Mastering Biology Chapter 48

leak channels

Diffusion of K+K+ through potassium channels that are always open (sometimes called leak channels) is critical for establishing the resting potential Furthermore, a resting neuron has many open potassium channels, but very few open sodium channels. Because Na+Na+ and other ions can't readily cross the membrane, K+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.

what distinguishes the type of information being transmitted?

All neurons transmit electrical signals within the cell in an identical manner. Thus a neuron that detects an odor transmits information along its length in the same way as a neuron that controls the movement of a body part. The particular connections made by the active neuron are what distinguish the type of information being transmitted. Interpreting nerve impulses therefore involves sorting neuronal paths and connections.

excitatory postsynaptic potential (EPSP)

An electrical change (depolarization) in the membrane of a postsynaptic cell caused by the binding of an excitatory neurotransmitter from a presynaptic cell to a postsynaptic receptor; makes it more likely for a postsynaptic cell to generate an action potential. At some chemical synapses, the ligand-gated ion channels are permeable to both K+K+ and Na+Na+ (see Figure 48.16). When these channels open, the membrane potential depolarizes toward a value roughly midway between EKEK and ENaENa. Because such a depolarization brings the membrane potential toward threshold, it is called an excitatory postsynaptic potential (EPSP).

inhibitory postsynaptic potential (IPSP)

An electrical change (usually hyperpolarization) in the membrane of a postsynaptic neuron caused by the binding of an inhibitory neurotransmitter from a presynaptic cell to a postsynaptic receptor; makes it more difficult for a postsynaptic neuron to generate an action potential. At other chemical synapses, the ligand-gated ion channels are selectively permeable for only K+K+ or Cl−Cl−. When such channels open, the postsynaptic membrane hyperpolarizes. A hyperpolarization produced in this manner is an inhibitory postsynaptic potential (IPSP) because it moves the membrane potential farther from threshold.

graded potential

An electrical response of a cell to a stimulus, consisting of a change in voltage across the membrane proportional to the stimulus strength. Sometimes, the response to hyperpolarization or depolarization is simply a shift in the membrane potential. This shift, called a graded potential, has a magnitude that varies with the strength of the stimulus: A larger stimulus causes a greater change in the membrane potential (​see Figure 48.11a and 48.11b). Graded potentials induce​ a small electrical current that dissipates as it flows along the membrane. Graded potentials thus decay with time and with distance from their source.

endorphins

Any of several hormones produced in the brain and anterior pituitary that inhibit pain perception. Endorphins are produced in the brain during times of physical or emotional stress, such as childbirth. In addition to relieving pain, they reduce urine output, decrease respiration, and produce euphoria as well as other emotional effects.

postsynaptic potential

At many chemical synapses, the receptor protein that binds and responds to neurotransmitters is a ligand-gated ion channel, often called an ionotropic receptor. 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.

glia (glial cells)

Cells of the nervous system that support, regulate, and augment the functions of neurons.

How does a stimulus alter the membrane potential?

Certain ion channels in a neuron, called gated ion channels, open or close in response to stimuli. When a gated ion channel opens or closes, it alters the membrane's permeability to particular ions (Figure 48.10). The result is a rapid flow of ions across the membrane, altering the membrane potential.

Nernst equation for human body temperature

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

Let's consider now what happens in a neuron when a stimulus causes closed voltage-gated ion channels to open:

If gated potassium channels in a resting neuron open, the membrane's permeability to K+K+ increases. As a result, net diffusion of K+K+ out of the neuron increases, shifting the membrane potential toward Ek(−90 mv at 37°C)Ek(-90 mv at 37°C). This increase in the magnitude of the membrane potential, called a hyperpolarization, makes the inside of the membrane more negative this is HYPERPOLARIZATION

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, the sodium channels remain inactivated, making the membrane temporarily refractory (not responsive) to further input. Consequently, the inward current that depolarizes the axon membrane ahead of the action potential cannot produce another action potential behind it. This is why action potentials do not travel back toward the cell body.

Some vertebrate neurons release dissolved gases as neurotransmitters.

