The Somatosensory System _ Boundless Anatomy and Physiology.pdf

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Unformatted text preview: Boundless Anatomy and Physiology Search Peripheral Nervous System The Somatosensory System General Organization of the Somatosensory System The somatosensory system is composed of the neurons that make sensing touch, temperature, and position in space possible. LEARNING OBJECTIVES Describe how the somatosensory system is composed of neurons that make sensing touch, temperature, and position in space possible KEY TAKEAWAYS Key Points The somatosensory system consists of primary, secondary, and tertiary neurons. Sensory receptors housed in the dorsal root ganglia project to secondary neurons of the spinal cord that decussate and project to the thalamus or cerebellum. Tertiary neurons project to the postcentral gyrus of the parietal lobe, forming a sensory homunculus. A sensory homunculus maps sub-regions of the cortical postcentral gyrus to certain parts of the body. Key Terms decussate: Where nerve bers obliquely cross from one lateral part of the body to the other. postcentral gyrus: A prominent structure in the parietal lobe of the human brain and an important landmark that is the location of the primary somatosensory cortex, the main sensory receptive area for the sense of touch. organization: The quality of being constituted of parts, each having a special function, act, o ce, or relation; to systematize. thalamus: Either of two large, ovoid structures of gray matter within the forebrain that relay sensory impulses to the cerebral cortex. The somatosensory system is distributed throughout all major parts of our body. It is responsible for sensing touch, temperature, posture, limb position, and more. It includes both sensory receptor neurons in the periphery (eg., skin, muscle, and organs) and deeper neurons within the central nervous system. Structure A somatosensory pathway will typically consist of three neurons: primary, secondary, and tertiary. . In the periphery, the primary neuron is the sensory receptor that detects sensory stimuli like touch or temperature. The cell body of the primary neuron is housed in the dorsal root ganglion of a spinal nerve or, if sensation is in the head or neck, the ganglia of the trigeminal or cranial nerves. . The secondary neuron acts as a relay and is located in either the spinal cord or the brainstem. This neuron’s ascending axons will cross, or decussate, to the opposite side of the spinal cord or brainstem and travel up the spinal cord to the brain, where most will terminate in either the thalamus or the cerebellum. . Tertiary neurons have cell bodies in the thalamus and project to the postcentral gyrus of the parietal lobe, forming a sensory homunculus in the case of touch. Regarding posture, the tertiary neuron is located in the cerebellum. Processing The primary somatosensory area of the human cortex is located in the postcentral gyrus of the parietal lobe. The postcentral gyrus is the location of the primary somatosensory area, the area of the cortex dedicated to the processing of touch information. At this location there is a map of sensory space referred to as a sensory homunculus. A cortical homunculus is the brain’s physical representation of the human body; it is a neurological map of the anatomical divisions of the body. The surface area of cortex dedicated to a body part correlates with the amount of somatosensory input from that area. For example, there is a large area of cortex devoted to sensation in the hands, while the back requires a much smaller area. Somatosensory information involved with proprioception and posture is processed in the cerebellum. Homunculus: Image representing the cortical sensory homunculus. It shows how the anatomical portions of the body, such as the tongue, elbow, and hip, are mapped out on the homonculus. The surface area of cortex dedicated to a body part correlates with the amount of somatosensory input from that area. Functions The somatosensory system functions in the body’s periphery, spinal cord, and the brain. Periphery: Sensory receptors (i.e., thermoreceptors, mechanoreceptors, etc.) detect the various stimuli. Spinal cord: A erent pathways in the spinal cord serve to pass information from the periphery and the rest of the body to the brain. Brain: The postcentral gyrus contains Brodmann areas (BA) 3a, 3b, 1, and 2 that make up the somatosensory cortex. BA3a is involved with the sense of relative position of neighboring body parts and the amount of e ort being used during movement. BA3b is responsible for distributing somatosensory information to BA1 and shape and size information to BA2. Tactile Sensation Touch is sensed by mechanoreceptive neurons that respond to pressure in various ways. LEARNING OBJECTIVES Describe how touch is sensed by mechanoreceptive neurons responding to pressure KEY