Ascending Neural Pathways Flashcards
Describe the neural pathway pattern of the somatosensory system that most conscious sensory tracts follow?
Primary afferent is a first order neuron and terminates in spinal cord or brain stem
The second order neuron projects to the thalamus
The third order neuron projects to the brain (somatosensory cortex)
Describe the sensation of light touch
Awareness of precise location of very delicate mechanical stimuli
- Stroking hairs on skin or feather on hairless skin.
Other phenomena: two-point sense, stereognosis and graphesthesia
Describe the sensation of pressure
Stimuli applied to subcutaneous structures.
Squeezing muscles or structures under the skin.
Describe the sensation of vibration sense
Flutter sensation through intact tissues
Can be elicited by placing a tuning fork on tissues
Describe the sensation of proprioception
Motion sense, position sense
Describe the sensation of pain and temp
Nociception (fast and slow)
What the fuck is nociception?
The sensory nervous system’s process of encoding noxious stimuli. In nociception, intense chemical (e.g., cayenne powder), mechanical (e.g., cutting, crushing), or thermal (heat and cold) stimulation of sensory nerve cells called nociceptors produces a signal that travels along a chain of nerve fibers via the spinal cord to the brain
What are some nonencapsulated peripheral receptors?
Free nerve endings
Merkel discs
Hair follicle receptors
What are some encapsulated peripheral receptors?
Meissner's corpuscles Pacinian corpuscles Ruffini corpuscles Neuromuscular spindles Neurotendinous spindles
Where are free nerve endings found?
Epidermis, cornea, gut, dermis, ligaments, joint capsules, bone, dental pulp, etc
Where are Merkel discs found?
The German Chancellory
Hairless skin
Where are hair follicle receptors found?
Hairy skin
Where are Meissner’s corpuscles found?
Dermal papillae of skin and palm and sole of foot
Where are Pacinian corpuscles found?
Dermis, ligaments, joint capsules, peritoneum, external genitallia, etc
Where are Ruffini corpuscles found?
Dermis of hairy skin
Where are neuromuscular spindles found?
Skeletal muscle
Where are neurotendenous spindles found?
Tendons
What is the stimulus, sensory modality, adaptability and fibres of free nerve endings?
Mechanoreceptor
Pain(fast) Pain(slow) Touch(crude) Pressure,? Heat and cold
Rapid
A delta,
C
What is the stimulus, sensory modality, adaptability and fibres of Merkel discs?
Mechanoreceptor
Touch
Slow
A beta
What is the stimulus, sensory modality, adaptability and fibres of hair follicle receptors?
Mechanoreceptor
Touch
Rapid
A beta
What is the stimulus, sensory modality, adaptability and fibres of Meissner’s corpuscles?
Mechanoreceptor
Touch
Rapid
A beta
What is the stimulus, sensory modality, adaptability and fibres of Pacinian corpuscles?
Mechanoreceptor
Vibration
Rapid
A beta
What is the stimulus, sensory modality, adaptability and fibres of Ruffini corpuscles?
Mechanoreceptor
Stretch
Slow
A beta
What is the stimulus, sensory modality, adaptability and fibres of Neuromuscular spindles?
Mechanoreceptor
Stretch - muscle length
Fast
A alpha,
A beta
What is the stimulus, sensory modality, adaptability and fibres of Neurotendinous spindles?
Mechanoreceptor
Compression - muscle tension
Fast
A alpha
What are the different structural sections of the spinal cord?
Upper cervical Lower cervical Thoracic Lumbar Sacral
What is white matter?
Axons
What neurons make up the ventral roots?
Motor
What neurons make up the dorsal roots?
Sensory
What neurons make up the spinal nerve?
mixed sensory/motor
What is grey matter?
Cell bodies
Describe the layout of grey/white matter in the spinal cord?
Grey matter forms a “butterfly” in a cylinder of white matter
Where are the descending tracts of neurons in the spinal cord?
In a triangle that fills all but the most distal space between the dorsal grey horn and ventral grey horn as well as distinct areas in the distal border of the spinal cord ventral to the grey matter.
What neurons are in the descending tract?
Motor
What neurons are in the amending tract?
Sensory
Where are the ascending tracts of neurons in the spinal cord?
All the area posterior to the grey matter medial to the bilateral extensions that make up the dorsal grey horn
The most distal and ventral aspects of the “triangle” of white matter between the dorsal and ventral grey root.
Also the lateral peripheries of white matter between the ventral grey horn and the lateral border of the spinal cord
Where in the brain does all this sensory information go?
Sensory information passes from the thalamus to the primary somatosensory cortex a region posterior to the central sulcus of the cerebral hemisphere.
Sensory information is somatotopically organized in this region of the cerebrum.
- Unequal distribution of the neural input which is representated by the sensory homunculus.
What is the order of the structures commanded in the primary motor cortex from the deepest part of the corpus callosum to the lateral sulcus?
