Introduction to Neurology. Functional Anatomy of the Spinal Cord O.L. Zharikova Associate Professor BSMU 2016 Plan Structural and functional divisions of the nervous system Principals of the structural organization of the central (CNS) and peripheral (PNS) nervous systems Development of the CNS Functional anatomy of the spinal cord Nervous System is a complex of specialized structures of nervous tissue • The main controlling and communicating system of the body • Coordinates and direct activities of other systems and adjusts them to changing conditions of environment • Provides ◦ Sensory input - detects changes occurring inside and outside the body (from receptors); ◦ Integration - conducts, processes, and interprets the sensory inputs in the nervous centers; ◦ Motor output - response to stimuli by activating effector organs. Organization of the nervous system. Topographic division: CNS (central nervous system) contains the majority of nerve cell bodies and synaptic connections ◦ Brain ◦ Spinal cord PNS (periferal nervous system): ◦ Pared nerves: 12 cranial & 31 spinal ◦ Their roots and ramifications within the body Nerve fibres and endings (motor and receptors), nerves, roots, branches, plexuses ◦ Ganglia CNS and PNS Structures Functions CNS provides: ◦ Regulation and coordination of the activity of all organs and systems ◦ Functional integrity (unity) of the body ◦ Higher mental activity - thinking, learning, memory, intelligence, est. PNS constitutes the link between the CNS and structures in the periphery of the body: ◦ transmits sensory information and motor impulses Functional division of the nervous system Autonomic (vegetative), ANS: Somatic (animal), SNS Provide conscious awareness of the external environment Acts largely voluntary Innervates skin, sensory organs, striated/skeletal muscles, (and muscles of some organs: tongue, pharynx and larynx) sympathetic & parasympathetic monitoring and controlling conditions in the internal environment, i.e. homeostasis Acts largely involuntary Innervates internal organs, glands, vessels, regulates contraction of smooth and cardiac muscles Sensory and Motor Divisions of PNS Both SNS and ANS have 2 functional divisions: Sensory (afferent) - transmits impulses to CNS ◦ Somatic (SNS) – from receptors in skin (exteroceptors), skeletal muscles and joints (proprioceptors) ◦ Visceral (ANS) – from receptors in internal organs and vessels (interoceptors) Motor (efferent) - transmits impulses from CNS to effectors ◦ Somatic – skeletal, striated muscles ◦ Visceral - smooth muscle, cardiac muscle, and glands Principles of Organization of the Nervous Tissue Neurons are basic structural & functional units of NS: Have ability to ◦ receive (excitability) and integrate incoming information ◦ transmit (conductibility) information to other neurons or effectors Highly specialized cells - lose their ability to divide after birth Last a life time, but if destroyed cannot be replaced Neuroglial cells (Glia) are essential for the normal functioning of nerve cells Neuroglial cells are more numerous than neurons ≈10 : 1 Neuron Structure Dendrites collect and conduct stimuli to the body • multiple branching processes increase surface area for collecting stimuli Body Axon (1 or 2) transmits the receptive zone, receives information signal away from the cell body • at the end has nerve or generates the electrical axon terminals (terminal impulse/signal buttons) biosynthetic center • • • • Synapse: Connection Between Neurons Information is passed between neurons at synapses usually by chemical means: 1. Release of neurotransmitters from pre-synaptic terminals of pre-synaptic neurons into the synaptic cleft 2. Binding neurotransmitters by the receptors of the post-synaptic neuron Types of Synapses NeuronTypes: Structural Classification Neurons show more variation in shape than any other cells in the body Bipolar Multipolar Interneurons & Special sensory neurons Motor neurons & interneurons (olfactory, gustatory, optic, acoustic, vestibular) Pseudounipolar Most sensory neurons Neuron Types: Functional Classification 1. Afferent/Sensory neurons transmit impulses from receptors to the CNS (cell bodies in sensory ganglia in PNS) ◦ Somatic ◦ Visceral 2. Efferent neurons transmit impulses away from the CNS to effectors ◦ Somatic efferent/motor neuron (cell bodies in motor nuclei in CNS) to skeletal muscles ◦ Visceral efferent/autonomic to smooth muscle, cardiac muscle, and glands (cell bodies in autonomic ganglia in PNS) 3. Interneurons: link the afferent and efferent neurons in a chain, majority form sensory (or relay) nuclei in CNS (up to 99.8%) Neuroglial cells (Glia) Provide structural and metabolic support for neurons: Nourishment Protection Separation Secretion Reparation • • • • In the CNS: Ependymal Oligodendrytes = Astrocytes Microglial cells In the PNS: • Schwann cells • Satellite cells Glial Cells of CNS Astrocytes (from Latin astra) form an additional layer around capillaries that creates selectively permeable Blood-Brain Barrier Ependymal cells line a cavity filled with CSF, cerebrospinal fluid, and participate in its production Oligodendrocytes form around axons a myelin sheath Glial cells of the CNS: Microglia Small phagocytes that