Somatosensory System.doc

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Anatomy and Physiology
Jacqueline Carnegie

Somatosensory System: Sensory Receptors and Sensation -sensory receptors are specialized to respond to changes in their environment (stimuli) -activation of a sensory receptor by an adequate stimulus results in graded potentials that trigger nerve impulses along the afferent PNS fibres that lead to the CNS -sensation is the awareness of the stimulus and perception is the interpretation of the meaning of the stimulus -both occur in the brain -sensory receptors are classified by: -the type of stimulus they detect -their body location -their structural complexity Classification by Stimulus -the class name typically indicates the type of stimulus 1) Mechanoreceptors -respond to mechanical force such as touch, pressure (including blood pressure), vibration, and stretch 2) Thermoreceptors -sensitive to temperature changes 3) Chemoreceptors -respond to chemicals in solution -i.e.: molecules smelled or tasted, changes in blood or interstitial fluid chemistry, etc. 4) Photoreceptors -respond to light energy -i.e.: in the retina of the eye 5) Nociceptors -respond to potentially damaging stimuli that result in pain -i.e.: extreme heat / cold, excessive pressure, inflammatory chemicals, etc., which are all interpreted as painful -these signals stimulate subtypes of thermoreceptors, mechanoreceptors, and chemoreceptors Classification by Location 1) Exteroreceptors -sensitive to stimuli arising outside the body -mostly found near or at the body surface -i.e.: touch, pressure, pain, and temperature receptors in the skin -i.e.: receptors of the special senses (vision, hearing, equilibrium, taste, smell) 2) Interoreceptors -known as visceroreceptors -respond to stimuli within the body (i.e.: from the internal viscera and blood vessels) -monitor a variety of stimuli (i.e.: chemical changes, tissue stretch, temperature, etc.) -their activity causes feelings of pain, discomfort, hunger, thirst, etc. 3) Proprioceptors -respond to internal stimuli -found in skeletal muscles, tendons, joints, and ligaments in connective tissue coverings of bones and muscles -advise the brain of body movements by monitoring the organs containing the receptors and how much the organs are being stretched Classification by Structural Complexity -overall receptor structure can be classified as simple and complex receptors -the majority are simple -simple receptors are modified dendritic endings of sensory neurons -found throughout the body and monitor general sensory information -complex receptors are sense organs -involve localized collections of cells associated with the special senses (vision, hearing, equilibrium, smell, and taste) Simple Receptors of the General Senses -general sensory receptors are mostly involved in tactile sensation (mix of touch, pressure, stretch, and vibration), temperature monitoring, and pain, as well as the "muscle sense" provided by proprioceptors -the receptors are either unencapsulated (free) nerve endings or encapsulated nerve endings Unencapsulated Dendritic Endings -free / naked nerve endings are present almost everywhere in the body -particularly abundant in epithelia and connective tissues -respond mainly to temperature and painful stimuli -some respond to tissue movement caused by painful stimuli -heat or cold outside the range of thermoreceptors activates nociceptors and is perceived as painful -nociceptors respond to pinch and chemicals released from damaged tissue -the itch sensation is mediated by free nerve endings -histamine is present at inflamed sites and activate the nerve endings -some nerve endings associate with epidermal cells to form tactile discs that function as light touch receptors -hair follicle receptors are free nerve endings that wrap around hair follicles and detect the bending of hairs Encapsulated Dendritic Endings -consist of one or more fibre terminals of sensory neurons enclosed in a connective tissue capsule -are mostly mechanoreceptors -Meissner's corpuscles are numerous in sensitive and hairless skin areas like the nipples, fingertips, and soles of the feet; they also act as receptors for discriminative touch and play a role in light touch reception like hair follicle receptors -also known as tactile corpuscles -Pacinian corpuscles (lamellated corpuscles) are scattered deep in dermis -respond to deep pressure when it is first applied -best suited for monitoring vibration -Ruffini endings are fond in the dermis, subcutaneous tissue, and joint capsules -respond to deep and