Chapter 2: Continued Flashcards
Thanksgiving
Mmmm
What are the main reasons for the negative membrane potential at rest
Negative charged protein anions are the main reason and make the cell -100 mv but potassium diminishes the negativity to -70mv at rest
The sodium potassium pump also contributes to the resting potential because its throwing three positive potassium ions out and bringing two in
How many different types of electrical responses can a nephron generate?
Graded electrical potential - small
Action potential - large
Is the membrane permeable? To what?
The membrane is selectively permeable, allowing some chemicals to pass more freely than others. The protein anions (A-) are negatively charged and are too large to pass so they stay inside the cell.
Sodium, potassium and chloride pass through specified channels
What protein channels are open / closed when the membrane is at rest?
Na+ channels are closed, and K+ channels are partially closed allowing the slow passage of K+. Some K+ comes inward due to electrical attraction. But more K+ leaks out down the concentration gradient
What does the resting potential of a neuron refer to?
The state of a neuron prior to the sending of a nerve impulse
What is the electrical charge of the membrane?
It is slightly negative on the inside relative to outside at -70 mv
What is an electrical gradient
The neuron membrane maintains an electrical gradient, which is a force acting on ions due to the difference in the electrical charge on the inside and outside of the cell.
What is the concentration gradient?
The difference in number of ions present inside relative to outside of the neuron
What is the electrical polarization?
At rest, the membrane maintains an electrical polarization (inside vs outside electrical charge difference), due to the 1) anions, 2) k+concentration and electrical gradients and 3) Na+/K+ pump
What kind of transport is the sodium-potassium pump
The sodium-potassium pump is a protein complex that continually pumps 3 sodium ions out of the cell, while drawing two potassium ions into the cell. It helps to maintain the electrical gradient
Because na+ and k+ ions are being moved against their concentration gradient they need energy for it to occur
What happens in a graded electrical potential?
The resting potential remains stable until the neuron is stimulated.
Hyperpolarization is due to an effluent of k+ ions (or an influx of CI- ions) making the inside more negative
Depolarization is due to an influx in Na+ ions
Typically occur on the dendrites and soma
What is an action potential?
Any stimulation beyond a certain level, called the threshold of excitation, produces sudden massive depolarization
Na+ channels open and permit a rapid massive flow of these ions into the cell.
Occurs in axon
What kind of channels are sodium channels?
Sodium channels are voltage activated channels whose permeability depends upon the voltage difference across the membrane
Requires high positive depolarization to occur
First step of an action potential at a molecular basis
At rest the Na+ channels are closed
The membrane at the axon hillock depolarizes corresponding to the arrival of the graded electrical potentials.
Second step of action potential at a molecular basis
When the action potential reaches the threshold, na+ channels open wide resulting in an influx of ions triggering the depolarization phase of the action potential
The inside state of the region of the neuron becomes more and more positive as na+ ions enter.
Gets more positive obv
Action potential at a molecular basis step 4
With increasing depolarization the electrical gradient for k+ flips, and the k+ channels open wide, permitting k+ to exit down its concentration and electrical gradient
Then, as the depolarization reaches its maximum, the Na+ channels close
Action potentials on a molecular basis Step 4
As K+ to exit the reporalization phase of the action potential occurs.
The membrane potential returns approaching threshold, and the k+ channels close
Action potential molecular basis step 5
The Na-K pump restores ionic distribution and the resting membrane potential to -70mv
When do sodium ions cross?
During the peak of the action potential
When do potassium ions cross
Later in the opposite direction, helping to return the membrane to its original negative inside polarization
What is the all-or-none law
It states that the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated the ap
What happens in a neuron after an action potential?
The neuron has a refractory period during which time the neuron resists the production of another action potential
What is the absolute refractory period?
It is the first part of the period in which the membrane cannot produce an action potential
What is the relative refractory period
It is the second part in which it take a stronger than usual stimulus to trigger An action potential
Where does an action potential begin?
In the neuron the action potential begins at the axon hillock, which is a swelling where the axon exits the soma
What is propagation of the action potential
Is the phrase used to describe the transmission of the action potential down the axon
Steps to an action potential
If the electrical responses added up can add to -50mv at the axon hillock then an action potential is initiated
Then it goes to the axonial endings where the neurotransmitter is being housed
Steps to an action potential
If the electrical responses add up to -50mv at the axon hillock then an action potential is intiated
Then it goes to the axonal endings where the neurotransmitter is being housed
The action potential is regenerated at each node of ranvier in the mylien
This is called saltatory conduction
This effect spreads the conduction of the impulse down the axon
How does the next node know when to fire?
