chapter 48 neurons, synapses and signaling

myelin sheath

-The electrical insulation that surrounds vertebrate axons -which causes an action potential's speed to increase

temporal summation occurs when

If two EPSPs are produced in rapid succession

glia, or glial cells

Most neurons are nourished or insulated by cells

Myelin sheaths are made by

glia— oligodendrocytes in the CNS and Schwann cells in the PNS -The membranes forming these layers are mostly lipid, which is a poor conductor of electrical current and thus a good insulator

Biogenic amines have a central role in a number of

nervous system disorders

presynaptic neuron

neuron that sends the signal

The summed effect of EPSPs and IPSPs determines

whether an axon hillock will reach threshold and generate an action potential

Postsynaptic potentials fall into two categories

-Excitatory postsynaptic potentials (EPSPs)are depolarizations that bring the membrane potential toward threshold a.(depolarization) -Inhibitory postsynaptic potentials (IPSPs)are hyperpolarizations that move the membrane potential farther from threshold (stop a neuron) b. (hyper polarization)

crash course def of action potential

-In order to send long distant signals to move along the axon, strong enough to trigger voltage

membrane potential

-Every cell has a voltage (difference in electrical charge) across its plasma membrane

Graded portential in a neuron can be

-Excitatory -inhibitory

Unlike most neurotransmitters, NO is not stored in

-NO is not stored in cytoplasmic vesicles, but is synthesized on demand -It is broken down within a few seconds of production

endorphins

-Neuropeptides include substance P and endorphins, which both affect our perception of pain

depolarization

-Opening other types of ion channels triggers -reduction in the magnitude of the membrane potential -n involves gated sodium channels. If a stimulus causes gated sodium channels to open, the mem-brane's permeability to Na+ increases. Na+ diffuses into the cell along its concentration gradient, causing a depolarization as the membrane potential shifts toward ENa (+62 mV at 37°C).

Resting potential can be modeled by an artificial membrane that separates two chambers

-The concentration of KCl is higher in the inner chamber and lower in the outer chamber -K+ diffuses down its gradient to the outer chamber -Negative charge (Cl-) builds up in the inner chamber

Termination of Neurotransmitter Signaling: 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 -the chemical synapse returns to its resting state

neurotransmitters

-The synaptic terminal of one axon passes information across the synapse in the form of chemical messengers

what toxins disrupt acetylcholine neurotransmission

-These include the nerve gas sarin and the botulism toxin produced by certain bacteria -Acetylcholine is just one of more than 100 known neurotransmitters -The remainder fall into four classes: amino acids, biogenic amines, neuropeptides, and gases

Termination of Neurotransmitter Signaling: The neurotransmitter molecules are cleared from

-the synaptic cleft -Blocking this process can have severe effects

threshold

If a depolarization increases the membrane potential to a level

At some chemical synapses, the ligand-gated ion chan-nels are permeable to

K+ and Na+ ca+

If a depolarization shifts the membrane potential sufficiently, it results in a massive change in membrane voltage

action potential in which ca+ enters

The combination of EPSPs through spatial and temporal summation can trigger

an action potential

hyperpolarization

an increase in magnitude of the membrane potential -makes the inside of the membrane more negative -Although opening potassium channels in a resting neu-ron causes hyperpolarization, opening some other types of ion channels has an opposite effect, making the inside of the membrane less negative

Mutations in genes that encode ion channels lead to

disorders affecting the nerves or brain—or the muscles or heart

In a resting neuron, the currents of K+ and Na+ar

equal and opposite, and the resting potential across the membrane remains steady

resting neuron

has a slight positive charge on the exterior and negative inside

Unipolar neuron

having or pertaining to one pole process -sensory

dendrites

highly branched extensions that receive signals from other neurons

The cell body of a postsynaptic neuron may receive inputs from

hundreds or thousands of synaptic terminals

As a result, action potentials are not generated in

in the regions between the nodes. Rather, the inward current produced dur-ing the rising phase of the action potential at a node travels within the axon all the way to the next node.

Glycine acts at

inhibitory synapses in parts of the CNS outside the brain

The rate at which action potentials are produced in a neuron is proportional to

input signal strength

Formation of the Resting Potential: the concentration of K+ is higher

inside the cell, while the concentration of more Na+is higher outside the cell

Interneurons

integrate (analyze and interpret) the information -bipolar neuron

Parkinson's disease

is associated with a lack of dopamine in the brain

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+ to diffuse in. The Ca2+ concentration in the terminal rises, causing synaptic vesicles to fuse with the terminal membrane and release the neurotransmitter.

