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Neuroanatomy - Pain Pathways

57 important questions on Neuroanatomy - Pain Pathways

What causes nociceptive pain?

Caused by activation of nociceptors by actual or potential tissue-damaging stimuli

What are the two subtypes of nociceptive pain and what are each of their characteristics?

1. Somatic pain - pain from skin, muscles, joints, or bones that is well localised
2. Visceral pain - pain from internal organs that is diffused. poorly localisable, and often referred

What causes neuropathic pain?

Caused by injury or dysfunction of the somatosensory nervous system (both central and peripheral)

What type of sensations is neuropathic pain described as?

Burning, shooting, or electric-like sensations

What causes inflammatory pain?

Caused by tissue inflammation and mediator release

How can inflammatory pain cause hyperalgesia?

Mediators released from inflammatory pain sensitizes nociceptors by lower their activation threshold. Thus, even mild stimuli can cause exaggerated pain, which is hyperalgesia.

How is inflammatory pain adaptive?

Inflammatory pain is adaptive because it promotes healing and protection of injured tissue by discouraging use of that tissue through sensations of pain.

What is functional/centralised pain and what causes it?

Functional/centralised pain is pain without clear nociceptive or neuropathic origin. It is driven by altered central processing and sensitisation.

List the 3 types of pain fibres with their corresponding stimuli/sensation.

1. Aδ fibres (a-delta) — fast pain
2. C-fibres — slow or sustained pain
3. Aβ fibres (a-beta) — non-painful stimuli (touch, light pressure, etc)

What type of receptors are each of the pain fibres?

1. Aδ fibres (a-delta) — mechanical receptor in skin
2. C-fibres — polymodal receptor in almost all tissues
3. Aβ fibres (a-beta) — thermal and mechanothermal receptors

What are the two roots of a spinal segment?

1. Anterior root
2. Posterior root

What type of fibres do each anterior and posterior root contain?

1. Anterior/ventral root - contains efferent/motor fibres to carry signals away from the CNS
2. Posterior/dorsal root - contains afferent/sensory fibres to carry signals to the CNS

What does it mean when symptoms “follow a dermatome”?

pain, numbness, or rash appears in a specific skin region corresponding to a single spinal nerve root
ex) pain in the circled areas indicate which spinal nerve root is responsible (C6=thumb, C7=middle finger, C8=pinky)

What is the clinical use of understanding dermatomes?

Helps localize the level of spinal nerve or spinal cord injury by identifying the area of skin experiencing pain without physical injury.

What determines the strength of a signal relayed from sensory receptors on the skin?

The strength of the stimulus that the receptor detects determines the strength of the signal

What is a characteristic that the dorsal column and spinothalamic pathway share?

Both pathways are three neuron pathways (first order, second order, and third order neurons)

What sensations is the dorsal column pathway responsible for?

Localised touch, proprioception

What sensations is the spinothalamic pathway responsible for?

Pain, temperature

Explain the three neuron chain of the spinothalamic tract

First order neuron - located in dorsal root ganglion (DRG)
Second order neuron - located in ventral horn of spinal cord, decussates in ventral commissure and ascend to ventral posterior nucleus of thalamus
Third order neuron - located in ventral posterior nucleus of thalamus, projects to the primary somatosensory cortex

How does the spinal cord and the PNS send information to the cerebrum?

Through the diencephalon

Where do the outputs from the cerebrum pass through?

Diencephalon

What are the two major regions that the cerebrum signals and forms the diencephalon?

Thalamus and hypothalamus

What area of the brain do all sensory information except smell pass through?

All sensory information, except smell, pass through the thalamus then to the cerebral cortex

What is the location of the thalamus and hypothalamus within the diencephalon?

Thalamus - on either side of the midline of the diencephalon
Hypothalamus - inferior and anterior to thalamus

What is Broadmann's area?

A classification system of different cerebral cortex regions according to their function

Which broadmann's area is responsible for relaying signals about pain?

Primary somatosensory cortex = broadmann's area 1-3

What is the anatomical location of the primary somatosensory cortex in the parietal lobe?

Post central gyrus

What is the sensory homunculus?

Mapping of the somatosensory receptors in the body to the somatosensory cortex

How do sensory afferents from the trigeminal nerve reach the cortex?

