Neurology 7 - Sensory Pathways Flashcards
What is the somatosensory cortexes function?
- The ability to interpret body sensations
- Mechanical, thermal, proprioceptive and nociceptive (not necessarily painful)
What does the function of mechanoreceptors depend on?
- Degree of specialisation (from free nerve endings to elaborate accessory structures)
- Location (in layers of skin, around hair shaft, in muscles and tendons)
- Physiological properties (activation threshold determines sensitivity (low threshold)
What is nociception?
- Provides information about unpleasant or harmful stimuli
- When this information is processed by the brain it is percieved as pain
- It can be measured
How is visceral pain transmitted?
- Carried peripherally by autonomic nerves, and centrally in the spinothalamic and dorsal columns pathways
- Localisation is mainly referred to the body wall at the same spinal level
How is somatic pain and termperature information transmitted?
Carried to the brain by the spinothalamic tract
List the characteristics of receptors involved in nociception
- Polymodal (mechanical, thermal or chemical stimulus)
- Free nerve endings
- High threshold
- Slow adapting (continue to fire impulses as long as the stimulus is present)
What is the function of taking information via the spinothalamic tract to the somatosensory I and II cortex?
Analysis of localisation and intensity of the noxious stimulus
What is the function of taking information via the spinothalamic tract to the forebrain?
Perception of pain
What is the function of taking information via the spinothalamic tract to the midbrain?
Inhibit pain
Where do the axons of the spinothalamic tract send collateral branches to?
- Brainstem (reticular formation)
- Thalamus (intralaminar nuclei)
- Hypothalamus and some cortex
- This triggers an increase in awareness and registers the unpleasantness of the stimulus
Describe central inhibition
- Descending pathway in the brainstem is triggered which inhibits the nociceptive pathway in the dorsal horn
- Pathway uses endogenous opiods and other transmitters
Describe the consequences of nociceptive dysfunction
- Reduced pain but disposed to increased injury
- Some changes may exacerbate pain (eg. windup in dorsal horn, thalamic syndrome and phantom pain)
List and define the major somatosensory modalities
- Touch (detection of light mechanical stimuli)
- Thermosensation (temperature)
- Nociception (noxious or potentially damaging stimuli)
- Proprioception (mechanical displacement of muscles and joints)
Where are free nerve endings used?
Thermoreceptors and nociceptors
Where are enclosed nerve endings used?
Mechanoreceptors
Describe the classification of sensory neurons.
- A beta are largest and fastest (mechanoreceptors of skin, touch and proprioception)
- A delta are middle size and middle speed, they convey pain and temperature
- C fibres are small and unmyelinated and therefore slow (temperature, pain and itch)
Define the term receptor
Transducers that convert energy from the environment into electrical stimulus
What type of fibres are used by thermoreceptors?
- A delta and C-fibres
- Free nerve endings
- TRP (transient receptor potential) ion channels - heat activated TRPV 1-4, cold activated TRPM8 and TRPA1
List the types of mechanoreceptors and their use
- Meissners corpuscle (fine discriminative touch, low frequency vibration - phasic)
- Merkel cells (light touch and superficial pressure - tonic)
- Pacinian corpuscle (deep pressure, high frequency vibration and tickling - phasic)
- Ruffini endings (continuous pressure or touch and stretch - tonic)
Define stimulus threshold
A threshold is the point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold)
Define stimulus intensity
Increase strength and duration of stimulus results in increased neurotransmitter release and greater frequency of action potential
What is the function of tonic receptors?
- Detect continuous stimulus strength
- Transmit impulses to the brain as long as the stimulus is present
- Keeps the brain informed of the status of the body (eg. Merkel cells, which slowly adapt)
What is the function of phasic receptors?
- Detect changes in stimulus strength
- Transmit impulses at the start and end of the stimulus - adapt quickly, fade in the middle
- Eg. Pacinian corpuscle, sudden pressure excites the receptor and transmits a signal again when the pressure is released.
Compare the receptive fields of the arm, fingers and back
- Arm has small receptive fields to allow detection of fine detail
- Fingers have densely packed mechanoreceptors with small receptive fields
- Large receptive fields allow the cell to detect changes over a wider area, less precise
Define receptive field
The receptive field is the region on the skin which causes activation of a single sensory neuron when activated
What is two point discrimination?
