BIOL #23: Nervous System Flashcards
Neuron
Most neurons have the same three parts:
1) Dendrites- receive electrical signals from the axons of adjacent cells.
2) Cell body (or soma)- which includes the nucleus and organelles, integrates the incoming signals from the dendrites and generates an outgoing signal.
3) Axon – transmits the outgoing signal to the dendrites of other neurons.
The end of the axon is usually divided into many branches.
- Each branched end of an axon transmits information to another cell at a junction called a synapse.
- The part of the axon branch that forms a synapse with another cell is called a synaptic terminal.
Highly branched axons can transmit information to many target cells.
At most synapses, chemical messengers, called neurotransmitters, pass information on to the next cell.
- The transmitting neuron is called the presynaptic cell.
- The receiving cell is called the postsynaptic cell.
Membrane Potential
Ions are always unequally distributed between the inside of cells and the surrounding environment, such that the inside of the cell is negatively charged relative to its outside.
- This charge difference (or voltage) is called the membrane potential and is measured in millivolts (mV).
- Membrane potentials are always expressed as inside-relative-to-outside.
- When a membrane potential exists, the ions on both sides of the membrane have potential energy. If the membrane were removed, ions would spontaneously move from the area of like charge to the area of unlike charge—causing a flow of charge, called an electric current.
Resting Potential
A neuron that is not sending a signal is referred to as resting and its membrane potential during this time is called its resting potential.
- In neurons, the resting membrane potentials are typically between -60 and -80 mV.
How Is the Resting Potential Maintained?
Resting potential exists in part because neurons have a high intracellular concentration of K+ and low intracellular concentrations of Na+ and Cl–.
The Na+ and K+ gradients of cells are maintained by sodium potassium pumps in the plasma membrane.
These pumps use the energy of ATP hydrolysis to actively transport Na+ out of the cell and K+ into the cell.
A sodium potassium pump transports three sodium ions out of the cell for every 2 potassium ions transported in, generating a net export of positive charge.
Even though sodium-potassium pumps can create a net negative charge inside the cell, every pump only changes the resting voltage by a few millivolts.
Another mechanisms is necessary to help maintain a voltage difference of 60-80 mV.
Ion channels, pores in the membrane that allows only specific ions to pass through, allow ions to diffuse across the membrane down their concentration gradient.
A resting neuron has a large number of open K+ channels but few open Na+ channels, making neurons more permeable to K+.
Because there is a higher concentration of K+ inside the resting neuron than outside, K+ will diffuse out, further increasing the net negative charge in the neuron.
The build up of the negative charge inside the cell stops when the excess negative charge inside the cell begins to exert an attractive force that opposes the flow of K+ out of the cell.
The net flow of K+ out of a neuron proceeds until the chemical and electrical forces are in balance, called the equilibrium potential for K+ (EK) – this produces a net negative charge in the neuron.
A hypothetical cell with only sodium ion channels would produce a net positive charge for the equilibrium potential of Na+ (ENa) because the higher concentration of Na+ outside the cell would cause a net movement into the cell.
- Because the resting potential of neurons has a net negative charge, it can be deduced that few sodium ions channels are open in this state.
Because the resting potential of a neuron is -60 to -80 mV, neither K+ or Na+ is at equilibrium in a resting neuron (ENa and EK do not match neuron resting potential), thus each ion still has a (potential) net flow.
Generating Nerve Impulses
Because neither K+ or Na+ is at equilibrium in a resting neuron, there is potential for membrane potential to move towards either ENa or EK.
Under conditions that allow Na+ to cross the membrane more readily, the membrane potential will move towards ENa and away from EK, which is what happens during the generation of a nerve impulse.
Changes in membrane potential occur because neurons contain gated ion channels, which open or close in response to stimuli.
The opening and closing of gated ion channels alters the membrane’s permeability to particular ions, which alters the membrane potential.
Opening gated K+ channels with a stimulus would increase the membrane’s permeability to K+ .
- Resulting in net diffusion of K+ out of the cell and shifting of membrane potential towards EK (-90 mV).
- This produces an increase in the magnitude of the membrane potential (i.e. inside the cell becomes more negative), called hyperpolarization.
- In a resting neuron, hyperpolarization results from any stimulus that increases the outflow of positive ions (or in the inflow of negative ions).
Opening gated Na+ channels with a stimulus would increase the membrane’s permeability to Na+ .
