Sensory Systems and Physiology of Pain Flashcards
Give examples of sensory receptors
Mechanoreceptors Chemoreceptors Thermoreceptors Nociceptors Proprioceptors
What is the name for the area to which a specific sensory receptor responds?
Receptive field
What does the term adequate stimulus refer to in relation to sensory receptors?
The type of stimulation to which they respond e.g. mechanoreceptors - touch
What initials happens to all adequate stimulus in a sensory receptor?
It is transduced into a graded potential called the receptor/generator potential
What information is carried in the APs from sensory receptors?
Information on the modality, intensity and location of the stimulus
Describe the relationship between the receptive field and sensitivity and acuity
The larger the receptive field, the less acuity that sensory receptor will be capable of, but it is no less sensitive
What is carried in A Beta fibres? Are they myelinated?
Touch, pressure, vibration
Large unmyelinated
What is carried in A Delta fibres? Are they myelinated?
Cold, fast pain
Myelinated
What is carried by C fibres? Are they myelinated?
Warmth, slow pain
Unmyelinated
What two types of afferent fibres mediate proprioception?
A Alpha
A Beta
What tract is used by the A alpha and A beta fibres of touch, pressure and vibration?
Dorsal column
Synapse in the cuneate & gracile nuclei
Decussate in the brainstem
What tract is used by the A delta and C fibres? What does it carry?
Spinothalamic tract - temperature, all pain
Decussate in dorsal horn
What is the result of damage to the dorsal column?
Loss of touch, vibration, proprioception below the lesion on ipsilateral side
What is the result of damage to the anterolateral quadrant?
Loss of nociceptive and temperature sensation below the lesion on the contralateral side
Where does sensory information ultimately terminate?
The somatosensory cortex (S1) of the post central gyrus
What does the arrangement of sensory information arriving in the somatosensory cortex produce?
The sensory homunculus
Describe the process of adaptation in sensory pathways
This is a process whereby sensory receptors are able to mediate their response to constant stimulation. If stimulus intensity is sustained over lengthened periods of time, the sensory receptors can adapt, preventing the continuous firing of unnecessary APs. Receptors can be either fast reacting or slow reacting e.g. the hat response
Describe the process of convergence in sensory pathways
This refers to the process whereby signals form multiple sensory receptors can converge at a synapse and continue towards the brain as a combined signal input
What are some benefits and downsides to convergence in sensory pathways?
The advantages are that it saves on neurons, and makes the transmission of sensory information more efficient in terms of the number of occupied neurons. However, the downsides are that it results in a reduction in acuity, and is the underlying principle of referred sensation
What are the two forms of convergent pathway in the sensory channels? Describe each
Specific ascending pathways - here receptor neurons synapse together with other neurons of the same adequate stimulus, and continue from the synapse in the spinal cord with information form multiple inputs but that are from the same type of stimulus. These pathways remain distinct through the second synapse in the midbrain and reach the cortex still as separate pathways
Nonspecific ascending pathways - here receptor neurons of different adequate stimuli synapse together, creating a post-synaptic pathway which contains information of more than one kind of sensory input e.g. touch and temperature merging together
Describe the process of lateral inhibition in sensory pathways
This refers to the principle that when one sensory fibre is activated, it causes synaptic inhibition of its neighbours, which are activated to a lesser degree by the original stimulus, in order to give better definition of borders, improving acuity and localisation of the input, and cleaning up sensory information
Give some of the different ways in which pain can be described
Sharp Stabbing Diffuse Throbbing Crushing
What are the two major forms of pain?
Acute
Chronic
Describe the analgesic action of NSAID drugs
NSAIDs inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins. Prostaglandins acting on prostanoid receptors on the surface of nociceptors increase the nociceptor sensitivity to braykinin acting on Beta-bradykinin receptors. This increased sensitivity causes stimulation of ion channels which allow entry to the nociceptor to the toxins that generate the depolarisation and subsequent APs - “pain” signals. Inhibition of the cyclo-oxygenase and prostaglandins therefore reduces the pain signals = analgesia
What is the (‘analgesic’) action of local anaesthetics?
They block sodium action potentials and therefore all axonal transmission - so are not strictly analgesic
Describe the principle of segmental controls with regard to analgesia
This is the principle that stimulation of other sensory fibres such as A alpha or beta fibres activates inhibitory interneurons which act on and inhibit, via an opiate-like mechanism, the A delta / C fibres carrying the ‘pain’ signals
Outline the theory of gate control and pain
The gate control theory of pain asserts that non-painful input closes the “gates” to painful input, which prevents pain sensation from traveling to the central nervous system. Therefore, stimulation by non-noxious input is able to suppress pain. The key contributor is the Substantia Gelatinosa, which essentials exerts an inhibitory effect on the nociceptive inputs to the ‘gate’. Mechanoreceptors stimulate the SG to inhibit = segmental control, descending inhibitory pathways inhibit directly as well as stimulating the SG to inhibit
Describe the descending pathways involved in gate control of pain
Descending pathways from the peri-aqueductal grey matter (PAG) and nucleus raphe magnus use inhibitory neurons to directly inhibit the nociceptive neurons as well as stimulating the SG to further inhibit - closing the gate
Describe the analgesic actions of opiates e.g. morphine
They reduce the sensitivity of the nociceptors, blocking transmission in the dorsal horn (hence epidural administration) and activating the descending inhibitory pathways = closing the gate
