Neurology - Somatosensory Flashcards
What is a sensory modality
A modality is a type of stimulus that has specialised receptors which transmit information through specific anatomical pathways to the brain
What are the different sensory modalities and what receptors detect them
Mechanoreceptors
- Touch
- Pressure
- Vibration
- Proprioception (joint position, muscle length, muscle tension)
Thermoreceptor
- Temperature
Nociceptor
- NociceptionWhat are the three types of sensory fibres
AB - thickest, myelinated, fastest transmission - mechanoreceptors of the skin
Ad - thick, myelinated, fast transmission - pain temperature
C - thin, slow, unmyelinated - temperature, pain and itch
A peripheral nerve will have all of these different fibres in it
What are the different types of nerve endings
C fibres have free nerve endings which are very close to the skin and are responsive to heat
Mechanoreceptor ending are encapuslated
What is the absolute threshold
The level of stimulus that produces a positive result 50% of the time
This will create a generator potential which will depolarise the nerve
The stronger the stimulus the more neurotransmitter is released
Explain the functionality of thermoreceptors
- Free nerve ending with high thermal sensitivity
- Change in temperature activates a family of transient receptor potential ion channels
- There are 4 heat activated TRP channels (TRPV1-4) ranging from low heat to high heat
- There are 2 cold activated TRP channels (TRPM8, TRPA1)
What are the different types of mechanoreceptors and what do they sense for
- Meissner’s corpuscle - fine discriminative touch
- Merkel cells - light touch and superficial pressure
- Pacinian corpuscle - detects deep pressure, vibration and tickling
- Ruffini endings - continuous pressure or touch and stretch
What are the two types of adaptation receptors
Tonic receptors
- Detect continous signal strength
- Continue to transmit impulses as long as the stimulus is present
- keeps the brain informed of the status of the body
- e.g. merkel cells - slowly adapt allowing for fine touch to be perceived
Phasic receptors
- Detect changes in stimulus strength
- Transmit an impulse at the start and at the end (when a chane is taking place)
- Also called movement receptors or rate receptors
- e.g. Pacinian corpuscle - sudden pressure excites receptor, transmits a signal again when pressure is released
What are somatosensory dermatomes
- Each spinal nerve has a specific dermatome on the skin
- Each spinal nerve innervates a certain level in the spinal cord
What are receptive fields
- The receptive field is the region of skin which causes activation of a single sensory neuron when activated
- There are different size receptive fields on the body e.g. smaller on the fingers to allow fine touch but much larger on the back
Define two point discrimination
The minimum distance at which two points are perceived as separate (relative to the size of the receptive field)
What are the different fibre types used for in nociception
Ad fibres mediate sharp, intense or first pain
- Myelinated
- Type 1 : ad-mechanoheat receptors (noxious mechanical and thermal stimuli)
- Type 2 : mechanoreceptors (noxious mechanical stimuli)
C fibres mediate dull, persistent or second pain
- Unmyelinated
- Respond to thermal, mechanical and chemical stimuli (polymodal)
Where are sensory cell bodies found within the body and face
In the body they are in the dorsal root ganglia
In the face they are in the trigeminal ganglia
How are the sensory pathways organised within the dorsal horn
Organised into layers - the rexed laminae (1-7)
What terminates into each of the lamina
Pain and temperature (Ad and C fibres) terminate in laminae 1-2 (superficial)
Innocuous mechanical stimuli (AB fibres (and Aa)) termiante in laminae 3-6 (deep)
What connects between the different laminae and adjacent peripheral inputs
Inter neurons
What is lateral inhibition and why does it occur
- Receptive fields can overlap and make it difficult to establish between two stimuli locations
- Lateral inhibition prevents overlap and facilitates pinpoint localisation of the stimulus
- It is mediated by interneurons within the dorsal horn of the spinal cord
What are the central sensory structures and where are they found
Primary somatosensory cortex
- In the postcentral gyrus
Secondary somatosensory cortex
- In the parietal operculum
Posterior parietal complex
- Spatial awareness of the bodyWhat is the function of the dorsal column system
- Transmits innocuous mechanical stimuli (fine discriminative touch, vibration)
- AB fibres enter via the dorsal horn and enter the ascending dorsal column pathways
- Information from the lower limbs and body (below T6) travel ipsilaterally along the gracile tract
- Information conveyed from the upper limbs and body (above T6) travel ipsilaterally along the cuneate tract
Where do first order neurons of the dorsal column system terminate
First order neruons terminate in the medulla
- Fibres in the gracile tract have their first synapse in the Gracile nucleus
- Fibres in the Cuneate tract have their first synapse in the Cuneate nucleus
Where do second order neurons of the dorsal column system cross and terminate
Second order neurons cross in the medulla
- Second order axons decussate in the caudal medulla
- Forms the contralateral medial lensicus tract
Second order neurons terminate in the thalamus
- Axons of second order neurons terminate in the ventral posterior lateral nucleus of the thalamus
- There is a topographical representation of the body in this nucleus as the limbs terminate more laterally
