none phenomenon.
voltage gated channels pass action potential down axon.
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first key player in action potentail is voltage gated NA channels. an activation gate and an inactivation gate. when activation gates open na goes inside. inaction gates can plug hole and keep Na from going down .
voltage gated K channels only have 1 gate. when open K goes outside, closed stays inside.
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If axon hillock membrane depolarizes to Threshold Stimulus (-55 mV):
The activation gate of voltage-gated Na+ channels open
Na+ rushes into cell
3. Na+ influx causes more depolarization which opens more adjacent Na+ channels → ICF less negative
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When Membrane potential reaches +30 mV (Action Potential peak):
- Na+ voltage-gated channel inactivation gates close(activation gate still open)
- Membrane permeability to Na+ declines to resting state
- Voltage-gated K+ channels open
- K+ exits the cell and internal negativity is restored (Repolarization)
ex
Some K+ voltage-gated channels remain open, allowing excessive K+ efflux
Inside of membrane more negative than resting state. This causes Hyperpolarization of the membrane (slight dip below resting voltage)
3. Voltage-gated Na+ channels begin to reset
propagation and conduction of action potentials
triggerzone where the action potentail starts.
positve A graded potential above threshold enters the trigger zone.
Its depolarization opens voltage-gated Na+ channels, Na+ enters the axon, and the initial segment of axon depolarizes.
Positive charge from the depolarized trigger zone spreads by local current flow to adjacent sections of the membrane, repelled by the Na+ that entered the cytoplasm and attracted by the negative charge of the resting membrane potential.
- The flow of local current toward the axon begins conduction of the action potential.
- When the trigger zone depolarizes, its Na+ channels open, allowing Na+ into the cell.
- This starts the positive feedback loop causing depolarization of adjacent membrane.
- The section of axon that has just completed its action potential is in its absolute refractory period, with its Na+ channels inactivated. For this reason, the action potential cannot move backward.
ex
Some K+ voltage-gated channels remain open, allowing excessive K+ efflux
Inside of membrane more negative than resting state. This causes Hyperpolarization of the membrane (slight dip below resting voltage)
3. Voltage-gated Na+ channels begin to reset
propagation and conduction of action potentials
triggerzone where the action potentail starts.
positve A graded potential above threshold enters the trigger zone.
Its depolarization opens voltage-gated Na+ channels, Na+ enters the axon, and the initial segment of axon depolarizes.
Positive charge from the depolarized trigger zone spreads by local current flow to adjacent sections of the membrane, repelled by the Na+ that entered the cytoplasm and attracted by the negative charge of the resting membrane potential.
- The flow of local current toward the axon begins conduction of the action potential.
- When the trigger zone depolarizes, its Na+ channels open, allowing Na+ into the cell.
- This starts the positive feedback loop causing depolarization of adjacent membrane.
- The section of axon that has just completed its action potential is in its absolute refractory period, with its Na+ channels inactivated. For this reason, the action potential cannot move backward.
absolute refractory period
When voltage-gated Na+ channels open, a neuron cannot respond to another stimulus
Absolute refractory period
Time from opening of voltage-gated Na+ channels until resetting of the channels
Ensures that each AP is an all-or-none event
Enforces one-way transmission of nerve impulses
relative refractory period
Follows absolute refractory period
Most Na+ voltage-gated channels have returned to their resting state
Some K+ voltage-gated channels still open
Repolarization is occurring
Threshold for AP generation is elevated
Inside of membrane more negative than resting state
Only exceptionally strong stimulus could stimulate an AP
graded potential
Graded potentials are variable-strength signals that travel over short distances and lose strength as they travel through the cell.
They are used for short-distance communication.
Graded potentials in neurons are depolarizations or hyperpolarizations that occur in the dendrites and cell body or, less frequently, near the axon terminals.
If a depolarizing graded potential is strong enough when it reaches the trigger zone of the axon hillock, the graded potential initiates an action potential.
These changes in membrane potential are called “graded” because their size, or amplitude, is directly proportional to the strength of the triggering event. A large stimulus causes a strong graded potential, and a small stimulus results in a weak graded potential.
