Physiology · MU Sofia

6. Local response

Covers graded potentials, including receptor, postsynaptic and local responses, and how their amplitude depends on stimulus intensity, their lack of threshold and their summation. It contrasts these with action potentials, which are all-or-none and conducted without decrement, and with electrotonic potentials that move the membrane towards threshold. The note walks through the ionic phases of the action potential, from resting potential to repolarisation, hyperpolarisation and refractory periods, and finishes with absolute and relative refractory periods.

Inside: 1 section · 7 labelled pictures · 12 flashcards · 6 min

Topic 6 of 104 in the MU Sofia syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 1 section, about 942 words (6 min of reading), with 12 flashcards.

In this note

  1. Introduction

Introduction

  • Graded potentials are generated in the membranes of the sensory receptors (receptor potentials) and the postsynaptic membranes in the chemical synapses (postsynaptic potentials). Graded potentials are also produced in the axonal and muscle membranes in response to weak, subthreshold stimuli (local responses).
  • Graded potentials are produced by selective opening/closure of ionic channels with different gating mechanisms: ligand- gated, mechano-gated, thermo-gated etc.
  • Graded responses can be depolarizing or hyperpolarizing, dependent on the type of ion channels opened/closed upon stimulation :
  • Depolarizing :
  • Na+ channel opening
  • Non-selective cation (Na+/K+/Ca2+) channel opening
  • K+ channel closure
  • Hyperpolarizing :
  • Cl- channel opening
  • K+ channel opening

Graded response Stimuli characteristics

  • Graded response amplitude depends on stimulus intensity (amplitude coding of stimulus intensity)
  • No threshold
  • The duration and shape of the graded responses are notstandard but depend on stimulus parameters
  • Effective over short distance
  • Can be summated* (temporal or spatial summation). They are thus involved in information processing.
  • Summation is the additive effect of electrical impulses in multiple numbers coming on a neuromuscular junction. Spatial summation involves simultaneous signals coming from multiple presynaptic neurons being received by a single postsynaptic neuron. Temporal summation involves a single presynaptic neuron rapid-firing signals to a postsynaptic neuron

Action Potentials

  • Generation Mechanism: Action potentials are generated when voltage-gated ion channels open in response to a change in membrane potential, primarily in axonal and muscle fiber membranes.
  • Characteristics:
  • Action potentials are all-or-none responses
  • An "all-or-none" action potential is a binary response in excitable cells: either a full-strength action potential occurs when the threshold is reached, or nothing happens when it's not reached. There are no partial action potentials.
  • The intensity of the stimulus is encoded by the frequency of the action potentials.
  • Conducted without decrement, ensuring signal integrity over long distances.
  • Initiation: Requires membrane depolarization to a threshold value, which can be triggered by electrical currents from the soma and dendrites or external electrical stimulation.

Electrotonic Potentials

  • Nature and Propagation: Passive changes in membrane potential that are conducted with decrement across the plasma membrane.
  • Features:
  • Not associated with changes in ion channel permeability.
  • The amplitude is graded according to the intensity of the stimulus.
  • Can be summated, which helps in increasing the overall membrane potential toward the action potential threshold.
  • Role: Essential in the propagation of graded responses like postsynaptic and receptor potentials, and in facilitating the generation of action potentials by moving the membrane potential towards the threshold.

Interrelation

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