Chapter 48: Neurons, Synapses, and Signaling

48.1: Neuron organization & structure reflect function & information transferLines of Communication

Neurons = nerve cells that transfer info within the bodyo Venom disrupts neuronal control of locomotion and respirationo Communication consists of long-distance electrical signals and short-

distance chemical signalso Structure lets neurons use pulses of electrical current to receive,

transmit, and regulate flow of info over long distances within the body Identity of type of info transmitted is encoded by connections made

o Interpreting signals involves sorting neuronal paths and connectionso Brain carries out higher-order processing in groups of neuronso Ganglia = simpler clusters of neurons

48.1: Neuron organization & structure reflect function in information transferIntroduction to Information Processing

Stages of information processing:1. Sensory input2. Integration3. Motor output

Central nervous system (CNS) organizes neurons that do integration

o Includes brain and nerve cord Peripheral nervous system (PNS) = neurons

that carry info in and out of CNS Nerves = neurons bundled together Types of neurons

o Sensory neurons transmit info from eyes and other sensors of internal and external info to processing centers in brain/ganglia

Neurons in brain/ganglia integrate (analyze/interpret) input

o Interneurons = neurons in the brain, form local circuits connecting neurons in the brain

o Motor neurons transmit signals to muscle cells causing contraction

Neuron Structure and Function Most neuron organelles are located in cell body Dendrites receive signals from other neurons Axon = extension that transmits signals to

other cellso Longer than dendrites

o Axon hillock = base where signals that travel down are generated

Synapse = branched end of axon that transmits info to other cell at junction

o Synaptic terminal = part of axon that forms specialized junction

o Neurotransmitters = chemical messengers that pass info from transmitting neuron to receiving cell

o Presynaptic cell = transmitting neurono Postsynaptic cell = cell that receives

the signal Glial cells = supporting cells that nourish neurons, insulate axons of neurons,

and regulate extracellular fluid surrounding neuronso Outnumber neurons 10-50-fold

48.2: Ion pumps and ion channels establish the resting potential of a neuronFormation of Resting Potential

Ions are unequally distributed between interior of cell and surrounding fluid

o Inside is negatively charged relative to outsideo Opposite charges = source of potential energy

Membrane potential = charge difference Resting potential = membrane potential of resting

neuron, usually between –60 and –80 mV Potassium ions (K+) and sodium ions (Na+) help form

resting potentialo Each ion type has a concentration gradient

across plasma membraneo In mammals: K+ high outside, Na+ high inside

Sodium-potassium pumps maintain Na+ and K+ gradients

o Use ATP hydrolysis energy to actively transport Na+ out & K+ in cell

o Transports three Na+ out of cell for every two K+ ions in

Ion channels = pores formed by clusters of specialized proteins that span the membrane and allow for ion movement

o Causes voltage differenceo Allows ions to diffuse through channels, carrying with them units of

electrical charge net movement generates membrane potentialo Selective permeability = only allow certain ions to passo Diffusion of K+ through open channels leads to net negative charge

inside cell buildup = source of membrane potential Concentration gradients represent chemical form of potential energy

Modeling the Resting Potential

Flow of K+ out of neuron proceeds until chemical & electrical forces balance Equilibrium potential (E[ion]) = magnitude of membrane voltage at

equilibrium for a particular ion, calculated by Nernst equation

o E[ion]=62mV (log[ion ]outside[ ion ]inside

)

Neither K+ nor Na+ is at equilibrium in resting neuron each ion has net flow (current) across membrane

o Resting potential remains steady K+ and Na+ currents are equal and opposite

o Ion concentrations also remain steady

48.3: Action potentials are the signals conducted by axonsHyperpolarization and Depolarization

Membrane potential of a neuron changes in response to variety of stimulio Record and graph changes as a function of timeo Changes occur because gated ion channels open/close in response to

stimuli, altering membrane permeability to certain ions Opening potassium channels increases membrane permeability to K+

o K+ net diffusion out of neuron increases shifting membrane potential toward EK

o Hyperpolarization = increase in magnitude of membrane potential Makes inside of the membrane more negative Results from stimulus that increases outflow of positive ions or

inflow of negative ionso Depolarization = reduction in magnitude of membrane potential

