Lect 4-synapse-8-

The Synapses

SMS1084Dr. Mohanad R. Alwan

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SYNAPSESA. Introduction1) A synapse is a junction between 2 nerve cells or a nerve cell and a

muscle cell. Usually these are formed between axon terminals and cell dendrites or body.

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B) Anatomy of A Synapse

1) Synaptic knob with transmitter vesicles and presynaptic membrane.

2) Synaptic cleft.

3) Postsynaptic membrane with receptors for transmitters.

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C) Function of The Synapse

1) Electrical impulse in knob causes influx of Ca++, vesicles rupture, release transmitters.

2) Transmitters diffuse across,cleft attach to receptors.

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3) Receptors open gates for ions.a) Can be directb) May be indirect via G-proteins

(which diffuse from receptor to ion channel.)

4) Transmitters can be broken down, taken up by presynaptic membrane or diffuse away.

Electrical Synapses

• Electrical synapses are formed when two neurons are connected by gap junctions (syncitia).

• These are very rare in mammalian adult nervous systems.– Primarily found in the control of certain eye

movements

• These are more common in the developing fetal nervous system.

Chemical Synapses

• Chemical synapses convert the electrical signal (AP) into a chemical signal (neurotransmitter) that is transmitted to the next cell.

• Chemical synapses are unidirectional.– Neurotransmitter released by one neuron– Neurotransmitter signal interpreted by other neuron

• The connection made at a chemical synapses contains a small gap (20-50 nm) between the connected neurons called a synaptic cleft.

View of The Chemical Synapse & Function

Neurotransmitter

• Neurotransmitter is made by the pre-synaptic neurone and is stored in synaptic vessels at the end of the axon.

• The membrane of the post-synaptic neurone has chemical-gated ion channels called neuroreceptors. These have specific binding sites for neurotransmitters.

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1) Acetylcholine was first discovered, there are many others.

2) Monoaminesa) The catacholamines: dopamine, norepinephrine, epinephrine?b) Serotonin

D) Transmitters

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3) Amino acidsa) glutamateb) GABA and glycine - inhibitory

4) Amines such as acetylcholine and histamine.

5) Polypeptides such as enkephalins and endorphins.

6) Gases such as nitric oxide and CO.

Neurotransmitters

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E) One-way Conduction1) Synapses make nervous system a one- way system - dendrites do not have neurotransmitter vesicles.2) Synaptic delay

POSTSYNAPTIC POTENTIALS

• The interaction of neurotransmitters with their receptors can cause changes in the membrane potential.

• The response to a given neurotransmitter depends on the type of receptor present on the postsynaptic site.

• Excitatory postsynaptic potentials cause a depolarization of the membrane (EPSP).

• Inhibitory postsynaptic potentials cause a hyperpolarization of the membrane (IPSP).

Types of Postsynaptic Potentials

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G) Synaptic Integration1) Most synapses involve many neurons. 2) Whether or not postsynaptic membrane has an action potential is dependent upon algebraic sum of EPSPs and IPSPs.

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3) Spatial summation occurs when numerous synaptic knobs release their transmitters.

4) Temporal summation can occur if neuron releases transmitter rapidly.

Neuromuscular Junctions

• Same stages as cholinergic synapses, but in this case the postsynaptic membrane is the muscle fibre membrane, (Sarcolemma). Depolarisation of the sarcolemma leads to contraction of muscle fibre.

Cholinergic Synapses

Acetylcholine is a common transmitter.

Synapses that have acetylcholine transmitter are called cholinergic synapses.

Some neurones form more than 1 synapse. • This is an electron

micrograph of synapses between nerve fibres and a neurone cell body.

What happens at a cholinergic synapse?

Stage 1

An action potential arrives at presynaptic membrane. Voltage gated calcium channels in the presynaptic membrane open, calcium ions enter the presynaptic neurone.

What happens at a cholinergic synapse? Stage 2

Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft.

What happens at a cholinergic synapse? Stage 3

Acetylcholine diffuses cross the synaptic cleft and binds to specific neuroreceptor sites in the post synaptic membrane.

What happens at a cholinergic synapse? Stage 4

Sodium channels open. Sodium ions diffuse into the postsynaptic membrane causing depolarisation, which may initiate an action potential.

What happens at a cholinergic synapse? Stage 5

Acetylcholinesterase breaks down acetylcholine. The products diffuse back into the presynaptic neurone where acetycholine is resynthesised using ATP from the mitochondria.

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H) Chemicals Which Affect The Synapse1) Many chemicals that affect nervous system do so by affecting

synapse. a) Clostridium botulinum toxin.b) Clostridium tetanii toxin.c) Caffeined) Anestheticse) Strychninef) Psychotropic drugs

The Role Of Membrane Ion Channels

Plasma membrane of neurons is filled with channels that allow specific ions to cross.

Ion channels fall into 1 of 2 categories:

Passive or leakage channels – usually open and allow specific ions to pass (i.e., K+).

Gated channels – only open when appropriate signal received. Chemically (ligand) gated channels – only open when

the appropriate chemical or neurotransmitter present. Voltage gated channels – only open when the membrane

voltage is at an appropriate level.

Types of Ion Channels

The Resting Membrane Potential

• If we measure voltage between the inside of a neuron and the outside we find that the neuron is more negative inside than outside with a potential of about –70 mV.

Source of The Resting Membrane Potential

• The resting membrane potential results from the concentrations of ions that are in & out of the cell and the permeability to those ions. The resting membrane potential is about –70 mV.

Membrane Potentials

• Neurons use electrical signals to receive, integrate and send information.

• Electrical potential of the membrane can change • There are two types of electrical changes to the membrane:

– Graded potentials - short distance– Action potentials - long distance (through the axon).

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