How is stimulus intensity encoded by action potentials in the nervous system?

Intro

Animals are equipped with a vast array of sensory systems that they use to monitor their internal and external environments.

-> Sensory transduction
1.Sensory transduction is the process by which properties within our external and internal environment become encoded as nerve impulses.

2. It is carried by specialised structures, namely sensory receptors.

Sensory receptors

-Range from single cells to complex sense organs.

-Types of receptors:

1.Chemoreceptors
a.Nose ->Olfactory receptors
b.Skin -> Nociceptors

2.Mechanoreceptors
a. Special senses (ear) - cochlear hair
b.Muscle and joints - muscle spindles
c. Skin and viscera - pacinian corpuscles

3.Photoreceptors
a. special senses (eye) - cones / rods

4.Electroreceptors

5.Magnetoreceptors

6.Thermoreceptors
a.skin - warm and cold receptors
b.CNS - temp sensing hypothalamuc neurons.

-All receptors transduce incoming stimuli into changes in membrane potential.
-> Receptor protein detects stimulus, ->Opening or closing of ion channel. ->Change in membrane potenial
->Signal sent to integrating centre. (CNS)

Sensory receptors 2

1.Telereceptors -> vision, hearing
2.Exteroceptors -> pressure, temp
3.Interoceptors ->blood pressure / oxygen

Chain of events
1. Chemical / Pressure / Light stimulus is received.
2. Causing a signal transduction pathway.
3. Ion channels open.
4. Change in membrane potential.
5. Signal sent to integrating centre.

2 Types of sensory receptor cells.
1. Sensory Neuron
2. Epithelial sensory receptor cells

Sensory neuron chain of events
1. Stimulus received by receptor protein, on the dendrite of the afferent axon of the PNS.
2. Depolarisation occurs. (generator potential)
3. Action potential causes signal to travel to integrating center.

Epithelial sensory receptor cell chain of events.
1. Stimulus received by receptor protein on epithelial sensory cell.
2. causes depolarisation -> receptor potential .
3. Ca2+ diffuses in.
4. Neurotransmitter released from vesicle in epithelial sensory cell.
5.Neurotransmitter taken in by Neurotransmitter receptor, on a dendrite of an afferent neuron.
6. Action potential, causes signal to travel to integrating centre.

Receptors may detect more than one stimulus modality

Adequate stimulus
-Preferred, most sensitive stim modal

Many receptors can be excited by other stimuli, if sufficiently strong.
-for example, pressure on eyelid -> perceive light.

Polymodal receptors
-Sensitive to more than one stimulus modality.
-For example, nociceptors; polymodal receptors for multiple types of pain.

Processing of sensory information

3 Neurons involved.
1. Sensory neuron, connects the sense organ (receptive field) and the interneuron.
2.Interneuron in the CNS connect to the motor neuron in the PNS.
3. Motor neuron transmits signal to effector.

*Receptive field - an area of the body surface where a stimulus could elicit a reflex.

Convergence vs Divergence

Between Primary sensory neurons and secondary sensory neurons until the CNS.

Basically: Primary neurons > Secondary neurons > CNS

10>3>1

Opposite of this is divergence.

Stimulus Encoding

All stimuli are ultimately converte into action potentials in a primary afferent neuron.

How can organisms differentiate among stimuli or detect the strength of the signal?

Sensory receptors and sensory neurons must encode four types of information.
-Stimulus modality
-Stimulus location
-Stimulus duration
-Stimulus intensity

Stimulus modality and location

-Receptor location encodes stimulus modality and location.

-Integrating center interprets modality and location.

-Modality
1. Theory of labeled lines
->Discrete pathways from sensory cell to integrating center.
2. Polymodal receptors are exceptions.
->Encode modality via temporal patterns of APs

-Location
->Theory of labeled lines.

Receptive Field and Location of stimulus.
-Receptive field: "region of the sensory surface that causes a response when stimulated."
-Smaller receptive field allows more precise location of the stimulus. (-> greater activity)

Improved ability to localise stimuli by:
-Using more than one sensory receptor cell to convey a signal.
-Lateral inhibition: Signals from neurons at the center of the receptive field inhibit neurons on the periphery.

Stimulus intensity

Sensory neurons code stimulus intensity by changes in action potential frequency.
- Strong stimuli -> high frequency.

Dynamic range
1. Range of stimulus intensities over which a receptor exhibits an increased response. (range between threshold intensity and receptor saturation)
2. Threshold of detection: weakest stimulus that produces a response in a receptor 50% of the time.
3. Saturation -> top of the dynamic range. (maximal response)

Dynamic Range and discrimination

Trade-off between dynamic range and discrimination.

1. Large dynamic range
-large change in stimulus causes a small change in AP frequency.
->large dynamic range
->poor sensory discrimination.

2.Narrow dynamic range
-small change in stimulus causes a large change in AP frequency.
->small dynamic range
->good sensory discrimination.

RANGE FRACTIONATION
->Groups of receptors work together to increase dynamic range without decreasing sensory discrimination.

Encoding logarithmically
->encode a wide range of stimulus intensities using a single receptor cell.
-Good discrimination at certain intensities.
-Poor discrimination at other intensities.

Stimulus duration

Two classes of receptors encode stimulus duration
1. Phasic
-Produce APs at the beginning or end of the stimulus.
- Encode change in stimulus, but not stimulus duration.

2.Tonic
-Produce APs as long as the stimulus continues.
-Encode duration of stimulus.
-Receptor adaptation -AP frequency decreases if stimulus intensity is maintained at the same level

Principles of Transduction

1. Stimulus
2. Receptor
3. Change in the ionic permeability of the afferent nerve ending.
4. Change in the membrane potential of the nerve ending.
5. Generation of action potentials in the afferent nerve terminal.
6. Propagation of action potentials to CNS.
7. Integration of information by CNS.

*The adequate stimulus leads ultimately to a change in the membrane potential known as the receptor potential

Basic steps in the transduction pathway.

The magnitude and duration of the receptor potential regulates the number and frequency of action potentials transmitted by the afferent nerve fibres to the CNS.

A weak stimulus will cause a sub-threshold receptor potential
which does not generate an AP.

Stimulus that generates
a super-threshold response will generate an AP.

Longer and stronger stimuli will increase the rate of AP firing/

Mechanoreceptors

Meissner's corpuscle: responsible for sensitivity to light tough and have the lowest threshold to vibrations lower than 50 Hertz. They are rapidly adaptive receptors.

Pacinian corpuscle: detect gross pressure changes and vibration and are rapidly adapting receptors. -> stimulated by higher frequencies, responds to the maintained skin indentation with a single AP.

Ruffini ending or Ruffini corpuscle: slowly adapting mechanoreceptor.

Merkels discs: Sustained response to pressure, Merkel nerve ending are slowly adapting.

Free nerve endings: are unencapsulated and have no complex sensory structures. Some respond to touch, but also to temperature.

Coding of stimulus intensity

The activation of a specific population of receptors will inform the CNS about the location and nature of the stimulus.

The intensity of the stimulus is coded by the number of active receptors and the number of action potentials that each receptor generates.

Adaptation

Rapid adapting receptors: respond to the onset of a stimulus with a few action potentials and then become quiet. (quiescent)

Slowly adapting receptors: respond by maintaining a steady flow of action potentials for the duration of the stimulus.

How is the intensity of the stimulus coded into action potentials?

How is stimulus intensity coded by the nervous system?

How are action potentials coded?

How does the neurons send information about the intensity of a stimulus?