Pain pathway

Stimulation of a nociceptor in the periphery of the body elicits action potentials in the first-order neuron, which transmits the signal to the second-order neuron in the dorsal horn of the spinal cord. From the spinal cord, the signal is transmitted to several regions of the brain. The most prominent ascending nociceptive pathway is the spinothalamic tract. Axons of the second-order sensory neurons project to the contralateral (opposite) side of the spinal cord and ascend in the white matter, terminating in the thalamus (see Figure 8.1). The thalamus contributes to the basic sensation or awareness of pain only; it cannot determine the source of the painful stimulus.

Signals are also transmitted to the reticular formation of the brainstem by way of the spinoreticular tract. The reticular formation plays an important role in the response to pain. First, it facilitates avoidance reflexes at all levels of the spinal cord and, second, it is responsible for the significant arousal effects of pain. Signals from the reticular formation cause an increase in the electrical activity of the cerebral cortex associated with increased alertness. Furthermore, it sends nerve impulses to the hypothalamus to influence its functions associated with sudden alertness, such as increased heart rate and

Somatosensory Cortex

(localization and perception of intensity of pain)

Nociceptive stimulus

(localization and perception of intensity of pain)

Nociceptive stimulus

^^ (behavioral and emotional ^ responses)

C fibers (slow) (1st ORDER)

Figure 8.1 The pain pathway. The pain signal is transmitted to several regions of the brain, including the thalamus; reticular formation; hypothalamus; limbic system; and somatosensory cortex. Each region carries out a specific aspect of the response to pain.

C fibers (slow) (1st ORDER)

^^ (behavioral and emotional ^ responses)

Figure 8.1 The pain pathway. The pain signal is transmitted to several regions of the brain, including the thalamus; reticular formation; hypothalamus; limbic system; and somatosensory cortex. Each region carries out a specific aspect of the response to pain.

blood pressure. These responses are mediated by the sympathetic nervous system.

Nerve signals from the thalamus and the reticular formation are transmitted to the limbic system as well as the hypothalamus. Together, these regions of the brain are responsible for behavioral and emotional responses to pain. The limbic system, in particular, may be involved with the mood-altering and attention-narrowing effect of pain.

The cell bodies of third-order sensory neurons are located in the thalamus. These neurons transmit the pain signal to the somatosensory cortex. The function of this region of the brain is to localize and perceive the intensity of the painful stimulus. Further transmission of the signal to the association areas of the cerebral cortex is important for the perception and meaningfulness of the painful stimulus.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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