protect CNS from bacteria and viruses. constitute 20% of the total glial cell population within the brain. Glial cells of the PNS: Schwann & Satellite cells Schwann cells form a myelin sheath around axons Nodes of Ranvier – gaps between myelinating cells allow electrical impulses to jump from one node to another (fast conduction) Satellite cells surround neurons within ganglia Myelinated Fibers Myelin sheath consists of plasma membrane (lipoprotein gives white color) wrapped multiple times around the axon ◦ Insulates the axon to prevent interference from nearby neurons ◦ Increases rate of conduction of an impulse Myelinated axons/fibers: ◦ In CNS - white matter (all cells’ axons) ◦ In PNS - axons of sensory and motor neurons Unmyelinated fibers in the PNS Bundles of axons that are wrapped very thinly Velocity of conducting signal is slower Neurons parts & Structural Components of NS In the PNS Neurons’ bodies form ganglia: In the CNS • Neurons’ bodies form gray matter ◦ Sensory ganglia of cranial and spinal nerves ◦ Autonomic (efferent) • Neurons’ processes/fibers ganglia • Neurons’ processes/fibers are bundled together to form: nerves, nerve fibers, roots, branches, plexuses form white matter, which consists of their bundles called fibers, fascicles, commissures, lemnisci, tracts… Structure of the Nerve Endoneurium wraps a single axon including the myelin sheath Perineurium wraps a fascicle, a small bundle of nerve fibers/axons. (peri = around) Epineurium wraps a bunch of fascicles. (epi = upon/top) CNS: Gray Matter Gray matter is formed by cells bodies of neurons together with unmyelinated processes and neuroglia: ◦ Clusters of nerve cell bodies form nuclei of the spinal cord and brain ◦ Neurons organized by layers form cortex of the cerebrum and cerebellum The certain regions of the cortex and nuclei serve as nervous centers - aggregations of neurons with similar anatomical connections and functions: Sensory – receive signals Integrative – integrate them Motor – send signals out Reflex and Reflex Arc In the NS signals from receptors to effectors travel along the chains of neurons that include nervous centers This chain of neurons is called reflex arc ◦ morphological basis of a reflex Reflex is a stereotyped pattern of response to a sensory stimulus: ◦ Somatic ◦ Visceral Mono- and polysynaptic reflex arcs • In the simplest (stretch) reflex sensory neurons directly synapse on motor neurons - monosynaptic • Most reflexes involve an interneuron - polisynaptic Development of the NS & Spinal Cord 1. 2. 3. 4 successive stages in the development of the spinal cord: Neural plate: thickening of the midline ectoderm; ≈16th day of embryonic development (stage of gastrulation) Neural groove: neural plate deepens forming the neural groove & folds on its sides; at their dorsal aspects the neural crests form;18th day Neural tube: neural folds and crests fuse by the 21st day (end of 3rd week); neural tube separates from the surface and neural crest starts to delaminate from the neural tube Future Spinal 22nd day The whole neural tube is enclosed normally by 4 weeks and the cells begin a period of rapid division. The front part will become the brain and the rest will become the spinal cord. Cell Differentiation in the Spinal Cord • Starts from the caudal portion of the neural tube 5 ½ weeks • By week 6, there are 2 clusters of neuroblasts: 1. Dorsal alar plate → interneurons; become Mantle layer posterior horn Marginal layer 2. Ventral basal plate → efferent (motor) neurons; become anterior horn Ependymal layer • Neural crest cells → sensory neurons; form Other derivatives of the neural dorsal root ganglia crest: autonomic neurons and adrenal medulla cells, Schwann cells, melanocytes in the skin est. Spinal Cord Growth At 2nd-3rd antenatal month: Vertebral column= Spinal cord Spinal cord ends at L3; Length = 14 cm 6th At antenatal month: Spinal cord ends at sacrum Spinal cord ends at L2; Lenth=43-45cm Development of the Brain 32 days 25 days newborn Congenital Malformations of the CNS Many different types of brain congenital defects (anomalies) are caused by improper closure of the neural tube: Anencephaly (“no brain") when the topmost portion of the tube fails to close Encephalocele is a protrusion of a part of the brain through a defect in the skull Congenital Malformation of the Spinal cord Failure of closure of the neural tube or its close position to the surface prevents full development of the vertebral arch → spina bifida (cleft spine); most often at the L5 or S1 ◦ low folic acid ingestion in 1st trimester It may cause protrusion of the spinal cord (or cauda equina) and its meninges Functional Anatomy of the Spinal Cord Functions of the Spinal cord Transmission of neural signals between the brain and the rest of the body through the conduction tracts ◦ sends sensations to the brain from the body ◦ returns motor commands to skeletal muscles and internal organs Coordination of somatic reflexes - segmental innervation of the body ◦ maintenance of posture and muscle tone, coordination of movements, protection Coordination of autonomic reflexes regulating: ◦ work of internal organs; ◦ functions of the bowel, bladder and sexual organs (controlled by autonomic centers