Biology Chapter 48 Neurons, Synapses, and Signaling

Refractory period

"Downtime" when a second action potential cannot be initiated

Acetylcholine

Common neurotransmitter in both invertebrates and vertebrates; vital for nervous system functions

For many mammalian neurons, the threshold is a membrane potential of about

-55 mV

A chemical synapse

1) An action potential arrives, depolarizing the presynaptic membrane 2) The depolarization opens voltage-gated channels, triggering an influx of Ca2+ 3) The elevated Ca2+ concentration causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft 4) The neurotransmitter binds to ligand-gated ion channels in the postsynaptic membrane, in example, binding triggers opening, allowing Na+ and K+ to diffuse through

The concentration gradient of ions across the plasma membrane represents:

A chemical form of potential energy that can be harnessed for cellular processes

Nodes of Ranvier

Gaps in myelin sheath

In most neurons, the concentration of K+ is higher inside the cell, while the concentration of Na+ is ____________ outside

Higher

Threshold

Action potential occur whenever a depolarization increase the membrane voltage to a particular value, called the threshold

Through spatial and temporal summation, several EPSPs can:

Combine to depolarize the membrane at the axon hillock to threshold, causing the postsynaptic neuron to produce an action potential

Hyperpolarization

Increase in the magnitude of the membrane potential, makes the inside of the membrane more negative

If a depolarization shifts the membrane potential sufficiently, the result is a massive change in membrane voltage called an ________________________

Action potential

Unlike graded potentials, action potentials have a constant magnitude and can regenerate in adjacent regions of the membrane

Action potentials can therefore spread along axons, making them well suited for transmitting a signal over long distances

At many chemical synapses, the receptor protein that binds and responds to neurotransmitters is a ___________________

Ligand-gated ion channel (inotropic receptor)

Consequence of refractory period

Limit the maximum frequency at which action potential can be generated. *Refractory period is due to the inactivation of sodium channels, not to a change in the ion gradient across the plasma membrane.

Equilibrium potential (Eion)

Magnitude of the membrane voltage at equilibrium for a particular ion

Saltatory conduction

Mechanism for propagating (jump) action potentials because action potential appears to jump along the axon from node to node.

Diffusion of K+ through potassium channels are always open (sometimes called leak channels) is critical for establishing the _____________ potential

Resting

Resting potential

Resting neuron--one that is not sending a signal. Membrane potential is typically between -60 and 80 mV

Potassium ions (K+) and sodium ions (Na+) play an essential role in the formation of resting potential

These ions each have a concentration gradient across the plasma membrane of a neuron.

How is neurotransmitter signaling terminated?

Both receptor activation and postsynaptic response cease when neurotransmitter molecules are cleared from the synaptic cleft. The removal of neurotransmitters can occur by simple diffusion or by other mechanisms. Ex: Inactivated by enzymatic hydrolysis or recaptured into the presynaptic cleft. Once this reuptake occurs, neurotransmitters are repackaged in synaptic vesicles or transferred to glia for metabolism or recycling to neurons

Axon hillock

Site where an action potential is initiated

In a resting neuron, hyper polarization results from:

Any stimulus that increases the outflow of positive ions or the inflow of negative ions

The sodium channels remain inactivated during the falling phase and the early part of the undershot

As a result, if a second depolarizing stimulus occurs during this period, it will be unable to trigger an action potential.

Several _______________, relatively short chains of amino acids, serve as neurotransmitters that operate via metabotropic receptors

Neuropeptides

Myelin sheaths are produced by two types of glia:

Oligodendrocytes in the CNS and Schwann cells in the PNS.

Glutamate

One of several amino acids that can act as a neurotransmitter

What stops the buildup of negative charge?

The excess negative charges inside the cell exert an attractive force that opposes the flow of additional positively charged potassium ions out of the cell. The separation of charge (voltage) thus results in an electrical gradient that counterbalances the chemical concentration gradient of K+

Once released, the neurotransmitter diffuses across the synaptic cleft, the gap that separates the presynaptic neuron from the postsynaptic cell.

Upon reaching the postsynaptic membrane, the neurotransmitter binds to and activates a specific receptor in the membrane

Action potentials arise because some of the ion channels in neurons are _________________________, opening or closing when the membrane potential passes a particular level

Voltage-gated ion channels

The net flow of K+ out of a neuron proceeds until the chemical and electrical forces are in balance

When the electrical gradient exactly balance the chemical gradient, no further net diffusion of K+ occurs across the membrane

Evolutionary adaptations of axon structure

Rate at which axons within nerves conduct action potential governs how rapidly can react to danger or opportunity. One such adaptation is a wider axon. Axon width matters because resistance to electrical current flow is inversely proportional to the cross sectional area of a conductor. A wide axon provides less resistance to the current associated with an action potential than does a narrow one.

