12-8

Neuronal Information Processing

  • Integration of Stimuli in Neurons

    • Individual neurons process information by integrating excitatory and inhibitory stimuli
    • Learning Outcome: Discuss the interactions that enable information processing in nervous tissue
    • A single neuron may receive inputs from thousands of synapses
    • Some neurotransmitters arriving at the postsynaptic cell can be either excitatory or inhibitory.

  • Response of Neuron to Stimuli

    • The net effect on membrane potential of the cell body, particularly at the axon hillock, determines neuron response
    • If depolarization occurs at the axon hillock:
    • Affects membrane potential at the initial segment
    • If threshold is reached, an action potential is generated and propagated along the axon
    • The axon hillock is responsible for integrating the stimuli affecting the cell body and dendrites

  • Integration Process

    • The integration process dictates the rate of action potential generation at the initial segment
    • It represents the simplest level of information processing in the nervous system
    • The postsynaptic cell's response depends on:
    • Behavior of stimulated receptors
    • Other stimuli affecting the cell simultaneously
    • Topics covered include:
    • Presynaptic inhibition
    • Presynaptic facilitation
    • Rate of action potential generation
    • Higher levels of information processing involve interactions among neurons

  • Postsynaptic Potentials

    • Defined as graded potentials that develop in the postsynaptic membrane in response to neurotransmitters
    • Major types of postsynaptic potentials:
    • Excitatory Postsynaptic Potential (EPSP)
      • A graded depolarization caused by neurotransmitters opening chemically gated ion channels
      • Example: Graded depolarization from the binding of acetylcholine (ACh)
      • Affects only the area immediately surrounding the synapse
      • Not all neurotransmitters have excitatory effects
    • Inhibitory Postsynaptic Potential (IPSP)
      • A graded hyperpolarization of the postsynaptic membrane
      • Example: IPSP may result from opening potassium ion channels
      • Requires a larger depolarizing stimulus to reach the threshold
      • Illustrates a situation where a depolarizing stimulus shifts the membrane potential from -70 mV to -60 mV, will normally produce an action potential; however:
      • If membrane is reset at -85 mV by an IPSP, the same stimulus shifts potential to -75 mV, below threshold

  • Integrating Postsynaptic Potentials

    • Individual EPSP has small effects, typically producing a depolarization of about 0.5 mV
    • Action potential requires local currents to depolarize at least 10 mV at the initial segment
    • Single EPSPs will not generate an action potential even if located at the axon hillock
    • EPSPs combine through a process known as summation
    • Summation integrates effects of all graded potentials affecting a membrane segment
    • EPSPs, IPSPs, and their interactions are essential in this discussion

  • Types of Summation

    • Temporal Summation
    • Addition of stimuli occurring in rapid succession at a single active synapse
    • Analogy: Using a bucket to fill a bathtub; one bucket alone isn't sufficient, but consistent effort will fill it
    • A typical EPSP lasts about 20 milliseconds, allowing a series of action potentials to arrive quickly
    • Effects of sequential EPSPs can combine, leading to increased depolarization
    • Spatial Summation
    • Occurs when multiple stimuli at different locations cumulatively affect membrane potential
    • Analogous to multiple buckets filling a bathtub at the same time
    • More active synapses at once increase localized effects, contributing to potential at the initial segment

  • Facilitation of Neurons

    • Defined as a state where membrane potential shifts closer to threshold, making it easier for subsequent depolarizing stimuli to trigger an action potential
    • The more significant the facilitation, the less stimulus is needed to reach threshold
    • Facilitation can result from:
    • Summation of EPSPs
    • Exposure to certain drugs (e.g., nicotine) that stimulate postsynaptic ACh receptors and enhance synaptic communication
    • Nicotine's role in enhancing dopamine release leading to addiction

  • Summation of EPSPs and IPSPs

    • Both EPSPs and IPSPs combine spatially and temporally
    • They activate distinct types of chemically gated ion channels, having opposing effects on membrane potential
    • Example: EPSPs add "water" to a bathtub while IPSPs remove it
    • If more EPSPs than IPSPs are present, the water level rises, leading to depolarization
    • Conversely, if IPSPs exceed EPSPs, water level and depolarization decline

  • Neuromodulation

    • Neuromodulators or hormones can modify postsynaptic membrane sensitivity to neurotransmitters
    • This results in either facilitation or inhibition, influencing CNS and PNS neurons

  • Presynaptic Regulation

    • Inhibition or facilitation can occur at presynaptic neurons via axoaxonic synapses
    • Presynaptic Inhibition:
      • For example, GABA inhibits voltage-gated calcium ion channel openings, reducing neurotransmitter release
    • Presynaptic Facilitation:
      • Enhances neurotransmitter release when an action potential arrives at the terminal
      • Example: Serotonin increases calcium ion channel activity, enhancing neurotransmitter effects

  • Rate of Action Potential Generation

    • Complex information translates into action potentials that propagate along axons
    • Frequency of action potentials can indicate intensity of stimuli
    • Lower frequency (1 per second) results in isolated muscle twitches
    • Higher frequency (100 per second) can induce sustained contractions
    • Sensory perception may differ based on the frequency of action potentials experienced
    • Factors affecting action potential generation:
    • Degree of depolarization above threshold correlates with action potential frequency
    • Membrane can respond to second stimuli after absolute refractory period ends
    • Prolonged depolarization increases action potential production rate

  • Absolute Refractory Period

    • Determines maximum theoretical frequency of action potentials, influenced by axon diameter
    • Shortest in large diameter axons, allowing for up to 2,500 per second
    • Real recorded frequencies in the body range from 50 to 1,000 per second

  • Summary of Information Processing

    • Basic components of nervous tissue and the significance of action potentials are now understood
    • Higher organizational levels in the nervous system will be addressed in upcoming chapters
    • Action potentials responsible for diverse sensory and motor experiences

  • Clinical Case: Franklin D. Roosevelt and Polio

    • Poliomyelitis caused by a virus that spreads easily and is mostly asymptomatic
    • Small percentage progress to central nervous system infections impacting motor neurons, causing flaccid paralysis
    • Ambiguities exist regarding Roosevelt's diagnosis:
    • Symptoms inconsistent with typical polio presentation
    • Paralysis was asymmetric and non-typical, recovery pattern was descending
    • Symptoms align more closely with Guillain Barre syndrome, which attacks peripheral nerve myelin
    • Discussion highlights the historical context and contemporary implications regarding polio eradication efforts

  • Related Clinical Questions

    1. Which CNS cells does the poliovirus target?
    • Motor neurons in the spinal cord.
    1. If Roosevelt had Guillain Barre syndrome, where was the pathology?
    • Attacks myelin covering of peripheral nerves in adults.

  • Clinical Terminology and Implications

    • Understanding differing presentations of neurological diseases is vital for diagnosis and treatment
    • Importance of advancing knowledge in neurology to inform public health initiatives like polio eradication efforts.