40 Questions
Which type of mechanoreceptors respond throughout the duration of the stimulus?
Merkel’s disks
What type of nociceptors are responsible for fast pain?
A-delta fibers
What is the function of thermoreceptors?
To detect changes in temperature
Which of the following receptors is responsible for signaling continuous touch of objects against the skin?
Merkel’s disks
What type of fibers are associated with free nerve endings?
Both A-delta fibers and C fibers
Which type of mechanoreceptors responds at the onset/offset of the stimulus?
Pacinian corpuscles
What is the function of A-delta mechanosensitive nociceptors?
To detect mechanical stimuli
Where are Ruffini’s corpuscles located?
All skin
Which pathway carries sensory information from the face, head, mouth, and nasal cavity?
Trigeminal Pathway
Where do the first-order axons in the Dorsal Column Pathway synapse with second-order neurons?
Medulla
What is the function of the Spinoreticular tract?
Mediates arousal and autonomic responses to pain
What is the name of the bundle of axons that decussate the midline of medulla and ascend the brainstem?
Medial lemniscus
What is the function of the Fibers in the dorsal columns?
Responsible for light touch and proprioception
Where do the second-order neurons in the Dorsal Column Pathway terminate?
Thalamus
What is the function of the Spinohypothalmic and spinotelencephalic tracts?
Evoke emotional behavior
What is the name of the two tracts that mediate sensation and proprioception?
Fasciculus gracilis and Fasciculus cuneatus
What type of stimuli is received by Lamina I of Rexed's laminae?
Noxious stimuli from cutaneous and deep somatic tissues via both A-delta and C fibers
What is the main site of action for opioids in the spinal cord?
Lamina II
What is the arrangement of fibers in the spinothalamic tract?
Fibers from the lower limb lie superficial, and fibers from the upper limb lie deep
Which type of pain is transmitted through the neospinothalamic tract?
Fast, mechanical pain
What is the function of the Tract of Lissauer?
To form the ascending and descending collaterals of pain fibers
What type of fibers terminate in Lamina V of Rexed's laminae?
WDR neurons and A-delta fibers
What is the function of the dorsal column pathway?
To transmit proprioception and mechanical information to the brain
What is the significance of the convergence of noxious and non-noxious stimuli in Lamina V?
It manifests clinically as referred pain
Where are the cell bodies of third-order neurons located?
Thalamic relay nuclei
What is the function of the periaqueductal gray matter?
Initiation of pain inhibition pathway
What neurotransmitter is released by the nucleus raphe magnus?
5-HT (serotonin)
What is the effect of enkephalin on nociception?
Inhibitory
Where do the axons of third-order neurons terminate?
Somatosensory cortex
What is the function of the nucleus raphe magnus?
Transmission of second-order signals
What is the effect of serotonin on nociception?
Inhibitory
Where do the descending pain pathway fibers synapse?
Rexed's lamina II
What is a major limitation of the Intensity Theory of Pain?
It fails to account for differences in pain perception among individuals.
Who first conceptualized the Intensity Theory of Pain?
Plato
According to the Pattern Theory of Pain, how are sensory impulses coded?
According to the number of receptors stimulated and the rate of their discharge
What is a key proposition of the Gate Control Theory of Pain?
That signals from primary afferents are transmitted to one of three regions in the spinal cord.
What is a major difference between the Specificity Theory of Pain and the Intensity Theory of Pain?
The Specificity Theory proposes that pain is a unique sensory experience.
What is a limitation of the Intensity Theory of Pain in relation to tactile perception?
It fails to account for the role of summation in pain perception.
Who postulated the Pattern Theory of Pain?
J.P.Nafe
What is a key feature of the Gate Control Theory of Pain?
It proposes that signals from primary afferents are transmitted to one of three regions in the spinal cord.
