© 2013 pearson education, inc. figure 11.16 synapses. axodendritic synapses dendrites cell body...

© 2013 Pearson Education, Inc.

Figure 11.16 Synapses.

Axodendriticsynapses

Dendrites

Cell body

Axoaxonalsynapses

Axon

Axosomaticsynapses

Axon

Axosomaticsynapses

Cell body (soma)of postsynaptic neuron


Varieties of Synapses: Electrical Synapses

• Less common than chemical– Electrically coupled (joined by gap junctions

that connect cytoplasm of adjacent neurons)• Very rapid• Unidirectional or bidirectional• Synchronize activity

– More abundant in:• Embryonic nervous tissue


Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.

Presynapticneuron

Action potentialarrives at axonterminal.

Mitochondrion

Axon terminal

Synapticcleft

Synapticvesicles

Postsynapticneuron

Postsynapticneuron

Presynapticneuron

1



Presynapticneuron


Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.

Mitochondrion

Axon terminal

Synapticcleft

Synapticvesicles

Postsynapticneuron

Postsynapticneuron

Presynapticneuron

1

2



Presynapticneuron


Voltage-gated Ca2+



Ca2+ entrycauses synapticvesicles to releaseneurotransmitterby exocytosis

Mitochondrion

Axon terminal

Synapticcleft

Synapticvesicles

Postsynapticneuron

Postsynapticneuron

Presynapticneuron

1

2

3



Presynapticneuron


Voltage-gated Ca2+



Ca2+ entrycauses synapticvesicles to releaseneurotransmitterby exocytosis

Neurotransmitter diffusesacross the synaptic cleft andbinds to specific receptors onthe postsynaptic membrane.

Mitochondrion

Axon terminal

Synapticcleft

Synapticvesicles

Postsynapticneuron

Postsynapticneuron

Presynapticneuron

1

2

3

4



Ion movement

Graded potential

Binding of neurotransmitter opension channels, resulting in gradedpotentials.

5



Enzymaticdegradation

Diffusion awayfrom synapse

Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse.

Reuptake

6


Synaptic Delay

• Time for NT to be released, diffuse across synapse, and bind to receptors– 0.3–5.0 ms

• Synaptic delay is rate-limiting step of neural transmission


Postsynaptic Potentials

• NT receptors cause graded potentials that vary in strength with:– Amount of NT released– Time nt stays in area


Table 11.2 Comparison of Graded Potentials and Action Potentials (1 of 4)


Postsynaptic Potentials

• Types of postsynaptic potentials – EPSP—excitatory postsynaptic potentials – IPSP—inhibitory postsynaptic potentials


Excitatory Synapses and EPSPs

• NT binding opens chemically gated channels• Allows simultaneous flow of Na+ and K+ in opposite

directions

• Na+ influx greater than K+ efflux net depolarization called EPSP (not AP)

• EPSP help trigger AP if EPSP is of threshold strength– Can spread to axon hillock, trigger opening of

voltage-gated channels, and cause AP to be generated


Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory.

An EPSP is a localdepolarization of the postsynaptic membranethat brings the neuroncloser to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing Na+ and K+ to pass through simultaneously.

Threshold

Stimulus

+30

0

–55

–70

Time (ms)10 20 30

Mem

bra

ne p

ote

nti

al (m

V)

Excitatory postsynaptic potential (EPSP)


Inhibitory Synapses and IPSPs

• Reduces postsynaptic neuron's ability to produce an action potential– Makes membrane more permeable to K+ or

Cl–• If K+ channels open, it moves out of cell• If Cl- channels open, it moves into cell

– Therefore neurotransmitter hyperpolarizes cell• Inner surface of membrane becomes more

negative• AP less likely to be generated


Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory.

Threshold

Stimulus

+30

0

–55

–70

Time (ms)10 20 30

Mem

bra

ne p

ote

nti

al (m

V) An IPSP is a local

hyperpolarization of the postsynaptic membranethat drives the neuronaway from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.

Inhibitory postsynaptic potential (IPSP)


Synaptic Integration: Summation

• A single EPSP cannot induce an AP• EPSPs can summate to influence

postsynaptic neuron• IPSPs can also summate • Most neurons receive both excitatory and

inhibitory inputs from thousands of other neurons– Only if EPSP's predominate and bring to

threshold AP


Two Types of Summation

• Temporal summation– One or more presynaptic neurons transmit

impulses in rapid-fire order• Spatial summation

– Postsynaptic neuron stimulated simultaneously by large number of terminals at same time


Figure 11.19a Neural integration of EPSPs and IPSPs.

Threshold of axon ofpostsynaptic neuron

Resting potential

0

Mem

bra

ne p

ote

nti

al (m

V)

–55–70

E1 E1

E1

No summation:2 stimuli separated in time cause EPSPs that do notadd together.

