Lecture 6 Outline - Tissues Part 4 Nerve tissue and the nervous system General characteristics Nervous system (NS) overview Neurons and supporting cells Basic structure and function of peripheral nerves Today?s most powerful supercomputer ?pales in comparison with the sophisticated circuitry and functionality of the human nervous system? (Tortora & Nielsen, 2009) General Characteristics MOST COMPLEX Nerve tissue: general characteristics Highly cellular in nature > 80 % of nerve tissue represented by nerve cells < 20 % of tissue is extracellular space Represents by 2 types 1. Neurons 2. Gilal cells. Total nervous system mass?2kg (4.5lbs) NS ? most complex of body?s systems Extremely complex architecture Highly organized network of billions of cells Nerve tissue: general characteristics Composed of two major cell types: Neurons The organic circuitry that? connects all the regions of the body to the central processing unit (brain) Transmission mechanism: Action Potential Capable of reaching great lengths Make extremely intricate connections with other cell types Nerve tissue: general characteristics Composed of two major cell types: Neurons (continued) (tansmitter) Responsible for most of the unique functions of NS, e.g., Sensing, thinking, remembering, and controlling Last a lifetime: No Miotic ability (cell division) Can?t Reproduce. Highly active: consume lots of energy requires good nutrient & oxygen supply. Nerve tissue: general characteristics Composed of two major cell types: Neuroglia (a.k.a. glial cells) Smaller cells than neurons Greatly Out# neurons (perhaps by 25 times) Support, Nourish, Protect Neurons Maintain interstatial fluid that bathes neurons Unlike neurons, neuroglia? continue to divide across the lifespan. Nervous System Overview/Organization Nerve Tissue: Nervous syst overview Central nervous system (CNS) Peripheral nervous system (PNS) Brain (~100 billion neurons) plus spinal column (~100 million neurons) Central Processor ?Higher Centers? CNS- brain and Spinal cord. All nerve structures outside CNS Cranial Nerves- come off brain head &neck spinal Nerves ?communication lines??to form CNS Anatomical organization: Nerve Tissue: NS overview Afferent system (sensory) SAT Efferent system (motor) MEA Conscious sensation or autonomic feedback Information flow is to the CNS Muscles, motor unit control, glandular secretions Information flow is from the CNS *Autonomic - involuntary Functional organization: Nerve Tissue: NS overview Autonomic nervous system (ANS) Somatic nervous system Involuntary in nature Both sensory and Motor functions Info to and from smooth and cardiac muscle and organs Both Sensory & Motor function Conscious sensation from receptors in skin, skeletal muscle joints, vision, hearing, taster, smell. Voluntary Motor Control PNS functional organization: Nerve Tissue: NS overview Sympathetic division Parasympathetic division Control under stressful situations ?flight or fight? responses exercise or emergency. Regulator of smooth muscle of CV system Control under Normal maintenance situations ?rest or digest? Regulator of smooth muscle of digestive tract and respiratory systems. Two branches of ANS motor function: CNS: brain + spinal cord Nerve Tissue: NS overview afferent efferent PNS Information flow at the spinal cord: Afferent transmission from receptors in periphery through the dorsal root of spinal nerve to the CNS Efferent transmission from the CNS out through the ventral root of spinal nerve to the periphery Nerve Tissue: NS overview *Communication Mechanism Neurons and Neuroglia Electrical conductors of the CNS & PNS Generally made up of two basic parts: Cell Body and nerve fibers (variable numbers of extensors) Nerve Tissue: Neurons Cell body: a.k.a. the soma or perikaryon located in brain and spinal cord and ganglion (ganglia) bulges Contains the nucleus Nerve Tissue: Neurons Nerve fibers - processes or extensions from soma are of two types: Nerve Tissue: Neurons Dendrites Soma Axon Dendrites- recpetive branches of neuron. Usually quite short Hundreds per neuron Conduct signals towards the soma Nerve Tissue: Neurons Axons Each motor neuron has 1 axon- although branching is possible. May be very long ? 1 meter or more. Conduct action potentials away from soma towards another neuron or muscle fibers, or ganglia Often encased in a? myelin sheath a lipid & protein electrically- insulated wrap *longest one is in the leg* Nerve Tissue: Neuroglia Contained in both CNS and PNS Smaller than neurons Much more plentiful than neurons, perhaps as much as 25 times more plentiful Unlike neurons, neuroglia are not electrically excitable From a scaffolding for neurons ? help support, nourish, and protect neurons Unlike neurons, neuroglia continue to divide across lifespan Neurons ? transmission for sensory or motor. Neurons never Divide. Nerve Tissue: Neuroglia Types of neuroglia in CNS (4 types) Astrocytes Star-shaped with many PSEUDOPIA Largest and most numerous of neuroglia Help maintain spatial orientation and shape. Wrap around capillaries and neurons ? help maintain ionic permablity & proteins. *knee extensor ? knee flexion- eccentric. Nerve Tissue: Neuroglia Types of neuroglia in CNS (4 types) Microglia Similar in shape to astrocytes Small cells with slender processes House-keeping function: As phagocytes and removes cellular debris. Nerve Tissue: Neuroglia Types of neuroglia in CNS (4 types) Ependymal cells Line cavities of brain and central canal of spinal cord Produce?CSF have cillia to monitor Nerve Tissue: Neuroglia Types of neuroglia in CNS (4 types) Oligodendrocytes Similar in shape to astrocytes but smaller and with fewer extensions Responsible for ?forming and maintaining myelin sheaths around CNS axons. It wraps around another neuroglia. Nerve Tissue: Neuroglia Types of neuroglia in PNS (2 types) Satellite cells Surround the cell bodies of PNS neurons Function not well understood but thought to help regulate ?exchanges of materials between neuronal cell bodies and interstatial fluid. Nerve Tissue: Neuroglia Types of neuroglia in PNS (2 types) Schwann cells Similar to oligodendrocytes in that they?form myelin sheaths around PNS Nerve Tissue: Neuroglia Schwann cells Myelination Nodes of Ranvier Small gaps *greatly speed Action Potential propagation via salatory conduction Peripheral nerves Nerve Tissue: Peripheral nerves Bundles of afferent and efferent axons Nerve trunk encased by epineuerium Fascicles encased by perineurium Each axon encased by endoneurium Sound familiar? Nerve Tissue: Peripheral nerves Scanning electron micrograph of nerve trunk cross-section vessels perineurium fascicle endoneurium fibers (individual axons)
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