Synapses, Neurotransmitters, Circuits, Memory, and Diseases

End of axon

Electrical signal cannot travel further; triggers neurotransmitter release

Neurotransmitter function

Stimulates new wave of electrical activity in next cell

Synapse

Junction between two neurons

Presynaptic neuron

First neuron in signal pathway; releases neurotransmitter

Postsynaptic neuron

Second neuron; responds to neurotransmitter

Axodendritic synapse

Presynaptic axon synapses with dendrite of postsynaptic neuron

Axosomatic synapse

Presynaptic axon synapses with soma of postsynaptic neuron

Axoaxonic synapse

Presynaptic axon synapses with axon of postsynaptic neuron

Synapse quantity

A single neuron can have thousands of synapses

Spinal motor neuron synapses

~10,000 axon terminals from other neurons

Spinal motor neuron dendritic synapses

~8,000 on dendrites

Spinal motor neuron somatic synapses

~2,000 on neurosoma

Cerebellar neuron synapses

Up to 100,000 per neuron

Chemical synapse

Uses neurotransmitters; allows decision-making; slower

Electrical synapse

Gap junctions; fast, direct transmission; no integration

Electrical synapse locations

Some neurons, neuroglia, cardiac and single-unit smooth muscle

Electrical synapse advantage

Faster signal transmission; no neurotransmitter delay

Electrical synapse disadvantage

Cannot integrate information

Neurotransmitters

Over 100 identified; act via chemical classes

Major neurotransmitter categories

ACh, amino acids (GABA, glutamate, aspartate), monoamines (NE, dopamine, serotonin, histamine, ATP), neuropeptides, gases (NO, CO), purines (ATP)

Acetylcholine (ACh)

Excitatory; formed from acetic acid and choline; triggers Na+/K+ flow

Amino acid neurotransmitters

Glycine, glutamate, aspartate, GABA

GABA

Inhibitory; opens Cl− channels; CNS function

Monoamines

Synthesized from amino acids; catecholamines: epinephrine, NE, dopamine

NE (Norepinephrine)

Excitatory; released by adrenergic synapses; CNS + ANS; depolarizing

Dopamine

CNS neurotransmitter; excitatory or inhibitory; Parkinson’s, cocaine

Serotonin

CNS neurotransmitter; affects attention and emotional states

Neuropeptides

Chains of 2–40 amino acids; stored in secretory granules; e.g., cholecystokinin, substance P

Gaseous neurotransmitters

NO and CO; inorganic; synthesized as needed; diffuse into postsynaptic neuron; no receptors

Purines

Adenosine, ATP

Drug effects on nervous system

Stimulate neurotransmitter receptors; affect perception, motor control, emotions

Psychoactive drugs

Treat depression, anxiety, OCD, ADD/ADHD; e.g., Prozac (SSRI)

Presynaptic axon terminal

Contains synaptic vesicles with neurotransmitters

Postsynaptic membrane

Contains neurotransmitter receptors; ligand-gated ion channels open when bound

Synaptic transmission

Neurotransmitter release, receptor binding, postsynaptic potential

Excitatory cholinergic synapse

ACh depolarizes postsynaptic cell; triggers AP if threshold reached

Inhibitory GABA-ergic synapse

GABA hyperpolarizes postsynaptic cell; reduces firing

Excitatory adrenergic synapse

NE binds GPCR; activates cAMP; slower but amplified

Neurotransmitter clearance

Enzymatic breakdown, reuptake, or diffusion ends signal

Neuromodulators

Chemicals (NO, neuropeptides) that modulate neuron groups long-term

Neural integration

Process, store, recall info; synapses allow decision-making

Summation

EPSPs/IPSPs combined at trigger zone to determine response

Temporal summation

EPSPs from one synapse over time; can reach threshold

Spatial summation

EPSPs from multiple synapses; combined to reach threshold

Presynaptic facilitation

Enhances transmitter release

Presynaptic inhibition

Reduces transmitter release

Neural coding

Qualitative: which neurons fire = stimulus type; Quantitative: firing rate & number = stimulus intensity

Neural pools & circuits

Groups of neurons controlling body functions (e.g., breathing, walking)

Discharge vs Facilitated zones

Discharge: neuron alone fires postsynaptic cells; Facilitated: needs other neurons

Diverging circuit

One neuron branches to many postsynaptic neurons

Converging circuit

Many inputs funnel to one neuron or pool

Reverberating circuit

Neurons stimulate each other in sequence; feedback loop maintains activity

Parallel after-discharge circuit

Input neuron diverges into chains with different synapse lengths; reconverge to output neuron

Serial vs Parallel processing

Serial: linear, one info flow; Parallel: diverging circuits, simultaneous processing

Memory trace/engram

Pathway of neurons storing memory

Synaptic plasticity

Ability of synapses to change strength or efficiency

Synaptic potentiation

Making transmission easier; basis for learning and memory

Immediate memory

Seconds-long echo of recent events; may use reverberating circuits

Short-term memory

Seconds to hours; facilitated synapses; tetanic stimulation enhances briefly

Posttetanic potentiation

Ca2+ buildup allows memory retrieval with little stimulation

Long-term memory

Lifetime storage; explicit (declarative) and implicit (procedural/emotional)

Long-term potentiation (LTP)

NMDA receptors, Ca2+ influx, new receptors and protein synthesis strengthen synapses

Long-term depression

Low-frequency stimulation, low Ca2+, protein dephosphorylation; synapses removed

Alzheimer disease

Memory loss, mood changes, cortical atrophy, plaques/tangles; ACh & NGF deficiency

Alzheimer treatment

Cholinesterase inhibitors, NGF, experimental β-amyloid clearance

Parkinson disease

Degeneration of dopamine neurons; tremors, rigidity, slow movement

Parkinson treatment

L-dopa, MAO inhibitors, physical therapy, sometimes surgery