somatic and proprioreceptive senses pacinian corpuscle
TRANSCRIPT
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Somatic and Proprioreceptive Senses
Pacinian corpusclehttp://www.science.mcmaster.ca
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Much of the text material is from, “Principles of Anatomy and Physiology, 14th edition” by Gerald J. Tortora and Bryan
Derrickson (2014). I don’t claim authorship. Other sources are noted when they are used.
Mappings of the lecture slides to the 12th and 13th editions are provided in the supplements.
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Outline
• Somatic sensations• Pain sensations• Proprioreceptive sensations• Somatic sensory pathways
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Somatic Sensations
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Somatic Sensations
• Somatic sensations result from the stimulation of sensory receptors in the:
- Epidermis, dermis, and subcutaneous layers of the skin—see the learning module on the integumentary system.
- Mucous membranes of body cavities open to the exterior, includ-ing the mouth, vagina, and anus.
- Skeletal muscles, tendons, and joints.
Chapter 16, page 550
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Somatic Sensations (continued)
• The four modalities of somatic sensations are tactile, thermal, pain, and proprioreception.
• Sensory receptors are unevenly distributed—some body areas are densely populated with receptors, while other areas have relatively few.
• The highest densities of somatic sensory receptors are found in the fingertips, lips, and tip of the tongue.
• Receptor densities are represented in the homunculus for the soma-tosensory projection area (discussed during the lecture on the brain).
Chapter 16, page 550
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Sensory Receptors in the Skin
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Tactile Sensations
• Tactile sensations include touch, pressure, vibration, itch, and tickle.
• Encapsulated mechanoreceptors with large-diameter, myelinated (type A) fibers mediate the sensations of touch, pressure, and vibra-tion.
• Free nerve endings with small-diameter, unmyelinated (type C) fibers mediate itch and tickle sensations.
• Type A fibers conduct action potentials to the central nervous system more rapidly than C fibers because they are myelinated and larger in diameter.
Chapter 16, page 550 Figure 16.2
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Touch Sensations
• Touch sensations result from the stimulation of tactile receptors in the skin and its subcutaneous layers.
• Meissner corpuscles and hair root plexuses are rapidly-adapting tac-tile receptors.
• Meissner corpuscles are especially sensitive at the onset of a touch.
• They are abundant in the fingertips, hands, eyelids, tip of the tongue, nipples, soles, clitoris, and penis.
• Hair root plexuses—which detect movement that disturbs hairs—are found in normally-hairy skin.
Chapter 16, page 550
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Meissner Corpuscle
Drawing and light micrograph
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Touch Sensations (continued)
• Merkel discs and Ruffini corpuscles are slowly-adapting tactile recep-tors.
• Merkel discs are sensitive to touch, and are densest in the fingertips, hands, lips, and external genitalia.
• Ruffini corpuscles are sensitive to stretching from the movement of the digits and limbs, and are most abundant in the hands and soles of the feet.
Digits = fingers including the thumb and toes.
Chapter 16, page 550
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Pressure Sensations
• Pressure is a sustained sensation usually felt over a larger surface area than touch.
• Pressure sensations occurs in response to the mechanical deformation of deep tissues.
• Pacinian corpuscles, Meissner corpuscles, and Merkel discs respond to mechanical pressure.
Deformation = a change from the normal size or shape of an anatomic structure due to mechanical forces that distort an
otherwise normal structure. (http://www.medterms.com)
Chapter 16, page 550
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Pressure Sensations (continued)
• Pacinian corpuscles adapt rapidly to mechanical pressure.
• They are widely distributed including in the:
- Dermis and subcutaneous layers of the skin- Submucosal membranes- Around joints, tendons, and muscles- Mammary glands- External genitalia- Some visceral organs and structures including the pancreas
and urinary bladder
Chapter 16, page 550
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Pacinian Corpuscle
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Drawing and light micrograph
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Vibration Sensations
• Sensations of vibration result from fast, repetitive sensory signals in tactile receptors.
• Meissner corpuscles respond to low-frequency vibrations and Paci-nian corpuscles respond to higher-frequency vibrations.
Chapter 16, page 550
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Itch Sensations
• Itch sensations result from the stimulation of free nerve endings by chemicals including bradykinin.