continuous pressure -muscle spindles are fusiform / spindle shaped proprioceptors found in the perimysium of skeletal muscle -each muscle spindle consists of intrafusal fibres (bundles of modified skeletal muscle fibres) -intrafusal fibres initiate a reflex that resists stretch -golgi tendon organs are proprioceptors located in tendons, close to the skeletal muscle insertion -consist tendon / collagen fibres -when tendon fibres are stretched by muscle contraction, the nerve endings are activated by compression; when activated, the contracting muscle is inhibited which causes the muscle to relax -joint kinesthetic receptors are proprioceptors that monitor stretch in the articular capsules that enclose synovial joints -made up of a combination of Pacinian corpuscles, Ruffini endings, free nerve endings, and receptors like Golgi tendon organs) -provide information on joint position and motion General Organization of the Somatosensory System -the somatosensory system receives inputs from exteroreceptors, proprioceptors, and interoreceptors -there are 3 main levels of neural integration that operate in the somatosensory system 1) Processing at the Receptor Level -a stimulus must excite a receptor and action potentials must reach the CNS -the stimulus energy must match the specificity of the receptor (i.e.: a touch receptor is responsive to pressure but not light) -the more complex the sensory receptor, the greater its specificity -the stimulus must be applied within a sensory receptor's receptive field -smaller field = greater ability for the brain to localize the stimulus site -the stimulus energy must be converted into the energy of a graded potential, known as a receptor potential -this process is known as transduction -membrane depolarizations that summate and directly lead to generation of action potentials in an afferent fibre are called generator potentials -a generator potential in the associated sensory neuron (a first-order neuron) must reach threshold so that voltage-gated sodium channels on the axon are opened and nerve impulses are generated and propagated to the CNS -many receptors exhibit adaptation, which is the change in sensitivity and nerve impulse generation in the presence of a constant stimulus -receptors responding to pressure, touch, and smell adapt quickly -receptors like Merkel's discks, Ruffini's corpuscles, and interoreceptors that respond to chemical levels in the blood respond slowly -pain receptors and proprioceptors do not exhibit adaptation 1) Phasic Receptors -fast adapting -act to report changes in the internal or external environment -e.g.: Pacinian and Meissner's corpuscles 2) Tonic Receptors -provide a sustained response with little to no adaptation -e.g.: nociceptors and proprioceptors (due to the protective importance of their information) 2) Processing at the Circuit Level -the 2nd level of integration -delivers impulses to the appropriate regions of the cerebral cortex for stimulus localization and perception -involves 1st, 2nd, and 3rd order neurons -axons of 1st-order sensory neurons link the receptor and circuit levels of processing -some branches take part in local spinal cord reflexes -other branches synapse with 2nd and 3rd-order neurons -impulses are sent along the dorsal column-medial lemniscal and spinothalamic ascending pathways -spinothalamic ascending pathways transmit pain, temperature, and coarse touch impulses (information is general, non-discriminatory, and involved in the emotional aspects of perception such as pleasure / pain) -dorsal column-medial lemniscal ascending pathway are involved in the discriminative aspects of touch, vibration, pressure, and conscious proprioceptrion (i.e.: limb and joint position) -proprioceptive impulses conducted by spinocerebellar tracts end at the cerebellum and the information is used to co-ordinate skeletal muscle activity 3) Processing at the Perceptual Level -interpretation of sensory input occurs in the cerebral cortex -the ability to identify and appreciate sensation depends on the specific location of target neurons in the sensory cortex -the thalamus projects fibres to the somatosensory cortex and sensory association areas -the brain always interprets activity of a specific sensory receptor as a specific sensation, no matter how it is activated -the main aspects of sensory perception are: a) Perceptual Detection -the ability to detect that a stimulus has occurred -the simplest level of perception -inputs from several receptors must be summed for perceptual detection to occur b) Magnitude Estimation -the ability to detect how