Biol 2401 Transmission at Synapses

Flashcards about neurons and synapses

Synapse

The site of communication between two or more neurons, mediating information transfer and modulating signals.

Estimated number of synapses in the human brain

Approximately 10^15

Pre-synaptic neuron

Neuron that sends the signal across a synapse.

Post-synaptic neuron

Neuron that receives the signal across a synapse.

Axodendritic synapse

Synapse where the signal travels from axon to dendrite.

Axosomatic synapse

Synapse where the signal travels from axon to cell body.

Axoaxonic synapse

Synapse where the signal travels from axon to axon.

Electrical Synapse

Type of synapse characterized by direct ion flow through gap junctions, resulting in fast transmission and synchronized activity.

Chemical Synapse

Type of synapse with a synaptic cleft; relies on neurotransmitters to transmit signals.

Neurotransmitters

Chemical messengers released from the presynaptic neuron that bind to receptors on the postsynaptic neuron.

Action at a Chemical Synapse

The presynaptic impulse is converted to a chemical signal (neurotransmitters) that crosses the cleft and influence the chemically gated ion channels in the postsynaptic neuron, and consequently the membrane potential in that area.

Action at the Post Synaptic End

The binding of neurotransmitters to specific receptors on the postsynaptic membrane leads to a change in ion permeability.

EPSP

Excitatory Postsynaptic Potential, a depolarization that increases the likelihood of an action potential.

IPSP

Inhibitory Postsynaptic Potential, a hyperpolarization that decreases the likelihood of an action potential.

Excitatory postsynaptic potential (EPSP)

A graded depolarization that moves the membrane potential closer to the threshold for firing an action potential (excitement).

Inhibitory postsynaptic potential (IPSP)

A graded hyperpolarization that moves the membrane potential further from the threshold for firing an action potential (inhibition).

Signal Integration

Integration and summing up of all EPSP’s and IPSP’s

Types of Signal Integration

Temporal and Spatial Summation

Temporal Summation

Occurs when the same neuron fires quickly in succession.

Spatial Summation

Occurs when different neurons fire in quick succession or at the same time.

Cholinergic Synapses

Synapses that use Acetylcholine (ACh) as neurotransmitters.

Why is terminating signals in a synapse important?

To prevent continuous stimulation by neurotransmitters.

Three main mechanisms for terminating the signal in a synapse.

Diffusion, Enzyme degradation, Re-uptake

AcetylCholine-Esterase

Enzyme on the postsynaptic membrane of cholinergic synapses that breaks down AcetylCholine.

SSRI drugs

Serotonin Specific Re-uptake Inhibitor; increases serotonin levels in the synapse by blocking re-uptake.

Modulation of NT Release

Changing the amount of neurotransmitter released into the synaptic cleft.

Facilitation

A neuron that is brought closer to threshold.

Pre-Synaptic Facilitation

Axo-axonic neuron releases serotonin which creates prolonged depolarization on the pre-synaptic neuron and increases Ca++ in the axon.

Pre-Synaptic Inhibition

Axo-axonic neuron causes an IPSP at the presynaptic membrane and v.g. Ca++ channels will close faster. This results in less Ca++ influx in the axon terminal and hence less exocytosis of NT vesicles

AcetylCholine

Opens c.g. Na+ channels and thus results in a fast depolarization and action.

Biogenic Amines

NT that are derived from Amino Acids - Tyrosine.

Amino Acids that function as Neurotransmitters

Glycine (creates IPSP’s) and glutamate ( creates EPSP’s)

Heavy Metal Poisoning

results in damage to neuroglia and demyelination. Examples are mercury, lead, arsenic

Multiple Sclerosis

results in demyelination of neurons in brain, spinal cord and is believed to be a defect of the immune system

Tay Sachs Syndrome

genetic abnormality that results in a build-up of gangliosides in lysosomes of neurons, Results in destructions of neurons; children die at early age.

Tetrodotoxin (TTX)

very potent v.g. Na+ channel blocker

Nerve Gas and some Insecticides

Inhibits the function of ACh-esterase

Marine animal Toxins that have an effect on multiple ion channels

Block c.g. Na+ channels in muscles and nerves + Block Ach-esterase function

IonoTropic Receptors

NT bind to membrane proteins called chemically gated channels on the post synaptic membrane and NT open or close ion channels

Metabotropic receptors

require intermediate membrane proteins to eventually open an ion channel