- Minimum distance at which two points are percieved as separate
- Related to the size of a receptive field (whether you can feel two points or one)
List the important dermatomes to remember
- C5 is the clavicle level
- C6 down the thumb and back of the index finger
- T4 level of the nipples
- T10 umbilicus
Where are cell bodies of the body and face found?
- Dorsal root ganglia for the body
- Trigeminal ganglia for the face
Describe the somatosensory pathway from the face
- Enters the brain at the pons
- Synapses in the trigeminal nucleus decussate
- Secondary project to the thalamus (ventral-posteriomedial nucleus)
- Tertiary sensory neurons project to the somatosensory cortex
Describe the somatosensory pathway from the lower and upper limbs
- Sensory information is carried at the cuneate fasciculus (upper limb - lateral) and gracille fasciculus (lower limb - medial)
- Crosses over at the medulla to form the medial lemiscus tract
- Connect to the thalamus, axons synapse with neurons in the ventral posteriolateral nucleas
- Tertiary neurons to the primary somatosensory cortex
How are dorsal horn neurons divided?
Those with axons that project to the brain (projection neurons) and those with axons that remain in the spinal cord (interneurons)
What is lateral inhibition?
- Lateral inhibition aids localisation of a stimulus by inhibiting neighbouring neurons becoming excited
- This enhances the effect of the stimulus
How is lateral inhibition mediated?
By inhibitory interneurons within the dorsal horn
Where do first order neurons terminate in the dorsal column medial lemiscus tract?
- Gracile tract have a synapse in the Gracile Nucleus
- Cuneate tract in cuneate nucleus
Where do second order neurons decussate in the dorsal column medial lemiscus tract?
- Caudal medulla
- Forms the contralateral medial lemniscus tract
Where do the axons of second order neurons terminate in the dorsal column medial lemiscus tract?
- Ventral posterior lateral nucleus of the thalamus
- Topographic representation of the body in the VPL (lower extremities are lateral)
Where do the axons of third order neurons terminate in the dorsal column medial lemiscus tract?
- 3rd order neurons from the VPL project to the somatosensory cortex
- Size of somatotopic area is proportional to the density of sensory receptors in that body region
Where do 1st order neurons terminate in the spinothalamic tract?
- In the dorsal horn
- Terminate upon entering the spinal cord, and then immediately decussate to form the spinothalamic tract
Where do 2nd order neurons terminate in the spinothalamic tract?
- Terminate in the ventral posterior lateral nucleus of the thalamus
What are the issues with sensory testing?
Psycholphysical assessment - it is subjective and affected by cooperation
What is the importance of qualitative sensory testing?
- Temperature and pain tests the integrity of the spinothalamic tract
- Testing discriminative touch and 2-point discrimination tests the dorsal column
List the affects of an anterior spinal cord lesion
- Blocked anterior spinal artery causes ischemic damage to the anterior part of the spinal cord
- Spinothalamic tract damage causes pain and temperature loss below the level of the lesion
- There is retained light touch, vibration and 2-point discrimination
Define pain
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
List the fibres involved in nociceptors
- A delta fibres mediate sharp, intense or first pain (type 1: noxious mechanical and type 2: noxious heat)
- C-fibres mediate dull aching or second pain (thermal, mechanical and chemical stimuli, reminding you to guard this site)
How are nociceptors tested?
- Pressure
- Thermal stimuli
- Sharp pin pricks
What happens in the process of spinal cord nociceptive processing?
- Glutamate is excitatory - major neurotransmitter for pain signalling
- Released in response to acute or persistent noxious stimuli
- Carried by a delta or C fibres to the dorsal part of the spinal cord
How is pain, both the sensory and emotional components, transmitted?
- Lateral spinothalamic tract (sensory component)
- Spinoreticular tract (emotional component)
Where is activity seen in the brain following fMRI and painful stimulus?