intense the stimulus is -perception increases as stimulus intensity increases c) Spatial Discrimination -allows for identification of the site or pattern of stimulation d) Feature Abstraction -the mechanism by which a neuron is tuned to one feature in preference to others -enables us to identify more complex aspects of sensation such as specific texture or shape e) Quality Discrimination -the ability to differentiate the submodalities of a sensation (i.e.: sweet, sour, bitter etc. for taste sensation) f) Pattern Recognition -the ability to recognize patterns in stimuli (i.e.: a melody, a familiar face, etc.) Perception of Pain -pain producing chemicals include histamine, K , ATP, acids, and bradykinin -enkephalins are inhibitory neurotransmitters that stop the pain signals generated by nociceptive neurons -involves the neurotransmitters glutamate and substance P, which activate 2nd-order sensory neurons, whose axons ascend to the brain by the spinothalamic tract and other anterolateral pathways Regeneration of Nerve Fibres -damage to nerve tissue is serious because mature neurons do not divide -if damage is sever or close to the cell body (soma), the entire neuron may die and other neurons that are stimulated by it may die as well -if the cell body remains intact, cut or compressed axons of peripheral nerves can regenerate 1) separated ends are sealed off -the axon and the myelin sheath distal to the injury site begin to disintegrate 2) phagocytes / macrophages degrades the axon and removes debris 3) surviving Schwann cells proliferate and release growth factors and express cell adhesion molecules that encourage axonal growth -they form a regeneration tube, which guides the regeneration axon across the gap to their original contacts 4) axons regenerate at 1.5 mm a day -the greater the distance between severed endings, the less the chance of recovery because adjacent tissues block growth White Matter -white matter of spinal cord composed of myelinated and unmyelinated nerve fibres that allow communication between different parts of the spinal cord and between the spinal cord and the brain -white matter nerve fibres run in 3 directions: 1) ascending up to higher centres (i.e.: sensory inputs) 2) descending down to the cord from the brain or within the cord to lower levels (i.e.: motor outputs) 3) transversely across from one side of the cord to the other (i.e.: commisural fibres) -ascending and descending fibres make up most of the white matter -white matter on each side of the cord is divided into 2 white columns or funiculi -named according to their position -dorsal (posterior) funiculi -lateral funiculi -ventral (anterior) funiculi -each funiculus contains several fibre tracts -each tract is made up of axons with similar destinations and functions -the name of the spinal tracts reveal their origin and destination -all major spinal tracts are part of multineuron pathways that connect the brain to the body periphery -ascending and descending tracts contain spinal cord neurons as well as peripheral neurons and neurons in the brain -some generalizations about tracts and the pathways they contribute to: 1) Decussation -most pathways cross from one side of the CNS to the other (decussate) at some point in their journey 2) Relay -most pathways consist of a chain of 2 or 3 neurons (a relay) that contribute to successive tracts of the pathway 3) Somatotopy -a precise spatial relationship among the tract fibres that reflects the orderly mapping of the body -e.g.: in an ascending sensory tract, fibres transmitting inputs from sensory receptors in the superior parts of the body lie lateral to those conveying information from inferior body regions 4) Symmetry -all pathways and tracts are paired symmetrically (right and left), with a member of the pair present on each side of the spinal cord or brain Ascending Pathways to the Brain -conducts sensory impulses upward through 3 successive chains of neurons to the brain -2nd order and 3rd order neurons are interneurons 1) First Order Neurons -cell bodies reside in ganglion (dorsal root or cranial) -conduct impulses from cutaneous receptors of the skin and from proprioceptors to the spinal cord or brain stem -synapse with 2nd-order neurons -impulses from the facial area are transmitted by cranialnerves -spinal nerves conduct somatic sensory impulses from the rest of the body to the CNS 2) Second Order Neurons -cell bodies reside dorsal horn of the spinal cord or the medullary nuclei -transmit impulses to the thalamus or to the cerebellum
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