Chapter 12 Somatic Sensory System PDF

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1 Chapter 12: The Somatic Sensory System Lecture Outline I. INTRODUCTION A. Input to the nervous system is provided by sensory receptors that detect such s timuli as touch, sound, light, pain, cold, and warmth. B. These receptors change sensory stimuli into nerve signals and the information conveyed in these signals is then processed by the nervous system. C. The components of the brain interact to receive sensory input , integration and  store the information , and transmit motor responses . II. SENSATION A. Sensation:  conscious or unconscious awareness of external or internal stimuli 1. Superficial Sensation: t ouch, temp, pain, pressure, etc from the surface of the body. AKA exteroceptive sensations 2. Deep Sensation: m  uscle, fascia, bone and joint pain, vibration, and proprioception (the detection of muscle and joint position, the detection of movement, and equilibrium→ 3 parts involved: the ear, cerebellum, and eyes) AKA interoceptive sensations B. Perception:  conscious awareness and interpretation of sensations. Sensation: the information that arrives from the senses. Perception: the conscious awareness of a sensation. C. Sensory Modality 1. The property by which one sensation is distinguished from another; there are two (2) classes: a. General Senses:  include both somatic  (touch, pressure, vibration, tickle, itch, warmth, cold, pain that arrives from receptors located in the skin and proprioception. Ex: a baroreceptor detects pressure found mainly in large muscular arteries that detect fluctuations in blood pressure-- carotid and aortic arch) and visceral  senses b. Special Senses:  include smell, taste, vision, hearing, and equilibrium. Receptors are located in sense organs such as the eyes and the ears. A sensory neuron only carries ONE modality. C. The Components of Sensation: to perceive a sensation, the following four (4) events MUST occur: 1. Stimulation of the Receptive Field  of a sensory neuron → any change in the environment that's capable of activating the receptive field of sensory neurons a. Receptive Field: a  rea within which a stimulus of appropriate quality and strength will cause a sensory neuron to initiate a receptor potential. 2. Transduction  (Conversion)  of the stimulus. A sensory receptor must receive the stimulus and transduce it into an electrical response. That response is a generator potential, which will lead to an action potential in that sensory neuron when threshold (-55 mV) is met. a. Generator Potential: g  raded potential (NOT an action potential). each type of sensory receptor can only transduce ONE type of stimulus. Ex: thermoreceptors→ only detect temp changes in the skin.

2 3. Impulse Generation  and Conduction T  he action potential is conducted along a sensory pathway from the PNS to the CNS a. First Order Neuron: a  sensory neuron that conducts impulses from a somatic receptor to the spinal cord and brainstem. b. Second Order Neuron: s ynapses with the first order neuron; conducts impulses from the spinal cord and medulla into the thalamus, where it synapses again. c. Third Order Neuron: s ynapses with the second order; from the thalamus to the somatosensory cortex in postcentral gyrus in the parietal lobe in the cerebrum. Where the conscious perception of the sensation is elicited. 4. Integration  of the sensory input. A region of the CNS must receive and integrate the information that’s carried by the AP into a sensation. This is the final pathway: when the CNS integrates the AP into a sensation. 5. Combined sensations : other sensations that are involved with identifying objects with your eyes closed; just by touch. → stereoagnosis. The ability to localize a cutaneous sensation: telling the location of something touching your arm → topognosis. The inability to judge shapes or forms of objects with the eyes closed → astereognosis D. Sensory Receptors 1. Classification of Sensory Receptors a. Microscopically: 1) Free Nerve Endings:  found everywhere in the skin and they detect pain (nociceptors), temperature, itch, tickle, touch, pressure, stretch. There are free nerve endings that function as thermoreceptors and nociceptors and mechanoreceptors → polymodality 2) Encapsulated Nerve Endings:  are the dendrites of first order neurons found in non-hairy parts of the skin. ex: lips, fingertips. These encapsulated nerve endings can detect well developed touch, pressure & vibration. 3) Separate Cells: synapse with first order neurons located in sense organs; hair cells in the ears, rods and cones in the retina, and gustatory receptor cells on taste buds.

b. According to Location: 1) Exteroceptors:  located at or near the body surface. Transmit information about the external environment. (hearing, vision, taste, smell, touch, pressure, vibration, temperature, pain) 2) Interoceptors o  r Visceroreceptors : l ocated in blood vessels and the viscera (walls of the organs) 3) Proprioceptors o  r Stretch Receptors:  located in muscles, tendons, joints, and t he internal ear and are stimulated by stretching or movement. Provide information about body position, muscle tension, joint position, and equilibrium.