In human males, for example, certain neurons release nitric oxide (NO) into the erectile tissue of the penis during sexual arousal. The resulting relaxation of smooth muscle in the blood vessel walls of the spongy erectile tissue allows the tissue to fill with blood, producing an erection. The erectile dysfunction drug Viagra works by inhibiting an enzyme that terminates the action of NO.

membrane potential

In neurons, as in other cells, ions are unequally distributed between the interior of cells and the surrounding fluid. As a result, the inside of a cell is negatively charged relative to the outside. This charge difference, or voltage, across the plasma membrane is calle​d the membrane potential, reflecting​ the fact that the attraction of opposite charges across the plasma membrane is a source of potential energy.

synaptic cleft

Neurotransmitter released from the synaptic terminus diffuses across the synaptic cleft, the gap that separates the presynaptic neuron from the postsynaptic cell. Diffusion time is very short because the gap is less than 50 nm across. Upon reaching the postsynaptic membrane, the neurotransmitter binds to and activates a specific receptor in the membrane.

Information processing by a nervous system occurs in three stages:

sensory input, integration, and motor output.

spatial summation

Summation can also involve multiple synapses on the same postsynaptic neuron. If such synapses are active at the same time, the resulting EPSPs can add together through spatial summation

2 types of synapses

Synapses are either electrical or chemical.

resting potential and what is it for a resting neuron?

The membrane potential characteristic of a nonconducting excitable cell, with the inside of the cell more negative than the outside. For a resting neuron—one that is not sending a signal—the membrane potential is called the resting potential and is typically between −60 and −80 millivolts (mV).​ -70 mV is what ppl usually say

cell body

The part of a neuron that houses the nucleus and most other organelles. Most of ​a neuron's organelles, including its nucleus, are located in the cell body. In a typical neuron, the cell body is studded with numerous highly branched extensions called dendrites (from the ​Greek dendron, tree). ​Together with th​e cell body, the dendrites receive signals from other neurons.​

synaptic terminal

The pa​rt of each axon branch that forms this specialized ju​nction is a synaptic terminal. At m​ost synapses, chemical messengers called ​neurotransmitters​ pass information from the transmitting neuron to the receiving cell. Cone snail venom is particularly potent because it interferes not only with electrical signaling along axons but also with chemical signaling across synapses.

Central Nervous System (CNS)

The portion of the nervous system where signal integration occurs; in vertebrate animals, the brain and spinal cord. In many animals, the neurons that carry out sorting, processing, and integration are organized in a central nervous system (CNS), which may include a brain or simpler clusters called ganglia.

threshold

The potential that an excitable cell membrane must reach for an action potential to be initiated.

sodium-potassium pump.

This pump uses the energy of ATP hydrolysis to actively transport 3 Na+ out of the cell and 2 K+ into the cell. Although this pumpi​ng generates a net export of positive charge, the pump acts slowly. The resulting change in the membrane potential is therefore quite small—only a few millivolts.

Summation applies as well to IPSPs:

Two or more IPSPs occurring nearly simultaneously at synapses in the same region or in rapid succession at the same synapse have a larger hyperpolarizing effect than a single IPSP. Through summation, an IPSP can also counter the effect of an EPSP

Action potentials are the signals conducted by axons

When a neuron responds to a stimulus, the membrane potential changes. Using intracellular recording, researchers can monitor these changes as a function of time. As you will see, such recordings have been central to the study of information transfer by neurons.

Electrical synapses

contain gap junction​s that ​allow electrical current to flow directly from one neuron to another. Such synapses often play a role in synchronizing the activity of neurons that direct rapid, unvarying behaviors. For example, electrical synapses associated with the giant axons of squids and lobsters facilitate swift escapes from danger. Electrical synapses are also found in the vertebrate heart and brain.

Information transfer at chemical synapses can be modified by altering either what or what?

either the amount of neurotransmitter that is released or the responsiveness of the postsynaptic cell. Such modifications underlie an animal's ability to alter its behavior in response to change and also form the basis for learning and memory.

Interneurons

form the local circuits connecting neurons in the brain or ganglia. Interneurons are responsible for the integration (analysis and interpretation) of sensory input.

what are myelin sheaths produced by?

glia Myelin sheath​​​​s are produced by glia: oligodendrocytes in the CNS and Schwann cells in the PNS. During development, these specialized glia wrap axons in many layers of membrane. The membranes forming these layers are mostly lipid, which is a poor conductor of electrical current and thus a good insulator.

