sensory systems - pain pathways

nociceptors and their adequate stimuli

nociceptors, the receptors for pain, are free nerve endings found in every tissue of the body except the brain and spinal cord

three types of nociceptors

• (1) mechanical nociceptors: which respond to intense mechanical stimuli, such as a pinch or puncture

• (2) thermal nociceptors: which respond to extreme thermal stimuli such as temperatures above 45°C or below 10°C; and (3) polymodal nociceptors: which respond to a variety of stimuli, including intense mechanical stimuli, extreme thermal stimuli, and chemicals released from damaged tissue

transduction in a nociceptor

transduction in nociceptors involves cation channels that open in response to noxious stimuli

• Ex: TRPV1† - a member of the transient receptor potential (TRP) vanilloid subfamily

• TRPV1 channels are found in the membrane of a polymodal nociceptor (Figure 9.14).

• they are cation channels that open in response to extreme heat or to capsaicin—the ingredient that makes chili peppers painfully hot

• opening these cation channels allows Na+ and Ca2+ ions to enter the nociceptor, which causes a depolarizing receptor potential to form in the nociceptor’s membrane

• if threshold is reached, an action potential is generated in the axon of the sensory neuron.

fast and slow pain

There are two types of pain: fast and slow

• fast pain: sharp, pricking sensation that is well localized

  • ex: the pain felt from a needle puncture or a knife cut to the skin is fast pain; signals for fast pain are transmitted along A-delta (Aδ) fibers (small, myelinated axons with conduction velocities ranging from 12 to 30 m/sec);

  • sensory neurons associated with mechanical or thermal nociceptors have Aδ fibers and are the source of the signals for fast pain

• slow pain: by contrast, is a dull, aching sensation that is poorly localized.

  • ex: is the pain associated with a minor toothache. Signals for slow pain are transmitted along C fibers (small, unmyelinated axons that have conduction velocities ranging from 0.5 to 2 m/sec)

  • sensory neurons associated with polymodal nociceptors have C fibers and are the source of the signals for slow pain.

anterolateral pathway

conscious awareness of pain sensation

• this is an ascending pathway; ascending pathways to the brain that are activated by nociceptors allow pain information to be processed by higher centers

• this pathway conveys input from nociceptors to the cerebral cortex via relays in the spinal cord and thalamus. This pathway is responsible for conscious awareness of pain sensations

reticular formation

the pathway to the reticular formation increases your level of arousal in response to a painful stimulus

limbic system

the pathway to the limbic system causes the emotional responses (fear, anxiety, etc.) that may occur after a painful experience

hypothalamus

the pathway to the hypothalamus elicits the autonomic responses (increased heart rate, sweating, etc.) that may accompany a painful incident

pain pathways

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• activation of first-order neurons of a given pain pathway causes the release of neurotransmitters that influence the activity of nearby postsynaptic neurons

• the two main neurotransmitters released from first-order pain neurons are glutamate and the neuropeptide substance P

• once glutamate and substance P are released, they activate the next neurons of the pain pathway

spinal reflex pathway

spinal reflex pathways that are activated by nociceptors provide unconscious protective responses when a noxious stimulus begins to damage the body

• ex: stepping on a tack or touching a hot burner on the stove elicits the flexor reflex, which quickly withdraws the affected limb away from the painful stimulus

• muscles are focused on removing the limb + happens quickly

somatic + visceral + referred pain

• somatic pain: arises from stimulation of nociceptors in skin, skeletal muscles, and joints

  • superficial somatic: skin pain

  • deep somatic: arises in muscles, joints, tendons

• visceral pain: results from stimulation of nociceptors in visceral (internal) organs.

  • in many instances of visceral pain, the pain is felt at a site other than the place of origin

  • ex: the pain of a heart attack typically is felt in the skin over the heart and along the left arm

    • this phenomenon is called referred pain

• referred pain: occurs because both somatic sensory and visceral sensory neurons often converge on second-order neurons of the same ascending pathway to the brain

  • pain that is felt at a location that was not the origin of the pain

  • occurs bc the neurons converge on the same 2nd order neuron