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THE NERVOUS SYSTEM & NERVOUS TISSUE Leonardo Da Vinci, 1508 Human Anatomy Sonya Schuh-Huerta, Ph.D. Nervous System • Master control & communication system • What makes us uniquely “human” Great intelligence Emotions & empathy Reasoning & problem solving Strategizing & predicting Culture (religion, etc.) … Nervous System • 3 Overlapping Functions: • Sensory receptors monitor changes inside & outside body • Change a stimulus • Gathered information sensory input • Processes & interprets sensory input • Makes decisions integration • Dictates a response by activating effector organs • Response motor output Nervous System Sensory input Integration Motor output Basic Divisions of Nervous System • Central nervous system (CNS) – Brain & spinal cord – Integrating & command center – Personality traits, emotions, intelligence, etc. Basic Divisions of Nervous System • Peripheral nervous system (PNS) – Outside the CNS – Consists of nerves extending from brain & spinal cord: • Cranial nerves • Spinal nerves – Peripheral nerves link all regions of body to CNS – Ganglia are clusters of neuronal cell bodies Basic Divisions of the Nervous System Brain CNS Spinal cord Nerves PNS Ganglia Sensory Input & Motor Output • Sensory (afferent) signals picked up by sensory receptors – Carried by nerve fibers of PNS to the CNS • Motor (efferent) signals are carried away from the CNS – From brain/spinal cord to organs (muscles, glands) Sensory Input & Motor Output • Divided according to region they serve: – Somatic body region – Visceral body region • Results in 4 main subdivisions of NS: – Somatic sensory – Visceral sensory – Somatic motor – Visceral motor (=autonomic nervous system) Types of Sensory & Motor Information Central nervous system (CNS) Peripheral nervous system (PNS) Brain and spinal cord Integrative and control centers Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS Somatic sensory fiber Visceral sensory fiber Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Somatic nervous system Skin Somatic motor (voluntary) Conducts impulses from the CNS to skeletal muscles Stomach Autonomic nervous system (ANS) Visceral motor (involuntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Mobilizes body systems during activity Sympathetic motor fiber of ANS Parasympathetic division Conserves energy Promotes housekeeping functions during rest Heart Structure Function Sensory (afferent) division of PNS Motor (efferent) division of PNS Parasympathetic motor fiber of ANS Bladder Basic Divisions of the Nervous System • Somatic sensory – General somatic senses receptors are widely spread • Touch • Pain • Vibration • Pressure • Temperature (receptors discussed later in Ch 14) Basic Divisions of the Nervous System • Somatic sensory (cont.) – Proprioceptive senses detect stretch in tendons & muscle • Body sense position & movement of body in space – Special somatic senses (Ch 16) • • • • Hearing Balance Vision Smell & Taste Basic Divisions of the Nervous System • Visceral sensory – General visceral senses stretch, pain, temperature, nausea, & hunger • Widely felt in digestive & urinary tracts, & reproductive organs – Special visceral senses • Taste & smell often considered special visceral senses Basic Divisions of the Nervous System • Somatic motor – General somatic motor signals contraction of skeletal muscles • Under our voluntary control! • Often called “voluntary nervous system” Basic Divisions of the Nervous System • Visceral motor – Regulates the contraction of smooth & cardiac m. – Makes up autonomic nervous system (ANS) – Controls function of visceral organs – Often called “involuntary nervous system” • “Fight or Flight” NS • =Autonomic nervous system (later, Ch 15) Nervous Tissue • Cells are densely packed & intertwined – 2 main cell types: • Neurons transmit electrical signals • Support cells (neuroglial or glial cells) – Nonexcitable – Support growth & function of neurons – Surround & wrap neurons The Neuron • The human body contains billions of neurons!!! – Basic structural unit of the Nervous System • Specialized cells that conduct electrical impulses along their plasma membrane – Nerve impulse (= action potential) The Neuron • Special characteristics: 1.) Excitability conduct electrical impulses 2.) Longevity can live & function for a lifetime! 