In neurons, graded potentials occur when chemical signals from other neurons open chemically-gated ion channels, allowing ions to enter or leave the neuron.
why do graded potentials lose their strength
Why do graded potentials lose strength as they move through the cytoplasm?
Current leak – the membrane of the neuron cell body has open leak channels that allow positive charge to leak out into the ECF. Some positive ions leak out of the cell across the membrane as the depolarization wave moves through the cytoplasm, diminishing the strength of the signal inside the cell.
Cytoplasmic resistance – the cytoplasm provides resistance to the flow of electricity
Graded potentials that are strong enough eventually reach the region of the neuron known as the trigger zone.
In efferent neurons and interneurons, the trigger zone is the axon hillock and the very first part of the axon, a region known as the initial segment.
In sensory neurons, the trigger zone is immediately adjacent to the receptor.
another name for axon
nerve fiber
speed of action potential conduction
Two key physical parameters influence the speed of action potential conduction in a mammalian neuron:
1. The diameter of the axon
- the larger the diameter of the axon, the faster an action potential will move
- large axons offer less total resistance
2. The resistance of the axon membrane to ion leakage out of the cell
- Nonmyelinated axons have low resistance to current leak because the entire axon membrane is in contact with the ECF and has ion channels through which current can leak.
- Myelinated axons limit the amount of membrane in contact with the ECF. In these axons, small sections of bare membrane (the nodes of Ranvier) alternate with longer segments with myelin sheaths. The myelin sheath creates a high resistance wall that prevents ion flow out of the cytoplasm.
Myelinated axons allow for Saltatory Conduction which is faster and is an effective alternative to large-diameter axons.
myelintated sheaths
lots of NA voltaged gated channels in high density in nodes of ranvier. when action potential reaches nodes they open cuz membrane potential becomes more positive. myelin blocks leak channels and keeps them insulated (as in keeps ions from leaking)
Cell-to-Cell Communication in the Nervous System
Neurons communicate at Synapses
Each synapse has two parts:
1. The axon terminal of the presynaptic cell
2. The membrane of the postsynaptic cell
In a neural reflex, information moves from a presynaptic cell to postsynaptic cell
The postsynaptic cells may be neurons or non-neuronal cells.
In most neuron-to-neuron synapses, the presynaptic terminals are next to dendrites or the cell body of the postsynaptic neuron.
chemical synapse
Majority of Synapses
Mainly use neurotransmitters to carry information from cell to cell.
Presynaptic axon terminals have many small synaptic vesicles filled with neuro-transmitters that are released on demand.
Neurotransmitters stored in vesicles in the axon terminal are released into the synaptic cleft by exocytosis.
Axon terminals also contain mitochondria to produce ATP for metabolism and transport.
neurotransmitter termination
pumped back through terminal, pumped away by glial cell
diffused out into the blood.
enzymes inactivate them
neurotransmitters
Language of the nervous system
50 or more neurotransmitters have been identified
Most neurons make two or more neurotransmitters
Neurons can exert several influences
Usually released at different stimulation frequencies
Classified by chemical structure and by function
acetylcholine
Neurotransmitters can be informally grouped into 7 classes.
1. Acetylcholine (ACh)
- First identified; best understood
Synthesized in axon terminal and made from acetyl-CoA and choline.
Neurons that secrete ACh and receptors that bind Ach are described as cholinergic.
Released at neuromuscular junctions and by some CNS and ANS neurons.
synthesis and recycling of acetylcholine
acetylcholine is made from choline and acetyl coa
in synaptic cleft ACh is broken down by acetylcholinase enzyme
choline transported back into axon terminal by cotransport of NA
recycled choline used to make ach.
mitochondira good source of choline
biogenic amines
- Biogenic Amines
Derived from single amino acids.
Catecholamines: Dopamine, norepinephrine (NE), and epinephrine
Synthesized from amino acid tyrosine
Neurons that secrete NE and receptors that bind norepinephrine are described as adrenergic.
Indolamines: Melatonin, Serotonin and histamine
Serotonin synthesized from amino acid tryptophan; histamine synthesized from amino acid histidine
Broadly distributed in brain and play roles in emotional behaviors and biological clock
Some ANS motor neurons (especially NE)
Imbalances associated with mental illness
