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Chapter 13. Spinal CordFunctions:Conduction – of information up and down the cordIntegration of received information, and deciding an appropriate outputLocomotion – coordinating the simple repetitive muscle contractionsReflexesSurface anatomy:Cylinder of nervous tissue – from foramen magnum to L1. From L2 to S5 → cauda equina (a bundle of nerve roots – pelvic organs + lower limbs).Here subarachnoid space is called lumbar cistern.Here pia mater becomes terminal filum (fibrous strand)There are 31 pairs of spinal nerves (1st pair passes b/w the skull and C1).2 enlargements: cervical (nerves of the upper limbs) and lumbar (lower limbs).*Lumbar enlargement → medullary cone → cauda equina.Cross-sectional anatomyGray matter:Horns – posterior (sensory input); anterior (motor output).Lateral horn (T2-L1) – sympathetic nervous system.Gray commissure – connects right and left sides (central canal in the middle).White matter:Columns (funiculi) – posterior, lateral, anterior.TractsAscendingGracile fasciculus joined by cuneate fasciculus at T6 and up:Pressure, proprioception.1st order nerve fibers go to medulla oblongata → 2nd order fibers → decussate → → thalamus → cerebral cortexSpinothalamic tract:Temperature, pain.1st order fibers end at posterior horn → 2nd order decussate → thalamus → cortexSpinocerebellar tract:Proprioception1st order fibers end at posterior horn → 2nd order travel ipsilateral side → cerebellum*Anterior → crosses over, but then crosses back in the brainstem.DescendingCorticospinal (pyramidal) tracts:Motor coordination for limb movements.Lateral tract → decussate in the lower medulla. Anterior → in the spinal cord.Tectospinal (extrapyramidal) tract:Controls the head during visual/auditor reflexesBegins in midbrain → decussates → neck.Nerve anatomy:Peripheral nervous system:Nerve fiber → Schwann cells (neurilemms + myelin) → basil lamina → endoneurium → fascicle → perineurium → epineurium (dense irregular connective tissue)Ganglion – cluster of neurosomas outside the CNS.Spinal Nerves8 cervical - 12 thoracic - 5 lumbar - 5 sacral - 1 coccygealProximal branches:anterior rootlets → anterior root (motor) | mergeposterior rootlets → posterior root with ganglion (sensory) |Distal branches (emerging from the intervertebral foramen)anterior ramus (larger) – innervates anterior and lateral skin, muscles of the trunk, gives rise to nerves of the limbs. (In thoracic region also forms an intercostal nerve).posterior ramus – innervates muscles and joints in that region of the spine, and skin of the backmeningeal branch – reenters the vertebral canal.Reflexesquick, involuntary, stereotyped reactions of muscles and glands to stimulation.Somatic reflexes – unlearned skeletal muscles reflexes (mediated by brain/brainstem/spinal cord).Reflex arc:Somatic receptors (skin/muscles/tendons) → afferent nerve fibers → intergrating center (gray matter of cord or brainstem) → efferent nerve fibers → effectors (muscles that carry out the response).Muscle spindle (the body's proprioceptor) – in the perimusium of a muscle.Made up of intrafusal fibers – have sarcomeres only at the ends.Nuclear chain fibers – single file of nuclei in the noncontractile region.Nuclear bag fibers – flatter, longer, have nuclei clustered in the baglike middle region.Nerve fibers:Sensory:(one) Primary afferent (group Ia) fiber – coils around the middle of fibers. Large, fast.(~8) Secondary afferent (group II) fibers – coild adjacent to ^.Smaller, slower.Motor (adjusts the length → sensitivity):Gamma motor neurons – connect to contractile ends of the intrafusal fibers. Relatively small and slow.Monosynaptic arc – there is only one synapse b/w afferent and efferent neuron (in spinal cord).Stretch reflex – when muscle is suddenly stretched it fights back – contracts, increases tone, stiffens.Usually an action of synergists and antagonists. Mediated primarily by the brain (*postural stability), but if the stretch is very sudden – spinal component is more pronounced.Tendon reflex – reflexive contraction of the muscle when the tendon is tapped.Polysynaptic reflex arc – signals travel over many synapses on their way back to the muscleWithdrawal reflex – quick contruction of flexor muscles to withdraw a limb from injurious stimulus.Requires sustained contraction – parallel after-discharge circuit → prolonged output.(*ipsilateral reflex arc)Crossed extension reflex – contraction of extensor muscles in the limb opposite from the one that is withdrawn – enables to keep the balance.Branches of the afferent nerve fibers cross from the stimulated side of the body to the contralateral side of spinal cord → exite/inhibit muscles of the contralateral limb.(*contralateral reflex arc – input and output are on opposite sides * intersegmental reflex arc – input and output occur at different levels of the spinal cord)Nerve Plexuses:Name Sensory innervation (distal) Motor innervation (proximal)Cervical plexus (C1 to C5):Phrenic nerve Diaphragm, pleura, pericardium DiaphragmBrachial plexus (C4 to T1)Musculocutaneous Skin of anterolateral forearmElbow jointBrachialis, biceps bracii,coracobrachialisAxillary Skin of lateral shoulder and armShoulder jointDeltoid and teres minorRadial Skin of posterior armPosterior and lateral forearm and wristJoints of elbow, wrist and handMainly extensor muscles of posterior arm and forearm (triceps bracii, extensor carpi radialis longus)Median Skin of lateral handTips of digits I-IVJoints of handMainly forearm flexorsUlnar Skin of palmar and median handDigits II-VJoints of elbow and handSome forearm flexorsLumbar plexus (L1 to L4)Femoral Skin of anterior, medial and lateral thigh and kneeSkin of medial leg and footHip and knee jointsIlliacusQuadriceps femorisSartoriusObturator Skin of medial thighHip and knee jointsObturator externusAdductorsSacral (L4, L5, S1) and Coccygeal plexuses (S4, S5, Co)Sciatic nerveTibial Skin of posterior leg. Plantar skin.Knee and foot jointsHamstringsSemitendinosus, semimembranosisGastrocnemius, soleusFibular (common)Skin of anterior distal leg. Dorsum of foot. Toes 1, 2.Knee jointTibialis anteriorToe extensors, dorsiflexorsPudental Skin of penis, scrotum.Clitoris, labia, lower vagina.Bulbospongiosus, levator ani. Sphincters (anal,


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MGC BIOL 1114K - Chapter 13

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