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Type of Stimulus Detected: 1) Mechanoreceptors: detect mechanical deformation of the receptor or of the cells adjacent to the receptor. Some mechanoreceptors act as nociceptors. 2) Thermoreceptors: detect changes in temperatures, some warm, others cold. 3) Nociceptors o  r Pain Receptors: detect damage to the tissues (physical or chemical damage) nociceptors are free nerve endings located in the skin, joint surfaces. If temperature drops below 10 degrees celsius, and above 40 degrees Celsius, nociceptors are stimulated. Most nociceptors adapt very little or not at all. 4) Photoreceptors o  r Electromagnetic Receptors: detect light on the retina of the eye 5) Chemoreceptors: detect taste in the mouth, smell in the nose, O2 & CO2 levels in the arterial blood and body fluids. Detects osmolarity of body fluids. 2. Adaptation in Sensory Receptors a. When a continuous sensory stimulus is applied, the receptors respond at a very rapid impulse rate at first, then at a progressively diminishing rate until finally many of them no longer respond at all. Dec in AP rate during a sustained stimulus. Some chemoreceptors and pain receptors never completely adapt. b. Receptors may be rapidly or slowly adapting. Rapid adaptation → phasic. These receptors are normally inactive and become active for a short time to provide information about the intensity and rate of change of a stimulus. Slow adapting → tonic → always active, show little adaptation. Remind you of an injury long after the initial damage has occurred. SOMATIC SENSATIONS A. Tactile Sensations 1. Tactile Sensations a. Touch: s timulation of tactile receptors in the skin or in tissues immediately beneath the skin such as subcutaneous layer (hypodermis or subcutaneous fascia) b. Pressure: stimulation of tactile receptors from the deformation of deep tissues c. Vibration: stimulation of tactile receptors from rapidly repetitive sensory signals d. Itch and Tickle: stimulation of tactile receptors in the superficial layers of the skin by a chemical mediator called bradykinin often as the result of a local inflammation. **very first thing that happens in an inflammation is blood vessels constrict for a very short time then dilates.

4 2. Tactile Receptors: a. Free nerve Endings: f ound everywhere in the skin and detect touch and pressure. (Itch, tickle, cold, warmth, pain.) b. Meissner’s Corpuscles:  encapsulated nerve endings that elicit large myelinated sensory nerve fibers, they perceive sensations of touch→ fine touch ex: a feather. Small receptive fields and they are rapidly adapting. They adapt within one second after contact. They are found on dermal papillae on non-hairy parts of the skin - lips, fingertips (dermis goes into epidermis, strengthens the junction between the two layers). 40% of the sensory innervation to each hand is gotten through these Meissner’s Corpuscles. Very sensitive to weak stimuli and are myelinated, sensory axon terminals. They follow spiral paths through irregular lamella. These lamellae are made by Schwann Cells. c. Merkel’s Discs o  r Type I Cutaneous Mechanoreceptors:  flattened portions of dendrites of sensory neurons. They contact the bottom layer of the epidermis, stratum basale. Very small receptive fields and adapt very slowly. 25% of the sensory innervation in the hands is through Merkel’s Discs. They are densely populated in the lips, fingertips, genitalia, all skin and hair follicles. Detect continuous touch of objects (therefore, their slow adaptation). TEXTURE* d. Ruffini’s End Organs o  r Type II Cutaneous Mechanoreceptors: a  single encapsulated myelinated nerve ending with multi- branched or spraining axonal endings. Large receptive fields, slowly adapting. 20% of the sensory innervation to the hands. Found in the skin, ligaments, and tendons. They signal continuous defamation of deeper tissue,  such as heavy and continuous touch, heavy and continuous pressure. Found in your joint capsules and important in degree of joint rotation. Respond to stretch for ex: limb movement e. Lamellated o  r Pacinian Corpuscles:  lie in the dermis, hypodermis, and in deep fascial tissues; only stimulated by very rapid movements of the tissues; rapid adapting. Adapt within a few hundredths of a second. They are responsible for the remaining 15% of sensory innervation to the hands. They are composed of a myelinated afferent axon terminal surrounded by concentric lamellae. The concentric lamellae contains a lymph like fluid-- capillaries and collagen fibers surrounding that afferent myelinated axon terminal. 3. Touch: continued a. Crude Touch:  the ability to perceive that something has simply touched the skin. Doesn’t tell you size or texture. b. Fine or Discriminative Touch:  the ability to recognize the exact location, shape, size, and texture of the stimulus. c. Itch and Tickle: continued a. Itch and tickle receptors are free nerve endings. b. Tickle is the only sensation that you may not elicit on yourself but only when someone else touches you. c. Itch sensation alerts your attention to some stimulus on the surface of the skin. These signals then excite a scratch reflex or

