Synaptic Transmission: Anatomy, Physiology, and Neurochemistry

Synapse

The specialized cellular junction where electrical or chemical nerve impulses are transmitted between a neuron and another target cell, such as a neuron, muscle, or gland.

Presynaptic neuron

The neuron that initiates and delivers a signal across a junction to another cell.

Postsynaptic neuron

The cell that receives transmission on the opposite side of a junction.

Role of astrocytes at the synapse

Glial cells that physically shield the junction from outside chemicals and actively exchange chemical substances with both the sending and receiving neurons.

Santiago Ramón y Cajal's anatomical discovery

Used Golgi staining techniques to demonstrate that a tiny physical gap separates the tip of an axon from the surface of the next neuron.

Charles Sherrington's deduction of synapses

Demonstrated that reflex response speed ($\sim 15\,\text{m/s}$) is slower than direct axonal conduction speed ($\sim 40\,\text{m/s}$), showing that a delay occurs where neurons communicate.

Temporal summation

The cumulative effect on a postsynaptic neuron produced by repeated stimuli arriving at the same synapse within a brief interval of time.

Spatial summation

The combined effect on a neuron resulting from synaptic inputs that arrive simultaneously from multiple separate anatomical locations.

Hebbian plasticity

The learning principle stating that the synaptic connection between neurons strengthens when they are repeatedly and persistently activated together ("neurons that fire together, wire together").

Electrical synapse

A junction where ionic current flows directly between cells through gap junctions (connexons), enabling rapid, bidirectional communication and synchronized firing without chemical release.

Chemical synapse

A junction where an impulse triggers the release of neurotransmitters across a fluid-filled cleft to target receptors, ensuring unidirectional signal transmission.

Primary advantage of chemical over electrical synapses

Enforces strict one-way (unidirectional) transmission from presynaptic membrane to postsynaptic receptors, preventing reverse signaling and allowing complex signal integration.

Excitatory neurotransmitters

Chemical messengers (such as glutamate) that depolarize the postsynaptic membrane, increasing the probability that an action potential will be generated.

Inhibitory neurotransmitters

Chemical messengers (primarily GABA) that hyperpolarize the postsynaptic membrane, reducing the likelihood of generating an action potential.

Excitatory postsynaptic potential (EPSP)

A graded depolarization of the postsynaptic membrane typically caused by an influx of sodium ions ($\text{Na}^+$).

Inhibitory postsynaptic potential (IPSP)

A graded hyperpolarization of the postsynaptic membrane typically caused by potassium ions ($\text{K}^+$) exiting the cell, making an action potential less likely.

Primary building blocks of neurotransmitters

Amino acids obtained through dietary protein intake.

Magnetic Resonance Spectroscopy (MRS)

A neuroscientific imaging method used noninvasively to quantify in vivo concentrations of molecules and neurotransmitters in the brain.

Mechanism of L-dopa

Acts as a chemical precursor that increases dopamine synthesis, serving as a primary treatment for Parkinson's disease.

Mechanism of hallucinogenic drugs like LSD

Chemically resembles serotonin and binds to serotonin type $2\text{A}$ ($5\text{-HT}_{2\text{A}}$) receptors, triggering stimulation at inappropriate times or for prolonged durations.

Mechanism of nicotine

Stimulates nicotinic acetylcholine receptors situated on dopamine-releasing neurons, thereby increasing dopamine release in the reward pathway.

Reuptake

The process where intact neurotransmitters are reclaimed from the synaptic cleft into the presynaptic neuron via specialized membrane transporter proteins to be reused.

Mechanism of stimulant drugs (e.g., cocaine and amphetamines)

Inhibit transporter proteins responsible for reuptake, preventing the reabsorption of dopamine, serotonin, and norepinephrine and prolonging their active effects.

Mechanism of acetylcholinesterase

An enzyme that breaks acetylcholine down into acetate and choline fragments within the cleft to terminate synaptic signaling.

Mechanism of cannabinoids

Bind to retrograde receptors on presynaptic neurons, inhibiting further neurotransmitter release and usually reducing anxiety.

Neurotransmitters vs. hormones (transmission route)

Neurotransmitters diffuse locally across a microscopic synaptic cleft directly to neighboring cells, whereas hormones travel through the bloodstream to distant tissues and organs.

Neurotransmitters vs. hormones (speed and duration)

Neurotransmitters act rapidly within milliseconds with brief effects, while hormones act more slowly over seconds to minutes and coordinate long-lasting changes.

Neurotransmitters vs. hormones (broadcast analogy)

Neurotransmitters function like a telephone call that connects directly to a specific target, whereas hormones function like a radio station broadcast received by any cell tuned to that signal.