• The chemicals involved in itch sensations are also associated with inflammatory responses.
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Chapter 16, page 550
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Tickle Sensations
• Free nerve endings in the skin are thought to mediate tickle sensa-tions.
• Tickle sensations don’t occur with attempts at self-tickling, possibly because of the active role of the cerebellum and other motor areas.
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Chapter 16, page 550
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Thermal Sensations
• Thermoreceptors are free nerve endings with receptive fields that are about 1mm in diameter.
• Cold receptors (type A fibers) are activated between 10º C and 40º C (50º F - 105º F).
• Warm receptors (type C fibers) are activated between 32º C and 48º C (90º F - 118º F).
• Note the overlap between the two temperature ranges for the cold and warm receptors.
• Thermoreceptors are located near the skin surface, and warm receptors are not as abundant as cold receptors.
Chapter 16, page 551
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Thermal Sensations (continued)
• Cold and warm receptors rapidly adapt after the onset of a thermal stimulus.
• The receptors continue to generate action potentials in response to prolonged thermal stimuli, but at a lower rate.
• Temperatures below 10º C (50º F) and above 48º C (118º F) also stim-ulate the pain receptors.
Chapter 16, page 551
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Pain Sensations
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Survival Value
• Pain is essential for survival because it serves as an important signal that tissue-damaging conditions may be present.
• An individual’s personal or subjective description of pain can help in the medical diagnosis of a disease or injury.
Chapter 16, page 551
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Nocireceptors
• Nocireceptors—the receptors for pain—are free nerve endings in all tissues of the body except the brain.
• The receptors can be activated by intense thermal, mechanical, or chemical stimuli.
• Tissue irritation or injury results in the release of certain chemicals such as prostaglandins, kinins, and K+ ions that stimulate the noci-receptors.
Chapter 16, page 552 Figure 16.2
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Nocireceptors (continued)
• Pain may persist even after the stimulus is removed because: 1) pain-mediating chemicals linger, and 2) pain receptors have very little sensory adaptation.
• Other conditions that can elicit pain include distension (stretching) of organs, prolonged muscular contractions, muscle spasms, and ischemia.
Ischemia = inadequate blood supply to an organ or part of the body.
Chapter 16, page 552
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Fast Pain
• Pain is classified as either fast or slow.
• The sensation of fast pain often occurs with 0.1 seconds after the stimulus is applied since the action potentials propagate along fast, type B fibers (myelinated and mid-size diameter).
• Fast pain consists of acute, sharp, or prickling sensations, such as from a needle puncture or skin cut.
• Fast pain originates in superficial tissues, but not from deep tissues and organs.
Acute = of short duration, but typically severe.
Chapter 16, page 552
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Slow Pain
• The sensation of slow pain begins about 1.0 seconds or longer after the stimulus is applied.
• The sensation gradually increases in intensity over several seconds to minutes.
• Action potentials for slow pain propagate along slower, type C fibers (unmyelinated and small diameter).
Chapter 16, page 552
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Slow Pain (continued)
• Slow pain often originates in deep tissues, including all organs except the brain.
• Slow pain can also originate in the skin.
• The pain can consist of chronic, burning, aching, or throbbing sensa-tions, which can be excruciating.
Chronic = persisting for a long time.
Chapter 16, page 552
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Fast versus Slow Pain
• Fast and slow pain can be experienced simultaneously, although with different onsets.
• When a person stubs stubs her or his toe, the long conduction dis-tance to the brain separates the onset of the two types of pain (fast pain before slow pain).
Chapter 16, page 552
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Pain Localization
• Fast pain can be precisely localized to the stimulated area, such as that of a pin prick.
• Since slow pain is typically spread over a large area, it cannot be as readily localized—often it is experienced as a throbbing sensation.
Chapter 16, page 552
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Referred Pain
• Visceral slow pain (such as from the heart) can be experienced in or adjacent to an organ, or in a surface area some distance away.
• The phenomenon is known as referred pain.
• The organ and the area of referred pain are generally served by the same spinal nerves and segment of the spinal cord.
• Pain associated with agina or a heart attack is sometimes felt in the skin overlying the heart, and along the inferior surface of the left arm.