3.) Do not divide fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception (olfactory & hippocampal neuron regeneration is an example) 4.) High metabolic rate require abundant oxygen & glucose! • Neurons die after 5 minutes without oxygen!!! The Cell Body • Cell body (= soma) – Perikaryon cytoplasm around the nucleus – Size of cell body varies from 5–140 µm – Contains usual organelles plus other structures • Chromatophilic bodies (= Nissl bodies) – Clusters of rough ER & free ribosomes – Stain darkly – Protein machinery – Renew membranes of the cell Chromatophilic (Nissl) bodies The Cell Body • Neurofibrils bundles of intermediate filaments – Form a network between chromatophilic bodies Neurofibril The Cell Body • Most neuronal cell bodies are: – Located within the CNS! – Protected by bones of skull & vertebral column • Ganglia (“knot in a string”) clusters of cell bodies – Lie along nerves in the PNS! Structure of a Typical Large Neuron Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Dendrites Neuron cell body Nucleus with nucleolus Neurofibril Nucleus Chromatophilic (Nissl) bodies (b) Nucleolus Nissl bodies Axon (impulse generating and conducting region) Axon hillock (a) Neurilemma Nuclei of neuroglial cells Impulse direction Schwann cell (one internode) Node of Ranvier Axon terminals (secretory region) Terminal branches Neuron Processes • Dendrites – Extensively branch from the cell body – Transmit electrical signals toward the cell body – Chromatophilic bodies only extend into basal part of dendrites & to the base of axon hillock – Function as receptive sites for receiving signals from other neurons Neuron Processes • Axon – Neuron has only one – Impulse generator & conductor axon hillock – Transmits impulses away from the cell body – Impulses travel to axon terminals – Chromatophilic bodies are absent – No protein synthesis in axon Neuron Processes • Axons – Neurofilaments, actin microfilaments, & microtubules • Provide strength & structure along length of axon • Aid in the transport of substances to & from the cell body – Axonal transport Neuron Processes • Axons – Branches along length are infrequent • Axon collaterals – Multiple branches at end of axon • Terminal branches – End in knobs called axon terminals (also called end bulbs or boutons) Nerve Impulse • Nerve impulse = action potential – Generated at initial segment of the axon (hillock) – Conducted along the axon electrical signal – Causes release of neurotransmitters at axon terminals • Neurotransmitters chemical signals (excite or inhibit neurons) – This is how the neuron receives & sends signals Synapses • Site at which neurons communicate • Signals pass across synapse in 1 direction • Presynaptic neuron – Conducts signal toward a synapse • Postsynaptic neuron – Transmits electrical signal away from a synapse 2 Neurons Communicating at a Synapse Presynaptic neuron Axon Axon terminal at synapse Synapse Dendrite (a) 2 neurons connected by synapses Postsynaptic neuron Types of Synapses • Axodendritic – Between axon terminals of one neuron & dendrites of another – Most common type of synapse! • Axosomatic – Between axons & cell bodies Synapses • Elaborate cell junctions • Synaptic vesicles in presynaptic neuron – Membrane-bound sacs containing neurotransmitters – Neurotransmitter released & binds to receptor & initiates depolarization of postsynaptic neuron – Mitochondria abundant in axon terminals • Synaptic cleft – Separates the plasma membrane of the 2 neurons The Synapse – Remember this at NMJ? Presynaptic axon Nerve impulses Microtubule Neurofilament Axon terminal Vesicle releasing neurotransmitter Mitochondrion Synaptic vesicles Synaptic cleft Postsynaptic dendrite (b) Enlarged view of the synapse Classification of Neurons • Structural classification – Multipolar possess more than 2 processes • Numerous dendrites & one axon – Bipolar possess 2 processes (1 axon, 1 dendrite) • Rare neurons • Found in some special sensory organs – Unipolar (pseudounipolar) possess 1 short, single process Neurons Classified by Structure Functional Classification of Neurons • Types of neurons based on functional classification: – Sensory Neurons – Motor Neurons – Interneurons Functional Classification of Neurons • Sensory (afferent) neurons – Transmit impulses toward the CNS • Virtually all are unipolar neurons • Cell bodies in ganglia outside the CNS – Short, single process divides into: » The central process runs centrally into the CNS » The peripheral process extends peripherally to the receptors Functional Classification of Neurons • Motor (efferent) neurons – Carry impulses away from the CNS to effector organs – Most Motor neurons are Multipolar – Cell bodies are within the CNS – Form junctions with effector cells • Interneurons (= association neurons) – Most are multipolar – Lie between/connect motor & sensory neurons – Confined to the CNS Neurons Classified by Function Supporting Cells • 6 types of supporting cells: – 4 in the CNS – 2 in the PNS • Provide supportive functions for neurons • Cover nonsynaptic regions of the neurons Neuroglia in the CNS • Neuroglia – Glial cells have branching processes & a central cell body – Outnumber neurons 10 to 1!!! – Make up half the mass of the brain – Can divide throughout life!!! – Have neuroglial adult stem cells Neuroglia in the CNS • Astrocytes most abundant glial cell type – Sense when neurons release glutamate – Extract blood sugar from capillaries for energy – Take up & release ions to control environment around neurons – Involved in synapse formation in developing neural tissue – Produce molecules necessary for neuronal growth (BDTF) – Propagate calcium signals involved with memory Neuroglia in the CNS Capillary Neuron Astrocyte (a) Astrocytes are the most abundant CNS neuroglia Neuroglia in the CNS • Microglia smallest & least abundant glial cell – Phagocytes the macrophages of the CNS – Engulf invading microorganisms & dead neurons – Derived from blood cells called monocytes Neuroglia in the CNS Neuron Microglial cell (b) Microglial cells are defensive cells in the CNS Neuroglia in the CNS • Ependymal cells • Line the central cavity of spinal cord & brain • Bear cilia help circulate the cerebrospinal fluid • Oligodendrocytes have few branches • Wrap their cell processes around axons in CNS – Produce myelin sheaths!!! Neuroglia in the CNS Fluid-filled cavity Ependymal cells Brain or spinal cord tissue (c) Ependymal cells line cerebrospinal fluid–filled cavities. Myelin sheath Process of oligodendrocyte Nerve fibers (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. Neuroglia in the PNS • Satellite cells surround neuron cell bodies within ganglia • Schwann cells surround axons in the PNS – Form myelin sheath around axons of the PNS Satellite cells Cell body of neuron Schwann cells (forming myelin sheath) Nerve fiber Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS. Myelin Sheaths • Segmented structures composed of the lipoprotein myelin whitish in color • Surround thicker axons • Form an insulating layer – Prevent leakage of electrical current • Increase the speed of impulse conduction! • Like the casing surrounding an electrical wire Myelin Sheaths in the PNS • Formed by Schwann cells • Develop during fetal period & in the first year of postnatal life • Schwann cells wrap in concentric layers around axon – Cover axon in a tightly packed coil of membranes • Neurilemma – Material external to myelin layers Myelin Sheaths • Nodes of Ranvier gaps along axon – Gaps in between adjacent Schwann cells – Thick axons are myelinated Nodes of Ranvier – Thin axons are unmyelinated • Conduct impulses more slowly • What happens at Nodes of Ranvier? Myelin Sheaths in the PNS An axon wrapped with a fatty insulating sheath formed from Schwann cells Myelin sheath Schwann cell plasma membrane Schwann cell cytoplasm Axon 1 A Schwann cell envelops an axon. Schwann cell nucleus Schwann cell cytoplasm Neurilemma 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. Neurilemma Myelin sheath Axon 3 The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. C section of a myelinated axon (TEM 30,000) Unmyelinated Axons in the