5 other maneuvers to rid yourself of the irritation. If that scratch reflex is strong enough, it can elicit pain. Then pain overrides/suppresses the itch signals in the spinal cord. 4. Clinical Application: Phantom pain  is the sensation of pain in a limb that has been amputated; the brain interprets nerve impulses arising in the remaining proximal portions of the sensory nerves as coming from the nonexistent (phantom)  limb; the neurons in the brain that once received input from the missing limb are still active. B. Pain Sensations 1. Pain: a protective mechanism for the body whenever the tissues are being damaged; it causes the individual to react and remove the pain stimulus 2. Fast Pain: 0  .1 sec. a  fter the stimulus; acute, sharp, electric, or prickly, ex: cut with a knife, stuck with a needle 3. Slow Pain : 1 sec. or more and increases slowly over many seconds or minutes; chronic, burning, aching, or throbbing, numbness, tingling. Usually associated with tissue destruction. Can be excruciating and can lead to suffering. 4. Somatic Pain: superficial pain  arises from the stimulation of receptors in the skin; deep pain  that arises from skeletal muscles, joints, and tendons. All sensory information in response to pain enters the spinal cord at the posterior root of the spinal nerve through the anterolateral pathway and finally terminates in the thalamus. Receptor → posterior root of spinal nerve → spinal cord → anterolateral pathway → thalamus. 5. Visceral Pain: any stimulus that excites pain nerve endings in diffuse areas of the viscera. Your organs have sensory receptors for no other modality other than pain and pressure. Causes for visceral pain: ● Ischemia-- the byproducts of tissue degeneration stimulates nociceptors. ● Chemical-- leaking gastric ulcer. Leaking gastric juice from a ruptured ulcer stimulates nociceptors. ● Spasm: if there is a spasm in the wall of a hollow organ→ mechanical activity stimulates nociceptors. Overstretching of a hollow organ can stimulating nociceptors. 6. Referred Pain:  pain felt in a part of the body, usually the skin, which is considerably removed from the tissues causing the pain, usually the visceral organs. When visceral pain fibers are stimulated, pain signals are conducted through the same posterior horn neurons in the spinal cord that conduct pain signals from the skin. You actually feel as though these internal pain signals originate in the skin when really it originated from an organ. This makes diagnoses very precise. D. Proprioceptive Sensations 1. Proprioceptors: receptors located in skeletal muscles, tendons, ligaments, in CT coverings of bones and muscles, in and around joints, and in the internal ear. They transmit nerve impulses related to muscle tone, movement of body parts, body position, and equilibrium.

6 a. Proprioceptive or Kinesthetic o  r Static Position Sense:  conscious orientation of specific body parts with respect to each other. b. Kinesthesia: t he study of the perception of body movements. 2. Joint Kinesthetic Receptors  : present around synovial joints and respond to pressure. They are Pacinian corpuscles in the CT outside the joint capsule. They are modified to respond to acceleration and deceleration of the joints during movement 3. Muscle Spindle Apparatus :  specialized  muscle fiber cells with less striations that are in parallel with muscle fibers. Composed of intrafusal fibers that contract when stimulated by a motor neuron. They monitor skeletal muscle length and they will trigger a stretch reflex if that length gets abnormally long. a. stimulated by motor neurons in the anterior gray horn of the cord 4. Golgi Tendon Organs: m  yelinated receptors f ound at the junction of a tendon and muscle; netlike arrangement within the fascicles of a tendon. Protects the tendon from excessive tension. They monitor tension developed during muscle contractions.They will prevent damage by decreasing tension on the muscle. IV. SOMATIC SENSORY PATHWAYS A. Somatic sensory pathways relay information from somatic receptors to the primary somatosensory cortex of the postcentral gyrus of the parietal lobe area in the cerebral cortex. Most input from the somatic receptors crossing over to the other side of the brainstem through decussation 1. The pathways consist of first-order, second-order, and third-order neurons (*look at diagram*) 2. Axon collaterals of somatic sensory neurons simultaneously carry signals into the cerebellum and the reticular formation of the brain stem. Pathway: The posterior column-medial lemniscus pathway→ sorts of info according the nature of the stimulus and the region of the body involved. Determines whether you perceive a given sensation as touch, pressure, or vibration. Steps: 1. First order neuron 2. Spinal cord 3. Medulla ON THE SAME SIDE through the gracile and cuneate fasciculus (ascending tracts in the white matter) 4. Second order neuron 5. Gracile and cuneate NUCLEUS in the medulla 6. Decussation: crossing over 7. They ascend medial lemniscus (ribbon-like) 8. Goes to the thalamus on the opposite side (because it already crossed over) 9. Third order neuron 10. The primary somatosensory cortex of the postcentral gyrus of the parietal lobe When the neurons in the primary somatosensory cortex are stimulated, you become aware of specific sensations originating from a specific location. B. Posterior Column-Medial Lemniscus Pathway to the Cortex 1. The nerve impulses for conscious proprioception and most tactile sensations ascend to the cortex along a common pathway formed by three-neuron sets.

