Lecture 14 : Somatic Sensation Flashcards
What are the 4 somatic sensations?
- Temperature - thermoreception
- Position of limbs - proprioception
- Touch - mechanoception
- Pain - nociception
Describe the organisation of the somatosensory cortex:
- It has a somatoroptic map, where body parts are mapped to specific cortical areas
- Area ∝ density of sensory receptors - fineness of discrimination and use
- This organisation is plastic
What is transduction?
- The conversion of a sensory stimulus into an action potential
- The receptor is depolarised and an action potential is initiated at the first node of ranvier if depolarisation is
sufficient to cross threshold - Action potential then propagates towards the cell body
Describe the 5 types of sensory receptors:
- Meissner’s Corpuscle (sensitive touch)
- Merkle’s Corpuscle (small receptive fields)
- Free nerve endings (pain)
- Pacinian Corpuscle (large receptive fields)
- Ruffini Corpuscle (touch, skin stretch)
Describe the number of receptors in hairy skin compared to glabrous skin:
Hairy skin has less sensory receptors than glabrous skin
Explain mechanoreceptor transduction:
- Mechanical stimulus
- Mechanosensitive channels open
- Influx of ions - Na+, Ca2+
- Action potentials generated at 1st node on the receptive end
What is a fast adapting receptor?
- Potential decays and AP’s stop
- Despite sustained stimulus = adaptation
What are rapidly adapting receptors?
- Only respond briefly, even if stimulus sustained
- Good for detecting movement and changing pressure
- Meissner’s and Pacinian corpuscles
What are slowly adapting receptors?
- Respond longer throughout a stimulus
- Good for signalling duration of stimulus
- Merkels discs and Ruffini endings
Describe the mechanism of mechanically gated ion channels:
E.g. Touch receptors
1. Physical stimulus
2. Membrane deformation
3. Ion channels open
4. Influx of ions
5. If stimulus is strong enough, this depolarises the membrane to threshold and an action potential is generated
Describe the mechanism of temperature gated ion channels:
G-protein coupled receptors
1. Activated by temperature - undergo physical transformation which causes the influx of ions
2. Different receptors are activated at different temperatures
3. Some receptors also be activated by ligands, such as menthol on the cool receptor or capsaicin on the heat receptor
What is lateral inhibition useful for?
- Mechanism to increase accuracy of sensory information
- Mediated by inhibitory interneurons
Explain the mechanism of lateral inhibition:
- The receptor that receives the most stimulus will fire the most APs
- This AP will synapse onto the inhibitory interneurons of neighbouring receptors
- Preventing the signals from these receptors from reaching the brain
- This results in APs only propagating to the brain from the neuron which had the most stimuli
What is modality?
Specific receptors are activated based on the type of stimulus e.g. touch vs temp.
What is intensity?
Coded by frequency of action potentials in each axon and the number of axons recruited/activated
What is duration?
Length of time action potentials are transmitted/sustained
- Slowly adapting receptors are better suited for coding stimulus duration
What is location?
- Smaller receptive fields provide better spatial resolution
- Fine-tuned by lateral inhibition to pinpoint location
- Somatotopic receptive fields in the somatosensory cortex correspond to the body part of stimulus origin
What are the 2 sensory pathways in which the somatosensory cortex receives sensory information?
- Dorsal column pathway
- Anterolateral pathway
Describe the dorsal column pathway:
- Touch and proprioception
- 1st axon in dorsal column
- Synapses onto 2nd axon in dorsal column nucleus
- Decussation at brainstem
- Synapses onto 3rd axon in the thalamus
- Information relayed to a specific area in the sensory cortex
Describe the anterolateral pathway:
- Pain and temperature
- Synapse and decussation in spinal cord
- Synapses in thalamus at 3rd axon