TAKEAWAYS Key Points Our sense of touch, or tactile sensation, is mediated by cutaneous mechanoreceptors located in our skin. There are four main types of cutaneous mechanoreceptors: Pacinian corpuscles, Meissner’s corpuscles, Merkel’s discs, and Ru ni endings. Cutaneous mechanoreceptors are categorized by morphology, by the type of sensation they perceive, and by the rate of adaptation. Furthermore, each has a di erent receptive eld. Key Terms receptive eld: The particular region of the sensory space (e.g., the body surface, space inside the ear) in which a stimulus will trigger the ring of that neuron. adaptation: A change over time in the responsiveness of the sensory system to a constant stimulus. Aβ ber: A type of sensory nerve ber that carries cold, pressure, and some pain signals. Aδ ber: Carries sensory information related to muscle spindle secondary endings, touch, and kinesthesia. A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. For instance, in the periodontal ligament, there are mechanoreceptors that allow the jaw to relax when biting down on hard objects; the mesencephalic nucleus is responsible for this re ex. In the skin, there are four main types in glabrous (hairless) skin: . Ru ni endings. . Meissner’s corpuscles. . Pacinian corpuscles. . Merkel’s discs. There are also mechanoreceptors in hairy skin. The hair cells in the cochlea are the most sensitive mechanoreceptors, transducing air pressure waves into nerve signals sent to the brain. Cutaneous Mechanoreceptors Cutaneous mechanoreceptors are located in the skin, like other cutaneous receptors. They provide the senses of touch, pressure, vibration, proprioception, and others. They are all innervated by Aβ bers, except the mechanoreceiving free nerve endings, which are innervated by Aδ bers. They can be categorized by morphology, by the type of sensation they perceive, and by the rate of adaptation. Furthermore, each has a di erent receptive eld: Ru ni’s end organs detect tension deep in the skin. Meissner’s corpuscles detect changes in texture (vibrations around 50 Hz) and adapt rapidly. Pacinian corpuscles detect rapid vibrations (about 200–300 Hz). Merkel’s discs detect sustained touch and pressure. Mechanoreceiving free nerve endings detect touch, pressure, and stretching. Hair follicle receptors are located in hair follicles and sense the position changes of hair strands. Ru ni Ending The Ru ni ending (Ru ni corpuscle or bulbous corpuscle) is a class of slowly adapting mechanoreceptors thought to exist only in the glabrous dermis and subcutaneous tissue of humans. It is named after Angelo Ru ni. This spindle-shaped receptor is sensitive to skin stretch, and contributes to the kinesthetic sense of and control of nger position and movement. It is believed to be useful for monitoring the slippage of objects along the surface of the skin, allowing the modulation of grip on an object. Ru ni endings are located in the deep layers of the skin. They register mechanical information within joints, more speci cally angle change, with a speci city of up to two degrees, as well as continuous pressure states. They also act as thermoreceptors that respond for a long time, such as holding hands with someone during a walk. In a case of a deep burn to the body, there will be no pain as these receptors will be burned o . Meissner’s Corpuscles Meissner’s corpuscles (or tactile corpuscles) are responsible for sensitivity to light touch. In particular, they have the highest sensitivity (lowest threshold) when sensing vibrations lower than 50 hertz. They are rapidly adaptive receptors. Pacinian Corpuscles Pacinian corpuscles (or lamellar corpuscles) are responsible for sensitivity to vibration and pressure. The vibrational role may be used to detect surface texture, e.g., rough versus smooth. Merkel Nerve Merkel nerve endings are mechanoreceptors found in the skin and mucosa of vertebrates that provide touch information to the brain. The information they provide are those regarding pressure and texture. Each ending consists of a Merkel cell in close apposition with an enlarged nerve terminal. This is sometimes referred to as a Merkel cell–neurite complex, or a Merkel disk receptor. A single a erent nerve ber branches to innervate up to 90 such endings. They are classi ed as slowly adapting type I mechanoreceptors. Proprioceptive Sensations Proprioception refers to the sense of knowing how one’s body is positioned in three-dimensional space. LEARNING OBJECTIVES Describe how propioception is the sense of the position of parts of our body in a three dimensional space KEY TAKEAWAYS Key Points Proprioception is the sense of the position of parts of our body and force being generated during movement. Proprioception relies on two, primary stretch receptors: Golgi tendon organs and muscle spindles. Muscle spindles are sensory receptors within the belly of a muscle that primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts. The Golgi organ (also called Golgi tendon organ, tendon organ, neurotendinous organ, or neurotendinous spindle) is a proprioceptive sensory receptor organ that is located at the insertion of skeletal muscle bers into the tendons of skeletal muscle. Key Terms alpha motor neuron: Large, multipolar lower motor neurons of the brainstem and spinal cord that are directly responsible for initiating muscle contraction. proprioreceptor: A sensory receptor that responds to position and movement and that receives internal bodily stimuli. posterior (dorsal) column-medial lemniscus pathway: A sensory pathway of the central nervous system that conveys localized sensations of ne touch, vibration, two-point discrimination, and proprioception from the skin and joints. Law of Righting: A re ex rather than a law, this refers to the sudden movement of the head to level the eyes with the horizon in the event of an accidental tilting or imbalance of the body. Golgi tendon organ: A proprioceptive sensory receptor organ that is located at the insertion of skeletal muscle bers into the tendons of skeletal muscle. muscle spindle: Sensory receptors within the belly of a muscle that primarily detect changes in the length of this muscle. proprioception: The sense of the position of parts of the body, relative to other neighboring parts of the body. Proprioception is the sense of the relative position of neighboring parts of the body and the strength of e ort being employed in movement. It is distinguished from exteroception, perception of the outside world, and interoception, perception of pain, hunger, and the movement of internal organs, etc. The initiation of proprioception is the activation of a proprioreceptor in the periphery. The proprioceptive sense is believed to be composed of information from sensory neurons located in the inner ear (motion and orientation) and in the stretch receptors located in the muscles and the joint-supporting ligaments (stance). Conscious proprioception is communicated by the posterior ( dorsal ) column–medial lemniscus pathway to the cerebrum. Unconscious proprioception is communicated primarily via the dorsal and ventral spinocerebellar tracts to the cerebellum. An unconscious reaction is seen in the human proprioceptive re ex, or Law of Righting. In the event that the body tilts in any direction, the person will cock their head back to level the eyes against the horizon. This is seen even in infants as soon as they gain control of their neck muscles. This control comes from the cerebellum, the part of the brain that a ects balance. Muscle spindles are sensory receptors within the belly of a muscle that primarily detect changes in the length of a muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles. Muscle spindle: Mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in spinal cord (middle), and expanded schematic (right). The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle bers. The sensory endings of a primary (group Ia) a erent and a secondary (group II) a erent coil around the non-contractile central portions of the intrafusal bers. The Golgi organ (also called Golgi tendon organ, tendon organ, neurotendinous organ or neurotendinous spindle) is a proprioceptive sensory receptor organ that is located at the insertion of skeletal muscle bers onto the tendons of skeletal muscle. It provides the sensory component of the Golgi tendon re ex. The Golgi organ should not be confused with the Golgi apparatus—an organelle in the eukaryotic cell —or the Golgi stain, which is a histologic stain for neuron cell bodies. Golgi tendon organ: The Golgi tendon organ contributes to the Golgi tendon re ex and provides proprioceptive information about joint position. The Golgi tendon re ex is a normal component of the re ex arc of the peripheral nervous system. In a Golgi tendon re ex, skeletal muscle contraction causes the agonist muscle to simultaneously lengthen and relax. This re ex is also called the inverse myotatic re ex, because it is the inverse of the stretch re ex. Although muscle tension is increasing during the contraction, alpha motor neurons in the spinal cord that supply the muscle are inhibited. However, antagonistic muscles are activated. Somatic Sensory Pathways The somatosensory pathway is composed of three neurons located in the dorsal root ganglion, the spinal cord, and the thalamus. LEARNING OBJECTIVES Describe the somatosensory area in the human cortex KEY TAKEAWAYS Key Points A somatosensory pathway will typically have three neurons: primary, secondary, and tertiary. The cell bodies of the three neurons in a typical somatosensory pathway are located in the dorsal root ganglion, the spinal cord, and the thalamus. A major target of somatosensory pathways is the postcentral gyrus in the parietal lobe of the cerebral cortex. A major somatosensory pathway