![]() ![]() These fibers lead all the way to the thalamus. Here they synapse with second-order neurons, which decussate and form the contralateral ascending spinothalamic tract. In this pathway, first-order neurons end in the posterior horn of the spinal cord near the point of entry. Light touch is the sensation produced by stroking hairless skin with a feather or cotton wisp, without indenting the skin crude touch is touch whose location one can only vaguely identify. The spinothalamic tract carries signals for pain, temperature, pressure, tickle, itch, and light or crude touch. The spinothalamic tract and some smaller tracts form the anterolateral system, which passes up the anterior and lateral columns of the spinal cord. Because of decussation, the signals carried by the gracile and cuneate fasciculi ultimately go to the contralateral cerebral hemisphere. Third-order fibers go from the thalamus to the cerebral cortex. In the medulla, second-order fibers of the gracile and cuneate systems decussate and form the medial lemniscus, a tract of nerve fibers that leads the rest of the way up the brainstem to the thalamus. Its fibers end in the cuneate nucleus on the ipsilateral side of the medulla oblongata. It carries the same type of sensory signals, originating from T6 and up (from the upper limbs and chest). It occupies the lateral portion of the posterior column and forces the gracile fasciculus medially. The cuneate fasciculus joins the gracile fasciculus at the T6 level. Proprioception is the nonvisual sense of the position and movements of the body. These fibers carry signals for vibration, visceral pain, deep and discriminative touch (touch whose location one can precisely identify), and especially proprioception from the lower limbs and lower trunk. It consists of first-order nerve fibers that travel up the ipsilateral side of the spinal cord and terminate at the gracile nucleus in the medulla oblongata of the brainstem. At T6, it is joined by the cuneate fasciculus, discussed next. Below vertebra T6, it composes the entire posterior column. The gracile fasciculus carries signals from the midthoracic and lower parts of the body. The names of most of them consist of the prefix spino- followed by a root denoting the destination of its fibers in the brain, although this naming system does not apply to the first two. The major ascending tracts are as follows. If a generator potential reaches the threshold level of depolarization, it will trigger one or more nerve impulses in the sensory neuron. It responds to a specific stimulus-a change in the internal or external environment-by producing a graded potential called a generator (or receptor) potential. The distal end of a sensory neuron (dendrite) or an associated sensory structure serves as a sensory receptor. Within the sensory division of the peripheral nervous system (PNS), sensory inputs are differentiated as general (widespread) or special (localized, i.e., the special senses). Other brain regions provide important integration for regulation of automatic movements. The cerebral cortex, the outer part of the brain, plays a major role in controlling precise voluntary muscular movements. Responses to the integrative decisions are brought about by motor activities (muscular contractions and glandular secretions). The sensory information is integrated and processed by interneurons in the spinal cord and brain. The sensory systems keep the central nervous system (the brain and the spinal cord) informed of changes in the external and internal environments. ![]() This pathway informs the central nervous system (the brain and the spinal cord) of stimuli within and around the body. The sensory (afferent) nervous system carries signals from various receptors (sense organs and simple sensory nerve endings) to the central nervous system (CNS). A ganglion (plural, ganglia) is a knotlike swelling in a nerve where the cell bodies of peripheral neurons are concentrated. Nerves emerge from the central nervous system (brain and spinal cord) through foramina of the skull and vertebral column and carry signals to and from other organs of the body. A nerve is a bundle of nerve fibers (axons) wrapped in fibrous connective tissue. The peripheral nervous system (PNS) consists of all the rest it is composed of nerves and ganglia.The central nervous system is discussed further in the other posts: Human brain and Spinal cord. The central nervous system (CNS) consists of the brain and spinal cord, which are enclosed and protected by the cranium and vertebral column.The nervous system has two major anatomical subdivisions: The sensory nervous system is part of the peripheral nervous system. Sensory Receptors Structural Classification.Sensory Receptors Functional Classification.Peripheral Sensory System Organs and Receptors.Posterior and anterior spinocerebellar tracts. ![]()
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