Somatic sensation Flashcards
What are the 2 subsystems of somatic sensory systems? What are their basic functions?
- Mechanosensory
- helps to identify shape, texture of surfaces ->
enables exploration of environment - monitors forces outside and inside of our body
e.g. muscles
- helps to identify shape, texture of surfaces ->
- Pain and temperature
- help to inform our system about potential
threat, injury
=> both contribute to proprioception - sense of our body
- help to inform our system about potential
How would you simply characterize basic organization?
- Pathways are forming parallel channels that reflect specialization arising right at the receptor level = labeled lines system (channel is labelled based on the information it carries)
- Two main pair of pathways:
- For the body (including back of the head) X for the face
What is the general organization of the first order neurons?
- types of cells, fibers, way of transmission further?
Dorsal root ganglion = home for cell bodies
- pseudomonopolar cells - one connection to the body while the other to CNS
Separation of fibers
- different receptor endings
1. Mechanisensory fiber - curved growth pattern
- Enters dorsal column (white matter) - along the course of te spinal cord
2. Temperature
- Enters dorsal horn and immediately makes a synaptic connection with another neuron
What can be said about the morphology of first order axons?
- Different morphological categories of first order axons that relate to their specialization
- E.g. proprioception (e.g. muscle spindle) have well myelinated axonx X pain perception = unmyelinated (C) or just a little
Recall how sensory transduction works in mechanosensory system.
- Channels pushed to open by the physical forces
- Going from receptor potential -> to spike potential
Which nerve endings are responsible for the sensation of light touch? (where, how, function, RF)
= small receptive field (especially sensitive to spatial stimulation)
= form and texture of light touch
- High innervation density = high spatial acuity
1. Meissner corpuscle
- nerve ending with Schwann cells
- At the place where dermis meets the epidermis
- perpendicular to the skin = sensitive to small pathes of skin
2. Merkel cell-neurite complex
- between dermis and epidermis
- generation of receptor potential -> release of neurotransmitters -> activating first-order neurons
What are the other nerve endings with larger receptive fields? (name, function, location, RF)
= large receptive field
= low innervation density = low spatial acuity
1. Ruffini corpuscle
- sensitive to strech e.g. extending digits
2. Pacilian corpuscle
- In subcutaneous layer
- Many layers of Swann cells around the nerve ending
- Buffered with extracellular fluid -> when pressure applied -> rapid activation -> quckly fluids redestribute to restore itself
- sensitive to vibration
How does the muscle spindle looks like?
- Formed by specialized muscle fibers and nerve endings
- Within the muscle spindle = intrafusal muscle fibers
- collection of nuclei bundled at the center -> contractile elements extending to the sides
How is muscle spindle supplied - what, location, overall function?
Supplied by:
1. Group Ia afferent neurons innervating the center - wrap around it
2. Group 2a afferent neuron terminating in flower spray ending at the contractile elements
=> sends signals to the spinal cord regarding stretching of the muscles
Which nerve ending is sensitive to muscle force? Where, how supplied?
Axon 1b afferent innervates the junction of the mucle and a tendon
- Branches within collagen fibers
- When muscles move -> collagen matrix gets stretched -> opens ion channels in nerve endings
=> sensitive to muscle force
Which parts of the brain receive info from the mechanosensory neurons (2)? Specific names, location?
- Somatosensory thalamus
= Ventral posterior complex of the thalamus- Ventral posterior lateral nucleus = body
- Ventral posterior medial nucleus = face
- Postcentral gyrus = Primary somatosensory cortex
- first part of the cortex getting mechanosensory info
- from paracentral lobule
What cytoarchitectural areas do we find in the S1? Somatotopy? Homunculus?
- Brodmann areas:
Higher order
- 1 = e.g. touch extending to multiple fingers
- 2 - shape
Physiological part
- 3a - proprioception
- 3b - cutaneous receptors (e.g. touch)
Somatotopy, Homunculus of S1?