is the dorsal column–medial lemniscal pathway. The postcentral gyrus is the location of the primary somatosensory area that takes the form of a map called the sensory homunculus. Key Terms parietal lobe: A part of the brain positioned superior to the occipital lobe and posterior to the frontal lobe that integrates sensory information from di erent modalities and is particularly important for determining spatial sense and navigation. reticular activating system: A set of connected nuclei in the brain responsible for regulating wakefulness and sleep-to- wake transitions. postcentral gyrus: A prominent structure in the parietal lobe of the human brain that is the location of the primary somatosensory cortex, the main sensory receptive area for the sense of touch. thalamus: Either of two large, ovoid structures of gray matter within the forebrain that relay sensory impulses to the cerebral cortex. A somatosensory pathway will typically have three long neurons: primary, secondary, and tertiary. The rst always has its cell body in the dorsal root ganglion of the spinal nerve. The second has its cell body either in the spinal cord or in the brainstem; this neuron’s ascending axons will cross to the opposite side either in the spinal cord or in the brainstem. The axons of many of these neurons terminate in the thalamus, and others terminate in the reticular activating system or the cerebellum. In the case of touch and certain types of pain, the third neuron has its cell body in the ventral posterior nucleus of the thalamus and ends in the postcentral gyrus of the parietal lobe. In the periphery, the somatosensory system detects various stimuli by sensory receptors, such as by mechanoreceptors for tactile sensation and nociceptors for pain sensation. The sensory information (touch, pain, temperature, etc.,) is then conveyed to the central nervous system by a erent neurons, of which there are a number of di erent types with varying size, structure, and properties. Dorsal root ganglion: Sensory nerves of a dorsal root ganglion are depicted entering the spinal cord. Generally, there is a correlation between the type of sensory modality detected and the type of a erent neuron involved. For example, slow, thin, unmyelinated neurons conduct pain whereas faster, thicker, myelinated neurons conduct casual touch. Ascending Pathways In the spinal cord, the somatosensory system includes ascending pathways from the body to the brain. One major target within the brain is the postcentral gyrus in the cerebral cortex. This is the target for neurons of the dorsal column–medial lemniscal pathway and the ventral spinothalamic pathway. Note that many ascending somatosensory pathways include synapses in either the thalamus or the reticular formation before they reach the cortex. Other ascending pathways, particularly those involved with control of posture, are projected to the cerebellum, including the ventral and dorsal spinocerebellar tracts. Another important target for a erent somatosensory neurons that enter the spinal cord are those neurons involved with local segmental re exes. dorsal root spinal nerve dorsal ramus white matter grey matter ventral ramus gray ramus white ramus ventral root Spinal nerve: The formation of the spinal nerve from the dorsal and ventral roots. Parietal Love: Primary Somatosensory Area The primary somatosensory area in the human cortex is located in the postcentral gyrus of the parietal lobe. This is the main sensory receptive area for the sense of touch. Like other sensory areas, there is a map of sensory space called a homunculus at this location. Areas of this part of the human brain map to certain areas of the body, dependent on the amount or importance of somatosensory input from that area. For example, there is a large area of cortex devoted to sensation in the hands, while the back has a much smaller area. Somatosensory information involved with proprioception and posture also target an entirely di erent part of the brain, the cerebellum. Cortical Homunculus This is a pictorial representation of the anatomical divisions of the primary motor cortex and the primary somatosensory cortex; namely, the portion of the human brain directly responsible for the movement and exchange of sensory and motor information of the body. Homunculus: Image representing the cortical sensory homunculus. Thalamus The thalamus is a midline symmetrical structure within the brain of vertebrates including humans; it is situated between the cerebral cortex and midbrain, and surrounds the third ventricle. Its function includes relaying sensory and motor signals to the cerebral cortex, along with the regulation of consciousness, sleep, and alertness. Intrathalmic adhesion n a i d e M a in m a L l a i d e M or i r e t An a Lamin VA ary l l u ed m l a rn Inte LD LP VL Pulvinar VI VPM VPL Medial...
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