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1H06 Anatomy & Physiology: Spinal Cord

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McMaster University
Health Sciences
Alexander Ball

CNS – SC Pathways Protective Structures 1. First layer – skull and vertebral column for CNS 2. Meninges – lie between bony encasement and nervous tissue in brain and spinal cord 3. Cerebrospinal fluid – suspends central nervous tissue and surrounds with shock-absorbing, hydraulic cushion Vertebral Column: spinal cord is located within vertebral canal of vertebral column Meninges  Three protective, connective tissue coverings encircling spinal cord and brain  From top to bottom: o Dura mater o Arachnoid mater o Pia mater  Spinal meninges surround spinal cord and continuous with cranial meninges (encircle the brain)  Spinal cord also protected with fat and connective tissue in epidural space 1. Dura Mater a. Thick and strong, made of dense irregular connective tissue b. Forms sac on foramen magnum in occipital bone, and continuous with meningeal dura mater of brain to second sacral vertebra 2. Arachnoid mater a. Thin covering made of collagen and elastic fibers b. Is spiderweb arrangement of delicate collagen and elastic fibers c. Deep to dura mater, continuous through foramen magnum with arachnoid mater of brain d. Between dura and arachnoid mater is subdural space, containing interstitial fluid 3. Pia mater a. Thin transparent connective, adheres to surface of spinal cord and brain b. Consists of thin squamous to cuboidal cells within bundles of collagen and elastic fibers c. Blood vessels supply oxygen and nutrients to spinal cord d. Denticulate ligaments (thickenings of pia mater) fuse with arachnoid mater and dura mater between anterior and posterior nerve roots of spinal nerves on each side Spinal Cord External Anatomy  Oval in shape, flattened slightly anteriorly and posteriorly th st  Cervical enlargement (superior) extends from 4 cervical vertebra to 1 thoracic vertebra  Lumbar enlargements (inferior) extends from 9 to 12 thoracic vertebra  Filum terminale – extension of pia mater that fuses with arachnoid and dura mater, anchoring spinal cord to coccyx  Spinal nerves – communication between SC and specific regions of body  Spinal cord is segmented because of 31 pairs of spinal nerves emerging from intervertebral foramina  8 pairs of cervical nerves  12 pairs of thoracic nerves  5 pairs of lumbar nerves  5 pairs of sacral nerves  1 pair of coccygeal nerves  Posterior root contains only sensory axons, which conduct impulses from sensory receptors in skin, muscles, and internal organs to CNS  Posterior (dorsal) root ganglion contains cell bodies of sensory neurons  Anterior (ventral) root and rootlets contain axons of motor neurons (conduct impulses from CNS to effectors) Spinal Cord Internal Anatomy  White matter consists of bundles of myelinated axons of neurons surrounding gray matter o Matter divided into three columns: anterior (ventral), posterior (dorsal), and lateral o Each column contains axon bundles with similar information o Sensory (ascending) tracts consist of axons that conduct impulses toward brain o Motor (descending) tracts consist of axons that conduct impulses from brain  Gray matter shaped like letter H or butterfly, consisting of dendrites and cell bodies of neurons, unmyelinated axons, and neuroglia o Culsters of neuronal cell bodies form nuclei o Sensory nuclei receive input from receptors of sensory neurons o Motor nuclei provide output to effectors via motor neurons  Matter subdivided into horns o Posterior gray horns contain cell bodies and axons of interneurons, and axons of incoming sensory neurons o Anterior gray horns contain somatic motor nuclei (clusters of cell bodies of somatic motor neurons that provide impulses for contraction of skeletal muscles) o In between are lateral gray horns  Anterior median fissure – wide groove on anterior side  Posterior median sulcus – narrow furrow on posterior side Spinal Cord Physiology (function) 1. Nerve impulse propagation 2. Integration of information  White matter tracts in spinal cord are highways for impulse propagation  Sensory input travels along tracts to brain, motor output travels from brain along tracts toward skeletal muscles and other effectors  Gray matter of spinal cord receives and integrates incoming and outgoing information  Often name of tract indicates position in white matter and where it begins and ends o Ex. Anterior corticospinal tract is located in anterior white column, beings in cerebral cortex (superficial gray matter of cerebrum of brain) and ends in spinal cord.  Nerve impulses from sensory receptors propagate up spinal cord to brain along two main routes on each side: spinothalamic tract and posterior column o Spinothalamic: pain, warmth, coolness, itching, tickling, deep pressure o Posterior column consists of gracile fasciculus and cuneate fasciculus o Conveys discriminative touch, light pressure, vibration, and conscious proprioception  Sensory systems keep CNS informed of changes in external and internal environments  Information is processed by interneurons in spinal cord and brain. Somatic Sensations  Cutaneous sensations – sensations from stimulating skin  Areas with highest density of somatic sensory receptors include tip of tongue, lips, and fingertips Tactile Sensations  Include touch, pressure, vibration, itch, and tickling  Mechanoreceptors attached to myelinated A fibers mediate these sensations  Itch and tickle sensations detected by free nerve endings attached to unmyelinated C fibers Touch  Stimulation of tactile receptors in skin  Rapid touch receptors: meissner corpuscles and hair root plexuses o Meissener corpuscles – located in demal papillae of hairless skin  Abundant in fingertips, ahnds, eyelids, tip of tongue, lips, nipples o Hair root – found in hairy skin (free nerve endings wrapped around hair follicles)  Detect movements on skin distubing hairs  Two types of slowly adapting touch receptors: Merkel discs and Ruffini corpuscles o Merkel discs: saucer-shaped flattened free nerve endings that make contact with Merkel cells of stratum basale  Abundant in fingertips, hands, lips, and external genitalia o Ruffini corpuscles: enlongated, encapsulated receptors deep in dermis, and in ligaments and tendons  Present in hands and soles Pressure  Sustained sensation felt over larger area than touch  Receptors contributing to sensations of pressure include Meissner corpuscles, Merkel discs, Pacinian corpuscles o Pacinian corpuscle – multilayered connective tissue enclosing dendrite Vibration  Rapidly repetitive sensory signals from tactile receptors  Receptors are Meissner corpuscles (lower-frequency) and Pacinian corpuscles (higher-frequency) Itch  Stimulation of free nerve endings by chemicals Tickle  Sensation arises only when someone else touches you Thermal Sensations  Free nerve endings with receptive fields on skin  Coldness and warmth tected by different receptors  Cold receptors located in stratum basale and attached to A and C fibers  Warm receptors located in dermis and attached to unmyelinated C fibers Pain Sensations  Nociceptors – receptors for pain  Free nerve endings found in every tissue of body except brain  Tissue irritation or injury releases chemicals stimulating nociceptors Types of Pain  Fast and slow  Fast: Myelinated A fibers, sharp and acute  Slow: second or more after stimulus applied, along unmyelinated C fibers  Superficial somatic pain – stimulation of receptors in skin  Deep somatic pain – receptors in skeletal muscles, joints, tendons  Visceral pain – stimulation of nociceptors in visceral organs Localization of Pain  Referred pain – pain felt in or just deep to skin that overlies stimulated organ  Ex. Pain of heart attack felt in skin over heart and along left arm Proprioceptive Sensations  Allow us to know where head and limbs are located and how they are moving even we are not looking at them  Kinesthesia – perception of body movements  Proprioceptors allow weight discrimination (assess weight of object)  Three types of proprioceptors: muscle
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