Ch. 6: Somatosensory System Flashcards
Spinoreticular Tract
- Divergent
- C Fibers
- Slow pain/temp
- Arouses due to pain
Spinoreticular Neurons
- 1st Order: periphery to dorsal horn
- Cross Midline
- 2nd Order: dorsal horn to reticular formation
Spinomesencephalic Tract
- Divergent
- C Fibers
- Slow Pain
- Through superior colliculus to orient head to pain
- Cause activation of brain to turn down pain signal (pain control)
Anterolateral Columns
- Spinothalamic
- Pain/Temp
- Conscious Relay
-Peripheral Neurons: A-Delta
, course touch
-Somatotopic organization
Divergent Pathways
- Pain/Temp
- mediate automatic movement, automatic response and emotional response to pain
- signal to autonomic and limbic centers
Spinomesencephalic Neurons
- 1st Order: Periphery to dorsal horn (with vertical branches to other levels)
- Cross Midline
- 2nd Order: dorsal horn to superior colliculus and periaqueductal grey
Spinothalamic Neurons
- 1st: periphery to dorsal horn
- Cross Midline
- 2nd: dorsal horn to thalamus
- 3rd: thalamus to parietal cortex
Unconscious Relay
- Body to subconscious (terminates in cerebellum)
- Via: Ia, Ib, II neurons
- Senses: unconscious proprioception
Fidelity of Pathways
High vs Low
Low Fidelity Pathways
- non-discriminative
- little somatotopic organization
Dorsal Column Pathway Neurons
- 1st Order: periphery to medulla
- 2nd Order: medulla to thalamus
- 3rd Order: thalamus to parietal cortex
High Fidelity Pathways
- Discriminitive
- high degree of somatotopic organization
Somatotopic Organization
-organized by location
Dorsal Column Pathway
- DC/ML
- Conscious (to cortex)
- Peripheral Neurons: Ia, Ib, A-beta, (II)
- Modalities: discriminative touch, conscious proprioception
Branches of 1st Order Neurons in Dorsal Column
1: to dorsal column (conscious proprioception)
2: to ventral horn (spinal level Mm reflex)
3: to cerebellum (lateral column)
Divergent Pathways
- Body–>subconscious
- Terminate in autonomic & Limbic centers
- C Neurons
- Sense: slow pain
Proprioception
- deep sensation from Mm and joints
- knowledge where body is in space
- have fastest conducting axons in body
Perception
- interpretation of sensation
- to make meaning of sensation
- occurs in cerebrum (parietal cortex)
Discriminative Touch
-can differentiate quality and location of touch
Exteroception
- feeling from the body’s surface
- Subset: pain/temp
Sensation
- actual signals coming in and being aware of them
- actual characteristics of an input
Modality-Specific receptors
- (pseudounipolar)
- Mechanoreceptors
- chemoreceptors
- thermoreceptors
Tonic receptors
receptors respond as long as stimulus is maintained
Routing of Sensation
- Conscious Awareness
- Automatic adjustment to posture and movement
Unconscious Awareness
- sensory signals to cerebellum and subcortical areas (limbic) are unconscious
- automatic adjustment to posture and movement
Nociceptors
- Subset of modality specific receptors
- sensitive to stimuli that damage or threaten to damage tissue
- stimulus–>pain
- have high threshold
Phasic receptors
- respond to a constant stimulus then stops
- tells when stimulus arrives, moves and leaves
Conscious Awareness
-signal goes to cerebral cortex and person becomes aware of it
Somatosensory Peripheral Neurons Morphology
pseudounipolar
Naming/Classification System of neurons
- Smaller letters and #’s=larger diameter and more myelination=faster
- Ia, Ib, II, A-Beta, A-Delta, C
Ia, Ib, II Sensory neurons
- proprioception
- muscle spindles and tendons
- fastest: Ia–Ib–II
- Signals from Mm and joints
- Myelinated
A-Beta
- exteroception
- Touch of skin and subQ
- myelinated
- axons from body surface
A-Delta
- fast localized pain and temp (discriminative)
- superficial and deep
- myelinated
- sharp pain
C
- slow non-localized pain and temp
- superficial and deep
- non-myelinated
- prolonged ache
Receptive Field
-Area of skin’s surface supplied by terminal branches of a single axon
Superficial Skin Sensation (4 categories)
- fine touch
- course touch
- Pain
- Temp
Cutaneous Fine touch
- Smaller receptive fields
- axons closer to surface
- better able to discriminate