Action potentials propagate more rapidly in myelinated axons or unmyelinated axons?

in myelinated axons because the time-consuming process of opening and closing of ion channels occurs at only a limited number of positions along the axon.

what happens when a neuron receives a stimulus?

the membrane potential changes. Rapid shifts in membrane potential are what enable us to see the intricate structure of a spiderweb, hear a song, or ride a bicycle. These changes, which are known as action potentials, will be discussed​ in concept 48.3. To un​derstand how they convey information, we need to explore the ways in which membrane potentials are formed, maintained, and altered.

Motor neurons

transmit signals to muscle cells, causing them to contract. Additional neurons that extend out of the processing centers trigger gland activity.

The major selective advantage of myelination is what?

its space efficiency. A myelinated axon 20 µm in diameter has a conduction speed faster than that of a squid giant axon with a diameter 40 times greater. Consequently, more than 2,000 of those myelinated axons can be packed into the space occupied by just one giant axon.

Sensory neurons

like those in the snail's siphon, transmit information about external stimuli (such as light, touch, or smell), and internal conditions (such as blood pressure or muscle tension).

acetylcholine

One of the most common neurotransmitters; functions by binding to receptors and altering the permeability of the postsynaptic membrane to specific ions, either depolarizing or hyperpolarizing the membrane. Indeed, a particular neurotransmitter can excite postsynaptic cells expressing one receptor and inhibit postsynaptic cells expressing a different receptor.

dendrites

One of usually numerous, short, highly branched extensions of a neuron that receive signals from other neurons. Together with th​e cell body, the dendrites receive signals from other neurons.​

brain

Organ of the central nervous system where information is processed and integrated.

After a response is triggered, the chemical synapse returns to its resting state. How does this happen?

- The key step is clearing the neurotransmitter molecules from the synaptic cleft. - Some neurotransmitters are inactivated by enzymatic hydrolysis -Other neurotransmitters are recaptured into the presynaptic neuron. - After this reuptake occurs, neurotransmitters are repackaged in synaptic vesicles or transferred to glia for metabolism or recycling to neurons. - Clearing neurotransmitter from the synaptic cleft is an essential step in the transmission of information through the nervous system. - Indeed, blocking this process can have severe consequences. For example, the nerve gas sarin triggers paralysis and death because it inhibits the enzyme that breaks down the neurotransmitter controlling skeletal muscles.

Evolutionary Adaptations of Axon Structure

- The rate at which the axons within nerves conduct action potentials governs how rapidly an animal can react to danger or opportunity. As a consequence, natural selection often results in anatomical adaptations that increase conduction speed. - One such adaptation is a wider axon. In the same way that a wide hose offers less resistance to the flow of water than does a narrow hose, a wide axon provides less resistance to the current associated with an action potential than does a narrow axon. - The evolutionary adaptation that enables fast conduction in vertebrate axons is electrical insulation, analogous to the plastic insulation that encases many electrical wires. 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.

What is the equilibrium potential for K+?

-90mV. The minus sign indicates that K+K+ is at equilibrium when the inside of the membrane is 90 mV more negative than the outside. At this potential, the cell is overly negative, there is no more K gradient, it won't allow anymore K+ to leave outside the cell.

hyperpolarization

A change in a cell's membrane potential such that the inside of the membrane becomes more negative relative to the outside. Hyperpolarization reduces the chance that a neuron will transmit a nerve impulse.

Depolarization

A change in a cell's membrane potential such that the inside of the membrane is made less negative relative to the outside. For example, a neuron membrane is depolarized if a stimulus decreases its voltage from the resting potential of −70 mV in the direction of zero voltage. In neurons, depolarization often involves gated sodium channels. If a stimulus causes gated sodium channels to open, the membrane's permeability to Na+Na+ increases. Na+Na+ diffuses into the cell along its concentration gradient, causing a depolarization as the membrane potential shifts toward ENaENa (+62 mV at 37°C). Although opening potassium channels in a resting neuron causes hyperpolarization, opening some other types of ion channels has an opposite effect, making the inside of the membrane less negative

nerves

A fiber composed primarily of the bundled axons of neurons. the shape of a neuron can vary from simple to quite complex, depending on its role in information processing. Neurons that have highly branched dendrites, such as some interneurons, can receive input through tens of thousands of synapses. Similarly, neurons that transmit information to many target cells do so through highly branched axons. When grouped together, the axons of neurons form the bundles we call nerves.

gated ion channels

A gated channel for a specific ion. The opening or closing of such channels may alter a cell's membrane potential.

neuron

A nerve cell; the fundamental unit of the nervous system, having structure and properties that allow it to conduct signals by taking advantage of the electrical charge across its plasma membrane.

