Somatosensory System: Pathways, Receptors, and Pain Mechanisms

What are somatosensory receptors responsible for?

Detecting stimuli from the body surface and internal tissues.

What do proprioceptors detect?

Body position and movement.

What do mechanoreceptors respond to?

Mechanical forces like pressure, touch, and stretch.

What is the function of rapidly adapting receptors?

They respond to the onset and offset of a stimulus.

Examples of rapidly adapting mechanoreceptors.

Pacinian corpuscle

Meissner's corpuscle

Hair follicle receptor

What do slowly adapting receptors do?

They respond continuously to a stimulus.

Examples of slowly adapting receptors.

Free nerve endings

Merkel's disk

Ruffini’s ending

What do Thermoreceptors respond to?

Temperature changes.

utilize transient receptor potential (TRP) ion channels, (respond to temperature and some chemicals)

Warm receptors

free nerve endings

responding to 30-43°C

increased frequency with increasing temperature

Cold Receptors

Possible free nerve endings

respond to 20-35°C

increase frequency with decreasing temperatures

Responses to static temperature

Cold and warm receptor ranges overlap

cold receptors are activated at 45°C→ can cause paradoxical cold (painfully hot sensation that makes it feel cold)

What are nociceptors?

(Pain receptors) that detect tissue-damaging or potentially damaging stimuli.

Free nerve endings

What types of fibers transmit signals from nociceptors?

A-delta or C fibers.

What are the three classes of nociceptors?

Mechanical nociceptors- pinching

thermal nociceptors- respond to temperatures of 44°C

polymodal nociceptors- respond to mechanical, extreme cold/heat and chemicals

What is the role of the somatosensory cortex?

one column - one modality

topographically oriented - body regions

the area devoted to a body region is NOT proportional to its size, but to its sensitivity (ACUITY) 

ex: homunculus ; lips have largest representation due to high sensitivity

What is the dorsal column-medial lemniscal pathway (posterior column) responsible for?

Transmitting sensations of touch, pressure, and proprioception.

Where does decussation occur in the dorsal column-medial lemniscal pathway (posterior column)?

In the Medulla.

-1st and 2nd order synapse- in the dorsal column nuclei (gracile and cuneate nuclei)

-the 2nd order neuron ascend via the medial lemniscus

What sensations does the spinothalamic tract carry?

Pain and temperature sensations.

Where does decussation occur in the spinothalamic tract?

In the Spinal cord (level of entry)

-1st and 2nd order neuron synapse in the dorsal horn

What is fast pain and how is it transmitted?

Transmitted by A-delta fibers (thin, lightly myelinated)

FAST - speed/efficiency

character: sharp pricking sensation 

easily localized!

What is slow pain and how is it transmitted?

Transmitted by C fibers (thin, unmyelinated)

SLOW

character: dull, aching sensation

poorly localized (broader detection of pain)

What are some chemicals that activate nociceptors?

Potassium

histamine

prostaglandins

bradykinin

serotonin

What is the gate-control theory?

It suggests that large-diameter myelinated A-beta fibers can inhibit pain signals.

ex: rubbing a sore spot, Transcutaneous electrical simulation (TENS)

What is endogenous analgesia?

A process involving descending pathways from the brainstem:

periaqueductal grey matter projects raphe magnus and reticular formation

these activate inhibitory interneurons at the 1st and 2nd order synapse in the dorsal horn.

What is hyperalgesia?

An enhanced pain response often caused by cytokines associated with illness (e.g. fever, fatigue)

What is phantom limb pain?

Pain perceived in an amputated limb, believed to involve activation of remaining neurons.

associated with stump pain and cut nerve endings

What is endogenous opioids (e.g. enkephalin)

block release of substance P from 1st order neurons

act as competitive inhibitors at opioid receptors on 2nd order neurons.

Pain perception

sensation produced by tissue damaging or potentially tissue damaging stimuli

elicits:

Autonomic responses (e.g., increased blood pressure, sweating).


Emotional responses (e.g., fear, anxiety)

Reflexive withdrawal from the stimulus

Pain perception is influenced by past experiences.