Lecture 8 - Sensory; LLT, Receptors, Modality/location/intensity

Sensory system

• Stimulus activates a sensor

• Afferent neuron stretches from that sensor to an integrating center (Interneuron), often in brain, spinal chord, or outside CNS (faster)

  • Sensory neuron = Afferent

  • Integrating center neuron = Interneuron

• Decision is made (un/conscious), and message is sent to an effector, causing the effect occur

  • Motor neuron = Efferent

Sensory stimuli are detected where? (2)

• Neurons

• Accessory cells

Sensory neuron

• Sensory neuron

  • Dendrites replaced by sensors

• Stimulus

• → received by receptor protein

• → produce a graded potential (generator potential)

• → reaches trigger zone (axon hillock), and if large enough, leads to an action potential

• → goes down axon

• → reaches synapse, sends message towards integrating center

Sensory epithelial cells (accessory cells)

• Completely separate from neuron, has a receptor on one end

• Receptor of cell receives a stimulus

• → Stimulus causes pressure

• → Pressure opens Na+ and K+ channels, creating a generator potential in the sensory cell

• → At the synapse, Ca2+ channels open

• → Vesicle go towards synapse, fuses with PM, and releases neurotransmitters into the synapse

• → NT touch receptor on the sensor of an Afferent Neuron

• → Graded potential goes towards integrating center

Sensory receptors (3)

• Convert incoming stimuli into changes in membrane potential

  • Chemoreceptors

  • Mechanoreceptors

  • Photoreceptors

Chemoreceptor

• Receives chemical stimulus

  • Taste, smell

• Chemical attaches to receptor protein

  • Often GPCR

• Opens ion channel

• Creates generator potential

• Sends message to integrating center

• Response

Mechanoreceptors

• Often baroreceptors - change in pressure

  • Skin, kidneys, vagus nerve, ears

• Instead of a ligand, a touch opens ion channel

• Leads to generator potential

• Sends message to integrating center

• Response

Photoreceptors

• Light stimulus

  • Specific receptor - photoreceptor

• Opens ion channel

• Leads to generator potential

• Sends message to integrating center

• Response

Sensory receptors - classified by stimulus modality

• Chemoreceptors - presence of chemicals in environment

• Mechanoreceptors - pressure and movement, including proprioception

• Photoreceptors - light

• Thermoreceptors - temperature

• Electroreceptors - electrical fields

• Magnetoreceptors - magnetic fields

Sensory receptors convert info about stimulus into - 4 features

• Action potentials ; encode 4 important features:

1. Stimulus modality

2. Stimulus location

3. Stimulus intensity

4. Stimulus duration

[Modality] Sensory pathways and stimulus modality - Labeled Line Theory (LLT)

• Generally, a particular afferent neuron is associated with one type of receptor ; one receptor recognizes one sense - Labeled Line Theory (LLT)

  • Each afferent neuron follows a particular pathway for integration - one receptor → message to integrating center → out effector ; always same sense

  • Brain recognizes the particular sense because it came from a particular receptor

  • The type of sensation you perceive is determined by which sensory pathway (line) is activated, not by the physical nature of the stimulus itself

• Theory isn’t perfect, has issues

[Modality] Polymodal receptors

• Sensitive to multiple sensory modalities - contradict LLT

  • Respond to more than one type of stimulus (modality) and produce the same type of sensory perception.

    • Thermal stimuli (heat/cold)

    • Mechanical stimuli (pressure, stretch)

    • Chemical stimuli (acids, capsaicin, irritants)

  • Despite different inputs, the same neuron fires.

• Ex: Nocireceptors (pain)

  • They can detect:

    • Extreme heat or cold

    • Mechanical damage (pinch, cut)

    • Chemical irritants (acid, capsaicin, inflammatory mediators)

  • All of these:

    • Activate the same nociceptive neuron

    • Send signals to the same CNS pathways

    • Are perceived as pain

  • Different stimulus → same percept

[Location] Receptive fields

• Touch - where is this sense coming from?

• Touch sensors (neurons) have sensors on dendrites, which spread out

  • The spread of the sensors - Receptive field

[Location] Receptive fields sensors sizes

• Receptive fields are big or small

  • Upper back - large receptive field

    • Less sensitive

  • Hand - small receptive field

    • Very sensitive

[Location] Receptive fields sensors overlap

• Receptive fields overlap to improve ability and to localize stimulus

  • Touch at one neuron - recognized for that neuron

  • Touch in between neurons - both neurons fire messages, brain recognizes touch in between the 2 neurons

• Help increase acuity of recognizing touch

[Location] Lateral Inhibition

• Some receptors have lateral inhibitory neurons

• Strong stimulus can stimulate multiple neurons →

  • Lateral inhibitory neurons can inhibit pathways of weaker stimulus, pinpoint where strong touch is

  • Makes strong signal feel stronger, weak signals feel weaker

[Location] Things that improve acuity

• Smaller spread

• Receptive field overlap

• Lateral inhibitory neurons contrasting signal strength

[Intensity]

• Receptor potential - how strong a touch is (degrades)

• Greater frequency in strong AP than weaker AP

[Intensity] Receptor Dynamic range

• Receptor will eventually get saturated ; can only sense so much [heat]

  • Dynamic range - sense differences in intensity until fully saturated

[Intensity] Receptor Dynamic range sizes

• Small dynamic range - receptor is really sensitive within that range

  • B - can produce a stronger response at weaker stimuli intensity (more sensitive), and will also get saturated quicker

• Large dynamic range - receptor is less within that range (but its

  • A

[Intensity] Range Fractionation

• Discrimination improves by distributing sensitivity amongst the receptor population

  • Different receptors that exist within a family and work together that sense different magnitudes of stimulus intensity / have different Dynamic ranges

• Recognize different intensities

[Intensity] Logarithmic encoding

• Allows for compromise between dynamic range and discrimination

• Sensitive at low ends of scale, less sensitive at the higher ends

• Allows for greater range, but sacrifices sensitivity on high end

  • Ex: Can EASILY recognize 1 lb from 2 lb from 10 lb, but not 100 lb from 200 lb, because they are too heavy, can’t really tell the difference

• Light, sound, and weight are encoded logarithmically by the nervous system