LESSON PLAN Sheet 1



PART 2: Spinal Cord and Spinal NervesTEACHER NOTESB. SPINAL NERVESConnection of Spinal Nerves to the Spinal CordSpinal nerves are the paths of communication between the spinal cord (CNS) & most of the body’s sensory receptors, muscles & glands (PNS). Parallel bundles of axons and associated neuroglia wrapped in CTThe 31 pairs of spinal nerves are named and numbered according to the region and level of the spinal cord from which they emerge (Fig 13.2)8 pairs of cervical nerves (C1-C8)12 pairs of thoracic nerves (T1-T12)5 pairs of lumbar nerves (L1-L5)5 pairs of sacral nerves (S1-S5)1 pair of coccygeal nerves (Co1)Because the spinal cord is shorter than the vertebral column (ends around L2), nerves that arise from the lumbar, sacral, and coccygeal spinal cord regions do not leave the vertebral column at the same level they exit the cord they do not necessarily align with their corresponding vertebraeCauda equina “horses tail” – roots of these spinal nerves angle inferiorly in the vertebral canal from the spinal cordSpinal roots (bundles of axons) are the two points of attachment that connect each spinal nerve to a “segment” (not clearly segmented) of the spinal cord (Fig 13.3). Spinal nerves are mixed nerves – both sensory and motor fncThe posterior or dorsal (sensory) root contains sensory nerve fibers and conducts nerve impulses from the periphery into the spinal cord; The posterior root ganglion contains the cell bodies of the sensory neurons from the periphery.The anterior or ventral (motor) root contains motor neuron axons and conducts impulses from the spinal cord to the peripheryThe cell bodies of motor neurons are located in the gray matter of the cord.Connective Tissue Covering of Spinal NervesSpinal nerve axons are grouped within CT sheathes (Fig 13.5).EndoneuriumNerve fiber = single axon within endoneuriumMesh of collagen fibers, fibroblasts (make collagen), macrophages Perineurium A fascicle is a bundle of fibers (many axons) within a perineurium.Thicker layer of CT – up to 15 layers of fibroblasts in a collagen network. EpineuriumA nerve is a bundle of fascicles within an epineurium.Fibroblasts and thick collagen fibers Numerous blood vessels are within the coverings.Distribution of Spinal NervesShortly after passing through its intervertebral foramen, a spinal nerve divides into several branches rami (Fig. 13.6).Posterior ramusDorsal branch – serves deep muscles and skin of posterior surface of trunkAnterior ramusVentral branch – serves muscles and structures of upper and lower limbs and skin of lateral and anterior surfaces of trunk Meningeal branch3rd branchReenters vertebral cavity through intervertebral foramenSupplies vertebrae, vertebral ligaments, blood vessels of spinal cord, and meninges. Rami communicantesOther branchesComponents of ANS – discussed later Plexuses Axons from the anterior rami of spinal nerves (except for thoracic nerves T2-T12) do not go directly into the body structures they supply. Instead, they join with many axons from anterior rami of adjacent nerves, forming networks on both sides called plexuses (Fig. 13.2 and Exhibits 13 A-D)T2-T12Anterior rami of T2-T12 do not form plexuses and are known as intercostal or thoracic nerves.Directly innervate structures they supply in the intercostal spaces.See examples on pg. 451 intercostal muscles and overlaying skin The posterior rami supply the deep back muscles and skin of the posterior aspect of the thorax.Cervical Plexus Figure 13.7Exhibit 13 AFormed by the roots of the 1st 4 cervical nerves (C1-C4) and some contributions from C5Supplies skin and muscles of head, neck, upper part of shoulders and chest. Connects with some cranial nervesAnterior rami of C3-C5 forms the phrenic nerveSupplies diaphragmDamage to spinal cord above origin of phrenic nerves = respiratory arrest. Breathing stops b/c phrenic nerves stop sending impulses to the diaphragmBranches of cervical plexus run parallel to 2 cranial nerves (accessory XI nerve and hypoglossal XII nerve)Brachial PlexusFigure 13.8Exhibit 13 B RTDCBRoots = anterior ramiTrunks (3) = joined rootsDivisions (anterior and posterior) = trunks divergeCords (3) = divisions unite in axillaeBranches = form main nerves The roots (anterior rami) of spinal nerves C5-C8 and T1Nerve supply for upper extremities and many neck and shoulder muscles.Many nerve disorders may result from injury to the brachial plexus (Figure 13.8 C)5 large terminal branches arise from the brachial plexus:Axillary nerve – innervates deltoid and teres minor.Musculocutaneous nerve – innervates biceps brachii and brachialisRadial nerve – innervates triceps brachii and forearm extensor muscles. Median nerve – innervates forearm flexors (except for flexor carpi ulnaris) and a few hand muscles. Ulnar nerve – innervates flexor carpi ulnaris and most hand muscles Lumbar PlexusFigure 13.9Exhibit 13 CFormed by anterior rami (roots) of spinal nerves L1-L4Supplies anterolateral abdominal wall, external genitals, part of lower extremitiesMinimal intermingling of fibers (unlike brachial plexus)Two important nerves arise from the lumbar plexus:Femoral nerve The largest nerve in the plexusInnervates quadriceps femoris musclesInjury to this nerve (stab or gunshot) is indicated by an inability to extend the leg and by loss of sensation in the skin over the anteromedial aspect of the thigh. Obturator nerve Innervates adductor muscles of hip jointInjury of this nerve is a common complication of childbirth. Results in paralysis of adductor muscles of leg and loss of sensations over medial aspect of thigh. Sacral PlexusFigure 13.10Exhibit 13 DRoots (anterior rami) of spinal nerves L4-L5 and S1-S4Supplies buttocks, perineum, part of lower extremitiesTwo important nerves arise from the sacral plexus:Sciatic nerve (tibial and fibular nerve branches)Largest nerve in plexus, also largest in bodyInnervates hamstrings and muscles of leg and footInjury results in sciatica pain extending from buttock down back of leg.Injury can occur due to a herniated (slipped) disc, dislocated hip, osteoarthritis of the lumbosacral spine, pressure from the uterus during pregnancy, or an improperly administered gluteal injection. Pudendal nerveInnervates perineum DermatomesThe skin over the entire body is supplied by spinal nerves that carry somatic sensory nerves impulses into the spinal cord.All spinal nerves except C1 innervate specific, constant segments of the skin; the skin segments are called dermatomes (Figure 13.11).Knowledge of dermatomes helps a physician determine which spinal cord segment or spinal nerve is malfunctioning.C. SPINAL CORD PHYSIOLOGYPrincipal Functions of the Spinal CordThe white matter tracts are highways for nerve impulse conduction to and from the brain.The gray matter receives and integrates incoming and outgoing information to perform reflexes.Sensory and Motor Tracts (discussed in more detail in chapter 16)Nerve impulses along tracts = first way spinal cord maintains homeostasis Figure 13.12 - principal sensory and motor tracts in the spinal cord. Sensory InformationTwo routes on each side of the cord: Sensory info receptors up spinal cord brainSpinothalamic tractConveys nerve impulses for pain, warmth, coolness, itching, tickling, deep pressure etc.Posterior column tract2 tracts (gracile fasciculus & cuneate fasciculus)For finer touch, light pressure, vibration, conscious proprioception (awareness of position)Motor InformationTwo types of descending tracts: Motor info brain down spinal cord effectors (muscles and glands)Direct pathways voluntary skeletal muscle mvmtIndirect pathways autonomic mvmts – coordination with visual stimuli ReflexesThe second way the spinal cord maintains homeostasis = serving as an integrating center for spinal reflexes in the gray matter (Fig 13.13).A reflex is a fast, predictable, automatic (involuntary) response to changes in the environment that helps to maintain homeostasis.TYPES OF REFLEXESInborn reflexes pulling hand away from heatLearned/Acquired reflexesDriving (car) responseLOCATIONCranialIntegration happens in the brain stem.Tracking mvmt of eyes while readingSpinalWhen integration happens in gray matter of spinal cord Patellar reflex (knee jerk) FUNCTIONAutonomic (visceral)Smooth muscles, glands, cardiac muscleDigestion, HR, waste removalSomaticContraction of skeletal musclesThe Reflex ArcA reflex arc is the simplest type of pathway; pathways are specific neuronal circuits and thus include at least one synapse.Reflexes help to maintain homeostasis by permitting the body to make exceedingly rapid adjustments to homeostatic imbalances.The five functional components of a reflex arc:ReceptorSensory response to stimulus produces a graded potential = generator (or receptor) potential. If threshold is reached = depolarization = nerve impulse Sensory neuronFound in gray matter of spinal cord or brain stemAxons conduct impulses from receptor to the integrating center. Integrating center neuronGray matter – allows conscious awareness that reflex has occurred Interneuron Relay impulses between sensory and motor neurons.Monosynaptic reflex arc one synapse in CNS between a sensory and motor neuron Polysynaptic reflex arc more than 2 types of neurons / synapses: sensory neuron synapses with 1+ interneurons before motor neuronMotor neuronIntegrating center propagates impulses out to motor neuron to stimulate a bodily response.Effector The part of the body