Sensory Pathways Flashcards
What are the somatosensory modalities
touch, thermosensation, nociception, proprioception
What is proprioception
mechanical displacement of muscles, joints
Which receptors have free nerve endings
thermoreceptors, nociceptors
Which receptors has enclosed nerve endings
mechanoreceptors
What are the 3 types of sensory neurons and what do they differ in
Abeta, Adelta, C fibres
they differ in diameter, speed (myelination), function
Describe Abeta fibres
largest, thick myelin (fastest), innocuous mechanical stimulation
Describe Adelta fibres
has myelin, noxious mechanical + thermal stimulation
Describe C fibres
smallest, unmyelinated, noxious mechanical + thermal + chemical stimulation
Sensory receptor definition
transducer that converts energy from the environment into neural APs
Describe thermoreceptors
Adelta + C fibres, free nerve endings, not uniformly distributed - this affects sensitivity
What are thermoreceptors linked to
transient receptor potential (TRP) ion channels
Which TRP ion channels are C fibres associated with
C fibres detect heat - so, TRPV1-4
Which TRP ion channels are Adelta fibres associated with
Adelta fibres detect cold - so, TRPM8, TRPA1
State the 5 types of mechanoreceptors
Merkel cells (light touch, superficial pressure), Pacinian corpuscle (deep pressure, tickling, high frequency vibration), Meissner’s corpuscle (low frequency vibration, fine + discriminative touch), Ruffni endings (continuous pressure, touch, stretch), hair follicles
Describe nociceptors
free nerve endings; Adelta for sharp (pin-prick) pain, cold; C fibre for dull aching pain, heat
Stimulus threshold definition
point of intensity at which a person can detect presence of stimulus 50% of the time (absolute threshold)
What is stimulus intensity
how fast a neutron fires
How do you get high stimulus intensity
increased stimulus strength AND duration leads to increased NT release and therefore increased stimulus intensity
Name the 2 types of adapting receptors
tonic, phasic
Describe tonic receptors and give an example
detects continuous stimulus strength, adapts v slowly, impulses continuously transmitted to brain as long as stimulus is present; example: Merkel cells
Describe phasic receptors and give an example
detect change in stimulus strength, adapt quickly, impulse transmitted at start and end of stimulus; example: Pacinian corpuscle: sudden pressure excites receptor and another impulse transmitted when pressure released
Receptive field definition
region on the skin which causes activation of a single sensory neuron when stimulated
3 examples of different receptive fields
fingers have densely packed mechanoreceptors with small receptive fields, arm has small receptive fields for detection of fine detail over small area - precise perception, back has large receptive fields for detection of changes over wider area
The smaller the receptive field, the … the precision of perception
greater
What is 2 point discrimination
minimum distance at which 2 points are perceives as separate; related to receptor field size
Where are cell bodies of the body located
dorsal root ganglion
Where are cell bodies of the face located
trigeminal ganglia
Info from lower body received at what spinal level and carried in which tract
lumbar level, gracile tract
info from upper body received at what spinal level and carried in which tract
cervical level, cuneate tract
info from face received at what level
pons
2 types of dorsal horn neurons
projections neurons (going to brain), interneurons (remain in spinal cord) *Abeta fibres are deep: laminas 3-5; Adelta + C fibres are superficial: laminas 1-2
What happens when receptive fields overlap
difficult to distinguish between the 2 stimuli locations
What prevents the overlap of these receptive fields
lateral inhibition; mediated by inhibitory interneurons within dorsal horn of spinal cord, leads to enhanced sensory perception
Describe the ascending pathway for touch, proprioception
Abeta fibres enter dorsal horn and enter dorsal column pathways. Info from lower/upper limbs travel ipsilaterally along gracile/cuneate tracts. 1st order neurons terminate in gracile/cuneate nucleus in medulla. 2nd order neurons decussate in caudal medulla, forming CONTRALATERAL medial leminscus tract. They terminate in ventral posterior lateral nucleus (VPL) of thalamus. 3rd order neutron projects from VPL to somatosensory cortex
What is crude touch
sensory modality that allows the subject to sense that something hastouched them, without being able to localise where they weretouched (contrasting finetouch)’’
Difference between fine and crude touch in terms of fibres
fine touch mediated by Abeta fibres (Meissner’s corpuscle), crude touch mediated by Adelta fibres (free nerve ending)
Pain and temperature sensations ascend within which part of the spinothalamic tract
lateral
Crude touch ascends within which part of the spinothalamic tract
anterior
Describe the ascending pathway for pain, temperature, crude touch
Spinothalamic (anterolateral) pathway.
1st order neurons terminate in the dorsal horn upon entering. 2nd order neurons decussate immediately in spinal cord, forming spinothalamic tract and ascend. The neurons terminate in VPL of thalamus. 3rd order neurons to somatosensory cortex
Clinical importance: examples of sensory testing
pin pricks to detect sharp vs blunt perception of stimulus; brushing to detect whether innocuous stimuli perceived as painful; graded tuning fork to identify normal/abnormal vibration sensation;
What does anterior spinal cord lesion result in
blocked anterior spinal artery causes ischaemic damage to anterior part of spinal cord - dorsal columns intact but damaged spinothalamic tract. Pain/temp loss below the level of lesion; retained light touch, vibration, 2-point discrimination
Which pathway is associated with sensational pain
lateral spinothalamic tract
Which pathway is associated with emotional pain
spin-reticular tract
What is gate control theory
when elbow gets hit, you activate C fibres. Rubbing the elbow stimulates Abeta fibres which blocks C fibres by stimulating inhibitory neurons. There is reduced C fibre input to somatosensory cortex, resulting in decreased stimulus intensity.