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

Gases such as nitric oxide (NO) are ___ in PNS

local regulators in the PNS

A single neurotransmitter may bind specifically to

more than a dozen different receptors

When gated K+ channels open, K+ diffuses out, making the inside of the cell more

negative

Gated ion channels and action potentials play an important role in

nervous system activity

In myelinated axons, Voltage-gated sodium channels are restricted to

nodes of Ranvier, gaps in the myelin sheath

bipolar neurons

one axon and one dendrite -interneurons

A neuron at resting potential contains many

open K+ channels and fewer open Na+channels; K+ diffuses out of the cell

Neurotransmitter binding causes ion channels to

open, generating a postsynaptic potential

how is Information is transmitted from neurons

presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell)

neuropeptides

relatively short chains of amino acids, also function as neurotransmitters

The neurotransmitter diffuses across the

synaptic cleft and is received by the postsynaptic cell

Conduction of Action Potentials: Inactivated Na+ channels behind the zone of depolarization prevent

the action potential from traveling backwards

Evolutionary Adaptations of Axon Structure: The speed of an action potential increases with

the axon's diameter

Through summation, an IPSP can counter

the effect of an EPSP

central nervous system (CNS)

where integration takes place; this includes the brain and a nerve cord or ganglia

ligand-gated ion channel,

at many chemical synapses the receptor protein that binds and respond to neurotransmitter

Action potentials propagate more rapidly in myelinated axons because

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

Opiates is used to

treat pain is a depressant

Hyperpolarization and Depolarization steps

(a) Graded hyperpolarizations producedby two stimuli that increasemembrane permeability to K+ (b) Graded depolarizations producedby two stimuli that increasemembrane permeability to Na+. (c) Action potential triggered by a depola-rization that reaches the threshold.

The presynaptic neuron does what

- synthesizes the neurotrans-mitter at each synaptic terminal,and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal

Modulated Signaling at Synapses: chemical synapses in which the receptor for the neurotransmitter is not part of an ion channel. At these synapses, the neurotransmitter binds to a

-(metabotropic) to a G protein-coupled receptor, activating a signal transduction pathway in the post-synaptic cell involving a second messenger

Neurons communicate with other cells at synapses At electrical synapses At chemical synapses Most synapses are

-At electrical synapses, the electrical current flows from one neuron to another through gap junctions (cardiac muscle) -At chemical synapses, a chemical neurotransmitter carries information between neurons -Most synapses are chemical synapses

Conduction of Action Potentials: what happens at the site where the action potential is generated

-At the site where the action potential is generated (usually the axon hillock), an electrical current depolarizes the neighboring region of the axon membrane

gated ion channels

-Changes in membrane potential occur because neurons contain -that open or close in response to stimuli

Opiates bind to

-the same receptors as endorphins and can be used as painkillers

The positive-feedback loop of channel opening and depolarization triggers an -what happens once initiated

-action potential whenever the mem-brane potential reaches threshold, about -55 mV for many mammals. -the action potential has a mag-nitude 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

Steps to a stimulus such as a spider crawling on your leg

-activate sensory neurons (unipolar neurons) -travels up axon of schanw cells -reaches spinal cord passed to motor neurons (multi polar neurons) -trigging you to kick it off -then it travels to your brain

refractory period result of after complete

-after an action potential, a second action potential cannot be initiated -The refractory period is a result of a temporary inactivation of the Na+ channels -After the refractory period is complete, depolarization of the axon hillock to threshold will trigger a new action poten-tial. In many neurons, action potentials last less than 2 mil-liseconds (msec), and the firing rate can thus reach hundreds of action potentials per second.

Graded potentials

-are changes in polarization where the magnitude of the change varies with the strength of the stimulus -Graded potentials induce a small elec-trical current that dissipates as it flows along the membrane. Graded potentials thus decay with time and with distance from their source.

Neurons Neurons use two types of signals to communicate

-are nerve cells that transfer information within the body -electrical signals (long-distance) and chemical signals (short-distance)

Generation of Action Potentials: Depolarization opens what what channels open first what blocks it? potassium channels remain

-both types of channels, but they respond independently and sequentially. -Sodium channels open first, initiatingg the action potential @-55 mv As the action potential proceeds, sodium channels remain open but become inactivated: @30mv a. A portion of the channel protein called an inactivation loop blocks ion flow through the open channel. -Sodium channels remain inactivated until after 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.

When an action potential arrives at a chemical synapse,

-depolarizes the plasma membrane at the synaptic terminal, opening voltage-gated channels that allow Ca2+ to diffuse in. The Ca2+ concentration in the terminal rises, causing synaptic vesicles to fuse with the terminal membrane and release the neurotransmitter.

ganglia

-in a central nervous system (CNS), which may include a brain or simpler clus-ters called ganglia. -Processing of information takes place in simple clusters of neurons or a more complex organization of neurons called a brain

Biogenic amines

-include norepinephrine, epinephrine, dopamine, and serotonin

Acetylcholine

-is a common neurotransmitter in vertebrates and invertebrates -muscle stimulation formation, and learning

Glutamate

-is one of several amino acids that can act as a neurotransmitter, in vertebrates and invertebrates

resting potential

-is the membrane potential of a neuron not sending signal - reflecting the fact that the attrac-tion of opposite charges across the plasma membrane is a source of potential energy. For a resting neuron—one that is not sending a signal

Gamma-aminobutyric acid (GABA)

-is the neurotransmitter at most inhibitory synapses in the brain

axon The cone-shaped base of an axon is called

-is typically a much longer extension that transmits signals to other cells at synapses -axon hillock

