Synapses and Neuromuscular Junctions

How does the transmission happen in synapses?

Signal travels from the presynaptic neuron to the postsynaptic neuron (effector cell)

How was the existence of neurotransmitters proven?

In 1921, by Otto Leowi using two frog hearts

What is needed for cells to be electrically coupled?

Cells must be almost the same size and joined by areas of contact with low electrical resistance. This allows impulses to be regenerated without interruption

How are two electrically coupled cells joined together?

Through gap junctions

What is the protein that makes gap junctions?

connexins

How do connexins form a gap junction?

Six connexions form a hemichannel, then two hemichannels from opposing cells come together to form a gap junction

What muscle has gap junctions?

Cardiac muscle

What are the axon endings on the presynaptic neuron called?

The terminal boutons

What is the space between the presynaptic and postsynaptic neurons?

synaptic cleft

What contains the neurotransmitters in the terminal bouton?

The synaptic vesicle

How are neuro transmitters released?

1. Action potentials reach axon terminals

2. Voltage gated Ca2+ channels respond to potential and calcium enters

3. Calcium binds to synaptotagmin, triggering fusion of synaptic vesicle to the plasma membrane for exocytosis

What do neurotransmitters do once they are released?

They bind to specific receptors

What does the binding of neurotransmitters to the receptor trigger?

The opening of ion channels in the post-synaptic membrane called chemically regulated channels

When the chemically regulated channels open, what do they produce?

A graded potential

If Na+ and Ca2+ channels open, what happens?

Post synaptic membrane becomes less negative; a graded depolarization called excitatory postsynaptic potential (ESPS)

What happens if Cl- channels open?

The postsynaptic membrane becomes more negative; graded hyperpolarization called inhibitory postsynaptic potential (IPSP)

Where are EPSP and IPSP produced?

At the dendrites but must go to the beginning of the axon to induce action potential

Why the inital part of the axon to start action potential?

They have a lot more Na and K channels

What is the devergence of neural pathways?

The fact that one neuron can make synapses with many others

Can axons converge into one neuron?

Yes, it is called convergence of neural pathways

What is spatial summation?

Release of neurotransmitters from 2 neurons

What is temporal summation?

Successive release of neurotransmitters from one neuron only

What are two excitatory neurotransmitters?

Glutamic Acid and Aspartic Acid

What hyperpolarizes the postsynaptic membrane?

Glycine and GABA

When and where is GABA and glycine used?

About 1/3 of the brain uses GABA. Glycine is used by the spinal cord, brain stem, and retina

What neurotransmitter is linked to motor control?

GABA

What is the result of GABA-releasing neuron deficiency?

Huntingtons disease

What is the excitatory neurotransmitter used by somatic motor neurons at the neuromuscular junction?

Acetylcholine (Ach)

What type of stimulation can it be at autonomic nerve endings?

excitatory or inhibitory due to the presence of different types of Ach receptors

What are the two Ach receptors?

1. Nicotinic Ach receptors (nicotine can bind them)

2. Muscarinic Ach receptors (muscarine can bind them)

What do nicotinic Ach receptors do?

They are found in the brain and control muscle contraction stimulation

What type of channel is the nicotinic Ach receptors?

Ligand-gated channel

What is a ligans-gated channel?

The receptor and channel are the same protein. When the neurotransmitter binds to the receptor, the channel opens

How many receptors does the ligand gated channel have?

two, only opens when both are activated

Where are muscarinic Ach receptors found and what do they do?

Smooth muscle cells, cardiac muscle cells, and glands. The receptor is a G protein-coupled receptor. The receptor and the channel are two separate things, and the G protein is an intermediary

What does the muscarinic GPCR dissociate into and what does it do?

dissociates into alpha and beta/gamma. the beta/gamma will either open or close the K+ channel

What is the role of Ach binding to the beta/gamma subunits in the heart muscle cells?

Opening the K+ channels and causing hyperpolarization

What does Ach binding ultimately lead to?

The production of EPSPs and IPSPs

How is free Ach inactivated?

Acetylcholinesterase, which is present in the post-synaptic membrane

What chemicals can inhibit AchE?

Nerve gas and fasciculins

What neurotransmitters are derived from amino acids?

Monoamines like dopamine and norepinephrine are derived from tyrosine

What is the family group of monoamines like dopamine and norepinephrine

Catecholamines

What amino acid is serotonin derived from?

Tryptophan

What amino acid is histamine derived from?

histidine

How is monoamine action stopped?

Reuptake into the presynaptic terminal bouton and then the degradation of it by an enzyme monoamine oxidase (MAO)

Do monoamines cause the opening of ion channels upon binding to their receptors

No, they act indirectly with the help of the second messenger: GPRCs

What are neurons called that use dopamine?

Dopaminergic receptors

What thing associated with dopamine is involced in motor control?

Nigrostriatal dopamine system

What dopaminergic system is involved in emotional reward?

Mesolimbic dopamine system

Where is seratonin used?

brain stem

What hallucinogens bind and activate the serotonin receptors?

LSD, Mezcaline, psilocybin