7 2. These neurons are a part of the posterior (dorsal) columns which consist of the gracile fasciculus and cuneate fasciculus. 3. Impulses conducted along this pathway are concerned with fine touch, proprioception, a  nd vibratory sensations . C. Anterolateral Pathways to the Cortex 1. The anterolateral  or spinothalamic pathways  carry mainly pain and temperature impulses. Poorly localized= crude touch, pressure, pain, temperature. An ascending pathway. Fibers cross over in the anterior white commissure of the spinal cord, turns upward, and then ascends to the brainstem in either the anterior spinothalamic tract of the lateral spinothalamic tract. Finally terminates in the postcentral gyrus somatosensory cortex in the parietal lobe. Made up of a 3 neuron set: the axons of first order neuron enter the spinal cord, they synapse with second order neurons in the posterior grey horn, they cross to the opposite side of the spinal cord and then begin their ascend with either the anterior or the lateral spinothalamic tract. The anterior tract: crude touch and pressure Lateral tract: pain and temperature 2. They also relay the sensations of tickle and itch and some tactile impulses. 3. Somatic sensory and motor maps in the cerebral cortex a. Specific areas of the cerebral cortex receive somatic sensory input from particular parts of the body and other areas of the cerebral cortex provide output instructions for movement of particular parts of the body. b. Precise location of somatic sensations occurs at the primary somatosensory area. c. The relative size of the regions in the somatosensory area is proportional to the number of specialized sensory receptors within the corresponding part of the tissue. D. Somatic Sensory Pathways to the Cerebellum 1. The posterior spinocerebellar  and the anterior spinocerebellar tracts  are the major routes whereby proprioceptive impulses reach the cerebellum. Ascending tracts. The posterior spinocerebellar tract contains second order neurons that DO NOT cross over to the opposite side of the spinal cord. Axons reach the cerebellar cortex through the inferior cerebellar peduncles. The anterior spinocerebellar tract is dominated by second order axons that HAVE crossed over to the opposite side of the spinal cord. However, there is still a significant number of UNCROSSED axons in the anterior tract. They reach the cerebellar cortex through the superior cerebellar peduncles. 2. Sensory impulses conveyed to the cerebellum are critical for posture, balance, and coordination of skilled movements. E. Clinical Application: Tertiary Syphilis  causes a progressive degeneration of the posterior portions of the spinal cord resulting in loss of somatic sensation and proprioception. 1. Causative Organism: Treponema Pallidum 2. Signs and Symptoms : uncoordinated gait, jerky body movements, paralysis, altered personality and judgement, and insanity.

8 V. SOMATIC MOTOR PATHWAYS A. Upper motor neurons extend from the cerebral cortex to anterior horn of the cord to the pons/medulla. The cell body of these neurons lie in the CNS and they synapse with lower motor neurons. The upper motor neurons (UMN) innervate a single motor unit in skeletal muscle. The activity of a UMN can facilitate or inhibit a LMN. B. Lower motor neurons (LMN) send their axons through spinal nerve roots and extend from the brainstem to the spinal cord to skeletal muscles. These lower motor neurons are called the final common pathway. LMN’s cell bodies lies in the motor nuclei, it would then trigger a contraction of an innervated muscle. Effector: Skeletal muscle. Axons of LMNs exit through the anterior root of the spinal nerve and terminate in skeletal muscles. C. Damage or destruction of a UMN eliminates voluntary and reflex control over that motor unit that it innervates → spastic paralysis. Spastic paralysis: the spinal motor neurons remain intact and the muscles of the opposite side of the body continue to be stimulated by spinal reflex activity. in an adult, positive babinski test (spread toes) D. Damage or destruction of LMN → flaccid paralysis, damage to the anterior horn. Nerve impulses do not reach the skeletal muscles on the same side of the body that are served by those neurons. You don’t get any voluntary or involuntary movement of the muscles. The muscle ...