Lecture 21 Somatosensory Pathways 1 Flashcards
Types of receptors in the somatosensory system that react to different types of stimulation
Thermoreceptors, Nocioreceptors, Mechanoreceptors, Proprioreceptors
Common features of the somatosensory pathways
[A] 1° 2°, 3° afférent neurons
1° (PNS) - pseudounipolar neuron
-> bring info from periphery (synapses with receptors), cell body in dorsal root ganglion
2° & 3° (CNS) - synapses with neurons in brain
[B] Decussation (spinal cord/brain stem) - allow better separation of tracts -> prevent against wiring errors compared to wiring on the same side
How are somatosensory tracts segregated
By somatotopic arrangement or by sensory modality
What is mechanoreception and what are the two pathways for it?
Sensation of skin distortion or mechanical pressure
Dorsal column-medial lemniscus tract (discriminative pathway)
Anterior spinothalamic tract (non-discriminative pathway)
Both relate to sensitivity of different receptors
Describe the dorsal column-medial lemniscus pathway
*neurons, pathway type, decussation, senses…,
Discriminative pathway
Fine touch, pressure, vibration, proprioception
1° neurons - fasciculus cuneatus -> upper body (~ lateral pathway)
- fasciculus gracilus -> lower body (~ medial pathway)
() -> somatotopic arrangement
2° neurons -> decussates in brain stem after synapse with 1°
- nucleus cuneatus, nucleus gracilus (info based on where they from) align with FC and FG
->travel along medial lemniscus tract (brain stem medulla oblongata )
3° neurons -> 2° synapse with in thalamus to primary sensory cortex
Describe the anterior spinothalamic tracts
*neurons, pathway type, decussation, senses…,
Non-discriminative pathway
Crude touch, pressure
1° neurons -> synapse in dorsal column of spinal cord
2° neurons -> decussates in spinal cord after synapse with 1
3° neurons -> 2° synapse with in thalamus to primary sensory cortex
Purpose of different mechanoreceptor types
List the 6 mechanoreceptors, their location and what they respond to
Enable perception of different sensation and sensitivity
Free nerve endings - epidermis - light skin contact
Tactile discs - epidermal base - light skin contact
Root hair plexus - deep in dermis - initial hair shaft contact
Encapsulated receptors
Tactile corpuscle - in dermis under epidermis - initial contact & low -frequency vibrations
Lamellated corpuscle - deep in dermis - initial contact & high frequency vibrations
Ruffini corpuscle - deep in dermis - dermis stretching and distortion
What do the six types of mechanoreceptors sense/ detect ?
[Fine touch]
Free nerve ending - touch, pressure, stretching
Merkel cells and Tactile discs - sustained touch and pressure, sensitive to fine touch
Free nerve endings of root hair plexus - hair movement and distortion
[Pressure & vibration]
Tactile corpuscle - light touch, movement, vibration, texture changes
Lamellated corpuscle - deep pressure, rapid vibration
[Deep pressure]
Ruffini corpuscle - tension (deep in skin)
How are action potentials triggered from the receptors?
Encapsulated receptors (surround fibres) - distortion of capsule and accessory cells (tactile corpuscle) & lamellae (collagen fibres separated by fluid - lamellated corpuscle) generate AP
Receptors have different activation thresholds need to be overcome. - receptor potential above activation threshold
Collagen movement / axon membrane stretch - axon stretch - channels open - Na+ influx - AP
What is adaptation in mechanoreception?
What are the two types of adaptation?
How fast receptor returns to normal
- Slow adapting - touch, pressure detection -> remain activated to continue sensing it
- Rapidly adapting - texture, vibration detection
Different physiology - allow to sense very accurately different stimuli
Define the role of receptive fields in mechanoreception
Area at which a receptor sense a stimuli
- area (accurate taction) - mechanoreceptors with small, accurate receptive fields (fingers)
What is thermoreception?
Describe thermoreceptors
Transport temperature info from periphery to brain
Free nerve endings -> transient receptor potential channels (TRP) - large receptor family -> different receptors sense different temperature ranges
- TRPM8 - cold
- TRPV3 & TRPV4 - warm temperature
Describe the pathway that carries thermoreception information/temperature sensation
Lateral spinothalamic tract
Non - discriminative pathways
Voluntary responses to temperature
3 neurons
Decussate in spinal cord (synapse before)
Synapse in thalamus
Primary sensory cortex
Describe the non - voluntary response to temperature
Autonomic response - hypothalamus included
Info from thermoregulatory center to effector organs
1. Skin artérioles - hot : dilate for heat to escape, cold: constrict to trap heat
2. Sweat glands
3. Skeletal muscles - shivering response
What is nocioception and what are the nerve fibres & receptors involved
React to noxious temperature, mechanical or chemical stimuli.
Receptor = TRPV1
Require higher stimuli to generate response
Nocioreceptors,
Aδ (first pain) - sharp immediate response, localised to particular response, higher conduction velocity, info to spinal cord (reflex), then carry info to brain
C (second pain) - dull, longer lasting pain, slow conduction velocity
