Somatosensation I Flashcards
What are the 5 functions of the somatosensory pathway?
→ Conveying sensation from the body
→ touch
→ proprioception ( sense of body position)
→ Heat and cold
→ pain and itch
How many pairs of spinal nerves do we have?
→ 31 pairs
How is the CNS connected to the body?
→ Spinal and cranial nerves
What is a nerve?
→ a bunch of fascicles
What do fascicles consist of?
→ Individual myelinated axons
What is the membrane of the fasciles called?
→ perineurium
What is the membrane of the spinal nerve called?
→ Epineurium
What is the membrane of each individual axon called?
→ Endoneurium
What do the 8 spinal nerves and the top of the thoracic nerves form?
→ Brachial plexus
→ Cervical plexus
What is the function of spinal nerves?
→ Carry information between the CNS and the periphery
How many pairs of nerves does the cervical region contain?
→ 8
What do the dorsal roots contain?
→ Sensory afferents
What do the ventral roots contain?
→ Motor efferents (somatic)
→ Autonomic effererents
What do dorsal and ventral roots unite to form?
→ Spinal nerve
What does white matter contain?
→ Ascending and descending axon tracts
What does grey matter contain?
→ Cell bodies
→ Dendrites
→ Synapses
Where do the spinal nerves pass through?
→ Intervertebral foramina
What do dorsal root ganglia contain?
→ All the cell bodies of all the sensory neurons of the somatosensory system
What do large fibres sense?
→ tactile and proprioceptive
What do small fibres sense?
→ Temperature
→ pain
→ itch
→ crude touch
What are dorsal root ganglia neurons called and why?
→ pseudo unipolar neurons
→ they have a single process
→ gives rise to an axon in both directions
What kind of adaptation does a Merkel disc have?
→ Slow adapting
How many large fibre afferents are there in the skin?
→ 4
What kind of touch are Merkels discs sensitive to?
→ Light touch and pressure
Where are Meissners corpuscles and Merkels discs located?
→ between the dermis and the epidermis
What happens to a Merkels disc if a stimulus is applied and maintained?
→ Action potentials fire
→if the stimulus is maintained then the action potentials continue to fire
What happens to a Meissners corpuscle if a stimulus is applied and maintained?
→ action potentials fire
→ if the stimulus is maintained the action potentials drop to 0
What are the cutaneous tactile afferents?
→ Meissners corpuscles
→ Merkels discs
What are the deep tactile afferents?
→ Ruffini
→ Pacinian
What type of fibre are tactile afferents?
→ A beta
large diameter, myelinated, 2nd fastest conducting
What do Pacinian corpuscles respond to?
→ Vibration
What do Ruffini corpuscles respond to?
→ Slipping
→ Non slipping
What are fast adapting afferents?
→ Meissner
→ Pacinian
What are slow adapting afferents?
→ Ruffini
→ Merkel
What do muscle spindles detect?
→ Changes in muscle length
What fibres detect change in length in muscles?
→ Large fibre systems
→A-alpha afferents
What senses muscle tension?
→ Golgi tendon organs
What is muscle length a cue for?
→ Proprioception
What type of neuron is a normal motor neuron?
→ Alpha
What are A delta fibres like?
→ thin
→ myelinated
What are C fibres like?
→ Thin
→ unmyelinated
What does a small receptive field mean?
→ THe stimulus is localised
What are the two major somatosensory pathways?
→ Dorsal column - medial lemniscal system
→ Spinothalamic tract
What does the dorsal column sense?
→ Mediates discriminative touch
→ vibration
→Inputs from A-β and A-α afferent fibres
→ proprioception
What does the spinothalamic tract sense?
→ Coarse touch
→ temperature
→coarse touch, temperature, pain
Inputs from A-δ and C fibres
→ pain
Describe the pathway from a tactile afferent in the upper body to the somatosensory cortex
→ Stimulus from the Merkel, Ruffini, Pacinian or Meissners occurs
→ The stimulus travels via 1st order neurons to the dorsal column (cuneatus)
→ the 1st order neuron travels up to the medulla (nucleus cuneatus) where it synapses with a 2nd order neuron
→ the 2nd order neuron decussates at the midline
axons from cells of the dorsal column nuclei arch toward the ventral and medial medulla, and decussate
→ the 2nd order neuron travels up the medial lemniscal tract of the brainstem to the VPL thalamus
→ 2nd order neuron synapses at the VP thalamus with a 3rd order neuron which carries it to the somatosensory cortex
What is the difference between the Cuneate and Gracile dorsal column?
→ Cuneate - upper body
→ Gracile - lower body
What inputs come to the ventral posterior medial nucleus?
→ Sensory inputs of the head and face
What inputs come to the ventral posterior lateral nucleus?
→ Sensory inputs from the body
Where is the somatosensory cortex?
→ Front of the parietal lobe
What are A-beta afferents?
large diameter, myelinated,
→2nd fastest conducting
Where are small receptive fields found?
→ tips of fingers
Describe the pathway from a tactile afferent in the upper body to the somatosensory cortex in the face
→ large diameter mechanosensory receptors from face picked by trigeminal ganglion
→ synapses onto the principal ipsilateral nucleus of trigeminal complex
→the 2nd order neuron decussates at the midline
→ the 2nd order neuron travels up the medial lemniscal tract of the brainstem to the VP thalamus
→relayed to somatosensory cortex
What is one difference between VPL and VPM?
→VPM- face
→VPL sensory info from the body
What are areas 1, 2,3 in the postcentral gyrus?
→ primary somatosensory cortex
What is area 4 in the central sulcus?
motor cortex
How are regional cortical differences identified?
→ relative thickness
→ cell size
→density
How are somatotopic maps duplicated?
duplicates 4 times each in the Browman’s area
Where does secondary somatosensory info go to?
→ to amygdala and hippocampus
Where does parietal somatosensory info go?
→ to motor and premotor cortical areas
What is the only sensory information which does not first synapse in the thalamus?
olfactory
What is the difference between muscle spindle and Golgi tendon organ?
The function of the GTO can be considered opposite of the muscle spindle, which serves to produce muscle contraction.
When a GTO is stimulated, it causes its associated muscle to relax by interrupting its contraction.