-light touch, vibration, pressure, hair displacement
Density of Receptors
- # of terminal axon branches in any receptive field
- smaller and densest in areas of body that explore environment (hands, feet, mouth)
Fine Touch
- Specialized receptor + A-beta neuron
- Cutaneous and SubQ
- Fine=discriminative
SubQ Fine Touch
- larger receptive fields
- touch, vibration, skin stretch
- less able to discriminate
Pain
- Free nerve ending plus A-delta and C fibers
- nociceptors
Course Touch
- Free nerve endings plus A-delta and C neurons
- know there is a touch but not specific location
Dermatome
-all sensory input from one dermatome (spinal level)
Spinal Nerve
- all sensory input to one spinal segment
- ex: C8 is part of the old neural tube that was connected to the 8th somite
Peripheral Nerve
-part of one dermatome and/or combo of parts of several dermatomes
If you can map a dermatome pattern of sensory loss:
it is a spinal nerve problem
A peripheral nerve always has____
2+ dermatomes in them
3 sensations transmitted by muscle spindles
- Muscle length (absolute)
- Change in muscle length
- Rate of change in muscle length
Temperature
- Free nerve ending plus A-delta and C fibers
- thermoreception
If you can map out a pattern of peripheral Nerve sensory loss:
it is a problem of peripheral nerves
Muscle Spindles
- transmit signals based on length of muscle and change in Mm length and rate of change of Mm length
- buried longitudinally in Mm
- increased # of signals when stretched (Mm lengthened)
Nuclear Bag Sensory Receptors
transmit change in Mm length and rate of change in Mm length
2 Mm fibers in Mm
Intrafusal & extrafusal
5 components of Mm spindle
- nuclear bag sensory receptors
- nuclear chain sensory rec
- Group Ia sensory axons
- Group II sensory axons
- Muscle fibers (extra/intrafusal)
Extrafusal Mm Fibers
- outside Mm spindle
- cause overall contraction of Mm and movement of body segment
- innervated by A-alpha axons
Nuclear Chain sensory receptors
Transmit:
- change in Mm length
- rate of change in Mm length
- absolute length
Intrafusal Fibers
- inside Mm spindle
- keep spindles sensitive to change of length while muscle is contracting
- innervated by A-gamma axons
A-gamma motor neurons
- innervate intrafusal Mm fibers
- (keep spindle sensitive to length change during muscle contraction)
Group Ia Sensory Axons
Transmit:
- change in Mm length
- rate of change in length
Group II Sensory Axons
Transmit:
-Absolute Mm Length
A-alpha motor neurons
Innervate extrafusal Mm fibers
to contract Mm
GTO
- golgi tendon organs
- specialized mechanoreceptors in Mm tendons
- depol with increased tenson in Mm and tendon
- Transmit via: Ib sensory axons
Capsule receptors
- Sense relative stretch on capsule
- especially passive movement & extremes of range
- joint movment
Free Nerve Endings
- no specialized receptors
- transmit pain & inflammation
- high threshold
- pain=excessive mechanical stimulation
- inflammation=excessive chemical stimulation (or pressure)
3 Joint receptors
- Capsule Rec
- Ligament Rec
- Free nerve Endings
Sensory Pathways to brain
3 types
- conscious relay
- divergent pathways
- unconscious relay
Ligament Receptors
Sense force or tension in ligaments
4 Proprioception Structures
- Muscle Spindles
- tendon rec
- joint rec (capsule/ligament/free nerve endings)
- cutaneous mechanoreceptors
Components of sensory pathways to Brain
- projection neurons
- interneurons
conscious relay
- body–>cerebral cortex
- Senses: light touch, conscious proprioception, fast pain
Projection Neurons
- multipolar neurons(relay messengers) in spinal cord and brain
- 2nd/3rd order neurons
- relay info from 1st order neurons to target
1st order Neurons
pseudounipolar neurons in periphery that get sensory info first
interneuron
- connect projection neurons together
- very short distances
- may modify signals (to send or not to send)
Branches of 1st order neurons in dorsal horn
- dorsal column
- ventral horn (Mm reflex)
- cerebellum
Association Cortex
- signal from post central gyrus to parietal lobe
- perception
Primary cortex
- pathway terminates in primary sensory area in post central gyrus of parietal lobe
- sensation