Where do third order neurons of the dorsal column system terminate
Third order neurons terminate in the somatosensory cortex
- Third order neurons from the VPL project into the somatosensory cortex
- Size of the somatotopic areas is proprtional to the density of sensory receptors in that body region
What is the function of the spinothalamic (anterolateral) pathway
- Pain and temperature sensations ascend within the lateral spinothalamic tract
- Crude touch ascends within the anterior spinothalamic tract
Where do first order neurons of the spinothalamic pathway terminate
First order neurons of the spinothalamic pathway terminate in the dorsal horn
- Primary afferent axons terminate upon entering the spinal cord
- Second order neurons decussate immediately in the spinal cord and form the spinothalamic tract
Where do second order neurons of the spinothalamic tract terminate
Second order neurons of the spinothalamic pathway terminate in the thalamus
- Second order neurons terminate in the ventral posterior lateral nucleus of the thalamus
- There is a topographic representation of the body (lower extremities are lateral)
What are the key differences between the spinothalamic tract and the dorsal column
The spinothalamic tract involves pain, temperature, and course touch and crosses in the spinal cord whereas the dorsal column is involved with light touch, vibration and 2-point discrimination and crosses over in the brainstem
Describe the pathway of pain
Pain is transmitted though the spinal cord to the parabrachial area in the brain stem and then to the limbic system
What cortexes are involved in pain
Primary somatosensory cortex, secondary somatosensory cortex, insula cortex, anterior cingulate cortex and prefrontal cortex
Also the amygdala, cerebellum, brainstem
How would you test the integrity of the ascending somatosensory pathways
- Use a set of tools to look into different sensory modalities
- This allows to detect what parts of the spinal cord are damaged, and from what level
What are electrical perceptual thresholds
Electric currents are sent through the skin and the patient can either feel it or not feel it, this can then be mapped for the whole body
Define pain
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
What are the different types of pain
- Nociceptive - tissue damage, typically acute (e.g. skin cut)
- Muscle - lactic acidosis, ischaemia (e.g. stretching, fibromyalgia)
- Somatic - well-localised (e.g. inflammation, infection)
- Visceral - deep, poorly localised (e.g. stomach, colon, IBS)
- Referred - from an internal structure/organ (e.g. angina)
- Neuropathic - dysfunction of the nervous system
- Chronic - pain lasting at least 6 months
How does chronic pain manifest
Chronic pain manifests as hyperalgesia (higher levels of pain) and allodynia (pain from stimuli that would not normally cause pain)
Also has an emotional aspect of anxiety and depression
What causes neuropathic pain
Neuropathic pain is caused by a lesion or disease of the somatosensory nervous system
- Pain in area of neurological dysfunction
- Sharp, burning, electric shocks
- Poor response to usual analgesic drugs (e.g. opiates)
Give some examples of neuropathic pain
- Radicular lower back pain (sciatica)
- Diabetic neuropathy
- Post herpetic neuralgia
- post-surgical pain
- HIV-induced neuropathy
- Chemotherapy induced neuropathy
- Complex regional pain syndrome (CRPS)
How is neuropathic pain diagnosed and assessed
Using a variety of screening tools
- Questionnaires
- Simple sensory testing devices (pin prick, brush, heat)
How can chronic pain be developed as a result of synaptic plasticity
- Initiated by NMDA receptor activation
- Ca2+ mediated synaptic plasticity in dorsal horn neurons
- Increased synaptic strength (efficacy)
- Reduced inhibitory influences on dorsal horn neurons
- Persistent activation of NMDA receptors can result in the development of chronic pain (e.g. arthritis)
What causes hyperalgesia and allodynia
The loss of inhibition on the pathway responsible for pain causes increased pain sensation
What is the neuropathic phenotype
The neuropathic phenotype is the type of pain and how it has changed
How can chronic pain be reduced
Chronic pain can be reduced by descending modulation using monoamines in the brainstem which inhibit spinal cord excitability
What are the two ways in which monoamines are released into the brainstem
PAG-RVM axis - 5-HT release Locus cereleus - In the pons - Uses noradrenaline as well as serotonin
How do opioids increase descending inhibition
- The PAG and RVM contain large amounts of u opioid receptors
- Endogenous opioids enhance the descending inhibition from the PAG-RVM axis
- This reduces pain transmission in the dorsal horn by inhibiting glutamate release
- This forms part of the endogenous analgesic system
How are descending control systems targeted for pain relief
- Opioids are used tp target the PAG-RVM axis
- SNRIs enhance descending noradrenergic inhibition which increases noradrenaline which activates more alpha 2 receptors which have an inhibitory effect
How is conditioned pain modulation used to measure the level of descending control in patients
The better the descending control the better the efficacy of duloxetine (SNRI)
How is transcranial direct current stimulation used to reduce chronic pain
This is non-invasive brain stimulation that causes changes in cortical excitability in the primary motor cortex that has been shown to reduce chronic pain in fibromyalgia and migraine patients
What are the three clusters of neuropathic pain
- Sensory loss
- Thermal hyperalgesia
- Mechanical hyperalgesia