Generation of Action Potentials: An action potential results from changes in

-membrane potential as ions move through voltage-gated channels -At resting potential 1.Most voltage-gated sodium (Na+) and potassium (K+) channels are closed -When an action potential is generated, 2.Voltage-gated Na+ channels open first, and Na+ flows into the cell 3.During the rising phase, the threshold is crossed, and the membrane potential increases During the falling phase, voltage-gated Na+channels become inactivated; voltage-gated K+channels open, and K+flows out of the cell 5.During the undershoot, membrane permeability to K+ is at first higher than at rest, then voltage-gated K+ channels close and resting potential is restored

Graded potential: inhibitory steps

-moves away from threshold by -neuron transmitters bind to a receptor on a chemically gated channel -gate opens and allows rush K out a neuron

Graded potential: Excitatory steps

-moves towards threshold depolarization By -neuro transmitter will bind to a receptor on chemically-gated channels - gate will open and let in NA deep

Vertebrates have two major classes of acetylcholine receptor

-one that is ligand gated and one that is metabotropic

Voltage-gated ion channels

-open or close in response to a change in voltage across the plasma membrane of the neuron

Termination of Neurotransmitter Signaling: The nerve gas sarin triggers

-paralysis and death due to inhibition of the enzyme that breaks down the neurotransmitter controlling skeletal muscles

what are the three steps to crate a signal in a neuron

-resting potential (-70mv) -graded potential (-55mv) trigger all or nun action potential -action potential

crash course action potential steps

-resting state a. stimulus occurs which trigger the sodium channels open -depolarization a.voltage gates sodium ions open positive sodium ions rush in -reploarization a. voltaged gated potassium ions open letting them out -hyperpolarization a. voltage drops negative closes channels

Nervous systems process information in three

-sensory input, integration, and motor output -example: a. The part of each axon branch that forms this specialized junction b.During the integration stage, networks of neurons in the snail brain process this information to determine if a fish is in fact pres-ent and, if so, where the fish is located c.During the integration stage, networks of neurons in the snail brain process this information to determine if a fish is in fact pres-ent and, if so, where the fish is located

A single EPSP is usually too small to why?

-trigger an action potential in a postsynaptic neuron -consider an EPSP arising at a single synapse. As a graded potential, the EPSP becomes smaller as it spreads from the synapse. Therefore, by the time a particular EPSP reaches the axon hillock, it is usually too small to trigger an action poten-tial

the sodium channels are what is the result of this process

-voltage gated, the increased depolarization causes more sodium channels to open, leading to an even greater flow of current. -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

peripheral nervous system (PNS)

-which carries information into and out of the CNS -The neurons of the PNS, when bundled together, form nerves

Conduction of an action potential. steps

1. An action potential is generated as Na+ flows inward across the membrane in one region 2.The depolarization of the action potential spreads to the neighboring region of the membrane, reinitiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward. 3.The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axo

what are the steps of Binding of a neurotransmitter to a metabotropic receptor activates

1. G protein-coupled receptor activates a G pro-tein, which in turn activates adenylyl cyclase the enzyme that converts ATP to cAMP 2.Cyclic AMP activates protein kinase A 3.which phosphorylates specific ion channel proteins in the postsynaptic membrane, caus-ing them to open or close.

Generation of Action Potentials: states

1. resting state 2.Depolarization 3.Rising phase of the action potential 4.Falling phase of the action potential 5.undershoot

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 (not shown) 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 the 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 outsid 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 close, 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

A chemical synapse steps

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 this example, binding triggers opening, allowing Na+ and K+ to diffuse through.

multipolar neuron

A neuron with a single axon and multiple dendrites; the most common type of neuron in the nervous system. -motor

Sodium-potassium pumps use the energy of

ATP to maintain these K+ and Na+ gradients across the plasma membrane -pumps in 2 potassium and three sodium

ligand-gated ion channels

Direct synaptic transmission involves binding of neurotransmitters in the postsynaptic cell

In spatial summation

EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together

synapse

Each branched end of an axon transmits information to another cell at a junction called a

summation

Individual postsynaptic potentials can combine to produce a larger potential

cell body

Most of a neuron's organelles are in

synaptic terminal

The part of each axon branch that forms this specialized junction

Changes in membrane potential

are called action potentials

Although inhaling CO can

be deadly, the vertebrate body synthesizes small amounts of it, some of which is used as a neurotransmitter

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 concentration ion channels gradients represent

chemical potential energy

the opening of ion channels in the plasma membrane converts

chemical potential to electrical potential

The resulting buildup of negative charge within the neuron is

the major source of membrane potential

postsynaptic neuron

the neuron on the receiving end of the synapse

The action potential causes the release of

the neurotransmitter

Action potentials in myelinated axons jump between

the nodes of Ranvier in a process called saltatory conduction

Neurotransmitter released from the synaptic terminus diffuses across

the synaptic cleft, the gap that separates the presynaptic neuron from the postsynaptic cell.

Conduction of Action Potentials: Action potentials travel in only one direction

toward the synaptic terminals

Sensory neurons

transmit information about external stimuli such as light, touch, or smell -unipolar

Motor neurons

transmit signals to muscle cells, causing them to contract -multipolar

action potential occurs where ?

trigger zone through axon