- Resulting in net diffusion of Na+ into the cell and shifting of membrane potential towards ENa (+62 mV).
- This produces a reduction in the magnitude of the membrane potential (i.e. inside the cell becomes less negative), called depolarization.
- In a resting neuron, depolarization results from any stimulus that increases the inflow of positive ions (or in the outflow of negative ions).
Graded Potentials
A graded potential is a simple shift in membrane potential in response to hyperpolarization or depolarization.
The magnitude of this shift corresponds to the magnitude of the stimulus.
Induces a small electrical current that leaks out of the neuron as it moves along the membrane, thus the signal decays with distance from the source.
These are NOT the nerve signals that travel along axons.
Action Potentials
An action potential is a massive change in membrane voltage due to sufficiently strong depolarization shifts in membrane potential.
Unlike graded potentials, action potentials have a constant magnitude and can regenerate in adjacent regions of the membrane, thus action potentials can spread along axons making them well-suited for transmitting a signal over long distances.
Action potentials arise because some of the ion channels in neurons are voltage-gated ion channels.
- These channels open and close when the membrane potential passes a particular level.
- If a depolarization event opens voltage-gated sodium channels, the resulting flow of Na+ into the neuron will result in further depolarization, causing more voltage-gated sodium channels to open
- This positive feedback mechanism triggers a fast, large magnitude response.
The particular level of membrane potential that voltage-gated ion channels must encounter for depolarization to occur is called the threshold.
- For mammalian neurons, the threshold of membrane potential is about -55 mV.
- Action potentials either occur fully or not at all, representing an all-or-none response to stimuli because of its positive feedback nature.
- Regardless of the size of the stimulus, the magnitude of the action potential does not change, thus it is the frequency of action potentials—rather than their size— that is the meaningful signal for information being transmitted within the nervous system.
+ e.g. in hearing, a louder sound results in more frequent action potentials being transmitted than a softer sound.
Andrew Huxley & Alan Hodgkins
During the 1940s and 1950s, Andrew Huxley & Alan Hodgkins performed experiments using giant neurons in squid to model how action potentials are generated, earning them a Nobel Prize.
Modeling Action Potentials
Membrane depolarization opens both Na+ and K+ channels, but they respond independently and sequentially:
- Sodium channels open first, creating depolarization and initiating an action potential.
- As the action potential proceeds, the sodium channel become inactivated due to conformation changes (inactivation loop).
- Sodium channels remain inactivated until the membrane returns to the resting potential and the channels close.
- Potassium channels open more slowly than sodium channels, but remain open and functional until the end of the action potential.
Modeling Action Potentials: Step 1
Resting State: When the membrane of the axon is at the resting potential, voltage-gated sodium and potassium channels are closed. Ungated channels remain open to maintain the resting membrane potential.
Modeling Action Potentials: Step 2
Depolarization: A stimulus opens some of the voltage-gated sodium channels. Na+ flows through those channels depolarizing the membrane. If the depolarization reaches the threshold, it triggers an action potential.
Modeling Action Potentials: Step 3
Rising phase of the action potential: Once the threshold is crossed, the positive-feedback cycle brings the membrane potential very close to ENa (Na+ influx makes the inside of the membrane positive with respect to the outside).
Modeling Action Potentials: Step 4
Falling phase of the action potential: Most sodium channels become inactivated, blocking the Na+ inflow. Most potassium channels open, permitting K+ outflow, which makes the inside of the cell negative again.
Modeling Action Potentials: Step 5
Undershoot: The sodium channels close but the potassium channels remain open. As the potassium channels close and the sodium channels become unblocked but remain closed, the membrane returns to the resting state. This stage is called ‘undershoot’ because the membrane potential is briefly closer to EK than it is to its resting membrane potential.
Refractory Period
The sodium channels remain inactivated during the falling phase and the early part of the undershoot phase.
- As a result, if a second depolarizing stimulus occurs during this period, it will be unable to trigger an action potential – this downtime, when a second action potential cannot be initiated, is called the refractory period.
- The refractory period sets a limit on the frequency at which action potential can be generated and ensures that all signals travel down the axon in a single direction.
- For most neurons, the interval between the onset of an action potential and the end of the refractory period is only 1-2 milliseconds (msec) – because action potentials are so brief, a neuron can produce hundreds per second.