Somatosensation Flashcards
What are the types of sensory nerves?
- Slowly adapting type I (SAI)
- Slowly adapting type II (SAII)
- Rapidly adapting type I (RAI)
- Rapidly adapting type II (RAII)
What are the types of sensory nerve endings and the fibre types that possess them?
- Naked ending: Fibres terminate without any accessory structures. Aδ, C fibres
- Encapsulated endings: Fibres terminate within accessory structures that aid in sensation. Aβ fibres
What type of accessory structure is associated with SAI fibres?
- Merkel cells
- Incomplete adaptation
- Contact (texture, edges, points)
What type of accessory structure is associated with SAII fibres?
- Ruffini endings
- Incomplete adaptation
- Tension, folding, joint movements
What type of accessory structure is associated with RAI fibres?
- Meissner’s corpuscules
- Complete adaptation
- Shearing
What types of accessory structure is associated with RAII fibres?
- Pacinian corpuscules
- Complete adaptation
- Deep pressure
What are accessory structures?
Accessory structures are structural components of sense organs which may play an importanty role in protection, conduction, concentration, analysis, sensitisation or inhibition; but are not directly involved in transduction.
What features makes Meissner’s corpuscules good at detecting shearing?
Mechanical coupling to papillary ridges in skin
Where are pacinian corpuscules found?
- Dermis of glabrous skin
- Connective tissues of muscles
- Periosteum of bone
- Mensentery of abdomen
What is the maximum sensitivity frequency of Pacinican corpuscules?
~200 - 250Hz
What is the maximum sensitivity frequency of Meissner’s corpuscules?
~10-40Hz
What is the sensitivity range of Merkel cells?
200μm - 1500μm skin deformation
Where are Ruffini endings found?
- Wrist
- Finger joints
- Skin folds of palm
What types of receptive fields possessed by each type of fibre?
- Type I: Small localised areas of high sensitivity
- Type II: Small area of high sensitivity surrounded by large area of low sensitivity
Which types of fibres are involved in high acuity tactility and why?
- SAI (Merkel cells) and RAI (Meissner’s corpuscules)
- These have the smallest receptive fields
What are the functions of slow/rapidly adapting fibres?
- Slow adapting fibres: Detection of details in stationsary objects such as edges, corners, curvature, points. Gives detail about shape, texture and size of object.
- Rapidly adapting fibres: Detectin of quickly changing stimuli such as vibrations and movement.
How is stimulus intensity encoded by different types of tactile fibres?
- Rapidly adapting: Recruitment, due to phase-locking not allowing frequency code.
- Slowly adapting: Frequency code.
Which fibres are concerned with temperature sensation?
Aδ, C fibres
How many colds and warm spots are there?
- ~700 cold
- ~24 warm
What types of receptor molecules are involved in temperature detection?
- Transient receptor potential (TRP) channels
- Warm (e.g. TRPV1)
- Cold (e.g. TRPM8)
What causes paradoxical cold?
>45oC, cold receptors are stimulated to give cold sensation.
What is the principle behind “labelled lines”?
Sensory experience is determined by the central connection of a particular sensory neurone, not the stimulus that stimulates it.
What are the types of axons found in nerve bundle and their associated conduction velocities?
- Aα: Myelinated motor fibres (80-120 m/s)
- Aβ: Myelinated somatic sensory fibres (40-80 m/s)
- Aδ: Thinly myelinated temperature and pain fibres (5-30 m/s)
- C: Unmyelinated temperature and pain fibres (0.5-2 m/s)
Which class of axons are affected by anoxia?
Aβ fibres (somatosensation)
Which class of axons are affected by local anaesthetics?
Aδ/C fibres (pain/temperature)
What are the functions of the spinal cord?
- Motor innervation to skeletal muscle (ventral root)
- Somatosensory innervation to skin (dorsal root)
- Automatic innervation to viscera
How many pairs of spinal nerves are there?
31
What is the nature of termination in the spinal cord for the somatosensation fibres?
- Both Aβ and Aδ/C fibres terminate in the dorsal horn of the grey matter.
- Aδ/C fibres terminate directly in layers I and II of the dorsal horn.
- Aβ fibres wind around dorsally and enter layers III-V of the dorsal horn.
