Thermal Sense (Temperature Sensation)
Temperature for living organisms is essential to maintain health and homeostasis (physiological balance within the body). Animals, and particularly humans, have the ability to sense heat and cold from the environment around them, which allows them to respond in an appropriate manner. Furthermore, the brain helps maintain the animal’s core temperature so that the body can protect itself during extreme temperature situations. In animals, temperature and pain sensations travel together in the nervous system. Specifically, these sensations are mediated by the same fiber types within the peripheral nervous system, travel in the spinal cord within the same pathway, and are perceived within the somatosensory cortex (postcentral gyrus of the brain). Regulation of body temperature is mediated by the hypothalamus, a deep brain structure located below the thalamus.
Anatomy and Physiology
The body’s sensory systems deal with temperature information being delivered to the nervous system by neurons that have special receptors for temperature stimuli that are generally not too painful, not too hot, and not too cold. These receptors are called thermoreceptors, which are free nerve endings found throughout the skin, the cornea (the covering of the eye), and the bladder. There are two fiber types used to sense temperature: C-fibers and A delta fibers. C-fibers are usually unmyelinated, meaning they lack a covering of insulation (myelin, produced by Schwann cells) around their axons. Thus, nerve impulses (action potentials) do not travel as fast in these types of fibers. A delta fibers are lightly myelinated, meaning they have a covering of insulation around their axons, allowing action potentials to travel quicker. For warm temperatures between 32° and 48°C (or 90° and 118°F), C-fibers will increase their firing rate of action potentials in response to being warmed. For cool or cold temperatures between 10° and 30°C (or 50° and 86°F), both C-fibers and A delta fibers are activated. Specifically, the C-fibers reduce their firing of action potentials when cooled, while A delta fibers increase their firing rate of action potentials. Nociceptors (pain receptors) can act as thermoreceptors to determine pain from temperature, such as when a person burns his hand. Additionally, nociceptors are activated as thermoreceptors when temperatures are too cold and cause pain.
Temperature signals travel within the C-fibers and A delta fibers to the spinal cord where they synapse on to neurons located in the dorsal horn. These neurons then project some of their axons to the same side (ipsilateral) and the majority of their axons to the opposite side (contralateral) of the spinal cord where they join the anterolateral system, the second major ascending (afferent) pathway that ends in the somatosensory cortex. Because temperature and pain information travel on both sides of the spinal cord, it acts as a backup system so if part of the spinal cord is damaged, not all temperature and pain sensation is lost. For the head and face, temperature information travels through cranial nerve V (trigeminal nerve), synapses in the trigeminal nucleus in the brainstem, and then terminates in the somatosensory cortex.
A body’s core temperature is regulated by the hypothalamus in response to peripheral and central input. The set point for human body temperature ranges from 36.4° to 37.2°C (or 97.5° to 99°F) and is called normothermia. Temperature regulation works by a feedback system that detects changes between peripheral thermoreceptors in the skin, spinal cord, and viscera, and central thermoreceptors in the anterior section of the hypothalamus. The central thermoreceptors in the hypothalamus change their firing rate based on the temperature of the blood or their local temperature. The hypothalamus then integrates temperature information from the autonomic nervous system, endocrine system, and skeletal muscles to determine if the core temperature is too hot (hyperthermia) and sweating needs to begin to cool down the body, or if the core temperature is too cold (hypothermia) and shivering needs to begin to burn energy and make body heat.
Jennifer L. Hellier
See also: Free Nerve Endings; Nociception; Sensory Receptors; Somatosensory Cortex; Somatosensory System; Thermoreceptors
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