1: Somatosensory Systems Flashcards
Describe the spinal trigeminal system.
Conveys pain, temperature, and crude touch information from the face and head
Synapse in spinal nucleus and tract of trigeminal
Describe the dorsal column-medial lemniscus (DCML) system. What receptors does it include?
Conveys well-localized touch, pressure, vibration, and joint position sense from the body
-> ipsilateral dorsal columns -> internal arcuate fibers (decussate) -> medial lemniscus
Receptors:
- Include Meissner corpuscles, Ruffini endings, and Pacinian corpuscles
- Associated with group II afferent fibers (well myelinated)
Describe the trigeminal mechanosensory system.
Conveys tactile, vibratory, and proprioceptive information from the face and head
Synapse in principle/chief nucleus of trigeminal
Describe the anterolateral system. What receptors does it include?
Conveys pain, temperature, and light touch from the body
Aka spinothalamic tract
Decussates almost immediately -> spinothalamic tract
Some remain ipsilateral as dorsolateral tract of Lissauer for several segments
Receptors:
- Include thermoreceptors, nociceptors, and mechanoreceptors
- All unencapsulated
- Associated with A-delta and C axons (type III and IV axons)
Define the concept of somatotopy.
The point-for-point correspondence of an area of the body to a specific point on the CNS
Organization by body region
Recognize the general organization of the sensory homunculus.
Sensory homunculus: a somatotopic representation in which the body is represented in proportion to its sensitivity
- Head closest to lateral fissure
- Toes and genitals in longitudinal fissure
- Representation of more sensitive areas (face/hand) much larger than for trunk/leg
Explain how pain is modulated by descending pain pathways.
Periaqueductal gray matter of the midbrain = most important pathway
Project primarily to substantia gelatinosa
Use NT like serotonin and norepinephrine to modulate pain
Activated by opiates from descending inputs, like raphe nuclei
Explain how pain is modulated by mechanosensory fibers.
Large mechanosensory fibers are stimulated -> activate inhibitory interneurons in dorsal horn -> suppress pain transmission
Known as “Gate control theory of pain”: mechanosensory fibers act as gate to modulate pain information
Why rubbing your toe after you stub it makes you feel better
Name the two pathways that convey unconscious sensory information. What is the general purpose of these unconscious sensations?
Dorsal spinocerebellar and cuneocerebellar tracts
Detect muscle stretch and tension, degree of excitability of spinal cord
Allow movements to be performed smoothly without conscious awareness
Describe the dorsal spinocerebellar tract.
1st order: Sensory nerve fibers mediating muscle stretch and tension information from LOWER LIMB (from muscle spindles and golgi tendon organs)
-Terminate in nucleus dorsalis (Clarke’s nucleus) in gray matter from C8 to L3
2nd order: ascend ipsilaterally in dorsal part of lateral funiculus
- Enter cerebellum through inferior cerebellar peduncle
- Terminate in spinocerebellum
Describe the cuneocerebellar tract.
1st order: upper limb equivalent of dorsal spinocerebellar tract
- Ascend in fasciculus cuneatus
- Terminate in external cuneate nucleus
2nd order: ascend ipsilaterally from external cuneate nucleus
- Enter cerebellum through inferior cerebellar peduncle
- Terminate in spinocerebellum
Distinguish encapsulated and non-encapsulated tactile mechanoreceptors. What does each detect best?
Encapsulated: encase nerve ending in specialized CT elements that affect how it responds to force
Non-encapsulated receptors: nerve endings that are either free or contact special skin cells (Merkle cells)
Name the types of non-encapsulated tactile mechanoreceptors and what each detects. (3)
Free nerve endings:
-Pain and temperature detection, itch
Merkel cell-neurite complexes: free nerve endings contacting Merkle cells
-Touch
Hair follicle receptors: specialized free nerve endings that wrap around hairs
-Very light touch, not well localized
Name the types of encapsulated tactile mechanoreceptors and what each detects. (3)
Meissner’s corpuscles: in dermal papilla; mostly for touch
Ruffini endings: in dermis; detection of stretch
Pacinian corpuscles: in subcutaneous layer; mostly for vibration, very rapidly adapting
Name the three types of proprioceptive mechanoreceptors and briefly state what they detect.
Muscle spindle receptors: muscle stretch and tension
Joint capsule receptors: joint position and speed of movement
Golgi tendon organs: muscle tension
Describe the general layout of a sensory tract for conscious sensations.
