Ascending Tracts in the Spinal Cord Flashcards
Where are the receptors for pain and temperature found?
in the dermis and epidermis of the skin and mucosa
Where are the cell bodies from the spinothalamic tract found?
the DRG
What is the sensation of pain and temperature conveyed by?
small myelinated (Aδ) and unmyelinated (C) fibres to the dorsal root
What do the Aδ and C fibres synapse with once in the spinal cord?
a second order neuron in the dorsal horn
What is the spinothalamic tract made up of?
axons crossing to the contralateral spinal cord via the anterior white commissure and ascending through the spinal cord, medulla, pons and midbrain as the spinothalamic tract
Where is the third order neuron of the spinothalamic tract?
in the thalamus within the diencephalon
Where does the spinaothalamic tract start and terminate respectively?
- starts in the dorsal horn
- terminates in the ventroposterior lateral nucleus (VPL) of the hypothalamus
What does the VPL do?
project axons through the posterior limb of the internal capsule to reach the somatosensory cortex
What is the internal capsule?
essentially a pathway for the axons to travel to the cerebral cortex
Where is the primary somatosensory cortex?
in the post central gyrus posterior to the central sulcus
What are the functions of the spinothalamic tract?
- carry pain and temperature from arms, legs and trunk
- carry crude/non-discriminative touch
What is crude/non-discriminative touch?
a sensory modality that allows the subject to sense that something has touched them, without being able to localise where they were touched
What does the spinoreticular tract do?
carry pain and crude touch sensations via polysynaptic relays to the brain stem reticular formation
What is the reticular formation?
loose network of neurons between major nuclei
Where do neurons in the reticular formation project to?
widespread areas of the cortex via the non-specific thalamic nuclei
What is the reticular activating system required for?
arousal of higher CNS centres e.g. increased wakefulness as a result of pain
What 3 sensations is the dorsal column medial lemniscus system pathway used by?
- fine touch
- conscious proprioception
- vibratory sense
What is fine touch?
a sensory modality that allows a subject to sense and localise touch and discriminate two points of a compass
What does proprioception allow?
someone to know exactly, and at all times, where the parts of the body are in space and in relation to each other
Where are the receptors for fine touch located?
in the dermis of the skin
Where are there the most receptors for fine touch?
in the fingertips and lips
Where are the receptors for proprioception located?
muscles, tendons and joints
Where do the fibres from all 3 sensations involved in the dorsal column medial lemniscus system pathway go?
towards the spinal cord in the peripheral nerves
Where do fibres from legs and lower trunk ascend and terminate respectively?
- ascend in the fasciculus gracilis of the dorsal column of the spinal cord
- terminate in the nucleus gracilis in the medulla oblongata
What do nucleus gracilis and nucleus cuneatus axons do?
cross to the contralateral medulla as the internal arcuate fibres and ascend through the medulla, pons, and midbrain as the medial lemniscus
Where do fibres of the medial lemniscus terminate?
in the VPL of the thalamus
What are functions of the dorsal column medial lemniscus system?
carry fine touch, vibration and proprioception from the arms, legs and trunk
How is the dorsal column medial lemniscus system function tested?
- tuning fork to test vibration
- figure writing on palm to test topognosis
- recognition of objects by palpation to test stereognosis
- discrimination of simultaneously applied punctate stimuli to test two-point discrimination
What are the signs of damage to the dorsal column medial lemniscus system before and after decussation?
- before = damage on the same side as injury
- after = damage on the opposite side
What can loss of proprioception lead to?
the inability to know where the limbs are i.e. patient may look down at feet when walking or stagger and fall
What is tabes dorsalis caused by?
demyelination by advanced syphilis infection
What do the bacteria implicated in tabes dorsalis do?
invade large, myelinated fibres, leading to the involvement of the dorsal column medial lemniscus pathway
What is the cerebellar tract a sensory tract for?
unconscious proprioception (sends signals to cerebellum instead of cerebral cortex)
What does the trigeminal nerve supply?
anything in front of the vertex-ear-chin line
What cranial nerve is the trigeminal nerve?