Excitatory synapse 1 (E1)


Inhibitory synapse (I1)

Time


Figure 11.19b Neural integration of EPSPs and IPSPs.

Threshold of axon ofpostsynaptic neuronResting

potential

0

Mem

bra

ne p

ote

nti

al (m

V)

–55–70

E1




Time

E1 E1

Temporal summation:2 excitatory stimuli closein time cause EPSPsthat add together.


Figure 11.19c Neural integration of EPSPs and IPSPs.


Resting potential

0

Mem

bra

ne p

ote

nti

al (m

V)

–55–70

E1




Time

E1 + E2

Spatial summation:2 simultaneous stimuli atdifferent locations causeEPSPs that add together.

E2


Figure 11.19d Neural integration of EPSPs and IPSPs.


Resting potential

0M

em

bra

ne p

ote

nti

al (m

V)

–55–70

E1




Time

Spatial summation ofEPSPs and IPSPs:Changes in membane potentialcan cancel each other out.

l1

E1 + l1l1


Integration: Synaptic Potentiation

• Repeated synapse increases ability of presynaptic cell to excite postsynaptic neuron– Ca2+ conc increases in presynaptic terminal

and postsynaptic neuron• Brief high-frequency stimulation partially

depolarizes postsynaptic neuron– Chemically gated channels allow Ca2+ entry– Ca2+ promotes more effective responses to

subsequent stimuli


Integration: Presynaptic Inhibition

• Excitatory nt release by one neuron inhibited by another neuron via an axoaxonic synapse

• Less neurotransmitter released• Smaller EPSPs formed


Neurotransmitters

• Language of nervous system• 50 or more nt have been identified• Most neurons make two or more nts

– Neurons can exert several influences• Usually released at different stimulation

frequencies• Classified by chemical structure and by

function


Classification of Neurotransmitters:Chemical Structure

• Acetylcholine (ACh)– First identified; best understood– Released at neuromuscular junctions ,by

some ANS neurons, by some CNS neurons– Synthesized from acetic and choline by

enzyme choline acetyltransferase– Degraded by enzyme acetylcholinesterase

(AChE)


Classification of Neurotransmitters: Chemical Structure • Biogenic amines

• Catecholamines– Dopamine, norepinephrine (NE), and epinephrine– Synthesized from amino acid tyrosine

• Indolamines– Serotonin and histamine– Serotonin synthesized from amino acid tryptophan;

histamine synthesized from amino acid histidine

• Broadly distributed in brain– Play roles in emotional behaviors and biological clock

• Some ANS motor neurons (especially NE)• Imbalances associated with mental illness



• Amino acids• Glutamate• Aspartate• Glycine• GABA—gamma ()-aminobutyric acid



• Peptides (neuropeptides)• Substance P

– Mediator of pain signals

• Endorphins– Beta endorphin, dynorphin and enkephalins– Act as natural opiates; reduce pain perception

• Gut-brain peptides– Somatostatin and cholecystokinin



• Purines – ATP!– Adenosine

• Potent inhibitor in brain• Caffeine blocks adenosine receptors

– Act in both CNS and PNS– Produce fast or slow responses– Induce Ca2+ influx in astrocytes


Classification of Neurotransmitters:Chemical Structure • Gases and lipids - gasotransmitters

• Nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide gases (H2S)

• Bind with G protein–coupled receptors in the brain• Lipid soluble• Synthesized on demand • NO involved in learning and formation of new

memories; brain damage in stroke patients, smooth muscle relaxation in intestine

• H2s acts directly on ion channels to alter function



– Endocannabinoids• Act at same receptors as THC (active ingredient in

marijuana)– Most common G protein-linked receptors in brain

• Lipid soluble• Synthesized on demand• Believed involved in learning and memory• May be involved in neuronal development,

controlling appetite, and suppressing nausea


Classification of Neurotransmitters:Function

• Great diversity of functions• Can classify by

– Effects – excitatory versus inhibitory– Actions – direct versus indirect


Classification of Neurotransmitters:Function

• Effects - excitatory versus inhibitory– Neurotransmitter effects can be excitatory

(depolarizing) and/or inhibitory (hyperpolarizing)

– Effect determined by receptor to which it binds• GABA and glycine usually inhibitory• Glutamate usually excitatory• Acetylcholine and NE bind to at least two receptor

types with opposite effects– ACh excitatory at neuromuscular junctions in skeletal

muscle– ACh inhibitory in cardiac muscle


Classification of Neurotransmitters: Direct versus Indirect Actions

• Direct action – Neurotransmitter binds to and opens ion

channels– Promotes rapid responses by altering

membrane potential – Examples: ACh and amino acids


Classification of Neurotransmitters: Direct versus Indirect Actions

• Indirect action – NT acts through second messengers, usually

G protein pathways– Broader, longer-lasting effects similar to

hormones– Biogenic amines, neuropeptides, and

dissolved gases


Figure 11.21 Indirect neurotransmitter receptor mechanism: G protein-inked receptors.

