Lecture 3 Flashcards
Somatic system:
- Sense (6)
- Info (4)
- Subsystems (3)
Senses: smell, taste, touch, audition, vision, proprioception
Info: modality, location, intensity, temporal info
Subsystems:
- fine touch, vibration, pressure,
- propriception
- temp, pain, non-discriminative touch
Transmission pathway (where are the fibers in the spinal cord?)
PATH (AKA):
↓Receptors (nerve endings)
↓afferent nerve fibers (axons)
↓afferent cell bodies (ganglia)
CNS circuits (CNS circuits)
*info via afferent axons in dorsal roots
Trigeminal ganglia & dorsal root ganglia (DRG)
- TG near head
- DRG in body PSEUDONIPOLAR
-> 1st synaptic terminal w/in spinal cord
Pseudonipolar neurons + sensory pseudonipolar neurons
PUNs general: 2branches, NO DENDRITES
(see 3.4)
Sensory PUNs: cell body in DRG
> central → dorsal horn of spinal cord
> peripheral → through SC to periphery
Sensory transduction steps
1) stimulus → depolarizing → POTENTIAL
2) APs generated in afferent fiber
3) APs travel on peripheral axon past cell body on central axon to spinal cord
Afferent fibers terminals (2 types)
Types:
> encapsulated by receptor cells
> free terminals (for pain!)
Basic mechanisms:
> stimulus changes cation channel
> permeability → generates
> depolarizing current
Piezo 1 & piezo 2
Mechanosensitive cation channels important in transduction
Specialization of somatic sensory afferents (4 properties)
1) Axon diameter → speed of signal, larger = faster
2) Receptive field → spatial accuracy
2-point discrimination: min distance for distinct stimuli
3) Temporal dynamics → speed of response
> rapid: fire quickly, then stop (for △ in ongoing stimulation)
> slow adapting: continue to fire w/ sustained stimulation(for attributes)
4) Quality of stimuli → specific stimuli types
TOUCH:
receptor (basic type)
haptics
stregnosis
Type: mechanoreceptors
Haptics: active touching & complex pattern
Stregnosis: capacity to identify an object via manipulation w/ your hand
Merkel cells afferents
- Slow adapting
- 25% of hand receptors
- samples info epidermis
- HIGHEST spatial resolution
- sensitive to: points, edges, curvature (form & texture info)
Meissner afferents
- Rapidly adapting
- 40% of hand receptors
- closer to skin surface Merkel
- less spatial resolution
- sensitive to: deformation, vibration, grip control (corpuscle deforms to trigger APs)
Pacinian afferents
- Rapidly
- 10-15% of hand receptors
- deep in the dermis (lower threshold {displacements as small as 10 mm!})
- large receptive fold
Ruffini afferents
- Slow adapting
- 20% of hand receptors
- In dermis, ligaments & tendons
- Sensitive to: cutaneous stretching, finger positon, hand conformation
Proprioception basics (what, integration, proprioception)
- Info about mechanical frces in the body
- Integrates w/ vestibular system
- Important for complex movement
- proprioceptors: low threshold mechanoreceptors
Proprioception speed vs touch
Larger diameter than touch
Wider & faster
(Bonus pain, free nerves, are smaller & slower)
Muscle spindles (intra vs extrafusal fibers)
Extrafusal:
- outside fusiform capsule
- α MN
- produce force
Intrafusal:
- inside fusiform capsule
- γ MN
- muscle spindles here !!!
-> motor supply for stretch receptor
Primary vs Secondary endings
Primary:
= Group 1a afferents
- Limb dynamics info
- (also senses length)
- Rapidly adapting
△ LENGTH & VELOCITY
Secondary:
- Group II afferents
- Static limb postion
- detect length
- sustained response (slow adapting)
STATIC INFO
Golgi tendon organs
- Low threshold mechanoreceptors
- △ muscle tension detection
- Group 1b afferents
- Along collagen fibers in series w/ extrafusal fibers
Central pathways: Tactile info
Via dorsal horn
Projection to lower medulla
AFFERENT
Central pathways: proprioceptive info
Via dorsal horn
AFFERENT & EFFERENT
-> dorsal & ventral horn
ventral toward MN pools
Dorsal towards cuneate nucleus → cerebellum
SPINOCEREBELLAR TRACT: Clarke’s nucleus → cerebellum*
inferior cerebellar peduncle
Central pathways: both
Gracile tract - lower body
Cuinate tract - upper body
Project to gracile & cuinate neucleus near medulla
1st order → 2nd → 3rd → cortex
1st order neurons
*Periphery → spinal cord
SIDE - Ipsilateral
BODY - In dorsal root & trigeminal ganglia
PATH - Cuneatus & gracile tracts // trigeminal tract
2nd order neurons
*Spinal cord → thalamus
SIDE - Ipsilateral
BODY - in gracile & cuneiform nuclei // trigeminal nucleus
PATH - Medial lemniscus // trigeminal lemniscus
3rd order neurons
*Thalamus → cortex
SIDE - Contralateral
BODY - In ventral posterior complex, medial thalamic & medial prabrachial nucleus
PATH - Internal capsule
Cortex (w/ pathways)
Cerebral cortex, primary somatosensory cortex, postcentral gyrus, PL // anterior cingulate cortex & insula
sensory info in thalamus
(order neurons, converging pathways, somatotopic representation) What layer?
3rd order neurons
Ascending sms. pathways converge in VPT
Somatotopic:
from body; lateral thalamus via medial lemniscus
from head; medial thalamus via trigeminal lemniscus
Project to layer 4 (VPT neurons)
Thalamus, S1, Broca’s areas
3b & 1: cutaneous stimulation
3a: proprioceptive stimulation
2: tactile & proprioceptive
- Rapid vs slow segregate within an area
Thalamus VPC Pathway
(see 3.28)
S1 Lesions
3b lesions: tactile issues
1&2 lesions: size. shape or texture issues