Genitals Toes Foot Leg Hip Trunk Neck Head Shoulder Arm Elbow Forearm Wrist Hand Little finger Ring finger Middle finger Index finger Thumb Eye Nose Face Upper lip Lips Lower lip Teeth, gums and jaw Tongue Pharynx Intra-abdominal
Describe somatosensory areas I and II
Is a distinct and separate spatial organisation in each area.
Area I much more extensive.
”Primary somatosensory cortex” almost always means area I.
Area I: Has a high degree of localisation of different body parts: see names in one of these flashcards somewhere
Area II: poor localisation.
what are the ascending pathways?
Dorsal column-medial lemniscus pathway
Anterolateral spinothalamic pathway
Dorsal and ventral spinocerebellar tracts
Describe the dorsal column-medial lemniscus pathway
This pathway allows the perception of:
Well localised touch sensations
Fine gradation of touch intensity
Vibration sense
Proprioception
via large myelinated axon with a velocity of ~30-110m/s to the brain.
Describe the first section of the dorsal column-medial lemniscus pathway
(1) First order axons enter the dorsal grey horn of spinal cord via dorsal roots
(2) Pass to either cuneate and gracile fasciculus
Above T6 enter fasciculus cuneatus (3b)
Below T6 enter fasciculus gracilis (3a)
What structural changes occur when axons in the dorsal column ascend?
New fibres are added to its lateral side. Therefore, axons from the cervical region lie lateral those of the sacral region
Describe the second part of the dorsal column-medial lemniscus pathway
(3) Synapse with second order neuron in either nucleus cuneatus or nucleus gracilis, respectively.
(4) Fibres then decussate via internal arcuate fibres to enter the medial lemniscus
Describe the third part of the dorsal column-medial lemniscus pathway
(5) Fibres of medial lemniscus ascend and synapse with third-order neurons of the ventral posterolateral (VPL) nucleus of the thalamus.
(6) The third order neurons leave the VPL passing through the internal capsule to terminate in the primary somatosensory cortex.
Describe the full progression of the dorsal column-medial lemniscus pathway
(1) First order axons enter the dorsal grey horn of spinal cord via dorsal roots
(2) Pass to either cuneate and gracile fasciculus
Above T6 enter fasciculus cuneatus (3b)
Below T6 enter fasciculus gracilis (3a)
(3) Synapse with second order neuron in either nucleus cuneatus or nucleus gracilis, respectively.
(4) Fibres then decussate via internal arcuate fibres to enter the medial lemniscus
(5) Fibres of medial lemniscus ascend and synapse with third-order neurons of the ventral posterolateral (VPL) nucleus of the thalamus.
(6) The third order neurons leave the VPL passing through the internal capsule to terminate in the primary somatosensory cortex.
Describe the anterolateral spinothalamic system
This pathway allows the perception of:
Poor localisation of touch sensations (crude touch)
Fast and slow pain
Temperature
Via small diameter myelinated and unmyelinated neurons, with lower velocities when compared to dorsal column-medial lemniscus pathway, up to 40m/s.
i.e., the anterolateral system is a cruder type of transmission system than dorsal column–medial lemniscal system. BUT certain modalities of sensation are transmitted only in this system ie pain, temperature, tickle, itch, and sexual sensations (in addition to crude touch and pressure).
Transmission of “less critical” sensory signals
Describe the first part of the anterolateral spinothalamic pathway
(1) First order axons enter the dorsal grey horn of the spinal cord via dorsal roots.
(2) Enter Lissauer’s tract to ascend or descend 1-2 spinal cord levels.
(3) Synapse in either lamina 1, 2 (substantia gelatinosa) or 5 of the dorsal grey horn.
Describe the second part of the anterolateral spinothalamic pathway
(4) Decussate via the anterior white commissure to enter the contralateral anterolateral spinothalamic tract.
- It can take 2-3 segments to reach the contralateral side.
What information is passed along the anterior spinothalamic tract
Crude touch and pressure
What information is passed along the lateral spinothalamic tract
(pain and temperature)
As this tract ascends, new fibres from higher levels are added to its medial side.
Describe the third part of. the anterolateral spinothalamic pathway
(5) Ascend to the ventral posterolateral (VPL) nucleus of the thalamus where they synapse with the third order neurons.
(6) The third order neurons leave the VPL passing through the internal capsule to terminate in the primary somatosensory cortex.
Similar to the dorsal column-medial lemniscus pathway
Nerve passes from ventral posterolateral nucleus of the thalamus throughout he internal capsule to brain
What is fast pain?
Felt within ~ 0.1s after a pain stimulus is applied.
AKA:
sharp pain,
pricking pain,
acute pain,
electric pain.
E.g., When a needle is stuck into the skin, when the skin is cut with a knife, or when the skin is acutely burned. It is also felt when the skin is subjected to electric shock.
