11. SOMATOSENSATION I Flashcards
What sensations does the somatosensory system detect?
Somatic sensations:
- Touch
- Proprioception (orientation, location etc.)
- Pain (Nociception), itch
- Temperature
What is the primary somatosensory cortex?
- The primary somatosensory cortex is involved with processing somatic information from receptors
- It consists of Brodman areas 1, 2, 3a & 3b
- It is located in the post central gyrus & lateral to the central sulccus
What do the dorsal roots contain?
- Sensory afferents
What do the ventral roots contain?
- Motor afferents
What does the dorsal root ganglion contain?
- Cell bodies of the sensory neurones of the somatosensory system
What do the dorsal ventral roots combine to form?
- Mixed spinal nerve
What are the two main classes of fibres of the somatosensory system?
- LARGE FIBRES - TACTILE & PROPRIOCEPTIVE
- large diameter, myelinated, fast conduction - SMALL FIBRES - PAIN, ITCH, CRUDE TOUCH
- small diameter, myelinated or unmyelinated, slow or medium conduction
Why is afferent fibre type important?
- The quality & type of sensation depends on the afferent fibre type
What are the 4 sensory afferents?
- A alpha fibres
- A beta fibres
- A delta fibres
- C fibres
What are the characteristics of A alpha afferents?
- Largest diameter
- Myelinated
- Fastest conduction velocity > 100m/s
What do A alpha afferents detect?
- Proprioception
What do A beta afferents detect?
- Tactile afferents/discrimination
- Discriminative touch
What are the characteristics of A beta afferents?
- Large diameter
- Myelinated
- 2nd fastest conduction velocity (30 -70m/s)
What afferents are cutaneous receptors?
- Cutaneous receptors are A beta afferents
What are the 4 types of cutaneous receptor?
- MERKEL’S DISCS
- MEISSNER’S DISCS
- RUFFINI CORPUSCLES
- PACINIAN ENDINGS
Which cutaneous receptors are superficial?
- Merkel’s discs
- Meissner’s corpuscles
Which cutaneous receptors are deep?
- Ruffini corpuscles
- Pacinian endings
What are the characteristics of the Merkel’s discs?
- High spatial resolution
- Fine tactile discrimination
- Detect texture & shape
What are the characteristics of Meissner’s corpuscles?
- Lower spatial resolution than Merkel’s discs
- But can still detect discriminative touch
What are the characteristics of Pacinian endings & Ruffini corpuscles?
- Less spatial resolution
- Can respond to stretch so are better for grip, texture, vibration
- Pacinian = Vibration
- Ruffini = Slipping or non-slipping
What’s the importance of receptive fields in sensation?
- Our ability to localise depends on sensory receptive fields
- Small receptive fields are needed for fine discrimination
- Many small receptive fields are better than few large receptive fields
- Merkel’s discs & Meissner’s corpuscles have small receptive fields
- Pacinian endings & Ruffini corpuscles have large receptive fields
Which cutaneous receptor would be best adapted to fine tactile discrimination?
- Merkel’s discs would be best for fine tactile discrimination
- They have small receptive fields & a high spatial resolution
- They are located at the finger tips
What are the characteristics of A delta afferents?
- Smallest diameter
- Thinly myelinated
- Moderate conduction velocity (<30m/s)
What are the characteristics of C afferents?
- Small diameter
- Unmyelinated
- Slow conduction (< 1m/s)
Which two sensory afferents have free nerve endings?
- A delta fibres
2. C fibres
What are the two central pathways of the somatosensory system?
- Dorsal columns- Medial Leminiscus (DCML)
2. Anterolateral system or Spinothalamic tract
What fibres does the DCML receive input from & what does it detect?
- The DCML receives input from A alpha & A beta afferents
- It detects proprioception, discriminative touch & vibration
What fibres does the STT receive inputs from & what does it detect?
- The STT receives inputs from A delta & C afferents
- It detects coarse touch (pressure), pain & temperature
Describe the pathway for the DCML
- A beta & alpha Sensory afferents enter the dorsal column of the spinal cord (dorsal roots)
- The branches from the dorsal roots project into the dorsal column nuclei of the medulla: Cuneate & Gracile nucleus
- The dorsal nuclei synapse in the mid-brain at the brainstem
- It then dessicates into the VENTRAL POSTERIOR NUCLEAR cortex of the thalamus
- Axons are then projected to the primary somatosensory cortex
DORSAL ROOTS -> CUNEATE & GRACILE NUCLEUS (MIDBRAIN) -> BRAINSTEM (MIDBRAIN) -> VENTRAL POSTERIOR NUCLEAR COMPLEX (THALAMUS) -> PRIMARY SOMATOSENSORY CORTEX
**Describe the pathway for the STT
- A delta & C fibre afferents terminate in the dorsal horn of the spinal cord
- The branches ascend into the medulla then into the midbrain. They then go into the VENTRAL POSTERIOR MEDIAL NUCLEUS of the thalamus
- Dorsal horn -> Medulla -> Midbrain -> Ventral posterior medial nucleus
What are the cortical connections of the primary somatosensory cortex?
- Amygdala
- Hippocampus
- Motor & Pre-motor cortical areas
