Anatomy & Physiology

Anatomy & Physiology Study Guide General sensory receptors can be classified by the type of stimulus that excites them: • Three types of nociceptor found in the body provide information on pain as related to extremes of temperature, mechanical damage, and dissolved chemicals. Myelinated Type A fibers carry fast pain. Slower Type C fibers carry slow pain. • Thermoreceptors are found in the dermis. Mechanoreceptors are sensitive to distortion of their membranes and include tactile receptors, baroreceptors, and proprioceptors. There are six kinds of tactile receptors in the skin, and three kinds of proprioceptors. Chemoreceptors include carotid bodies and aortic bodies. Separate pathways carry somatic sensory and visceral sensory information: • Sensory neurons that deliver sensation to the CNS are referred to as first-order neurons. These synapse on second-order neurons in the brain stem or spinal cord. The next neuron in this chain is a third-order neuron, found in the thalamus. • Three major somatic sensory pathways carry sensory information from the skin and musculature of the body wall, head, neck, and limbs: the posterior column pathway, the spinothalamic pathway, and the spinocerebellar pathway. • The posterior column pathway carries fine touch, pressure, and proprioceptive sensations. The axons ascend within the fasciculus gracilis and fasciculus cuneatus and relay information to the thalamus via the medial lemniscus. Before the axons enter the medial lemniscus, they cross over to the opposite side of the brain stem. This crossing over is called decussation. • The spinothalamic pathway transports inadequately localized perceptions of temperature, touch, pressure, and pain. The axons intersect in the spinal cord and rise within the anterior and lateral spinothalamic tracts to the ventral nuclei of the thalamus. Abnormalities along the spinothalamic pathway can lead to referred pain, which are inaccurate localizations of the source of pain. • The spinocerebellar pathway, including the posterior and anterior spinocerebellar tracts, carries sensations to the cerebellum concerning the position of muscles, tendons, and joints. • Visceral sensory pathways carry information collected by interoceptors. Sensory information from cranial nerves V, VII, IX, and X is delivered to the solitary nucleus in the medulla oblongata. Dorsal roots of the spinal nerves T1–L2 transport visceral sensory data from organs between the diaphragm and the pelvic cavity. Dorsal roots of spinal nerves S2–S4 carry sensory information from more inferior structures. Olfaction, the sense of smell, involves olfactory receptors responding to chemical stimuli: • The olfactory organs contain the olfactory epithelium with olfactory receptors, supporting cells, and basal (stem) cells. The surfaces of the olfactory organs are coated with the secretions of the olfactory glands. • The olfactory receptors are highly modified neurons. • Olfactory reception involves detecting dissolved chemicals as they interact with odorant- binding proteins. • In olfaction, the arriving information reaches the information centers without first synapsing in the thalamus. Achieve Page 201 of 368 ©2018