2

1

3 4

5a

5b

5c

Ligand (1st messenger)

Receptor G protein Enzyme 2ndmessenger

Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race.

Neurotransmitter (1st messenger) binds and activates receptor.

Receptor

G protein

Adenylate cyclase Closed ion channel Open ion channel

GDP

Receptor activates G protein.

G protein activates adenylate cyclase.

Adenylate cyclase converts ATP to cAMP (2nd messenger).

Active enzyme

cAMP changes membrane permeability by opening or closing ion channels.

cAMP activates enzymes.

cAMP activates specific genes.

Nucleus


Channel-Linked (Ionotropic) Receptors: Mechanism of Action

• Ligand-gated ion channels• Action is immediate and brief• Excitatory receptors are channels for small

cations– Na+ influx contributes most to depolarization

• Inhibitory receptors allow Cl– influx that causes hyperpolarization


Basic Concepts of Neural Integration

• Neurons function in groups• Groups contribute to broader neural

functions• There are billions of neurons in CNS

– Must be integration so the individual parts fuse to make a smoothly operating whole


Organization of Neurons: Neuronal Pools

• Functional groups of neurons– Integrate incoming information received from

receptors or other neuronal pools– Forward processed information to other destinations

• Simple neuronal pool– Single presynaptic fiber branches and synapses with

several neurons in pool– Discharge zone—neurons most closely associated

with incoming fiber– Facilitated zone—neurons farther away from

incoming fiber


Presynaptic(input) fiber

Facilitated zone Discharge zone Facilitated zone

Figure 11.22 Simple neuronal pool.


Types of Circuits

• Circuits– Patterns of synaptic connections in neuronal

pools• Four types of circuits

– Diverging– Converging– Reverberating– Parallel after-discharge


Figure 11.23a Types of circuits in neuronal pools.

Input Diverging circuit• One input, many outputs• An amplifying circuit• Example: A single neuron in the brain can activate 100 or more motor neurons in the spinal cord and thousands of skeletal muscle fibers

Many outputs


Figure 11.23b Types of circuits in neuronal pools.

Converging circuit• Many inputs, one output• A concentrating circuit• Example: Different sensory stimuli can all elicit the same memory

Input 1

Input 2 Input 3

Output


Figure 11.23c Types of circuits in neuronal pools.

Reverberating circuit• Signal travels through a chain of neurons, each feeding back to previous neurons• An oscillating circuit• Controls rhythmic activity• Example: Involved in breathing, sleep-wake cycle, and repetitive motor activities such as walking

Output

Input


Figure 11.23d Types of circuits in neuronal pools.

Output

Input Parallel after-discharge circuit• Signal stimulates neurons arranged in parallel arrays that eventually converge on a single output cell• Impulses reach output cell at different times, causing a burst of impulses called an after-discharge• Example: May be involved in exacting mental processes such as mathematical calculations


Patterns of Neural Processing: Serial Processing• Input travels along one pathway to a

specific destination• System works in all-or-none manner to

produce specific, anticipated response• Example – spinal reflexes

– Rapid, automatic responses to stimuli– Particular stimulus always causes same

response– Occur over pathways called reflex arcs

• Five components: receptor, sensory neuron, CNS integration center, motor neuron, effector


Figure 11.24 A simple reflex arc.

Interneuron

Spinal cord (CNS)

Stimulus

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Response

1

2

3

4

5


Patterns of Neural Processing: Parallel Processing

• Input travels along several pathways• Different parts of circuitry deal

simultaneously with the information– One stimulus promotes numerous responses

• Important for higher-level mental functioning

• Example: a sensed smell may remind one of an odor and any associated experiences


Developmental Aspects of Neurons

• Nervous system originates from neural tube and neural crest formed from ectoderm

• The neural tube becomes CNS– Neuroepithelial cells of neural tube proliferate

to form number of cells needed for development

– Neuroblasts become amitotic and migrate– Neuroblasts sprout axons to connect with

targets and become neurons


Axonal Growth: Finding the Target

• Growth cone at tip of axon interacts via:– Cell surface adhesion proteins provide anchor points– Neurotropins that attract or repel the growth cone– Nerve growth factor (NGF) keeps neuroblast alive

• Must find target and right place to form synapse– Astrocytes provide physical support

• About 2/3 of neurons die before birth– If do not form synapse with target– Many cells also die due to apoptosis

(programmed cell death) during development


Figure 11.25 A neuronal growth cone.

© 2013 pearson education, inc. figure 11.16 synapses. axodendritic synapses dendrites cell body...

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