The Somatosensory system and chronic pain Flashcards
Define the somatosensory modalities
type of sensory stimulus - touch, temperature, nociception, proprioception
What are the receptors for these modalities
touch - mechanoreceptor, temperature - thermorecpetor
nociception - nociceptor
Describe the structure of mechanoreceptors
Merkel disks is for light touch and superficial pressure,
Meissner’s corpuscles is for fine discriminative touch
Ruffini endings is for continuous pressure of touch or stretch
Pacinian corpuscles is for deep sustained pressure vibration and tickling
Describe thermoreceptors
specific temperature activate family of transient receptor potential (TRP) channels heat -activated : 1,2,3,4, cold-activated : TRPM8, TRPA1 TRPV1 - activated by capsaicin TRPM8 activated by cold substances
Describe Nociceptors
respond to damaging or potentially damaging stimuli
There are three types
Thermal (extreme temp)
mechanical (intense pressure)
Polymodal (respond to thermal, mechanical and chemical)
How are itch stimulated
due to low frequency firing of ncociceptors
specific receptors sensitive to histamine which produce the unpleasant sensation that urges us to scratch and inducing pain
How do receptors generate an electrical signal from a physical or chemical stimulus ?
• receptors have specific cation channels on the nerve endings membrane, which open in response to the stimulus and if intensity of stimulus is high enough to reach threshold for activating sodium channels, and lead to depolarisation and any depolarisation above the threshold will induce a train of action potentials. Each action potential causes release of neurotransmitters at the synapse with the second neuron of the afferent pathway
What happens when there is a longer and stronger stimulus
- A stimulus of greater duration and amplitude will induce a larger and longer generator potential.
- The increased generator potential is shown here as a larger depolarisation.
- The frequency of action potentials is proportional to the duration and intensity of the stimulus.
- Greater transmission along primary afferent.
- Increased release of neurotransmitters at the synapse with the second neuron of the afferent pathway.
what is adaptation
• Adaptation - whether there is a decrease in sensitivity in the presence of a constant stimulus
can be phasic or tonic
Explain phasic receptors
adapt quickly after detecting a change in stimulus strength
• Strongly when the stimulus begins but soon stop firing
• Only fire again when the stimulus turns off
• Referred as movement receptors / rate receptors and provide dynamic information about a stimulus
Explain tonic receptors
do not adapt or adapt very slowly during duration of the stimulus
• Detect continuous stimulus and continue to transmit impulses to the brain as long as the stimulus is present
• Keep brain constantly informed of the status of the body
• Provide information about static qualities of a stimuli
§ Eg. Merkel cells slowly adapt allowing for sustained pressure and fine touch to be perceived.
what is receptive field
region on the skin which causes activaion of a single sensory neuron when stimulated
how is receptive field being tested
using 2-point discrimination test to test minimum distance at which two points are perceived as separate
compare small receptive fields and large receptive fields
small receptive fields allow detection of finer detail and are important in precise perception while larger receptive fields allow detection over a wider area and present in body where precise perception is not important
What is saltatory conduction
propagation of action potential along myelinated axons from one node of Ranvier to the next node, increasing conduction velocity of action potential
What are the three types of primary afferents
A beta fibres are wide myelinated fibres and transmit fast signal of mechanical information to spinal cord
A delta fibres are medium myelinated fibres that transmit fast signal about pain and temperature
C-fibres are small unmyelinated afferents with slow conduction velocity about temperature, pain and itch information
What is the difference between somatosensory system and our special senses (sight, hearing ,smell)
• Somatosensory receptors are located throughout the body instead of densely distributed to a specific site (e.g. eye, ear, nose).
Describe the dorsal column pathway
• Carriesfine discriminative touchandvibrationsensations from the large diameter, fast conductingA-betafibres from the spinal cord to the somatosensory cortex.
1. There are some local connections with interneurons in the dorsal horn but the majority of theAβ fibresprimary afferent neurons enter the ascendingdorsal column pathway. 2. Primary afferent neurons make their first synapse with 2nd order neurons in themedulla. • 2nd order neuronsdecussate(cross the midline) in the caudal medulla and form the contralateralmedial lemniscuswhich then projects up through the brainstem to thethalamus. 3. These 2nd order axons then terminate in thethalamus. • Finally, there are 3rd order neurons from the thalamus which project to thesomatosensory cortexwhere the size of the somatotopic area is proportional to the density of sensory receptors in that body region which is also referred to as the somatosensory homunculus.
Describe spinothalamic pathway
• information from smaller diameterA-deltaandunmyelinated C-fibrescarrying information aboutpainandtemperature.
1. Primary afferent axons terminate upon entering thespinal cord, they synapse in thedorsal hornof the grey matter. 2. 2nd order neuronsdecussate immediatelyin the spinal cord and form the contralateralspinothalamic tract. The 2nd order neurons terminate in thethalamus 3. 3rd order neurons from thethalamusproject to thesomatosensory cortexwhere the size of the somatotopic area is proportional to the density of sensory receptors in that body region which is also referred to as the somatosensory homunculus.
What fibres are involved in transmitting pain signals
A-delta - medium diameter and myelinated
type 1 - respond to mechanical pain and mediate first acute response to sharp pinprick pain
type 2 - lower heat pain threshold and transmit first acute response to heat pain
only responsive to mechanical stimulation at very high intensities
C fibres - small diameter and unmyelinated
mediate dull, persistant or second pain
Explain the signalling when a pain signal is detected
- primary afferent neurons synapse in the dorsal horn, releasing glutamate which binds to post-synaptic glutamate receptors (i.e. AMPA and NMDA receptors).
* This causes a depolarisation of 2nd order projection neurons, which cross the midline and then enter ascending pathways to the brain, the spinothalamic tract (or antero-lateral system).- The first synapse in the pain pathway is open to lots of modulation by endogenous analgesic (pain-relieving) systems,
explain the pathway to achieve analgesia
- Myelinated A-beta fibres are activated.
- These signals flood the pathway and stimulate the substantia gelatinosa in the dorsal horn of the spinal cord.
- The gate swings closed.
- The C (small, nociceptive) fibre signals are stopped at the gate and the pain signals are not transmitted to the thalamus.
- Analgesia is achieved. (inability to feel pain)
function of surspinal modulation of pain
- activated in response to a noxious stimulus and release monoamine neurotransmitters – noradrenaline and serotonin
* regulate the overall amount of pain felt
* Can contribute to development of chronic pain
Describe the ascending pathway in sursprinal modulation of pain
- reaching the thalamus with nociceptive information, also reach the amygdala (and other parts of the limbic system), with the emotional component of pain
* the unpleasant feeling, anxiety and fear that accompanies a noxious stimulus, including autonomic activation associated with fight-or-flight responses