Norepinephrine

A neurotransmitter involved in arousal, as well as in learning and mood regulation G protein-coupled receptors modulate the responsiveness and activity of postsynaptic neurons in diverse ways. Binding of norepinephrine to its G protein-coupled receptor activates a G protein, which in turn activates adenylyl cyclase, the enzyme that converts ATP to cAMP​ Cyclic AMP ​activates protein kinase A, which phosphorylates specific ion channel proteins in the postsynaptic membrane, causing them to open or close. Because of the amplifying effect of the signal transduction pathway, the binding of one norepinephrine molecule can trigger the opening or closing of many channels.

refractory period

A period, immediately following a response to stimulation, during which a cell or organ is unresponsive to further stimulation. The "downtime" when a second action potential cannot be initiated is called the refractory period. Note that the refractory period is due to the inactivation of sodium channels, not to a change in the ion concentration gradients across the plasma membrane. The flow of charged particles during an action potential involves far too few ions to change the concentration on either side of the membrane significantly.

summation

A phenomenon of neural integrationin which the membrane potential of the postsynaptic cell is determined by the combined effect of EPSPs or IPSPs produced in rapid succession at one synapse or simultaneously at different synapses. For instance, two EPSPs may occur at a single synapse in rapid succession. If the second EPSP arises before the postsynaptic membrane potential returns to its resting value, the EPSPs add together through temporal summation. If the summed postsynaptic potentials depolarize the membrane at the axon hillock to threshold, the result is an action potential

Neuropeptides

A relatively short chain of amino acids that serves as a neurotransmitter. operate via G protein-coupled receptors

voltage-gated ion channel

A specialized ion channel that opens or closes in response to changes in membrane potential.

ligand-gated ion channel (ionotropic receptor)

A transmembrane protein containing a pore that opens or closes as it changes shape in response to a signaling molecule (ligand), allowing or blocking the flow of specific ions; also called an ionotropic receptor.

oligodendrocytes

A type of glial cell that forms insulating myelin sheaths around the axons of neurons in the central nervous system.

Schwann cells

A type of glial cell that forms insulating myelin sheaths around the axons of neurons in the peripheral nervous system.

axon

A typically long extension, or process, of a neuron that carries nerve impulses away from the cell body toward target cells. the extens​ion that transmits signals to​ other cells. Axons are often much longer than dendrites, and some, such as those that reach from the spinal cord of a giraffe to the muscle cells in its feet, are over a meter long. The specialized structure of axons allows them to use pulses of electrical current to transmit information, even over long distances. The cone-shaped base of an axon, called the axon hillock, is typically where signals that travel down the axon are generated. Near its other end, an axon usually divides into many branches.

Why is inactivation of channels required during an action potential?

Because they are voltage gated, the sodium channels open when the membrane potential reaches the threshold of −55 mV−55 mV and don't close until the resting potential is restored. They are therefore open throughout the action potential. However, the resting potential cannot be restored unless the flow of Na+Na+ into the cell stops. This is accomplished by inactivation. The sodium channels remain in the "open" state, but sodium ions cease flowing once inactivation occurs. The end of Na+Na+ inflow allows K+K+ outflow to repolarize the membrane. The sodium channels remain inactivated during the falling phase and the early part of the undershoot. As a result, if a second depolarizing stimulus occurs during this period, it will be unable to trigger an action potential.

myotonia and epilepsy

For example, mutations affecting voltage-gated sodium channels in skeletal muscle cells can cause myotonia, a periodic spasming of those muscles. Mutations affecting sodium channels in the brain can cause epilepsy, in which groups of nerve cells fire simultaneously and excessively, producing seizures.

Nodes of Ranvier

Gap in the myelin sheath of certain axons where an action potential may be generated. In saltatory conduction, an action potential is regenerated at each node, appearing to "jump" along the axon from node to node. In myelinated axons, voltage-gated sodium channels are restricted to gaps in the myelin sheath called nodes of Ranvier (see Figure 48.14). Furthermore, the extracellular fluid is in contact with the axon membrane only at the nodes. As a result, action potentials are not generated in the regions between the nodes. 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

G protein-coupled

However, there are also chemical synapses in which the receptor for the neurotransmitter is not part of an ion channel. At these synapses, the neurotransmitter binds to a G protein-coupled receptor, activating a signal transduction pathway in the postsynaptic cell involving a second messen​ger Becau​se the resulting opening or closing of ion channels depends on one or more metabolic steps, these G protein-coupled receptors are also called metabotropic receptors.

