Synapses

What happens when the action potential gets to the end of a neuron?

Where two neurons meet it is called a synapse, where a neuron and muscle meet it is called a neuromuscular junction

-          Both similar in structure and function

Synapses and neuromuscular junctions

-          Electrical synapse

o   Two cells whose cytosols are in contact via lots of gap junctions

o   So action potential simply moves from one cell to next via ion flow through gap junctions

§  Cardiac muscle

-          Chemical synapse

o   Two cells not directly connected or touching

§  Typical chemical synapse (Fig. 43.14)

-          presynaptic neuron has enlarged knob

-          full of vesicles containing a chemical called a neurotransmitter

o   some are made in cell body and travel down to end of axon along microtubules, others made at axon terminal

-          presynaptic membrane and postsynaptic membrane

-          separated by synaptic cleft

Neurotransmission at synapses

- Release of neurotransmitter

-          arrival of action potential at presynaptic membrane causes

-          Ca2+ channels to open and Ca2+ rushes into cell

-          In response to this Ca2+ surge, vesicles to fuse and release their contents into synaptic cleft

-          Postsynaptic effects

·         receptors on postsynaptic membrane bind neurotransmitter

·         neurotransmitters can be excitatory or inhibitory

·         excitatory result in depolarizing postsynaptic cell

o   excitatory postsynaptic potential (EPSP)

·         inhibitory result in hyperpolarizing

o   inhibitory postsynaptic potential (IPSP)

·         a given axon may make both, but usually only one

Types of neurotransmitter receptors

-          ionotropic receptors

·         ligand gated ion channels

·         what opens them up is neurotransmitter binding to the receptor

-          metabotropic receptors

·         bind neurotransmitter

·         send signals inside cell that allow ion channels to be opened

IPSPs and EPSPs again

-          so, an excitatory PSP is produced when an ion channel opens that depolarizes the postsynaptic cells

·         like a Na⁺ channel

-          an inhibitory PSP is produced when an ion channel opens that hyperpolarizes the postsynaptic cells

·         like a Cl^- channel

Synaptic Integration

-          a single neuron may be receiving inputs from many other neurons (Fig. 43.15)

-          some excitatory, some inhibitory

-          make various ion channels open

-          what the ions do to membrane potential at beginning of axon (axon hillock), whether threshold potential is reached, determine if action potential is sent

-          axons contacting nearer cell body have more impact on this

Neurotransmitters

-          acetylcholine

o   can be excitatory or inhibitory depending on whether it binds to nicotinic or muscarinic ACh receptor, respectively

o   so in muscle, which express nicotinic, ACh is a excitatory

o   in heart, which express muscarinic, ACh is inhibitory

-          biogenic amines

o   monoamines – derivatives of amino acids like dopamine, epinephrine,  norepinephrine and serotonin

§  dopamine and serotonin involved of all kinds of things in CNS

·         too little dopamine is Parkinsons

·         too much may be involved in schizophrenia

§  Epinephrine and norephinephrine

o   Act as hormones as well as neurotransmitters

-          simple amino acids like GABA, glycine, glutamate

-          glutamate and Chinese restaurant syndrome

-          Box on page 927 about GABA receptor

-          Receptor is 5 subunits

o   But humans have genes for 16 different

o   So there can be many GABA receptors, slightly different from each other (Fig. 43.18)

- neuropeptides

        - endorphins and enkephalins

        - mood, behavior, pain perception

-          gases – nitric oxide and carbon monoxide

o   not stored in vesicles, but enzymes that make these are regulated to produce

o   diffuses to nearby cells

o   sets in motion signal transduction cascade which results in an effect

§  like dilation of blood vessels

·         in heart – nitroglycerin is converted to NO, relaxes smooth muscle around blood vessels, more blood to heart

·         in penis – results in blood pouring in to penis under certain stimuli

·         Viagra inhibits an enzyme that slows the muscle relaxing effects of NO

Turning off signal at synapse

-          want to get neurotransmitter out of synaptic cleft

o   enzyme

-          acetylcholinesterase.  Inhibitor curare is deadly poison

-          monoamine oxidase destroys monoamine neurotransmitters.  MOA inhibitors are used as antidepressants

o   diffusion

o   reuptake by presynaptic neuron

§  Prozac is a serotonin reuptake inhibitor

§  Cocaine is a dopamine reuptake inhibitor