nervous system works because information flows from neuron to neuron

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The Synapse

• Nervous system works because information flows from neuron to neuron

• Neurons functionally connected by synapses– Junctions that mediate information transfer

• From one neuron to another neuron• Or from one neuron to an effector cell


Synapse Classification

• Axodendritic—between axon terminals of one neuron and dendrites of others

• Axosomatic—between axon terminals of one neuron and soma of others


PLAY Animation: Synapses

Important Terminology

• Presynaptic neuron– Neuron conducting impulses toward synapse– Sends the information

• Postsynaptic neuron (in Pns may be a neuron, muscle cell, or gland cell)– Neuron transmitting electrical signal away from

synapse– Receives the information

• Most function as both


Figure 11.16 Synapses.

Axodendriticsynapses

Dendrites

Cell bodyAxoaxonalsynapses

Axon

Axosomaticsynapses

Axon

Axosomaticsynapses

Cell body (soma)of postsynaptic neuron


Chemical Synapses

• Specialized for release and reception of chemical neurotransmitters

• Typically composed of two parts – Axon terminal of presynaptic neuron

• Contains synaptic vesicles filled with neurotransmitter – Neurotransmitter receptor region on postsynaptic

neuron's membrane• Usually on dendrite or cell body

• Two parts separated by synaptic cleft– Fluid-filled space

• Electrical impulse changed to chemical across synapse, then back into electrical


Synaptic Cleft

• 30 – 50 nm wide (~1/1,000,000th of an inch)

• Prevents nerve impulses from directly passing from one neuron to next


PLAY Animation: Neurotransmitters

Synaptic Cleft

• Transmission across synaptic cleft – Chemical event (as opposed to an electrical

one)– Depends on release, diffusion, and receptor

binding of neurotransmitters– Ensures unidirectional communication

between neurons


Information Transfer Across Chemical Synapses• AP arrives at axon terminal of presynaptic

neuron • Causes voltage-gated Ca2+ channels to open

– Ca2+ floods into cell • protein binds Ca2+ and promotes fusion of

synaptic vesicles with axon membrane• Exocytosis of neurotransmitter into synaptic cleft

occurs– Higher impulse frequency more released


Information Transfer Across Chemical Synapses• Neurotransmitter diffuses across synapse• Binds to receptors on postsynaptic neuron

– Often chemically-gated ion channels • Ion channels are opened• Causes an excitatory or inhibitory event

(graded potential)• Neurotransmitter effects terminated


Termination of Neurotransmitter Effects

• Within a few milliseconds neurotransmitter effect terminated in one of three ways– Reuptake

• By astrocytes or axon terminal – Degradation

• By enzymes– Diffusion

• Away from synaptic cleft


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+




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 movementGraded 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



Presynapticneuron


Voltage-gated Ca2+




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

Mitochondrion

Axon terminal

Synapticcleft

Synapticvesicles

Postsynapticneuron

Ion movement

Graded potentialEnzymaticdegradation

Reuptake

Postsynapticneuron

Diffusion awayfrom synapse

Binding of neurotransmitter opension channels, resulting in gradedpotentials.

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

Presynapticneuron

1

2

3

4

5

6


Synaptic Delay

• Time needed for neurotransmitter 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

• Neurotransmitter receptors cause graded potentials that vary in strength with– Amount of neurotransmitter released and– Time neurotransmitter stays in area


Postsynaptic Potentials

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


Excitatory Synapses and EPSPs

• Neurotransmitter 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


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


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


Neurotransmitters

• Language of nervous system• 50 or more neurotransmitters have been

identified• Most neurons make two or more

neurotransmitters– 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


Classification of Neurotransmitters:Chemical Structure • 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


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


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


Presynaptic(input) fiber

Facilitated zone Discharge zone Facilitated zone

Figure 11.22 Simple neuronal pool.


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

nervous system works because information flows from neuron to neuron

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