Study Notes
Mechanoreceptors
- Divided into two functional groups: rapidly adapting and slowly adapting
- Respond to stimulation differently:
- Rapidly adapting: respond at the onset and offset of the stimulus
- Slowly adapting: respond throughout the duration of the stimulus
- Four major types of mechanoreceptors:
- Meissner's corpuscles (rapidly adapting)
- Pacinian corpuscles (rapidly adapting)
- Merkel's disks (slowly adapting)
- Ruffini's corpuscles (slowly adapting)
Thermoreceptors
- Function to detect changes in temperature using two types of receptor cells:
- Warm receptors: sense temperatures between 30-45°C
- Cold receptors: sense temperatures between 17-27°C
- Poor indicators of absolute temperature
- Sense of temperature comes from the comparison of signals from the two types of receptors
Nociceptors
- Relatively unspecialized nerve cells with "free endings" that initiate the sensation of pain
- Conduction along the axons is relatively slow compared to that of the mechanoreceptors
- Split into two pathways:
- Fast pain: A-delta fibers
- Slow pain: C fibers
- Present in both somatic and visceral tissues
- Three major classes of nociceptors:
- A-delta mechanosensitive nociceptors
- A-delta mechanothermal nociceptors
- Polymodal nociceptors
Somatic Sensory Receptors
- Receptor type, location, function, and associated axon:
- Meissner's corpuscles: glabrous skin, touch and vibration, A-beta fibers
- Pacinian corpuscles: subcutaneous tissue, vibration, A-beta fibers
- Merkel's disks: all skin, continuous touch, A-beta fibers
- Ruffini's corpuscles: all skin, continuous heavy touch and pressure, A-beta fibers
- Thermoreceptors: skin, temperature, A-delta fibers (cold) and C fibers (warm)
- Free nerve endings: skin and organs, pain, itch, touch, pressure, A-delta fibers and C fibers
Spinal Cord Anatomy
- Gray matter:
- Dorsal horn (sensory)
- Ventral horn (motor)
- Lateral (autonomic)
- White matter
- Central canal
Rexed's Laminae
- Lamina I: receives noxious stimuli from cutaneous and deep somatic tissues via A-delta and C fibers, relays pain and temperature
- Lamina II: receives noxious stimuli from C fibers and A-delta fibers, relays pain, temperature, and mechanical information
- Lamina III/IV: receives input from A-beta and A-delta fibers, relays proprioception, mechanical, pain, and temperature sensations
- Lamina V: receives noxious and non-noxious, somatic and visceral stimuli from A-delta fibers and C fibers, relays pain and referred pain
Ascending Pathways
- Spinothalamic tract: carries pain and temperature information to the thalamus
- Dorsal column pathway: carries sensation and proprioception information to the thalamus
- Trigeminal pathway: carries sensory information from the face, head, mouth, and nasal cavity
Alternative Pain Pathways
- Spinoreticular tract: mediates arousal and autonomic responses to pain
- Spinomesencephalic tract: may activate antinociceptive pathways
- Spinohypothalamic and spinotelencephalic tracts: evoke emotional behavior
- Spinocervical tract: alternative pathway for pain
Third-Order Neurons
- Cell body located in the thalamic relay nuclei
- Axons synapse in the thalamus with second-order neurons
- Axons terminate in the somatosensory cortex
- Send fibers to areas of the postcentral gyrus of the parietal cortex and superior wall of the Sylvian fissure
Descending Pain Pathway
- Responsible for pain inhibition
- Descending neurons originate in the periventricular and periaqueductal gray
- Transmit through the nucleus raphe magnus to the substantia gelatinosa
- Releases both enkephalin and GABA
Key Inhibitory Neurotransmitters
- Enkephalin: released by the periaqueductal gray matter, inhibitory
- Serotonin: released by the nucleus raphe magnus, inhibitory
Theories of Pain
- Intensity Theory: pain is perceived only if the stimulus is sufficient in intensity and/or frequency
- Pattern Theory: sensory impulses are coded according to the number of receptors stimulated and the rate of their discharge
- Gate Control Theory: signals produced in primary afferents from stimulation of the skin are transmitted to one of three regions in the spinal cord, and the transmission of pain signals is controlled by the activation of inhibitory neurons.
Learn about the different types of mechanoreceptors, their functions, and how they respond to stimulation. Understand the characteristics of rapidly adapting and slowly adapting mechanoreceptors.
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