Saltatory conduction is just the perception its leaping from node to node
First event of an action potential is the sodium channel at the first node in the axon hillock
So sodium is in high concentration outside the cell
So positive sodium ions wont go inside because its negative and it wants to go down its concentration and electrical gradient
This leads to depolarization
What is the myelin sheath
Insulting material of fats and proteins
Nodes of ranvier
At each node, the action potential is regenerated by a chain of positively charged ions pushed along by the previous segment
Saltatory conduction
Describes the leaping of the action potential from node to node
Provides rapid conduction of impulses
Conserves energy
What does myelin do?
With no myelin the inside positive charge would decay with distance
With myelin inside positive charge is sustained above threshold permitting ap at next mode
Think of a stoned dropped in water producing a circular wave of passive spread
Arrival of action potential at the axonal ending
Synaptic vesicles fuses with the presynpatic membrane and the neurotransmitters are released into the synapse and bind tot he receptors on the cell body and/or dendrites
This causes channels nearby to open dependent on the type of neurotransmitter positive / negative channel —> going to cause hyperpolarization or depolarization
The arrival of the action potential into axonal ending causes
Triggers voltage gated Ca++ channels to open. The influx of Ca+ ions induces the migration of transmitter filled vesicles towards the pre synaptic membrane
What is exocystosis
It refers to the excretion of the neurotransmitter from the presynaptic terminal into the synapse; the synaptic vesicles fuse with the membrane and rupture releasing their contents into he synapse
The neurotransmitters quickly diffuse across the synapse chemically binding to the matching receptor
Each activated receptor in turn triggers ion channels to open in the receiving cells membrane, allowing ions to flood across, changing the electrical potential across the post synaptic membrane.
What did Charles Scott Sherrington discover
He coined the term synapse to describe the specialiazed gap that existed between neurons.
He conducted his research investigating hoe neurons communicated by studying reflexes
He used behavioral observations to infer major synaptic properties
1857-1952
Observed difference in speed conduction in a reflex arc from measured axon potentials
Speed along axon versus at synapse(slower and more variable)
Temporal summation
Sherrington observed that repeated stimuli over a short period of time produced a stronger response
This led to the idea of temporal summation or that repeated stimuli when occurring close enough in time can have a cumulative effect and can produce an action potential if the EPSPs combine to exceed threshold
Spatial summation
Sherrington saw that several stimuli on a similar location produced a reflex when a single stimulus did not
This led to the idea of spatial summation that synaptic input at the same time from several locations can have a cumulative effect and trigger an action potential
What is an inhibitory post-synaptic potential
Occurs when synaptic input selectively opens the gates for positively charged potassium ions to leave the cell of for negatively charged chloride ions to enter the cells.
Serves as an action “brake”, that suppresses excitation
Sherrington 3 major findings
- Reflexes are slower than the conduction along an axon
- Several weak stimuli presented at slightly different times or slightly different locations produces a stronger reflex than a single stimulus does
John Carew Eccles
A student of Sherrington developed convincing evidence that signals do not spark across synapse but rather operate by releasing packages of chemicals called neurotransmitters
They travel across the synapse and allow communication between neurons
They bind to receptors on the post-synaptic membrane and induce a graded electrical potential
Loewi
A pharmacologist was the first to show that the electrical impulses generated by nerve cells are transmitted to other cells via a chemical messenger
Identified the neurochemical transmitter as vagusstoff (vagal substance) because it was released from the vagus nerve where it was electrically stimulated
Later when the chemical structure was determined would be given the name acetylcholine
What is an antagonist
Drugs block the effects of neurotransmitters
Agonist
Drugs mimic or increase the effects of neurotransmitters
Steps of chemical events at the synapse
1a. Synthesis of peptide neurotransmitters and vesicles
1b. Synthesis of smaller neurotransmitters such as acetylcholine
- Transport of peptide neurotransmitters
- Action potential causes calcium to enter, releasing neurotransmitter
- Neurotransmitter binds to receptor
- Separation from receptors
- Reputable of neurotransmitter by transporter protein
- Postsynaptic cell releases retrograde transmitters that slow further release from presynaptic cells
- Negative feedback sites respond to retrograde transmitter or to presynaptic cells own transmitter
Ssris do what
Act to increase the amount of serotonin that is permitted to hang around in the synapse by blocking the reuptake mechanism
Effect is to inhibit excessive or inappropriate emotions or moodiness
Also tricked the dopamine transports into retrieving serotonin into dopamine vesicles
