Lecture 17 - Synapses and Information Processing

What are the types of synapses

1. Electrical

2. Chemical

Electrical synapses

Synapses that are closer together and connected through gap junctions (allow passage of ions)

• Produces continuous local current and action potential propagation

Chemical synapses

Action potential MAY or MAY NOT be propagated to post synaptic cell

depends on : AMOUNT of neurotransmitter released and SENSITIVITY of the postsynaptic cell

What are the classes of neurotransmitters?

1. Excitatory neurotransmitters

2. Inhibitory neurotransmitters

Excitatory neurotransmitters

Causes DEPOLARIZATION of postsynaptic membranes and promote action potentials

Inhibitory neurotransmitters

Causes hyperpolarization of post synaptic membranes AND suppresses action potentials

Acetylcholine (ACh)

Promotes action potentials but inhibits cardiac neuromuscular junctions

Cholinergic synapse

Synapses that release ACh includes:

1. All neuromuscular junctions with skeletal muscle

2. Many synapses in CNS

3. All neuron to neuron synapses in the PNS

4. All neuromuscular and neuroglandular junctions of ANS parasympathetic division

Events at cholinergic synapse

1. Action potential arrives and depolarizes synaptic terminal

2. Calcium ions enter synaptic terminal and trigger exocytosis of ACh (more is produced than needed)

3. ACh binds to receptors and depolarizes postsynaptic membrane

4. AChE BREAKS ACh into acetate and chlorine if there is an excess of it

Synaptic delay

0.2 -0.5 milliseconds that occur between the arrival of action potential at synaptic terminal and effect on postsynaptic membrane

• FEWER synapses means FASTER responses

• Reflexes may involve ONLY ONE synapse

Synaptic fatigue

Occurs when neurotransmitter CANNOT recycle fast enough to meet demands of intense stimuli

• Synapse is inactive UNTIL ACh is replenished

List important neurotransmitters

1. Norepinephrine

2. Dopemine

3. Serotonin

4. Gamma Aminobutyric Acid (GABA)

Norepinephrine

A neurotransmitter that is released by adrenergic synapses and has an excitatory and depolarizing effect. It is widely distributed in the brain and portions of the ANS

Dopamine

A CNS neurotransmitter that might be excitatory or inhibitory. Involved in Parkinson’s disease and cocaint addiction

Serotonin

CBS beurotransmitter that affects attention and emotional states

Gamma aminobutyric acid (GABA)

Functions in the CNS and has an inhibitory effect

Neuromodulators

Other chemicals that are released by the synapse and have a similar function to neurotransmitters

Neuromodulators characteristics

• Long term effects that are slow to appear

• Responses involve multiple steps

• Affects presynaptic and postsynaptic membranes

• Released alone OR with a neurotransmitter

Neuropeptides

Neuromodulators that bind to receptors and activate enzymes

Opioids

Neuromodulators in the CNS that bind to the same receptors as opium and morphine. Pain relief

Information processing

Many dendrites receive neurotransmitter messages simultaneously (some excitatory and others inhibitory)

• Net effect on axon hillock determines if action potential is produced

Postsynaptic potential

Graded potentials developed in a postsynaptic cell

Excitatory postsynaptic potential (EPSP)

Graded depolarization of postsynaptic membrane

Inhibitory postsynaptic potential (IPSP)

Graded hyperpolarization of post synaptic membrane

Inhibition

A neuron that receives many IPSPs is inhibited from producing an action potential because the stimulation needed to reach threshold is increased

Temporal summation

Rapid and repeated stimuli at ONE synapse

Spatial summation

Many stimuli arrive at MULTIPLE synapses