Somatosensory System and Pain Lecture Notes PDF

SOMATOSENSORY SYSTEM and PAIN Sensations and Receptors The somatosensory system processes information about; • Touch • Position Mechanoreceptors • Pain Nociceptors • Temperature Thermoreceptors Two pathways for of somatosensory information to the CNS The dorsal column system • Fine touch • Pres...

SOMATOSENSORY SYSTEM and PAIN Sensations and Receptors The somatosensory system processes information about; • Touch • Position Mechanoreceptors • Pain Nociceptors • Temperature Thermoreceptors Two pathways for of somatosensory information to the CNS The dorsal column system • Fine touch • Pressure • Two point discrimination • Vibration • Proprioception (limb position) • The dorsal column system consists mainly of group I and II nerve fibers. The anterolateral system (Spinothalamic pathway) • Pain • Temperature • Light touch • The anterolateral system consists mainly of group III and group IV fibers. Fourth-order neuron in the somatosensory cortex 1. The first-order neuron is the primary afferent neuron. Their axons synapse on somatosensory receptor cells. 2. The second-order neuron axons of anterolateral system (spinothalamic pathway) cross the midline in the spinal cord, Axons of dorsal colum system cross the midline in the brain stem and ascend to the contralateral thalamus (ıt has map). 3. The third-order neuron is located in the thalamus. The thalamus has a somatotopic arrangement of somatosensory information. This decussation means that somatosensory information from one side of the body is received in the contralateral thalamus. 4. The fourth-order neuron is located in the somatosensory cortex. • Somatosensory cortex and other associative cortical areas integrate complex information. • The somatosensory cortex has a somatotopic representation, or “map,” similar to that in the thalamus. • This map of the body is called the somatosensory homunculus. The face, hands, and fingers, are densely innervated by somatosensory nerves and where sensitivity is greatest. Mechanoreceptors An important characteristic of each receptor is the type of adaptation that it exhibits. • Very rapidly and rapidly adapting receptors changes in the stimulus and changes in velocity. • Slowly adapting receptors intensity and duration of the stimulus. Thermoreceptors • Slowly adapting receptors • Detect changes in skin temperature. • The two classes of thermoreceptors; Cold receptors and warm receptors • Each type of receptor functions over a broad range of temperatures, with some overlap in the moderate temperature range. • When the skin is warmed above 36°C, the cold receptors become quiescent, and when the skin is cooled below 36°C, the warm receptors become quiescent. The adequate stimulus for a warm receptor is warming, which results in an increase in their action potential discharge rate. What about at extremely warm or cold? • If skin temperature rises to damaging levels (above 45°C), warm receptors become inactive; • At temperatures above 45°C polymodal nociceptors will be activated. • Likewise, extremely cold (freezing) temperatures also activate nociceptors. Other Hot or cold sensation • Transduction of warm temperatures involves transient receptor potential (TRP) channels are activated capsaicin, an ingredient in spicy foods. (This phenomenon explains why people describe the taste of chili peppers as “hot.”) • Transduction of cold temperatures involves a different TRP channel, TRPM8, which is also opened by compounds like menthol (which gives a cold sensation). Nociceptors • It is activated by extreme stimulus of pressure, temperature, noxious or pain. 1. Thermal or mechanical nociceptors (TRPV or TRPM8 channels); are supplied by myelinated A-delta afferent nerve fibers and respond to mechanical stimuli such as sharp, pricking pain 2. Polymodal nociceptors; are supplied by unmyelinated C fibers and respond to high-intensity mechanical or chemical stimuli and hot and cold stimuli. Damaged skin releases a variety of chemicals which initiate the inflammatory response. The blood vessels become permeable, and, as a result, there is local edema and redness of the skin. Axons release substances that sensitize the nociceptors to stimuli. This sensitization process, called hyperalgesia, is the basis for reduced threshold for pain. Fast pain, Slow pain • Fast pain (e.g., pin prick) is carried on A delta, group II, and group III fibers, has a rapid onset and offset, and is precisely localized. • Slow pain (e.g., burn) is carried on C fibers and is characterized as aching, burning, or throbbing pain that is poorly localized. Referred pain • Referred pain is of visceral origin that is misperceived as pain arising from a somatic location. • The pain is “referred” according to the dermatomal rule, which states that sites on the skin are innervated by nerves arising from the same spinal cord segments as those innervating the visceral organs. • Thus according to the dermatomal rule, ischemic heart pain (angina) is referred to the chest and shoulder, • The gallbladder pain is referred to the abdomen, kidney pain is referred to the lower back, and so forth. Exam preparation • Negative and positive feedback examples… Example: Which of the following has a positive feedback mechanism? a) b) c) d) e) Extracellular or intracellular major ion concentrations…. What are the cellular transport types? Examples are important…. let’s study well • Action potential events….. • İon effects on AP • Junctional types and functions • Muscle contraction energy sources • Excitation-Contraction Coupling events • Muscle contraction and relaxation, as well as important enzymes involved • Which of the following is correct information about the effects of the autonomic nervous system on the body? let’s study well • Which of the following brain areas is directly involved in …. • Glial cells functions… • Which of the following information about ……. is incorrect? • Brain region functions… • Good luck☺)