Lecture Human anatomy and physiology - Chapter 11: Fundamentals of the nervous system and nervous tissue (part c)

In this chapter, students will be able to understand: Define synapse, distinguish between electrical and chemical synapses by structure and by the way they transmit information, distinguish between excitatory and inhibitory postsynaptic potentials, describe how synaptic events are integrated and modified,...

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PowerPoint ® Lecture Slides

prepared by Janice Meeking, Mount Royal College

Nervous Tissue: Part C

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The Synapse

from one neuron:

• To another neuron, or

• To an effector cell

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The Synapse

toward the synapse

away from the synapse

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Types of Synapses

neuron and the dendrite of another

neuron and the soma of another

• Axoaxonic (axon to axon)

• Dendrodendritic (dendrite to dendrite)

• Dendrosomatic (dendrite to soma)

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Dendrites Cell body

Axon

Axodendritic synapses

Axosomatic synapses

Cell body (soma) of postsynaptic neuron

Axon

(b)

Axoaxonic synapses

Axosomatic synapses

(a)

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Electrical Synapses

• Neurons are electrically coupled (joined by gap junctions)

• Communication is very rapid, and may be

unidirectional or bidirectional

• Are important in:

• Embryonic nervous tissue

• Some brain regions

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Chemical Synapses

neurotransmitters

• Axon terminal of the presynaptic neuron, which contains synaptic vesicles

• Receptor region on the postsynaptic neuron

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Synaptic Cleft

and postsynaptic neurons

from one neuron to the next

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Synaptic Cleft

• Is a chemical event (as opposed to an

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Information Transfer

channels

promotes fusion of synaptic vesicles with

axon membrane

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Information Transfer

receptors (often chemically gated ion channels) on the postsynaptic neuron

excitatory or inhibitory event (graded potential)

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Action potential arrives at axon terminal.

Voltage-gated Ca 2+

channels open and Ca 2+

enters the axon terminal.

Ca 2+ entry causes neurotransmitter- containing synaptic vesicles to release their contents by exocytosis.

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

Ca 2+

Synaptic vesicles

Axon terminal

Mitochondrion

Postsynaptic neuron

Presynaptic neuron

Synaptic cleft

Ca 2+

Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the

postsynaptic membrane.

Binding of neurotransmitter opens ion channels, resulting in graded potentials.

Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse.

Ion movement Graded potential

Reuptake

Enzymatic degradation

Diffusion away from synapse

1 2

3

4

5

6

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Ca 2+

Axon terminal

Mitochondrion

Synaptic cleft

Ca 2+

1

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Ca 2+

Axon terminal

Mitochondrion

Synaptic cleft

Ca 2+

1 2

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Ca 2+

Axon terminal

Mitochondrion

Synaptic cleft

Ca 2+

1 2

3

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Ca 2+

Axon terminal

Mitochondrion

Synaptic cleft

Ca 2+

1 2

3

4

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Reuptake

Neurotransmitter effects are terminated

by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse.

6

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Ca 2+

Axon terminal

Mitochondrion

Synaptic cleft

Ca 2+

Binding of neurotransmitter opens ion channels, resulting in graded potentials.

Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse.

Ion movement Graded potential

Reuptake

1 2

3

4

5

6

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Termination of Neurotransmitter Effects

neurotransmitter effect is terminated

• Degradation by enzymes

• Reuptake by astrocytes or axon terminal

• Diffusion away from the synaptic cleft

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Synaptic Delay

across the synapse, and bind to receptors

5.0 ms)

neural transmission

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Postsynaptic Potentials

• Amount of neurotransmitter released

• Time the neurotransmitter is in the area

1 EPSP—excitatory postsynaptic potentials

2 IPSP—inhibitory postsynaptic potentials

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Excitatory Synapses and EPSPs

chemically gated channels that allow

directions

net depolarization

is of threshold strength and opens the

voltage-gated channels

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An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold

Neurotransmitter binding opens chemically gated ion channels, allowing the simultaneous pas- sage of Na + and K +

Time (ms) (a) Excitatory postsynaptic potential (EPSP)

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Inhibitory Synapses and IPSPs

of membrane becomes more negative)

produce an action potential

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An IPSP is a local hyperpolarization of the postsynaptic membrane and drives the neuron away from AP threshold

Neurotransmitter binding opens K + or Cl – channels.