Involves gated sodium channels Results from stimulus causing gated sodium channels in

resting neuron to open increasing permeability to Na+ Diffuses into cell along concentration gradient

Graduated Potentials and Action Potentials Graded potential = shift in membrane potential as a response to

hyperpolarization/depolarizationo Magnitude varies with strength of stimuluso Induce small electrical current that leaks out of neuron as it flows

o Decay with distance from source Action potential = massive change in membrane voltage

o Have constant magnitude and can regenerate in adjacent regions of the membrane can spread along axons over long distances

o Arise because voltage-gated ion channels open and close when membrane potential passes a certain level due to positive feedback

o Occur when depolarization decreases membrane voltage to threshold Once initiated action potential has magnitude independent of

strength of triggering stimulus All-or-none response because depolarization opens channels

causing more depolarizationGeneration of Action Potentials: A Closer Look

Membrane depolarization opens both channels but they respond independently and sequentially

o Na+ ions open first initiating action potentialo As it proceeds Na+ channels become inactive (loop of channel protein

moves blocking ion flow through opening) until after membrane potential finishes

1. When membrane of axon is at resting potential, voltage gated sodium channels are closed

2. Stimulus depolarizes membrane some gated Na+ channels open causing further depolarization more Na+ diffuses back into cell

3. Rising phase = once threshold is crossed positive-feedback cycle brings membrane potential close to Ena

4. Falling phase: voltage gated sodium channels inactivate halting Na+ inflow and voltage-gated K+ channels open

a. Bring falling membrane potential to Ek5. Undershoot = membrane’s permeability to K+ Is higher to test

a. Gated K+ channels close returning to resting potential

Sodium channels remain inactivated during falling phase/early undershoot

If second depolarizing stimulus occurs during this time it won’t trigger an action potential = refractory period (“downtime”)

o Sets limit on max frequency at which action potentials can generate

o Ensures that all signals in axon travel in one direction (cell body terminals)

o Due to inactivation of sodium channels (not change in ion gradients)

o Interval = 1-2 milliseconds Frequency that a neuron generates action potentials

varies by inputo Differences convey info about signal strengtho Only variable in transmission of info by axono Ex: louder sounds = more frequent action

potentials Mutations in genes that encode ion channel proteins

can cause disorderso Type depends on where in the body the gene

is expressedConduction of Action Potentials

At the axon hillock where an action potential is initiated Na+ inflow creates an electrical current that depolarizes neighboring part of axon membrane

o Depolarization is big enough to reach threshold causing action potential to be reinitiated repeated along length of axon

Action potentials only move in one direction, from axon hillock toward synaptic terminal

o Because zone of repolarization caused by K+ outflow follows traveling zone of depolarization

o Current that depolarizes axon membrane ahead of action potential can’t produce another action potential behind it

Evolutionary Adaptations of Axon Structure Axon diameter impacts speed of action potentials

o Increased width = increased speedo Because resistance to electrical current flow is inversely proportional

to cross section area of conductoro Wide axon has less resistance to current with action potential

Myelin sheath = electrical insulation that surrounds vertebrate axono Causes the depolarizing current to spread farther along the axon

sooner, bringing more distant regions to threshold faster o Allow vertebrate axons with narrow diameters to still be fasto Produced by glia cells

Oligodendrocytes in CNS Schwann cells in PNS

o Wrap axons in layers of lipid membraneo Voltage-gated sodium channels are restricted to nodes of Ranvier =

gaps in myelin sheath Extracellular fluid is only in contact with axon membrane here Action potentials are not generated in regions between nodes Time-consuming process of opening/closing ion channels only

occurs at nodes makes it fastero Saltatory conduction = mechanism for action potential propagation

where action potential jumps along axon from node to nodeo Space efficient: makes smaller axons that are mylinated more efficient

43.4: Neurons communicate with other cells at synapsesInfo is transmitted at synapses

Electrical synapses have gap junctions which allow electrical current to flow directly from one neuron to another

o Synchronize neuron activity responsible for rapid unvarying behavior Chemical synapses involve the release of a chemical neurotransmitter by

presynaptic neurono Synthesizes and packages neurotransmitter into membrane bound

synaptic vesicleso Arrival of action potential at synaptic terminal depolarizes plasma

membrane, opening voltage-gated channels allowing Ca2+ to diffuse into terminal

Causes synaptic vesicles to fuse with terminal membrane releasing neurotransmitter

o Neurotransmitter diffuses across synaptic cleft = gap that separates presynaptic neuron from postsynaptic cell

