NEURAL CODING
Overview of Topics
Lesson Objectives
Kinds of Neural Coding Possible
Anatomical Basis for Neural Coding
Function of Receptors
Concepts
Lesson Objectives
There is no direct input of physical energy into the nervous system. Rather, receptors detect and tranduce this energy into neural impulses. While neural impulses vary as a function of the length of the myelin segments, they otherwise are identical. The brain must function to decode these impulses much like a Moris code operator does.
- How do the receptor systems code information?
- What are the primary features of the CNS decoding process?
- Explain qualitative coding
- Explain quantitative coding
- Explain the similarities and differences between receptor, generator, and graded potentials.
- Discuss lateral inhibition and describe its function.
Basic Problem
- Defining the stimulus
- Coding of information by receptor
- Decoding of information at higher levels
- All receptor events transduced to neural impulse
- Same kind of neural event for all stimuli.
- Survival depends on transducer's specialization.
Kinds of neural coding possible
Qualitative coding
- Modality coding (Muller,1826- specific energy of
nerves).
- Receptor elements (i.e., rods and cones).
- Pattern of excitation
Quantitative coding
- Frequency of single unit
- Number of units
- Threshold change
Spatial Localization (receptor distribution): Spontaneous responses (on, off, on-off). (Adaptation, Habituation, & Fatigue)
Anatomical basis for neural coding
- Spatial connections between units
- Higher centers
- Receptors (Hartline- lateral inhibition).
- Mutual inhibition between neighboring ommatidia
- Each fires at rate related to S intensity
- Each inhibits neighbors
- Inhibition of rate of firing
- Output is function of all above.
- Von Bekesy extended this concept to all receptors (Mach Bands).
Function of Receptor
Generator potential - not spike (small, steady depolarization in neuron).
- Stationary, non-propagating.
- Greater the generator potential, the more frequent the spikes.
- Transducer process sometimes includes other intermediate events:
- Vision - chemical breakdown
- Audition - graded response probably arises from accessory elements.
- Adequate stimulus - energy to which the receptor responds
- Differential sensitivity - (only respond to stimuli within a range).
Cells function in groups in CNS - as a result, new functions arise.
Concepts
- Basic problem is specifying the stimulus.
- Receptors behave as "pass filters" admitting only certain kinds of information to the CNS. Receptors, thus, determine to which stimuli the organism can respond.
- An adequate stimulus is an energy form to which the receptor
responds most readily.
- A receptor transduces physical energy into an electrical change,
the intensity being coded by frequency modulation.
- The major problem is to understand how all receptors (tranducers) can convert sensory information into neural impulses which can be decoded and interpreted in the CNS.
Terms to Know
| transduction |
modality coding |
receptor elements |
| quatalitive coding |
quantitative coding |
on fibers |
| off fibers |
on/off fibers |
lateral inhibition |
| Mach bands |
transduction |
recruitment |
| adaptation |
habituation |
fatigue |
| ERG |
generator potential |
limulus eye |
| ommatidia |
adequate stimulus |
single unit |