Positive feedback loop of depolarization

If a depolarization increases the membrane potential to a level called threshold, the voltage-gated sodium channels open. The resulting flow of Na+Na+ into the neuron results in further depolarization. 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

action potential​

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. Action potentials arise because some of the ion channels in neurons are voltage-gated.

saltatory conduction

Rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane. Action potentials propagate more rapidly in myelinated axons because the time-consuming process of opening and closing of ion channels occurs at only a limited number of positions along the axon. This mechanism for propagating action potentials is called saltatory conduction (from t​he Latin saltare, to leap) beca​use the action potential appears to jump from node to node along the axon.

Why, then, is there a membrane potential of −60 to −80 mV in a resting neuron if the NaK pump is so slow?

The answer lies in ion movement through ​ion channels​, pores formed by clusters of specialized proteins that span the membrane. Ion channels allow ions to diffuse back and forth across the membrane. As ions diffuse through channels, they carry with them units of electrical charge. Furthermore, ions can move quite rapidly through ion channels. When this occurs, the resulting current—a net movem​ent of positive or ​negative charge—generates a membrane potential, or voltage across the membrane.

integrating center

The axon hillock is the neuron's integrating center, the region where the membrane potential at any instant represents the summed effect of all EPSPs and IPSPs. Whenever the membrane potential at the axon hillock reaches threshold, an action potential is generated and travels along the axon to its synaptic terminals. After the refractory period, the neuron may produce another action potential, provided the membrane potential at the axon hillock once again reaches threshold.

axon hillock

The cone-shaped base of an axon, called the axon hillock, is typically where signals that travel down the axon are generated. Near its other end, an axon usually divides into many branches.

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+

synapse

The junction where a neuron communicates with another cell across a narrow gap via a neurotransmitter or an electrical coupling.

equilibrium potential

The magnitude of a cell's membrane voltage at equilibrium; calculated using the Nernst equation. When our model neuron reaches equilibrium, the electrical gradient will exactly balance the chemical gradient so that no further net diffusion of K+K+occurs across the membrane. The magnitude of the membrane voltage at equilibrium for a particular ion is called that ion's equilibrium potential (EionEion).

chemical synapses

The majority of synapses are chemical synapses, which rely on the release of a chemical neurotransmitter by the presynaptic neuron to transfer information to the target cell. While at rest, the presynaptic neuron synthesizes the neurotransmitter at each synaptic terminal, packaging it in multiple membrane-enclosed compartments called synaptic vesicles. When an action potential arrives at a chemical synapse, it depolarizes the plasma membrane at the synaptic terminal, opening voltage-gated channels that allow Ca2+Ca2+ to diffuse in. The Ca2+Ca2+ concentration in the terminal rises, causing synaptic vesicles to fuse with the terminal membrane and release the neurotransmitter.

The frequency of action potentials conveys information:

The rate at which action potentials are produced in a particular neuron is proportional to input signal strength. In hearing, for example, louder sounds result in more frequent action potentials in neurons connecting the ear to the brain. Similarly, increased frequency of action potentials in a neuron that stimulates skeletal muscle tissue will increase the tension in the contracting muscle. Differences in the number of action potentials in a given time are in fact the only variable in how information is encoded and transmitted along an axon.

peripheral nervous system (PNS)

The sensory and motor neurons that connect to the central nervous system. The neurons that carry information into and out of the CNS constitute the peripheral nervous system (PNS). Neurons of both the CNS and PNS require supporting cells called glial cells, or glia (from a Greek word meaning "glue")

myelin sheath

Wrapped around the axon of a neuron, an insulating coat of cell membranes from Schwann cells or oligodendrocytes. It is interrupted by nodes of Ranvier, where action potentials are generated.

Ganglia

a cluster (functional group) of nerve cell bodies

what is the threshold for many mamals?

−55 mV The positive-feedback loop of channel opening and depolarization triggers an action potential whenever the membrane potential reaches threshold, about −55 mV for many mammals. 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 stimuli.