Somatosensory pathways Flashcards
What is somatosensory system responsible for mediating?
Touch and proprioception
What are the mechanosensitive channels thought to be?
TRPs probs
Role for piezo2 channels
What are the 4 types of mechanoreceptor in glabrous skin?
Pacinian corpuscles, Meissner’s corpuscles, Merkel’s discs, and Ruffini’s endings
Which receptors have very small receptive fields? Which are large?
Found that Meissner’s corpuscles and Merkel’s discs had very small receptive fields while Ruffini’s endings and Pacinian capsules had very large
Describe rapidly adaptive mechanoreceptors vs slowly adapting
Rapidly adapting mechanoreceptors respond quickly at first but then stop firing as the stimulus continues
Slowly adapting mechanoreceptors produce a more sustained response.
Draw the table summarising the receptors
OneNote
Describe Pacinian corpuscles
Myelinate Abeta enclosed by concentric layers of connective tissue
What are the three types of follicle receptor in non-glabrous skin?
Hair guard
Hair tylotrich
Hair down
Which nerves are involved?
Abeta subgroup
C fibers → normally involved in pain but thought they could be involved in pleasant touch
Where do the nerve endings go?
dorsal horn of spinal cord
Describe pathway to the cortex
dorsal horn (where they can synapse with second order neurons to form a reflex arc) → travel up the ipsilateral dorsal column → terminate in the medulla in the dorsal column nuclei → decussate before continuing to ascend on the contralateral side of the medial lemniscus → VPL of the thalamus → internal capsule → ipsilateral postcentral gyrus of cerebral cortex (S1)
How are the dorsal columns within the spinal cord organised?
- fibres conveying info about lower limbs = medially and travel upwards as gracile tract
- fibres about upper limbs = more lateral cuneate tract
- these terminate in the gracile nucleus and the cuneate nucleus of the dorsal column nuclei respectively
How does information from the face reach S1?
It is, however, the trigeminothalamic system that is responsible for conveying tactile information about the face
Primary sensory neurons in trigeminal nerve ganglion
Enter at the level of the pons
Terminate on neurons in the trigeminal brainstem complex
Decussate and enter the ventral posterior medial (VPM) nucleus of the thalamus via the trigeminal meniscus
Delivered to the ipsilateral areas of the primary and secondary somatosensory cortices
Describe anterior choroidal syndrome
Anterior choroidal syndrome
Occlusion of the anterior choroidal artery supplies the posterior limb of the internal capsule
Causes contralateral hemihypesthesia (reduction in sensation on one part of the body)
Other clues: contralateral hemiplegia (paralysis on one side) and homonymous hemianopia (blindness)
Draw somatosensory pathway
OneNote
Describe wallenburg syndrome
Injury to the lateral medulla
- loss of pain and temperature contralaterally while touch is spared
- PICA is thrombosed, leads to ischemia and contralateral loss of pain and temperature sensation, touch is not lost because it has a different blood supply and doesn’t travel in the same channel → dorsal column supplied by the small arteries of the vertebral basilar circulation
Where is touch information altered on the pathway?
In both the dorsal column and the thalamic nucleus
- common alteration is contrast enhancement which serves to amplify differences in activity of neighbouring neurons
- one mechanisms achieving this is lateral inhibition (neighbouring neurons inhibit one another, thus enhancing spatial contrast and allowing fine tactile discrimination)
Where is the somatosensory cortex located?
In the postcentral gyrus of the parietal lobe
What is S1 formed from?
Brodmann’s areas 1, 2, 3a and 3b
What order do the Brodmann’s areas come in in S1?
3a, 3b, 1, 2 in that order, anterior → posterior
Draw the diagram from OneNote
Which part of S1 do most afferents terminate in?
3a and 3b which in turn project to areas 1, 2 and S2
Where is S2 located?
Posterior parietal lobe
What are the crude roles of these areas?