What is the function of the dorsal column-medial lemniscus (DC-ML) system?
Central pathway through which touch and proprioceptive information from Aβ fibres are relayed.
What is the structure of the DC-ML system?
- As Aβ fibre enters the spinal cord, it bifurcates into a short branch entering the dorsal horn and a long branch ascending in the dorsal column.
- Below the mid-thoracic level (T7), nerve fibres enter the gracile fascicle on the medial aspects of the dorsal column.
- Above the mid-thoracic level, the dorsal column is divided by a sulcus and subsequent spinal nerve entering the column enters a more lateral cuneate fascicle.
- Nerve fibres in the cuneate/gracile fascicles synapse in the cuneate/gracile nuclei of the medulla oblongata respectively, together forming the dorsal column nuclei.
- Axons from the dorsal column nuclei cross the midline to ascend as the medial lemniscus contralaterally, being joined by fibres from the trigeminal nuclei (receiving sensory afferents from head).
- Fibres synapse in the ventroposterior medial/lateral nuclei of the thalamus before projecting into the primary/secondary somatosensory cortex.
What is the topographic arrangement of fibres in the DC-ML system?
- Medial-most fibres from the legs and sacral region.
- Lateral-most fibres from the arm.
What is the function of the spinothalamic (ST) system?
Primary pathway through which temperature and nociceptive information from Aδ and C fibres are relayed to brain.
What is the structure of the ST system?
- Nociceptive fibres entering spinal cord give both ascending and descending branches into Lissauer’s tract.
- Branches from Lissauer’s tract enter the grey matter and synapse with second order cells principally in substantia gelatinosa. This synapse allows for gating mechanisms (e.g. gate-control of pain).
- Fibres originating from lamina I ascend in the lateral spinothalamic tract while fibres originating from lamina V ascend in the ventral spinothalamic tract; both crossing over at the midline to ascend on the contralateral side. Together, these 2 tracts form the anterolateral system.
- At the medial lemniscus, fibres from the trigeminal nucleus cross the midline to join the tract.
- From the medial lemniscus, fibres in the spinothalamic tract synapse in the ventral posterior lateral/medial nuclei of the thalamus before projecting various areas of the brain.
Which areas of the brain do fibres in the ST system project?
- Primary somatosensory nucleus
- Insula
- Anterior cingulate cortex (ACC)
What additional tracts are given off by the anterolateal tract of the ST system?
- Spinoreticular tract: Originates from laminae VII and VIII and terminates in the reticular formation and the thalamus.
- Spinomesencephalic tract: Originates from laminae I and V and terminates in the mesencephalic reticular formation and the periaqueductal grey of the midbrain.
What are the origins of sensory inputs into various parts of the thalamus from the DC-ML and ST systems?
Ventroposteromedial nucleus: Head
Ventroposterolateral nucleus: Body
What types of clinical conditions are associated with spinal cord transections?
- Spinal cord hemisection (Brown-Sequard syndrome):
1. Ipsilateral loss of tactility and proprioception.
2. Contralateral loss of pain and temperature. - Posterior column syndrome (e.g. tabes dorsalis):
1. Pain and temperature unaffected.
2. Bilateral loss of tactility and proprioception. - Syringomyelia (fluid-filled cavity in spinal cord):
1. Bilateral loss of pain and temperature.
2. Tactility and proprioception unaffected. - Total transection:
Bilateral loss of all senses.
What are the principles behind referred pain?
Neurones in lamina V of spinal cord synapse with both Aδ/C fibres from skin and visceral afferents.
What is the location of the primary somatosensory cortex?
Posterior to the central sulcus in post-central gyrus.
What are the Brodmann’s areas associated with primary somatosensory cortex (S1)?
3a, 3b, 1, 2
What is the nature of connectivity in S1?
- Layers I-III project to higher cortical areas via pyramidal cells and are involved with association.
Layer IV receive afferent inputs from the thalamus.
Layer V sends efferent projections to the thalamus.
Layer VI sends efferent projections to other subcortical regions.
What is referred pain?
Pain felt in a part of the body away from the actual source of pain.
What is the astereognosis and what is it caused by?
- Patient is able to feel objects, but are unable to use tactile information to describe the shape of object.
- Lesion in posterior parietal region of somatosensory cortex.