that responds to the motor neuron.Somatic reflex = skeletal muscleAutonomic (visceral) reflex = smooth muscle, cardiac muscle, glands Because reflexes are usually very predictable, they provide important information about your NS health. Somatic spinal reflexes include the stretch reflex, tendon reflex, flexor (withdrawal) reflex, and crossed extensor reflex; all exhibit reciprocal innervation. KEY TERMS: IspilateralReflex response on the same side of the body that is stimulated. ContralateralReflex occurs on the opposite side of the body from the stimulus. Ex. Crossed extensor reflex.MonosynapticOne synapse in CNS between sensory and motor neuronPolysynapticSensory neuron synapses with one or more interneurons before synapsing with the motor neuron.Reciprocal innervation When one muscle contracts, its antagonistic muscle relaxes. Ex. Biceps relaxes when triceps contracts. Stretch Reflex (fig. 13.14)Contraction of skeletal muscle (effector) in response to stretching muscle. Brain controls how vigorously a muscle responds to stretching.Elicited by tapping on tendons attached to muscles (elbow, wrist, knee, ankle) IpsilateralImportant in maintaining muscle tone (small degree of stretching at rest) and coordination during exercise.Sensory receptors: muscle spindles – sensitive to muscle length. Monosynaptic reflex arcReciprocal innervation One sensory neuron & on motor neuronAll monosynaptic arcs are ipsilateralThe stretch reflex pathway itself is monosynapticHowever, the control of the antagonistic muscles (at the same time) is polysynaptic Inhibitory interneuron synapses with a motor neuron that relaxes the antagonistic muscles. RI Prevents conflict between opposing muscles and coordinates body mvmts EXAMPLEPatellar reflex – operates as a feedback mechanism to control muscle length by causing muscle contraction. Tendon Reflex (Fig 13.15)Prevents damage to muscles and tendons as a result of stretching. Operates as a feedback mechanism to control muscle tension by causing muscle relaxation when muscle force becomes too extreme. Less sensitive than the stretch reflex.IpsilateralSensory receptors: tendon organs – sensitive to muscle tension.Polysynaptic reflex arcTendon organ depolarized when stimulatedNerve impulse enters spinal cordIntegrating center – sensory neuron inhibitory interneuron inhibits (hyperpolarizes) the motor neuron muscle relaxes (releases tension)Sensory neuron also synapses with excitatory interneuron motor neuron controlling antagonistic muscles Relaxation of muscle attached to tendon organ and simultaneous contraction of antagonists. Somewhat the opposite of stretch reflex Flexor Reflex Crossed Extensor Reflexes (Fig. 13.16)Flexor reflex is a protective withdrawal reflex that moves a limb to avoid painContraction of flexor muscles to move a limb to avoid injury or pain.Works with crossed extensor reflex to maintain balance. IpsilateralSensory receptor: dendrites of pain-sensitive neuron Polysynaptic reflex arcIntersegmental reflex arcPain sensory receptor (dendrites) of pain-sensitive neuron nerve impulses to spinal cord interneurons extending into spinal cord segments motor neurons in diff segments ACh released contracts effector muscles withdrawal of limb from damaging stimulus.Requires the contraction of many muscle groups to move an entire limb away quickly = many motor neurons sending impulses simultaneously. Initiates crossed extensor reflex to prevent losing balanceEXAMPLEStepping on a tackCrossed Extensor Reflex (Fig 13.17)Balance-maintaining reflex synchronized extension of joints of one limb and flexion of joints in opposite limb. ContralateralHelps to maintain balance during the flexor reflex. Polysynaptic reflex arcIntersegmental reflex arcReciprocal innervation Step on tack Stimulated pain-sensitive neuron sensory response in right foot nerve impulses to spinal cord stimulates several interneurons synapse with motor neurons on left side of spinal cord and in many segements incoming pain signals cross to opposite side through interneurons motor neurons stimulated ACh released innervated extensor muscles of opposite (left) side contract weight placed on this side supporting body. Synchronize extension of contralateral limb with withdrawal (flexion) of stimulated limb. Clinical Importance of Reflexes (see page 467)Reflexes are often used for diagnosing disorders of the nervous system and locating injured tissue.If a reflex is absent, or abnormal, the damage may be somewhere along a particular conduction pathway.Among the clinically important reflexes are the Patellar reflex, Achilles reflex, Babinski reflex, Abdominal reflex and Pupillary reflex. ................
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