Area 3 - shape and texture discrimination
Area 2 - shape
Area 1 - texture
Describe the somatotopic organisation of S1
Feet, legs, arms, trunk, face → medial to lateral
What does the somatosensory association get input from?
Somatosensory, visual and auditory systems
What does slowly adapting mean
means surface stimulation causes continuous APs until removed (firing frequency
directly proportional to extent of stimulus)
What does rapidly adapting mean?
same stimulus causes a
transient AP then silence followed by a small AP when removed
What sort of fibres do cold receptors have?
A delta
What sort of fibres do warm receptors have?
C fibres
Where does lateral inhibition occur?
Dorsal column nuclei
What do lesions in S1 impair?
Somatosensation (dysfunction in texture, size and shape discrimination)
What indicates that the spinal cord is only partially damaged?
there must be some preservation of sensation or motion in the areas
innervated by S4 to S5 (eg. voluntaryexternal anal sphinctercontraction) – the nerves in this area are
connected to the very lowest region of the spinal cord, and retaining sensation and function in these
parts of the body indicates that the spinal cord is only partially damaged.
What do lesions at the lumbrosacral level cause?
result in decreased control of legs and hips, genitourinary system, anus
What do lesions of thoracic area cause?
in addition to problems found in lower-level injuries, thoracic spinal lesions can affect
muscles in the trunk (the lower the level of injury, the less extensive its effects):
What do injuries from T1-T8 cause?
inability to control abdominal muscles, may affect trunk stability
What do injuries from T9-T12 cause?
partial loss of trunk and abdominal muscle control
What do injuries at the cervial level result in?
full or partialtetraplegia(aka. quadriplegia);
depending on specific location and severity of trauma, limited function may be retained:
C1-C4 – full paralysis of the limbs; cannot breathe without mechanical ventilation
C5 – paralysis of wrists, hands and triceps; difficulty coughing and clearing secretions
C6 – paraysis of wrist flexors, triceps and hands; respiratory function as above
What does complete transection cause?
Complete transection is a complete tear in the spinal cord; it results in absence of movement,
sensation, and body organ function below the level of the injury. Commonly thoracic or cervical
- Complete injury (all function lost) may also occur even if spinal cord is not severed
Describe Brown-Sequard syndrome
This syndromeoccurs when the spinal cord is injured on one side much more than the other
- It is rare for the spinal cord to be truly hemisected (severed on one side), but partial lesions due to
penetrating wounds (such as gunshot or knife wounds) or fractured vertebrae or tumors are common
- On the ipsilateral side of the injury (same side), the body loses motor function,proprioception, and
senses of vibration and touch
- On the contralateral (opposite side) of the injury, there is a loss of pain and temperature sensations
Describe syringomyelia
- Syringomyelia is a generic term referring to a disorder in which acystor cavity forms within thespinal
cord – this cyst, called asyrinx, can expand and elongate over time, destroying the spinal cord - The damage may result in pain,paralysis, weakness; stiffness in the back, shoulders, and extremities
- It may also cause a loss of the ability to feel extremes of hot or cold, especially in the hands
- The disorder generally leads to a cape-like loss of pain and temperature sensation along back and arms
- Each patient experiences a different combination of symptoms – these symptoms typically vary
depending on the extent and, often more critically, to the location of the syrinx within the spinal cord - A syrinx may also cause disruptions in theparasympatheticandsympathetic nervous systems, leading
to abnormal body temperature or sweating, bowel control issues, or other problems. - If the syrinx is higher up in the spinal cord or affecting the brainstem as in syringobulbia,vocal cord
paralysis, ipsilateral tongue wasting,trigeminal nervesensory loss, and other signs may occur. - Classically, syringomyelia spares thedorsal column/medial lemniscusof the spinal cord, leaving
pressure, vibration, touch andproprioceptionintact in the upper extremities.
What are the spinal levels of the dermatomes and basic reflexes
Biceps - C5
Triceps - C7
Knee - L3/4
Ankle - S1