PNS (b) Unmyelinated axons in PNS Axons that are not covered with an insulating sheath Schwann cell Schwann cell Axons Schwann cell nucleus 1 A Schwann cell surrounds multiple axons. Neurilemma Axons c s of unmyelinated axons (TEM 11,000) 2 Each axon is encircled by the Schwann cell plasma membrane. Myelin Sheaths in the CNS • Oligodendrocytes form the myelin sheaths in the CNS – Have multiple processes – Coil around several different axons – Can also see coiled layers of myelin & Nodes of Ranvier Nerves • Nerves cable-like organs in the PNS – Consist of numerous axons wrapped in connective tissue – Axon is surrounded by Schwann cells, that are then surrounded by connective tissue • You’ll see many nerves in lab under the microscope & models Nerves • Endoneurium layer of delicate connective tissue surrounding the axon • Perineurium connective tissue wrapping surrounding a nerve fascicle – Nerve fascicles groups of axons bound into bundles (just like in skeletal muscle!) • Epineurium whole nerve is surrounded by tough fibrous sheath Structure of a Nerve Axon Myelin sheath Endoneurium Perineurium Blood vessels Endoneurium Perineurium Fascicle Blood vessels Fascicle (b) Nerve fibers Epineurium Schwann cell nucleus Axon Myelin (a) Node of Ranvier Like an electrical wire/cable (c) Gray & White Matter in the CNS • Gray matter – Is gray-colored & surrounds hollow central cavities of the CNS – Forms butterfly-shaped region in spinal cord • Dorsal half contains cell bodies of interneurons • Ventral half contains cell bodies of motor neurons – Primarily composed of neuronal cell bodies, dendrites, unmyelinated axons gray – Surrounds white matter of CNS in cerebral cortex & cerebellum Gray & White Matter in the CNS • White matter – Lies external to the gray matter of CNS – Composed of myelinated axons white – Consists of axons passing between specific regions of the CNS – Tracts are bundles of axons traveling to similar destinations Gray & White Matter in the CNS Gray & White Matter in the CNS PNS Sensory (afferent) fiber Spinal nerve Motor (efferent) fiber CNS Gray matter Short unmyelinated interneurons Cell bodies of interneurons and motor neurons Neuroglia White matter Fiber tracts of myelinated and unmyelinated axons Hollow central cavity Integration Between the PNS & CNS • CNS & PNS are functionally interrelated • Nerves of the PNS – Information pathways to & from body periphery • Afferent PNS fibers respond to sensory stimuli • Efferent PNS fibers transmit motor stimuli from CNS to muscles & glands Integration Between the PNS & CNS • Nerves of the CNS – Composed of interneurons that: • • • • Process & receive sensory information Direct information to specific CNS regions Initiate appropriate motor responses Transport information from one area of the CNS to another Reflex Arcs • Reflex arcs simple chains of neurons – Explain reflex behaviors – Determine structural plan of the nervous system – Responsible for reflexes • Rapid, autonomic motor responses – Can be visceral or somatic 5 Essential Components of the Reflex Arc • 1.) Receptor site where stimulus acts • 2.) Sensory neuron transmits afferent impulses to the CNS • 3.) Integration center consists of one or more synapses in the CNS • 4.) Motor neuron conducts efferent impulses from integration center to an effector • 5.) Effector muscle or gland cell – Responds to efferent impulses • Contracts or secretes something Components of a Reflex Arc Stimulus Skin 1 Receptor Interneuron 2 Sensory neuron 3 Integration center 4 Motor neuron 5 Effector Spinal cord (in cross section) Types of Reflexes • Monosynaptic reflex – Simplest of all reflexes – Just 1 synapse – The fastest of all reflexes!!! • ie. Knee-jerk reflex Monosynaptic Reflex 1 Sensory (stretch) receptor 2 Sensory (afferent) neuron 3 4 Motor (efferent) neuron 5 Effector organ (a) Monosynaptic stretch reflex Types of Reflexes • Polysynaptic reflex – More common type of reflex – Most have a single interneuron between the sensory & motor neuron (= 3 neurons) • Withdrawal reflexes Polysynaptic Reflex 1 Sensory receptor 2 Sensory (afferent) neuron 3 Interneuron 4 Motor (efferent) neuron 5 Effector organ (b) Polysynaptic withdrawal reflex Simplified Design of the Nervous System • 3-neuron reflex arcs – Basis of the structural plan of the nervous system – Similar reflexes are associated with the brain Simplified Design of the Nervous System • Sensory neurons located dorsally – Cell bodies outside the CNS in sensory ganglia – Central processes enter dorsal aspect of the spinal cord • Motor neurons located ventrally – Axons exit the ventral aspect of the spinal cord Simplified Design of the Nervous System • Interneurons located centrally – Synapse with sensory neurons – Interneurons are confined to CNS – Long chains of interneurons between sensory & motor neurons Simplified Design of the NS Gray matter White matter Withdrawal reflex. A painful stimulus triggers nerve impulses in a sensory neuron, which initiate the polysynaptic withdrawal reflex. Cerebrum Brain stem Sensory neuron Cervical spinal cord Motor neuron Interneuron Parallel processing. Simultaneously, the nerve impulses travel on an axon branch that extends into the white matter. This ascending fiber carries the nerve impulses to the brain. Disorders of the NS • There are many! • Multiple sclerosis – Common cause of neural disability – Symptoms: fatigue, numbness, tingling, pain, blurred vision, lack of coordination, & paralysis – Cause is incompletely understood – An autoimmune disease: • • • • Immune system attacks myelin around axons in CNS Varies widely in intensity More women than men affected When men are affected, disease develops quicker & is more devastating Neuronal Regeneration in the PNS • Neural injuries may cause permanent dysfunction • If axons alone are destroyed, cell bodies often survive & axons may regenerate – PNS macrophages invade & destroy axon distal to the injury • Schwann cells form regeneration tube • Axon filaments grow peripherally from injured site • Partial recovery is sometimes possible Regeneration of the Peripheral Nerve Fiber Endoneurium Schwann cells Droplets of myelin 1 The axon becomes fragmented at the injury site. Fragmented axon Site of nerve damage Schwann cell Macrophage Aligning Schwann cells form regeneration tube 3 Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells. Fine axon sprouts or filaments 2 Macrophages clean out the dead axon distal to the injury. Schwann cell Single enlarging axon filament Site of new myelin sheath formation 4 The axon regenerates, and a new myelin sheath forms. Neuronal Regeneration In CNS neuroglia never form tubes to guide axon growth & may hinder axon growth with growth-inhibiting chemicals – No effective regeneration after injury to the spinal cord & brain!!! – This is why injuries to the CNS are so damaging BUT…. Current stem cell research has great potential for generating functional neurons & glia and treating many diseases of the nervous system Stem cells Neurons growth factors Culture stem cells in vitro transplantation into patient Grow & characterize cells Adult cells can be transformed into embryonic-like cells = Induced Pluripotent Stem Cells (iPSCs) & this could prevent immune rejection Clinical Trials on Stem Cells for Spinal Cord Injuries Underway Nervous Tissue Throughout Life • Nervous system develops from: the dorsal ectoderm – Invaginates to form the neural tube & neural crest • These cells divide & become neuroblasts Embryonic Development of NS Ectoderm (a) 28 days. Neural tube and Neural neural crest form tube from invaginating ectoderm. Neural crest (b) Week 5. Neuroepithelial cells of the neural tube divide and migrate externally to become neuroblasts and neuroglia. Neuroblast s Neuroepithelial cells Neuroepithelial cells Sensory neurons from neural crest Alar plate: interneurons Axons form white matter Neuroepithelial cells Basal plate: motor neurons Central cavity (c) Week 6. Neural crest cells form the sensory neurons. Dorsal neuroblasts form the alar plate (future interneurons). Long axons extending from the interneurons form the white matter. Ventral neuroblasts form the basal plate (future motor neurons). Questions…? S. Schuh-Huerta What’s Next? Lab: Nervous Tissue & CNS Mon Lecture: Finish NS tissue/CNS Mon Lab: CNS Wed Lecture & Lab: CNS