Chain of three neurons–
1st order: conveys sensation from periphery to CNS
- Axon in peripheral nerve
- Cell body in DRG or cranial nerve sensory ganglion
2nd order: relays signal from spinal cord/brain stem to thalamus
-Has collateral branches to other brain regions: mediate reflex effects, provide important inputs to other sensory/motor regulating systems
3rd order: from thalamus to cerebral cortex for conscious processing
How do sensory tracts for unconscious sensations differ from those for conscious sensations?
The most direct pathways involve 2 neuron pathways
End in cerebellum (mostly spinocerebellum)
Terminate primarily ipsilaterally
What is the only area of skin that doesn’t receive sensory information from spinal nerves? What innervates this area?
The face and anterior 1/2 of the scalp
Innervated by CN V - trigeminal nerve
Name the four somatosensory pathways that conduct conscious information and briefly state what they conduct.
Tactile, vibratory, and proprioceptive information:
- BODY - Dorsal column-medial lemniscus system (DC-ML)
- FACE - Trigeminal mechanosensory system
Pain, temperature, and crude touch information:
- BODY - Anterolateral system (aka spinothalamic tract)
- FACE - Spinal trigeminal system
Name the two types of thermoreceptors. Give a range for each.
Heat receptors: activated between 29-45C
Cold receptors: activated between 5-40C
What do nociceptors detect?
Detect INTENSE mechanical, thermal, AND chemical stimuli
Give rise to perception of pain no matter how they are stimulated, if stimulus passes their high threshold
What two types of pain do nociceptors conduct?
Fast pain: sharp, well localized, stinging
-Conducted on A-delta (type III) fibers
Slow pain: dull, aching, burning
-Conducted on C (type IV) fibers
What is the most well known nociceptor? What activates it?
TRPV1 receptor: activated by head, acid, and capsaicin
Describe the sensitization of nociceptors. Define allodynia and hyperalgesia.
Prostaglandins and bradykinin (inflammatory mediators), substance P lower the threshold of nociceptors
Serves a protective function to safeguard from further damage
-> Allodynia: perception of an innocuous stimulus as painful
-> Hyperalgesia: painful stimuli in damaged area are perceived as more painful than normal
What is substance P?
A neurotransmitter produced by nociceptors
Causes vasodilation, swelling, and release of histamine from mast cells
A nociceptor stimulant that -> sensitization
What is the dorsolateral tract of Lissauer?
Aka posterolateral tract
A tract located at the edge of the dorsal horn comprised of lightly myelinated collateral branches of the small dorsal root ganglia neurons
Result: synapse between 1st and 2nd order neurons in anterolateral system occurs over 4-6 spinal segments
What is the substantia gelatinosa?
The second layer of the dorsal horn of the spinal cord
Packed with tiny neurons
Site of the synapse between dorsolateral tract of Lissaeur neurons and second order neurons in the anterolateral system
Also site of synapse of first order neurons with interneurons
Where do first order fibers from the anterolateral system synapse?
Mostly in the marginal zone and lamina V of the dorsal horn gray matter
Describe the course of second order fibers of the ALS.
Axons decussate in anterior/ventral white commissure (few remain ipsilateral)
Join ascending spinothalamic tract
-Sacral fibers lateral, more superior fibers medial
Where do second order fibers of the spinothalamic tract synapse?
Most synapse in ventral posterolateral (VPL) nucleus of thalamus
Few in brainstem (reticular formation, periaqueductal gray matter): pain reflexes, pain modulation
Few in intralaminar thalamic nuclei: project broadly into cortex, participate in emotional aspects of pain
What are the three main nuclear groups of the thalamus?
Anterior, medial, lateral
Describe the path of third order neurons in the anterolateral system: cell bodies, projection, termination.
Cell bodies: VPL nucleus of thalamus
Project through posterior limb of internal capsule
Terminate: mostly in primary somatosensory cortex in postcentral gyrus
What determines the amount of sensory cortex devoted to a particular region?
The amount of sensory cortex devoted to a particular region is determined by the DENSITY of receptors in the region, not its size
Result of lesions to the spinothalamic tract
Contralateral loss of pain starting a few segments below lesion
Diffuse, poorly localized pain comes back after a while via alternative pathways
Describe the path of first order neurons in the DCML system: cell bodies, projection, termination.
Cell bodies: dorsal root ganglia
Main part of axon enters/ascends in dorsal columns
-Axons from legs medial (fasciculus gracilis), axons from arms lateral (fasciculus cuneatus) (this is opposite of ALS)
Terminate in caudal medulla (nucleus gracilis or cuneatus)
At what level do the fasciculus gracilis and fasciculus cuneatus form?
T6
Describe the path of second order neurons in the DCML system: cell bodies, projection, termination.
Cell bodies: gracile or cuneate nuclei of caudal medulla
Fibers pass ventromedially as INTERNAL ARCUATE FIBERS, decussate, and become contralateral MEDIAL LEMNISCUS
Ascend to terminate in VPL of the thalamus
Describe the path of third order neurons in the DCML system: cell bodies, projection, termination.
Cell bodies: VPL of thalamus
Pass through posterior limb of internal capsule
Terminate in primary somatosensory cortex (postcentral gyrus)
What is use-dependent cortical plasticity?
The cortex is constantly remapped throughout an individual’s life based on experiences
Increased use -> increased cortical representation
Result of lesions of dorsal column pathways. Define dorsal column ataxia and Romberg sign.
Deficits of fine, discriminative touch and vibratory sense
Can’t identify objects by touch
Loss of position sense
Dorsal column ataxia: clumsy movement, particularly when vision is limited
Romberg sign: unsteadiness with the eyes closed
List the general functions of the trigeminal nerve.
Sensory innervation to most of the head
Control of muscles of mastication
Innervates dura, most cranial blood vessels
Where are trigeminal nerve sensory cell bodies located? What is unusual about one of these locations?
Most in trigeminal (semilunar) ganglion in lateral wall of cavernous sinus
Proprioceptive afferent fibers have cell bodies in MIDBRAIN
UNIQUE: this is the only sensory ganglion inside the CNS
Where does the trigeminal nerve enter the brainstem?
Pierces the middle cerebellar peduncle in the mid-pons
Separate sensory and motor nerve roots
Name the three nuclei of the trigeminal brainstem complex and list their functions.
All three make up a system of three elongated nuclei
- Mesencephalic nucleus: near the midbrain
- Proprioception from teeth and jaws - Principle nucleus: in the pons
- Well localized touch and pressure sense - Spinal nucleus: extends from the upper spinal cord to near the pons
- Pain and temperature sensation
Describe the course of trigeminal mechanosensory system 1st order neurons.
1st order: Large, heavily myelinated fibers from face
-> chief sensory nucleus
Why don’t unilateral CNS lesions result in complete loss of sensations on one side of the head?
Ipsilateral fibers in the posterior trigeminothalamic tract provide a route around these lesions
Describe the course of spinal trigeminal system 1st order neurons.
How does the course of these neurons affect the results of damage?
1st order: small diameter sensory fibers from face/head -> enter pons -> descend in spinal tract as far as C5-> terminate in spinal nucleus
Damage to lateral part of medulla or upper cervical spinal cord -> ipsilateral loss of pain and temperature sensation, with touch sensation entirely intact
Describe the course of trigeminal mechanosensory system 2nd order neurons.
2nd order: chief sensory nucleus -> decussate -> anterior/ventral trigeminothalamic tract (aka trigeminal lemniscus) -> ventral posteromedial (VPM) nucleus of thalamus
-Some fibers ascend ipsilaterally as the posterior/dorsal trigeminothalamic tract
Describe the course of trigeminal mechanosensory system 3rd order neurons.
3rd order: VPM of thalamus to face and head areas of somatosensory cortex (inferior portion of postcentral gyrus)
Describe the course of spinal trigeminal system 2nd order neurons.
2nd order: spinal nucleus of trigemial -> ventral trigeminothalamic tract (aka trigeminal lemniscus) -> ventral posteromedial (VPM) nucleus of thalamus
Describe the course of spinal trigeminal system 3rd order neurons.
3rd order: VPM of thalamus -> facial area of postcentral gyrus
What other nerves synapse in the spinal nucleus of the trigeminal?
Other cranial nerves that have GSA function (VII, IX, X) (-> referred pain in head)
Upper cervical nerve roots
Involved in many cranial reflexes
List and describe the trigeminal reflexes. (6)
Corneal reflex: a consensual reflex of eye closure (CN VII) to stimulation of the cornea
Tearing: irritation of eyes -> motor limb is GVE parasympathetic fibers in CN VII
Cough and sneeze: complex motor limb
Oculocardiac reflex: slowing of heart to pressure on globe; motor limb in CN X
Jaw jerk reflex: monosynaptic reflex with sensory limb in mesencephalic V and motor in motor root of V
Consequences of damage to dorsal spinocerebellar tract
Incoordination in walking that is NOT corrected by having the eyes open (vs problem with dorsal columns)
Like a drunk person walking