After crossed it binds to and activates receptor in membraneGeneration of Postsynaptic Potentials

Ligand-gated ion channel/ionic receptor = receptor protein that binds and responds to neurotransmitters

o Clustered in postsynaptic cell membrane opposite synaptic terminal

o Binding of neurotransmitter to part of receptor opens channel and allows ions to diffuse across postsynaptic membrane postsynaptic potential = graded potential in postsynaptic cell

Excitatory postsynaptic potential (EPSP) = when a ligand-gated ion channel permeable to both K+ and Na+ opens depolarizing membrane potential

o Moves membrane potential toward threshold Inhibitory postsynaptic potential (IPSP) = when a ligand-gated ion channel

selectively permeable to only K+ or Cl– opens hyperpolarizing postsynaptic membrane

o Moves membrane potential farther from threshold Mechanisms to clear neurotransmitter molecules from synaptic cleft limit

duration of post-synaptic potentialso Some transported back into presynaptic neuron, others repackaged

into synaptic vesicles, transported into glia for fuel, removed by diffusion or enzyme that hydrolyses

Summation of Postsynaptic Potentials Since postsynaptic potential becomes smaller with distance from synapse, a

single ESP is usually too small to trigger and action potential Temporal summation = when two EPSPs occur at a single synapse so close in

time that the membrane potential hasn’t returned to resting potential yet Spatial summation = when

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

Allows several EPSPs to combine to depolarize membrane to produce an action potential

Can also happen with IPSPs to counter the effect of an EPSPModulated Signaling at Synapses

In some synapses the receptor for the neurotransmitter isn’t part of an ion channel neurotransmitter binds to metabotropic receptor

Binding activates a signal transduction pathway in postsynaptic cell involving a second messenger

o Second messenger modulates responsiveness of postsynaptic neurons o Ex: norepinephrine binds to metabolic receptor activating G protein,

activating adenylyl cyclase converting ATP to cAMP activating protein kinase A phosphorylating ion channel proteins making them open/close

Effects are slower but last longerNeurotransmitters

100+ neurotransmitters, belong to 5 groups Response depends on kind of receptor expressed

by postsynaptic cell One neurotransmitter can bind to multiple types of

receptors Acetylcholine involved in muscle stimulation,

memory formation, learningo Ligand-gated ion channel type functions at

neuromuscular junction where motor neurons synapse with skeletal muscle cells

o When released by motor neurons and binds it opens the ion channel producing an EPSP

Acetylcholinesterase = enzyme that terminates activity

o Receptors found in PNS and CNSo Can be inhibited by toxins (ex: Botox)o Found in the heart: activates signal

transduction pathway G proteins inhibit adenylyl cyclase & open potassium channels reducing heart rate

Amino Acid neurotransmitters active in CNS and PNS (senior project!!!)

o Glutamate = most common in CNS Has excitatory effect Involved in long-term memory

o Gamma-aminobutyric acid (GABA) = neurotransmitter at inhibitory synapses in the brain

Binding increases membrane permeability to Cl– IPSP Used to reduce anxiety

o Glycine acts at inhibitory synapses in CNS outside brain Biogenic Amines synthesized from amino acids

o Includes norepinephrine = excitatory neurotransmitter in autonomic nervous system (PNS)

o Dopamine and serotonin released to affect sleep, mood, attention, and learning

Hallucinatory drugs produce effect by binding to receptorso Role in nervous system disorders and treatments

Neuropeptides = short chains of amino acids that operate with metabotropic receptors

o Substance P = key excitatory neurotransmitter that deals with perception of pain

o Endorphins decrease pain perception Produced during times of stress Relieve pain, decrease urine output, depress respiration, and

produce euphoria Gases that are dissolved can be released to act as local regulators

o Nitric oxide (NO) is synthesized on demand Diffuses into target cells, produces change, and is broken down Stimulates an enzyme to synthesize a second messenger that

affects cellular metabolism Released during sexual arousal

o Body produces small amounts of carbon monoxide to act as a neurotransmitter to regulate release of hypothalamic hormones or act as an inhibitor