L13. Somatic Sensation Flashcards
Somatic sensation
- Temperature (thermoreception)
- Position of limbs (proprioception)
- Touch (mechanoreception)
- Pain (nociception)
Peripheral receptors –> somatosensory cortex
- Coding of sensory information
Somatotopic representation in sensory cortex?
- Ordered representation of body
- Area is proportional to density of receptors (fineness of discrimination) and use
- Margins of representations are modifiable (“plastic”)
Sensory receptors?
Convert physical stimulus to action potentials in primary sensory neuron (transduction)
- Thermoreceptors (temperature) = different types more sensitive to warm or cold temperature ranges
- Proprioceptors (position of limbs) = muscle spindles, Golgi tendon organs, joint capsule) skin mechanoreceptors
- Mechanoreceptors (touch) = sensitive to mechanical deformation
- Nociceptors (pain) = respond to extreme mechanical, temperature, or chemical stimuli
Different sensory receptors on skin?
- Meissner’s corpuscles = sensitive touch
- Merkle’s corpuscle = small receptive fields
- Free nerve endings = pain
- Pacinian corpuscle = large receptive fields
- Ruffini corpuscle = touch, skin stretch
Mechanoreceptor transduction?
Potential decays (and action potentials stop) despite sustained stimulus (adaptation)
Touch receptor adaptation?
Duration:
“Rapidly adapting” receptors
- Only respond briefly, even if stimulus sustained
- Good for detecting movement and changing pressure
- Meissner’s and pacinian corpuscles
“Slowly adapting” receptors
- Respond longer throughout a stimulus
- Good for signalling duration of stimulus
- Merkel’s discs and ruffini endings
Temperature ion gated channels?
- Activated by temperature
- Different receptors are activated at different temperatures
- Can also be activated by ligands
Menthol –> cool receptor
Capsaicin –> heat receptor
Intensity coding?
Intensity increase = action potential increase = increase neurotransmitter = increased feeling
Lateral inhibition?
A fundamental mechanism to increase accuracy of sensory information
Mediated by inhibitory interneurons
In somatic sensation, lateral inhibition localises sensation to a restricted area of skin
Lateral inhibition is used wherever accurate location of a stimulus is required
Modality?
Specificity of receptors
Intensity?
Frequency of action potentials in each axon
AND
Number of axons activated
Duration?
Length of time action potentials are sustained
Location?
Mapping of receptive fields of individual primary afferents to specific cortical locations (somatotopic representation)
AND
Lateral inhibition
Dorsal column pathway (sensory)?
Fine touch and proprioception
1st afferent neuron is the touch and proprioceptive endings
1st axon in dorsal column then synapses in dorsal column nucleus and then decussates at the brainstem to synapse at the thalamus (2nd axon), then at the sensory cortex (3rd axon)
Anterolateral pathway (sensory)?
Pain and temperature
1st afferent neuron are the pain nerve endings
Synapse and decussation in the spinal cord, then onto 2nd axon to a synapse in the thalamus, and a final 3rd neuron in the sensory cortex
