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
How many pairs of nerves does the thoracic region contain?
→ 12
How many pairs of nerves does the lumbar region have?
→ 5
How many pairs of nerves does the sacral region have?
→ 5
How many pairs of nerves does the coccygeal region have?
→ 1
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
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
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
→ proprioception
What does the spinothalamic tract sense?
→ Coarse touch
→ temperature
→ 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
→ the 2nd order neuron travels up the medial lemniscal tract of the brainstem to the thalamus
→ 2nd order neuron synapses at the 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
