12 March 2025

Relationship Between Stimulus Strength and Neurotransmitter Release

• Single Action Potential

  • A single action potential reaching the axon terminal releases a constant amount of neurotransmitter.

  • Release can be increased or decreased based on several factors:

    • Strength and frequency of the stimulus.

    • Activity at dendrites and cell body.

    • Action potential frequency at axon hillock and initial segment, which affects neurotransmitter release.

• Action Potential Frequency and Stimulus Information

  • Neurons utilize action potential frequency to indicate the duration and strength of stimuli.

  • Sensory Adaptation:

    • A decrease in action potential frequency can occur during sensory adaptation leading to diminished perception.

    • To perceive stimuli again, an increase in frequency of action potentials is necessary.

  • Coding of Stimulus Strength:

    • The stronger the stimulus, the higher the frequency of action potentials:

      • More action potentials per second leads to greater neurotransmitter release.

Neurotransmitter Release and Graded Potentials

• Graded Potentials

  • Only require a threshold value (approximately -55mV) to trigger action potentials.

    • Strong stimuli create higher graded potentials (e.g., greater than -20mV).

  • Action potentials are all-or-nothing:

    • Consistent amplitude, varying frequencies based on stimulus strength.

    • Example:

      • 4 action potentials/sec for weak stimuli leading to limited neurotransmitter release.

      • Higher frequencies (e.g., 8 action potentials/sec) for stronger stimuli resulting in increased neurotransmitter release.

Application to Motor Neurons and Smooth Muscle Control

• Motor Neurons & Blood Vessels

  • Increased neurotransmitter release affects the contraction of smooth muscle in blood vessel walls:

    • More neurotransmitter = contraction (vasoconstriction).

    • Less neurotransmitter = relaxation (vasodilation).

  • Frequency of action potentials directly influences muscle response:

    • Signaling can be recorded as action potentials (e.g., 9 per millisecond).

• Force of Contraction and Frequency

  • Tonic contractions (sphincters) are maintained until relaxation is needed.

    • Example:

      • Closed pyloric sphincter due to tonic contraction when food is digested.

      • Relaxation allows for food passage into the small intestine.

  • The degree of contraction changes responsively:

    • Greater release of neurotransmitters results in stronger contractions, while less leads to relaxation.

Integration within the Gastrointestinal Tract

• Smooth Muscle of the GI Tract

  • Tonic contractions can be prolonged and are controlled by the frequency and amount of neurotransmitter release.

  • Graphs will show the relationship between electrical signals (action potentials), force of contraction, and whether sphincters are open or closed.

• Vasodilation vs. Vasoconstriction

  • Peaks and troughs in contraction force can indicate vasodilation (lower force) and vasoconstriction (greater force).

Conclusion and Exam Preparation

• End of nervous system material reviewed with focus on neurotransmitters and actions in the body.

  • After spring break, the discussion on nervous system two will begin:

    • Detailed examination of neurotransmitters, receptors, and communication within the nervous system (both central and autonomic divisions).

  • Encourage students to work on questions and clarify doubts before exam day.