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Lecture # 20: The Spinal Cord and Spinal Nerves Objectives: 1- Describe the major structural features and functions of the spinal cord. 2- Name the three coverings of the central nervous system and give the characteristics, location, and function of each. 3- Identify ventral root, dorsal root, and dorsal root ganglion and give the composition of each. 4- Discuss white matter, gray matter, tracts, and nuclei. 5- Differentiate among motor nerves, sensory nerves, mixed nerves, cranial nerves, and spinal nerves. 6- Describe the anatomy of a reflex arc. (Chapter 13) Functions of the Spinal Cord Conduction: Bundles of fibers passing information up and down spinal cord (ascending and descending tracts) , connecting different levels of the trunk with each other and with the brain. Locomotion: Walking involves repetitive, coordinated actions of several muscle groups. Central pattern generators are pools of neurons in the cord providing control of flexors and extensors that cause alternating movements of the lower limbs. Reflexes: They are involuntary, stereotyped responses to stimuli, such as the withdrawal of a foot from pain involves brain, spinal cord and peripheral nerves. Surface Anatomy of the Spinal Cord Foramen magnum Cervical enlargement Medullary cone Cauda equina Lumbar enlargement L1 – L2 Terminal filum It attaches the spinal cord to the coccyx and prevents vertical movements Meninges of the Spinal Cord Epidural space (Space for the epidural anesthesia) Dura mater Subdural space Arachnoid mater Subarachnoid space (It contains cerebrospinal fluid)) Pia mater Pia mater Arachnoid mater Dura mater Cross Sectional Anatomy of the Spinal Cord Gray matter It contains the bodies and dendrites of neurons. Therefore, it contains little myelin. It is composed of bundles of axons called tracts. Therefore, it White matter contains abundance of myelin. They carry sensory Posterior (dorsal) information root of spinal nerve Posterior root ganglion They contain the bodies of sensory neurons Posterior They contain bodies of gray horns sensory somatic neurons and sensory visceral neurons Lateral gray horns They contain bodies of visceral motor neurons Spinal nerve They carry both sensory and motor information Anterior (ventral) root of spinal nerve They carry motor information Anterior They contain bodies of gray horns somatic motor neurons Cerebral cortex Somato-sensory neurons Visceral-sensory neurons Visceral-motor neurons Somato-motor neurons Cerebral cortex Thalamus Hypothalamus SS VS VM SM Somatic sensory fiber Somatic motor fiber Visceral sensory fiber Visceral motor fiber White Matter and Spinal Tracts Posterior (dorsal) columns or funiculi Lateral columns or funiculi Ascending tracts (They carry sensory information up the cord) Anterior (ventral) columns or funiculi Ascending tracts or fasciculi Descending tracts or fasciculi Descending tracts (They carry motor information up the cord) Some Ascending Pathways Gracile Fasciculus and Cuneate Fasciculus They carry signals for vibration, visceral pain, deep and discriminative touch, and conscious proprioception (sensations of limbs and trunk positions and movements). Spinothalamic Tract It carries signals for somatic pain, pressure, temperature, light touch, tickle, and itch. Spinocerebellar Tracts They carry unconscious proprioceptive signals from limbs and trunk up to the cerebellum. Postcentral gyrus of cerebrum It contains the primary somatosensory cortex for touch, pain and temperature. Thalamus Cerebellum Spinocereberall Tracts Cerebellum Medulla oblongata Spinal cord Cerebrum Pons Medulla oblongata Cerebellum It coordinates rapid, automatic adjustments of postural muscles, and programs and finetunes motor commands to produce smooth movements. Axon of first order neuron (in posterior root) They carry unconscious proprioceptive signals from limbs and trunk up to the cerebellum (contralateral and ipsilateral). Axons of second order neurons Body of second order neuron (in posterior gray horn) Body of first order neuron (in posterior root ganglion) Spinal cord Descending Tracts Motor homunculus of the precentral gyrus (Body parts are in proportion to the amount of motor cortex dedicated to their control) Corticospinal Tracts They carry motor signals from the precentral gyrus of the cerebral cortex for precise, finely coordinated limb movements (contralateral) Pyramidal cells (Upper motor neurons) Precentral gyrus of cerebrum (motor cortex that controls precise coordinated limb movements) Postcentral gyrus of cerebrum (somatosensory or somesthetic cortex) Medulla oblongata Anterior corticospinal tracts (decussation in spinal cord) Lower motor neurons To skeletal muscles Midbrain Decussation in medulla oblongata Lateral corticospinal tracts (roughly 85% of the axons undergo decussation at the medulla oblongata)) Spinal cord To skeletal muscles Some Pathways of the Central Nervous System Tract