- SI, SII
- Insula cortex
- Anterior cingulate cortex
- Prefrontal cortex
- Amygdala
- Cerebellum
- Brainstem
Describe the gate control theory of pain
- Inhibition of primary afferent inputs before they are transmitted to the brain through ascending pathways
- This explains why you want to rubbing a part of the body that has been hit before the feeling of pain (rubbing stimulates A beta neurons which inhibit the transmission of C fibres)
List the descending control pathways of pain and the key areas of the brain involved
- Strong emotions can inhibit pain
- Electrical analgesia can reduce pain
- Periaqueductal grey
- Facilitation and inhibition of nociceptive processing in the dorsal horn
- Monoamines have a role (serotonin and noradrenaline)
- Opiods respond the same way, as well as placebo’s
What are the types of chronic pain?
- Nociceptive pain (related to the skin, muscles, bones or viscera) - arthritis, headache, fractures
- Neuropathic pain (related to a lesion or disease in the somatosensory system) - sciatica, post surgical, trauma
- Mixed - osteoartritis and low back pain
What is peripheral sensitisation?
- Damaged tissue releases inflammatory soup (chemicals such as neurotransmitters, peptides, lipids, proteases and cytokines)
- These produce inflammation, and also modulate excitability of nociceptors to make them more receptive to pain
- Decreases the threshold to stimuli at the site of injury
What is central sensitisation?
- Decreases the threshold to peripheral stimuli at an adjacent site to the injury
- Expands the receptive field causing spontaneous pain
What is allodynia?
Pain due to a stimulus that does not normally provoke pain
What is hyperalagesia?
Increased pain from a stimulus that normally provokes pain (primary at the site and secondary when around the site)
How is diagnosis and assessment of neuropathic pain performed?
- Must have an injury or disease to the somatosensory system
- Symptoms must be present in an area consistent with the sensory involvement
- Identify either sensory loss or sensory pain
- Neuropathic pain questionairres are used to assist this
What drugs are effective in neuropathic pain?
- Serotonin selective re-uptake inhibitors are not effective
- Serotonin noradrenaline re-uptake inhibitor and tricyclic antidepressants are more useful
- Noradrenaline has a protective effect, while serotonin is a facilitator of chronic pain
How does conditioned pain modulation occur?
- Pain inhibits pain (eg. pressure pain and heat)
- This is because the threshold increases or the pain decreases
What is the function of the dorsal column system?
- Conveys innocuous mechanical stimuli (fine discriminative touch and vibration)
- Uses A beta fibres
List the two spinothalamic pathways
- Pain and temperature ascend within the lateral spinothalamic tract (a delta)
- Crude touch ascends within the anterior spinothalamic tract (A beta)
Define the term adaptation
- The decline of the electric responses of a receptor neuron over time in spite of the continued presence of a stimulus of constant strength.
- This change is apparent as a gradual decrease in the frequency of action potentials generated.
What is the representation of the body like in the ventral posterior lateral nucleus of the thalamus?
- Topographic representation
- Lower extremities are lateral
Compare the localisation of pain and temperature to that of touch and vibration
- Size of somatotopic areas is proportional to the density of receptors in that body region in regard to touch and vibration
- Pain and temperature location is not as precise
What type of receptors detect touch and proprioception?
Mechanoreceptors
Compare the receptive fields of nociception and touch
- Touch is much more specific
- Nociception much larger fields
Where do the third order neurones go in the spinothalamic tract?
- Relays through the ventral posteriolateral nucleus to the primary somatosensory cortex
How are the spinothtamic tract and dorsal column different?
- In spinothalamic, information from the lowest parts are lateral and the highest parts are medial
- In the dorsal column the information from lower is medial and from higher is lateral
Describe the pathway of nociceptive information from the face
- Comes via the trigeminal nerve
- Into trigeminal ganglion at the level of the pons
- Goes down alongside trigeminal nucleus, synapses in the trigeminal nucleus at the level of the medulla
- This second neuron then joins the medial end of the spinothalamic tract, to synapse with the ventral postereomedial nucleus and finally primary somatosensory cortex
Which sensory neurons decussate?
Second order neurons
Why is overlapping of sensory neurones receptive field important?
- As this allows the precise location of a stimulus to be detected
- Depending on the exact location of the stimulus, different neurons will be stimulated at different frequencies
How does sensory convergence affect receptive field size?
If many primary neurons converge to a single sensory neuron, the receptive field will be larger