Sensory afferents of CN V (trigeminal) project to the trigeminal nucleus complex in the brainstem, which continues projecting to the cortex.

What does the trigeminal ganglia (TG) contain?

Neuronal cell bodies of sensory neurons projecting to the orofacial structures

Where does the opathalmic branch (V1) of CN V project to?

V1 projects to forehead skin and dura via opthalmic nerve

Where does the maxillary branch (V2) of CN V project to?

V2 projects to skin, mucosa, and teeth of upper jaw via maxillary nerve

Where does the mandibular branch (V3) of CN V project to?

V3 projects to skin, mucosa, and teeth of lower jaw (including tongue mucosa)

With which region are pain-sensing nociceptors from the orofacial area highly connected?

The caudal region of the trigeminal nucleus complex = trigeminal subnucleus caudalis (Vc)

Where do neurons of the trigeminal subnucleus caudalis (Vc neurons) signal to after receiving signals?

They relay signals to various ascending pain pathways in the brain.

Which nerve innervates the tongue mucosa and carries taste signals?

The chorda tympani nerve (branch of CN VII facial nerve)

Explain the general pathway of taste signals

Chorda tympani nerve —> nucleus tract solitarius in brainstem —> central taste pathways

What are some causes of persistent orofacial pain?

Tissue or nerve injury, irradiation, chemotherapy (oral mucositis), and oral cancer.

What can direct injury of peripheral nerves (e.g., trauma or tooth extraction) lead to?

Painful post-traumatic trigeminal neuropathy (PTTN).

Why is trigeminal neuralgia a distinct entity of chronic pain?

Beacuse trigeminal neuralgia is derived from compression of the central root of the trigeminal nerve

Which cranial nerve primarily detects pain from the head, face, oral cavity, meninges, and cranial blood vessels via nociceptors?

Trigeminal nerve (CN V)

Where do nociceptors of CN VII (facial nerve) carry pain from?

Small areas around the ear

Where do nociceptors of CN IX (glossopharyngeal) carry pain from?

Posterior tongue, pharynx

Where do nociceptors of CN X (vagus) carry pain from?

External ear canal, dura of posterior cranial fossa

Summarise the trigeminal sensory system for pain

Face/Head Nociceptors → Trigeminal Ganglion of CN V (1st order neuron) → Spinal Trigeminal Nucleus (2nd order neurons) → via decussation Contralateral VPM Thalamus (3rd order neurons)→ Primary Somatosensory Cortex

Where does the 1st order neuron of the trigeminal pain pathway enter the brainstem at?

At the level of the pons

After entering the brainstem through the pons, where does the trigeminal sensory fibres descend to?

Trigeminal sensory fibres descend to the spinal tract of the trigeminal nerve to synapse with the 2nd order neurons in the spinal trigeminal nucleus

What sensations of the face is relayed by the spinal trigeminal nucleus?

Spinal trigeminal nucleus is the key relay station for pain and temperature sensations from the face

Where do second order neurons of the trigeminal pain pathway decussate and ascend through?

2nd order neurons decussate at the spinal trigeminal nucleus and ascend through the trigeminothalamic tract to reach the contralateral thalamus

What does the ventral posteromedial nucleus (VPM) of thalamus contain?

Third order neurons of the trigeminal pain pathway

Where do third order neurons of the trigeminal pain pathway project to?

3rd order neurons project to the primary somatosensory cortex in the postcentral gyrus

What is the function of the primary somatosensory cortex?

Localisation and interpretation of intensity and quality of facial pain

Which areas of the brain contribute to the emotional and affective components of pain perception?

Associated cortical and limbic areas

What does the descending pain modulatory system do?

It allows the organism to function enough to respond to a pain source by reducing the pain signal through neuronal inhibition, known as spinal gating mechanism/gate theory of pain.

What is periaqueductal gray (PAG)?

Area of grey matter in the midbrain involved in descending pain control pathway

Explain the input, relaying, and output of periaqueductal gray (PAG)

Nociceptive input from various structures —> relayed to rostral ventral medulla (RVM) —> neurons of RVM send a signal down the spinal cord —> signal activates endogenous opiate system —> pain is suppressed

What does raphe-spinal tract do in terms of spinal gating mechanism?

Raphe-spinal tract consist of descending pain modulatory pathway neurons

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Neuroanatomy

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