Module B: Spinal Cord and Sensory Processing Flashcards
Common Features of Somatosensory pathways
- Somatosensory receptors that sense environment - 1°, 2° and 3° afferent neurons - Decussation - Include a thalamic nucleus
1° afferent neuron
- psuedounipolar neuron - peripheral axon that innervates one receptor - central process that synapses with a 2° afferent neuron
2° afferent neuron
synapses with 3° afferent neuron in thalamus
3° afferent neuron
synapses with neurons in the cerebral cortex
Decussation
- occurs in spinal cord or brain stem - allows better seperation of tracts (more robust against wiring errors than same-sided schemes)
Mechanoreceptor
- is a sensory receptor that responds to mechanical pressure/distortion - leads to pressure sensitive action potential - different types allow perception of different sensation and sensitivity - can be encapsulated or unencapsulated
Types of mechanoreceptor (6)
- free nerve endings - Merkel cells and tactile discs - free nerve endings of root hair plexus - Ruffini corpuscle - tactile corpuscle - lamellated corpuscle
Mechanoreceptor: Free nerve endings
- touch - pressure - stretching
Mechanoreceptor: Merkel cells and tactile discs
- detect sustained touch and pressure - sensitive to fine touch
Mechanoreceptor: Free nerve endings of root hair plexus
- movement and distortion of hair
Mechanoreceptor: Ruffini corpuscle
- tension deep in the skin
Mechanoreceptor: Tactile corpuscle
- light touch - movement - vibration - changes in texture
Mechanoreceptor: Lamellated corpuscle
- deep pressure - most sensitive to rapid vibrations
Slow adapting receptors
- produce sustained response to static stimulation
- slow to return to normal firing (tonic)
- useful for detecing touch and pressure
Rapidly adapting receptors
- produce transient response
- quickly return to normal firing (phasic)
- useful for texture and vibration
Receptive fields
- small, accurate fields needed for accurate taction
e. g. finger tips, multiple Merkel cells and tactile corpuscles - 2 point discrimination ~2mm
Proprioreception
- sense of movement and body position
- concious (awareness of body position, control of voluntary movements)
- unconcious (righting reflex)
- nervous system requires constant feedback from the muscles and joints to control movemkent
Proprioreceptors
- Golgi Tendon organ - monitors tension, pressure and joint movement
- Neuromuscular spindle - detects rate and size of changes in length of muscle and generates supraspinal responses to control muscle contraction and spinal reflexes
Nocioceptors
- free nerve endings
- pain receptors, sensed by a number of parts of the brain
- Sensory discriminative - Allow detection of location, intensity and quality of pain. Small sensory field.
- Affective motivational - fear and anxiety associated with pain
What sensory fibres innervate mechanoreceptors?
type Aβ and Aδ
What sensory fibres innervate proprioceptors?
Type Aα and Aβ
Peripheral comatosensory axons
- innervate receptors
- there are different types that have different diameters and myelination, which alters their conduction velocities.
Dermatome
- the area innervated by a single posterior root is the sum of the receptive fields of the primary afferents.
(primary afferents collect to form a posterior root to enter the spinal cord)
Sensory modality
fibres are arranged according to the information carried
Somatotopic arrangement
fibres are arranged according to site of origin
Medial-lateral rule
inferior nerves travel more medially
Anatomical principles by which tracts are segregated are…
- Sensory modality
- Somatotopic arrangement
- Medial-lateral rule
Dorsal Column - Medial Lemniscus Tract
- First order neurons - axons enter spinal cord through dorsal root, and ascend Gracile (below T6) or Cuneate (above T6) Fasciculus
- Second order neurons - from gracile and cunneate nuclei, axons travel up medial lemniscus to synapse in thalamus
- Third order neurons - axons carry information to primary sensory cortex
Spinothalamic Tracts
- First order neurons - axons enter spinal cord through dorsal root and synapse at dorsal horn
- Second order neurons -cross to opposite side of tract and ascend appropriate spinothalamic tract to the thalamus
- Third order neurons - axons carry information to the primary sensory cortex
Termination of neurons in the thalamus
- Second order neurons synapse with the Ventroposterior Nucleus (VPN) of the thalamus
- terminate in different regions depending on sensory information they are carrying - contains precise maps based on system they belong to
Spinocerebellar Tracts…
- Conveys unconcious proprioceptive and cutaneous information to the cerebellar
- Important to control muscle contraction for movement
Posterior Tract - carries information from lower limbs and body
Anterior Tract - integrated proprioceptive information with descending inputs
Spinal border cells - integrate information from lower limb, descending inputs and from flexor reflex arcs
Spinocerebellar Tract neurons
- First order neurons - enter spinal cord via dorsal root, synapse in dorsal horn
Second order neurons in….
Anterior Tract - Primary afferants synapse with spinal border cells, decussate and travel to cerebellum via superior cerebellar peduncle
Posterior Tract - afferants synapse in Clarke’s nucleus then ascend to the cerebellum vis the inferior cerebellar peduncle