Chapter 16, page 552 Figure 16.3
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Pain-Relieving Drugs
• For acute pain, analgesic drugs such as aspirin and ibuprofen block the formation of prostaglandins that stimulate the nocireceptors.
• Local anesthetics such as Novacaine® may provide temporary pain relief by blocking action potentials along the axons of nocireceptors.
• Morphine and other opiates alter the quality of pain perception in the brain—the pain is still sensed, but it is no longer perceived as being as distressing.
• Antidepressant drugs are sometimes used to help treat chronic pain by reducing the emotional component, which can exacerbate the pain sensation.
Chapter 16, page 553
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Phantom Limb Sensation
• A person who has lost a limb may continue to experience itching, pressure, tingling, and pain sensations as if the limb still existed.
• This medical condition is known as phantom limb sensation.
Chapter 16, page 551
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Causes?
• The cerebral cortex might continue to interpret the action potentials from the proximal portions of the sensory neurons that had carried action potentials from the limb.
• Another possible explanation is that the brain’s networks of neurons that generate sensations of body awareness may remain active and give false body sensations.
• Yet another explanation involves dendritic reorganization in the pri-mary somatosensory cortex, as covered in the videotape, “Secrets of the Mind.”
Chapter 16, page 551
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Treatment
• Phantom limb sensations are often often reported as intense and pain-fully-distressing sensations.
• This pain is often not resolved by traditional pain medication therapies.
• Electrical nerve stimulation, acupuncture, and biofeedback sometimes are helpful.
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Proprioreception
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Proprioreceptors
• Proprioreceptors provide information to the brain about the location and movement (kinesthesia) of the head and limbs.
• In skeletal muscles and tendons, they provide information about the amount of contraction, tension on the tendons, and positions of the joints.
• Specialized hair cells in the inner ear sense the orientation and posi-tion of the head, as discussed in the information package for the aud-itory and vestibular system.
• The brain continually receives nerves impulses from proprioreceptors since they adapt slowly and very slightly.
Chapter 16, page 553
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Types of Proprioreceptors
• Muscle spindles within skeletal muscles
• Tendon organs within tendons
• Joint kinesthetic receptors within synovial joint capsules
• Specialized hair cells in the vestibular system within the inner ear (covered in another learning module)
Synovial = a joint surrounded by a thick, flexible membrane into which a viscous fluid is secreted to provide lubrication.
Chapter 16, page 553
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Muscle Spindles
• Muscle spindles monitor changes in skeletal muscles length in order to control stretch reflexes.
• The brain establishes muscle tone by adjusting how vigorously muscle spindles respond to the stretching of skeletal muscles.
• A muscle spindle has slowly-adapting sensory nerve endings wrapped around 4-to-10 intrafusal muscle fibers.
Muscle spindle = a stretch receptor found in vertebrate muscle.
Intrafusal muscle fibers = skeletal muscle fibers that make-up the muscle spindle, and is innervated by gamma motor
neurons.
Chapter 16, page 553 Figure 16.4
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Muscle Spindles (continued)
• Muscle spindles are interspersed among skeletal muscle fibers and are aligned parallel with them.
• They are densest in the skeletal muscles that control fine movements such as those of the hands.
• Fewer muscle spindles are found in the skeletal muscles involved in coarse movements such as the major muscle groups of the arms and legs.
• The tiny muscles of the inner ear are the only skeletal muscles that do not have muscle spindles.
Chapter 16, page 553 Figure 16.4
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Muscle Spindles (continued)
• A sudden and prolonged stretching of the intrafusal muscle fibers stimulates the sensory nerve endings of the muscle spindles.
• Action potentials propagate to the primary somatosensory area of the cerebral cortex to enable the conscious awareness of limb posi-tions and movements.
• Action potentials also propagate to the cerebellum to helps coordi-nate muscle contractions.
Chapter 16, page 553 Figure 16.4
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Muscle Spindles (continued)
Chapter 16, page 553 Figure 16.4
• Muscle spindles contain gamma motor neurons to adjust the tension of the muscle spindles based on variations in skeletal muscle length.
• When a muscle shortens, gamma motor neurons stimulate the intra-fusal fibers to contract slightly.