Action potentials are not generated in regions between the nodes due to contact with extracellular fluid

Rather, the inward current produced during the rising phase of the action potential at a node travels within the axon all the way to the next node. There, the current depolarizes the membrane and regenerates the action potential.

The role of voltage-gated ion channels in the generation of an action potential

1) Resting state: The gated Na+ and K+ channels are closed. Ungated channels maintain the resting potential 2) Depolarization: A stimulus opens some sodium channels. Na+ inflow through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential 3) Rising phase of action potential: Depolarization opens most sodium channels, while the potassium channels remain closed. Na+ influx makes the inside of the membrane positive with respect to the outside 4) Falling phase of the action potential: Most sodium channels become inactivated, blocking Na+ inflow. Most potassium channels open, permitting K+ outflow, which makes the inside of the cell negative again 5) Undershoot: The sodium channels are closed, but some potassium channels are still open. As these potassium channels close and the sodium channels become unblocked (though still closed) the membrane returns to its resting state

Summary of action potential:

1) When membrane of axon is at resting potential, most voltage-gated sodium channels are closed. Some potassium channels are open, but most voltage-gated potassium channels are closed. 2) When a stimulus depolarizes the membrane, some gated sodium channels open, allowing more Na+ to diffuse into the cell 3) Once the threshold is crossed, the positive feed-back cycle rapidly bring the membrane potential close to ENa. This stage of the action potential is called the rising phase 4) Two events prevent the membrane potential from actually reaching ENa: Voltage-gated sodium channels inactivate soon after opening, halting Na+ inflow; and most voltage-gated potassium channels open, causing a rapid outflow of K+. Both events quickly bring the membrane potential back toward Ek. This stage is called the falling phase 5) In the final phase of action potential, called undershoot, the membrane's permeability to K+ is higher than at rest, so the membrane potential is closer to Ek than it is at the resting potential. The gated potassium channels eventually close, and the membrane potential returns to the resting potential.

Inhibitory postsynaptic potential (IPSP)

A hyper polarization produced is an IPSP because it moves the membrane potential further from threshold

Depolarization

A reduction in the magnitude of the membrane potential Ex: Gated sodium channels opening, the membrane's permeability to Na+ increases, Na+ diffuses into the cell along its concentration gradient, causing a depolarization as the membrane shifts toward ENa +62 mV

Signaling at a synapse brings about a response that depends on both the neurotransmitter released from the presynaptic membrane and the receptor produced at the postsynaptic membrane

A single neurotransmitter may bind specifically to more than a dozen receptors, including inotropic and metabotropic types.

Basis of membrane potential

Although there is a substantial concentration gradient of sodium across the membrane, very little net diffusion of Na+ occurs because there are very few open sodium channels. In contrast, the many open potassium channels allow a significant net outflow of K+. Because the membrane is only weakly permeable to chloride and other anions, this outflow of K+ results in a net negative charge inside the cell

In neurons, as in other cells, ions are unequally distributed between the interior of the cells and surrounding fluid

As a result, the inside of a cell is negatively charged relative to the outside

Ion channels allow ions to diffuse back and forth across the membrane

As ions diffuse through channels, they carry with them units of charge. Any resulting net movement of positive or negative charge will generate a membrane potential, or voltage across the membrane

Synaptic vesicles

At each terminal, the presynaptic neuron synthesizes the neurotransmitter and packages it in multiple membrane-enclosed compartments

Conducting of action potentials

At site where an action potential is initiated, Na+ inflow during the rising phase creates an electrical current that depolarizes the neighboring region of the axon membrane. The depolarization is large enough to reach threshold, causing an action potential in the neighboring region. This process is repeated many times along the length of the axon. Because an action potential is an all-or-none event, the magnitude and duration of the action potential are the same at each position along the axon. The net result is the movement of a nerve impulse from the cell body to the synaptic terminal. Domino effect

Once initiated, the action potential has a magnitude that is independent of the strength of the triggering stimulus

Because action potentials either occur fully or do not occur at all, they represent an all-or-none response to a stimulus. This all-or-none property reflects the fact that depolarization opens voltage-gated sodium channels, causing further depolarization. The positive-feedback loop of channel opening and depolarization triggers an action potential whenever the membrane potential reaches threshold

