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, and how might axo-axonic synapses influence action potential generation?

• Axosomatic: axon → soma.

• Axodendritic: axon → dendrite.

• Axo-axonic: axon → another axon.

• Axo-axonic synapses can modulate neurotransmitter release from the presynaptic terminal → influence AP generation indirectly (by enhancing or inhibiting transmitter release).

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?

1. Stimulate one sensory neuron → single EPSP in motor neuron (below threshold).

2. Stimulate many sensory neurons → EPSPs sum, larger depolarization → reach threshold → action potential in motor neuron.

• Demonstrates summation of excitatory inputs.

How can inhibition of motor neurons be studied in the flexor (hamstring) muscle?

• Inhibition studied via flexor muscle (biceps femoris) pathway.

1. Stimulate a single inhibitory interneuron → produces a small IPSP (slight hyperpolarization) in motor neuron.

2. Stimulate multiple sensory neurons → activates many inhibitory interneurons → results in a larger IPSP (greater hyperpolarization).

• Demonstrates summation of inhibitory inputs similar to excitatory summation, but decreases motor neuron excitability.

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.

• IPSPs can counteract EPSPs and prevent firing.

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, 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 influence action potential firing frequency?

• Summed inputs → if large enough depolarization → neuron fires AP train.

• AP frequency proportional to degree of depolarization at spike-initiation zone.

What happens to firing frequency as depolarization increases?

• Increased depolarization → more action potentials.

• Beyond a certain depolarization, firing rate plateaus (max frequency reached).

What does a single motor neuron action potential produce in muscle?

• One motor neuron AP → single muscle twitch.

• Twitch lasts up to ~120 ms (brief contraction).

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 force (e.g., lifting), but can cause cramps if uncontrolled.