in conus medullaris) ◦ blood pressure and body temperature Spinal reflexes Are reflexes, involuntary stereotyped patterns of response to sensory stimuli, involving the spinal cord Somatic reflexes provide muscular responses 1. Contribute to ordinary muscle activity (such as the “knee” reflex) 2. Protective responses (such as withdrawing the hand after touching a hot surface) * Visceral (autonomic) reflexes will be discussed in ANS Spinal Somatic Reflex Arcs: Monosynaptic reflexes Tendon / stretch reflex mediated by a 2–neuron chain Caused by percussion of the tendon of the muscle Extensor contraction is important for muscle tone and upright body posture Biceps C5-6 Each is served by specific Brachioradial (supinator jerk) C5-6 Triceps C6-7 spinal cord segments Quadriceps (knee jerk) L3-4 Achills (ankle jerk) S1-2 Spinal Somatic Reflex Arcs: Polisynaptic reflexes Flexor reflex mediated by a 3–neuron chain Noxious cutaneous stimulation of the limb causes its flexion withdrawal (protection) Requires coordinated action of several joints and involves several spinal cord segments Autonomic and Somatic Spinal Reflex Arcs Functional Zones/Nuclei of the Gray Matter somatic sensory (SS) - posterior horn visceral sensory (VS) - posterior horn visceral motor (VM) - lateral horn autonomic (sympathetic) neurons (C8-L2) somatic motor (SM) - anterior horn • gray commissure — encloses central canal, connects masses of gray matter Functional division of White Matter in the Spinal Cord White matter is divided into 3 funiculi • Fibers run in 3 directions: ascending, descending, transversely (horizontally) • Bundles of fibers form conducting tracts Conduction tracts (pathways) are formed by nerve processes (axons) with similar functions and connections • ◦ Tract names usually reveal their origin and destination Long: afferent & efferent Short conducting tracts form the proper fibers - Posterior funiculus: ascending afferent sensory tracts Anterior funiculus: primary descending efferent motor tracts Lateral funiculus: both Next to the gray matter: proper fibers – connect centers within the spinal cord White commissura: proper fibers and fibers of some long conducting tracts Somatotopic Organization of the Spinal cord Segments of the Spinal Cord Segmental Innervation of the Skin Dermatome Map of the Body Dermatome - skin area innervated by the sensory fibers from a single spinal segment/root/nerve ◦ bands around the trunk and along the limbs. Cervical segments/spinal nerves innervate neck and arms Thoracic - chest and body, Lumbar – lower back & legs Each dermatom is supplied from 3 segments Segmental Innervation of Muscles A group of muscles primarily innervated by the motor fibers of a single nerve segment/mtor root is called a myotome. • • In a spinal cord injury, complete or partial paralysis occurs in the areas of the body that are controlled by the nerves associated with the damaged segment and those below the damaged segment The higher the injury the more area of paralysis Reticular Formation - Is one of the integrative systems of the CNS loosely organized network, which consists of clusters of neurons bodies interspersed among small bundles of myelinated axons extends throughout the brain stem from the spinal cord to hypothalamus Includes small and large nuclei (more than 1000) Receives collaterals from afferent tracts; being activated activates cortex of the brain (RAS) via thalamus or directly needed for arousal from sleep and maintaining wakefulness helps to maintain consciousness filters out insignificant information Has afferent and efferent connections with all parts of the brain and the spinal cord: process incoming sensations and sends outgoing motor commands; ◦ Conducts and process nonlocalized pain ◦ Activates or inhibits motor neurons of the spinal cord, regulates muscle tone ◦ Initiate involuntary motor responses to stimuli ◦ Regulates visceral reflexes Reticular Formation Location & Connections 51 Meninges of the brain and spinal cord The CNS is protected and isolated by multiple structures: Bones: vertebral canal and bony cranium Meninges: Three connective tissue membranes surround the CNS – dura mater, arachnoid mater & pia mater: ◦ Provide protection, stability and shock absorption for the CNS parts and their blood vessels Dura mater forms partitions for parts of the brain and venous sinuses within the skull Denticulate ligaments of pia mater help to suspend the spinal cord The pia mater contains blood vessels and capillaries that are responsible for nourishing the brain and formation of blood-brain barrier prevents entry of harmful materials from the bloodstream ◦ Participate in the formation and circulation of the cerebrospinal fluid (CSF) 1552 Cerebrospinal fluid CSF bathes the inside and outside of the spinal cord and the brain and provides a liquid support and cushion Nourishes brain and spinal cord, removes waste Carries hormonal and chemical signals between parts of the CNS • A lumbar puncture (spinal tap) is performed to test CSF (or reduce pressure) between spinous processes of L3-L4 or L4-L5 vertebrae • A needle should be inserted into the subarachnoid space • The epidural anesthesia is made into the dural sac to reach the roots of spinal nerves
