14. Synaptic Integration Part 1-Postsynaptic potentials

How does synaptic integration occur in the CNS and how does it differ from the neuromuscular junction?

• CNS neurons communicate via chemical synapses (like neuromuscular junctions).

• Each CNS neuron receives thousands of excitatory & inhibitory inputs.

• Muscle cells: innervated by only one motor neuron → no integration needed.

• CNS neurons must integrate all inputs to determine output (firing).

• Knowledge of synaptic integration largely from alpha motor neurons in vertebrate spinal cord.

What are the main synapse types in the CNS?

• Axosomatic: axon → soma.

• Axodendritic: axon → dendrite.

• Axo-axonic: axon → another axon.

How might axo-axonic synapses influence action potential generation?

• Presynaptic axon synapses directly onto another axon terminal.

• This direct input depolarizes the terminal enough to open voltage-gated Ca²⁺ channels.

• Ca²⁺ influx → neurotransmitter release even without a full action potential in the postsynaptic neuron’s axon.

• Eliminates reliance on graded potentials (no decay) and bypasses need for an AP to reach the terminal.

• Result: any signal from the presynaptic neuron reliably triggers neurotransmitter release.

How does the patellar (knee-jerk) reflex work?

• Tapping the patellar tendon stretches the quadriceps, activating stretch receptors.

• These receptors send signals through an afferent neuron to the spinal cord.

• That afferent neuron does two things:

  • Excites an efferent motor neuron → contracts the quadriceps → leg kicks forward.

  • Excites an inhibitory interneuron, which hyperpolarizes the efferent motor neuron to the hamstring → hamstring relaxes.

• This spinal reflex happens without involving the brain, enabling fast, automatic movement.

What is the function of inhibitory synapses?

• Inhibitory synapses prevent the postsynaptic neuron from firing an action potential.

• They hyperpolarize or stabilize the membrane potential below threshold.

How can excitatory transmission be studied in the knee-jerk reflex?

• Each stretch receptor connects to multiple afferent neurons.

• Stimulating one afferent neuron produces a small EPSP in the postsynaptic motor neuron—not enough for an action potential.

• Stimulating all afferent neurons together causes their EPSPs to summate, reaching threshold.

• This summation triggers a full action potential in the motor neuron → muscle contraction.

• Therefore, the reflex relies on the combined input of many afferent neurons, not just a single one.

How do inhibitory interneurons ensure hamstring relaxation during the patellar reflex?

• One inhibitory interneuron firing → small IPSP in the hamstring motor neuron → slight hyperpolarization (not enough to prevent contraction).

• Stimulating all sensory afferents → strong excitation of the inhibitory interneurons → large IPSP in the hamstring motor neuron.

• This strong hyperpolarization prevents any action potential, even if excitatory signals arrive.

• Key point: Inhibitory interneurons themselves receive EPSPs (they must be excited), but they produce IPSPs in the hamstring motor neuron.

What are EPSPs and what causes them?

• EPSPs: depolarizations that increase likelihood of postsynaptic AP.

• Caused by neurotransmitter release → binds to postsynaptic receptors → membrane depolarization.

• Single EPSPs: small (<1 mV), below threshold.

• Thousands of EPSPs can sum in space & time to reach threshold.

• Ion flux: likely Na⁺ influx (sometimes Ca²⁺).

What are IPSPs and how do they affect postsynaptic neurons?

• IPSPs: hyperpolarizations that decrease likelihood of AP.

• Can sum with EPSPs → reduce EPSP amplitude.

• Can reduce probability of reaching threshold.

• Ion flux: likely Cl⁻ influx or K⁺ efflux.

How do EPSPs and IPSPs interact to determine if an action potential occurs?

• Balance between excitatory (EPSP) and inhibitory (IPSP) inputs changes over time.

• Neurotransmitters bind receptors → open/close ion channels → change conductance → alter current flow.

• Summation of all inputs determines membrane potential → whether threshold is reached → AP generation.

How do spatial and temporal summation influence action potential generation?

• Spatial summation: EPSPs from different synapses on dendrite combine → may reach threshold.

• Temporal summation: repeated EPSPs from same synapse in quick succession overlap → may reach threshold.

• Real cases: Spatial + temporal summation through same sets of axons

How do postsynaptic currents spread, and where is integration most effective?

• Currents spread electrotonically toward axon hillock (spike initiation zone).

• Potentials decay with distance from synapse.

• Axon hillock: lower threshold due to high density of voltage-gated Na⁺ channels → key site for AP initiation.

• If current starts at point A (further) vs point B (closer), A decays more.

How does synaptic integration use action potential frequency to encode signal strength?

• Action potentials are all-or-nothing, so their amplitude never changes.

• What does change is frequency: stronger depolarization at the soma/axon hillock → more frequent APs.

• Weak depolarization just above threshold → low-frequency firing (longer gaps due to relative refractory period).

• Strong depolarization → high-frequency firing, APs occur as soon as channels recover.

• Postsynaptic cells interpret frequency, not AP size, as the signal strength (e.g., intense pain = very high firing rate).

What happens to firing frequency as depolarization increases?

• Increased depolarization → more action potentials.

• Beyond a certain depolarization, firing rate plateaus (absolute refractory period).

What happens in a muscle fiber when a single motor neuron fires one action potential?

• One motor neuron AP → one AP in the skeletal muscle fiber.

• This produces a single muscle twitch, shown on a myogram.

• A twitch has two phases:

  • Contraction phase: muscle tension rises.

  • Relaxation phase: tension falls back to baseline.

• The exact timing varies by muscle type, but the key idea is: one AP → one brief contraction (twitch).

What is wave summation and how does it lead to tetanus?

• Wave summation: repeated stimulation → less relaxation time → stronger contraction.

• Caused by high AP frequency in motor neuron.

• Tetanus: sustained contraction (no relaxation) due to rapid stimuli.

• Useful for sustained tension (e.g., lifting), but can cause cramps if uncontrolled.