Excitatory postsynaptic potential (EPSP)

Because depolarization brings the membrane potential toward threshold it is called EPSP

If a depolarization opens voltage-gated sodium channels, the resulting flow of Na+ into the neuron results in further depollarization

Because the sodium channels are voltage gated, the increased depolarization causes more sodium channels to open, leading to an even greater flow of current. The result is a process of positive feedback that triggers a very rapid opening of many voltage-gated sodium channels and the marked temporary change in membrane potential that defines an action potential

The neurotransmitters grouped as ___________________ are synthesized from amino acids and include norepinephrine which is made from tyrosine

Biogenic amines

Membrane potential

Charge different or voltage is called membrane potential. Attraction of opposite charges across the plasma membrane is a source of potential energy

The major of synapses are:

Chemical synapses, which involve the release of chemical neurotransmitter by the presynaptic neuron

A membrane permeable to a single type of ion can be calculated using a formula called the Nernst equation. At human body temperature (37 degrees C) and for an ion with a net charge of 1+, such as K+ or Na+, the Nernst equation is

E ion = 62mV(log[ion] outside/[ion]inside)

Spatial summation

EPSP produced nearly simultaneously by different synapses on the same postsynaptic neuron adding together

Myelin sheath

Electrical insulation that surround vertebrate axons

In vertebrate axons that has narrow diameters can still conduct action at high speed because:

Electrical insulation; insulation causes the depolarizing current associated with an action potential to travel farther along the axon interior, bringing more distant regions to the threshold sooner

Changes in the membrane potential occur because neurons contain _____________________, ion channels that open or close in response to stimuli

Gated ion channels

An action potential that starts at the axon hillock moves along the axon only toward the synaptic terminals. Why?

Immediately behind the traveling zone of depolarization caused by Na+ inflow is a zone of depolarization caused by K+ outflow. In the depolarized zone, the sodium channels remain inactivated. Consequently, the inward current that depolarizes the axon membrane ahead of the action potential cannot produce another action potential behind it. This prevents action potential from traveling backward toward the cell body

The opening or closing of gated ion channels alters the membrane's permeability to particular ions which in turns alters the ____________________

Membrane potential

Why is there a voltage difference of 60 to 80 mV in a resting neuron?

Movement through ion channels, pores formed by clusters of specialized proteins that span the membrane.

In vertebrates there are two major classes of acetylcholine receptor

One is a ligand-gated ion channel, which functions at the vertebrate neuromuscular junction, the site where the motor neuron forms a synapse with a skeletal muscle cell. When acetylcholine released by the motor neurons bind this receptor, the ion channel opens, producing an EPSP. This excitatory activity is soon terminated by acetylcholinesterase, an enzyme in the synaptic cleft that hydrolyzes the neurotransmitter

Graded potential

Shift in membrane potential due to response to hyper polarization or depolarization. Has a magnitude that varies with the strength of the stimulus: a larger stimulus causes a greater change in the membrane potential. Graded potentials induce a small electrical current that leaks out of the neuron as it flows along the membrane. Graded potentials thus decay with time and with distance from their source

Sodium-potassium pump

The Na+ and K+ gradients are maintained by this pump; uses the energy of ATP hydrolysis to actively transport Na+ out of the cell and K+ into the cell Transports three Na+ out of the cell for every two K+ that it transport in Pumping generates a net export of positive charge but resulting voltage different is only a few millivolts

The arrival of an action potential at a synaptic terminal depolarizes the plasma membrane, opening voltage gated channels that allow Ca2+ to diffuse into the terminal

The resulting rise in Ca2+ concentration in the terminal causes some of the synaptic vesicles to fuse with then terminal membrane, releasing the neurotransmitter

Ligand-gated ion channel

These receptors are clustered in the membrane of the postsynaptic cell, directly opposite the synaptic terminal. binding of the neurotransmitter (the receptor's ligand) to a particular part of the receptor opens the channel and allows specific ions to diffuse across the postsynaptic membrane. The result is a postsynaptic potential, a graded potential in the postsynaptic cell

Ion channels that convert this chemical potential energy to electrical potential energy can do so because:

They have selective permeability, allowing only certain ions to pass

Because Na+ and other ions can't readily cross the membrane, K+ outflow leads to a net negative charge inside the cell

This buildup of negative charge within the neuron is the major source of the membrane potential

Temporal summation

When EPSP add together due to the postsynaptic neuron's membrane potential not returning to the resting potential before the arrival of the second EPSP