Fast-sharp pain is NOT felt in most deep tissues of the body.
Elicited by mechanical and thermal pain
What is slow pain?
Begins after 1s or more after pain stimulus applied, then increases slowly over 1+s or min.
AKA:
slow burning pain,
aching pain,
throbbing pain,
nauseous pain,
chronic pain.
Is usually associated with tissue destruction.
Can lead to prolonged, almost unbearable suffering.
Slow pain can occur both in the skin and in almost any deep tissue or organ.
Elicited by mechanical, thermal and chemical pain
What picks up painful stimuli?
All painful stimuli is picked up my nociceptors
free nerve endings in the superficial skin and in some internal tissues (e.g. periosteum, arterial walls, joint surfaces and cranial vault).
Nociceptors show little to no adaptation
But can demonstrate increased sensitivity (hyperalgesia)
What are the two pathways for pain transmission into the CNS?
Fast-sharp pain pathway
Slow-chronic pain pathway
Describe the fast-sharp pain pathway
Neospinothalamic Tract
Transmitted via Aδ fibres at ~5-30m/s, mainly mechanical + acute thermal pain.
Terminate mainly in lamina I (lamina marginalis) of the dorsal horns and they excite second order neurons of the neospinothalamic tract.
The second order neurons give rise to long fibres that cross immediately to opposite side of cord through anterior commissure and then turn upward passing to the brain in the anterolateral columns.
Describe the slow-chronic pain pathway
Paleospinothalamic Pathway (older system)
Mostly transmitted via unmyelinated C fibres at ~0.5-2m/s. Although it does transmit some signals from type Aδ fibres too.
The peripheral fibres terminate in spinal cord almost entirely in laminae II (substantia gelatinosa).
Most signals then pass-through 1 + additional short fibre neurons within dorsal horns before entering (mainly) lamina V.
Then give rise to long axons that mostly join fibres from fast pain pathway, passing first through anterior commissure to the opposite side of the cord, then upward to the brain in the anterolateral pathway.
What is referred pain?
Pain perceived in a location remote from tissue causing pain.
E.g., pain in a visceral organ referred to an area on body surface.
What is the probable mechanism of referred pain?
Branches of visceral pain fibres synapse in spinal cord on same 2nd-order neurons that receive signals from skin.
When visceral pain receptors are stimulated, pain signals from viscera are conducted through some of the same neurons conducting pain signals from skin, so person feels sensations originate in skin.
Describe the input and basics of the dorsal and ventral spinocerebellar tracts
Associated with unconscious proprioception
Gets input from:
- Muscle Spindles (muscle tension)
- Golgi organs (proprioception)
- Touch receptors
Used by the cerebellum to co-ordinate movements and posture
Describe the pathway of the dorsal spinocerebellar tract (DSCT)
First order axons enter dorsal grey horn via dorsal roots and synapse in Clarke’s nucleus/column.
- Common route for first order neurons of both pathways
Second order axons ascend in the ipsilateral dorsal spinocerebellar tract to the cerebellar cortex.
Conveys input from trunk and lower limbs
Describe the pathway of the ventral spinocerebellar tract (VSPT)
First order axons enter dorsal grey horn via dorsal roots and synapse in Clarke’s nucleus/column.
Common route for first order neurons of both pathways
Majority second order axons decussate and ascend in the contralateral ventral spinocerebellar tract then crosses back over to the ipsilateral cerebellum.
Minority ascend ipsilaterally to the cerebellum.
Conveys input from trunk, upper limbs and lower limbs.
What is anterior spinal cord syndrome?
Hyperextension injury
Infarction of anterior spinal artery
Loss of bilateral pain, temp and crude touch below
Maintain bilateral fine touch, vibration and proprioception
What is posterior spinal cord syndrome?
Penetration injury
Posterior spinal artery occlusion
Multiple sclerosis
Ipsilateral loss of Fine touch, vibration and proprioception
Maintain contralateral Fine touch, vibration and proprioception
Maintain bilateral pain temp and course
What is Brown-Séquard syndrome?
Penetrating trauma
Same level – ipsilateral total loss of sensation
1-2 levels below – Contralateral loss of pain, temp and course touch
1 level below – ipsilateral loss of fine touch, vibration and proprioception
caused by damage to one half of the spinal cord, i.e. hemisection of the spinal cord resulting in paralysis and loss of proprioception on the same (or ipsilateral) side as the injury or lesion, and loss of pain and temperature sensation on the opposite (or contralateral) side as the lesion
What are the important dorsal columns of the ascending tracts?
Gracile fasciculus (3a) Cuneate fasciculus (3b)
What are the important spinocerebellar tracts of the ascending tracts?
Posterior (4a)
Anterior (4b)
What are the important parts of the anterolateral system of the ascending tracts?
Lateral Spinothalamic tract (5a)
Anterior Spinothalamic tract (5b)