Spinothalamic Tract (Sensory, ascending) Origin Somatic receptors for pain, pressure, temperature, light touch. Spinocereberall Proprioceptors in muscles, tendons, Tracts (Sensory, ascending) Corticospinal Tracts (Motor descending) and joints Precentral gyrus of cerebrum (motor cortex) 1rst order neuron 2nd order neuron 3rd order Destination neuron Posterior root ganglion Posterior Thalamus gray horn Somatosensory (somesthetic) cortex in postcentral gyrus of brain Posterior root ganglion Posterior gray horn Cerebellum (cerebellar cortex) Upper motor neurons Pyramidal cells of precentral gyrus of cerebrum (motor cortex) Not present Lower motor neurons Motor neurons of anterior horn of spinal cord Skeletal muscles Decussation In spinal cord Posterior tracts: none Anterior tracts: in spinal cord Function It carries sensory signals for somatic pain, pressure, temperature, light touch, tickle, and itch (contralateral). They carry unconscious proprioceptive signals from limbs and trunk up to the cerebellum (contralateral and ipsilateral) They carry motor signals from the precentral gyrus of the cerebral cortex for precise, finely coordinated limb Anterior tracts: movements (contralateral) Lateraltracts: in medulla oblongata in spinal cord General Anatomy of Nerves and Ganglia Sensory pathway Motor pathway Posterior root ganglion Somatosensory neurons Sensory pathway Spinal nerve Posterior root Epineurium Loose connective tissue that surrounds one nerve Blood vessels Anterior root Motor pathway General Anatomy of Nerves Rootlets Posterior root Posterior root ganglion Anterior root Up to 20 layers of overlapping Perineurium squamous cells that surrounds each fascicle Unmyelinated nerve fiber Myelinated nerve fiber Spinal nerve Myelin Endoneurium Loose connective tissue that surrounds each fiber Fascicle Bundle of nerve fibers Epineurium Loose connective tissue that surrounds each nerve Spinal Nerves Proximal Branches: Posterior root (sensory) Anterior root (motor) Spinal nerve They are mixed nerves, carrying sensory and motor information. They emerge through the intervertebral foramina. Distal Branches: Posterior ramus It innervates muscles and joints in the region of spine and skin of back. Anterior or ventral ramus Shortly after they exit the vertebral column, they branch and combine to form large PLEXUSES. They innervate anterior and lateral skin, and muscles of trunk and limbs. Meningeal ramus It innervates meninges, vertebrae and spinal ligaments. The Cervical Plexus C1 Hypoglossal nerve (XII) C2 Lesser occipital nerve C3 Great auricular nerve Transverse cervical nerve C4 Ansa cervicalis: Anterior root Posterior root Roots C5 Supraclavicular nerves Phrenic nerve Innervates the diaphragm The Brachial Plexus Posterior scapular nerve C5 Lateral cord Posterior cord Medial cord Suprascapular nerve C6 Axillary nerve Musculocutaneous nerve Lateral cord C7 Posterior cord Median nerve Medial cord C8 Radial nerve T1 Musculocutaneous nerve Axillary nerve Radial nerve Long thoracic nerve Roots Median nerve Ulna Ulnar nerve Median nerve Radial nerve Radius Trunks Anterior divisions Posterior divisions Cords Ulnar nerve Superficial branch of ulnar nerve Digital branch of ulnar nerve Digital branch of median nerve The Lumbar Plexus Hip bone Sacrum Roots Femoral nerve Anterior divisions L1 Pudendal nerve Posterior divisions Sciatic nerve Femur L2 Iliohypogastric nerve Anterior view Ilioinguinal nerve From lumbar plexus L3 Genitofemoral nerve Tibial nerve From sacral plexus Obturator nerve Common fibular nerve Superficial fibular nerve L4 Lateral femoral cutaneous nerve Deep fibular nerve L5 Femoral nerve Obturator nerve Lumbosacral trunk Fibula Tibia Tibial nerve Medial plantar nerve Lateral plantar nerve Posterior view Sacral and Coccygeal Plexuses Lumbosacral trunk L4 Roots Anterior divisions Posterior divisions L5 S1 S2 Superior gluteal nerve Inferior gluteal nerve S3 S4 S5 Sciatic nerve: Common fibular nerve Tibial nerve Co1 Posterior cutaneous nerve Pudendal nerve Spinal Reflexes Events in a Reflex Arc Action potential Receptor potential The Muscle Spindle The muscle spindles inform the brain of muscle length and body movement This enables the brain to send motor commands back to the muscles that control coordinated movement, corrective reflexes, muscle tone, and posture Muscle spindles are proprioceptors: specialized sense organs to monitor the position and movement of the body parts The Stretch Reflex 4 3 Primary afferent neuron stimulates alpha motor neuron to extensor muscle The stretch reflexes maintain the muscles at the suitable length, and help to maintain equilibrium and posture. 6 Primary afferent neuron excited + _ 7 5 2 Muscle spindle is stimulated 1 Extensor muscle stretched Alpha motor neuron stimulates extensor muscle to contract Extensor Flexor 8 Flexor muscle (antagonist) relaxes Primary afferent neuron stimulates inhibitory interneuron Interneuron inhibit alpha motor neuron to flexor muscle