- Somatotopic map = mapping of the body on the cortex
- Paracentral lobule = feet -> dorsal part - lower extremity -> trunk -> shoulder -> S curve = hands -> below that - face
- Cortical magnification = magnifying representation of the body over the cortex -> increased sensitivity
- E,g, lips, hands
How does cortical processing progresses from the S1?
Secondary SC
- inferior posterior parietal cortex
- input:
=> further elaborated e.g. memories, emotions
Parietal areas 5, 7
- posterior to superior parietal lobule
- spatial mapping, orientation -> drives our motor system e.g. visually guided reach
What are the pairs of pathways for conscious awareness of mechanosensory division?
- Post-cranial = body
- Mechanical stimuli: dorsal-column medial lemniscal system
- Pain. temperature: anterolateral system
- Face
- Mechanical: principal/chief sensory nucleus of the trigeminal complex
- Pain and temperature: spinal nucleus of trigeminal complex
Give a brief introduction to the dorsal-column medial lemniscal pathway - which neurons and nuclei does it hit?
Involves 3 neurons (from skin to cortex)
1. Dorsal root ganglion neuron
-> enters dorsal column of the spinal cord
-> until it gets to caudal part of medulla
= Dorsal column nuclei
2. Second order neuron/axon
-> decussation
=> Ventral posterior lateral nucleus (VPL)
3. Third order neuron
=> Post-central gyrus
What is the difference between first-order neurons in cervical and lumbar part of the spinal cord?
- dorsal column
- tract + nuclei
- which body parts
- Lumbar: the neuron enters spinal cord via the dorsal horn and continues in the MEDIAL dorsal column = Gracile tract (lower body)
-> eventually meet the Gracile nucleus - Cervical: they contribute to the LATERAL dorsal column = Cuneate tract (form at the upper thoracic cord)
-> Cuneate nucleus
How do we call the structure when gracile and cuneate nuclei connect to the second neuron and decussate the midline?
What do they form after crossing?
= Internal arcuate fibers
= medial lemniscus
Give a brief introduction to the principal nucleus of trigeminal complex pathway - which neurons and nuclei does it hit?
- Ganglion cell of the trigeminal nerve
-> enters brainstem at the ventrolateral surface of the pons
=> Principal/chief nucleus - Second order neuron = Trigeminal lemniscus
-> decussates
=> Ventral posterior medial nucleus (VPM) - Third order neuron
=> Post-central gyrus (only facial part)
Give a brief introduction to the spinocerebellar pathway - which neurons and nuclei does it hit?
Both for lower and upper body (what’s the difference)?
Proprioceptic receptor e.g. musle spindle
1. Dorsal root ganglion cell
-> enters spinal cord
(-> gives rise to local collaterals)
-> Dorsal column until Thoracic segment
=> Clarke’s nucleus
2. Second order neuron
-> In dorso-lateral white matter = Dorsal spinocerebellar tract
=> Inferior cerebellar peduncle
=> Cerebellum
For upper body - doesn’t reach Clarke’s nucleus
-> ascends via dorsal column
=> External cuneate nucleus
2. Second order neuron
=> Inferior cerebellar peduncle
=> Cerebellum
Look at how are Gracile and Cuneate tracts positioned in cervical slice.
Look at how are Clarke’s nucleus and dorsal spinocerebellar tract positioned in thoracic slice.
NOTE: cerebellum gets ipselateral input due to spinocerebellar tract
Look at how are External cuneate nucleus positioned in slice of brainstem.
Look at where would we find nuclei with second order neurons in a slice of brainstem.
NOTE: nucleui are surrounded by the corresponding tracts
What structure do we see when Gracile tract crosses the midline (=internal arcuate fibers)?
What’s the nucleus corresponding to this nerve?
-> grows axons to the trigeminal lemniscus -> as we ascend it will move to the position of Ventral posterior medial nucleus
Look closer at the medial lemniscus.
Left Medial lemniscus
- Where do the axons have soma?
- Right dorsal column nuclei (+ upper part - cuneate, lower - gracile)
- Where do the axons have synapses?
- Left ventral posterior lateral nuclei