• Gamma motor neurons keep the intrafusal fibers taut to maintain the sensitivity of the muscle spindles to the stretching of the skeletal mus-cle.
Taut = pulled or drawn tight; under tension.
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Muscle Spindles (continued)
• The intrafusal fibers are surrounded by extrafusal skeletal muscle fibers.
• These fibers, supplied by alpha motor neurons, are active during stretch reflexes.
Chapter 16, page 553 Figure 16.4
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Tendon Organs
• Tendon organs are located at the junctions of tendons and skeletal muscles.
• They are involved in tendon reflexes to protect tendons and muscles from excessive tension.
• Tendon organs contain sensory nerve endings that are intertwined with the collagen fibers of the tendon.
Chapter 16, page 553 Figure 16.4
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Tendon Organs (continued)
• When external tension is applied to a skeletal muscle, tendon organs generate action potentials that propagate into the CNS.
• The resulting tendon reflex decreases muscle tension through muscle relaxation.
Chapter 16, page 554 Figure 16.4
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Joint Kinesthetic Receptors
• Several types of joint kinesthetic receptors are found at the synovial joints.
• They consist of free nerve endings and Ruffini capsules that respond to pressure.
• Pacinian corpuscles in the connective tissue respond to acceleration and deceleration of joints during movement.
• Ligaments contain receptors similar to tendon organs to prevent ex-cessive strain on a joint.
Chapter 16, page 554
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Somatic Sensory Pathways
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Sensory Pathways
• Somatic sensory pathways relay information from sensory receptors to the cerebellum and primary somatosensory area of the cerebral cortex via the thalamus.
• The sensory pathways have first-, second-, and third-order neurons.
• First-order neurons propagate action potentials from the somatic sen-sory receptors into the spinal cord or brainstem.
Chapter 16, page 555
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Sensory Pathways (continued)
• Second-order neurons propagate action potentials from the spinal cord or brainstem to the thalamus.
• The axons cross-over in the medulla oblongata before entering the thalamus.
• The higher brain centers receive somatosensory information from the contralateral (opposite) sides of the body.
• Third-order neurons propagate action potentials from the thalamus to the primary somatosensory area on the ipsilateral (same) side of the brain.
Chapter 16, page 556
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• Action potentials from somatic sensors ascend to the cerebral cortex via three pathways.
- Posterior column-medial lemniscus pathway (spinal cord)- Anterolateral or spinothalamic pathway (spinal cord)- Trigeminothalamic pathway (cranial nerve V)
• Sensory information reaches the cerebellum via the spinocerebellar tracts.
Figures 16.5, 16.6, and 16.7Chapter 16, page 556
Sensory Pathways (continued)
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Relay Stations
• Relay stations are collections of nuclei within the CNS where neurons synapse with other neurons as part of a sensory or motor pathway.
• The thalamus is the major relay station for many of the sensory path-ways.
Chapter 16, page 556
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Primary Somatosensory Area
• The input from somatic senses can be mapped to the primary soma-tosensory area of the cerebral cortex.
• The primary somatosensory area (Brodmann’s areas 1, 2, and 3) is located immediately posterior to the central fissure in the cerebral cortex.
Chapter 16, page 558 Figure 16.8
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Primary Somatosensory Area (continued)
• The somatic sensory map, known as a homunculus, represents the somatic sensations from the opposite side of the body.
• The external surfaces of the body that have the greatest densities of somatic sensory receptors, such as the hands and lips, are most well-represented in the homunculus.
Homunculus = a very small human-like object.
Chapter 16, page 559 Figure 16.8
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Primary Motor Area
• A somatic motor map, a second homunculus, can be depicted for the primary motor area of the motor cortex located anterior to the central fissure (Brodmann’s area 4).
• The two homunculi have similarities and differences, as shown on the slide.
Chapter 16, page 561 Figure 16.8
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Sensory and Motor Homunculi
http://brainmind.com
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Cerebellum
• The anterior and posterior spinocerebellar tracts provide propriorecep-tive information to the cerebellum.
• These tracts and the cerebellum are involved in posture, balance, and coordination of skilled movements.
• Cerebellar sensory input is not consciously perceived if it does not also involve projection to the cerebral cortex.
Chapter 16, page 565