Time (ms) (b) Inhibitory postsynaptic potential (IPSP)

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Integration: Summation

potential

canceling each other out

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Threshold of axon of postsynaptic neuron

Excitatory synapse 1 (E 1 ) Excitatory synapse 2 (E 2 ) Inhibitory synapse (I 1 )

(b) Temporal summation:

2 excitatory stimuli close

in time cause EPSPs that add together.

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(d) Spatial summation of EPSPs and IPSPs:

Changes in membane potential can cancel each other out.

(c) Spatial summation:

2 simultaneous stimuli at different locations cause EPSPs that add together.

E 1

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Integration: Synaptic Potentiation

• Repeated use increases the efficiency of

• Ca 2+ activates kinase enzymes that promote more

effective responses to subsequent stimuli

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Integration: Presynaptic Inhibition

neuron may be inhibited by the activity of

another neuron via an axoaxonic synapse

EPSPs are formed

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Neurotransmitters

neurotransmitters, which are released at different stimulation frequencies

identified

function

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Chemical Classes of Neurotransmitters

• Released at neuromuscular junctions and

some ANS neurons

• Synthesized by enzyme choline

acetyltransferase

• Degraded by the enzyme acetylcholinesterase (AChE)

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• Catecholamines

• Dopamine, norepinephrine (NE), and epinephrine

• Indolamines

• Serotonin and histamine

biological clock

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• GABA—Gamma ( )-aminobutyric acid

• Glycine

• Aspartate

• Glutamate

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• Act in both the CNS and PNS

• Produce fast or slow responses

• Induce Ca2+ influx in astrocytes

• Provoke pain sensation

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• Nitric oxide (NO)

• Synthesized on demand

• Activates the intracellular receptor guanylyl cyclase to cyclic GMP

• Involved in learning and memory

• Carbon monoxide (CO) is a regulator of cGMP

in the brain

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Functional Classification of

Neurotransmitters

• Neurotransmitter effects may be excitatory

(depolarizing) and/or inhibitory (hyperpolarizing)

• Determined by the receptor type of the postsynaptic neuron

• GABA and glycine are usually inhibitory

• Glutamate is usually excitatory

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Neurotransmitter Actions

• Neurotransmitter binds to channel-linked receptor and opens ion channels

• Promotes rapid responses

• Examples: ACh and amino acids

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• Promotes long-lasting effects

• Examples: biogenic amines, neuropeptides, and dissolved gases

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Neurotransmitter Receptors

1 Channel-linked receptors

2 G protein-linked receptors

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Channel-Linked (Ionotropic) Receptors

cations

that causes hyperpolarization

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Ion flow blocked

Closed ion channel

(a) Channel-linked receptors open in response to binding

of ligand (ACh in this case).

Ions flow Ligand

Open ion channel

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G Protein-Linked (Metabotropic) Receptors

often prolonged and widespread

those that bind biogenic amines and

neuropeptides

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G Protein-Linked Receptors: Mechanism

receptor

second messengers, e.g., cyclic AMP, cyclic

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G Protein-Linked Receptors: Mechanism

• Open or close ion channels

• Activate kinase enzymes

• Phosphorylate channel proteins

• Activate genes and induce protein synthesis

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2

Receptor activates G protein.

3

G protein activates adenylate cyclase.

4

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

cAMP changes membrane permeability

by opening or closing ion channels.

5b

cAMP activates enzymes.

5c

cAMP activates specific genes.

Active enzyme GDP

5a

(b) G-protein linked receptors cause formation of an intracellular second messenger (cyclic

AMP in this case) that brings about the cell’s response.

Nucleus

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Nucleus

1

2

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G protein activates adenylate

1

2 3

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Neurotransmitter

(1st messenger) binds

and activates receptor.

Adenylate cyclase converts ATP to cAMP

1

2 3 4

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cAMP changes membrane permeability by opening and closing ion channels.

Nucleus

1

2 3 4

5a

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cAMP activates specific genes.

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