V
Where does the trigeminal nerve go?
the spinal trigeminal nucleus in the medulla oblongata
What do the trigeminal nerve afferents carry sensation from?
- ophthalmic division, V1
- maxillary division, V2
- mandibular division, V3
Where do axons in the trigeminal nerve terminate?
- spinal trigeminal nucleus
- chief sensory nucleus
- mesencephalic nucleus
What does the spinal trigeminal nucleus supply?
sensory and motor innervation to the muscles of mastication
Where is the spinal trigeminal nucleus?
in the medulla oblongata and cervical spinal cord
How does the spinal trigeminal nucleus receive pain and temperature sensation?
through the trigeminal and the glossopharyngeal nerve
What do axons from the spinal trigeminal nerve do?
cross to join the contralateral spinothalamic pathway and terminate in the ventroposterior medial nucleus of the thalamus (VPM)
What is the chief sensory nucleus responsible for?
fine touch
Where is the chief sensory nucleus?
the pons
How does the chief sensory nucleus receive fine touch and vibration sensation from the face?
via 3 branches of trigeminal nerve
What do axons from the chief sensory nucleus do?
cross to join the contralateral medial lemniscus and terminate in the VPM of the thalamus
What is the mesencephalic nucleus responsible for?
proprioception
Where is the mesencephalic nucleus?
in the midbrain
What does the mesencephalic nucleus contain?
cell bodies of axons of the mandibular branch (V3) of the trigeminal nerve that carry proprioceptive input and projects axons to neurons of the trigeminal motor nucleus
How is the function of the spinal trigeminal nucleus tested?
- pinprick to test pain
- warm and cold vials to test temperature
How is the function of the chief sensory nucleus tested?
- tuning fork to test vibration
- wisp of cotton to test fine touch
How is the function of the mesencephalic nucleus tested?
jaw-jerk reflex
What does the VPM of the thalamus do?
- relay pain and temperature input from the contralateral face
- project axons through genu of the internal capsule to reach the somatosensory cortex
What is pain referred to?
dermatomes supplied by the same posterior roots that carry visceral afferents to spinal cord
What is referred pain?
when pain is experienced at the surface of the body some distance from the affected organ
Give examples of referred pain
- a person experiencing a heart attack feels pain that radiates along the inner aspect of the left arm
- pain from the parietal pleura and diaphragm is experienced at the shoulders near the root of the neck
- pain from the appendix is felt in the umbilicus initially and eventually moves to the right iliac fossa when the appendix is inflamed enough to touch the body wall
- pain originating from the ureters is felt in the flank and inguinal area
- pain from the gallbladder may be felt not only below the ribs, but also in the shoulder area
What is pathological pain?
continuous, intractable, frequently excruciating, burning pain with or without peripheral nerve damage
What does pathological pain result from?
damage to sensory pathways, not from nociceptor activation
What happens if a peripheral nerve is cut and does not reach its target?
it can form a tangle (neuroma) which is very sensitive to pain
How does phantom pain occur?
nerve fibres at the stump are frequently squeezed by the scar tissue, and this pain stimulus passes to the sensory cortex, which interprets it as coming not from the stump area but from the skin areas of the fingers or toes of the missing limb
What can phantom limb pain result from?
plasticity changes in the somatosensory cortex
What is the antinociceptive pathway?
a descending suppressor against pain starting at the cerebral cortex
What is the gate control theory of pain?
a mechanism in the spinal cord in which pain signals can be let through to the brain or restricted to the spinal cord
How do small fibres open the pain ‘gate’?
they activate posterior horn neurons that transmit pain centrally
How do large, myelinated fibres close the pain ‘gate’?
they activate interneurons that inhibit neurons in posterior horn that transmit pain
What does electrical stimulation of the midbrain periaqueductal grey do?
produce analgesia without affecting fine touch
What does enkephalin do?
suppress pain transmission from the first order to second order neurons in the pain pathway
